Recent development trends in metal forming

archives of civil and mechanical engineering 19 (2019) 898–941

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Review

Recent development trends in metal forming Z. Gronostajski a, Z. Pater b, L. Madej c,*, A. Gontarz b, L. Lisiecki c, A. Lukaszek-Solek c, J. Luksza c, S. Mróz e, Z. Muskalski e, W. Muzykiewicz c, M. Pietrzyk c, R.E. Sliwa d, J. Tomczak b, S. Wiewiórowska e, G. Winiarski b, J. Zasadzinski c, S. Ziólkiewicz f a

Wroclaw University of Science and Technology, 27 Wybrzeze Wyspianskiego St., 50-370 Wroclaw, Poland Lublin University of Technology, 38D Nadbystrzycka St., 20-618 Lublin, Poland c AGH University of Science and Technology, 30 Mickiewicza Av., 30-059 Kraków, Poland d Rzeszow University of Technology, 12 Powstanców Warszawy Av., 35-959 Rzeszów, Poland e Czestochowa University of Technology, 69 Generala Jana Henryka Dabrowskiego St., 42-201 Czestochowa, Poland f INOP – Metal Forming Institute, 14 Jana Pawla II, 61-139 Poznan, Poland b

article info

abstract

Article history:

Major, recent developmental trends in the field of metal forming are presented in the paper

Received 28 January 2019

both from experimental and numerical point of view. First, progress made in metal forming

Accepted 15 April 2019

processes such as: rolling of long flat products, cross wedge rolling, open die forging, die

Available online 4 May 2019

forging, extrusion, drawing, and stamping are addressed. Then, the study provides information on the current trends in the application of numerical modeling in the field of metal

Keywords:

forming. Presented discussion of the particular issues, is confronted with the authors' own,

Rolling

recently elaborated, solutions. © 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.

Forging Extrusion Drawing Stamping

1.

Introduction

Metal forming is one of the oldest technology known to mankind. Already in 4500 B.C., metal forming techniques were applied to copper, with the use of hammers made of strong stones. Steel began to be forged at a larger scale, after it was produced in bloomeries, in about 1000 B.C. In the 8th century B. C., coins began to be manufactured (from silver or electron

alloy) in processes resembling primitive die forging. The Middle Ages are the times of the development of forging due to the popularization of water wheel, which started to be used in forging shop to power the hammers for open die forging (since the 14th century) as well as rolling mills applied initially in the production of lead plates (since the 17th century). The further development of metal forming technologies was connected with the introduction of a steam engine, which began to be applied to power machines such as: rolling mills (Wilkinson,

* Corresponding author. E-mail address: [email protected] (L. Madej ). https://doi.org/10.1016/j.acme.2019.04.005 1644-9665/© 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.

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1783), hammers (Nasmyth, 1839) or presses (Haswell, 1861). At the beginning of the 20th century, forging machines started to be equipped with individual electric drives. Next, automatization and numerical control were introduced to create the metal forming industry of today. At present, metal forming is an important part in the production of machine components as well as ready to use products, mainly owing to such advantages as: economy of material and labor, repeatability of dimensions, possibility of producing the desired internal stresses and of providing the objects with in use properties often unreachable by means of other manufacturing methods e.g. casting or machining. Thus, metal forming operations are often applied in the industry, especially in large lot production. The popularity of this technology can be proved by a statistical car, which, in 90%, consists of components made by metal forming processes. Therefore, in the recent years, significant progress is being observed in the development of modern metal forming processes. This is reflected in high value journal publications as well as granted patents. There is, however, no comprehensive study, which discusses the progress in this field in the recent years. With that, an idea was created at the Metal Forming Division of Metallurgy Committee of the Polish Academy of Sciences to provide a review on the matter that would summarize the achievements and identify directions of future developments in metal forming.

2.

Rolling processes

The rolling processes belong to the group of the most efficient production technologies and can be in general classified as rolling of long and flat products or forge rolling of shaped components.

2.1.

Shape rolling

Shape rolling processes are applied in the production of long products characterized by simple cross-sections (e.g. round, square, flat) as well as ribbed bars and wire rods [1,2]. Besides that shape rolling found wide range of applications in the production of complicated profiles, such as angle bars, channel bars, I-beams and rails [3–5]. The shape of a final product is provided by means of cooperating rolls with properly designed grooves. Shape rolling is often used to obtain final products with controlled mechanical properties [2,6,7]. As mentioned, rolling is most often applied to produce bars with simple crosssections [1,2,8], which are used as final products or semiproducts for further processing by drawing, die forging or extrusion. Therefore, the paper pays particular attention to the recent achievements in this field of applications. The precision of shaped rolled products is described by deviations between the measured cross section dimensions and their nominal values. In case of round and square products as well as flat rods both height and width are characteristic dimensions while for ribbed rods, the running meter is most often used. In recent years, a clear increase in the customers demand for rolled products within the negative deviations from required tolerances is observed. The newlyconstructed and recently modernized shape rolling mills are

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capable of producing parts with dimensional accuracy lower than 1/4 DIN standard what directly results in significant metal economy as well as mass reduction of final products and their assemblies [8]. The main factors affecting dimensional accuracy of final products, beside rolling mill parameters, are grove shapes, deformation pass design and rolling temperatures [9]. The latter ones, play particularly important role. Variations in the billet temperature along the length of furnace can be in the range of 30 8C to even 90 8C [10]. A non-uniform temperature distribution in the rolled band significantly affects recorded force-energy parameters, results in nonuniform wear of the rolls, and most of all influences plastic flow of metal during subsequent rolling passes. As a result final product may fail to meet the customers' requirements. As seen in Fig. 1a and b, a temperature drop along the length of the band by about 50 8C causes a momentary increase in rolling torque and power. At the same time such temperature drop influences dimensional accuracy what can be seen in Fig. 1c and d. In this case a drop of temperature by 50 8C along the length of a rolled oval band with diameter 70 mm, resulted in an increase of the band width of 0.8 mm in the areas of a lower temperature [10]. Therefore, temperature is a very important factor, with regard to both the rolling mill stand safety as well as final product dimensional accuracy [10]. The precision of shaped rolled products also depends on the applied system of passes and grove shapes. In order to unify the deformation distribution in the rolled band, modifications of the classic passes are more often applied [1,8]. The effect of the application of modified stretching passes on the possibility of obtaining final products with narrowed dimensional tolerances as well as on the decrease of the rolling force parameters for the two rolling variants has been discussed in [8]. In the paper, the first variant is a horizontal oval-vertical-oval-horizontal-oval-circle system often called ‘‘universal’’ system of passes, which enables shape rolling of a wide range of products with various dimensions. In the second variant for the horizontal and vertical oval passes, modification of shapes was introduced and new dimensions of the passes were determined. In the design of the modified passes, the similarity of the surface areas of the modified passes with those of the classic passes were used. The modification consisted, among other things, of the application of three construction radii, while the typical oval passes have only one radius. Examples of geometrical changes of the rolled 14 mm diameter bar in subsequent rolling are shown in Fig. 2. Comparisons of products obtained from the two investigated variants clearly showed that introduced modifications narrow dimensional tolerances of the final product (no ovality of the final product, regardless of the applied steel). Also recorded mean unit pressures acting on rolls during deformation was lower in the second variant of the technology what directly leads to reduction of the rolls wear. The surface quality of the rolled product was also improved [11]. The reduction of rolls wear is often addressed in the literature by means of different approaches. In [1] an interesting approach based on combination of regression analysis and the probability density function distribution was used to optimize the oval pass shape.

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Fig. 1 – Comparison of the change in the value of the torque (a) and power of rolling (b) as well as temperature distribution and its influence on the band width in the exit from the roll gap for the two cross sections (c and d) [10].

Fig. 2 – Shape and dimensions of the crosswise templets collected from the bands after particular roll passes: (a) steel C45, variant I, (b) steel C45, variant II, (c) steel X2CrNi19-11, variant I, and (d) steel X2CrNi19-11, variant II.

In the recent years, the demand for ribbed bars for concrete reinforcement has increased. That resulted in improvement of the rolling technology.as consumers require products with appropriate structure, shape as well as dimensions but at a low price at the same time. One of the ways of reducing the production costs of ribbed bars is increasing the production capabilities by using e.g. rolling with longitudinal band separation (multi-slit rolling process) instead of the traditional

single-stand rolling technology. The major advantage of the technology is that it can be introduced into the industrial practice both in the new continuous rolling mills and in the already existing modernized ones without significant investment costs. The technology allows separation of the billet at the rolling train into two, three, four or five strands for subsequent rolling of ribbed bars. The main difficulties encountered in the technology are related to the selection of the appropriate shape of the slitting passes, which are responsible for the shaping of the particular strands and the bridges which connect them. The issue of low durability of the rolling equipment used during slitting passes is another a disadvantage of the process [15]. Since the beginning of the 21st century, many scientific centers have been conducting research addressing these issues. The studies concentrated mainly on the optimization of the shape of slitting passes and the selection of the system of these passes [12–15]. Adding additional slitting passes and proper modification of the geometry of other rolls leads to reduction of deformation value and eventually wear. Example of the modified technology is presented in Fig. 3. The proposed slitting pass system used during the rolling process can also lower force-energy parameters, which reduces the load of the main drives for the group of finishing stands of the analyzed rolling mill [12,13]. During designing of the shape rolling technology, beside obtaining appropriate shape of the final product required by the customers, particular attention has been payed to the final mechanical properties. These properties in a classical ap-

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Fig. 3 – System of slitting passes for three-strand rolling of ribbed bars: (a) classic, (b) modified; H – horizontal rolls stand and V – vertical rolls stand.

proach can be obtained after the rolling process through the use of thermal treatment. However, in recent years, the concept of accelerated cooling of the band between rolling stands has been introduced [2,6,16]. The use of this technology for e.g. 26 mm diameter bars, rolled under the conditions of a continuous mill D370 provided the possibility to obtain a lower rolling end temperature what directly influence the properties of final product. The use of accelerated cooling limited the growth of the austenite grain, which ensured a reduction of its size from about 20 mm for the classic process to about 15 mm [16] and causes homogenization of the microstructure at the cross-section of the rolled bars from steel S355. This leads to increase in the impact energy value from 8 to 60 J for 40 8C and from 30 to 92 J for 0 8C. A similar approach to the one analyzed has also been discussed in study [6].

2.2.

Rolling of flat products

Manufacturing of flat products in the form of hot rolled thick and thin plates, cold rolled thin sheets and strips as well as coated sheets is presently the second largest (after long products) assortment of products obtained with the use of the rolling technology. Among the products mentioned above, a part of them is directly used for the constructions of various kinds (e.g. shipbuilding or power industry), but a significant majority, after cutting, is used as semi-products for further processing in e.g. rolling, forming and bending. Advanced rolling technologies must ensure not only required geometry (e.g. flatness) of the plate and sheet, but also required more elevated mechanical properties, which are obtained e.g. by using new advanced types of steel. This is connected with the automatization and robotization of the production processes as well as the aim of lowering the kerb weight with an increased level of safety (e.g. automotive industry).

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During the rolling of thick plates, there is a problem with the ‘‘penetration’’ of the deformation in the whole thickness of the strip. Inhomogeneous deformation in the thickness results in a non-uniform morphology of the microstructure, which worsens the mechanical properties. In recent years, in order to homogenize the microstructure in the whole volume of the rolled strip, a new technology of rolling thick and thin plates has been developed, i.e. Gradient Temperature Roll (GTR), which ensures an increase of the deformation uniformity in the whole thickness of the strip. The concast slab, and next the rolled strip, are subjected to intensive cooling between the particular passes in such a way so as to lower the surface temperature to about 800 8C. In turn, the core has a much higher temperature, which can be equal even as much as 1100 8C. The temperature gradient obtained in this way causes the prescribed deformation to accumulate in the strip's core, which contributes to obtaining of fine austenite grains as well as fine and homogenous ferrite plates. Both ensures an improvement of mechanical properties in the whole thickness of the plate [17,18]. When geometrical accuracy of sheets and plates is of interest an asymmetric rolling can be used. This process is known for many years, although being still developed and modified, [19]. Introduction of asymmetry in the form of a difference in the peripheral speeds or diameters of the rolls lowers the rolling force and, as a result of improvement of the sheets' flatness, reduces the dimensional deviations in their length and width. In recent years, this technology has also been analyzed in respect of an improvement of the properties of the rolled products [20,21]. The use of asymmetrical process of rolling for austenitic steels intensified the recrystallization and refinement of the structure in the central layer for temperatures above 900 8C. In turn, for rolling temperatures below 900 8C, even with the use of a 60% rolling reduction, in a symmetrical rolling process, the phenomenon of dynamic recrystallization, occurring in the asymmetrical process, was not observed. A significant effect of asymmetrical rolling on the mechanical properties was observed also in the case of sheets rolled from non-ferrous metals [21]. As a result of the activation of shear bands, in the deformed material, a stronger grains refinement takes place in the areas of their presence, which improves the mechanical properties, as compared to the sheets obtained in a symmetrical process. At present, the steel processing industry (including sheets) requires from the semi-products to exhibit better and better properties both in respect of quality (proper geometry) and functionality (mechanical properties, but also, more and more often, a reduced level of residual stresses). The sheets, after they have been unwind from the coil, usually are characterized by lack of flatness as well as a high and non-uniform level of residual stresses. This is, among other things, a remain of the rolling process and bending into coil. In the case of hot rolled sheets, it is also related to cooling, which causes non-uniform temperatures of the sheets along their width (the edges cooled more intensively than the central part), which especially generates geometrical defects and increases the level of residual stresses (residual stresses type I). There are two groups of defects strip flatness (mainly non-uniform strip thickness) and strip straightness deviations such as: waviness, bow shaped faults and strip camber. In the cold rolling the most common shape deviation is a strip waviness which is

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measured by a wave height and a wave length. In order to minimize such defects, in the recent years, a finishing pass with a very small deformation, usually from 0.5 to 4%, the socalled skin pass, has been used by the industry (temper rolling) [22,23]. Usually, the skin pass is applied after the annealing of the sheet in order to homogenize the level of residual stresses on the upper and lower surface of the sheet through softening of the surface areas, to improve the sheet's flatness and to obtain a specific sheet texture. Surface softening of the sheet lowers the non-uniformity of the deformation in the width of the profile formed from such a strip; also, the bow and camber type defects are limited. The use of the skin pass technology during the cold rolling of sheets, beside mentioned advantages, also lowers the operational costs, increases the elasticity of the mill and improves the surface quality of the sheet. The latter is especially important in the case of use of such sheets as a semi-product for the forming and stamping processes. Currently, an increasing interest in light construction materials characterized by high strength, with a simultaneous preservation of good plastic properties can be noticed. And so, more and more often, non-ferrous metal alloys are applied, especially those of aluminum and magnesium. The processes of metal forming of magnesium alloys are unique in their character, and their proper implementation in the industrial practice is very difficult. The main limitation of their application is the low plasticity of these alloys at low temperatures. Try to avoid it, the twin-roll casting (TRC) technology has been developing. With the technology fine-grained feedstock materials, which are then hot rolled into thin sheets can be obtained. This process combines metal solidification followed by rolling into one production stage. In this way, the number of passes as well as the thermal treatment are limited compared to the conventional rolling process. This results in an improvement of the mechanical and plastic properties of the products in comparison with those obtained in the classic technology of sheet rolling [24,25]. The properties of an Mg alloy sheet made in the TRC technology is significantly affected by the rolling parameters. The application of higher deformation values in the particular passes and a lower rolling temperature leads to a higher strength, lower elongation. In turn, higher rolling temperatures improve the deformability but worsen the strength of the sheet. It has also been proven that the quality of the final product is strongly affected by the amount of impurities in the feedstock material [25]. As far as hot rolling of thin strips is considered, the most important challenges can be summarized in four groups, as follows:  Dimensional accuracy, which involves problems of homogeneity through the width and through the length of the strip, as well as stability of rolling.  Low level of residual stresses, which can be obtained by controlling the laminar cooling and coiling parameters.  Improved microstructural parameters and mechanical properties, which are obtained by thermomechanical rolling and complex cooling strategies.  Rolling of new materials with exceptional properties – Advanced High Strength Steels are used as an example in the present paper.

There are, in general, two strategies in development of hot strip mills. The first is focused on development of conventional rolling mills (Fig. 4a), which are generally composed of furnace (1), descaler (2), slab sizing press (3), roughing train (4), roller table (5), finishing train (6), laminar cooling (7) and coiler (8). Development of these mills involves an increase of the power of motors, new gauge control systems and complex cooling strategies including ultra-fast cooling, which allows to decrease coiling temperature below 400 8C. The second strategy involves compact hot mills (CSP – Compact Strip Production), which is a novel technology developed in 1980s for casting-hot-rolling of thin slabs (Fig. 4b). This technological innovation follows converter steelmaking and continuous casting technology in the steel industry. The CSP provides a more compact line and simpler procedure as compared with the traditional hot rolling technology. Both conventional hot strip mills and CSP trains are now capable of reducing the final thickness of the strip to 1 mm. Hot rolled strips can be either used directly for manufacturing final products or can be subjected to further cold rolling process (Fig. 4c). In the former case the final microstructure is obtained by applying complex cooling strategies in the laminar cooling. In the latter case hot rolled strips are subjected to typical cooling cycles resulting in ferrite–pearlite microstructures. There is no CPS mill operating in Poland and this process is not discussed further here, see [26] for details on the development of the CPS technology and [24] for its application to magnesium alloys. The focus in the present paper is on development of new routes for conventional hot strip mills with manufacturing of AHSS strips used as an example. Multiphase microstructure of the AHSS can be obtained either during laminar cooling after hot rolling (thicker strips) or during continuous annealing after cold rolling (thinner strips). Hot strip rolling is a process specifically designed to control the product microstructure and properties. The process called thermomechanical process control (TMPC) is of particular importance. The TMPC rolling is used to obtain both the desired shape of the product and its final properties without further processing. The microstructure is sensitive to the process parameters. Thus, understanding and controlling the influence of these parameters is crucial for obtaining the desired product properties. A typical the TMCP rolling mill configuration is shown in Fig. 4a. It illustrates wide possibilities for varying the process parameters including rolling schedule, rolling temperatures, cooling temperatures and rates, etc. The property control is mainly achieved in the finishing mill and by laminar cooling afterwards. The influence of both is briefly discussed below. The TMCP aims at maximum grain refinement, which can be reached either by the austenite grain refinement or by increase the effective area of grain boundaries or increase the density of nucleation sites for ferrite. Rolling deformation results in an increased energy level of the material due to the accumulated dislocations generated by plastic deformation. The elevated temperatures during hot rolling lower this stored energy by recovery, recrystallization and grain growth processes. In this context the non-recrystallization temperature TnRX, above which static recrystallization between passes

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Fig. 4 – Typical process steps for TMCP rolling mill configuration (a) and Compact Strip Production (b) followed by cold rolling, continuous annealing and manufacturing of final product (c).

Table 1 – Key aspects regarding innovative hot strip rolling processes [28]. Market demands Improved homogeneity of mechanical properties Improved homogeneity of mechanical properties Reduction of geometrical tolerances Cost efficiency vs. Mechanical properties cost reduction due to decrease of alloying elements content Increased dimensional feasibility

New grades development and production Weight reduction

Needs at rolling mill Cooling homogeneity/control of process conditions Rolling stability + cooling homogeneity Rolling stability Lower alloying elements, increased cooling capacity and efficiency (UFC) Adaptation of the metallurgical concept Increased power/torque on HSM, different technologies (CSP for thinner product), rolling stability, coiling capacity for thicker products. Low coiling temperature (200–400 8C) Production of new grades with enhanced mechanical properties

occurs, is of main interest. TnRX depends on the deformation, the cooling rate and the inter-pass times between roll stands. Only decelerated recrystallization appear below this temperature due to strain-induced precipitation of second-phase particles. In case of retarded recrystallization the work hardened (pancaked) austenite provides a high number of nucleation sites for the phase transformation resulting in very fine ferrite grains. The control volume fractions of constituent phases are the main challenge in manufacturing AHSS strips. The transformation kinetics is needed to reach this goal. By using dilatometric results the austenite to ferrite fraction is deduced. When the material reaches the defined fractions, the fast cooling begins and the material is cooled down to martensite start temperature (Ms). In general it can be stated that

Limitations Classical cooling is heterogeneous Different thermal path from head to tail & heterogeneous coil cooling High investment required

Investment required and increase of energy consumption Different thermal path from head to tail Difficulty to control coiling temperature in this range of temperature Complex concept

decreasing hot deformation temperature or/and increasing strains shifts the transformations start temperatures toward higher levels and lowers the Ms, what pronounce finer ferrite grains and finer martensite blocks. The effect of the TMCP rolling compared to conventional rolling was thoroughly investigated by the researchers, see [27] for the review in the area. A variety of pass schedules and mill characteristics together with the operating capacity and related constraints were analyzed and innovative rolling routes as well as modern equipment was proposed, while the general setup of the mill (Fig. 4b) remained unchanged. Key aspects regarding innovative rolling processes are summarized in Table 1. It is seen that a number of challenges is large and they involve various aspects of the process. General discussion of all these items is presented in [28]. Two aspects

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connected with: (i) rolling stability due to temperature control and (ii) new grades development (AHSS, bainitic) by sophisticated and more intensive cooling routes and decreasing the coiling temperature, are presented below. AHSS grades compose of dual phase (DP), complex phase (CP), transformation induced plasticity (TRIP) and martensitic (M) steels. It can be stated that the production of DP steels is well investigated. However, the process window in hot rolling of these steels is tight as only very short times less than 10 s are allowed on the existing run-out tables, according to their limited length. The pursued objective of research in [28] was to accomplish a higher flexibility in terms of rolling AHSS strips. This was intended by the intercritical rolling in the temperature range of the austenite–ferrite transformation. This process exhibits the potential of producing new, high performance hot rolled products and the ability to produce thinner strips for a given maximum rolling force. Beyond this, the cooling rate, which is necessary to get the desired amount of martensite can be reduced significantly. Controlled accelerated cooling treatment is the crucial part of the hot strip mill and it decides about product microstructure and properties. Conventional laminar flow cooling (LFC) systems use one of the following strategies: front cooling, rear cooling and sparse cooling. Since the cooling schedules required for the AHSS microstructures often cannot be reached in the existing LFC systems for typical velocities and thicknesses of strips, new systems were proposed, such as: reinforced cooling (RC), compact cooling (CC) and ultra-fast cooling (UFC). They can achieve a wide range of cooling rates at the exit of the rolling mill, the main objective being a reduction of the time between the last rolling pass and the run-out table. It was observed that through the rational allocation of the UFC and LFC, more flexible cooling path control and cooling strategies could be obtained. The new systems strengthen the control capability of grain refinement and precipitation behavior by the front located UFC. Meanwhile, due to the appropriate set of intensive laminar boxes, the rear LFC cooling capacity can be further improved and meet the new demands. In consequence, the mechanical properties of the steels get a great leap forward due to the cooling strategies, which can decrease costs and create economic benefits for the steel companies. The UFC technology allows for the realization of high cooling rates and has a remarkable effect on grain refinement and precipitation during phase transformation. By applying the TMCP technology followed by the UFC combined with the traditional LFC and suitable control strategy, steel products with high added value can be processed. This technology allows for the production of high strength material at a reduced amount of alloying and microalloying elements. High strength level (750 MPa) can be obtained for a lean C–Mn– Nb chemistry, with an attractive strength-ductility balance, high toughness, as well as interesting fatigue properties in the as hot-rolled condition. All discussed problems are particularly important for other than DP multiphase steels. Investigation of various hot rolling routes cannot be performed on the industrial mill and industrially realized pass schedules are difficult to acquire due to the existing competitive situation and resultant confidentiality. Therefore, it is only possible to gain indirect information by using numerical simulations as well as physical simulations in

laboratory conditions. The former can be performed by using e.g. VirtRoll hybrid computer system described in [28,29]. VirtRoll, which is a digital twin of the industrial mill, is composed of two parts: (i) a web-based module allowing design of an arbitrary rolling mill and (ii) computing module dedicated to numerical simulations of designed manufacturing cycle. The VirtRoll system combines models, data and knowledge bases and inverse approach to design of optimal process parameters with respect to required final properties. It allows configuration of a rolling line composed of basic equipment like furnaces, descalers, rolling stands, laminar cooling, coilers, etc. Dependently on selected materials specific numerical models can be loaded by the system. Information about materials, models and devices available for hot rolling mill designers is stored in the database, which is the crucial part of the system. Details concerning the material models in the database can be found in [30] and coefficient in these models for all steels investigated in [28] are given in [29]. Flexibility and a content of the database are the factors, which decide about the reliability and usefulness of the whole system. On the other hand physical simulations of new rolling routes can be performed on thermo-mechanical simulator Gleeble 3800. Special focus can be turned to the identification of an enlargement of the working space enabling the more flexible and energy efficient production of existing steel grades and to production of new grades. As it has been mentioned, hot rolled strips require diverse and flexible control of cooling path in order to take full advantages of strengthening mechanisms, such as fine grain strengthening, precipitation strengthening and transformation strengthening. Results of physical and numerical simulations performed to develop optimal cooling strategies are described below. Bainitic steel and AHSS strips were considered. Different schedules were investigated for each group of steel grades [29]:  Bainitic steels. Four steels with various content of Nb, Ti and Mo were tested. Two variants of physical simulations were considered. In variant 1, presented schematically in Fig. 5a, lower temperatures of deformation were applied. Variant 2, which is not presented here, was characterized by higher temperatures of deformation. Grain size prior to the first deformation (after soaking) was 67 mm. Cooling from the last deformation temperature to the holding temperature was at the rate of 20 8C/s. Three holding temperatures during cooling, 400, 450 and 500 8C for cooling versions a, b and c respectively, were used for each variant. These temperatures correspond to the coiling temperatures in the industrial mill.  AHSS: Four steels with various combinations of Nb, Ti and Mo were tested. Three schedules were considered (Fig. 5b). The four first passes were the same in all the cases. The difference was in the interpass time between the two last deformations and in the time before accelerated cooling. Schedule (a) was a reference schedule, schedule (b) was the shortest interpass time between the two last deformations and schedule (c) was the shortest time between the last deformation and immediate accelerated cooling. It allowed to obtain different state of the austenite at the beginning of phase transformations. Moreover, two coiling temperatures

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Fig. 5 – Simulated rolling schedules for bainitic steels (a) and AHSS grades (b) [29].

Fig. 6 – Sample images of the microstructure for 0.18%Ti bainitic steel variant 2a (a) and variant 2b (b) [28].

(CT) were considered (500 and 600 8C). The objective was to see the influence of the CT on the subsequent microstructure and properties. Samples were quenched at various stages of the process and microstructure was analyzed. Mechanical properties of all samples were measured. Full set of micrographs and results of properties measurement is presented in [28] and only the most important results and conclusions are discussed below. Bainitic steels. Typical microstructures are shown in Fig. 6a for variant 1 and in Fig. 6b for variant 2. The microstructure contains allotrimorphic ferrite (<10%), degenerated upper and lower bainite, and blocky martensite. It is seen that, generally, coiling at the highest temperature of 500 8C results in a coarser microstructure and higher blocky martensite fraction compared to coiling at 450 and 400 8C. Some diversification of strength of the samples was observed depending on the coiling temperature, namely, decreasing the CT temperature resulted in yield strength decrease and elongation increase, but the lowest strength was obtained for the CT of 450 8C (variants b). Steel with 0.18Ti performs better in terms of strength versus ductility relation. Steel with 0.03Nb and 0.18Ti showed the highest mechanical properties. For low end of rolling temperatures (variant 1) yield stress (YS) around 840 MPa and Ultimate Tensile Strength (UTS) around 1110 MPa were obtained. These properties were, however, combined with slightly lower elongation, which was due to the highest amount of blocky martensite. The most plausible explanation of all the observation is that longer time of thermomechanical processing resulted in more intense precipitation of TiC in the austenite. This, however, lowers

the precipitation strengthening effect of TiC in bainitic ferrite, see [28] for details. AHSS. For all the samples ferrite was the main constituent of the microstructure (from 60 to 90% depending on the grade and process conditions). The rest of the microstructure consisted of pearlite and/or martensite. No significant effect of the rolling schedule (time between two last deformations and time before cooling) was observed mainly due the fact that austenite is already highly non-recrystallized for the reference rolling schedule. In all cases austenite was highly deformed leading to fine but heterogeneous microstructures. Concerning mechanical properties, better results were obtained for a coiling temperature of 600 8C. The higher level of properties (YS = 810 MPa and UTS = 1130 MPa) was obtained for the TiMo grade and CT of 600 8C. Lower mechanical properties of the ‘‘no Mo’’ comparing to ‘‘Mo’’, which is due to higher fractions of hard phases in the latter. In some cases, important gap between mechanical properties was observed without significant variations in the microstructure suggesting an important effect of precipitation strengthening. This effect seems more pronounced for Ti grades. All rolling/cooling schedules were simulated by mentioned VirtRoll system and selected results for the bainitic steel are presented in Fig. 7. Two cases were considered, conventional rolling with the final temperature of 950 8C (variant 1) and a new route with additional fast cooling before stands 5 and 6 (variant 2). Time-temperature profiles for both variants are shown in Fig. 7a and changes of the austenite grain size are shown in Fig. 7b. There data were used as starting point for simulations of laminar cooling variants a, b and c, see Fig. 7c. All these plots show capabilities of using combination of

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Fig. 7 – Time–temperature profiles (a), grain size changes (b) and kinetics of phase transformation (c) for variants 1 and 2 (T – temperature, F – ferrite, B – bainite).

Fig. 8 – Methods of forge rolling: (a) longitudinal, (b) cross, and (c) skew.

TMPC, UFC and LFC to design process routes leading to a variety of microstructures. Other aspects of the hot strip rolling design, which were not discussed above and are of scientific and practical interests, include:  Investigation and analysis of the residual stresses caused be temperature heterogeneity.  Substantial through-thickness microstructure and texture inhomogeneity, which can occur due to the imposed through-thickness gradients in shear, deformation, and temperature.  The use of a coil box, which can help to limit the variation of finish rolling temperature along the length of the strip. More details on all these aspects can be found in [28].

2.3.

Forge rolling

The processes of forge rolling are usually realized under the conditions of hot metal forming, where, with respect to the positioning of the rollers and the rotation direction can be classified as follows:  longitudinal rolling (Fig. 8a), during which the treated object is in translational motion and the rollers arranged in parallel rotate in the opposite direction;  cross rolling (Fig. 8b), where the treated object rotates opposite to the rollers arranged in parallel, which rotate in the same direction;  skew rolling (Fig. 8c), during which the treated object is in translational-rotational motion (screw motion), and the rollers, arranged askew, rotate in the same direction.

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Fig. 10 – Results of modeling, in the Forge program, of the process of rolling a stepped shaft, with the marked temperature distribution (8C).

laboratory tests of longitudinal rolling of a lever preform made of the 2014 aluminum alloy. The [38] discusses the results of theoretical and experimental tests in the scope of cold longitudinal rolling of a compressor blade made of the Inconel-718 alloy. In this case, the research was directed to the improvement of the rolling accuracy by way of taking into account the elastic deformations occurring in the tool-rolling mill system, in the drafts.

2.3.2. Fig. 9 – View of the production of motorcar front axle beams (top) and the exemplary products (bottom) [35].

2.3.1.

Longitudinal rolling

Longitudinal forge rolling constitutes a popular type of rolling and is applied mostly for the formation of preforms of drawn forgings, preliminarily forged on forging presses. This process is currently quite well-known and so the number of new scientific papers referring to it is relatively low. However, after 2008, owing to the possibilities provided by computer modeling, new types of passes used in this rolling method have been developed, including oval-flat and oval-diamond passes [31,32]. The use of such passes gave a possibility to homogenize distribution of deformation intensity at the crosssection of the elongated strand. A lot of focus [33,34] has been put to issues related to the longitudinal rolling of motor truck front axle beams. At present, these components are flash rolled in three passes, and next, they are bent and forged on a press to obtain the predescribed shape. In China only, as many as 30 front axle production lines have been activated (Fig. 9), including longitudinal rolling mills and presses with the load of 25– 40 MN. The annual production efficiency of these lines varies in the range of 50,000–100,000 beams, depending on their overall dimensions. The material output is 85–92% and the durability of the rolling segments equals 8000–15,000 preforms, being twice as high as the durability of the dies in which the beams are bent and forged [35]. The literature also provides some information on the longitudinal rolling of components made of non-ferrous metals. And so, the studies [36] discuss research in the scope of rolling of a lever preform made of the AZ31 grade magnesium alloy. In turn, article [37] presents the results of

Cross rolling

Among the cross rolling technologies, the one mostly applied in the forging industry is the cross-wedge rolling (CWR), which is used to form components such as stepped axles and shafts, as well as to produce preforms, which are next forged at forging presses. This state of knowledge in the scope of CWR until 2010 has been discussed in detail in the study [39]. In the last decade, there have been numerous studies in the scope of CWR, in which the possibilities provided by numerical modeling have been widely used. A large part of these studies [40–44] have been focused on the research of the limitations in the course of the CWR process, in the form of: uncontrolled slide, narrowing of the formed core of the treated object, bending of the product and cracking of the material. Also, based on FEM analyses, the force parameters have been determined [45,46] as well as the change in the temperature [47], the microstructure [48], the stress [49] and the deformation state [50] in the rolled product. The development of computer programs used for the modeling of metal forming processes, as well as the increasing computational capabilities of the computer units created the possibility to stimulate very complex cases of CWR processes. An example in this scope can be the process of forming a driving shaft presented in Fig. 10, where the treated object is rolled by means of two rollers. The extreme steps are rolled with the cross-section reduction equaling 91%, which requires the use of a two stage rolling process. In the first stage, the forming of the pins is realized on the intermediate diameter, and in the second stage – on the final diameter. Taking into consideration the expansion of the technological possibilities of CWR, in recent years, several research works have been performed. A part of these studies referred to the forming of components with steps whose cross-sections are different than the circular one (e.g. square, hexagonal, oval). The making of elements with such steps requires the use of tools with specially profiled calibrating surfaces [51,52]. The characteristic of the mentioned forming processes is an alternating change of forces, which is a consequence of the

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cyclically changing draft. Examples of products, which do not fulfill the conditions of axial symmetry but can be produced by the CWR methods are also shafts with toothed rings or worm windings. In this case, the rolling tools are equipped with special inserts (placed directly behind the calibration zone) used to form the windings. The results of the research performed so far show that, in this way, it is possible to produce shafts with normal and skew toothing [53] as well as with a worm winding [54,55]. An emphasis has also been made on the development of new methods for manufacturing hollow elements, which are currently becoming popular in the machines construction. As a result of the quite numerous studies performed in this area [56–58], it has been established that:  the CWR process of hollow elements can be disturbed by the product being crushed or narrowed, as well as significantly deformed in its internal surface;  the wedge angles should be selected differently than in the case of full products (it is recommended to use smaller angles of flare and larger shaping angles);  in order to eliminate the ovalization of the cross-section, the sizing zone should be elongated in such a way so that the formed product can make at least 3 rotations in it.

Fig. 11 – A CWR factory producing transmission shafts [64].

volume of the material closed in the pass, which creates the grooves of the cooperating rollers. Tools with such shape are difficult to design and manufacture. That is why the discussed rolling technology is currently used to form products of simple shapes, such as rings for rolling bearings or balls. The limited scope of the use of skew rolling translates directly to a small number of scientific papers referring to this issue and published in the last decades in the subject literature. The studies recorded in this area referred mainly to the FEM analysis of the process of rolling balls [65] and the designing of tools securing the ball forming process [66]. In recent years, at Lublin University of Technology, a new method of producing steel balls has been developed, called wedge rolling (HWR). This method uses wedges, which are helically wound over rollers in one or two coils, which makes it possible to produce balls in a continuous way. The performed research [67] has fully confirmed the purpose of the developed forming concept. Taking into account the results obtained in the scope of HWR of balls, this technology could be applied in the manufacture of other axial symmetrical products, such as: joining elements [68], stepped axles and shafts [69], or bodies of rotary knives (Fig. 12).

With the aim of improving the possibilities of forming long products, a lot of attention has been paid to the development of the multi-wedge rolling technology, during which the product is formed by more than one pair of wedges. The application of such a solution results in shortening of the tools, increase of the forming forces and complication of the tools' construction (the shape of the side wedges must take into account the elongation of the product caused by the operation of the central wedges) [59]. Beside the long products, such as railway carriage axles [49], this method can also be used to produce several short products at the same time, such as balls for ball mills [60]. The CWR method is applied mostly to form steel products. However, recently, several research works have been performed on the expansion of this technology for products made of non-ferrous metals and their alloys. In this scope, so far, attempts have been made at the rolling of elements made of aluminum [61] and titanium [62] alloys, superalloys based on nickel, copper balls [63] and shafts made of a zinc alloys used in nuclear reactors [63]. The present popularity of the CWR process in the forging industry is proven by the fact that, in China only, about 300 production lines based on this technology have been activated. The total production capacity of these lines equals about 400,000 t/year. Also, in this country, approximately 20 professional CWR factories have been opened, each of them having the annual production capacity at the level of over 10,000 tons [64]. An example of such a factory, in which the CWR method is used to produce transmission shafts, has been shown in Fig. 11.

A wide range of applications of forgings is explained by their high quality, good efficiency of forging equipment as well as a relatively low amount of scrap after forming. In comparison to other production technologies, forgings, especially after thermal treatment, are characterized by exceptional mechanical properties and a fine-grained structure. Therefore, they are often used for manufacturing of the machine parts, which are exposed to the excessive loads. The amount of the global production of forgings reaches 26 million tons, of which the hot closed die forged constitute 19 million tons, the closed die forged parts 644 thousand tons and the open die forgings 6 million tons [72]. Fig. 13 shows the share of the particular countries in the global tonnage production. It is worth noticing that the forging industry is dominant in the regions with a strongly developed automotive industry, i.e. in Europe, USA, Japan as well as China.

2.3.3.

3.1.

Skew rolling

Skew rolling, in the industrial practice, is rarely applied. The reason for this is the fact that the rollers have screw grooves of varying shapes and steps, determined by the constancy of the

3.

Forging processes

Open die forging

The characteristic feature of the open die forging processes is the possibility of producing elements of very large overall

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Fig. 12 – Numerical simulation of the HWR process of a rotary knife body, with the marked temperature distribution (8C).

Fig. 13 – Production of forgings: (a) globally, (b) Europe, and (c) globally open die forgings, in 2016 [112].

dimensions (much larger than in the case of e.g. die forging) as well as optimized mechanical properties (exceeding the parameters of cast products) [70,71]. The high customer requirements often make the open die forging technology the only one to manufacture a given detail of a significant size. Open die forging is distinguished by a wide spectrum of manufactured products/forgings from modern materials, with a focus on the fulfillment of the increasing demand on the local and global market. The current tendency forces the manufactures to deliver products, which are maximally processed and of the possibly highest quality. The production rate of open die forged large-size parts depends on the demand generated by such branches as the power industry (wind, nuclear and thermal power plants) and sea transport. The main assortment is constituted by shafts, rings, disks, bushings, as well as end caps and shells. The biggest global producers of heavy open die forgings made of the heaviest ingots are: Japan Steel Works in Japan, China First Heavy Industries and China Erzhong in China, Doosan in South Korea, Le Creusot in France, OMZ Izhora in Russia, Pilsen steel, Vítkovice Heavy Machinery in Czech, Celsa Huta Ostrowiec and Kuznia Swobodna Stalowa Wola in Poland, Sheffield Forgemasters International Ltd.in Great Britain, and Larsen & Toubro, Bharat Forge Ltd. in India [73].

Materials for the open die forging are usually in the form of slabs and multi-angular forging ingots. They ensure the optimal purity (through e.g. minimization of inclusions and impurities), small inner porosity and a uniform structure in the whole volume of the detail. The studies [73–75] performed in the scope of steel manufacturing (directional solidification technologies) and casting (of sub-arcs, ingot mold casings and top swages) have enabled optimization of the parameters and control of the process as well as increase capabilities of the casting technology. Nevertheless, there are technological problems connected with the creation/occurrence of various kinds of internal discontinuities in the process of ingot solidification (e.g. defects, internal voids). This issue is quite largely discussed in scientific papers referring to the analysis of the behavior of internal discontinuities during the deformation process. Usually, two phenomena are considered: mechanical closing [76,77] and pressure welding [78], which, in consequence, enables the final bonding of the inner surfaces (elimination, complete healing/integration) of the voids. These issues are addressed by the two different approaches: macroscopic (process scale) and micro-analytical (defect scale) [76]. The performed investigations are based on a theoretical analysis, as well as numerical and physical modeling, with the use of artificially introduced discontinuities into the deformed

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Fig. 14 – Forging simulation in the QForm – 16 ton ingot, in combined dies: (a) ingot with introduced internal voids in the head area and (b and c) following forging stages with visible voids reduction [107].

Fig. 15 – Analysis of the distribution of porosity in the hollow ingot: (a) after solidification and (b–d) after subsequent stages of forging on a mandrel, from THERCAST® and FORGE® [108].

material (Fig. 14) [77,79,80]. This makes it possible to formulate empirical and analytical models [81], semi-analytical models [82] and phenomenological models of the material's behavior. A tool, which helps in the evaluation of internal voids behavior in heavy ingots during forging are computational software, such as DEFORM [80,83], QForm [84] or FORGE® [80]. Another important problem related to the closing of material voids is their behavior in the case of a complex sequence of forging operations including upsetting and cogging, consisting of many drafts prescribed in the consecutive passes [80]. Some of the voids that were closed in a given pass later can reopen in the following forming stages. Preliminary cooling, degree of deformation, feed rate and tool shapes have significant influence on forging performance and voids elimination [80]. For example research results from [80] directly showed that shaped dies used at initial stages of forging significantly reduce amount of voids due to closing process. Limiting the plastic flow of the material during the deformation process favors the deformation's penetration to the core of the forged material. On the other hand, this solution is rarely applied in the industrial practice, as it causes a rapid increase of the force compared to the forging in flat dies with similar deformation levels. Generated three-dimensional compression stresses in the central part of the ingot by the wide die heavy blow forging (WHF) method also leads to effective void closure in the cogging process [85]. The deformation process of large forging ingots modify also the crystalline structure of the material and increases its mechanical properties. At present, the trends in the forging ingot production aim at manufacturing elements of an increasing mass. The increase in the power of energy aggregates is connected with the

necessity of producing their construction elements (such as shafts, rings, etc.) of increasing dimensions. So far, they have been produced as assembled elements (e.g. through welding or bonding) out of several smaller forgings. However, manufacturing monolithic parts significantly improves their durability owing to the preservation of structure continuity. The technologies of producing such elements show not only the wide possibilities of the forging technologies in the scope of the given assortment, but also the advanced progress of the forging industry. A good example can be manufactures from China and Japan, which produce forgings from ingots with the mass exceeding 600 tons. Another solution becoming more and more popular among the open die producers is the use of hollow ingots with axial ports (Fig. 15). These are usually products forged on a mandrel, such as pierced shafts, rings and bushings [86–88]. Casting an ingot with an opening is connected with material economy and simplicity as additional operations such as piercing or drilling of an opening in the technological cycle are not required. Another effective way of achieving material economy is ensuring an improved quality of the ingot before the forging process. A verified method of improving the ingot structure is electroslag remelting [89], which is successfully applied in the case of large-size ingots made of nickel alloys. The process consists of subjecting the solidified ingot to gradual remelting, and the liquid metal is then additionally passed through a layer of slag, which causes its purification. After the repeated solidification, the element characterizes in a much lower content of non-metallic inclusions. While the process requires the engagement of additional aggregates and the extra energy consumption for the repeated material remelting, the obtained material exhibits high purity what positively affects

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Table 2 – Compilation of selected open die forged equipped with high pressure force presses [97]. Country Japan South Korea China India Russia Czech Rep. Poland UK the USA

Company

Forging Press (ton)

Max. ingot (ton)

Japan Steel Works Doosan China Erzhong L&T OMZ Izoha Vitkovice Celsa Huta Ostrowiec Sheffield Forgemasters N. American Forgemasters

14,000 17,000 16,000 15,000 15,000 12,000 8000 10,000 9000

650 540 650 300 600 250 130 200 170

its plasticity. As a result of it is possible to perform forging with a significant limitation of required material to fully fill the shaped die, what in the case of expensive materials, is crucial from the economical point of view. Recently also intensive development of robotization and automatization is clearly visible in the area of open die forging. This fact is largely related to the concept of Industry 4.0, which has been introduced in recent years [90,91]. Under production conditions, there is a pursuit of full integration of all the equipment participating in the production process, from heating furnaces, presses and manipulators to measurements equipment controlling the product quality. Specialized systems make it possible to track the parameters of the machines in real time through the collection and processing of data provided by the particular stations along the production line. Such an approach, beside the data collection for statistical purposes, makes it possible to eliminate or improve some weak points in the technological chain [92]. Also predictive maintenance is possible with the use of such complex systems. From the point of view of the environment protection, the key issue in the open die forging is limitation of the carbon dioxide and nitrogen oxide emissions [92]. With that reduction of the fuel consumption in gas furnaces (usually used to heat the elements during open die forging) without disturbing the production continuity has to be achieved. The solutions in this

area are based on the assembly of specialized burners enabling the heat recovery from combustion gases, which reduces the fuel consumption by up to 30% [92]. It is also important to precisely control the temperature inside the furnace chamber [93,94] and to properly arrange the heated products [95]. Hydraulic presses, as the machines which are the most frequently used for the open die forging of large-size elements, are also systematically equipped with energy-saving power supply systems [96], whereas the integration of these devices with modern manipulators accelerates the process and minimizes the risk of damage. Mentioned hydraulic presses are presently more often used for open die forging (Table 2), of the nominal force exceeding 100 MN (10,000 tons) – Fig. 16. In Poland, the press of the highest load is a machine of the nominal working force up to 80 MN (8000 tons) [72]. For the production of solid products, shaped/stepped shafts or thick-walled pipes, swaging machines with hydraulic drive are more often used especially with a four-anvil system, enabling deformation with large drafts and feeds. They ensure not only precision (narrowed dimensional tolerances), repeatability, fully automated forging, but also a homogeneous finegrained microstructure [99,100]. The development of swaging machines is also directed to the automatization and energy saving of utilization.

Fig. 16 – Generalized diagram and main overall dimensions of a modern hydraulic press with the nominal force of 170 MN (17,000 tons) in Doosan, South Korea [98].

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Fig. 17 – Current solutions proposed by forging manipulator producers in the context of open die forging: (a) automatic circular manipulator in the working area separated by a laser curtain [101] and (b) schematic representation of the process of bending a preform with the use of flat tools and manipulator [102].

A mass increase of the produced preforms is also possible due to developments in the industry supplying forging manipulators (Fig. 17). Complex automatization and increase in maneuvering capabilities of rail manipulator allow to carry preforms up to 200 tons[101]. Developments in the circular manipulators, which carry lighter forgings. is directed toward full automatization as well as the possibility to deform preforms with the use of manipulator jaws and press anvils as seen in Fig. 17b [102]. The increase in the working requirements of open die forgings and the common application of advanced materials have created the necessity of a strict control of the quality of the formed elements, both after and during the forging process [103]. Especially useful are optical measurement devices (Fig. 18a) used for the analysis of the product dimensions and modern defects detectors that can identify even internal discontinuities [103–105]. These solution replaces the manual measuring devices, which enabled mainly local measurements of the geometry and forced the operator to closely approach the heated detail [106]. Optical systems are also used for the analysis of the vibrations and undesirable deformations of forging machine components during the forging process (Fig. 18b) [105].

3.2.

Close die forging

3.2.1.

Close die forging of steel forgings

Since the end of the last century, there is a continuous development in the closed die forging industry. The requirements for reduction in the production costs, increased quality of products, increased flexibility of product as well as fulfillment of the environment protection requirements are

the main factors affecting competitiveness and therefore new developments [109]. The North American Forging Industry (NAFI) has reported a growing risk for the American forging, being a result of the global competition as well as the development of new production processes, such as the casting (from modified cast iron and high-alloy cast steels) and the efficiency of the machining equipment. A threat to the forging industry is also non-metallic construction materials, widely used in the automotive and aircraft industry. NAFI has pointed out that the cost-effectiveness of the American forging depends on the close cooperation of associated industries, joint research and development programs in the scope of cost reduction and material use increase [110]. At the same time, the strong pursuit of production cost reduction has led, in 1990s, to the tendency to transfer the production to Asian countries with low labor costs. However, the authors [111] note that such policy may not be effective, presenting examples showing that the cooperation with local forging producers is mutually beneficial, both economically (e.g. through the use of knowledge and experience, which lower the implementation costs, and quality guarantee), but also through the development of technologies lowering the costs of mechanical treatment or selection of new materials. Based on [109,111,113,115–117] it seems justifiable to state that the development of modern die forging will be directly related to the development of precision forging – Fig. 19. The basic feature of a precision forging process is limitation of the material consumption and the machining time, as well as improvement of the strength properties as a result of a better grain pattern [114]. Initially, precision forging referred mostly to cold forging processes. Currently, however, more often there is a pursuit of

Fig. 18 – Quality control by means of optical systems: (a) a forging model after scanning [106] and (b) control of the press vibrations and analysis of the undesirable tool displacement [105].

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Table 3 – Comparison of characteristic parameters of die forging processes [113]. Criteria

Hot forging

Forging weight Shape Roughness Additional thermal treatment

<60 kg Any 100 mm –

Softening pressures Energy costs Tolerances Tool costs Tool durability (number of items from impression)

Low High General Low 5000–10,000

Fig. 19 – Examples of precision forgings made by IFUM [115].

high quality and dimensional accuracy in a hot forging process, which is close to that obtained by cold forging. An alternative for hot forging is warm forging, i.e. a forging process performed within the temperature range above ambient temperature and below the recrystallization one. The selection of the appropriate die forging technology takes into account many factors, such as the scale of production, the type of the forging material, the quality requirements, the available equipment, etc. The authors [113], quoting Sheljaskov from 1996, compiled the criteria for the selection of the proper technology in a tabular form. It should be noted that a significant progress obtained in the scope of tool materials, tool manufacture methods and new lubricants as well as the frequent choice of dies made of sintered carbides has caused an improvement of durability by approx. 100% in respect of the values presented in Table 3. The precision forging is conducted in close dies. The precision of the forging in such a process is affected by many interconnected factors, such as the preform temperature, the tool temperature, the material type, the tool construction as well as the characteristics of the forging press (e.g. slide velocity, body rigidity). In this process, another construction of dies is applied (Fig. 20), in which the parting plane is designed so that the die working area throughout the process is confined. In the case of complex forgings, it is recommended to use split dies (Fig. 21). Such a solution provides the possibility to use compensators for flash material [114].

Warm forging

Cold forging

<10 kg Rotational-symmetric 50 mm Additional surface treatment is not usually applied Medium Medium Precise High 10,000–20,000

<2 kg Rotational-symmetric 10 mm Forcing High Low Very precise High 20,000–50,000

Fig. 20 – Half of a die for precise forging with exemplary forgings (INOP).

Fig. 21 – Diagram of die forging in split dies [114].

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Fig. 22 – Multi-motion press (a) and exemplary forgings (b).

The method of close die forging with split dies was successfully used for the production of e.g. bevel gears or spiders. The charge located in the cavity between the upper and lower die, can be, depending on the construction, simultaneously pressed by the upper and lower punch. The material flows in the crosswise direction, with a relatively small contact area with the punches, and so, the forming load is proportionally lower than in the case of forging in open dies. This method requires a multiaxial motion of the forging device causing an independent movement of the upper and lower punch. A solution fulfilling the requirements of such a process has been developed and implemented at the Metal Forming Institute in Poznan (Fig. 22a). The presented hydraulic press has three independent servo-motors enabling an independent movement of the die closing and a controlled movement of the upper and lower die. The movements of the punches are steered by a control unit making it possible for the punches to move according to the prescribed program determining the direction and speed of the punch movement. Such an approach enables a higher control of the forming process through the creation of local material deformation zones. Fig. 22b shows exemplary forgings obtained by the developed press. Another concept of the realization of a closed die forging process was developed in Nichidai [118]. In order to obtain relative motions of the upper and lower punch and the dies of the forging press, the pantograph mechanism was applied (Fig. 23). The upper punch is pressed to the bottom by the slide of the mechanic press, and the lower punch is placed motionless on the press table. The dies are located at the middle plate connected by the pantograph mechanism with the upper and lower plate, moving downwards with half a speed of the slide. This device is used for precision forming of e.g. bevel gears.

Fig. 23 – Device used for forging in open dies based on the pantograph mechanism [118].

During the design of the technological process of precision forging, the elastic deformation of the tools should be considered. The size of the elastic deformation of the tool in a cold forging process can be within the scope of 50–500 mm, which is a quantity exceeding the acceptable tolerances concerning the making of a forging at the level of 10 mm. The changeability of the tool deformation is strictly connected with the, variable unit pressure operating on the tool walls during the forming. In order to compensate for the effect of elastic deformation of the tools on the dimensional errors of the forgings, the authors [114,118] proposed to construct tools with elastic die inserts. Fig. 24 shows a solution for an application of an elastic die for gears forging. During the forming, the die insert is pressed through the punch into a cylindrical die casing, causing its shrinking and a precise representation of the tooth impression on the forging. After the relief of pressure, the die insert is ejected from the

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Fig. 24 – Concept of tool construction with an elastic die insert [118].

Fig. 26 – Hollow forging [119]. Fig. 25 – Hollow shafts [120].

cylindrical clamping ring and next, the forming of the tooth surface begins. When the diameter of the press-formed core die is ejected from the die casing, it undergoes elastic expansion, which facilitates the removal of the forging from the die.

3.2.1.1. Hollow forgings. A reply of the forging industry to the requirements of the automotive industry related to the reduction of the vehicle weight with a simultaneous preservation or improvement of safety are the developed technologies of forming hollow shafts (Fig. 25). Hollow elements are mainly applied as: driving shafts in gear boxes and toothed gears, axle shafts in vehicle power transmission systems and electric motor shafts. At present, the main recipients of hollow elements are the automotive industry and the aircraft industry. This results from the elevated ecological standards as well as the continuous increase of fuel prices, which forces the constructors to search for solutions improving the efficiency of power transmission systems while reducing their weight. Even hollow components can be produced efficiently in cold forging. The shaft shown in Fig. 26 is produced in two halves by backward cup extrusion and reducing. Next two halves are joined by friction welding. The hollow shaft presented in Fig. 27 is formed when a tube billet undergoes multiple forward tube extrusion and heading operations. Alternative technologies as: cross-wedge rolling, forging in swaging machines or hydroforming also provide encouraging results in the scope of forming geometrically complex hollow products [120,122–124].

Fig. 27 – Hollow forging [119].

A general disadvantage of all mentioned processes is the necessity to use of a pipe segment as a preform, which increases the costs in comparison to classical bar preform. The processes of hot or warm forging on technological lines equipped on transfer presses are currently becoming the only economically justified technologies of manufacturing hollow products in series production [121]. The realization of a die forging process on a press, from an initial material in the form of a section of the rod, is implemented in several technological procedures including the processes of upsetting, extrusion and piercing. As a result of a properly selected sequence of technological procedures, hollow forging of a shaft with narrowed tolerances can be obtained.

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3.2.2. Die forging processes for parts made of non-ferrous metals Although steel is the most commonly applied construction material, again non-ferrous metals and their alloys are irreplaceable in many applications. Aluminum, titanium, copper, magnesium and nickel alloys are key materials when it comes to forging. Die forging processes for these materials have been used for a long time. A comprehensive description of these processes is given in [125]. Although the forging of non-ferrous metals is very similar to the forging of steel, it is usually more difficult to perform due to specific properties of non-ferrous metals. Given the limitations primarily resulting from high variability of properties with respect to forging parameters and narrow ranges of hot forming temperatures for these materials, the production of complex-shaped forgings is often more difficult than in the case of steel, if not impossible. This justifies conducting research aimed at optimizing the existing methods and testing new more effective ways of producing forgings from these materials. In recent years one can observe a growing interest in aluminum, titanium and magnesium alloys on the part of global research centers and production plants. This results from an increased use of parts made of these alloys in the aircraft and automotive industries where mass reduction is of significant importance. In light of the above, it is justified to present recent achievements in the development of die forging processes for this group of alloys. The main advantage of aluminum alloys, especially the 2000, 6000 and 7000 series, is their high relative strength, i.e. the strength-to-density ratio. For this reason, aluminum alloy forgings are primarily used in the automotive and aerospace industries. Due to their advantages, such as high abrasion and corrosion resistance, good electrical and thermal conduction, low contraction, high impact strength at low temperatures, high light and heat beam reflection coefficient, the resistance to impact spark generation and incombustibility, aluminum alloys are also used in the electrical, chemical, food, shipbuilding and construction industries, among others. The wide range of applications of these materials lead to a growing increase in the global production of aluminum and its alloys. Over the last 20 years, this production increased from about 50 million tons in 1998 to over 125 million tons in 2017. The highest increase was recorded in China, where the production of aluminum tripled in the last 10 years, reaching over 70 million tons in 2017 [126]. Among all parts made of aluminum alloys in automotive structures, the use of forged parts increased from 3% in 2012 to 5% in 2016 [127]. Aluminum alloy forgings are produced by the traditional flash forging methods, mainly in forging presses, but also with the use of hammers. The properties of these materials, such as good workability, relatively low hot-forming temperatures, low deformation resistance and lack of scale, make the forging of aluminum alloys considerably easy. On the other hand, the production of forged parts requires access to sophisticated equipment and devices that are different than those used for steel forging. A good example are billet heating furnaces ensuring precise temperature control and distribution, hyperquenching and aging lines, etching rooms and many others. For this reason, the standard technical equipment of forging

plants used for producing steel forgings is insufficient to manufacture forgings from aluminum alloys. The current development of technologies for producing aluminum alloy forgings focuses on taking advantage of the casting and forging processes. The shape of cast preforms is close to that of the finished product [128]. The application of a forging operation to obtain the desired final shape and size of a forging results in the production of parts with good structure quality and mechanical properties that are characterized by lower anisotropy compared to the forging of extruded bars. In addition, the production costs are 15–20% lower than those of the standard forging process [129]. The authors of [130] reported interesting results of investigations conducted under industrial conditions. They propose a forging process for producing a suspension element from alloy EN AW 6082, in which the applied billet heating temperature is higher than in the standard forging process and the hyperquenching operation is omitted. As a result, they obtained a product with better structure quality (without surface coarse-granularity), and higher impact strength and fatigue properties. The proposed solution is an interesting approach to modernization of the forging process, allowing for reduced production costs; this however requires maintaining very strict temperature conditions at the production stage. For several years particularly aircraft industry was interested in the production of forgings from Al-Li alloys (e.g. 2090, 2091, 2297, 8090, 8091). These alloys have good strength properties along with 8–10% lower density, and higher elasticity modulus and corrosion resistance than standard aluminum alloys. Despite their lower workability and high price, forgings made of these alloys have already been used in commercial applications, e.g. in the EH101 helicopter from Agusta Westland [131], which proves that it is a promising direction of development. Titanium alloys have high relative strength, can be treated at high temperatures and are characterized by very good corrosion resistance and good biocompatibility. As a result, forgings made of these materials are widely used in the aircraft, power, shipbuilding and automotive industries as well as in medicine. Die forging of titanium alloys resembles typical forging operations for steel; nevertheless, it is more difficult due to high deformation resistance and relatively low workability of these materials. The design of die shape is here of key importance. Due to different material flow characteristics, titanium alloys do not fill the die as good as steel; as a result, bigger filet radii should be applied preventing sudden changes in the die shape dimensions. Consequently, depending on the forging geometry, it is necessary to use more dies characterized by the lowest roughness possible. A typical process for producing a Ti6Al4V forging is described in [132]. To discover new methods of improving the workability of titanium alloys in hot forming processes, forging processes are investigated under both isothermal and quasi-isothermal conditions. The tools are heated to a temperature that is close to or slightly lower than that of the billet, which significantly improves the workability of the material in the forming process, thus making it possible to produce complexshaped forgings and reduce both material consumption and forming forces at the same time [133,134]. An important problem that has to be solved regarding isothermal forging

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processes concerns the low durability of tools and low process efficiency. The study [135] discusses the industrially applied isothermal forging process for producing TiAl alloy blades for high-speed compressors and aircraft engine turbines. The billet and the tools were heated to 1150 8C. The forming process was performed at a low speed of the slide, at a strain rate of about 103 s 1. Under such conditions, the forging operation lasted 5–10 min. In order to prevent oxidation, tools made of heat-resistant molybdenum alloy were kept in protective atmosphere. Positive results of both industrial and product certification tests made it possible to implement series production. This example shows that – despite the high forming temperature – the isothermal forging of titanium alloys holds the promise of wider use, particularly in small batch production. Some research studies focus on a forging method for preforms made of metal powders. The advantages of this process include reduced costs and a shortened production cycle, as well as forgings have the desired structure and good mechanical properties [136,137]. There are also continuous developments in the area of superplasticity of titanium alloys. Under suitable thermal and mechanical conditions, it is possible to obtain a fine-grained structure exhibiting high elastic deformation capacity [138,139]. Various research works pointed to the possibility of taking advantage of titanium alloy superplasticity in forging processes; however, it is difficult to find studies which confirm the practical applications of obtained test results in this area. Magnesium alloys have the lowest density out of all construction materials. As a result, forged parts made of these materials are applied in the aerospace, automotive, sports and recreation industries. Due to their capacity for vibration damping and electromagnetic radiation absorption, magnesium alloys are widely used in the electromagnetic and electrical industries. In recent years these materials have also been increasingly applied in medicine. A serious limitation to the use of magnesium alloys is their low corrosion resistance. Recent progress in corrosion protection methods and development of new materials with enhanced strength and functional properties led to a growing interest in the manufacturing of products, including forgings, made of magnesium alloys. Between 2005 and 2015 an over twofold increase in the global use of magnesium was recorded [140]. It is predicted that by 2020 every car will have from 100 to 160 kg of parts made of these materials [141]. Most magnesium alloy products are manufactured by casting methods; however, for

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applications requiring good mechanical and functional properties, forging is applied. Due to a narrow range of processing temperature and high thermal conductivity of magnesium alloys, the forging of magnesium alloys is relatively difficult to perform. For this reason, there are few forging plants in the world that deal with the production of magnesium alloy forgings. The most widely used forging process is the one involving the use of hydraulic presses or low-speed mechanical presses under isothermal or quasi-isothermal conditions [142]. Tool are heated with the use of special heating systems, which ensures the production of forgings with shape and size that are close to those of the finished product (near-net shape), as well as with relatively large overall dimensions. Owing to its advantages, this forging method is constantly being developed. Examples of forged parts produced by isothermal forging are shown in Fig. 28 [143–145]. Disadvantages of this process include high tool cost and low efficiency due low strain rates. In recent years, numerous studies have been performed to investigate die forging processes for magnesium alloys involving the use of forging machines with work tools operated at higher speeds [142,146], including die forging hammers and screw presses. Both theoretical studies and experimental tests demonstrated that these forging machines can be used for die forging of some alloys with small contents of alloying additions e.g. AZ31, AZ41, AZ61. Examples of forgings manufactured at ZOP Co. Ltd Forging Plant (Poland) by the developed techniques are shown in Fig. 29. The use of die forging hammers and screw presses ensures higher manufacturing efficiency and allows plants equipped with standard machinery to launch the production of forged parts made of magnesium alloys. An interesting recent development in the research on metal forming of magnesium alloys is a multiaxial forging [148,149]. Multiaxial deformation ensures reduced grain size along with an increased workability and strength of alloys. As an example presented in [148] show that with this method, the grain of the hard-to-deform magnesium alloy WE43 can be reduced from 25 mm to 6 mm, as a result of which it is possible to obtain elongation equal to 475% by uniaxial tensile testing at 375 8C and a strain rate of 3  10 4 s 1. The large number of promising results in this area creates a potential of their practical applications. Also interesting are the results of studies on combined die casting and forging processes [150,151], a technique which combines the advantages of the two processes. The casting method enables the production of forgings of complex shapes,

Fig. 28 – Examples of forgings manufactured in hydraulic presses with heated dies: (a) bracket with ribs made of AZ31B alloy [143], (b) rim made of AZ80 alloy [144], and (c) cantilever beams made of AZ80 alloy [145].

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Fig. 29 – Examples of forgings made of magnesium alloys: (a) levers made of AZ31B alloy by a hammer forging, (b) bracket made of AZ61A alloy by a screw press, and (c) handles made of AZ41 alloy by a screw press [147].

Fig. 30 – The general scheme of the extrusion process.

whereas the final forging operation removes casting flaws and ensures a product with structure and properties characteristic of forged parts. Another direction of development for magnesium alloys is the design of new alloys. Alloying additions in the form of rareearth metals increase the strength properties and corrosion resistance of formed materials [152,153]. Alloys containing lithium are another promising materials, since lithium is the only element that decreases the density of magnesium alloys and positively affects the workability, which makes such alloys suitable for the production of forgings [154].

4.

Extrusion of metallic materials

Similar as previously, in the recent years, there has been a significant technical and technological progress in the scope of extrusion of metals and alloys as well as metallic composite materials. This refers mainly to the idea of perfecting the existing processes (Fig. 30.) and aiming at the implementation of new ones, which would be close to ‘‘ideal’’, but also at developing new special purpose extrusion processes. The basis for the development of extrusion processes is the growing need to manufacture sophisticated products with specific features, both in respect to required mechanical properties and the inner structure (macro- and microstructure). The production capabilities and lowering the manufacturing costs are also equally important factors.

Extrusion processes depending on the requirements can be classified from different point of view as presented in Table 4. Analysis of the possibilities of forming different types of metallic materials in the extrusion process as well as existing needs for the production of more and more sophisticated products with high technical requirements (e.g. light and strong at the same time, with homogeneous microstructure, etc.) and simultaneously, the search of economically justifiable technological solutions, have become the basis for the elaboration of new extrusion processes, including special extrusion processes, such as: -

continuous extrusion (e.g. CONFORM, LINEX, EXTROLLING), hydrostatic extrusion, explosive extrusion, extrusion with liquid phase, isothermal extrusion.

The most popular method of extrusion is hot extrusion of long products from non-ferrous metals, mainly aluminum and its alloys. Also, there is an increased interest in products extruded from magnesium alloys and composite materials because of the demand for advanced products for special applications [155]. In vase of steels, a significant role in the metal forming industry is played by the hot and cold extrusion of short products - so-called press forging. Development in the extrusion technology is primarily related to:

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Table 4 – Types of extrusions processes. Type of extrusion process Hot extrusion Cold extrusion Long extrusions Short extrusions Extrusion of open sections (full) Extrusions of closed sections (hollow) Co-extrusion (direct extrusion) Indirect extrusion Direct-indirect extrusion (combined) Extrusion with active friction Side extrusion Extrusion with oscillating die (KOBO) Extrusion through singleimpression dies Extrusion through multiimpression dies Extrusion of non-ferrous metals Extrusion of steels Extrusion of metal composites Extrusion through regular dies (without welding) Extrusion through porthole dies (extrusion welding) Extrusion of full billet Extrusion of hollow billet

Division criterion Extrusion temperature Length of extruded product Shape of extruded product cross-section Kinematics of tool movement in extrusion process

Number of simultaneously extruded products

Type of extruded material

Type of applied extrusion dies

Type of extrusion billet

- perfection of the technology in the whole extrusion cycle (homogenization, temperature and rate parameters, solutioning on a press), - development of extrusion welding, - perfection of the extrusion tools (porthole dies),

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- perfection of the continuous extrusion technology (CONFORM), - development of new extrusion methods (KOBO, Friction Extrusion), - extrusion of composite materials, including powder mixtures, - development of recycling methods based on the extrusion of the initial material in different forms, including e.g. chips, - development of numerical methods used to design tools and describe (simulate) the course of the extrusion process. The scope is also put on improvement of the whole technological cycle, from billet homogenization, selection of temperature, heating and deformation rates to heat treatment of extruded sections. New solutions in the scope of homogenization of billets refer to the selection of the homogenization cycle parameters (temperature – time) in order to obtain the desired billet structure with the phase particle dispersion not exceeding 0.5– 1 mm, which guarantees good plasticity of the billet in relation with the extrusion parameters. Fig. 31 shows the structure of billets after homogenization [156]. When selection of proper extrusion temperature-velocity condition is investigated the maximization of the speed of the metal's flow out of the die and maintaining isothermicity of the process, which leads to the efficiency of the extrusion process should be in particular addressed [157]. Fig. 32 schematically shows the conditionings deciding about the maximization of the speed of the metal's flow out of the die opening during the extrusion. A guarantee of obtaining high and uniform mechanical properties of the extruded product is the application of controlled oversaturation of the press coasting, followed by their artificial aging.

Fig. 31 – Microstructure of the billet made from 6082 alloy after casting – (a and b) image after homogenization at 535 C for the time of 8 h (c and d).

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Fig. 34 – Continuous extrusion of coatings on power and telecommunication cables. Fig. 32 – Scheme of the relationship between the material velocity at the die opening during extrusion and the process temperature with respect to the press force parameters and the properties of the extruded metal.

The newest extrusion lines are equipped with presses with coasting on which it is possible to perform oversaturation of the sections with a controlled cooling rate, both along the section's length and the cross-section [158] as seen in Fig. 33. Extrusion is most commonly used to manufacture so-called closed sections, often with small wall thickness. They are manufactured by the extrusion welding method. The basic difficulty limiting a wider application of this method for pipes production or closed sections of with various cross-sections is the low weldability of some alloys. Owing to an original device for testing weldability [159], which enables the selection of such welding parameters and which guarantee high quality of the welds, extrusion welding has lately been mastered for high magnesium-silicon alloys from the 6xxx series, as well as a selected group of alloys from 5xxx, 7xxx and 2xxx series. A lot of research has also been devoted to improvement and designing of tools for extrusion, including porthole dies. The most important problems include the selection of the welding chamber geometry, minimization of the frictional resistance during the metal flow and the durability of the calibrating straps. Also important is the mechanical strength of the die in the context of its final exit geometry [160].

Fig. 33 – Oversaturation of extruded sections with controlled cooling rate: 1 – puller, 2 – air nozzle, and 3 – main cooling chamber.

Tracing the development of the processes of metal and alloy extrusion in the last decade, it is hard to point to any spectacular successes in the creation of completely new extrusion processes, which have been used in practice under industrial conditions. And so, in the analysis of the progress made in continuous extrusion, which dates back to early 1970s, it should be noted that, except for the CONFORM method, other ways of continuous extrusion have not been successfully implemented. In the case of the use of the CONFORM method to produce different products, in the recent years, continuous application of extruded coatings on power and telecommunication cables has been proposed [161] as well as sections made of copper for the power industry (Fig. 34). An interesting extrusion process, which strongly affects the structure of the extruded products and helps lower the extrusion forces is the KOBO process [162], whose schematics are shown in Fig. 35. The process uses a cyclic movement of the die causing a change in the deformation path during the extrusion. The process is characterized by a significant reduction of the extrusion force and the possibility to realize a ‘‘cold’’ extrusion process. Another advantage is possibility to obtain monolithic products from scrap materials, e.g. from machining chips, which after proper compaction there are subjected to consolidation what provide an attractive form of recycling (Fig. 35b and c) [163]. A novelty of the recent years is the Friction Extrusion (FE) process [164], shown in Fig. 36, which has been inspired by the plasticity effect in the process of friction stir welding (FSW). The process can be used for production of wires from metals and metallic composites, however presently, the FE process is at the stage of research under laboratorial conditions. Extruding composites from powder mixtures within the scope of powder metallurgy is another interesting area of application. The industrial practice includes extrusion of composites from a preliminarily pressed charge, extrusion of a charge located in a casing (extrusion of a so-called cartridge), and possibly, extrusion of loosely poured powder mixture directly into the press container. Also promising are the results of the research on the use of porthole dies to extrude closed sections from composite mixtures based on aluminum alloys reinforced with hard particles, e.g. SiC [165].

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Fig. 35 – (a) Scheme of KOBO extrusion with an oscillating die, (b) extrusion of metal chips in the KOBO process – chip before and after billet compaction and (c) an extruded monolithic profile.

extruding metals is the Faculty of Non-Ferrous Metals of AGH University of Science and Technology, Kraków [169].

5.

Fig. 36 – Scheme of the Friction Extrusion (FE) process.

Progress in these area will contribute to the development of recycling methods based on extrusion. Even currently, it is possible to perform recycling of aluminum alloy chips through their hot extrusion in order to extrude full quality products [163,166]. It should also be mentioned, that recent progress in the extrusion technology would not be possible without the excessive use of numerical methods to design tools and evaluate material behavior under extrusion conditions, e.g. [167,168]. Introduced benchmarking options (ICEB) resulted in significant progress in developed numerical models, that became more reliable. To sum up, it is also worth pointing out that, beside the current reports on the progress in the extrusion of metals and alloys as well as metallic composites presented in the subject literature, a very important role is played by the periodical conferences and congresses held on a global scale, including Extrusion Technology (ET) and Aluminum 2000. The research in the scope of extrusion is performed at many scientific centers in the world. In Poland, the leading scientific center with an over 50 year tradition, working with the processes of

Drawing

The conventional wire drawing technology is usually devoted for processing of simple axiasymmetrical products. The main disadvantage of the process is associated with the friction, at the interface between the drawing tool and deformed material, which results in heterogeneity of properties at the drawn wires cross-section. Increasing the product properties during conventional wire drawing by modifications introduced to the die, including their hardness, surface smoothness and shape of the working zone, or changes to the processing parameters and lubricants have been investigated for many years and it seems that they have reached their technological maturity. Therefore, in recent years alternative drawing technologies such as accumulated angular drawing (AAD) with a non-linear drawing system, ultrasonic vibrations assisted drawing or ‘‘warm’’ drawing of hardly deformable non-ferrous alloys attracted a lot of attention. Intensive research is also carried toward adaptation of well-known drawing technologies to modern steel grades with multi-phase structures (e.g. AHSS steels), which become more frequently used due to their elevated properties.

5.1.

Non-conventional drawing technologies

In the drawing processes, a lot of attentions was put on forming, very hard materials with low plasticity. These materials deformed in the conventional way, cause the occurrence of various technological problems, such as: excessive tool wear, formation of surface scratches and cracks, improper circularity of the product, etc. At the same time, the need for products with special properties has increased, e.g.: ultrafine structure, specific deformation heterogeneity (related

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Fig. 38 – Change in the drawing force in time in the case of drawing aluminum wire through a split die with the use of ultrasound vibration perpendicular to the drawing axis: (a) drawing with ultrasounds FcSr = 157.27 N; (b) drawing without ultrasounds FcSr = 290.3 N. Drawing rate = 1 m/s; elongation l = 1.85; drawing without a lubricant [172].

Fig. 37 – Different arrangements of drawing dies: (a) linear; (b) crank; (c) stepped [170].

to specific heterogeneity of properties). Therefore there is a continuous search for manufacturing technology of very thin wires (below 0.1 mm in diameter) made of steel or alloys with low plasticity. One of the developed solutions that addresses that issue is an Accumulative Angular Drawing (AAD). The essence of this method consists in multi-step drawing through a system of three arched drawing dies, whose mutual arrangement of the axes can be changed in a controlled manner [170,171]. As a result of the drawing process, shown schematically in Fig. 37, a significant non-homogeneous accumulation of deformation can be observed, as the drawn material undergoes deformation. This is a direct outcome of the flow through the dies, bending, twisting and burnishing. The largest total equivalent deformation occurs in the surface layers, becoming gradually smaller toward the axis of the product. In the AAD process, a material with a significant nonhomogeneous properties distribution at the cross-section is obtained, which is a result of the non-homogeneity of deformation. At the same time, in the material after deformation, high, accumulated, plastic deformation energy is present. As a result, the material annealed after the drawing exhibits diversified yet very fine grain at the cross-section. The properly selected parameters of the AAD process make it possible to obtain a product with high strength and good plastic properties. Such a character of mechanical properties results from the controlled heterogeneity of properties, which, in turn, is a result of the significant difference in the reinforcement between the surface layers and the layers in the vicinity of the drawn product's axis.

Drawing performed with the use of longitudinal or circumferential ultrasound vibration of the drawing die is another example of nonconventional process, which increases deformation characteristics of materials with low plasticity. To produce vibration with the usual frequency of about 20 kHz, piezoelectric or magnetostrictive generators with the acoustic power of a few kilowatts are applied. The use of ultrasound vibration lowers the drawing force, improves the surface quality and increases the plasticity of the drawn material. Drawing though a split die with the use of ultrasound vibrations is presented in [172–175]. The process consists in drawing the product through a die axially divided into two halves. On one die ultrasound vibration perpendicular to the drawing axis are imposed, which significantly changes the deformation conditions. In the material, compressive stresses occur, produced by the vibrating die. Fig. 38 shows an exemplary oscillogram of the changes in the force of drawing aluminum wire through a split vibrating die as well as a die which does not perform perpendicular ultrasound vibrations. The conducted research showed that the use of vibration leads to a clear reduction of the drawing force. Depending on the applied elongation and drawing rate, the reduction of the drawing force was within the scope of 80%. The increase of elongation and drawing rate reduces the effect of drawing force reduction. At the same time, the construction of a split drawing die creates the risk of the occurrence of flash on the drawn material. It should, also, be emphasized that each reduction in drawing force reduces the accumulated deformation thus causes an increase in plasticity. The study [173] presents the results of investigations of wire drawing in a tandem die system, where a split die is in front of, or behind, a monolithic die. It should be remembered that drawing through tandem system dies makes it possible to create a back tension between the cooperating dies. Drawing through heated dies also attracted a lot of attention throughout last 10 years. For example, the research team led by Prof. A. Milenin has developed a technology of producing very thin wires from the MgCa 0.8 alloy by way of drawing through a heated die [176,177]. The MgCa 0.8 alloy

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belongs to the group of biocompatible alloys, which can be successfully applied in medicine, e.g. as surgical sutures. Unfortunately, it exhibits very low plasticity, which makes it impossible to obtain very thin wires by the traditional drawing method. In order to increase the plasticity of the MgCa 0.8 alloy, a modification of the well-known process of warm drawing in a heated die was applied. The use of a heated die made it possible to obtain wires from the MgCa 0.8 alloy with 0.1 mm in diameter. The technology was extended with continuous annealing operation to manufacture thin wires with diameter of 0.05 mm [176,177].

5.2.

Modern materials in drawing industry

At present, the developed technologies of obtaining products from steel with a multi-phase structure, containing martensite, bainite or retained austenite, belonging to the group of high strength steels AHSS (Advanced High Strength Steel) [178–180] refer almost exclusively to the processes of mechanical and thermo-mechanial treatment of sheets by way of rolling, whereas there are no descriptions of the technologies of producing wire rods and wires from these steel types. Since 2006, a team at the Department of Drawing and Metal Products of Czestochowa University of Technology have been performing research on the possibilities of the use of TRIP type steel in the drawing process. Obtaining a TRIP type structure in a wire rod is possible, as in the case of sheet rolling, through the use of regular cooling of the wire rod directly after the hot rolling process. Accelerated-controlled cooling of the wire rod can be carried out with the use of a properly efficient cooling system Stelmor. With the developed technology TRIP structure wire rods with diameter 5.5–8.0 mm were obtained [181]. It was also shown that wires from TRIP steel with the content of 0.1– 0.4% C have the same level of mechanical properties wires from steel with a ferritic–pearlitic structure and the carbon content of 0.45–0.78% C (Fig. 39) [181–185]. Example of final products out of these materials are e.g. fasteners (screws). Performed research confirmed that final products from TRIP type steel with the content of 0.1% C, have comparable

Fig. 39 – Change in the tensile strength Rm of wires from TRIP type steel (0.29% C) and wires from high carbon steel GD75A with a ferritic–pearlitic structure as a function of homogeneous deformation eH [182].

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level of mechanical properties determined by the standard PNEN ISO 898-1 to products from the properties class 8.8, without additional heat treatment process (Table 5). Currently, intensive research is being performed on the implementation of TRIP type steel into the industrial production of fasteners and also, there is a search of further possibilities of their application.

6.

Sheet forming

The progress observed in the last decade in the area of press forming concerns mainly the shaping of high strength materials as well as non-conventional methods of sheet forming. The driving force is especially the automotive industry, where through the increasing introduction of lighter materials, the vehicle mass can be reduced, which thus limits also the CO2 emission [187].

6.1.

Modern materials

The first group of materials is constituted by AHSS type high strength steels. These steels have been the subject of research for several years now, and most of them have been implemented into the large lot production [188]. The biggest progress in the recent years in the area of press forming refers to the shaping of manganese-boron steels (22MnB5) in the process of hardening. The process is presently performed in two different variants: the direct and the indirect hot stamping method. In the case of direct hot stamping, the semi-product is heated in a furnace, and next it is transferred onto a press, where it is formed and hardened in the tool (Fig. 40a). During indirect hot stamping, first the die stamping is only coldformed and next it undergoes only hardening and calibration (Fig. 40b). Due to economic reasons, mainly the direct hot stamping technology is applied at the industrial scale [189]. A properly carried direct hot stamping process requires a high cooling rate capabilities, i.e. at least 27 K/s, which provides a possibility to obtain martensitic structure. Depending on the applied material, this provides tensile strength in the range of 1000–1900 MPa [190]. Direct hot stamping is a complex technology and therefore its proper design requires extensive knowledge of not only the press forming process but also the structural phenomena taking place during the cooling. Many varying parameters cause that not all stamping presses can be used for the technology. The most important problem is the selection of the thermo-mechanical parameters of the process, such as the forming temperature, the deformation degree, the heating and cooling rates, the austenitization temperature and the hardening time. All these factors are critical for the final properties of the product [191]. Another very important factor is the proper design of the tools, especially the selection of a material of a properly high conductivity coefficient and the appropriate distribution of the cooling channels (Fig. 41). During the design of the tools, it is very helpful to apply mathematical modeling of the whole process. This technology is currently used for the form pressing of an increasing number of elements, and so there is also the problem of tool durability [192]. The basic

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Table 5 – Mechanical properties of screws made from steel with the TRIP effect and medium carbon screws in the properties class 8.8 according to PN-EN ISO 898–1 [186]. Strength properties Rm R0.2

Steel TRIP (C < 0.1%)

Carbon steel with ferritic–pearlitic structure (0.15% < C < 0.55%)

836 MPa 678 MPa

800 MPa 640 MPa

mechanisms of tool wear during the hot press forming of a 22MnB5 steel sheet coated with Al–Si is adhesion. The material from the sheet coating is transferred onto the die surface through mechanical and chemical operation, thus forming layers with the thickness of over 50 mm on the tools. In the case of forming with a 22MnB5 coating, mainly abrasive wear is present [193]. As not all applications require high strength in the whole product (e.g. in some areas, good deformability is more important), in recent years, the idea of partial press hardening has been created, where only a part of the die stamping is hardened; the remaining part has good plastic properties. Of course the design of such process requires extensive knowledge of the phase transformations and it is difficult to perform. For this reason, this method is still not applied in the industry [194]. Other materials, which may enable a high reduction of the die stamping mass are TWIP steels with a high content of manganese, i.e. over 25% wt. They represent a new type of material from the point of view of its properties – high strength, good deformability and ability of energy absorption. In TWIP type steels, the reinforcement runs on the basis of the generation of twins as well as nano-twins of deformation [195]. The literature provides, however, only simple research of the press-formability [196] or springing [197] of these materials. As in the press forming processes, the cost of material is very important, which is high in the case of TWIP steels due to a high content of manganese, and so, despite their very good properties, their practical applications are highly doubtful.

Another group of materials for which a great progress has been observed in the area of press forming are sheets made of light alloys, mainly magnesium as well as high strength aluminum from the 7xxx group. At present, some examples of the use of magnesium are: Daimler-Chrysler – 7G-Tronic automatic gear box, BMW – engine block, steering wheel, casing of momentum transfer (however, all these elements are casted) [198]. The processes of bending and press forming of magnesium alloys pose significant problems, as they need to be performed at temperatures above 200 8C [199]. There is also contradictory information on the resistance of elements made of magnesium alloys to dynamic loads [200]. In the study [201], for the press forming of the AZ31 alloy, the authors used a work station enabling independent heating of the forming tools (Fig. 42). During the press forming process, the pressing force on the counter-punch is regulated with the hydraulic cushion of the press. The designed station makes it possible to perform tests in the scope of 20–450 8C with the maximal shaping punch motion speed of 10 mm/min and the maximal rate of 2 mm/s. At the shaping temperature of 300 8C, obtained products showed no visible defects in the form of delaminations for the punch speed of 10 mm/s. The selection of press forming temperature is, however, dependent on the speed of the process. With very high sensitivity to the deformation rate, lowering the forming temperature requires a reduction of the deformation rate. The tests of the energy consumption of samples made of the AZ31 magnesium alloy showed that magnesium exhibits good energy consumption but only with low displacements, when the deformations are small. Because of this as well as the fact that the process of its forming is very difficult and expensive, the use of magnesium for press-formed elements is questionable at this stage. Recent years have also showed intensive research on the forming of aluminum alloys from the 7xxx series, especially the 7075 alloy, which, in the T6 state, has its strength close to that of high strength steel. The knowledge of the deformability

Fig. 40 – Methods of press hardening (a) direct hot stamping and (b) indirect hot stamping [189].

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finally aging [203]. sThe idea of this technology as well as the Bpillar press-formed according to it have been presented in Fig. 44a.

6.2.

Fig. 41 – Schematics of a press forming tool.

Fig. 42 – Tools for sheet forming of magnesium and the bracket made from magnesium alloy AZ31.

and thermal treatment of the 7075 alloy is quite extensive. Among other things, the tests show that up to the temperature of 140 8C, the alloy characterizes in a slight drop of strength with a simultaneous small deformability. Its best plasticity is in the case of 220 8C; however, at this temperature, a very rapid elimination of the T6 state as well as loss of strength are observed [202]. This knowledge does not, however, translate to an industrial use of this alloy, as only slight changes in temperature may cause drastic changes of properties. The course of the aging process in the annealed state differs from the deformed annealed state as well as the warm-deformed state. The literature provides information about a technology of warm forming of the 7075 aluminum alloy developed by LKR Leichtmetallkompetenzzentrum Ranshofen GmbH (in 2012) Austria Metal GmbH (Fig. 43a). There are also analyses of the concept of shaping the 7xxx aluminum alloys, which is close to the 22MnB5 boron-manganese steel. The process consists in preparing the blank material, hardening in cold tools and

Non-conventional sheet metal forming methods

The search of non-conventional solutions in sheet forming technologies usually results from the necessity of surmounting the limitations posed by the traditional solutions (cracking, sheet wrinkling, shape defects, drawpieces surface flaws), as well as the desire to intensify the process (increasing the material deformation degree or the technological safety factor with the predetermined deformation), and often the need to find a way of forming a product, which is difficult or simply impossible to form by means of traditional methods. This has been the reason of a dynamic development of e.g. hydraulic forming methods: internal (Hydroforming) and external (especially Hydromec), as well as sheet forming methods with high deformation rates (explosive and electrodynamic forming). It also resulted in the appearance of such solutions as deep drawing with a temperature gradient, with the use of tools (dies, blankholders) with a shaped working surface, with a controlled blankholder pressure, with the use of ultrasound vibration of the tools or sheet forming with active friction forces. Most of these solutions lead to the sheet metal forming processes being performed under the conditions of lowered tensile stresses in the critical area of the formed semi-product by way of lowering the material's deformation resistance and/or increasing the load capacity of the drawpiece wall. The interest in many of the mentioned non-conventional sheet metal forming methods, like hydroforming, observed in the recent years, results mainly from the need to produce products often of complicated shapes or structures, from harddeformable steels as well as aluminum or titanium alloys, usually for the automotive and aircraft industry [204,205]. Multi-layer panels [204] or ‘‘honeycomb’’ type structures [205] significantly improve the rigidity factor to the mass of the formed construction elements. An important direction of development contributing a new quality in the implementation of non-conventional sheet metal forming methods are hybrid solutions, which combine the advantages of the related technologies. An example can be the combination of hydroforming with hot forming, the socalled Warm Hydroforming (WHF) [206]. This solution makes it possible to obtain products with complicated shapes from high strength materials. A solution to the appearing problems connected with the sealing of the system at high temperatures can be the use of another type of shaping medium, for example sand or ceramic granulate [207]. In turn, supporting the hydroforming process with ultrasound vibration of the tools [208] results in the lowering of the material plastic flow resistance. Another interesting alternative solutions to sheet forming are:

6.2.1.

Single Point Incremental Forming

This concept was developed to enable manufacturing of small series of parts using universal one tool. The main idea of this technology is the forming of the sheet by means of a tool in the form of a punch along the assumed path, as shown in Fig. 44. In

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Fig. 43 – Schematics of forming process of aluminum 7075 (a) the hot forming developed by LKR Leichtmetallkompetenzzentrum Ranshofen GmbH (in 2012) Austria Metal GmbH, (b) press hardening process applied for aluminum 7075 and the B-pillar manufactured by the method and (c) cold forming in saturation state.

Fig. 44 – Main idea of Single Point Incremental Forming (SPIF) [215].

heated material, which determines the absorption of the provided heat, and also to know the emissivity factor of the treated material. The technology of hot spinning of Hastelloy C-276 alloy with the use of high power diode laser developed at the Metal Forming Institute in Poznan [218] made it possible to obtain a product in line with the set assumptions (Fig. 45c), characterized by minimal springback and a better surface quality than a product made by cold spinning with the use of the necessary inter-operational heat treatment.

6.2.3. Single Point Incremental Forming (SPIF) the sheet of metals is deformed by the relative movement between the tool and the sheet. The possibility of moving the punch in three directions x, y, z is used in most machines and it is enough to obtain complicated shapes. Different variants of the technology are also investigated including Two Point Incremental Forming, SPIF with partial/full dies or SPIF with pressurized fluid support [215].

6.2.2.

Spinning with laser heating

For materials whose cold forming is impossible or very difficult, the hot spinning technology is applied, which, in the case of traditional solutions, contributes to a faster wear of the shaping tools (mandrel and roller). A minimization of this effect is possible by way of heating of the formed material by means of laser (Fig. 45b), which provides the possibility to direct a focused beam in the area of the current location of the shaping roller. Such a solution enables a full control of material heating during the spinning process. Here, it is important to properly adjust the laser beam in respect of the

Forming of non-rotational products through spinning

The creation of the technology of non-axisymmetric spinning, as well as spinning of products with a non-circular section or the so-called tooth-shaped spinning, and their further development, has begun to question the limitations of the spinning technology in its traditional understanding as an axisymmetric process [219]. A classification of these nonconventional processes has been proposed on the basis of the relative position of the rotating axes, the section geometry and the variability of the wall thickness of the spinned part (Fig. 46). These innovative processes have found their application mainly in the production of parts with complex geometries from pipes or sheets for the automotive industry. Aluminum alloys as well as low carbon steel are usually used in the process. The study [220] describes the peculiarities of the numerical modeling by finite element method in the case of non-circular spinning for the variant of steering the displacement of the forming roller. The simulation results were compared with the reference experiments performed for two types of nonrotational mandrels (Fig. 47). For the case of a ‘‘Tripode’’

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Fig. 45 – Spinning: (a) schematic of the process (1 – shaped material, 2 – spinning roller, 3 – pressure pad, 4 – centering pilot, 5 – mandrel, 6 – stages of material forming); (b) MZH-500 spinner with a diode laser; (c) product made of Hastelloy C-276 alloy obtained by the method of spinning with laser heating [218].

Fig. 46 – Classification of non-conventional spinning processes [219].

mandrel, it was established that it is possible to simulate noncircular spinning with an error below 5% in respect of the minimal wall thickness. It also turned out that it was possible to predict the non-circular spinning processes, critical from the point of view of the material wrinkling, which was confirmed for the ‘‘Pagoda’’ case.

6.2.4.

Electromagnetic forming

The forming of sheet metals and thin-walled products with electromagnetic field energy [221,222] has the biggest application potential among the high energy methods. Under slightly differing names, various modifications and applications of this forming technique can be recognized: ElectroMagnetic Forming (EMF), ElectroMagnetic Puls Forming (EMPF), Electromagnetic pulse assisted progressive deep drawing, Incremental electromagneticassisted stamping (IEMAS). The process consists in the use of the Lorentz forces operating onto metal elements placed in an impulse magnetic field (Fig. 48). The forming process takes

place without the participation of intermediate masses (noncontact technique), at the strain rate of up to 2  103 s 1. The industrial applications of electromagnetic forming include the processes of forming (drawing, necking, flaring), joining (crimping, welding/bonding) and cutting as well as their combinations in connection with conventional technologies [221,224]. The forming of sheet metals is usually carried out with the use of pancake inductors (Fig. 49), whereas the forming of tubes or walls of deep-drawn cups – with the use of coil-shaped inductors, placed outside the semi-product during the reducing (Fig. 48) or inside, in the case of expanding or flanging. This method can be used to form materials with good electrical conductance. Beside copper, this group of materials of significant technological importance also includes aluminum and magnesium. This makes this technology especially attractive for the automotive industry, which is searching for solutions lowering the vehicle mass [225,226] as well as in the

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Fig. 47 – Shapes of the mandrels used in the non-circular spinning experiments: (a) ‘‘Tripode’’ and (b) ‘‘Pagoda’’ [220].

Fig. 48 – Schematic diagram of the device for electrodynamic metal forming [223]: I – energy member, II – technological member (1 – high voltage transformer, 2 – rectifier, 3 – capacitor battery, 4 – connecting ducts, 5 – control system, 6 – inductor, 7 – semi-product, i – inductor current, ip – current in semi-product, B – magnetic induction, p – pressure).

Fig. 49 – Schematics of electromagnetic forming of sheets [221].

area of designing and producing of energy-saving constructions based on bionic [227]. The lack of friction between the material formed in the EMPF technology and the tools as well as the high pressures (magnetic pressure of values exceeding 250–300 MPa in the time of milliseconds) favors obtaining high surface quality of the product, which is especially important in the case of initial materials with an applied coating, as well as high dimensional precision and repeatability of the product shape, e.g. as a result of reduced springback.

In response to the expectations of the automotive industry concerning the materials and technologies of their processing leading to a reduction of mass, the authors of the study [228] analyzed the process of electromagnetic forming (bulging) of a perforated aluminum sheet. During the shaping of shell products made of perforated sheets, regardless of the applied plastic working method, one should remember about their unique properties – strong directivity, dependent on the type of perforation (on the directional character of the geometrical features of the openings mesh) as well as heterogeneity of deformation, enforced by the structure of the center [229,230]. The directions of the development of the EMF/EMPF method are mostly inspired by its limitations: first application only to materials with high electrical conductivity, second – application to workpieces with shapes and sizes limited by the dimensions and power of the coil as well as the cost of the instrumentation. The boundary below which a direct electromagnetic plastic working is impossible is considered to be the value of electrical conductivity of structural steel (9,3 MS/ m) [224]. This results from the excessive heating of materials with high resistivity and the reduction of the magnetic impact. The limitation resulting from low electrical conductivity can be eliminated through the use of an intermediate element (‘‘driver sheet’’), made from a material with high conductance, placed between the inductor and the formed material. The use of a moderator enables the treatment of high-alloy steels, titanium and even materials, which do not conduct electric current. The effectiveness of the method depends on the type of moderator material (its deformation resistance, electrical conductivity) and its geometrical parameters (thickness of the sheet as well as the transverse dimensions in reference to the dimensions of the coil) [231]. A higher electrical conductivity of the moderator strengthens the Lorentz forces, whereas a lower deformation resistance reduces the energy absorbed for its free deformation. The forming of large components by means of small inductors and low discharge energy is possible through electromagnetic incremental forming (EMIF) [232]. Owing to this method, it is possible to manufacture precision products with complex shapes. Hybrid solution, known as ElectroMagneticAssisted Stamping (EMAS) or Incremental electromagnetic-assisted stamping (IEMAS), combines the advantages of conventional forming with rigid tools and electromagnetic forming [233,234]. In the study [234], the forces of the electromagnetic field are used to support the forming of both the stretched bottom part and the drawn flange part of the formed product, whose shape can be symmetrical or asymmetrical. Fig. 50 shows an exemplary distribution of the electromagnetic coils 1–3. At Wroclaw University of Science and Technology, a new method of electromagnetic sheet metal forming has been developed, which uses multiple continuous capacitor discharges with a strictly determined frequency. The developed technology makes it possible to increase the maximal depth of the drawpieces by over 30% and to realize the processes of continuous press forming. It also expands the scope of applications and increases the effectiveness of electromagnetic forming with the use of conventional techniques [222].

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Fig. 50 – Possible locations of the coils in electromagnetically supported stamping [234].

6.2.5.

Press forming with the electroplastic effect

In the case of less deformable materials, it is possible to use the method of forming with the electroplastic effect. The latter is induced with the flow of pulsatory high density current. In metallic materials, it can cause a radical drop of the yield stress. The study [235] proposes an electrically supported process of embossing micro-channels on metal elements. Both the experiments and the numerical simulations showed that, in a forming process supported by electric current with high density, the stress was reduced in the workpiece, and the depth of the produced channel was increased. Such advantages of the electroplastic treatment as high technological effectiveness, energy saving, deformability increase and change of the metal's microstructure can stimulate the interest in this technology as well as its development.

6.2.6.

Sheet microforming

Microforming refers to the process of shaping with plastic working methods of parts or their features with at least two dimensions in a submillimeter range. A clear, increasing, demand for microelements especially from the electronic industry is observed for the last few years. Microelements mean large productivity with a lower material and energy consumptions. The microforming issues, similar to the ones in traditional technologies, can be divided into four groups: material, process, tools and machines [209]. In the case of the material the grain size is the most important parameter, which limits the geometry of microelements. The flow stress, limit strains and anisotropy should also be considered, as in the small scale material cannot be treated as a homogeneous continuum. In the manufacturing processes the main problems are associated with friction effects (large contact surface between tools and material to volume ratio), forming forces and springback. For the tools area the problem of very small contours and very high quality of the surface have to be mentioned. Finally, in the case of forming machines there are series of difficulties related to handling of microelements e.g. inserting or removing components from tools. A thorough analysis of the developments of the microforming technologies has been performed in [209–211]. The subject of research for over a quarter of the century have been the problems resulting from the miniaturization itself (size effects), as well as the phenomena occurring during the process at the micro scale. Problems, which should be considered in the design and development of a microforming system have been summarized in Fig. 51 [209].

Fig. 51 – Problems related to the size effect in microforming processes [209].

The most popular microforming processes are: extrusion, forging, drawing and sheet forming. The latter one is particularly addressed within the paper. The microforming of sheets, both in reference to the testing of their mechanical properties and the spatial forming, requires subtle modifications of the shaping tools or micromachines construction, as well as special technological solutions and precise process control. As it has been demonstrated in the study [211], the effect of improvement of the material's deformability during micro deep drawing was obtained as a result of applying a laser to heat the flange of the formed sample (Fig. 52). Both the experiment and the numerical simulations showed a significant influence of the size effect on the cracking character at the micro/meso scale. The forming limit curve (FLC) shifts downwards with the decreasing thicknessgrain size ratio. In the last decade, a progress in the research carried out in the area of microforming toward advanced hybrid technologies for the microscale was also observed. During an ultrasonic forming process in [212] the pressure forming the microproduct (trapezoidal micro-channels) was transferred onto a thin sheet by means of powder, plasticized as a result of the friction effect, caused by ultrasonic vibration. In the study [213], the method of induction bulging was used to form the micro-channels in a thin copper sheet. During laser microforming [214], where the energy source is the impact wave

Fig. 52 – Concept of a tool with laser heating of the sample's flange during micro deep drawing [211].

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microscale bulging, whereas the study [217] discusses the physics of the laser microforming phenomenon. Finally it has to be mentioned, that research works on development of all presented modern technologies are also closely supported by capabilities offered by numerical modeling techniques.

7.

Fig. 53 – Schematic of indirect forming with a laser impulse [214].

generated by the laser, the way of indirect forming with a laser impulse was examined, the idea of which is presented in Fig. 53. The study [216] presents an analysis of the effect of the laser energy on the micro-deformability of pure copper during

Numerical modeling in metal forming

As demonstrated throughout the paper numerical modeling became an important tool in the area of metal forming. Initially predictions of mechanical states including strains, stresses as well as resulting forces were under consideration. The slab [236] and upper bound [237] methods should be mentioned as primary tools used in an industry at that time. In subsequent years modeling of a microstructure evolution during forming [238] and cooling [239] by Johnson–Mehl–Avrami–Kolmogorov (JMAK) type models received a lot of attention. Finally, the work of Kobayashi

Fig. 54 – The idea of multiphysics character of the models (a) and schematic illustration of development of models leading to uncoupled and fully coupled multiscale approach (b) [30].

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on finite element (FE) flow formulation provided an invaluable input to this field [240,241] and made numerical modeling easily accessible. Then, during the end of XX and beginning of XXI century this FE concept was combined with mentioned microstructure evolution models providing the fully coupled thermal–mechanical–microstructural tool for metal forming simulations and process optimization. Therefore, during the last decade research in the computer aided development of metal forming operations has been primarily focused on improving description of material behavior under cold and hot forming conditions [30,243] as well as on development of automatic optimization procedures that could be applied to the complex combined processes [242]. Numerical modeling of metal forming today requires multiphysics models (Fig. 54a) and multiscale approaches (Fig. 54b). The former combines mechanical, thermal and metallurgical components in one model. The latter combines phenomena occurring in micro scale with the macro scale description of the process. From practical point of view both issues are extremely important as they increase predictive capabilities of numerical simulations (Fig. 55 [27]). However, it is crucial that at the same time, computing cost have to be maintained at the acceptable level. With this regard, issues related to numerical simulations efficiency have also gained a lot of attention in the area of metal forming modeling [244]. Presently not only general information on material state and its microstructure are required. On the contrary sophisticated numerical models, where material structure is considered not only in an implicit manner but also directly in an explicit way, are essential to form complex new metallic

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materials (e.g. multiphase steels and non-ferrous light metals alloys). Thus, conventional mean field models based on closed from analytical equations (e.g. conventional flow stress models, internal variable models, Avrami type evolution models) [245] are gradually substituted by more elaborated full field models [246–248]. In this case a microstructure morphology as well as phenomena occurring during forming are precisely simulated at various length scales often exceeding predictive capabilities of experimental investigation techniques. Therefore, the Computational Materials Science (CMS) with emerging the Digital Materials Representation (DMR) concept [249] have been intensively investigated during the last decade as an approach that can offer a support for description of a material behavior during forming of new products with special in-use properties. The definition according to [250] states that the Digital Material Representation is a material description based on a set of measurable quantities that provides the necessary link between simulation and experiment. Thus, the objective of the DMR is to provide a digital model of the microstructure where all important features are represented explicitly. That way comparison with a metallographic investigation is straightforward as seen in Fig. 56. To address such complex models of modern materials, new set of numerical approaches designed to couple phenomena occurring at various length and time scales have also been proposed [251]. In the metal forming area, to describe material behavior at the macro scale the conventional continuum solutions are frequently used [30]. That includes mesh based (finite element

Fig. 55 – Natural relation between computational time and predictive capabilities of models [27].

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Fig. 56 – A concept of the digital material representation approach [27].

Fig. 58 – A concept of the two scale finite element and cellular automata upscaling model of a dynamic recrystallization during extrusion [264].

method, finite volume method, boundary element method etc.) [242,244,252,253] as well as mesh free approaches (smoothed particle hydrodynamic, element-free Galerkin method, material point method, moving particle finite element method, finite cloud method, boundary cloud method etc.) [254,255]. Similar versatility is observed in the micro scale analysis techniques that can deal with proper description of material behavior at the lower length scales, namely mezo- or micro scales. Series of previously mentioned continuum but also discrete methods (Monte Carlo, lattice Bolztman, cellular automata, etc.) are equally popular [256,258,259].

During the last decade coupling of these computational techniques to provide both multiphysics and multiscale material response (Fig. 54) has proven enormous predictive capabilities. Therefore, combinations of a variety of numerical approaches: finite element (FEM), cFigtal plasticity finite element (CPFEM), extended finite element (XFEM), finite volume (FVM), boundary element (BEM), mesh free, multi grid methods, Monte Carlo (MC), Cellular Automata (CA), Molecular Dynamics (MD), Molecular Statics (MS), Level set methods, Fast Fourier Transformation (FFT) etc. have been applied to practical metal forming simulations [248,257,262].

Fig. 57 – A concept of upscaling and concurrent multiscale numerical models [27,245].

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In general multiscale modeling techniques, can be classified into two groups: upscaling and concurrent approaches as seen in Fig. 57 [261]. These multiscale approaches became the basis of intensively developed Integrated Computational Materials Engineering (ICME) [260]. In the ICME three fundamental aspects of materials design: Manufacturing, Design and Materials are combined. In the metal forming community the ICME concept has been additionally extended to the Integrative Computational Materials and Process Engineering (ICMPE) approach, which combines multiscale modeling and through process simulation in one comprehensive approach [263]. As mentioned in [263], in the upscaling class of methods, constitutive models at higher scales are constructed from observations and models at lower, more elementary scales. In

Fig. 59 – A concurrent multiscale model of incremental forging concept [269] based on the digital material representation approach and the CPFEM approach.

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the fully coupled upscaling modeling, information from macro is then used to recalculate material micro state. An example of practical realization of a two scale finite element and cellular automata upscaling model of dynamic recrystallization during an extrusion of Al/Mg [264] is presented in Fig. 58. Conceptually different approach to multiscale modeling is represented by concurrent models. In this case, the computational domain is simultaneously decomposed to deal with phenomena associated to particular length/time scale (Fig. 57). Therefore, with these methods a detailed investigation of material flow at various length scale became possible as can be seen in Fig. 59, where concurrent multiscale model based on the digital material representation approach and the CPFEM approach is presented [265]. This class of models is also often used when cold forming operations are investigated e.g. wire drawing [266], stamping [267] or rolling [268]. As presented in Fig. 55, an increase in computational time, which is usually unacceptable for industrial practice, is the major drawback of these multiscale models. Research on the computational efficiency is carried out from the applied computer science point of view. The increasing power of new computer processors or graphical cards, as well as development of alternative methods and strategies for computational simulations, create new ways to address this issue, and provide possibilities for multiscale modeling to become a common industrial practice. There are in general two concepts that have been explored as seen in Fig. 60:  Reduced-order-modeling concept (ROM) [270], where in order to reduce the computational costs to affordable level simplifications to material or computational domain des criptions are introduced. Developed model reduction strategies range from purely physical or analytical approaches to black-box methods (e.g. artificial neural networks) [271]. The sensitivity analysis is also a common representative of this class of methods [272].

Fig. 60 – Major concepts in computational models cost reduction [30].

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Fig. 61 – Future trend of coexistence of mean and full field approaches in practical industrial application [245].

 New high performance computer architectures and grid distributed computational environments. In this case reimplementation of already available numerical models is essential to take benefits from such high computer power. During last ten years parallelization techniques became a standard approach for e.g. metal forming commercial finite element software providers [244]. As presented, the combination of material science and applied computer science brought to metal forming community a possibility to use complex full field multi scale models on more regular basis. It have to be emphasized, that these models can often provide predictions of material behavior in the conditions, which are difficult or even impossible to be monitored under laboratory conditions. With this level of accuracy, they have recently started to be used as a major source of data for development of simplified mean field approaches for practical applications (Fig. 61). The primary goal of such an approach is minimization of time consuming and expensive laboratory investigations, which are usually used to provide sufficient amount of data to propose mentioned mean field approaches. It seems that this will be a large research area for the next decade.

8.

Conclusions

An overview of selected metal forming methods, which have been the subject of research and implementation into the industry in the last 10 years was addressed in the paper. It can be concluded from the performed analysis that the basic driving force of the last decade's progress in this field is the forming of new materials, especially high strength steels with a multi-phase microstructures containing martensite, bainite or austenite, belonging to the group of AHSS (Advanced High Strength Steels) as well as non-ferrous alloys with aluminum, magnesium and titanium as primary representatives. A significant progress can also be observed in the area of non-conventional methods of metal forming, such as:

incremental forming, electromagnetic press forming, drawing with the use of ultrasounds, CONFORM extrusion, etc., which take advantage of the more and more extensive knowledge of the effect of deformation on the shaped material as well as the higher possibilities of process control, e.g. the movement of the tools. Another area which has a very significant effect on the progress in metal forming is robotization and automatization, which significantly increase the efficiency of the production processes as well as the quality of the product. Final components gained repeatable narrowed dimensional tolerances as well as a fine-grained microstructure. It should be emphasized that the development of metal forming is supported, to a large extent, by numerical simulations. Historically, this tool was used for the prediction of mechanical states, including strains, stresses and forces. Thermal coupling of the mechanical models became possible in mid-1970s and the temperature dependence of the material properties could be introduced in the models. Current models enable prediction of the microstructure development, and fully coupled thermal-mechanical-metallurgical approaches are available. Optimization of the processes with an objective function based on the product properties has also become possible. A multiscale approaches to modeling, which are commonly used today, makes it possible to investigate the phenomena occurring in the material microstructure accounting for the current local changes of parameters in the technological processes. In Poland, the driving force of the progress in metal forming technologies is especially the automotive industry, which, through the introduction of increasingly lighter materials, aims at lowering the vehicle mass, and thus, also, limiting the CO2 emission. However, influence of other industrial branches also cannot be disregarded.

Conflict of interest None declared.

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Ethical statement The work has been prepared according to common ethical standards.

Funding body None declared.

Acknowledgements None declared.

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