Semisolid Processes

5.06

Semisolid Processes

G Govender, H Mo¨ller, and OFRA Damm, Council for Scientific and Industrial Research, Pretoria, South Africa Ó 2014 Elsevier Ltd. All rights reserved.

5.06.1 5.06.2 5.06.2.1 5.06.2.1.1 5.06.2.1.2 5.06.2.1.3 5.06.2.1.4 5.06.2.1.5 5.06.2.2 5.06.2.2.1 5.06.2.2.2 5.06.3 5.06.3.1 5.06.3.1.1 5.06.3.1.2 5.06.3.1.3 5.06.3.1.4 5.06.3.1.5 5.06.3.1.6 5.06.3.1.7 5.06.3.1.8 5.06.3.1.9 5.06.3.1.10 5.06.3.1.11 5.06.3.1.12 5.06.3.1.13 5.06.3.1.14 5.06.3.2 5.06.3.2.1 5.06.3.2.2 5.06.3.2.3 5.06.3.2.4 5.06.3.2.5 5.06.3.2.6 5.06.3.2.7 5.06.3.2.8 5.06.3.3 5.06.3.3.1 5.06.3.3.2 5.06.3.3.3 5.06.3.3.4 5.06.3.3.5 5.06.3.3.6 5.06.3.3.7 5.06.3.4 5.06.4 5.06.4.1 5.06.4.2 5.06.4.3 5.06.4.4 5.06.5 5.06.6 5.06.7 References

Introduction Thixoprocessing Preparation of Feedstock Mechanical Stirring Magnetohydrodynamic Stirring Grain Refinement Thermomechanical Methods Other Methods of Producing Feedstock Reheating of Billets Radiation/Convection Heating Induction Heating Rheoprocessing Nucleation-Driven Processes New Rheocasting Cooling Slope Direct Thermal Subliquidus Casting Continuous Rheoconversion Process (CRP) Self-Inoculation Method (SIM) In-Ladle Direct Thermal Control Rheocontainer Process (RCP) Cup-Cast Method Helical Curve Duct (HCD) Serpentine Pouring Channel (SCP) Inverted Cone-Shaped Channel Process Controlled Nucleation Process The Damper Cooling Tube Method Rheocasting Systems Based on Nucleation and Active Contact Stirring/Shear Advanced Semisolid Casting Technology, Honda Semisolid Rheocasting (SSR) Process Low Superheat Pouring with a Shear Field (LSPSF) Process The Swirled Enthalpy Equilibration Device Rheomolding Rheometal Process Gas-Induced Semisolid Metal Process Melt Spreading and Mixing Technique (MSMT) Nucleation and Active Noncontact Stirring The Hitachi Process Advanced Rheocasting Process a.k.a Hong-Nano Casting Method In-Mold Rheocasting Process Novel Hot Chamber Rheodiecasting Process Multielectromagnetic Stirring Continuous Preparation Process Slurry on Demand (SoD) The Council for Scientific and Industrial Research Rheocasting System Summary on Rheocasting Systems Forming Methods Casting Forging Extrusion Joining Thixomolding Semisolid Free Forming Technology Summary

Comprehensive Materials Processing, Volume 5

http://dx.doi.org/10.1016/B978-0-08-096532-1.00516-1

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5.06.1

Semisolid Processes

Introduction

Semisolid processing was first discovered at the Massachusetts Institute of Technology (MIT) (1–3) in 1971 during the study of hot tearing of steel during solidification. During modeling of this phenomenon using Sn–15 wt.%Pb using a Couette rheometer (1,3), it was established that the viscosity increased very slowly with increased solid fraction. The mechanical stirring induced by the rheometer resulted in a spherical grain structure, which accounted for the material displaying a thixotropic behavior. The industrial importance of this discovery was recognized immediately and two feasible processing routes were demonstrated, ‘rheocasting’ and ‘thixocasting’ (4,5). These and other related processes are now more commonly known as semisolid metal (SSM) forming or processing. The basic requirements for a metal or alloy to be processed by SSM processing are l

A nondendritic or spherical primary grain structure Solid–liquid region l Suitable solid fraction l

The development of semisolid processes revolved around attaining a globular microstructure when the metal alloy is in the semisolid state. These can conveniently be classified into four main processing routes, i.e., thixoforming (thixocasting), rheoforming (rheocasting), thixomolding (6), and a fourth process, compocasting (7,8), which is a variant of SSM processing used for the production of composite material. The rheo and thixo processing routes have two steps; the first is to prepare semisolid slurries and the second is the forming of the semisolid slurry into a component (1). l

Thixoprocessing is a two-stage process that involves the preparation of a solid feedstock material that when heated into the semisolid region will have a globular microstructure and formed into a product (Figure 1) (1,4,9,10). l Rheoprocessing is a single-step process that involves cooling the liquid metal to the semisolid state under controlled conditions to obtain a semisolid material with the required round grain microstructure followed immediately by a forming operation (Figure 1) (1,4,10). l Thixomolding is very similar to plastic injection molding but in this process magnesium alloy flakes are sheared and heated into the semisolid state before injecting into a mold (Figure 2) (6,10). Compocasting is the process developed for producing composite materials. The reinforcing phase is added to the metal matrix in the semisolid state and cast (7,8)

Figure 1 Schematic illustration of (a) the thixo- and (b) rheo processing methods. Reproduced from Mehrabian, R.; Flemings, M. C. Die Casting of Partially Solidified Alloys. Trans. Am. Foundry Soc. 1972, 80, 173–182; Decker, R. F. The Technology and Commercialisation of Thixomolding®. In Proceedings of the 12th International Conference on Semi-solid Processing of Alloys and Composites, Cape Town, South Africa, 8–11 October 2012, Solid State Phenomena (192–193); 2013; pp 47–57.

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Figure 2 Schematic illustration of the thixomolding process. Reproduced from Decker, R. F. The Technology and Commercialisation of Thixomolding®. In Proceedings of the 12th International Conference on Semi-solid Processing of Alloys and Composites, Cape Town, South Africa, 8–11 October 2012, Solid State Phenomena (192–193); 2013; pp 47–57.

5.06.2

Thixoprocessing

In the early commercialization efforts of SSM forming the main process implemented and researched was thixoforming (1,9,10–14). The thixoforming process consists of three steps: 1. SSM billet or feedstock preparation; 2. Reheating of the billet; and 3. The forming process (Figure 1).

5.06.2.1

Preparation of Feedstock

Many different methods have been devised for producing SSM feedstock that when heated into the semisolid range would produce a globular microstructure that displays thixotropic properties: mechanical stirring, magnetohydrodynamic (MHD) stirring, spray casting, liquidus casting, ultrasonic treatment, grain refinement, and thermomechanical processing or combinations of these methods (1,9,10,11,14). The four production routes that have received the most attention are mechanical stirring, MHD stirring, grain refinement and chill casting, and thermomechanical processing (10). These methods have been used to produce feedstock from a variety of ferrous and nonferrous alloys, although the main focus has been on aluminium alloys and in particular the aluminium–silicon casting alloys. Of all the processes, the MHD continuous casting method emerged as the preferred route for the volume production of SSM feedstock, especially for the Al–Si casting alloys. The advantages of the thixoprocessing route is that the quality of the feedstock, chemical composition, and gas content can be tightly controlled, thereby ensuring tight quality control over the SSM feedstock material.

5.06.2.1.1

Mechanical Stirring

The early method of producing billets was based on mechanical stirring, which originated at MIT (1,7,12,13). The mechanical agitation was achieved by means of impellers, augers, paddles, or multiples of agitators mounted on a central rotating shaft. The early techniques based on batch production (Figure 3) (12) eventually evolved into a continuous process (Figure 4) (1,13). In the continuous process, the liquid metal flows through an annulus between an outer cylinder and a stirring rod where it is stirred and cooled. The slurry flows from the bottom of the rod and can be either solidified as a continuous billet or formed into a shape (rheocasting). This method of feedstock preparation was not commercialized due to a number of technical difficulties such as contamination due to oxidation and reaction of the melt with the stirring device, especially in the case of aluminium (1,10).

5.06.2.1.2

Magnetohydrodynamic Stirring

The MHD method of producing SSM feedstock became the preferred method of producing SSM feedstock because it addressed a number of the technical problems of the mechanical stirring route (10). The MHD process was developed by ITT Corporation in the United States in the mid- to late 1980s and is described in a number of patents held by Alumax Inc. (1,14–16). The MHD continuous casting process is very similar to other conventional direct chill continuous casting processes with the exception that the metal is stirred by a rotating electromagnetic field before and during solidification (Figure 5), thus influencing the nucleation and solidification processes to avoid dendritic growth. Variations of this idea, such as a vertical (Figure 5(a)) or circumferential stirring

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Figure 3 Schematic illustration of the early feedstock preparation technique. Reproduced from Flemings, M. C.; Mehrabian, R.; Spencer, D. B. Composition and Methods for Preparing Liquid-solid Alloys for Casting and Casting Methods Employing the Liquid-solid Alloys. US Patent 3,948,650, 1976.

(Figure 5(b)) or combinations of both have been used. The billets are either cast vertically or horizontally (1). The MHD continuous casting technique was adapted by primary metal suppliers, Pechiney (17), SAG, and Ormet in the 1990s, and these made feedstock in various diameters and quality available (14). Only SAG is currently still producing and supplying feedstock produced via MHD for thixoforming on a commercial basis.

5.06.2.1.3

Grain Refinement

Chemical grain refinement is an alternative to active mechanical and MHD stirring techniques – it achieves the production of feedstock with the required globular microstructure by controlling the nucleation process. Grain refinement can be achieved through the use of chemical grain refiners or other techniques such as the so-called cooling slope, spray forming, etc., all of which create a large number of nuclei that limit dendritic growth and promote the formation of the desired globular microstructure. In particular, the use of grain refiners in conjunction with a high cooling rate results in the formation of semidendritic primary grains, which ripen and become globular upon reheating (10). Grain-refined billets are limited to diameters of approximately 90 mm to ensure sufficiently high cooling rates. Although grain refinement offers a number of advantages over active stirring, a significant amount of grain refiner is required to obtain the fine grain microstructure (18–20), which adds significant cost to the material. It was also shown that upon reheating, grain growth and coalescence negated the effect of grain refiners (20). Today, grain refinement is not used extensively to produce SSM feedstock material, although in some applications it is used in conjunction with other techniques to obtain the desired properties (21).

5.06.2.1.4

Thermomechanical Methods

The thermomechanical method of preparing feedstock is a solid state method. Two distinct methods have evolved, i.e., straininduced melt activation (SIMA) (Figure 6(a)) (1,22–24) and recrystallization and partial remelting (RAP) (Figure 6(b)) (1,22,25). Both techniques rely on inducing a critical strain in the material to initiate recrystallization. The SIMA process involves extruding the material above the recrystallization temperature followed by a cold working operation to attain the critical strain required (23,24) while the RAP process employs warm extrusion below the recrystallization temperature to obtain the critical strain (25). There are several technical limitations for the practical application of the thermomechanical methods of SSM feedstock preparation. For instance, the size of billets that can be produced with the required strain is limited by the size of the press or extruder.

Semisolid Processes

Figure 4

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Schematic illustration of the continuous rheocaster. Reproduced from Fan, Z. Semisolid Metal Processing. Int. Mater. Rev. 2002, 47 (2), 49–85.

Figure 5 Schematic illustration of the MHD casting process. (a) vertical stirring and (b) circumferential stirring. Reproduced from Hirt, G.; Kopp, R. Thixoforming: Semi-solid Metal Processing; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2009.

Moreover, the amount of strain in the cross-section of the billet is not uniform, which results in an inconsistent microstructure after recrystallization and partial melting. This problem was partially resolved by the equal channel angular pressing method, which can produce severe, uniform strain through the cross-section of the billet (26,27). Although thermomechanical routes can produce high-quality feedstock, they are significantly more expensive than the MHD stirring process and the billet diameter size is limited to approximately 70 mm, as noted earlier. However, the technique is suitable for thixoforming of high melting temperature alloys (1) and for smaller niche applications. This technique is currently being used by Thixoforge Inc. in the United States for the commercial production of SSM components.

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Figure 6 Schematic illustrations of (a) the SIMA process (reproduced from Atkinson, H. V. Alloys for Semi-solid Processing. In Proceedings of the 12th International Conference on Semi-solid Processing of Alloys and Composites, Cape Town, South Africa, 8–11 October 2012, Solid State Phenomena (192–193); 2013; pp 16–27; Young, K. P.; Kyonka, C. P.; Courtois, J. A. Fine Grained Metal Composition. US Patent 4,415,374, 1983) and (b) the RAP process (reproduced from Atkinson, H. V. Alloys for Semi-solid Processing. In Proceedings of the 12th International Conference on Semi-solid Processing of Alloys and Composites, Cape Town, South Africa, 8–11 October 2012, Solid State Phenomena (192–193); 2013; pp 16–27; Kirkwood, D. H.; Sellars, C. M.; Boyed, L. G. E. Thixotropic Materials, US Patent 5,133.811, 1992).

5.06.2.1.5

Other Methods of Producing Feedstock

A multitude of other techniques to produce SSM feedstock has been developed during the intensive search for cheaper feedstock material during the 1980s and 1990s. These include Passive stirring – molten metal is passed through stationary porous media (ceramic spheres) that induce shear in the melt during solidification, which prevents dendritic growth and results in a large number of nuclei (14). Spray casting (Osprey process) – a nonagitation technique where molten metal is atomized by a high velocity stream of gas and collected on a moving substrate (1,21,28). Liquidus casting – also known as low superheat casting where a melt just above the liquidus is poured into a mold (1,21). Ultrasonic treatment – a high-power ultrasonic vibration is applied to the solidifying melt (1,29). Powder compaction – billets are produced by compacting powder into a billet followed by sintering and partial remelting (30). Single slug production – this technique can use MHD stirring to produce single billets (31,32). The aim was to develop a simple process that foundries can use to make billets when required and also allow for in-house recycling of scrap.

5.06.2.2

Reheating of Billets

The reheating of the billets is a critical step in the thixoforming process because it influences the final properties obtained in the components produced. The billet must be heated to the desired SSM temperature as fast as possible, must attain a homogeneous temperature distribution with minimal grain growth, and the process must be highly repeatable (14). The two main heating systems that evolved from these requirements were radiation/convection heating and induction heating.

5.06.2.2.1

Radiation/Convection Heating

Radiation/convection heating was successfully used in industrial heating of billets for semisolid forming by Moschini (14,33) and the process was one of the first to be used successfully in the mass production of automotive components. The technology had the advantage of low capital costs and a robust/simple process control structure but it was not adopted widely because of the slow heating cycle that limited productivity (14).

5.06.2.2.2

Induction Heating

Induction heating emerged as a preferred reheating process mainly due to the higher heating rates that could be achieved. However, induction heating is also associated with overheating of the skin and corners of the billet and a significant amount of research was done on different coil designs and heating strategies to ensure that billet was reheated homogenously (14,34–36). Careful control of the power input and induction frequencies in the range of 250–1000 Hz were found to achieve sufficient penetration of the billet and limit the thermal gradients in the billet. With adequate process controls in place, induction heating can provide high heating rates with a high degree of control over billet temperatures and properties (10,14). Two distinct design philosophies evolved around multicoil systems (Figure 7): Single power supply to single coil – this offered the advantage of precise control on individual billets, optimized cycle time, dwell times could be created during production down times, and the billets could be heated in the vertical or horizontal position. The main disadvantages of this system were the higher capital costs and additional maintenance costs. l Single power supply to multiple coils – the main advantage of this system was reduced capital cost but the control options were limited and the billets could only be heated in the vertical position. Further control could be effected by changing the coil geometry to refine the heating profile. l

Although the vertical heating systems are cheaper, the horizontal systems provide a number of technical advantages (36) that result in more uniform mechanical properties of the resultant SSM billets and components.

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Figure 7 (a) Induction design philosophies. (b) Schematic illustration of a single power source to single coil layout. (c) Schematic illustration of a single power source to multiple coil layout. Reproduced from Hirt, G.; Kopp, R. Thixoforming: Semi-solid Metal Processing; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2009.

5.06.3

Rheoprocessing

Rheocasting as a process was defined in the early days of SSM research (3), but was not pursued as a potential commercially viable process until the mid-1990s. Its development was mainly driven by the high cost of the thixoprocessing route, which resulted in waning interest in SSM forming from industry. Rheoprocessing methods first attracted attention in the mid-1990s, when Hitachi proposed their “new semiliquid metal casting process” at the 4th International Conference on Semi-Solid Processing in 1996 (37). An explosion of new rheocasting (NRC) processes followed since the early 2000s, fueled by the introduction of the first commercial rheocasting process by UBE Industries, Japan, and waning interest in thixoprocessing due to its high cost. The main developments were around methods to produce SSM slurries for use in conventional forming processes, with the aim to produce SSM slurries directly from liquid with a globular structure and uniform temperature distribution for the forming process of choice. It became evident that the main mechanism to achieve this was through the control of the nucleation and grain growth processes (38). In order to achieve the globular structure, sufficient nucleation was required while the growth needed to be controlled to prevent dendritic growth and also final primary grain size. This mechanism dictated the evolution of the processes that can be classified into three broad process categories: 1. Nucleation 2. Nucleation and active contact stirring/shearing 3. Nucleation and active noncontact stirring

5.06.3.1 5.06.3.1.1

Nucleation-Driven Processes New Rheocasting

NRC is the oldest commercial rheocasting process. It was patented in 1996 by Japanese machine builder UBE Industries and presented at the GIFA International Foundry Trade Fair 1999. The process involves the casting of slightly superheated molten metal into a holder, producing a large number of nuclei, which are made to grow into a globular microstructure through targeted slow cooling (Figure 8). This was followed by a temperature adjustment using induction heating and subsequent forming. The NRC process was the first rheocasting process to be industrialized and was licensed to a number of component manufacturers, including Stampal s. p. A. in Italy (39–41). A disadvantage of the process is the fact that the slurry-making process is integrated with the high-pressure

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Figure 8 Schematic illustration of the NRC rheocasting process (UBE). Reproduced from Innovative Casting Process Clearing the Way for New Casting Possibilities “New Rheocasting”; Ube Industries Ltd, Marketing brochure: Japan, 1999.

die casting (HPDC) machine – in other words, the process is not compatible with existing HPDC machines and infrastructure. This substantially drives up the capital cost of the process, and it is currently not widely available on a commercial basis.

5.06.3.1.2

Cooling Slope

This process originated in Japan as a method to prepare feedstock for thixoforming (42) but has since been investigated as a method to prepare semisolid slurries for rheocasting by the Foundry Institute of Aachen University, Germany, and other institutes. In the CSP process, slightly superheated slurry is poured over an inclined and tempered channel into an insulated container (Figure 9). Copious amounts of nuclei are generated on the cooling slope where the metal temperature drops quickly into the semisolid range. These nuclei are carried into the container by the metal flow where the material is subjected to slower cooling to the desired SSM temperature (42,43). The slurry can be used in subsequent forming processes or frozen for use in the thixoprocessing route. Although this process is technically very simple, it can be prone to gas pick up and oxide formation which will impact negatively on mechanical properties.

5.06.3.1.3

Direct Thermal

In this method, the melt is poured into a cylindrical metallic mold with a high thermal conductivity, which causes the melt to cool rapidly into the semisolid temperature range (44,45). The mold has a low thermal mass and high thermal conductivity so that it cools slowly in air. After holding for a short time, the metal is quenched to retain the microstructure.

5.06.3.1.4

Subliquidus Casting

Subliquidus casting (SLC) was introduced and developed in 2001 at THT Presses, Inc., Dayton, Ohio, United States. The process was described at international SSM conferences in 2002 and 2004. In this process, degassed, modified, and grain-refined melt is introduced into the shot sleeve at a temperature only a few degrees above liquidus. Nucleation and grain growth to produce a slurry occurs in the shot sleeve. The slurry is then injected through a gate plate directly into the die cavity, as illustrated in Figure 10 (46,47). One of the claimed advantages is that the gate plate concept provides a wide range of opportunities for positioning gates to minimize both flow and solidification shrinkage feeding distances.

Figure 9 Schematic illustration of the cooling slope process. Reproduced from Haga, T. Semi-solid Roll Casting of Aluminium Alloy Strip by Melt Drag Twin Roll Caster. J. Mater. Process. Technol. 2001, 111, 64–68.

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Figure 10 Schematic illustration of the subliquidus casting (SLC) method. Reproduced from Jorstad, J. L. Future Technology in Die Casting. Die Cast. Eng. September 2006, 18–25.

Products that have been successfully demonstrated or produced include steering knuckles, control arms, valve bodies, torque converters, heat exchangers, disc brake rotors and calipers, transmission pump parts, and transmission input housings.

5.06.3.1.5

Continuous Rheoconversion Process (CRP)

This process was developed by Bühler Druckguss AG, Uzwil, Switzerland, together with the Advanced Casting Research Center (ACRC), Metal Processing Institute, Worcester Polytechnic Institute, Worcester, Massachusetts, United States. The process employs a liquid mixing technique in a specially designed ‘reactor’ that provides copious nucleation and forced convection during the initial stages of solidification (Figure 11) (48–50). This slurry-on-demand process was scaled up for industrial applications but has not been adopted on an industrial scale. Research performed at the ACRC suggested that the CRP could successfully be used over a wide process window to create highly globular semisolid slurries.

5.06.3.1.6

Self-Inoculation Method (SIM)

In this method, solid cubes of 5 mm  5 mm  5 mm of the alloy being processed are added to the melt as an inoculant. The cooling and mixing is achieved from the flow of the metal through a cooling channel (Figure 12) (51).

Figure 11 Schematic illustration of the continuous rheoconversion process (CRP) process and a laboratory prototype. Reproduced from Pan, Q.; Jha, M.; Apelian, D. Low Cost Energy Efficient Methods for Manufacture of Semi-solid (SSM) Feedstock. Final Technical Report; Worcester Polytechnic Institute, October 2005. Report Number DE-FC36-021D14232.

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Figure 12 Schematic illustration of the self-inoculation method (SIM) process. Reproduced from Bo, X.; Yuandong, L.; Ma, Y.; Yuan, H. Effects of Novel Self-inoculation Method on the Microstructure of AM60 Alloy. China Foundry 2011, 8 (1), 121–126.

5.06.3.1.7

In-Ladle Direct Thermal Control

The in-ladle direct thermal control rheocasting process is a codevelopment of the Advanced Material Center, Korea Institute of Industrial Technology, and the Department of Advanced Material Engineering, Sungkyunkwan University, Korea. The process has been developed for aluminium alloys. The liquid metal is slightly superheated in a furnace and the shot weight prepared by pouring the necessary volume into a ladle. The semisolid slurry made during the transferring time is poured into the shot sleeve of the die casting machine (52).

5.06.3.1.8

Rheocontainer Process (RCP)

This process was also developed at the Foundry Institute of Aachen University, Germany. Liquid aluminium is poured into an aluminium container with a wall thickness of 1 mm. The alloy cools to the target processing temperature in the container. The top side is protected from the atmosphere by using a cover with Argon. The container is transferred together with the semisolid billet to the shot chamber of a die casting machine and pressed into a die (Figure 13) (53).

Figure 13 Schematic illustration of the rheocontainer process, Aachen University. Reproduced from Pahlevani, F.; Endo, Y.; Yoakawa, J.; Itamura, M.; Kikuchi, M.; Nagasawa, O.; Anzi, K. Development of Cup Cast Method; Semi-solid Slurry Preparation without External Stirring. Solid State Phenom. 2006, 116–117, 358–361.

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5.06.3.1.9

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Cup-Cast Method

The cup-cast method (CCM) was a collaborative development between l

Department of Metallurgy, School of Engineering, Thoku University, Japan Nano-Cast Co., Tokyo, Japan l Tokyo Rika Manufacturing Co. Ltd, Japan l

In this method, globular solid particles are prepared by controlling the turbulence and heat distribution in the melt through pouring instead of applying certain external stirring forces. Embryos of solid particles are created on the inner surface of the cup and evolve to nuclei. These nuclei are separated from the cup’s wall by flow of the melt and move to the other parts of the melt with low temperature and composition gradient. The CCM was developed by using a new and simple shape of the cup (54).

5.06.3.1.10

Helical Curve Duct (HCD)

The helical curve duct process is based on the cooling channel, serpentine channel, and CRPs. The only difference to these processes is the flow path for the liquid metal. The slurry production method is based on creating sufficient nucleation by pouring low superheat molten metal into a helical channel (Figure 14) (55). The SSM slurry is collected in a container or shot sleeve of an HPDC machine for further processing or forming into a component. The quality of the SSM slurry is dependent on the melt temperature and length of the helix.

5.06.3.1.11

Serpentine Pouring Channel (SCP)

The molten metal is poured through a serpentine channel, which results in nucleation. Further mixing and cooling occurs as the metal flows through the channel into the shot sleeve of an HPDC machine for forming (Figure 15). The number of bends and pouring temperature influences the final quality of the slurry produced (56).

Figure 14 The schematic illustration of key apparatus for HCD process: (a) front view; (b) top view; and (c) cross-sectional view. Reproduced from Wang, M.; Yang, X.; Guo, V. Investigation of Semi-solid Slurries Prepared by Helical Curve Duct. Solid State Phenom. 2013, 192–193, 379–385.

Figure 15 Serpentine channel process (SCP). Reproduced from Mao, W. M.; Chen, Z. Z.; Liu, H. W.; Li, Y. G. Preparation and Rheo-die Casting of Semi-solid A356 Aluminum Alloy Slurry through a Serpentine Pouring Channel. Solid State Phenom. 2013, 192–193, 404–409.

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5.06.3.1.12

Inverted Cone-Shaped Channel Process

The inverted cone-shaped channel process is another process based on nucleation and grain evolution as the metal flows through a conical channel where nucleation occurs. The slurry is collected in a cup for further processing or quenching to produce thixoprocessing billets (Figure 16) (57).

5.06.3.1.13

Controlled Nucleation Process

This process is researched at the University of Southern Queensland and James Cook University, Queensland, Australia. The process uses solidification conditions rather than mechanical or electromagnetic stirring (EMS) to control nucleation, nuclei survival, and grain growth to achieve a fine-grained and nondendritic microstructure for semisolid forming. The main principle is to maximize grain density in the melt and promote grain growth (58).

5.06.3.1.14

The Damper Cooling Tube Method

Molten alloy is poured at the predetermined temperature into a container, and the hydraulic head is kept at a constant height for molten alloy (Figure 17). The cooling device is used to adjust the cooling rate and control the nucleation rate. A wedge block functions as a stirring system to produce a slurry with uniform temperature distribution. Two heating systems are used; one to clear blockages and another to adjust the slurry temperature and control solid fraction. The slurry is collected in a cup for further processing (59).

5.06.3.2 5.06.3.2.1

Rheocasting Systems Based on Nucleation and Active Contact Stirring/Shear Advanced Semisolid Casting Technology, Honda

Developed and patented by Honda Ltd in the early 2000s, this process involves the mechanical stirring of an aluminium melt and the transfer of quantities of the semisolid slurry to a conventional HPDC machine for component production (Figure 18(a)) (60,61). It is apparent that the process was established specifically for the production of aluminium diesel engine blocks (Figure 18(b)). These require high strength due to the high combustion pressures that characterize the diesel engine, and are therefore difficult to produce through conventional HPDC. The process has been commercialized in Japan with the first engines installed in the 2003 Honda Accord. More recently, in 2009, Honda has established a plant in the United Kingdom to produce the engine blocks.

5.06.3.2.2

Semisolid Rheocasting (SSR) Process

This process was originally developed at the MIT and involved the immersion of a cold rotating rod for a short duration into a melt held slightly above its liquidus temperature in order to drop the temperature of the melt below its liquidus to facilitate nucleation

Figure 16 Schematic illustration of the inverted cone channel process. Reproduced from Yang, B.; Mao, W. M.; Zeng, J. N.; Song, X. J. Effect of the Parameters in Inverted Cone-shaped Pouring Channel Process on the Microstructure of Semi-solid 7075 Aluminium Alloy Slurry. Solid State Phenom. 2013, 192–193, 415–421.

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Figure 17 Schematic illustration of the damper cooling tube method. Reproduced from Xie, S. S.; Yang, H. Q.; Wang, Hao; LI XG, L. I. L. Damper Cooling Tube Method to Manufacture Semi-solid Slurry of Magnesium Alloy. In Proceedings of the 8th International Conference on Semi Solid Processing of Alloys and Composites, Limassol, Cyprus; Alexandrou, A., Apelian, D., Eds.; 2004.

and impart the necessary shear forces to produce the SSM microstructure (Figure 19) (62,63). The rod is then removed and the slurry cooled further. Once the desired temperature is reached, the slurry is transferred into an HPDC machine and injected into the die.

5.06.3.2.3

Low Superheat Pouring with a Shear Field (LSPSF) Process

Developed at Nanchang University, China, this is a controlled nucleation and limited growth technique using a specially designed rotating barrel (64,65). The process uses solidification conditions to control nucleation, nuclei survival, and grain growth by means of low superheat pouring, vigorous mixing, and rapid cooling during the initial stage of solidification combined with thereafter a much slower cooling (64,65). The low superheat pouring with a shear field process has been shown to effectively produce highquality SSM slurry. The process can be applied to a wide range of Al alloys (Figure 20).

5.06.3.2.4

The Swirled Enthalpy Equilibration Device

The swirled enthalpy equilibration device (SEED) process was developed by Alcan Ltd around 2006. The original SEED process is a liquid-based slurry-making process that involves the extraction of a controlled quantity of enthalpy to generate liquid/solid slurry and then draining away excess liquid to form a compact, self-supporting slug for casting (Figure 21(a)) (66,67). A new version of the SEED process was developed recently that eliminates the drainage step (Figure 21(b)) (67). The slurry is transferred to the shot sleeve of an HPDC machine (Figure 21(c)) or formed using other forming techniques.

5.06.3.2.5

Rheomolding

The original rheomolding concept was introduced by Wang et al. (68). It is very similar to the ThixomoldingÒ process but in this process liquid metal is fed into a barrel with a screw (Figure 22). The molten metal is cooled and sheared by the screw to produce a globular semisolid microstructure for subsequent forming. This concept led to a number of patents and rheocasting processes that will be described briefly in this section. These processes have limited applicability to aluminium alloys because of the high reactivity of aluminium with iron-based alloys (i.e., the construction material of the processing equipment). Their application has thus been limited primarily to magnesium alloys. 5.06.3.2.5.1 Rheodiecasting Process The rheodiecasting (RDC) process was developed by BCAST at Brunel University, United Kingdom. It innovatively combines a well-established twin-screw slurry maker with the existing cold chamber HPDC process. The RDC equipment consists of three basic functional units, a twin-screw slurry maker, a standard cold chamber HPDC machine, and a central control unit (Figure 23(a)) (69). BCAST has also developed the MCAST process for conditioning liquid metal at temperatures either above or below the alloy liquidus using a high-shear twin-screw mechanism (Figure 23(b)) (70). The MCAST process has now been combined with the twin

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Figure 18 (a) Schematic illustration of the advanced semisolid casting technology rheocasting process developed by Honda (reproduced from Sakamoto, K.; Hamazoe, N.; Ohwada, K.; Suzuki, A. Method and Apparatus for Manufacturing Semi-solidified Metal. European Patent EP 1050353, issued November 24, 2004). (b) Diesel engine block cast with Honda rheocasting technology (reproduced from Kuroki, K.; Suenaga, T.; Tanikawa, H.; Masaki, T.; Suzuki, A.; Umemoto, T.; Yamazaki, M. Establishment of a Manufacturing Technology for the High Strength Aluminum Cylinder Block in Diesel Engines Applying a Rheocasting Process. In Proceedings of the 8th International Conference on Semi Solid Processing of Alloys and Composites, Limassol, Cyprus; 2004).

roll casting (TRC) process to form an innovative technology, namely the melt-conditioned TRC process for casting Al-alloy and Mgalloy strips. The processing of aluminum alloys does pose challenges due to the reaction of molten aluminum with the screw and barrel which will reduce the life of the barrel and contaminate the alloy with Fe. 5.06.3.2.5.2 Taper barrel Molten alloy with a low superheat is poured into the processing vessel, which comprises an external tapered barrel and a rotating internal taper barrel. The metal is cooled into the semisolid state while undergoing shear. The metal flows between the tapered barrels and the gap between the inner and outer barrels can be controlled by moving the inner barrel up or down (Figure 24) (71). The slurry can be collected at the slurry outlet for forming or cooled for the production of thixoforming billets. 5.06.3.2.5.3 Rotating Barrel Rheomolding Machine Process Developed at the School of Materials Science and Engineering, University of Science and Technology, Beijing, China, the rotating barrel rheomolding machine process is claimed to be an improved technique for preparing and rheomolding semisolid aluminium slurry. The device comprises two relative-rotating conical barrels with uniquely designed grooves that generate high shear rates and high intensity turbulence (Figure 25) (72). When the correct dose of alloy is poured into the gap of two relative-rotating barrels, the liquid is quickly cooled to a predetermined processing temperature to obtain the desired solid fraction. 5.06.3.2.5.4 Forced convection rheomolding process The forced convection rheomolding process is very similar to the original rheomolding process that was developed by Wang et al. (68,73). Molten metal is poured into the barrel containing a rotating screw where the metal is cooled into the semisolid state under controlled cooling and shear. A forced convection flow is induced in the barrel, which assists in refining the grain structure into a globular structure (Figure 26).

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Figure 19 Schematic illustration of the semisolid rheocasting (SSR) process. Reproduced from Yurko, J. A.; Martinez, R. A.; Flemings, M. C. Development of the Semi-solid Rheocasting (SSR) Process. In Proceedings of the 7th International Conference on Semi-solid Processing of Alloys and Composites, Tsukuba, Japan; 2002; pp 659–664; Martinez, R. A.; Flemings, M. C. Evolution of Particle Morphology in Semi-solid Processing. Metall. Mater. Trans. A 2005, 36 (8), 2205–2210.

Figure 20 Schematic illustration of the low superheat pouring with a shear field (LSPSF) process. Reproduced from Guo, H. M.; Yang, X. J. Efficient Refinement of Spherical Grains by LSPSF Rheocasting Process. Mater. Sci. Technol. 2008, 24 (1), 55–63.

5.06.3.2.6

Rheometal Process

Developed around 2004 at Jönköping University in Sweden, this process involves the pouring of a liquid melt into an insulated container. An amount of solid alloy is attached to a stirrer and immersed and stirred into the melt. The added solid alloy is normally relatively cold, i.e., relative low enthalpy; it will absorb the heat from the melt, which will partially or totally melt away and leave behind a semisolid slurry (Figure 27) (74,75). It has been shown that 120 kg of slurry (A356 alloy) can be produced for SSM processing in less than 30 s.

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Figure 21 Schematic illustration of the SEED process. Reproduced from Côté, P.; Larouche, M.; Chen, X. G. New Developments with the SEED Technology. Solid State Phenom. 2013, 192–193, 373–378. (a) The original SEED process. (b) The new SEED process. (c) A typical HPDC cell layout with the SEED process.

The rheometal process has been commercialized through the formation of a spin-off company, RheoMetal AB in 2005.

5.06.3.2.7

Gas-Induced Semisolid Metal Process

This is an alternative process for producing semisolid slurry and utilizes the combination of local rapid heat extraction and agitation achieved by the injection of fine gas bubbles through a graphite diffuser (Figure 28) (76–78). The process was developed at the Department of Mining and Materials Engineering, Prince of Songkla University, Thailand, and the Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States. One of the claimed advantages is the fact that the gas-induced semisolid (GISS) metal process only requires minor modifications to the die casting machine and the casting process. It has been demonstrated on a number of different forming processes successfully (78). The GISS process is being commercialized via GISSCO Co. Ltd, established in 2009.

5.06.3.2.8

Melt Spreading and Mixing Technique (MSMT)

Research at the National Engineering Research Center for Non-Ferrous Metal Composites, General Research Institute for NonFerrous Metals, Beijing, China, and Inner Mongolia University of Technology, College of Materials Science and Engineering, Huhehaote, China, focuses on the production of semisolid slurry for mass production with uniformly fine, nondendritic

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Figure 22 Schematic illustration of the rheomolding process. Reproduced from Wang, K. K.; Peng, H.; Wang, N.; Wang, S. Method and Apparatus for Injection Molding of Semi-solid Metals. US Patent 5,501,266, 1996.

microstructures (79). The molten metal is poured onto a rotating disc and thrown onto the walls of the slurry chamber. The pouring speed and rotating disc speed are controlled. Nucleation takes place homogeneously and the supercooled metal ensures sufficient nucleation occurs. The SSM slurry is collected in a receiving crucible for further processing (Figure 29).

5.06.3.3 5.06.3.3.1

Nucleation and Active Noncontact Stirring The Hitachi Process

The Hitachi process was first introduced in the mid-1990s as alternative process to the thixoprocessing route (37). The process encompassed ladling molten metal into a vertical injection shot sleeve, where it is cooled and electromagnetically stirred to the semisolid state before injection into the die. A subsequent design was released in 2002, which essentially worked on the same principle but used an electromagnetic pump to supply metal to the shot sleeve (Figure 30) (80).

5.06.3.3.2

Advanced Rheocasting Process a.k.a Hong-Nano Casting Method

This process was developed by Yonsei University in Korea and is reportedly in the commercialization phase in Korea and Japan. The molten slurry is poured into a slurry-making vessel to which an electromagnetic field is applied in order to prevent the formation of initial solidification layers at the inner vessel wall in the early stage of cooling (81). The temperature distribution in the slurry-making vessel can be uniformly maintained during the cooling stage below the liquidus temperature, leading to an increase in the heterogeneous nucleation density, and resulting in a high quality of metallic slurry with very fine and uniform globular structures. Commercial rheodiecasting machines based on this method were developed in 2006, and are now reportedly being used in manufacturing aluminium automotive casting parts. The Hong-nano casting method (H-NCM) rheodiecasting machines were built by Sanjo Seiki and UBE Ind. Ltd. The following commercial uses and trials are known: l

Tokyo-Rikka Co., Ltd (Oil pressure component, production, 2006). Kia Motors Co. applied and tested the H-NCM system to cast an engine support bracket. l Lower arm produced by the H-NCM rheodiecasting tested by Hyundai Motors Co. (2006). l

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Figure 23 (a) Schematic illustration of the twin-screw RDC process, Brunel University, UK (reproduced from Fan, S. J. Z.; Bevis, M. J. Semi-solid Processing of Engineering Alloys by Twin Screw Rheomoulding Process. Mater. Sci. Eng. A 2001, 299, 210–217). (b) Schematic illustration of the MCAST process (reproduced from Zuo, Z. B.; Xia, M. X.; Lian, S. M.; Wang, Y.; Scamans, G. M.; Fan, Z. Y. Grain Refinement of DC Cast AZ91D Mg Alloy by Intensive Melt Shearing. Mater. Sci. Technol. 2011, 27 (1), 101–107).

Figure 24 Schematic illustration of the taper barrel process. Reproduced from Zhang, F.; Kang, Y.; Yang, L.; Ding, R. Taper Barrel Rheomoulding Process for Semi-solid Slurry Preparation and Microstructure Evolution of A356 Alloy. Trans. Nonferrous Soc. China 2010, 20, 1677–1684.

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Figure 25 Schematic illustration of the rotating barrel rheocasting method. Reproduced from Seo, P. K.; Lee, S. M.,; Kang, C. G. A New Process Proposal for Continuous Fabrication of Rheological Material by Rotational Barrel with Stirring Screw and Its Microstructural Evaluation. J. Mater. Process. Technol. 2009, 209, 171–180.

Figure 26 Schematic illustration of the forced convection rheomolding process. Reproduced from Zhou, B.; Kang, Y.; Zhang, J.; Gao, J.; Zhang, F. Forced Convection Rheomoulding Process for Semi-solid Slurry Preparation and Microstructure Evolution of 7075 Aluminium Alloy. Solid State Phenom. 2013, 192–193, 422–427.

Figure 27 Schematic illustration of the rheometal process. Reproduced from Wessén, M.; Cao, H. The RSF Technology – A Possible Breakthrough for Semi-solid Casting Processes. Metall. Sci. Technol. 2007, 25 (2), 22–28; Payandeh, M.; Jarfos, A. E. W.; Wessén, M. Effect of Superheat on Melting Rate of EEM of Al Alloys during Stirring Using the RheoMetal Process. Solid State Phenom. 2013, 192–193, 422–427.

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Figure 28

Schematic illustration of the gas-induced semisolid (GISS) process. Reproduced from www.gissco.com.

Figure 29 Schematic illustration of the melt spreading and mixing technique (MSMT) process. Reproduced from Lou, S.; Keung, W. C.; Kang, Y. Theory and Application Research and Development of Semi-solid Forming in China. Trans. Nonferrous Metals Soc. China 2010, 20, 1805–1814.

5.06.3.3.3

In-Mold Rheocasting Process

The process has been developed by the Advanced Material Center, KITECH, Yeonsu-gu, Korea, and Future Cast Co., Korea. In-mold rheocasting requires no additional processing equipment apart from the HPDC machine, no grain refinement procedure, and no additional cycling time to produce slurry on demand (82). The slurry is produced in the shot sleeve with EMS.

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Figure 30 Schematic illustration of the Hitachi rheocasting process. (a) High-pressure die casting cell layout and (b) shot sleeve design. Reproduced from Kaneuchi, T.; Shibata, R.; Ozawa, M. Development of New Semi-solid Metal Casting Process for Automotive Suspension Parts. In Proceedings of the 7th International Conference on Semi-solid Processing of Alloys and Composites, Tsukuba, Japan; September 2002; pp 145–150.

5.06.3.3.4

Novel Hot Chamber Rheodiecasting Process

This process was developed by the Korea Institute of Industrial Technology, Korea, and Future Cast Co., Korea. As for the in-mold rheocasting process, the hot chamber rheodiecasting process is one of the new rheodiecasting processes that require no additional processing equipment, no molten metal control, and no additional cycling time to produce slurry on demand. The slurry is produced in the hot chamber HPDC machine nozzle under EMS (83). The process has been specifically designed for the mass production of magnesium 3C’s applications, electromagnetic interference (EMI) shields, and cell phone cases. Results achieved reportedly showed high efficiency in energy, production cost, productivity, and process stability for mass production.

5.06.3.3.5

Multielectromagnetic Stirring Continuous Preparation Process

The process was developed by General Research Institute for Nonferrous Metals, China, who has several years’ experience in vertical continuous casting with EMS. The modified multielectromagnetic stirring continuous preparation process combines noncontact EMS and an annular chamber with specially designed profiles to make in situ high-quality semisolid slurry (84,85).

5.06.3.3.6

Slurry on Demand (SoD)

The slurry-on-demand method of preparing SSM slurries was developed by AEMP Corporation in the United States in the early 2000s (86). This technique used the combination of initial nucleation on the walls of a cup followed by EMS as shown in Figure 31.

5.06.3.3.7

The Council for Scientific and Industrial Research Rheocasting System

The Council for Scientific and Industrial Research (CSIR) rheocasting system was developed by CSIR in South Africa in the early 2000s to prepare semisolid slurries of metal alloys (87–89). Molten metal is poured into a metal cup and is subjected to controlled cooling under induction heating/stirring and air cooling (Figure 32(a)). The induction heat serves to assist in controlling the cooling rate and agitates the melt to ensure that a slurry with a uniform temperature and globular microstructure is achieved. The process has been demonstrated on a semi-industrial scale (Figure 32(b) and 32(c)). The processing flexibility was demonstrated by the range of alloys processed on the system as well as a unique achievement of demonstrating an SSM microstructure in pure aluminium (90,91).

5.06.3.4

Summary on Rheocasting Systems

Since the mid-1990s, there has been a proliferation of NRC processes in the search of developing a cost-effective method of producing semisolid slurries with the required properties repeatedly and cost-effectively. These have been captured in patents and are presented in the International Conferences on Semi-Solid Forming since the 4th conference in 1996. The previous sections presented a broad outline of processes that have been published, but there many more processes that have been proposed and are in various stages of development. It is notable that few of these processes have achieved significant industrial applications to date. Those that have been successful have addressed particular niche applications involving demanding combinations of high component integrity, strength, and geometrical complexity.

5.06.4

Forming Methods

Semisolid processing as described in the introduction, encompasses preparing a slurry with the appropriate properties, the processes are described in Sections 5.06.3.2 and 5.06.3.3, which can be formed into a final product. The final step of forming the final product is common for both the thixo and rheo slurry preparation routes. Once the semisolid slurry has been processed, it can be formed using commonly available forming processes such as casting, forging, rolling, or extrusion.

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Figure 31 Schematic illustration of the slurry formation mechanism for the slurry on demand (SoD) process. Reproduced from Norville, S. M. D.; Lombard, P. J.; Lu, J.; Wang, S. Apparatus for and Method of Producing On-demand Semi-solid Material for Castings. US Patent 6,845,809 B1, 2005.

Figure 32 The CSIR rheocasting system (RCS) (courtesy of CSIR, South Africa). (a) Schematic illustration of the laboratory scale prototype, (b) schematic illustration of the industrial scale prototype, and (c) CSIR-RCS incorporated into an HPDC cell.

5.06.4.1

Casting

HPDC is a common process for forming nonferrous metal with low melting points. It is particularly suited for the high-volume production of comparatively complex near-net shape parts. Thixocasting via the HPDC route is currently being commercially used by V-Forge (92) and SAG (93) while the rheocasting route is being pursued by a number of research centers and to a lesser extent commercially. Casting processes with a significant solid fraction use the thixotropic behavior of the slurry to achieve laminar filling and hence produce high-integrity castings. The high-pressure die casting of SSM slurries can be achieved using cold chamber HPDC machines with horizontal injection and horizontal clamping, vertical injection with vertical clamping, and vertical injection with vertical clamping. More recently, the slurry manufacturing process, in particular low solid fraction slurries, has been used in the in-sand castings as well (94). Slurries in this form can be used in a much wider range of casting processes.

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5.06.4.2

131

Forging

SSM forging can be applied to a wider alloy range including high melting point materials such as ferrous (95,96) and nonferrous metals including titanium alloys (97) as well as the lower melting point nonferrous alloys of aluminium, copper, Zn, and others. The forging process uses conventional forging presses but the slurry is either prepared via the thixo or rheo processing methods. The solid fractions are higher than that of SSM casting but the process offers the benefit of lower forging pressures.

5.06.4.3

Extrusion

Semisolid extrusion of various alloys has been demonstrated using standard extrusion equipment. The semisolid slurry is again prepared either via the thixo or rheo processing techniques and extruded using standard extrusion presses. Initial SSM extrusion processes focused on using the thixoprocessing route (thixoextrusion); however, it has been demonstrated that the rheoprocessed metal (rheoextrusion), especially the low melting point alloys, can also be extruded (78,98). For high melting point alloys, thixoextrusion would be preferred.

5.06.4.4

Joining

Semisolid joining involves heating the surfaces to be joined into the semisolid range before fusing the pieces together. The benefits are that metal is joined at a much lower temperature and joints with large cross sections can be fused in one operation (95,99).

Figure 33 Schematic illustration of the semisolid free forming technology. Reproduced from Rice, C. S. Solid Freeform Fabrication Using Semi-solid Processing. MSc Theis, MIT, June 1995.

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5.06.5

Thixomolding

This process was invented in the early to mid-1990s by Dow Chemical Company and commercialized by Thixomat Incorporated. The process is analogous to plastic injection molding. Magnesium chips are fed into a hopper and through a feed screw under a protective atmosphere. The chips are progressively heated in the screw by means of external heater bands. Once the material has reached the desired consistency at the front of the screw (typically only 5–10% solid fraction), it is injected into the die under high pressure to form a part (6,100,101). The process does not require melting facilities because the material is processed from the solid. The process is currently used mainly in the electronics industry but has been demonstrated across a much wider industry sector (6,101). There are currently about 50 licensees operating 260 machines in 10 countries.

5.06.6

Semisolid Free Forming Technology

Semisolid free forming was developed as a rapid prototyping technique to produce metal parts in the early to mid-1990s (102,103). The method involves preparing slurry under mechanical stirring and extruding the slurry and depositing on a moving table to build the part. Although some work on the technical feasibility was undertaken in the late 1990s, there are no known commercial applications (Figure 33).

5.06.7

Summary

Since the discovery of semisolid processing more than 40 years ago, a huge variety of semisolid processing techniques have been developed. Two processing routes, initially identified by Flemings in the early 1970s, have developed at differing rates. In early development of SSM processing, from the early 1970s to mid-1990s, the main focus was on the development of thixocasting or thixoprocessing methods. This encompassed feedstock preparation techniques, reheating into the semisolid ranges and forming into final products. This early research was predominantly done in Europe and America with a relatively small amount of work in the East, Japan and Korea. The process was embraced by the automotive industry as a potential alternative to HPDC and forging in order to produce high-integrity components. The high cost of the feedstock material, in particular, resulted in the process losing favor with industry, except for very niche applications where structural integrity of the component was more important. This led to the development of the rheoprocessing technologies. There has been an explosion of new methods of preparing semisolid slurries since the mid-1990s. Again, the process development was driven by Europe, America, and Japan up to the early 2000s but since then research and commercialization efforts have been driven by the East, especially China. A large number and variety of rheocasting or slurry preparation techniques have been proposed, patented, and developed over the past decade and half, which are largely captured in the International Conferences on Semi-Solid Forming. This chapter provides an overview of the SSM processes that have been developed.

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