- Email: [email protected]
M Ti6Al4V alloy forging
Journal of Alloys and Compounds xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Design and verification of thermomechanical parameters of P/M Ti6Al4V alloy forging Marek Wojtaszek ⇑, Tomasz S´leboda AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Av. Mickiewicza 30, 30-059 Cracow, Poland
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Powder metallurgy Titanium alloy Compaction Forging Numerical simulation Properties
a b s t r a c t This work is focused on the design of technology of forging high-quality Ti6Al4V alloy by means of powder metallurgy methods. A mixture of elemental powders, with the chemical composition of that of Ti6Al4V alloy, was used as a starting material for the investigation. Powder mixtures were fully densified by hot compaction under precisely controlled conditions. The mechanical properties of the obtained compacts were examined. The mechanical behaviour of the investigated alloy powder compacts was evaluated by compression test under various thermomechanical conditions using Gleeble simulator. The microstructure of powder compacts as well as P/M alloy samples deformed in compression tests was examined. All data obtained from the experimental tests were applied as boundary conditions for numerical simulation of forging of selected forgings. Basing on the results of both plastometric tests and simulations, thermomechanical parameters of the investigated alloy forging were determined. Designed parameters of forging technology were verified by forging trials performed in industrial conditions. The quality of the obtained forgings was examined by means of computed tomography. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The application of titanium and its alloys is growing in the areas such as aviation, space exploration, shipbuilding, chemical industry and many other branches of economy [1,2]. The main reason is a combination of favourable properties of these alloys, such as low specific gravity (4.5 g/cm3), high specific strength, resistance to crack propagation, good low-temperature toughness, fatigue strength and excellent corrosion resistance. The limitations to the applicability of titanium alloys include low heat conduction, low machinability [3–5] as well as high costs. The most commonly used titanium alloy is Ti6Al4V. Beside the advantages specific to titanium, this alloy also shows heat resistance, relatively good formability, weldability and biocompatibility. Usually, the deformation of this alloy proceeds by forging or extrusion processes [6]. The application of powder metallurgy (P/M) to the manufacturing of products of titanium alloys is a known technology [7]. The use of P/M methods in respect to hard-to-deform alloys may result in a better controllability of the forming process parameters [8–12]. The type of initial powders is, however, a significant issue that should be taken into consideration. The applied types include
⇑ Corresponding author. Tel.: +48 (12) 617 38 72; fax: +48 (12) 617 33 39. E-mail address: [email protected] (M. Wojtaszek).
powders obtained by liquid alloy atomization (pre-alloying method) and those obtained by mixing of elemental powders (blended elemental powder metallurgy – BEPM). High production cost can be a significant limitation in the case of producing P/M Ti6Al4V alloy parts. Considerably lower costs are generated in the case of manufacturing products based on the mixture of elemental powders. However, there is a potential risk of inhomogeneity of chemical composition of the product and that is why a special care should be taken of a uniform distribution of powder mixture ingredients [13] as well as the proper forming route [14]. One of the possible methods of processing such materials is a combination of powder metallurgy and metal forming techniques, what ensures the advantages of both. The application of fully densified powder compacts as charge for further processing allows using the equipment already existing in commercial metal forming companies without the necessity of its modification. Contrary to densification processes, where the dimensions of the press chamber restricts the geometry of the product, multi-stage forging enables modification of the geometry of preform and significant enhancement of the range of dimensions and shapes of the final products. Moreover, forging process enables application of various cooling procedures for the forged part, what significantly influences its microstructure and properties. This work discusses the possibilities of design and application in industrial conditions for the technology of forging Ti6Al4V alloy by means of blended elemental powder metallurgy route [14].
http://dx.doi.org/10.1016/j.jallcom.2014.01.161 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
2
M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
2. Material under investigation and research methods Ti6Al4V powder was obtained by means of mechanical mixing of elemental powders. The powder mixtures were applied as the initial material for the investigations. The powders were blended for 10 h in a conical mixer. WC balls were additionally used to intensify the mixing process. The testing included the assessment of powder morphology and the basic chemical analysis (EDS) performed with application of Hitachi TM3000 scanning electron microscope. The analysis of the chemical composition of random sample of the mixture of elemental powders was performed to determine the correctness of the applied blending conditions for obtaining the chemical composition of commercial Ti6Al4V alloy. Forming of highly consolidated P/M compacts was realized by hot sintering under pressure, using the test stand by Thermal Technology Inc. Ti6Al4V powder mixtures were sintered for 3 h at the temperature of 1200 °C, under the pressure of 25 MPa, and under an argon atmosphere. Sintering resulted in obtaining cylindrical specimens with an approximate height of 50 mm and a diameter of 75 mm. The relative density of compacts (Archimedes method) as well as their selected mechanical properties were analysed. The hardness of specimens was measured using the Vickers method (Tukon 2500 hardness tester), applying the load of 1 kg. The mechanical properties were determined with application of uniaxial compression test (Zwick Z250 testing machine). The material after hot compaction was subjected to axisymmetrical compression tests with application of Gleeble 3800 simulator. The specimens of 10 mm diameter and 12 mm height were used in compression tests. The plastometric tests were realized with thermal and mechanical parameters given in Table 3. The material was heated to the testing temperature, held for a specified period of time, and then subjected to deformation and subsequent cooling. Varying temperatures and strain rates were assumed for the specimens. The microstructure of a compact and the specimens after deformation at different thermomechanical parameters was observed using Leica DM4000M optical microscope. The cross-sections were observed after twostage etching (1st stage: 6% HF, 2nd stage: 2% HF + 2% HNO3 + 96% H2O). Numerical modelling of forging of the selected forging was performed with application of the finite element method, using QForm 2D/3D commercial software. Numerical simulations were designed to ensure that all parameters being modelled are attainable in industrial conditions. A crank press of 1000 ton capacity was assumed in the simulation. The press slide velocity was 1 m s 1, and WCLV steel was assumed as the die insert material. The temperature of dies was 300 °C, and the hot working lubricant (graphite–water slurry) was introduced, with the appropriate coefficient of friction of 0.23. The obtained results of experimental investigations and numerical simulation were verified basing on die forging tests realized in industrial conditions. The forging process was carried out using a crank press at HSW Kuz´nia Stalowa Wola Ltd., Poland. For a representative specimen, during heating and cooling, the temperatures were recorded inside the specimen (hole with a thermocouple) and on the surface (pyrometer). The obtained characteristic allowed to maintain the thermal conditions during the tests close to those evaluated numerically. The quality of forgings was assessed by means of non-destructive testing, with application of computed tomography (CT). Scanning was realized using v tome 450 tomograph with the resolution of 40 lm.
3. Results Morphology of the powder under investigation, being the mixture of elemental powders of titanium, aluminium and vanadium, is presented in Fig. 1a. The mixture consists of particles showing irregular surface, diversified shape and the size between 10 and 200 lm. Results of EDS analysis are shown in Fig. 1b. The chemical composition of the investigated alloy is given in Table 1. The mass fractions of titanium, aluminium and vanadium in a randomly selected powder sample (Fig. 1b) are close to those specific for
Table 1 Chemical composition of Ti6Al4V alloy (ISO 5832/3). O
V
Al
Fe
H
C
N
Ti
<0.20
3.5–4.5
5.5–6.75
<0.30
<0.0015
<0.8
<0.05
Bal.
Table 2 Selected properties of Ti6Al4V alloy in the state after sintering of elemental powders. Density (g/cm3)
Relative density (%)
Hardness (HV)
Rp0.2 (MPa)
Rm (MPa)
Young’s modulus (GPa)
4.40 ± 0.01
99.36 ± 0.16
339 ± 11
1013 ± 31
1029 ± 36
104.6 ± 5.3
Table 3 The parameters used in Gleeble 3800 simulator compression tests. Heating rate (°C/s)
Holding time before compression (s)
Temperature of tests (°C)
Strain rate (s 1)
Cooling rate (°C/s)
2.5
10
800, 900, 950, 1000
0.01, 0.1, 1, 10, 100
Approx. 50
Ti6Al4V alloy (Table 1), thus confirming the proper distribution of individual elemental powders in a mixture. The results of density measurements as well as the selected mechanical properties of the material after hot compaction are put together in Table 2. Basing on the results of plastometric tests, the material flow characteristics were elaborated. The selected examples of the effect of temperature and strain rate on the character of true stress vs. true strain curves are collected in Fig. 2. The microstructure of the material was observed after hot compaction under pressure and after deformation in varying temperature and strain rate conditions. The microstructure of the material after hot compaction is presented in Fig. 3a, while Fig. 3b and c shows the examples of microstructures obtained after compression tests in Gleeble simulator. The flow curves were applied to the description of material behaviour in variable thermomechanical conditions of numerical modelling of hot die forging. A ring with a flange was selected as the forging to be analysed. The simulations were carried out assuming different variants of heating, holding and cooling operations as well as variable forging process parameters, being however realizable in industrial conditions in each case. Fig. 4 presents only selected results of modelling, obtained, as an example, for the temperature of 900 °C and the tool speed of 1 m s 1. Basing on the results of a complex analysis, the material geometry was specified as well as thermal and mechanical forging conditions, favourable for the analysed alloy and selected forging, were evaluated. Their verification was carried out basing on forging tests in industrial conditions. The model showing the selected shape of a forging
Fig. 1. Morphology of Ti6Al4V powder (a) and results of EDS chemical analysis of powder (b).
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
(b)
400
300
5
200
4
3
100
0
0,0
400
nr 1 2 3 4 temperature, °C 800 900 950 1000
nr 1 2 3 4 5 strain rate, s-1 0.01 0.1 1 10 100
2 1 0,1
0,2
0,3
0,4
0,5
0,6
true strain ε
0,7
0,8
0,9
1,0
true stress, MPa
true stress, MPa
(a)
3
300
1 200
2 3
100
0
4
0,0
0,1
0,2
0,3
0,4
0,5
0,6
true strain ε
0,7
0,8
0,9
1,0
Fig. 2. Example true stress vs. strain curves obtained from compression tests performed using Gleeble simulator at the temperature of 900 °C (a) and at the strain rate of 10 s 1 (b).
Fig. 3. Microstructure of P/M Ti6Al4V alloy in the state after hot sintering (a), after deformation at the temperature of 800 °C and strain rate of 10 s the temperature of 900 °C and strain rate of 10 s 1 (c) and after deformation at the temperature of 900 °C and strain rate of 100 s 1 (d).
together with the photograph of a hot-forged product are presented in Fig. 5. The example CT images taken in selected sections of a forging, are shown in Fig. 6. 4. Discussion Basing on the results of EDS analysis it was confirmed that mass fractions of the basic ingredients of the mixture of elemental powders are close to those required for the investigated alloy. It should be stated that in case of powder mixtures, the results obtained from EDS analysis can be questionable, since the different specific gravity of individual ingredients may cause segregation during taking samples, thus affecting the results. However, no such problem was observed in this work. It was shown that hot sintering of Ti6Al4V powder under pressure, with the assumed process parameters, resulted in obtaining the material showing the relative den-
1
(b), after deformation at
sity of a solid material and good mechanical properties (Table 2). The observations of a character of flow curves (Fig. 3) showed high sensitivity of the material to the changes of deformation temperature (Fig. 3a) and strain rate. Metallographic examination of compacts and specimens after deformation allows to establish the relationship between the thermomechanical conditions of deformation process and the resulting state of microstructure. The observations of a microstructure of the alloy after sintering showed the precipitations of a-phase occurring in a form of lamellae of average length of about 50 lm and width of a few lm (Fig. 3a). The deformation of the material caused the refinement of a-phase lamellae distributed within the b-phase matrix (Fig. 3b and c). The observations of microstructure of compacts and specimens after deformation did not show symptoms of cracking or the occurrence of pores, thus qualitatively confirming the results of density measurements.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
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Fig. 4. Distributions of mean stress (a), effective strain (b) and temperature (c) obtained from FEM numerical simulation of forging the sinter of Ti6Al4V alloy at the temperature of 900 °C.
Fig. 5. Shape of the forging selected for manufacturing in industrial conditions: the model (a) and the photograph of a real forging (b).
Fig. 6. Schematic representation of the location of selected sections (a, b) and images of these sections of a forging obtained using computed tomography (CT) method sections through YZ plane (c) and through XZ plane (d).
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
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The distribution of mean stresses obtained as a result of the FEM numerical simulation of forging at the temperature of 900 °C and the tool speed of 1 m s 1 (Fig. 4a), presented as an example, is rather uniform within the whole forging, with the exception of the web. However, in case of the analysed forging of a ring with a flange, the web region is not a problem as it is destined for being cut away as a discard. This testifies for a proper selection of the shape of contour impression and the dimensions of feedstock. Large gradient of the effective strain (Fig. 4b) results from the shape of a forging. In the final forging stage, significant temperature rise was observed in the flash region (Fig. 4c), resulting from conversion of the work of plastic deformation into the heat. The temperature drop was observed in regions contacting with the dies, the temperature of which is at the level of about 300 °C. Except these regions, the temperature distribution within the volume of a forging is quite uniform. Basing on the complex analysis of the results of FEM numerical modelling, considering the character of flow curves of the alloy being analysed and its microstructure as influenced by deformation conditions, thermomechanical parameters were elaborated for the process of hot die forging of a forging with required shape. The temperature of 900 °C was established as the most favourable forging temperature. In order to ensure the dissolution of alloying elements and to homogenize the microstructure, it was assumed to apply preliminary heating of a feedstock to the temperature of 1000 °C. Considering low heat conduction of titanium and large temperature difference between a feedstock and a die, the obtaining of required forging temperature by means of cooling allowed to avoid problems connected with the occurrence of temperature gradients within the material volume, as it was indicated in the results of FEM simulation. The forging process was realized in two operations and the finished forgings were air-cooled. Basing on the preliminary observations of the shape and surface condition of forgings, it was stated that the deformation process was realized properly and the parameters selected basing on numerical modelling were correct. These observations were confirmed basing on the results of non-destructive testing. The analysis of CT images did not show cracks or discontinuities within the material volume, in the range of applied resolution. Moreover, no pores were observed, thus confirming qualitatively the results of density testing. 5. Conclusions Basing on the results of the realized investigations it can be stated, that the proposed technology of hot sintering under pressure, applied to the mixture of elemental powders with mass fractions corresponding with Ti6Al4V alloy, allows to manufacture the product showing the density of a solid material with the same chemical composition. This was determined by the density measurements and then confirmed by metallographic observations and computed tomography. Since the mechanical properties of a material in this
5
condition are high, it can be used as a finished product or as a semi-finished product for further processing. The flow curves of the compacts of Ti6Al4V alloy, obtained with application of Gleeble simulator, describe the behaviour of this material subjected to deformation in different thermomechanical conditions. They are fundamental for the preparation of a material database. In such form, they can be used for the description of boundary conditions in FEM numerical modelling. The results of numerical modelling, together with microstructure observations, allowed to elaborate favourable parameters of forging of the analysed alloy. Their correctness was verified in industrial conditions by means of manufacturing forgings with a selected shape. The tests proved to be successful – flawless forgings were produced, the quality of which was confirmed by means of non-destructive testing. The authors claim that the presented results of investigations indicate that the proposed technology can find its application to the manufacturing of structural components from initial material in a form of the mixture of elemental powders. The potential fields of application for the presented results include particularly automotive industry and aviation. Acknowledgements Financial support of Structural Funds in the Operational Programme –nnovative Economy (IE OP) financed from the European Regional Development Fund – Project WND-POIG.01.03.01-12004/09 is gratefully acknowledged. References [1] C. Chunxiang, H. BaoMin, Z. Lichen, L. Shuangjin, Mater. Des. 32 (2011) 1684– 1691. [2] O.M. Ivasyshyn, A.V. Aleksandrov, Mater. Sci. 44 (2008) 311–327. [3] Y. Sheng, Z. Guo, J. Hao, D. Yang, Procedia Eng. 36 (2012) 299–306. [4] P. Heinl, L. Müller, C. Körner, R.F. Singer, F.A. Müller, Acta Biomater. 4 (2008) 1536–1544. [5] M.J. Donachie Jr. (Ed.), Titanium and Titanium Alloys Source Book, Am Soc Metals, OH, 1982, pp. 265–269. [6] S. Bednarek, A. Łukaszek-Sołek, J. Sin´czak, Arch. Civ. Mech. Eng. 8 (2) (2008) 13–20. [7] C. Haasea, R. Lapovoka, N.H. Pang, Y. Estrina, Mater. Sci. Eng. A – Struct. 550 (2012) 263–272. [8] T. S´leboda, Steel Res. Int. 79 (2008) 493–498. [9] T. S´leboda, J. Kane, RN. Wright, NS. Stoloff, D.J. Duquette, Mater. Sci. Eng. A – Struct. 368 (2004) 332–336. [10] T. Fujita, A. Ogawa, Ch. Ouchi, H. Tajima, Mater. Sci. Eng. A – Struct. 213 (1996) 148–153. [11] W. Chen, Y. Yamamoto, W.H. Peter, M.B. Clark, S.D. Nunn, J.O. Kiggans, T.R. Muth, C.A. Blue, J.C. Williams, K. Akhtar, J. Alloys Comp. 541 (2012) 440–447. [12] Y. Liu, LF. Chen, HP Tang, CT. Liu, B. L, BY. Huang, Mater. Sci. Eng. A – Struct. 418 (2006) 25–35. [13] M. Wojtaszek, J. Durak, Metall Foundry Eng. 33 (2007) 23–31. [14] M. Wojtaszek, T. S´leboda, Thermomechanical processing of P/M Ti6Al4V alloy, in: Conf. Proc. Metal 2013, 22nd Int. Conf .Metal Mater., May 15th–17th, Brno, Czech Republic, 364–369.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Design and verification of thermomechanical parameters of P/M Ti6Al4V alloy forging Marek Wojtaszek ⇑, Tomasz S´leboda AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Av. Mickiewicza 30, 30-059 Cracow, Poland
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Powder metallurgy Titanium alloy Compaction Forging Numerical simulation Properties
a b s t r a c t This work is focused on the design of technology of forging high-quality Ti6Al4V alloy by means of powder metallurgy methods. A mixture of elemental powders, with the chemical composition of that of Ti6Al4V alloy, was used as a starting material for the investigation. Powder mixtures were fully densified by hot compaction under precisely controlled conditions. The mechanical properties of the obtained compacts were examined. The mechanical behaviour of the investigated alloy powder compacts was evaluated by compression test under various thermomechanical conditions using Gleeble simulator. The microstructure of powder compacts as well as P/M alloy samples deformed in compression tests was examined. All data obtained from the experimental tests were applied as boundary conditions for numerical simulation of forging of selected forgings. Basing on the results of both plastometric tests and simulations, thermomechanical parameters of the investigated alloy forging were determined. Designed parameters of forging technology were verified by forging trials performed in industrial conditions. The quality of the obtained forgings was examined by means of computed tomography. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The application of titanium and its alloys is growing in the areas such as aviation, space exploration, shipbuilding, chemical industry and many other branches of economy [1,2]. The main reason is a combination of favourable properties of these alloys, such as low specific gravity (4.5 g/cm3), high specific strength, resistance to crack propagation, good low-temperature toughness, fatigue strength and excellent corrosion resistance. The limitations to the applicability of titanium alloys include low heat conduction, low machinability [3–5] as well as high costs. The most commonly used titanium alloy is Ti6Al4V. Beside the advantages specific to titanium, this alloy also shows heat resistance, relatively good formability, weldability and biocompatibility. Usually, the deformation of this alloy proceeds by forging or extrusion processes [6]. The application of powder metallurgy (P/M) to the manufacturing of products of titanium alloys is a known technology [7]. The use of P/M methods in respect to hard-to-deform alloys may result in a better controllability of the forming process parameters [8–12]. The type of initial powders is, however, a significant issue that should be taken into consideration. The applied types include
⇑ Corresponding author. Tel.: +48 (12) 617 38 72; fax: +48 (12) 617 33 39. E-mail address: [email protected] (M. Wojtaszek).
powders obtained by liquid alloy atomization (pre-alloying method) and those obtained by mixing of elemental powders (blended elemental powder metallurgy – BEPM). High production cost can be a significant limitation in the case of producing P/M Ti6Al4V alloy parts. Considerably lower costs are generated in the case of manufacturing products based on the mixture of elemental powders. However, there is a potential risk of inhomogeneity of chemical composition of the product and that is why a special care should be taken of a uniform distribution of powder mixture ingredients [13] as well as the proper forming route [14]. One of the possible methods of processing such materials is a combination of powder metallurgy and metal forming techniques, what ensures the advantages of both. The application of fully densified powder compacts as charge for further processing allows using the equipment already existing in commercial metal forming companies without the necessity of its modification. Contrary to densification processes, where the dimensions of the press chamber restricts the geometry of the product, multi-stage forging enables modification of the geometry of preform and significant enhancement of the range of dimensions and shapes of the final products. Moreover, forging process enables application of various cooling procedures for the forged part, what significantly influences its microstructure and properties. This work discusses the possibilities of design and application in industrial conditions for the technology of forging Ti6Al4V alloy by means of blended elemental powder metallurgy route [14].
http://dx.doi.org/10.1016/j.jallcom.2014.01.161 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
2
M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
2. Material under investigation and research methods Ti6Al4V powder was obtained by means of mechanical mixing of elemental powders. The powder mixtures were applied as the initial material for the investigations. The powders were blended for 10 h in a conical mixer. WC balls were additionally used to intensify the mixing process. The testing included the assessment of powder morphology and the basic chemical analysis (EDS) performed with application of Hitachi TM3000 scanning electron microscope. The analysis of the chemical composition of random sample of the mixture of elemental powders was performed to determine the correctness of the applied blending conditions for obtaining the chemical composition of commercial Ti6Al4V alloy. Forming of highly consolidated P/M compacts was realized by hot sintering under pressure, using the test stand by Thermal Technology Inc. Ti6Al4V powder mixtures were sintered for 3 h at the temperature of 1200 °C, under the pressure of 25 MPa, and under an argon atmosphere. Sintering resulted in obtaining cylindrical specimens with an approximate height of 50 mm and a diameter of 75 mm. The relative density of compacts (Archimedes method) as well as their selected mechanical properties were analysed. The hardness of specimens was measured using the Vickers method (Tukon 2500 hardness tester), applying the load of 1 kg. The mechanical properties were determined with application of uniaxial compression test (Zwick Z250 testing machine). The material after hot compaction was subjected to axisymmetrical compression tests with application of Gleeble 3800 simulator. The specimens of 10 mm diameter and 12 mm height were used in compression tests. The plastometric tests were realized with thermal and mechanical parameters given in Table 3. The material was heated to the testing temperature, held for a specified period of time, and then subjected to deformation and subsequent cooling. Varying temperatures and strain rates were assumed for the specimens. The microstructure of a compact and the specimens after deformation at different thermomechanical parameters was observed using Leica DM4000M optical microscope. The cross-sections were observed after twostage etching (1st stage: 6% HF, 2nd stage: 2% HF + 2% HNO3 + 96% H2O). Numerical modelling of forging of the selected forging was performed with application of the finite element method, using QForm 2D/3D commercial software. Numerical simulations were designed to ensure that all parameters being modelled are attainable in industrial conditions. A crank press of 1000 ton capacity was assumed in the simulation. The press slide velocity was 1 m s 1, and WCLV steel was assumed as the die insert material. The temperature of dies was 300 °C, and the hot working lubricant (graphite–water slurry) was introduced, with the appropriate coefficient of friction of 0.23. The obtained results of experimental investigations and numerical simulation were verified basing on die forging tests realized in industrial conditions. The forging process was carried out using a crank press at HSW Kuz´nia Stalowa Wola Ltd., Poland. For a representative specimen, during heating and cooling, the temperatures were recorded inside the specimen (hole with a thermocouple) and on the surface (pyrometer). The obtained characteristic allowed to maintain the thermal conditions during the tests close to those evaluated numerically. The quality of forgings was assessed by means of non-destructive testing, with application of computed tomography (CT). Scanning was realized using v tome 450 tomograph with the resolution of 40 lm.
3. Results Morphology of the powder under investigation, being the mixture of elemental powders of titanium, aluminium and vanadium, is presented in Fig. 1a. The mixture consists of particles showing irregular surface, diversified shape and the size between 10 and 200 lm. Results of EDS analysis are shown in Fig. 1b. The chemical composition of the investigated alloy is given in Table 1. The mass fractions of titanium, aluminium and vanadium in a randomly selected powder sample (Fig. 1b) are close to those specific for
Table 1 Chemical composition of Ti6Al4V alloy (ISO 5832/3). O
V
Al
Fe
H
C
N
Ti
<0.20
3.5–4.5
5.5–6.75
<0.30
<0.0015
<0.8
<0.05
Bal.
Table 2 Selected properties of Ti6Al4V alloy in the state after sintering of elemental powders. Density (g/cm3)
Relative density (%)
Hardness (HV)
Rp0.2 (MPa)
Rm (MPa)
Young’s modulus (GPa)
4.40 ± 0.01
99.36 ± 0.16
339 ± 11
1013 ± 31
1029 ± 36
104.6 ± 5.3
Table 3 The parameters used in Gleeble 3800 simulator compression tests. Heating rate (°C/s)
Holding time before compression (s)
Temperature of tests (°C)
Strain rate (s 1)
Cooling rate (°C/s)
2.5
10
800, 900, 950, 1000
0.01, 0.1, 1, 10, 100
Approx. 50
Ti6Al4V alloy (Table 1), thus confirming the proper distribution of individual elemental powders in a mixture. The results of density measurements as well as the selected mechanical properties of the material after hot compaction are put together in Table 2. Basing on the results of plastometric tests, the material flow characteristics were elaborated. The selected examples of the effect of temperature and strain rate on the character of true stress vs. true strain curves are collected in Fig. 2. The microstructure of the material was observed after hot compaction under pressure and after deformation in varying temperature and strain rate conditions. The microstructure of the material after hot compaction is presented in Fig. 3a, while Fig. 3b and c shows the examples of microstructures obtained after compression tests in Gleeble simulator. The flow curves were applied to the description of material behaviour in variable thermomechanical conditions of numerical modelling of hot die forging. A ring with a flange was selected as the forging to be analysed. The simulations were carried out assuming different variants of heating, holding and cooling operations as well as variable forging process parameters, being however realizable in industrial conditions in each case. Fig. 4 presents only selected results of modelling, obtained, as an example, for the temperature of 900 °C and the tool speed of 1 m s 1. Basing on the results of a complex analysis, the material geometry was specified as well as thermal and mechanical forging conditions, favourable for the analysed alloy and selected forging, were evaluated. Their verification was carried out basing on forging tests in industrial conditions. The model showing the selected shape of a forging
Fig. 1. Morphology of Ti6Al4V powder (a) and results of EDS chemical analysis of powder (b).
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
(b)
400
300
5
200
4
3
100
0
0,0
400
nr 1 2 3 4 temperature, °C 800 900 950 1000
nr 1 2 3 4 5 strain rate, s-1 0.01 0.1 1 10 100
2 1 0,1
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true stress, MPa
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true strain ε
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Fig. 2. Example true stress vs. strain curves obtained from compression tests performed using Gleeble simulator at the temperature of 900 °C (a) and at the strain rate of 10 s 1 (b).
Fig. 3. Microstructure of P/M Ti6Al4V alloy in the state after hot sintering (a), after deformation at the temperature of 800 °C and strain rate of 10 s the temperature of 900 °C and strain rate of 10 s 1 (c) and after deformation at the temperature of 900 °C and strain rate of 100 s 1 (d).
together with the photograph of a hot-forged product are presented in Fig. 5. The example CT images taken in selected sections of a forging, are shown in Fig. 6. 4. Discussion Basing on the results of EDS analysis it was confirmed that mass fractions of the basic ingredients of the mixture of elemental powders are close to those required for the investigated alloy. It should be stated that in case of powder mixtures, the results obtained from EDS analysis can be questionable, since the different specific gravity of individual ingredients may cause segregation during taking samples, thus affecting the results. However, no such problem was observed in this work. It was shown that hot sintering of Ti6Al4V powder under pressure, with the assumed process parameters, resulted in obtaining the material showing the relative den-
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(b), after deformation at
sity of a solid material and good mechanical properties (Table 2). The observations of a character of flow curves (Fig. 3) showed high sensitivity of the material to the changes of deformation temperature (Fig. 3a) and strain rate. Metallographic examination of compacts and specimens after deformation allows to establish the relationship between the thermomechanical conditions of deformation process and the resulting state of microstructure. The observations of a microstructure of the alloy after sintering showed the precipitations of a-phase occurring in a form of lamellae of average length of about 50 lm and width of a few lm (Fig. 3a). The deformation of the material caused the refinement of a-phase lamellae distributed within the b-phase matrix (Fig. 3b and c). The observations of microstructure of compacts and specimens after deformation did not show symptoms of cracking or the occurrence of pores, thus qualitatively confirming the results of density measurements.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
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M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Fig. 4. Distributions of mean stress (a), effective strain (b) and temperature (c) obtained from FEM numerical simulation of forging the sinter of Ti6Al4V alloy at the temperature of 900 °C.
Fig. 5. Shape of the forging selected for manufacturing in industrial conditions: the model (a) and the photograph of a real forging (b).
Fig. 6. Schematic representation of the location of selected sections (a, b) and images of these sections of a forging obtained using computed tomography (CT) method sections through YZ plane (c) and through XZ plane (d).
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161
M. Wojtaszek, T. S´leboda / Journal of Alloys and Compounds xxx (2014) xxx–xxx
The distribution of mean stresses obtained as a result of the FEM numerical simulation of forging at the temperature of 900 °C and the tool speed of 1 m s 1 (Fig. 4a), presented as an example, is rather uniform within the whole forging, with the exception of the web. However, in case of the analysed forging of a ring with a flange, the web region is not a problem as it is destined for being cut away as a discard. This testifies for a proper selection of the shape of contour impression and the dimensions of feedstock. Large gradient of the effective strain (Fig. 4b) results from the shape of a forging. In the final forging stage, significant temperature rise was observed in the flash region (Fig. 4c), resulting from conversion of the work of plastic deformation into the heat. The temperature drop was observed in regions contacting with the dies, the temperature of which is at the level of about 300 °C. Except these regions, the temperature distribution within the volume of a forging is quite uniform. Basing on the complex analysis of the results of FEM numerical modelling, considering the character of flow curves of the alloy being analysed and its microstructure as influenced by deformation conditions, thermomechanical parameters were elaborated for the process of hot die forging of a forging with required shape. The temperature of 900 °C was established as the most favourable forging temperature. In order to ensure the dissolution of alloying elements and to homogenize the microstructure, it was assumed to apply preliminary heating of a feedstock to the temperature of 1000 °C. Considering low heat conduction of titanium and large temperature difference between a feedstock and a die, the obtaining of required forging temperature by means of cooling allowed to avoid problems connected with the occurrence of temperature gradients within the material volume, as it was indicated in the results of FEM simulation. The forging process was realized in two operations and the finished forgings were air-cooled. Basing on the preliminary observations of the shape and surface condition of forgings, it was stated that the deformation process was realized properly and the parameters selected basing on numerical modelling were correct. These observations were confirmed basing on the results of non-destructive testing. The analysis of CT images did not show cracks or discontinuities within the material volume, in the range of applied resolution. Moreover, no pores were observed, thus confirming qualitatively the results of density testing. 5. Conclusions Basing on the results of the realized investigations it can be stated, that the proposed technology of hot sintering under pressure, applied to the mixture of elemental powders with mass fractions corresponding with Ti6Al4V alloy, allows to manufacture the product showing the density of a solid material with the same chemical composition. This was determined by the density measurements and then confirmed by metallographic observations and computed tomography. Since the mechanical properties of a material in this
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condition are high, it can be used as a finished product or as a semi-finished product for further processing. The flow curves of the compacts of Ti6Al4V alloy, obtained with application of Gleeble simulator, describe the behaviour of this material subjected to deformation in different thermomechanical conditions. They are fundamental for the preparation of a material database. In such form, they can be used for the description of boundary conditions in FEM numerical modelling. The results of numerical modelling, together with microstructure observations, allowed to elaborate favourable parameters of forging of the analysed alloy. Their correctness was verified in industrial conditions by means of manufacturing forgings with a selected shape. The tests proved to be successful – flawless forgings were produced, the quality of which was confirmed by means of non-destructive testing. The authors claim that the presented results of investigations indicate that the proposed technology can find its application to the manufacturing of structural components from initial material in a form of the mixture of elemental powders. The potential fields of application for the presented results include particularly automotive industry and aviation. Acknowledgements Financial support of Structural Funds in the Operational Programme –nnovative Economy (IE OP) financed from the European Regional Development Fund – Project WND-POIG.01.03.01-12004/09 is gratefully acknowledged. References [1] C. Chunxiang, H. BaoMin, Z. Lichen, L. Shuangjin, Mater. Des. 32 (2011) 1684– 1691. [2] O.M. Ivasyshyn, A.V. Aleksandrov, Mater. Sci. 44 (2008) 311–327. [3] Y. Sheng, Z. Guo, J. Hao, D. Yang, Procedia Eng. 36 (2012) 299–306. [4] P. Heinl, L. Müller, C. Körner, R.F. Singer, F.A. Müller, Acta Biomater. 4 (2008) 1536–1544. [5] M.J. Donachie Jr. (Ed.), Titanium and Titanium Alloys Source Book, Am Soc Metals, OH, 1982, pp. 265–269. [6] S. Bednarek, A. Łukaszek-Sołek, J. Sin´czak, Arch. Civ. Mech. Eng. 8 (2) (2008) 13–20. [7] C. Haasea, R. Lapovoka, N.H. Pang, Y. Estrina, Mater. Sci. Eng. A – Struct. 550 (2012) 263–272. [8] T. S´leboda, Steel Res. Int. 79 (2008) 493–498. [9] T. S´leboda, J. Kane, RN. Wright, NS. Stoloff, D.J. Duquette, Mater. Sci. Eng. A – Struct. 368 (2004) 332–336. [10] T. Fujita, A. Ogawa, Ch. Ouchi, H. Tajima, Mater. Sci. Eng. A – Struct. 213 (1996) 148–153. [11] W. Chen, Y. Yamamoto, W.H. Peter, M.B. Clark, S.D. Nunn, J.O. Kiggans, T.R. Muth, C.A. Blue, J.C. Williams, K. Akhtar, J. Alloys Comp. 541 (2012) 440–447. [12] Y. Liu, LF. Chen, HP Tang, CT. Liu, B. L, BY. Huang, Mater. Sci. Eng. A – Struct. 418 (2006) 25–35. [13] M. Wojtaszek, J. Durak, Metall Foundry Eng. 33 (2007) 23–31. [14] M. Wojtaszek, T. S´leboda, Thermomechanical processing of P/M Ti6Al4V alloy, in: Conf. Proc. Metal 2013, 22nd Int. Conf .Metal Mater., May 15th–17th, Brno, Czech Republic, 364–369.
Please cite this article in press as: M. Wojtaszek, T. S´leboda, J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.161