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Numerical simulation on warm deep drawing of magnesium alloy AZ31 sheets
Materials Science and Engineering A 499 (2009) 40–44
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Numerical simulation on warm deep drawing of magnesium alloy AZ31 sheets L.M. Ren a , S.H. Zhang a,∗ , G. Palumbo b , D. Sorgente b , L. Tricarico b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Department of Mechanical & Management Engineering (DIMeG), Polytechnic of Bari, 70126 Bari, Italy
a r t i c l e
i n f o
Article history: Received 12 March 2007 Received in revised form 30 August 2007 Accepted 22 November 2007 Keywords: Numerical simulation Warm deep drawing Magnesium alloy AZ31 Fracture
a b s t r a c t Warm forming of magnesium alloys has attracted much attention due to the very poor formability of Mg alloys at room temperature. In the present paper, the warm deep drawing of magnesium alloy AZ31 (3 wt.% Al, 1 wt.% Zn) sheets was studied by both the experimental approach and the finite element analysis. The results indicated that the formability of the AZ31 sheets could be improved significantly at elevated temperatures. Sound cups could be formed at 150 ◦ C with the highest punch speed of 6 mm/min, while when the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest punch speed of 120 mm/min. Finite element analyses were performed to investigate the effects of the process parameters on the drawability of rectangular cups and to predict the formation of the process defects. The reasonable agreement between the numerical simulation results and experimental data validated the accuracy of the finite element analysis. © 2008 Published by Elsevier B.V.
1. Introduction Recently, magnesium alloy materials have been widely applied in automotive and electronic industries as the lightest weight structural materials [1]. Sheet metal forming is expected to be effective as environmentally conscious processing technology for magnesium alloys. Deep drawing is an important sheet metal forming process, which can remarkably improve the productivity and the qualification of the products [2,3]. Because of its hexagonal close-packed (HCP) crystal structure, the major difficulty in plastic forming of Mg alloy is its very poor formability at room temperature. However, many research activities proved that it shows excellent ductility and formability at elevated temperatures ranging from 150 to 300 ◦ C [4–9]. In recent years, quite a few efforts have been made to study the warm deep drawing (WDD) processes of magnesium alloys. For instance, Yoshiohara et al. developed a local heating and cooling system to improve the formability of magnesium alloy sheets [3]. Chen et al. investigated the WDD of magnesium alloy square cups by experimental and finite element (FE) modeling methods [4]. Considering most applications of magnesium alloys in the electronics industry bear rectangular shapes, such as the coverings of notebook computers, mobile phones and MD players, the WDD of rectangular cups (40 mm × 25 mm) with magnesium alloy AZ31 sheets was investigated by both the experimental approach and the
∗ Corresponding author. Tel.: +86 24 8397 8266; fax: +86 24 2390 6831. E-mail address: [email protected] (S.H. Zhang). 0921-5093/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.msea.2007.11.132
finite element analysis in the present paper in order to gain more insights into the formability of Mg alloy AZ31. 2. Finite element model Finite element simulations of the WDD process was conducted using the commercial software package MSC.Marc. A finite element model of the tools and blank was developed, as shown in Fig. 1. Only one-quarter of the geometry was modeled due to the symmetric boundary conditions. Data from uniaxial tensile tests performed at different temperatures (from room temperature to 300 ◦ C) and different strain rates (in the range 0.001–0.1 s−1 ) were collected to define the mechanical properties of the Mg alloy. The large strain additive and updated Lagrange procedure was adopted because of the large deformation of the WDD process. The Full Newton–Raphson iterative procedure was chosen to solve the iteration process and non-linear equations of motion. The other simulation parameters are listed in Table 1. 3. Experimental work of drawing the rectangular cups WDD experiments were performed at the Laboratory of the DIMeG at Bari. The designed and manufactured equipment, as shown in Fig. 2, was assembled on the material test machine Instron 4485. As heating system, six 300 W power heaters were positioned inside the equipment. Three thermocouples were used to acquire the temperature in specified positions of the blank, the die and the blank holder (BH). Punch load, stroke, temperature and the thinning
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Fig. 3. Temperature distribution of the drawn cup. Fig. 1. Finite element model of WDD.
Table 1 Material properties and process parameters applied in FE simulation Material of blank Thickness (mm) Young’s modulus (GPa) Poisson ratio Interface Heat Transfer coefficient (N/(s K mm)) Heat conductivity (N/(s K)) Specific heat capacity (mm2 /(s2 K)) Factor to convert plastic deformation energy to heat Friction coefficient, blank-punch Friction coefficient, blank-die Friction coefficient, blank-blank holder
AZ31 0.7 44.8 0.35 4.5 Temperature dependent Temperature dependent 0.95 0.1 0.05 0.05
were obtained by means of Lab View and an optical measure system to validate the numerical simulation results. Different forming temperatures and punch speeds (PS) were selected to evaluate the effect of the most important process parameters affecting the WDD of Mg alloy AZ31 rectangular cups. The tests were performed with temperature ranging from room temperature to 250 ◦ C. During the heating phase, the punch was kept far from the heaters and the blank was clamped between the BH and the die for a short time before drawing. When the temperature of the blank reached the expected value, heating stopped and the test started. The water-based lubricant PTFE was smeared uniformly on the surfaces of the BH and the female die in contact with the blank.
Fig. 4. Temperature evolutions of different locations on the blank.
4. Results and discussion Fig. 3 shows the temperature distribution of the part formed at 200 ◦ C with the PS of 60 mm/min. Fig. 4 displays the relations between punch stroke and temperature evolutions of different locations on the blank. According to the simulation results, the region of the blank in contact with the punch has lower
Fig. 2. Final assembly of the WDD equipment.
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L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
Fig. 5. Thinning along the diagonal of the cup drawn at 180 ◦ C with the PS of 90 mm/min.
temperature compared with the region in contact with the die and the BH (Fig. 3). From Fig. 4, large drop in temperature in the wall (A) and bottom (B and D) of the blank was observed. It is because Mg alloy AZ31 has high thermal conductivity and low specific heat capacity. Thus for the region in contact with the punch at room temperature, heat lost rapidly in a short time. It is highlighted the punch force needed to cause deformation is applied to the bottom of the cup. This force is transmitted through the cup wall to the flange. Thus, fracture occurs in the cup wall just above the punch radius. This position transmits the largest deformation forces during the forming process. With decreasing of the temperature near the punch corner, the strength of the blank here was increased and the thinning was decreased. Meanwhile, the higher temperature of flange leads to the decrease in the strength of the blank. Therefore, it is effective for warm deep drawability of Mg alloy with different temperature distribution in the different forming regions of the blank. Moreover, it also can be seen in Fig. 4 that temperature evolution of the flange (C) of the blank was close to that of the BH. Considering the majority of the deformation occurs in the flange area of the cup, this result confirmed that the heating method adopted in the present study was effective to guarantee high and constant temperature conditions of flange area during WDD tests. The defects mentioned above such as necking and fracture could be predicted from the thinning, as shown in Fig. 5, which shows the thinning along the diagonal of formed part with the forming temperature at 180 ◦ C and the PS of 90 mm/min. From Fig. 5, the maximum thinning was observed on the walls contacting with punch corners and the maximum thickness was located in the blank near die corners. Hence, it could be predicted that in deep drawing of rectangular cups, fracture occurs in the cup wall along the diagonal of drawn cup and in the vicinity of the punch corners. A comparison of the thinning along the sections of a part at the middle stage of forming between simulation and experimental results is shown in Fig. 6. The trend of percentage change in thickness predicted by simulation agrees with that obtained from experiments with the same process parameters. Although there is deviation between the simulation and experimental results, the simulation can predict the thinning and then estimate the thickness of the regions that are difficult to be measured precisely by the CCD sensors of optical measure system, such as the flange which was clamped between the die and BH. The formability of magnesium alloy sheet at elevated temperature is significantly affected by the processing parameters.
Fig. 6. Thinning along the sections of OA and OB. (a) Results from FE simulation and (b) results from experiment.
Among them the forming temperature, the PS, the BH force and the lubrication are probably the most relevant. Based on the previous investigation, a set of WDD equipment was redesigned and manufactured in the present paper. In addition, a more suitable and proper lubricant of water-based PTFE was adopted. It may be noted from the tests and analyses that the highest drawn depth were always obtained with the lowest PS and higher PS values lead to early fracture of the blank, as shown in Fig. 7(a). However, a sound cup could not be formed even with a low PS of 1.5 mm/min when the forming temperature was too low to activate the deformation mechanisms, as shown in Fig. 7(b). In order to gain more insights into the formability of Mg alloy AZ31, different forming temperatures and PS are selected to evaluate their effects on the forming of Mg alloy AZ31 sheets. Fig. 8 shows the punch stroke–load curves obtained from numerical simulations and experiments at different forming temperature and with the same PS of 60 mm/min. The maximum punch load obtained from finite element modeling was higher than the load obtained in experiment for corresponding temperatures. However, the simulations could predict that the lower temperature leads to a higher punch load with the same PS. Fig. 9 shows the punch stroke–load curves plotted according to the experiments. It is noted from Fig. 9 that the maximum punch load increased as the PS is increased. That is because the yield strength increases with higher forming speed. With the punch traveling down, due to the decreasing of the flange area, the force required to pull the blank into the die will decrease. It should also be noted that there is not remarkable difference in the punch load at a lower temperature with different
L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
43
Fig. 9. Punch load–stroke curves from experiments with various process parameters.
Fig. 7. Drawn cups with different parameters. (a) PS = 90 mm/min, 180 ◦ C and (b) PS = 1.5 mm/min, room temperature.
PS (Fig. 9). This result agrees well with the research results obtained by Doege and Droder [7], which implies that the influence of PS is less significant, if the experiment is carried out at a lower temperature. According to a large number of tests and finite element analyses, an elementary process window could be drawn as shown in Fig. 10. As mentioned above, if the test is performed at a higher temperature, the effect of forming speed is much significant. This indicates that PS can exhibit a prominent influence on the formability of Mg AZ31 sheets, when the temperature factor is not dominating. Sound cups could be formed with a higher PS at a higher forming temperature. As shown in Figs. 10 and 11, it is noted that sound cups could be formed at 150 ◦ C with the highest PS of 6 mm/min, while when
Fig. 10. Process window.
Fig. 11. Drawn cup at 250 ◦ C with the PS of 120 mm/min.
the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest PS of 120 mm/min. 5. Concluding remarks
Fig. 8. Punch load–stroke curves from numerical simulations and experiments performed with the PS of 60 mm/min.
In this study, WDD of magnesium alloy AZ31 sheets was studied by both the experimental approach and the finite element analysis. It was confirmed that the most important factor affecting the deep drawability of Mg alloy sheet is temperature. And if the test is performed at a higher temperature, the effect of forming speed
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L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
is much significant. Sound cups could be formed at 150 ◦ C with the highest PS of 6 mm/min, while when the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest PS of 120 mm/min. The investigations confirmed that the deep drawing of magnesium parts with heatable tools for industrial applications is possible. Acknowledgements The authors wish to thank the following Italian Institutions: Ministero Attivita` Produttie, Istituto Commercio Estero and Conferenza RettoriUniversita` Italiane for financing the present research activity. The authors are grateful to Eleventh-five scientific support project of China for the financially support.
References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] Q.F. Chang, D.Y. Li, Y.H. Peng, X.Q. Zeng, Int. J. Mach. Tools Manuf. 47 (2007) 436–443. [3] S. Yoshiohara, K.I. Manabe, H. Nishimura, J. Mater. Process. Technol. 170 (2005) 579–585. [4] F.K. Chen, T.B. Huang, C.K. Chang, Int. J. Mach. Tools Manuf. 43 (2003) 553– 1559. [5] S.H. Zhang, K. Zhang, Y.C. Xu, Z.T. Wang, Y. Xu, Z.G. Wang, J. Mater. Process. Technol. 185 (2007) 147–151. [6] G. Palumbo, D. Sorgente, L. Tricarico, S.H. Zhang, W.T. Zheng, J. Mater. Process. Technol. 191 (2007) 342–346. [7] E. Doege, K. Droder, J. Mater. Process. Technol. 115 (2001) 14–19. [8] H. Palaniswamy, G. Ngaile, T. Altan, J. Mater. Process. Technol. 146 (2004) 52–60. [9] S.H. Zhang, L.M. Ren, L.X. Zhou, Y.C. Xu, G. Palumbo, L. Tricarico, Mater. Sci. Forum 546–549 (2007) 333–336.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Numerical simulation on warm deep drawing of magnesium alloy AZ31 sheets L.M. Ren a , S.H. Zhang a,∗ , G. Palumbo b , D. Sorgente b , L. Tricarico b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Department of Mechanical & Management Engineering (DIMeG), Polytechnic of Bari, 70126 Bari, Italy
a r t i c l e
i n f o
Article history: Received 12 March 2007 Received in revised form 30 August 2007 Accepted 22 November 2007 Keywords: Numerical simulation Warm deep drawing Magnesium alloy AZ31 Fracture
a b s t r a c t Warm forming of magnesium alloys has attracted much attention due to the very poor formability of Mg alloys at room temperature. In the present paper, the warm deep drawing of magnesium alloy AZ31 (3 wt.% Al, 1 wt.% Zn) sheets was studied by both the experimental approach and the finite element analysis. The results indicated that the formability of the AZ31 sheets could be improved significantly at elevated temperatures. Sound cups could be formed at 150 ◦ C with the highest punch speed of 6 mm/min, while when the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest punch speed of 120 mm/min. Finite element analyses were performed to investigate the effects of the process parameters on the drawability of rectangular cups and to predict the formation of the process defects. The reasonable agreement between the numerical simulation results and experimental data validated the accuracy of the finite element analysis. © 2008 Published by Elsevier B.V.
1. Introduction Recently, magnesium alloy materials have been widely applied in automotive and electronic industries as the lightest weight structural materials [1]. Sheet metal forming is expected to be effective as environmentally conscious processing technology for magnesium alloys. Deep drawing is an important sheet metal forming process, which can remarkably improve the productivity and the qualification of the products [2,3]. Because of its hexagonal close-packed (HCP) crystal structure, the major difficulty in plastic forming of Mg alloy is its very poor formability at room temperature. However, many research activities proved that it shows excellent ductility and formability at elevated temperatures ranging from 150 to 300 ◦ C [4–9]. In recent years, quite a few efforts have been made to study the warm deep drawing (WDD) processes of magnesium alloys. For instance, Yoshiohara et al. developed a local heating and cooling system to improve the formability of magnesium alloy sheets [3]. Chen et al. investigated the WDD of magnesium alloy square cups by experimental and finite element (FE) modeling methods [4]. Considering most applications of magnesium alloys in the electronics industry bear rectangular shapes, such as the coverings of notebook computers, mobile phones and MD players, the WDD of rectangular cups (40 mm × 25 mm) with magnesium alloy AZ31 sheets was investigated by both the experimental approach and the
∗ Corresponding author. Tel.: +86 24 8397 8266; fax: +86 24 2390 6831. E-mail address: [email protected] (S.H. Zhang). 0921-5093/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.msea.2007.11.132
finite element analysis in the present paper in order to gain more insights into the formability of Mg alloy AZ31. 2. Finite element model Finite element simulations of the WDD process was conducted using the commercial software package MSC.Marc. A finite element model of the tools and blank was developed, as shown in Fig. 1. Only one-quarter of the geometry was modeled due to the symmetric boundary conditions. Data from uniaxial tensile tests performed at different temperatures (from room temperature to 300 ◦ C) and different strain rates (in the range 0.001–0.1 s−1 ) were collected to define the mechanical properties of the Mg alloy. The large strain additive and updated Lagrange procedure was adopted because of the large deformation of the WDD process. The Full Newton–Raphson iterative procedure was chosen to solve the iteration process and non-linear equations of motion. The other simulation parameters are listed in Table 1. 3. Experimental work of drawing the rectangular cups WDD experiments were performed at the Laboratory of the DIMeG at Bari. The designed and manufactured equipment, as shown in Fig. 2, was assembled on the material test machine Instron 4485. As heating system, six 300 W power heaters were positioned inside the equipment. Three thermocouples were used to acquire the temperature in specified positions of the blank, the die and the blank holder (BH). Punch load, stroke, temperature and the thinning
L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
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Fig. 3. Temperature distribution of the drawn cup. Fig. 1. Finite element model of WDD.
Table 1 Material properties and process parameters applied in FE simulation Material of blank Thickness (mm) Young’s modulus (GPa) Poisson ratio Interface Heat Transfer coefficient (N/(s K mm)) Heat conductivity (N/(s K)) Specific heat capacity (mm2 /(s2 K)) Factor to convert plastic deformation energy to heat Friction coefficient, blank-punch Friction coefficient, blank-die Friction coefficient, blank-blank holder
AZ31 0.7 44.8 0.35 4.5 Temperature dependent Temperature dependent 0.95 0.1 0.05 0.05
were obtained by means of Lab View and an optical measure system to validate the numerical simulation results. Different forming temperatures and punch speeds (PS) were selected to evaluate the effect of the most important process parameters affecting the WDD of Mg alloy AZ31 rectangular cups. The tests were performed with temperature ranging from room temperature to 250 ◦ C. During the heating phase, the punch was kept far from the heaters and the blank was clamped between the BH and the die for a short time before drawing. When the temperature of the blank reached the expected value, heating stopped and the test started. The water-based lubricant PTFE was smeared uniformly on the surfaces of the BH and the female die in contact with the blank.
Fig. 4. Temperature evolutions of different locations on the blank.
4. Results and discussion Fig. 3 shows the temperature distribution of the part formed at 200 ◦ C with the PS of 60 mm/min. Fig. 4 displays the relations between punch stroke and temperature evolutions of different locations on the blank. According to the simulation results, the region of the blank in contact with the punch has lower
Fig. 2. Final assembly of the WDD equipment.
42
L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
Fig. 5. Thinning along the diagonal of the cup drawn at 180 ◦ C with the PS of 90 mm/min.
temperature compared with the region in contact with the die and the BH (Fig. 3). From Fig. 4, large drop in temperature in the wall (A) and bottom (B and D) of the blank was observed. It is because Mg alloy AZ31 has high thermal conductivity and low specific heat capacity. Thus for the region in contact with the punch at room temperature, heat lost rapidly in a short time. It is highlighted the punch force needed to cause deformation is applied to the bottom of the cup. This force is transmitted through the cup wall to the flange. Thus, fracture occurs in the cup wall just above the punch radius. This position transmits the largest deformation forces during the forming process. With decreasing of the temperature near the punch corner, the strength of the blank here was increased and the thinning was decreased. Meanwhile, the higher temperature of flange leads to the decrease in the strength of the blank. Therefore, it is effective for warm deep drawability of Mg alloy with different temperature distribution in the different forming regions of the blank. Moreover, it also can be seen in Fig. 4 that temperature evolution of the flange (C) of the blank was close to that of the BH. Considering the majority of the deformation occurs in the flange area of the cup, this result confirmed that the heating method adopted in the present study was effective to guarantee high and constant temperature conditions of flange area during WDD tests. The defects mentioned above such as necking and fracture could be predicted from the thinning, as shown in Fig. 5, which shows the thinning along the diagonal of formed part with the forming temperature at 180 ◦ C and the PS of 90 mm/min. From Fig. 5, the maximum thinning was observed on the walls contacting with punch corners and the maximum thickness was located in the blank near die corners. Hence, it could be predicted that in deep drawing of rectangular cups, fracture occurs in the cup wall along the diagonal of drawn cup and in the vicinity of the punch corners. A comparison of the thinning along the sections of a part at the middle stage of forming between simulation and experimental results is shown in Fig. 6. The trend of percentage change in thickness predicted by simulation agrees with that obtained from experiments with the same process parameters. Although there is deviation between the simulation and experimental results, the simulation can predict the thinning and then estimate the thickness of the regions that are difficult to be measured precisely by the CCD sensors of optical measure system, such as the flange which was clamped between the die and BH. The formability of magnesium alloy sheet at elevated temperature is significantly affected by the processing parameters.
Fig. 6. Thinning along the sections of OA and OB. (a) Results from FE simulation and (b) results from experiment.
Among them the forming temperature, the PS, the BH force and the lubrication are probably the most relevant. Based on the previous investigation, a set of WDD equipment was redesigned and manufactured in the present paper. In addition, a more suitable and proper lubricant of water-based PTFE was adopted. It may be noted from the tests and analyses that the highest drawn depth were always obtained with the lowest PS and higher PS values lead to early fracture of the blank, as shown in Fig. 7(a). However, a sound cup could not be formed even with a low PS of 1.5 mm/min when the forming temperature was too low to activate the deformation mechanisms, as shown in Fig. 7(b). In order to gain more insights into the formability of Mg alloy AZ31, different forming temperatures and PS are selected to evaluate their effects on the forming of Mg alloy AZ31 sheets. Fig. 8 shows the punch stroke–load curves obtained from numerical simulations and experiments at different forming temperature and with the same PS of 60 mm/min. The maximum punch load obtained from finite element modeling was higher than the load obtained in experiment for corresponding temperatures. However, the simulations could predict that the lower temperature leads to a higher punch load with the same PS. Fig. 9 shows the punch stroke–load curves plotted according to the experiments. It is noted from Fig. 9 that the maximum punch load increased as the PS is increased. That is because the yield strength increases with higher forming speed. With the punch traveling down, due to the decreasing of the flange area, the force required to pull the blank into the die will decrease. It should also be noted that there is not remarkable difference in the punch load at a lower temperature with different
L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
43
Fig. 9. Punch load–stroke curves from experiments with various process parameters.
Fig. 7. Drawn cups with different parameters. (a) PS = 90 mm/min, 180 ◦ C and (b) PS = 1.5 mm/min, room temperature.
PS (Fig. 9). This result agrees well with the research results obtained by Doege and Droder [7], which implies that the influence of PS is less significant, if the experiment is carried out at a lower temperature. According to a large number of tests and finite element analyses, an elementary process window could be drawn as shown in Fig. 10. As mentioned above, if the test is performed at a higher temperature, the effect of forming speed is much significant. This indicates that PS can exhibit a prominent influence on the formability of Mg AZ31 sheets, when the temperature factor is not dominating. Sound cups could be formed with a higher PS at a higher forming temperature. As shown in Figs. 10 and 11, it is noted that sound cups could be formed at 150 ◦ C with the highest PS of 6 mm/min, while when
Fig. 10. Process window.
Fig. 11. Drawn cup at 250 ◦ C with the PS of 120 mm/min.
the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest PS of 120 mm/min. 5. Concluding remarks
Fig. 8. Punch load–stroke curves from numerical simulations and experiments performed with the PS of 60 mm/min.
In this study, WDD of magnesium alloy AZ31 sheets was studied by both the experimental approach and the finite element analysis. It was confirmed that the most important factor affecting the deep drawability of Mg alloy sheet is temperature. And if the test is performed at a higher temperature, the effect of forming speed
44
L.M. Ren et al. / Materials Science and Engineering A 499 (2009) 40–44
is much significant. Sound cups could be formed at 150 ◦ C with the highest PS of 6 mm/min, while when the forming temperature was increased up to 250 ◦ C, sound cups could be drawn with the highest PS of 120 mm/min. The investigations confirmed that the deep drawing of magnesium parts with heatable tools for industrial applications is possible. Acknowledgements The authors wish to thank the following Italian Institutions: Ministero Attivita` Produttie, Istituto Commercio Estero and Conferenza RettoriUniversita` Italiane for financing the present research activity. The authors are grateful to Eleventh-five scientific support project of China for the financially support.
References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] Q.F. Chang, D.Y. Li, Y.H. Peng, X.Q. Zeng, Int. J. Mach. Tools Manuf. 47 (2007) 436–443. [3] S. Yoshiohara, K.I. Manabe, H. Nishimura, J. Mater. Process. Technol. 170 (2005) 579–585. [4] F.K. Chen, T.B. Huang, C.K. Chang, Int. J. Mach. Tools Manuf. 43 (2003) 553– 1559. [5] S.H. Zhang, K. Zhang, Y.C. Xu, Z.T. Wang, Y. Xu, Z.G. Wang, J. Mater. Process. Technol. 185 (2007) 147–151. [6] G. Palumbo, D. Sorgente, L. Tricarico, S.H. Zhang, W.T. Zheng, J. Mater. Process. Technol. 191 (2007) 342–346. [7] E. Doege, K. Droder, J. Mater. Process. Technol. 115 (2001) 14–19. [8] H. Palaniswamy, G. Ngaile, T. Altan, J. Mater. Process. Technol. 146 (2004) 52–60. [9] S.H. Zhang, L.M. Ren, L.X. Zhou, Y.C. Xu, G. Palumbo, L. Tricarico, Mater. Sci. Forum 546–549 (2007) 333–336.