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Die motion control for die-quench forging process of AA6061 aluminum alloys
CIRP Annals - Manufacturing Technology 65 (2016) 297–300
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Die motion control for die-quench forging process of AA6061 aluminum alloys Hiroshi Utsunomiya (2)*, Koki Tada, Ryo Matsumoto Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Quenching Aluminum Hot stamping
In die-quench forging of heat-treatable aluminum alloys, a billet is held at high temperature for solutiontreatment, then compressed and rapidly cooled simultaneously between cold dies to form supersaturated solution. This process is similar to hot stamping of aluminum alloy sheets, however a workpiece billet is relatively thicker and thickness strain is larger. Although many applications of slidemotion control on a servo press were proposed, it has not been well discussed as a method to control microstructure and properties of the workpiece. In die-quench forging of AA6061 billet, effect of die speed is discussed to achieve higher hardness with uniform distribution. ß 2016 CIRP.
1. Introduction Metal forming processes on servo presses have many advantages [1]. Slide-motion control is used to reduce forming load and machine vibration as well as to improve formability and lubricity. The most successful metallurgical application is hot stamping of high-strength steel sheets. Austenite is transformed into martensite by rapid cooling between the dies, i.e., die quenching, so that products with higher strength and less spring back are manufactured at small load [2,3]. As the rapid cooling is achieved by sandwiching a hot sheet between cold dies, a servo press is desirable machine because sandwiching after stamping is readily realized. Another application is a production of super-saturated solid solution (S.S.S.S.) of heat-treatable alloys from solution treatment [4–6]. This process has advantages in productivity by removing reheating process, and in shorter aging duration and in higher strengthening due to strain aging effect. However workpieces for hot stamping or die quenching have been limited to thin sheets or foils because of cooling rate. If die quenching is applicable to thick workpiece such as plates, billets and bars, it could be used widely in industries to manufacture thick and stronger components and parts. Therefore feasibility studies of die-quench forging of bulk metals are important. The authors introduced dies of carbide tool with higher thermal conductivity and reported that die-quench forging of steel billets is feasible [7]. It was found that Al-Mg-Si is preferable alloy for die-quench forging. 16 mm-thick AA6061 billets were reduced heavily (50% in height) in die-quench forging and formation of supersaturated solid solution was confirmed [8].
* Corresponding author. E-mail address: [email protected] (H. Utsunomiya). http://dx.doi.org/10.1016/j.cirp.2016.04.132 0007-8506/ß 2016 CIRP.
By the way, in die-quench forging, hot, warm and cold workings are applied at once in very short duration so that microstructure and properties of forged products may be different from those of conventional products. They should be sensitive to the die motion. In this study, effects of forging speed on hardness before/after artificial aging are investigated fundamentally in die-quench forging process of AA6061 alloy. Cylindrical billets were used for axisymmetric analysis, though studies with complicated shapes are required in future for industrial applications. 2. Experiment 2.1. Workpiece Cylindrical billets were machined from an extruded Al–Mg–Si alloy (A6061-T6) bar. The chemical composition was Al–0.9%Mg– 0.57%Si–0.2%Fe–0.29%Cu–0.09%Cr. The billet was 16 mm in diameter D0, and 8 mm in height h0. 2.2. Die-quench forging A 450 kN servo press (H1F45, Komatsu Ind. Corp.) was used. The dies were made of carbide tool (WC-20mass%Co, thermal conductivity = 70 W m1 K1, surface roughness Ra = 0.02–0.04 mm). They were used under dry conditions (without lubrication). First, the billet was held in a box furnace at 823 K for 1.8 ks for solution treatment. In order to avoid sudden temperature drop by touching with cold dies before forging, the billet was supported by steel wires as shown in Fig. 1. The billet was transferred from the furnace to the press, and cooled in air slightly until surface temperature became 788 K, then forged between the dies. 788 K is the lowest solution-treatment temperature for this alloy [9]. The billet height was compressed from 8 mm to 4 mm (Dh/h0 = 50%) with three slide motions schematically shown in Fig. 2. Although
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Fig. 1. Schematic illustrations of experimental setup.
Fig. 3. Measured temperature change in the water quenching (WQ), the air cooling (AC) and the die-quench forging (DQ). Fig. 2. Schematic illustrations of slide motion.
prescribed speed was constant as seen in Fig. 2, actual speed was not exactly constant. The average speed was 2.5, 7.4 and 17.4 mm/s so that the initial strain rate was 0.21, 0.84 and 2.52 s1, respectively. When the upper die reached the bottom dead center, the die-motion was stopped and the billet was sandwiched for 8 s for cooling. After opening the dies, the forged billet was ejected and held in air. Temperature change of the billet throughout the process was measured by a CA thermocouple welded at the center on the side surface as shown in Fig. 1. For comparison, water-quenching (WQ) and air-cooling (AC) were also performed from the solution treatment without forging. 2.3. Aging treatment The die-quenched billets as well as the water-quenched (WQ) and the air-cooled (AC) billets were aged in a silicone oil bath at 448 K. Aging duration was 0.25, 0.5, 1, 2, 4, 8, 16 and 32 h. It means 0.9, 1.8, 3.6, 7.2, 14.4, 28.6, 57.6 and 115.2 ks (kiloseconds). 2.4. Hardness test Vickers hardness was measured along the symmetrical axis on polished longitudinal section of the billet. Indentation was conducted in center region (within 250 mm distant from the mid-plane) and in surface region (250–500 mm distant from the end surface). The indentation load was 0.49 N (50 gf) and the holding time was 15 s.
17.4 mm/s was confirmed by thermal analysis (TG-DTA) and transmission electron microscopy (TEM) in previous study [8]. It is notable that cooling rate is higher when forging speed is faster. 3.2. Deformation in die-quench forging Change in macrostructure on longitudinal section during diequench forging at 17.4 mm/s is shown in Fig. 4. From the image of the as water-quenched billet without deformation (Fig. 4(a)), it is found that coarse grains were elongated in the axial direction and that grains near the surface are finer in size. This may be due to strain introduced in machining process from an extruded bar to cylindrical billets. Forging was interrupted at Dh/h0 = 20% and a halfway-deformed billet was sampled and observed in Fig. 4(b). The billet shows barreling of side surface. Metal around the midplane mainly flows in the radial direction, while grain fibers keep vertical beneath the end surfaces so triangular less-deformation zones are clearly observed. It implies that cooling rate near the surfaces is higher. After 50% compression (Fig. 4(c)), macrostructure looks more inhomogeneous through the height. Radial grain flow around the mid-plane is more clearly observed. This is due to the less deformation zones as well as temperature gradient during forging. In other words, deformation is concentrated around the
3. Results 3.1. Continuous cooling transformation (CCT) diagram Continuous cooling curves during die-quench forging (DQ), water quenching (WQ) and air cooling (AC) are compared in Fig. 3. Time t in x-axis starts from the moment when the billet was cooled below 788 K and the forging started. A solid C curve in the figure indicates precipitation start of b00 Mg2Si phase in case without deformation, which was predicted by a commercial software JMatPro (Sente Software Ltd., UK). It is predicted that the billet in air cooling (AC) shows precipitation after 30 s. All the billets in die-quench forging (DQ) and that in water quenching (WQ) were quickly cooled to below 323 K within 10 s. Even at the lowest speed (2.5 mm/s), compression was finished before t = 2 s, and sandwiching was finished before t = 10 s. At the highest speed (17.4 mm/s), temperature change shows some fluctuations due to short sampling time of 100 Hz. Formation of supersaturated solid solution in the billets water-quenched and die-quench forged at
Fig. 4. Change in macrostructure on longitudinal section of billet during forging (v ¼ 17:4 mm=s). ¯
H. Utsunomiya et al. / CIRP Annals - Manufacturing Technology 65 (2016) 297–300
center where temperature is relatively higher because surface regions are more cooled by cold dies. 3.3. Hardness around center Hardness around the center of the billets before aging and that after peak aging are compared in Fig. 5. The peak aging duration and peak hardness of the water-quenched billet (WQ) and the three DQ billets forged at 2.5, 7.4 and 17.4 mm/s are 57.6, 3.6, 7.2, 7.2 ks and 108, 126, 121, 115 HV, respectively. All the die-quench forged billets show higher hardness than the water-quenched billet (WQ). Peak aging time is remarkably shortened from 57.6 ks of WQ to 3.6–7.2 ks of die-quench forging (DQ). It is found that not only precipitation is faster but also peak hardness is higher in the billets forged with slower speed. In case of slow forging, a billet is cooled for longer duration during forging. Therefore forging temperature is lower so that more dislocations are introduced, work hardening is larger and strain aging is enhanced. 3.4. Hardness distribution through the height
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sensitive to the forging speed, while hardness around the center increases with decreasing forging speed. Therefore slow forging results in non-uniform hardness distribution. It is supposed that, in slow forging, the billet is deformed at lower temperature so more dislocations are introduced around the center. It is notable that the hardness difference between center and surface is reduced by aging, although both the hardness increases considerably. 3.5. Effects of forging speed on macrostructure of billet Macrostructures of the billets forged at different speeds are compared in Fig. 7. The macrostructure of the highest speed was already presented in Fig. 4(c). Regions beneath end surfaces are cooled rapidly by contacting with cold dies so that deformation is not severe. On the other hand, central region is compressed axially and elongates in the radial direction. Such effect is more pronounced at higher forging speed (Fig. 4(c)), because the billet is compressed at relatively higher temperature where temperature difference between center and surface is larger so that deformation is concentrated around the center.
Hardness around the center and that near the surfaces are compared in Fig. 6. When forging speed is faster, hardness distribution is narrower although the macrostructure is more inhomogeneous as seen in Fig. 3. Hardness near the surface is not
Fig. 7. Macrostructure on longitudinal billet as a function of forging speed.
4. Discussion
Fig. 5. Hardness at the center of billet before and after artificial aging.
Fig. 6. Hardness at the center and surface as a function of forging speed.
The above-mentioned results show that microstructure, hardness and aging behavior of die-quench forged billets are very sensitive to forging speed on a servo press. This is because the speed influences temperature change during forging. However it is not easy to understand the behavior only from a cooling curve shown in Fig. 3, because cooling curve just show chronological temperature change and deformation is not considered. On the other hand, stress-strain curve is very useful to discuss mechanical behavior of materials, however it does not include thermal effects. In die-quench forging, a billet is cooled and forged at once so simultaneous consideration of all the changes in deformation (strain), load (stress) and temperature is required because this process is real thermo-mechanical treatment. Changes in stress, strain and temperature against forging duration are shown in Fig. 8(a)–(c). In Fig. 8(a), true strain is calculated from the initial height and the slide position experimentally measured by a laser displacement meter under assumption of uniform deformation without barreling. It is found that the forging is completed within 2 s. True stress calculated from compression load is shown in Fig. 8(b). Load increases and shows maximum before sandwiching. Maximum stress increases with decreasing forging speed. Stress relief is greater in case of faster forging. Enlarged cooling curves around the forging process are shown in Fig. 8(c). Fig. 9(a)–(c) are combined diagrams of strain, stress and temperature based on Fig. 8. Conventional stress-strain curve is shown in Fig. 9(a). True stress against temperature is shown in Fig. 9(b). These figures imply the billet is deformed at lower temperature in case of slower forging. Temperature against true strain is shown in Fig. 9(c). Forging duration t is added on the
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Fig. 8. Changes in (a) strain, (b) stress and (c) temperature with time.
Fig. 9. Relationships among stress, strain and temperature obtained by combining data shown in Fig. 8.
curves in Fig. 9(b) and (c). From Fig. 9(c), it is found that cooling rate is higher when forging speed is faster. However, if temperatures at same strain are compared, temperature is higher when forging speed is faster. Therefore the results of Fig. 9(a) and (b) seem to be controversial to the results of Fig. 3 and Fig. 8(c). This is because cooling rate per strain dT/de does not always agree with nominal cooling rate dT/dt. Real deformation temperature is important. According to Fig. 9(c), if forging speed is fast, deformation is applied mostly in hot or warm working. In this case, even deformation is inhomogeneous as seen in Fig. 4, work hardening is negligible so that hardness distribution after forging is small. On the other hand, if forging speed is slow, applied deformation is mostly warm or cold working so work hardening plays an important role. Under these conditions, the temperature gradient results in hardness distribution in products. It is concluded that die-quench forging at slow speed makes deformation temperature lower and introduces more dislocations and hardens central region, then increases hardness after artificial aging due to strain aging. However hardness distribution through the height is wider. 5. Conclusion Die-quench forging of Al–Mg–Si alloy AA6061 cylindrical billets (16 mm in diameter and 8 mm in height) has been performed on a servo press with carbide tool WC-Co dies. Effects of forging speed on temperature change, deformation and aging behavior have been investigated. When forging speed is slower, nominal cooling rate is lower. The billet is deformed at lower temperature so that hardness around the center is higher and that hardness after aging is also
higher. However hardness distribution through the height is wider because the billet was deformed under large temperature gradient. If slide motion is controlled for metallurgical applications, it is recommended to consider changes in stress, strain and temperature at once. Beside cooling curve, temperature-strain curve is useful to distinguish hot/warm/cold working and to predict microstructure and properties of forged billets.
References [1] Osakada K, Mori K, Altan T, Groche P (2011) Mechanical Servo Press Technology for Metal Forming. CIRP Annals 60:651–672. [2] Karbasian H, Tekkaya AE (2010) A Review on Hot Stamping. Journal of Materials Processing Technology 210:2103–2118. [3] Lechler J, Merklein M (2008) Hot Stamping of Ultra High Strength Steels as a Key Technology for Lightweight Construction. Materials Science and Technology 3:1698–1709. [4] Garrett RP, Lin J, Dean TA (2005) Solution Heat Treatment and Cold Die Quenching in Forming AA6xxx Sheet Components: Feasibility Study. Advanced Materials Research 6–8:673–680. [5] Maeno T, Fujii H, Mori K, Sato M (2010) Control of Slide Motion in Hot Impression Die Forging of Aluminum Alloy Billets Using Servo Press. Steel Research International 81:354–357. [6] Jeon JY, Matsumoto R, Utsunomiya H (2014) Feasibility Study on Die Quenching of AA2024 Aluminum Alloy Billet Using Servo Press. Advanced Materials Research 922:286–291. [7] Matsumoto R, Osumi Y, Utsunomiya H (2012) Selective Die Quenching of Hot Forged Steel Product Using High and Low Thermal Conductivity Tools on a Servo Press. Steel Research International 207–210. (Special Edition). [8] Tada K, Matsumoto R, Utsunomiya H, Watanabe K, Matsuda K (2015) Microstructure Evolution of Al–Mg–Si Aluminum Alloy in Die-Quench Forging Process. Proceedings of 128th Conference of Japan Institute of Light Metals (2015), 351–352. [9] Microstructure and Properties of Aluminum Alloys, (1991), Japan Institute of Light Metals: 477. (in Japanese).
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Die motion control for die-quench forging process of AA6061 aluminum alloys Hiroshi Utsunomiya (2)*, Koki Tada, Ryo Matsumoto Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Quenching Aluminum Hot stamping
In die-quench forging of heat-treatable aluminum alloys, a billet is held at high temperature for solutiontreatment, then compressed and rapidly cooled simultaneously between cold dies to form supersaturated solution. This process is similar to hot stamping of aluminum alloy sheets, however a workpiece billet is relatively thicker and thickness strain is larger. Although many applications of slidemotion control on a servo press were proposed, it has not been well discussed as a method to control microstructure and properties of the workpiece. In die-quench forging of AA6061 billet, effect of die speed is discussed to achieve higher hardness with uniform distribution. ß 2016 CIRP.
1. Introduction Metal forming processes on servo presses have many advantages [1]. Slide-motion control is used to reduce forming load and machine vibration as well as to improve formability and lubricity. The most successful metallurgical application is hot stamping of high-strength steel sheets. Austenite is transformed into martensite by rapid cooling between the dies, i.e., die quenching, so that products with higher strength and less spring back are manufactured at small load [2,3]. As the rapid cooling is achieved by sandwiching a hot sheet between cold dies, a servo press is desirable machine because sandwiching after stamping is readily realized. Another application is a production of super-saturated solid solution (S.S.S.S.) of heat-treatable alloys from solution treatment [4–6]. This process has advantages in productivity by removing reheating process, and in shorter aging duration and in higher strengthening due to strain aging effect. However workpieces for hot stamping or die quenching have been limited to thin sheets or foils because of cooling rate. If die quenching is applicable to thick workpiece such as plates, billets and bars, it could be used widely in industries to manufacture thick and stronger components and parts. Therefore feasibility studies of die-quench forging of bulk metals are important. The authors introduced dies of carbide tool with higher thermal conductivity and reported that die-quench forging of steel billets is feasible [7]. It was found that Al-Mg-Si is preferable alloy for die-quench forging. 16 mm-thick AA6061 billets were reduced heavily (50% in height) in die-quench forging and formation of supersaturated solid solution was confirmed [8].
* Corresponding author. E-mail address: [email protected] (H. Utsunomiya). http://dx.doi.org/10.1016/j.cirp.2016.04.132 0007-8506/ß 2016 CIRP.
By the way, in die-quench forging, hot, warm and cold workings are applied at once in very short duration so that microstructure and properties of forged products may be different from those of conventional products. They should be sensitive to the die motion. In this study, effects of forging speed on hardness before/after artificial aging are investigated fundamentally in die-quench forging process of AA6061 alloy. Cylindrical billets were used for axisymmetric analysis, though studies with complicated shapes are required in future for industrial applications. 2. Experiment 2.1. Workpiece Cylindrical billets were machined from an extruded Al–Mg–Si alloy (A6061-T6) bar. The chemical composition was Al–0.9%Mg– 0.57%Si–0.2%Fe–0.29%Cu–0.09%Cr. The billet was 16 mm in diameter D0, and 8 mm in height h0. 2.2. Die-quench forging A 450 kN servo press (H1F45, Komatsu Ind. Corp.) was used. The dies were made of carbide tool (WC-20mass%Co, thermal conductivity = 70 W m1 K1, surface roughness Ra = 0.02–0.04 mm). They were used under dry conditions (without lubrication). First, the billet was held in a box furnace at 823 K for 1.8 ks for solution treatment. In order to avoid sudden temperature drop by touching with cold dies before forging, the billet was supported by steel wires as shown in Fig. 1. The billet was transferred from the furnace to the press, and cooled in air slightly until surface temperature became 788 K, then forged between the dies. 788 K is the lowest solution-treatment temperature for this alloy [9]. The billet height was compressed from 8 mm to 4 mm (Dh/h0 = 50%) with three slide motions schematically shown in Fig. 2. Although
H. Utsunomiya et al. / CIRP Annals - Manufacturing Technology 65 (2016) 297–300
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Fig. 1. Schematic illustrations of experimental setup.
Fig. 3. Measured temperature change in the water quenching (WQ), the air cooling (AC) and the die-quench forging (DQ). Fig. 2. Schematic illustrations of slide motion.
prescribed speed was constant as seen in Fig. 2, actual speed was not exactly constant. The average speed was 2.5, 7.4 and 17.4 mm/s so that the initial strain rate was 0.21, 0.84 and 2.52 s1, respectively. When the upper die reached the bottom dead center, the die-motion was stopped and the billet was sandwiched for 8 s for cooling. After opening the dies, the forged billet was ejected and held in air. Temperature change of the billet throughout the process was measured by a CA thermocouple welded at the center on the side surface as shown in Fig. 1. For comparison, water-quenching (WQ) and air-cooling (AC) were also performed from the solution treatment without forging. 2.3. Aging treatment The die-quenched billets as well as the water-quenched (WQ) and the air-cooled (AC) billets were aged in a silicone oil bath at 448 K. Aging duration was 0.25, 0.5, 1, 2, 4, 8, 16 and 32 h. It means 0.9, 1.8, 3.6, 7.2, 14.4, 28.6, 57.6 and 115.2 ks (kiloseconds). 2.4. Hardness test Vickers hardness was measured along the symmetrical axis on polished longitudinal section of the billet. Indentation was conducted in center region (within 250 mm distant from the mid-plane) and in surface region (250–500 mm distant from the end surface). The indentation load was 0.49 N (50 gf) and the holding time was 15 s.
17.4 mm/s was confirmed by thermal analysis (TG-DTA) and transmission electron microscopy (TEM) in previous study [8]. It is notable that cooling rate is higher when forging speed is faster. 3.2. Deformation in die-quench forging Change in macrostructure on longitudinal section during diequench forging at 17.4 mm/s is shown in Fig. 4. From the image of the as water-quenched billet without deformation (Fig. 4(a)), it is found that coarse grains were elongated in the axial direction and that grains near the surface are finer in size. This may be due to strain introduced in machining process from an extruded bar to cylindrical billets. Forging was interrupted at Dh/h0 = 20% and a halfway-deformed billet was sampled and observed in Fig. 4(b). The billet shows barreling of side surface. Metal around the midplane mainly flows in the radial direction, while grain fibers keep vertical beneath the end surfaces so triangular less-deformation zones are clearly observed. It implies that cooling rate near the surfaces is higher. After 50% compression (Fig. 4(c)), macrostructure looks more inhomogeneous through the height. Radial grain flow around the mid-plane is more clearly observed. This is due to the less deformation zones as well as temperature gradient during forging. In other words, deformation is concentrated around the
3. Results 3.1. Continuous cooling transformation (CCT) diagram Continuous cooling curves during die-quench forging (DQ), water quenching (WQ) and air cooling (AC) are compared in Fig. 3. Time t in x-axis starts from the moment when the billet was cooled below 788 K and the forging started. A solid C curve in the figure indicates precipitation start of b00 Mg2Si phase in case without deformation, which was predicted by a commercial software JMatPro (Sente Software Ltd., UK). It is predicted that the billet in air cooling (AC) shows precipitation after 30 s. All the billets in die-quench forging (DQ) and that in water quenching (WQ) were quickly cooled to below 323 K within 10 s. Even at the lowest speed (2.5 mm/s), compression was finished before t = 2 s, and sandwiching was finished before t = 10 s. At the highest speed (17.4 mm/s), temperature change shows some fluctuations due to short sampling time of 100 Hz. Formation of supersaturated solid solution in the billets water-quenched and die-quench forged at
Fig. 4. Change in macrostructure on longitudinal section of billet during forging (v ¼ 17:4 mm=s). ¯
H. Utsunomiya et al. / CIRP Annals - Manufacturing Technology 65 (2016) 297–300
center where temperature is relatively higher because surface regions are more cooled by cold dies. 3.3. Hardness around center Hardness around the center of the billets before aging and that after peak aging are compared in Fig. 5. The peak aging duration and peak hardness of the water-quenched billet (WQ) and the three DQ billets forged at 2.5, 7.4 and 17.4 mm/s are 57.6, 3.6, 7.2, 7.2 ks and 108, 126, 121, 115 HV, respectively. All the die-quench forged billets show higher hardness than the water-quenched billet (WQ). Peak aging time is remarkably shortened from 57.6 ks of WQ to 3.6–7.2 ks of die-quench forging (DQ). It is found that not only precipitation is faster but also peak hardness is higher in the billets forged with slower speed. In case of slow forging, a billet is cooled for longer duration during forging. Therefore forging temperature is lower so that more dislocations are introduced, work hardening is larger and strain aging is enhanced. 3.4. Hardness distribution through the height
299
sensitive to the forging speed, while hardness around the center increases with decreasing forging speed. Therefore slow forging results in non-uniform hardness distribution. It is supposed that, in slow forging, the billet is deformed at lower temperature so more dislocations are introduced around the center. It is notable that the hardness difference between center and surface is reduced by aging, although both the hardness increases considerably. 3.5. Effects of forging speed on macrostructure of billet Macrostructures of the billets forged at different speeds are compared in Fig. 7. The macrostructure of the highest speed was already presented in Fig. 4(c). Regions beneath end surfaces are cooled rapidly by contacting with cold dies so that deformation is not severe. On the other hand, central region is compressed axially and elongates in the radial direction. Such effect is more pronounced at higher forging speed (Fig. 4(c)), because the billet is compressed at relatively higher temperature where temperature difference between center and surface is larger so that deformation is concentrated around the center.
Hardness around the center and that near the surfaces are compared in Fig. 6. When forging speed is faster, hardness distribution is narrower although the macrostructure is more inhomogeneous as seen in Fig. 3. Hardness near the surface is not
Fig. 7. Macrostructure on longitudinal billet as a function of forging speed.
4. Discussion
Fig. 5. Hardness at the center of billet before and after artificial aging.
Fig. 6. Hardness at the center and surface as a function of forging speed.
The above-mentioned results show that microstructure, hardness and aging behavior of die-quench forged billets are very sensitive to forging speed on a servo press. This is because the speed influences temperature change during forging. However it is not easy to understand the behavior only from a cooling curve shown in Fig. 3, because cooling curve just show chronological temperature change and deformation is not considered. On the other hand, stress-strain curve is very useful to discuss mechanical behavior of materials, however it does not include thermal effects. In die-quench forging, a billet is cooled and forged at once so simultaneous consideration of all the changes in deformation (strain), load (stress) and temperature is required because this process is real thermo-mechanical treatment. Changes in stress, strain and temperature against forging duration are shown in Fig. 8(a)–(c). In Fig. 8(a), true strain is calculated from the initial height and the slide position experimentally measured by a laser displacement meter under assumption of uniform deformation without barreling. It is found that the forging is completed within 2 s. True stress calculated from compression load is shown in Fig. 8(b). Load increases and shows maximum before sandwiching. Maximum stress increases with decreasing forging speed. Stress relief is greater in case of faster forging. Enlarged cooling curves around the forging process are shown in Fig. 8(c). Fig. 9(a)–(c) are combined diagrams of strain, stress and temperature based on Fig. 8. Conventional stress-strain curve is shown in Fig. 9(a). True stress against temperature is shown in Fig. 9(b). These figures imply the billet is deformed at lower temperature in case of slower forging. Temperature against true strain is shown in Fig. 9(c). Forging duration t is added on the
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Fig. 8. Changes in (a) strain, (b) stress and (c) temperature with time.
Fig. 9. Relationships among stress, strain and temperature obtained by combining data shown in Fig. 8.
curves in Fig. 9(b) and (c). From Fig. 9(c), it is found that cooling rate is higher when forging speed is faster. However, if temperatures at same strain are compared, temperature is higher when forging speed is faster. Therefore the results of Fig. 9(a) and (b) seem to be controversial to the results of Fig. 3 and Fig. 8(c). This is because cooling rate per strain dT/de does not always agree with nominal cooling rate dT/dt. Real deformation temperature is important. According to Fig. 9(c), if forging speed is fast, deformation is applied mostly in hot or warm working. In this case, even deformation is inhomogeneous as seen in Fig. 4, work hardening is negligible so that hardness distribution after forging is small. On the other hand, if forging speed is slow, applied deformation is mostly warm or cold working so work hardening plays an important role. Under these conditions, the temperature gradient results in hardness distribution in products. It is concluded that die-quench forging at slow speed makes deformation temperature lower and introduces more dislocations and hardens central region, then increases hardness after artificial aging due to strain aging. However hardness distribution through the height is wider. 5. Conclusion Die-quench forging of Al–Mg–Si alloy AA6061 cylindrical billets (16 mm in diameter and 8 mm in height) has been performed on a servo press with carbide tool WC-Co dies. Effects of forging speed on temperature change, deformation and aging behavior have been investigated. When forging speed is slower, nominal cooling rate is lower. The billet is deformed at lower temperature so that hardness around the center is higher and that hardness after aging is also
higher. However hardness distribution through the height is wider because the billet was deformed under large temperature gradient. If slide motion is controlled for metallurgical applications, it is recommended to consider changes in stress, strain and temperature at once. Beside cooling curve, temperature-strain curve is useful to distinguish hot/warm/cold working and to predict microstructure and properties of forged billets.
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