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Cold forging of high strength aluminum alloys and the development of new thermomechanical processing
Journal of Materials Processing Technology 80 – 81 (1998) 156 – 160
Cold forging of high strength aluminum alloys and the development of new thermomechanical processing Ola Jensrud a,*, Ketill Pedersen b a
Raufoss Technology AS, Department of Materials Technology, Box 77, N-2831 Raufoss, Norway b SINTEF Materials Technology, Norway
Abstract Cold forging is a process suitable for manufacturing low-cost and high quality automotive components in high strength aluminium alloys. This method is particularly suitable for parts with narrow geometrical tolerances, good concentricity, smooth surface finish and for near net shape products. However, an increasing request for producing components at a lower cost requires even more economical production processes. Forming in the warm condition is an alternative process that has the advantages of producing rather complicated geometrical shapes in less operation steps compared to cold forming. In addition, warm forming at moderate temperatures has all the benefits of cold forming including good control of the microstructure and thereby improved strength and ductility. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Warm forming; AlMgSi alloy; Compression testing; Backward forging
1. Background
1.1. Cold forging Cold forging is a production process often used when forming tubular components in high strength aluminium alloys where high concentricity and close tolerances are required [1]. Very often, the forming is carried out by a combination of forward and backward cold forging. If the geometry is complex, several severe forming operations follow one another that also involves intermediate soft annealing and lubrication. This often requires an interruption of the production line that is both time-consuming and expensive (Fig. 1(a)). A new alternative production process is therefore developed [2,3]. The forging is accomplished at moderate temperatures where it is possible to form rather complicated geometrical shapes in less forming operations without leaving the production line to soft-anneal and re-lubricate the workpiece (Fig. 1(b)). Several papers have been published on cold forging, warm forging and a combination of the two methods in steel [4–6] in the past 10 years. However, less work has been accomplished on warm forging of age pardonable * Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00219-2
aluminium alloys. Warm forging of steel typically occurs in the temperature range from 750 to 850°C depending on the material and the main purpose is to lower the yield stress and increase the formability. Age pardonable aluminium alloys can be heat-treated to reduce the yield stress. However, soft annealing is both time-consuming and expensive. The motivation for forming of AlMgSi alloys in warm condition is therefore somewhat different from steel and the main purpose is to reduce the number of heat-treatment steps. Attention should however be given to the fact that the hardening mechanisms in age-pardonable aluminium alloys are complicated and will be a combination of deformation hardening and precipitation hardening.
1.2. Research objecti6es The main objectives in this investigation have therefore been to compare the process parameters in an alternative and more cost-effective process (warm forging) with the parameters used in the present process (cold forging). An important issue is that the flow stress during forming is equal or less than in the present process and that the mechanical properties in the final products are approximately the same as in the existing process.
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
157
Fig. 1. A sketch of the different forging processes. (A) Cold forging of material in O temper and (B) warm forging of material in W temper.
2. Material behaviour The material used in this investigation is an AA6082 alloy that has been extruded from a billet with a diameter of 29 to a diameter of 178 mm. The casting and extrusion are performed by normal industrial procedures. The alloy has been homogenised to give a high density of dispersoids to prevent recrystallization during extrusion. An extrusion bar with a non-recrystallized fibrous grain structure and a fine subgrain structure is thereby achieved. The nominal composition of the material is given in Table 1. The typical mechanical properties are also tabulated in Table 2. However, by obtaining proper heat treatment and quenching before ageing, it is possible to increase the values by 10%. Material from 6xxx series ages naturally during storing at room temperature. In industrial practice, it will always take from several hours to weeks from when the extrusion is accomplished to the forming being out. During this period, the material ages naturally and the hardness of the material increases causing a reduction in formability. Therefore, before forging in cold condition, the material has to be annealed below solvus temperature to develop a soft material and a stable particle structure that will not give any increase in the strength of the material during room temperature storing. The annealing (O temper) is time-consuming and expensive, however, it has the advantage of making the material soft, stable and easy to form. Forming high strength aluminium alloys in as extruded and water quenched condition (W temper) reTable 1 The chemical composition in weight percentages
quires either a very short storing period at room temperature or a new solution heat treatment immediately before forming. In both cases, the material is unstable and unpredictable concerning the forging load and the final mechanical properties. Forming at an elevated temperature is an alternative method that combines a reduction in load with the ability to age harden the final component to the specified properties. Even though the material will remain unstable, experiments indicate that the process gives repeatable results. The period between water quenching and forming is also found to be of less importance. However, the time and temperature where the forming is carried out is critical. If the material is kept too long at a high forming temperature, a reduction in the ageing potential after forming will be obtained and it will not be possible to reach the required mechanical properties without subsequent solution heat treatment. On the contrary, a low forging temperature results in loads beyond the capacity of the forming equipment. Therefore, it is necessary to establish forming parameters that fulfil both the load requirements and a reasonable hardening potential for subsequent ageing.
3. Compression testing Compression tests have been carried out to establish sufficient process parameters for forging at elevated temperatures. Samples with a diameter of 10 mm and a height of 15 mm have been deformed at temperatures ranging from 200 to 300°C in an servohydraulic 880 Table 2 Typical mechanical properties for T6 condition
Alloy
Si
Fe
Mn
Mg
Al
Alloy
s0.2
su
ef
VHN10
AA6082
1.00
0.20
0.60
0.65
Bal
AA6082
300 MPa
340 MPa
12%
115
158
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
Fig. 2. True stress – strain curves for compression of AA6082 at elevated temperatures. The broad line is O temper.
MTS. testing machine. An environment chamber was used for heating the equipment to the actual forming temperature. A testing procedure is designed that uses parameters near to industrial practise. The procedure is implemented into the computer system of the testing machine. Each test could then be repeated easily and the results from identical samples show good correlation. The samples were heated to elevated temperatures, quenched in water immediately after deformation to prevent any further precipitation and stored at room temperature for a short period before ageing. The entire thermomechanical sequence from heating to watercooling took 1 min. Fig. 2 shows the variations in the compression yield curves for increasing temperatures. The yield stress at a compression strain of 1 is reduced from 200 to 100 MPa, when the temperature increases from 225 to 300°C. The broad line represents the stress – strain curve for soft annealed material deformed at room temperature (O-temper). The true stress – strain curves for material deformed at moderate temperatures seem to level out to a greater extent than the cold-formed material. This reduction in work hardening is more easily shown
Fig. 4. Hardness curves after deformation at 225 and 300°C, respectively, to different strains 0.2, 0.4, 0.6, 0.1 and subsequent ageing at 175°C.
in the work hardening curves in Fig. 3. The observed reduction in work hardening with temperature could be due to recovery taking place at elevated temperatures. Any recovery occurring during forming will increase the formability and reduce the number of forming steps. However, to gain full advantage of the recovery, one may have to form at temperatures where precipitation also takes place. If the temperature is too high, the particles being precipitated will be the non-coherent equilibrium phase which is too large to give any strengthening effect in the final product. The alloying elements used for age hardening are then being depleted. Therefore, an optimum temperature has to be chosen to satisfy both requirements. Fig. 4 indicates that the forming temperature should be close to 225 rather than 300°C. For material deformed at 300°C, the obtainable hardness after ageing is only half of that observed for material deformed at 225°C. Deformed material obtains strength from precipitation of coherent particles and during forming from accumulation of dislocations. However, at small compression strains, the precipitation hardening dominates, while at large strains the deformation hardening is the main source for strengthening. At moderate strains, the strength will be determined by a combination of both mechanisms.
4. Forming of tubes
4.1. Method
Fig. 3. Work hardening (ds/do) versus strain curves for compression of AA6082 at elevated temperatures. The broad line is O temper.
In order to verify the practical use of the forming procedures established by compression testing, backward forging of tubes with an outer diameter of 29 mm and wall thickness of 1 mm have been chosen. This is a generic component with a large surface expansion, i.e. new surface area divided by original surface area. A schematic sketch of the tool set-up is shown in Fig. 5.
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
The container is equipped with three heating elements mounted in a circle to ensure a constant temperature of the tooling. The workpiece was heated in an induction furnace and the punch was heated to the correct temperature by moving the punch to the bottom of the container to obtain contact between the piston and the heated container. The time and temperature cycles were tuned to correspond to what is achievable in industrial practise. The equipment is installed in an 800 ton hydraulic press. To obtain precise measurements, the press is equipped with a high accuracy external load cell and an external sensor for measuring displacement. In addition, a labtech-notebook data acquisition system was used for collecting and calculating forging load, ram position and ram speed. The workpieces were lubricated using calciumaluminat/sodiumstearate. The lubrication of the workpieces was carried out in a commercial production line.
4.2. Experimental results Fig. 6 shows the forging load versus the ram position for cold forging and warm forging of tubes using the same alloys as described above. Material which is cold forged has been soft annealed before forming (O-temper), while the material forged at elevated temperatures is formed in condition as extruded and water quenched at the press (W-temper). The curves represent load versus ram position when forming cups with a height of 200 and 250 mm, respectively. The heights correspond to an expansion in surface area of 29 and 32 times the original surface area. It was possible to achieve expansion above what is reported here. However, when the height of the tubes exceed 200 mm, an opening which was too short between the upper and lower die in the laboratory press made it impossible to remove the
Fig. 5. Schematic draws of the tool set-up for backward forging.
159
Fig. 6. Force vs. ram position for backward extrusion of tubes with an outer diameter of 29 mm and wall thickness of 1 mm.
tubes from the punch without dismantling the equipment. The two curves shown in Fig. 6 show the same tendency as for the compression test. By choosing proper forming parameters, even a reduction in forging load is obtained. Material formed at room temperature also experiences a peak load at the beginning of the test. This peak seems to be lacking when forming at elevated temperatures. The ram rates for all of the tests were set to 20 mm s − 1. This is a high deformation rate and will result in an increase in temperature of the workpiece during forming.
5. Discussion The cold forging of the AA6082 alloy requires soft annealing of the material to obtain sufficient formability at loads that are achievable in regular forging equipment. Soft annealing is both time-consuming and expensive. In addition, stable particles which are too coarse to give any increase in the mechanical properties of the final product are formed during soft annealing. Therefore, the material has to be solution heat treated after cold forming to recrystallize the material and dissolve the coarser particles. Subsequent artificially ageing in the temperature range from 150 to 200°C will form hardening precipitates and give the material the specified mechanical properties. However, by forging at elevated temperatures, extruded billets having the specified dimension can be forged without any further heat treatments. This means that the forging temperature is high enough to bring most of the elements that have precipitated at room temperature and caused an increase in strength, into solid solution. Storing material of age pardonable alloys at room temperature will always result in a certain increase in yield strength. As can be seen from the
160
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
Fig. 7. The Vickers hardness versus strain after deformation and after deformation and subsequent ageing at 225°C for 30 min.
compression test and the following hardness curves, the temperature used for forging in this experiment is sufficiently high to fulfil the hardness requirement and even reduce the forging yield stress compared to cold forging. Also, the work hardening is reduced when forging at elevated temperature. However, some work hardening may be beneficial to avoid local strain instability. The reduction in load is also verified by the load versus ram position curves for tube forming, which in this experiment even shows a 20% reduction in forging load. Even though cold forging means that the workpiece is inserted into the container in a cold condition, the temperature can locally increase due to the conversion of deformation work into heat. An increase of several hundred degrees can sometimes be obtained depending on the punch rate and the geometry of the final product. What is commonly called cold forging is actually forming with a certain increase in temperature during the forging process. However, due to the subsequent solution heat treatment, the increase in temperature during forging will have little effect on the mechanical properties of the product. When forging at elevated temperatures, the material will be exposed to higher temperature. The compression tests indicate that the forging cycle chosen will not .
effect the final hardness of the material, even though one must expect a certain increase in forging temperature during deformation in the latter case also. Forging at elevated temperatures creates a deformation structure, which influences the subsequent precipitation. At medium strains, the response to ageing is still present, even though maximum hardness is reached after a comparatively short annealing time (Fig. 7). The strength obtained is a combination of work hardening and precipitation hardening. At higher strains, beyond 0.5, only a small effect of ageing and saturation hardness seems to be reached. The maximum hardness is the same at low strains as at high strains and the short ageing time used in this investigation does not overage the material at any stages. However, the effect of forming followed by precipitation ageing on fracture toughness and ductility has to be investigated.
6. Conclusions The principal basis for a cost-effective warm forging production line is established. The experiments show that by using a forging temperature of 250°C, a reduction in forging load is obtained without any reduction in hardness of the final product.
References [1] S. Skog, K. Asbøll, Cold forging aluminium components, Aluminum 67 (1992) 442 – 446. [2] O. Jensrud, Warm deformation and the age hardening response in two aluminium alloys. Proc. NATO ASI Advanced Light Alloys and Composites, Zakopane, Poland, 1997. [3] T. Welo, S.R. Skjervold, O. Jensrud, K. Pedersen, Cold forging and grain size control in an Al-1.2wt%Si alloy. Metal. Forming, Krakow, 1992. [4] M. Hirsschvogel, H.v. Dommelen, Some application of cold and warm forging, J. Mater. Process. Technol. 35 (3 – 4) 1992. [5] E. Korner, R. Knodler, Possibilities of warm extrusion in combination with cold extrusion, J. Mater. Process. Technol. 35 (3–4) 1992. [6] S. Sheljaskov, Current level of development of warm forging technology, J. Mater. Process. Technol., (46) (1/2) 1994.
Cold forging of high strength aluminum alloys and the development of new thermomechanical processing Ola Jensrud a,*, Ketill Pedersen b a
Raufoss Technology AS, Department of Materials Technology, Box 77, N-2831 Raufoss, Norway b SINTEF Materials Technology, Norway
Abstract Cold forging is a process suitable for manufacturing low-cost and high quality automotive components in high strength aluminium alloys. This method is particularly suitable for parts with narrow geometrical tolerances, good concentricity, smooth surface finish and for near net shape products. However, an increasing request for producing components at a lower cost requires even more economical production processes. Forming in the warm condition is an alternative process that has the advantages of producing rather complicated geometrical shapes in less operation steps compared to cold forming. In addition, warm forming at moderate temperatures has all the benefits of cold forming including good control of the microstructure and thereby improved strength and ductility. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Warm forming; AlMgSi alloy; Compression testing; Backward forging
1. Background
1.1. Cold forging Cold forging is a production process often used when forming tubular components in high strength aluminium alloys where high concentricity and close tolerances are required [1]. Very often, the forming is carried out by a combination of forward and backward cold forging. If the geometry is complex, several severe forming operations follow one another that also involves intermediate soft annealing and lubrication. This often requires an interruption of the production line that is both time-consuming and expensive (Fig. 1(a)). A new alternative production process is therefore developed [2,3]. The forging is accomplished at moderate temperatures where it is possible to form rather complicated geometrical shapes in less forming operations without leaving the production line to soft-anneal and re-lubricate the workpiece (Fig. 1(b)). Several papers have been published on cold forging, warm forging and a combination of the two methods in steel [4–6] in the past 10 years. However, less work has been accomplished on warm forging of age pardonable * Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00219-2
aluminium alloys. Warm forging of steel typically occurs in the temperature range from 750 to 850°C depending on the material and the main purpose is to lower the yield stress and increase the formability. Age pardonable aluminium alloys can be heat-treated to reduce the yield stress. However, soft annealing is both time-consuming and expensive. The motivation for forming of AlMgSi alloys in warm condition is therefore somewhat different from steel and the main purpose is to reduce the number of heat-treatment steps. Attention should however be given to the fact that the hardening mechanisms in age-pardonable aluminium alloys are complicated and will be a combination of deformation hardening and precipitation hardening.
1.2. Research objecti6es The main objectives in this investigation have therefore been to compare the process parameters in an alternative and more cost-effective process (warm forging) with the parameters used in the present process (cold forging). An important issue is that the flow stress during forming is equal or less than in the present process and that the mechanical properties in the final products are approximately the same as in the existing process.
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
157
Fig. 1. A sketch of the different forging processes. (A) Cold forging of material in O temper and (B) warm forging of material in W temper.
2. Material behaviour The material used in this investigation is an AA6082 alloy that has been extruded from a billet with a diameter of 29 to a diameter of 178 mm. The casting and extrusion are performed by normal industrial procedures. The alloy has been homogenised to give a high density of dispersoids to prevent recrystallization during extrusion. An extrusion bar with a non-recrystallized fibrous grain structure and a fine subgrain structure is thereby achieved. The nominal composition of the material is given in Table 1. The typical mechanical properties are also tabulated in Table 2. However, by obtaining proper heat treatment and quenching before ageing, it is possible to increase the values by 10%. Material from 6xxx series ages naturally during storing at room temperature. In industrial practice, it will always take from several hours to weeks from when the extrusion is accomplished to the forming being out. During this period, the material ages naturally and the hardness of the material increases causing a reduction in formability. Therefore, before forging in cold condition, the material has to be annealed below solvus temperature to develop a soft material and a stable particle structure that will not give any increase in the strength of the material during room temperature storing. The annealing (O temper) is time-consuming and expensive, however, it has the advantage of making the material soft, stable and easy to form. Forming high strength aluminium alloys in as extruded and water quenched condition (W temper) reTable 1 The chemical composition in weight percentages
quires either a very short storing period at room temperature or a new solution heat treatment immediately before forming. In both cases, the material is unstable and unpredictable concerning the forging load and the final mechanical properties. Forming at an elevated temperature is an alternative method that combines a reduction in load with the ability to age harden the final component to the specified properties. Even though the material will remain unstable, experiments indicate that the process gives repeatable results. The period between water quenching and forming is also found to be of less importance. However, the time and temperature where the forming is carried out is critical. If the material is kept too long at a high forming temperature, a reduction in the ageing potential after forming will be obtained and it will not be possible to reach the required mechanical properties without subsequent solution heat treatment. On the contrary, a low forging temperature results in loads beyond the capacity of the forming equipment. Therefore, it is necessary to establish forming parameters that fulfil both the load requirements and a reasonable hardening potential for subsequent ageing.
3. Compression testing Compression tests have been carried out to establish sufficient process parameters for forging at elevated temperatures. Samples with a diameter of 10 mm and a height of 15 mm have been deformed at temperatures ranging from 200 to 300°C in an servohydraulic 880 Table 2 Typical mechanical properties for T6 condition
Alloy
Si
Fe
Mn
Mg
Al
Alloy
s0.2
su
ef
VHN10
AA6082
1.00
0.20
0.60
0.65
Bal
AA6082
300 MPa
340 MPa
12%
115
158
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
Fig. 2. True stress – strain curves for compression of AA6082 at elevated temperatures. The broad line is O temper.
MTS. testing machine. An environment chamber was used for heating the equipment to the actual forming temperature. A testing procedure is designed that uses parameters near to industrial practise. The procedure is implemented into the computer system of the testing machine. Each test could then be repeated easily and the results from identical samples show good correlation. The samples were heated to elevated temperatures, quenched in water immediately after deformation to prevent any further precipitation and stored at room temperature for a short period before ageing. The entire thermomechanical sequence from heating to watercooling took 1 min. Fig. 2 shows the variations in the compression yield curves for increasing temperatures. The yield stress at a compression strain of 1 is reduced from 200 to 100 MPa, when the temperature increases from 225 to 300°C. The broad line represents the stress – strain curve for soft annealed material deformed at room temperature (O-temper). The true stress – strain curves for material deformed at moderate temperatures seem to level out to a greater extent than the cold-formed material. This reduction in work hardening is more easily shown
Fig. 4. Hardness curves after deformation at 225 and 300°C, respectively, to different strains 0.2, 0.4, 0.6, 0.1 and subsequent ageing at 175°C.
in the work hardening curves in Fig. 3. The observed reduction in work hardening with temperature could be due to recovery taking place at elevated temperatures. Any recovery occurring during forming will increase the formability and reduce the number of forming steps. However, to gain full advantage of the recovery, one may have to form at temperatures where precipitation also takes place. If the temperature is too high, the particles being precipitated will be the non-coherent equilibrium phase which is too large to give any strengthening effect in the final product. The alloying elements used for age hardening are then being depleted. Therefore, an optimum temperature has to be chosen to satisfy both requirements. Fig. 4 indicates that the forming temperature should be close to 225 rather than 300°C. For material deformed at 300°C, the obtainable hardness after ageing is only half of that observed for material deformed at 225°C. Deformed material obtains strength from precipitation of coherent particles and during forming from accumulation of dislocations. However, at small compression strains, the precipitation hardening dominates, while at large strains the deformation hardening is the main source for strengthening. At moderate strains, the strength will be determined by a combination of both mechanisms.
4. Forming of tubes
4.1. Method
Fig. 3. Work hardening (ds/do) versus strain curves for compression of AA6082 at elevated temperatures. The broad line is O temper.
In order to verify the practical use of the forming procedures established by compression testing, backward forging of tubes with an outer diameter of 29 mm and wall thickness of 1 mm have been chosen. This is a generic component with a large surface expansion, i.e. new surface area divided by original surface area. A schematic sketch of the tool set-up is shown in Fig. 5.
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
The container is equipped with three heating elements mounted in a circle to ensure a constant temperature of the tooling. The workpiece was heated in an induction furnace and the punch was heated to the correct temperature by moving the punch to the bottom of the container to obtain contact between the piston and the heated container. The time and temperature cycles were tuned to correspond to what is achievable in industrial practise. The equipment is installed in an 800 ton hydraulic press. To obtain precise measurements, the press is equipped with a high accuracy external load cell and an external sensor for measuring displacement. In addition, a labtech-notebook data acquisition system was used for collecting and calculating forging load, ram position and ram speed. The workpieces were lubricated using calciumaluminat/sodiumstearate. The lubrication of the workpieces was carried out in a commercial production line.
4.2. Experimental results Fig. 6 shows the forging load versus the ram position for cold forging and warm forging of tubes using the same alloys as described above. Material which is cold forged has been soft annealed before forming (O-temper), while the material forged at elevated temperatures is formed in condition as extruded and water quenched at the press (W-temper). The curves represent load versus ram position when forming cups with a height of 200 and 250 mm, respectively. The heights correspond to an expansion in surface area of 29 and 32 times the original surface area. It was possible to achieve expansion above what is reported here. However, when the height of the tubes exceed 200 mm, an opening which was too short between the upper and lower die in the laboratory press made it impossible to remove the
Fig. 5. Schematic draws of the tool set-up for backward forging.
159
Fig. 6. Force vs. ram position for backward extrusion of tubes with an outer diameter of 29 mm and wall thickness of 1 mm.
tubes from the punch without dismantling the equipment. The two curves shown in Fig. 6 show the same tendency as for the compression test. By choosing proper forming parameters, even a reduction in forging load is obtained. Material formed at room temperature also experiences a peak load at the beginning of the test. This peak seems to be lacking when forming at elevated temperatures. The ram rates for all of the tests were set to 20 mm s − 1. This is a high deformation rate and will result in an increase in temperature of the workpiece during forming.
5. Discussion The cold forging of the AA6082 alloy requires soft annealing of the material to obtain sufficient formability at loads that are achievable in regular forging equipment. Soft annealing is both time-consuming and expensive. In addition, stable particles which are too coarse to give any increase in the mechanical properties of the final product are formed during soft annealing. Therefore, the material has to be solution heat treated after cold forming to recrystallize the material and dissolve the coarser particles. Subsequent artificially ageing in the temperature range from 150 to 200°C will form hardening precipitates and give the material the specified mechanical properties. However, by forging at elevated temperatures, extruded billets having the specified dimension can be forged without any further heat treatments. This means that the forging temperature is high enough to bring most of the elements that have precipitated at room temperature and caused an increase in strength, into solid solution. Storing material of age pardonable alloys at room temperature will always result in a certain increase in yield strength. As can be seen from the
160
O. Jensrud, K. Pedersen / Journal of Materials Processing Technology 80–81 (1998) 156–160
Fig. 7. The Vickers hardness versus strain after deformation and after deformation and subsequent ageing at 225°C for 30 min.
compression test and the following hardness curves, the temperature used for forging in this experiment is sufficiently high to fulfil the hardness requirement and even reduce the forging yield stress compared to cold forging. Also, the work hardening is reduced when forging at elevated temperature. However, some work hardening may be beneficial to avoid local strain instability. The reduction in load is also verified by the load versus ram position curves for tube forming, which in this experiment even shows a 20% reduction in forging load. Even though cold forging means that the workpiece is inserted into the container in a cold condition, the temperature can locally increase due to the conversion of deformation work into heat. An increase of several hundred degrees can sometimes be obtained depending on the punch rate and the geometry of the final product. What is commonly called cold forging is actually forming with a certain increase in temperature during the forging process. However, due to the subsequent solution heat treatment, the increase in temperature during forging will have little effect on the mechanical properties of the product. When forging at elevated temperatures, the material will be exposed to higher temperature. The compression tests indicate that the forging cycle chosen will not .
effect the final hardness of the material, even though one must expect a certain increase in forging temperature during deformation in the latter case also. Forging at elevated temperatures creates a deformation structure, which influences the subsequent precipitation. At medium strains, the response to ageing is still present, even though maximum hardness is reached after a comparatively short annealing time (Fig. 7). The strength obtained is a combination of work hardening and precipitation hardening. At higher strains, beyond 0.5, only a small effect of ageing and saturation hardness seems to be reached. The maximum hardness is the same at low strains as at high strains and the short ageing time used in this investigation does not overage the material at any stages. However, the effect of forming followed by precipitation ageing on fracture toughness and ductility has to be investigated.
6. Conclusions The principal basis for a cost-effective warm forging production line is established. The experiments show that by using a forging temperature of 250°C, a reduction in forging load is obtained without any reduction in hardness of the final product.
References [1] S. Skog, K. Asbøll, Cold forging aluminium components, Aluminum 67 (1992) 442 – 446. [2] O. Jensrud, Warm deformation and the age hardening response in two aluminium alloys. Proc. NATO ASI Advanced Light Alloys and Composites, Zakopane, Poland, 1997. [3] T. Welo, S.R. Skjervold, O. Jensrud, K. Pedersen, Cold forging and grain size control in an Al-1.2wt%Si alloy. Metal. Forming, Krakow, 1992. [4] M. Hirsschvogel, H.v. Dommelen, Some application of cold and warm forging, J. Mater. Process. Technol. 35 (3 – 4) 1992. [5] E. Korner, R. Knodler, Possibilities of warm extrusion in combination with cold extrusion, J. Mater. Process. Technol. 35 (3–4) 1992. [6] S. Sheljaskov, Current level of development of warm forging technology, J. Mater. Process. Technol., (46) (1/2) 1994.