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The influence of the thermomechanical processing and forming parameters on superplastic behaviour of the 7475 aluminium alloy
Journal of Materials Processing Technology 118 (2001) 397±402
The in¯uence of the thermomechanical processing and forming parameters on superplastic behaviour of the 7475 aluminium alloy A. Smoleja,*, M. GnamusÏb, E. SlacÏekc a
Department of Materials and Metallurgy, University of Ljubljana, Ljubljana, Slovenia b Litostroj, Foundry, Ljubljana, Slovenia c IMPOL, Aluminium Industry, Slovenska Bistrica, Slovenia
Abstract Aluminium AA7475 alloy (Al±Zn±Mg±Cu) was thermomechanically treated by four procedures which included various combinations of rolling, solution annealing, quenching and overageing. The in¯uence of these treatment procedures on the size and the stability of crystal grains and on superplastic properties of the alloy, such as the highest elongation without localised neck, strain rate sensitivity values, and ¯ow stresses were determined. The highest elongations were over 1000%. With simpli®ed thermomechanical treatment without separate solution annealing and overageing, a sheet with elongations up to 590% was made in industrial conditions. In¯uences of the parameters of superplastic forming (working temperature, strain rate) on superplastic properties of the alloy were determined too. # 2001 Published by Elsevier Science B.V. Keywords: AA7475 alloy; Thermomechanical treatment; Superplasticity
1. Introduction Superplasticity is a property of metallic materials which enables them to achieve very high elongations during tensile testing. The condition for superplastic behaviour of the material is that ®ne crystal grains of a size below 10 mm must remain stable at high working temperatures over 0.5Tm. Superplastic forming (SPF) takes place at low strain rates of 10 2±10 5 s 1, and at low ¯ow stresses [1±7]. Deformation mechanism of SPF was extensively investigated and explained [2,3,8±11]. The basic mechanisms which occur during the plastic forming are grain boundary sliding (GBS) accommodated by slip, and GBS accommodated by diffusional ¯ow. Among various aluminium alloys with superplastic properties, there are technically important Al±Zn±Mg±Cu alloys of the AA7XXX group, next to Al±Mg±X and Al±Li±X groups. In references and in practice, the 7475 and 7075 alloys are the most often cited. Wert et al. [12] developed a special thermomechanical treatment which gives in the standard 7075 alloy a very ®ne microstructure as a condition for SPF. The procedures of thermomechanical treatment, including hot rolling, solution annealing, overageing, rolling at temperatures below 2008C, and recrystallisation annealing, are used also for the grain re®ning in the 7475 alloy *
Corresponding author.
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 0 6 - 2
[6,7,13,14]. There exist numerous investigations on the 7475 and 7075 superplastic alloys [15±24]. By a changed thermomechanical treatment and changed SPF parameters, various highest elongations were achieved [6,7,21±24], such as 2100% with the 7075 alloy [23], and over 1700% with the 7475 alloy [24]. But the original thermomechanical treatment and also some later proposed modi®cations cannot be easily achieved with greater amounts of materials in industrial conditions of manufacturing. This is especially valid for the quenching of material after the solution annealing, for the overageing, and rolling at temperatures below 2008C. According to available information, even better superplastic properties can be obtained with the 7475 alloy by a simpler technology of thermomechanical treatment [24]. This paper discusses the possibilities of manufacturing 7475 alloy in industrial conditions by various thermomechanical treatments. The applied thermomechanical treatment procedures varied in the number and the demanding of single technological steps. Superplastic properties of sheets, thermomechanically treated in various ways, were determined by measuring the grain size, ¯ow stresses, and the highest achieved elongations. Also in¯uences of some SPF parameters are described, such as the in¯uence of working temperature and strain rate on ¯ow stresses and plasticity of material. But the paper does not deal with the mechanism of superplastic deformation.
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A. Smolej et al. / Journal of Materials Processing Technology 118 (2001) 397±402
Table 1 Chemical composition of the tested 7475 alloy (in wt.%) Si
Fe
Cu
Mg
Cr
Zn
Ti
Al
0.05
0.08
1.53
2.25
0.20
5.63
0.04
90.17
2. Experimental work The tested 7475 alloy was semicontinuously cast in slab with dimensions 200 800 1000 mm3 (Table 1). Slab was hot rolled at 4008C to the thickness of 10 mm, after the homogenisation. The hot rolled sheet was the starting material for four various thermomechanical treatments: 1. Material A. Four-hour annealing of sheet at 4858C, cooling in water, 8 h annealing at 4008C, cooling in water, rolling at about 2008C to the final thickness of 1.4 mm, annealing half-an-hour at 4808C. This procedure was identical with the thermomechanical treatment proposed by Wert et al. [12]. 2. Material B. Four-hour annealing of sheet at 4858C, cooling in air, 8 h annealing at 4008C, cooling in air, rolling at about 2008C to the thickness of 1.5 mm, annealing half-an-hour at 4808C. 3. Material C. Four-hour annealing at 4008C, cooling in air, rolling at about 2008C to the thickness of 1.5 mm, annealing half-an-hour at 4808C. 4. Material D. Ten-hour annealing at 4608C, cooling in furnace, cold rolling to the thickness of 1.4 mm, annealing half-an-hour at 4808C. Test pieces with gauge length of 10 mm and gauge width of 5 mm were cut out of the sheet (Fig. 3). Tensile tests for determining superplastic properties were made on the Gleeble 1500 machine where tensile deformation took place in horizontal direction. The tests were made at working temperatures 500±5308C, initial strain rates of 2:5 10 4 1:5 10 3 s 1 , and with constant strain rates 7:5 10 5 7:5 10 4 s 1 .
Fig. 1. Flow stresses in relation with true strains for materials A±D. Initial strain rate at 5158C was 5 10 4 s 1 .
Fig. 2. Elongations with the 7475 alloy in the states A±D, at various initial strain rates, and temperature of 5158C.
material A, and 590% with the material D. All the test pieces, regardless on the thermomechanical treatment, were deformed without local necking (Fig. 3). It is known that the alloys of the Al±Zn±Mg±Cu group can be most ef®ciently superplastically worked at very low strain rates. Surprisingly, the plasticity of material was reduced when strain rates were very low (Fig. 2).
3. Results of investigations 3.1. Influence of thermomechanical treatment on superplastic properties of the alloy The in¯uence of thermomechanical treatment procedures on the superplastic properties of the alloy was determined by measuring ¯ow stresses, the highest elongations, and strain rate sensitivity values. Flow stresses in the applied testing conditions did not exceed 10 MPa. Stresses were the lowest with the material A which was manufactured by the most demanding thermomechanical treatment. Further order was: B, C and D (Fig. 1). As the ¯ow stresses were lower, the highest achieved elongations were higher (Fig. 2). At the working temperature 5158C, and the strain rate of 5 10 4 s 1 , elongation of 1000% was achieved with the
Fig. 3. Plasticity of the 7475 alloy in the states A±D at the initial strain rate 2:5 10 4 s 1 , and temperature 5158C.
A. Smolej et al. / Journal of Materials Processing Technology 118 (2001) 397±402
399
Fig. 4. Size of crystal grains after various thermomechanical treatments of the 7475 alloy.
An important indicator of SPF is the strain rate sensitivity value (m). These values were determined by the rate change test within the stepped increase of cross-head velocity at constant elongation of 20% between two changes [5]. Higher m values were with the materials being treated by a more demanding thermomechanical treatment. The alloy in the state A had the value m 0:60 at the optimal strain rate 5 10 4 s 1 and the temperature 5158C. For the other procedures, the values of m were higher than 0.30. Superplastic properties of the material depend on the size and shape of crystal grains and their stability during annealing and SPF. Change of the grain size is a consequence of static growth which occurs during the heating of material to the working temperature, and a consequence of dynamic growth which takes place during the SPF. Static growth of grains was determined in isothermally annealed test pieces at temperatures 480 and 5158C, at time intervals up to 120 min. Dynamic grain growth was determined by SPF at various elongations between 55 and 830%. The alloy in various thermomechanical states had various starting sizes of crystal grains. Recrystallised grains were elongated in the direction of deformation. Their average lengths were from 8 mm for the procedure A to 15 mm for the procedure D (Fig. 4). Finer crystal grains caused the increase of elongations during the SPF (Fig. 5). The size of crystal grains increased approximately linearly with the increased time of isothermal annealing (Fig. 6). The magnitude of static growth of grains was approximately 0.4 mm/h at 5158C for the material in the state A, and 2 mm/h for the material in the state D. The grain size varied also during the SPF as a consequence of static and dynamic growth (Fig. 7). The alloy with the ®nest grains after the A thermomechanical treatment had also the most stable grains during the SPF.
shifted maximum towards lower deformations. The in¯uence of strain rates and strains on the ¯ow stress during SPF is due to several in¯uential parameters, such as changed mechanism of SPF, static and dynamic grain growth, deformation hardening, and varying strain rate at pulling with
Fig. 5. Elongations of the alloy depending on the size of grains at temperatures 500 and 5158C.
3.2. Influence of working parameters on superplastic properties of the alloy Flow stress increased with the increasing strain rate (Fig. 8). The stress±strain curves exhibited stress maximum which depended on the strain rate. Increased strain rates
Fig. 6. Variation of grain size during isothermal annealing of the alloy in the states A and D at 5158C (l: grain axis in the direction of working, t: grain axis normal to the direction of working).
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Fig. 7. Variation of grain size during SPF of the alloy in the states A and D at initial strain rate 5 10 4 s 1 , and 5158C.
Fig. 8. Flow stress variation with strain at various initial strain rates for material A at 5008C.
constant cross-head velocity [25,26]. At constant true strain rate, the ¯ow stress is changed with strain in a different way than during elongation obtained by pulling with constant cross-head velocity (Fig. 9). The strain rate is, next to temperature, the main parameter which in¯uences the SPF of a material. The optimal strain rate was 5 10 4 s 1 at 5158C for the majority of test pieces (Fig. 2). Tensile tests with a ®xed initial strain rate which was changing with increasing strain and the tests with constant true strain rate in¯uenced the SPF of the alloy in different ways (Fig. 10). The elongations achieved at a constant strain rate were lower, the highest values of elongations were shifted to lower strain rates, when compared with the ®xed initial and changing strain rate. In a suitable way, thermomechanically treated alloy A exhibited good superplastic properties at working temperatures above 5008C. Increase of working temperature up to 5308C enabled the highest elongations at higher strain rates (Fig. 11). The temperature increase of 308C in that temperature interval enabled to achieve equal elongations at three times higher
Fig. 9. Flow stress variation with strain for material A which was tested with initial strain rate of 7:5 10 4 s 1 and constant strain rate of 7:5 10 4 s 1 at 5158C.
Fig. 10. The highest elongations of the alloy A at 5158C at fixed initial and constant strain rate.
Fig. 11. The higher elongations of alloy A in relationship to initial strain rates, and to work-in temperatures (": no failure).
strain rates. Essential is the result at the initial strain rate of 1:5 10 3 s 1 which shows that increased temperature enables the working of 7475 alloy technologically and commercially with more suitable strain rates.
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4. Discussion The basic mechanism for material ¯ow during the SPF of an alloy of the Al±Zn±Mg±Cu group is the sliding process on grain boundaries which includes grain rotation and exchange of sites [1,5,9]. The size, shape, and stability of grains are thus from the viewpoint of material the main parameters which in¯uence the superplasticity. A suitable thermomechanical treatment enables in the 7475 alloy to achieve very ®ne grains which are the basic condition for good workability. During the overageing of the alloy, after the hot rolling, solution annealing, and quenching, there are formed relatively coarse MgZn2 precipitates in the matrix, being of the size up to 1 mm. Matrix in the neighbourhood of precipitates is highly deformed during further rolling at temperatures below 2008C. Densely distributed, highly deformed regions are the preferential sites for formation of the recrystallisation nuclei which create ®ne-grained microstructure during the recrystallisation annealing [12]. Such an effective thermomechanical treatment is not easy to be performed in industrial conditions. In our research, the basic thermomechanical treatment was changed with the intention of ®nding to which degree simpli®ed procedures reduce superplastic properties of the alloy. The size of crystal grains in the tested alloy varied a great deal during the thermomechanical treatment procedures. The ®nest grains were found in sheet which was manufactured according to the original and the most demanding procedure A. Omitting or changing of single technological steps, which caused formation of coarse precipitates, also caused an increase of crystal grain size and their instability for dynamic growth. The highest elongations, strain rate sensitivity values, and ¯ow stresses during the SPF depended on the stability and size of crystal grains. At the beginning of SPF, the stress increased with strain. Deformation hardening in this region of forming was mainly the consequence of dynamic growth of grains [26]. The material in the state A was formed with the lowest stresses since there were the most stable grains. Flow stress was reduced above a certain strain. Stress reduction was the consequence of lower growth rate of grains, and mainly of reduced strain rate. In the test pieces, where the overageing step was limited or omitted, higher stresses were needed for forming. All the materials in various states showed low static growth of grains and therefor it did not in¯uence the superplastic properties. Related to the grain size, the best plasticity of materials and the highest strain rate sensitivity values were achieved. The highest elongation of over 1000% had the alloy in the state A. With the material in the state D where overageing coincided with slow cooling from the annealing temperature after hot rolling, the achieved elongations were up to 590%. These elongations are still acceptable for industrial practice if the simple process of thermomechanical treatment is taken into account.
401
The in¯uence of strain rate on the ¯ow stress and plasticity of the alloy is mainly the consequence of its in¯uence on the SPF mechanism. At lower strain rates, the main accommodation process is represented by the diffusion of atoms along grain boundaries if the GBS mechanism prevails. Increased strain rate reduces the time available for diffusion. Accommodation process cannot anymore follow the process of forming, and therefore the true ¯ow stress is increased. During the SPF also the simultaneous dynamic grain growth occurs which also causes the increase of true stress. The reason is bigger crystal grains which cause longer paths of atoms in the accommodation mechanism of the GBS. In tensile tests with constant strain rates, the highest elongations at lower strain rates were obtained when compared with elongations where equal initial strain rate was applied. But this is only an apparent phenomenon since the initial strain rate of 5 10 4 s 1 in SPF is already at elongation of 100% reduced to 2:5 10 4 s 1 , while at elongation of 500% it has value of 8:3 10 5 s 1 , and at an elongation of 1000% even only 4:5 10 5 s 1. The results show that it is possible to achieve higher elongations at a certain initial strain rate, which later varies, than with the constant strain rate. In references, the most often cited temperature for the SPF of 7475 alloy is 5168C (9608F). This investigation showed that higher elongations of material were achieved with higher strain rates at 5308C, without the danger of incipient melting, as already cited [26]. 5. Conclusions 1. The 7475 alloy was thermomechanically treated in four various ways. The highest elongations of over 1000% were achieved with test pieces where the procedure of treatment consisted of solution annealing, quenching in water, and overageing. With the test pieces which were thermomechanically treated by the simplest procedure and which could be easily performed also in industrial conditions, the elongations of up to 590% were obtained. 2. The optimal strain rate for the alloy in all the states had the value of 5 10 4 s 1 at 5158C. 3. The highest elongations were achieved at the working temperature of 5308C. At that temperature, the highest elongations were achieved at higher strain rates than those used at 5158C. References [1] J.W. Edington, Microstructural aspects of superplasticity, Metall. Trans. A 13 (1982) 703±715. [2] N. Ridley, Superplastic microstructures, Mater. Sci. Technol. 6 (1990) 1145±1156. [3] H.C. Heikkenen, T.R. McNelly (Eds.), Superplasticity in Aerospace, The Metallurgical Society, Warrendale, PA, 1988.
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[4] W.F. Hosford, R.M. Caddell (Eds.), Metal Forming: Mechanics and Metallurgy, Prentice-Hall, London, 1983. [5] I.I. Novikov, V.K. Portnoj, SuperplastizitaÈt von Legierungen, VEB Deutscherverlag fuÈr Grundstoffindustrie, Leipzig, 1981. [6] R.K. Mahindhara, H.S. Yang, A.K. Mukherjee, The effect of grain size and prestrain on the superplastic behaviour of a 7475 aluminium alloy, Z. Metallkd. 85 (1994) 715±722. [7] D.H. Shin, C.S. Lee, W.-J. Kim, Superplasticity of fine-grained 7475 Al alloy and a proposed new deformation mechanism, Acta Mater. 45 (1997) 5195±5202. [8] T.G. Langdon, The mechanical properties of superplastic materials, Metall. Trans. A 13 (1982) 689±701. [9] O.D. Sherby, J. Wadsworth, Superplasticity and superplastic forming processes, Mater. Sci. Technol. 1 (1985) 925±937. [10] K.A. Padmanabhan, J. Schlipf, Model for grain boundary sliding and its relevance to optimal structural superplasticity. Part 1. Theory, Mater. Sci. Technol. 12 (1996) 391±399. [11] V.V. Astanin, S.N. Faizova, K.A. Padmanabhan, Model for grain boundary sliding and its relevance to optimal structural superplasticity. Part 2. Evidence for cooperative grain/interphase boundary sliding and plain interface formation, Mater. Sci. Technol. 12 (1996) 489±494. [12] J.A. Wert, N.E. Paton, C.H. Hamilton, M.W. Mahoney, Grain refinement in 7075 aluminium by thermomechanical processing, Metall. Trans. A 12 (1981) 1267±1276. [13] N.E. Paton, C.H. Hamilton, US Patent 4,092,181 (1977). [14] A.K. Ghosh, C.H. Hamilton, Influence of material parameters and microstructure on superplastic forming, Metall. Trans. A 13 (1982) 733±743. [15] A.J. Shakesheff, P.G. Partridge, Effect of superplastic deformation on the grain size and tensile properties of Al±6.2 Zn±2.5 Mg±1.7 Cu (7010) alloy sheet, J. Mater. Sci. 20 (1985) 2408±2416.
[16] K.R. Mahidhara, A.K. Mukherjee, Superplastic deformation behaviour of a fine-grained aluminium alloy 7475, Mater. Sci. Eng. 80 (1986) 181±193. [17] Y. Hirose, Y. Miyagi, M. Hino, T. Eto, Superplastisches werkstoffverhalten der hochfesten aluminiumlegierung 7475, Aluminium 63 (1987) 386±389. [18] D.H. Shin, S.C. Maeng, Superplastic behaviour of 7475 aluminium alloy, J. Mater. Sci. Lett. 8 (1989) 1380±1382. [19] S. Mehta, P.K. Sengupta, K.J.L. Iyer, K. Nair, Studies of the superplasticity of high strength aluminium alloy, Aluminium 68 (1992) 234±237. [20] J. Xinggang, C. Jianzho, M. Longxiang, Grain refinement and superplasticity of high strength 7475 aluminium alloy, Mater. Sci. Technol. 9 (1993) 493±496. [21] D.H. Shin, K.S. Kim, D.W. Kum, S.W. Nam, New aspects on the superplasticity of fine-grained 7475 aluminium alloy, Metall. Trans. A 21 (1990) 2729±2737. [22] X. Yang, H. Miura, T. Sakai, Evolution of fine grained microstructure and superplasticity in warm-worked 7075 aluminium alloy, Mater. Trans. JIM 37 (1996) 1379±1387. [23] J. Xinggang, W. Qingling, C. Jianzhong, M. Longxiang, A study of the improvement of superplasticity of 7075 alloy, Metall. Trans. A 24 (1993) 2596±2597. [24] J. Xinggang, C. Janzhong, M. Longxiang, A study of grain refinement and superplasticity of high strength 7475 Al alloy, Z. Metallkd. 84 (1993) 216±219. [25] M.W. Mahoney, C.H. Hamilton, A.K. Ghosh, Development of forming limits for superplastic formed fine grain 7475 Al, Metall. Trans. A 14 (1983) 1593±1598. [26] C.H. Hamilton, Simulation of static and deformation-enhanced grain growth effect on superplastic ductility, Metall. Trans. A 20 (1989) 2783±2792.
The in¯uence of the thermomechanical processing and forming parameters on superplastic behaviour of the 7475 aluminium alloy A. Smoleja,*, M. GnamusÏb, E. SlacÏekc a
Department of Materials and Metallurgy, University of Ljubljana, Ljubljana, Slovenia b Litostroj, Foundry, Ljubljana, Slovenia c IMPOL, Aluminium Industry, Slovenska Bistrica, Slovenia
Abstract Aluminium AA7475 alloy (Al±Zn±Mg±Cu) was thermomechanically treated by four procedures which included various combinations of rolling, solution annealing, quenching and overageing. The in¯uence of these treatment procedures on the size and the stability of crystal grains and on superplastic properties of the alloy, such as the highest elongation without localised neck, strain rate sensitivity values, and ¯ow stresses were determined. The highest elongations were over 1000%. With simpli®ed thermomechanical treatment without separate solution annealing and overageing, a sheet with elongations up to 590% was made in industrial conditions. In¯uences of the parameters of superplastic forming (working temperature, strain rate) on superplastic properties of the alloy were determined too. # 2001 Published by Elsevier Science B.V. Keywords: AA7475 alloy; Thermomechanical treatment; Superplasticity
1. Introduction Superplasticity is a property of metallic materials which enables them to achieve very high elongations during tensile testing. The condition for superplastic behaviour of the material is that ®ne crystal grains of a size below 10 mm must remain stable at high working temperatures over 0.5Tm. Superplastic forming (SPF) takes place at low strain rates of 10 2±10 5 s 1, and at low ¯ow stresses [1±7]. Deformation mechanism of SPF was extensively investigated and explained [2,3,8±11]. The basic mechanisms which occur during the plastic forming are grain boundary sliding (GBS) accommodated by slip, and GBS accommodated by diffusional ¯ow. Among various aluminium alloys with superplastic properties, there are technically important Al±Zn±Mg±Cu alloys of the AA7XXX group, next to Al±Mg±X and Al±Li±X groups. In references and in practice, the 7475 and 7075 alloys are the most often cited. Wert et al. [12] developed a special thermomechanical treatment which gives in the standard 7075 alloy a very ®ne microstructure as a condition for SPF. The procedures of thermomechanical treatment, including hot rolling, solution annealing, overageing, rolling at temperatures below 2008C, and recrystallisation annealing, are used also for the grain re®ning in the 7475 alloy *
Corresponding author.
0924-0136/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 0 6 - 2
[6,7,13,14]. There exist numerous investigations on the 7475 and 7075 superplastic alloys [15±24]. By a changed thermomechanical treatment and changed SPF parameters, various highest elongations were achieved [6,7,21±24], such as 2100% with the 7075 alloy [23], and over 1700% with the 7475 alloy [24]. But the original thermomechanical treatment and also some later proposed modi®cations cannot be easily achieved with greater amounts of materials in industrial conditions of manufacturing. This is especially valid for the quenching of material after the solution annealing, for the overageing, and rolling at temperatures below 2008C. According to available information, even better superplastic properties can be obtained with the 7475 alloy by a simpler technology of thermomechanical treatment [24]. This paper discusses the possibilities of manufacturing 7475 alloy in industrial conditions by various thermomechanical treatments. The applied thermomechanical treatment procedures varied in the number and the demanding of single technological steps. Superplastic properties of sheets, thermomechanically treated in various ways, were determined by measuring the grain size, ¯ow stresses, and the highest achieved elongations. Also in¯uences of some SPF parameters are described, such as the in¯uence of working temperature and strain rate on ¯ow stresses and plasticity of material. But the paper does not deal with the mechanism of superplastic deformation.
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Table 1 Chemical composition of the tested 7475 alloy (in wt.%) Si
Fe
Cu
Mg
Cr
Zn
Ti
Al
0.05
0.08
1.53
2.25
0.20
5.63
0.04
90.17
2. Experimental work The tested 7475 alloy was semicontinuously cast in slab with dimensions 200 800 1000 mm3 (Table 1). Slab was hot rolled at 4008C to the thickness of 10 mm, after the homogenisation. The hot rolled sheet was the starting material for four various thermomechanical treatments: 1. Material A. Four-hour annealing of sheet at 4858C, cooling in water, 8 h annealing at 4008C, cooling in water, rolling at about 2008C to the final thickness of 1.4 mm, annealing half-an-hour at 4808C. This procedure was identical with the thermomechanical treatment proposed by Wert et al. [12]. 2. Material B. Four-hour annealing of sheet at 4858C, cooling in air, 8 h annealing at 4008C, cooling in air, rolling at about 2008C to the thickness of 1.5 mm, annealing half-an-hour at 4808C. 3. Material C. Four-hour annealing at 4008C, cooling in air, rolling at about 2008C to the thickness of 1.5 mm, annealing half-an-hour at 4808C. 4. Material D. Ten-hour annealing at 4608C, cooling in furnace, cold rolling to the thickness of 1.4 mm, annealing half-an-hour at 4808C. Test pieces with gauge length of 10 mm and gauge width of 5 mm were cut out of the sheet (Fig. 3). Tensile tests for determining superplastic properties were made on the Gleeble 1500 machine where tensile deformation took place in horizontal direction. The tests were made at working temperatures 500±5308C, initial strain rates of 2:5 10 4 1:5 10 3 s 1 , and with constant strain rates 7:5 10 5 7:5 10 4 s 1 .
Fig. 1. Flow stresses in relation with true strains for materials A±D. Initial strain rate at 5158C was 5 10 4 s 1 .
Fig. 2. Elongations with the 7475 alloy in the states A±D, at various initial strain rates, and temperature of 5158C.
material A, and 590% with the material D. All the test pieces, regardless on the thermomechanical treatment, were deformed without local necking (Fig. 3). It is known that the alloys of the Al±Zn±Mg±Cu group can be most ef®ciently superplastically worked at very low strain rates. Surprisingly, the plasticity of material was reduced when strain rates were very low (Fig. 2).
3. Results of investigations 3.1. Influence of thermomechanical treatment on superplastic properties of the alloy The in¯uence of thermomechanical treatment procedures on the superplastic properties of the alloy was determined by measuring ¯ow stresses, the highest elongations, and strain rate sensitivity values. Flow stresses in the applied testing conditions did not exceed 10 MPa. Stresses were the lowest with the material A which was manufactured by the most demanding thermomechanical treatment. Further order was: B, C and D (Fig. 1). As the ¯ow stresses were lower, the highest achieved elongations were higher (Fig. 2). At the working temperature 5158C, and the strain rate of 5 10 4 s 1 , elongation of 1000% was achieved with the
Fig. 3. Plasticity of the 7475 alloy in the states A±D at the initial strain rate 2:5 10 4 s 1 , and temperature 5158C.
A. Smolej et al. / Journal of Materials Processing Technology 118 (2001) 397±402
399
Fig. 4. Size of crystal grains after various thermomechanical treatments of the 7475 alloy.
An important indicator of SPF is the strain rate sensitivity value (m). These values were determined by the rate change test within the stepped increase of cross-head velocity at constant elongation of 20% between two changes [5]. Higher m values were with the materials being treated by a more demanding thermomechanical treatment. The alloy in the state A had the value m 0:60 at the optimal strain rate 5 10 4 s 1 and the temperature 5158C. For the other procedures, the values of m were higher than 0.30. Superplastic properties of the material depend on the size and shape of crystal grains and their stability during annealing and SPF. Change of the grain size is a consequence of static growth which occurs during the heating of material to the working temperature, and a consequence of dynamic growth which takes place during the SPF. Static growth of grains was determined in isothermally annealed test pieces at temperatures 480 and 5158C, at time intervals up to 120 min. Dynamic grain growth was determined by SPF at various elongations between 55 and 830%. The alloy in various thermomechanical states had various starting sizes of crystal grains. Recrystallised grains were elongated in the direction of deformation. Their average lengths were from 8 mm for the procedure A to 15 mm for the procedure D (Fig. 4). Finer crystal grains caused the increase of elongations during the SPF (Fig. 5). The size of crystal grains increased approximately linearly with the increased time of isothermal annealing (Fig. 6). The magnitude of static growth of grains was approximately 0.4 mm/h at 5158C for the material in the state A, and 2 mm/h for the material in the state D. The grain size varied also during the SPF as a consequence of static and dynamic growth (Fig. 7). The alloy with the ®nest grains after the A thermomechanical treatment had also the most stable grains during the SPF.
shifted maximum towards lower deformations. The in¯uence of strain rates and strains on the ¯ow stress during SPF is due to several in¯uential parameters, such as changed mechanism of SPF, static and dynamic grain growth, deformation hardening, and varying strain rate at pulling with
Fig. 5. Elongations of the alloy depending on the size of grains at temperatures 500 and 5158C.
3.2. Influence of working parameters on superplastic properties of the alloy Flow stress increased with the increasing strain rate (Fig. 8). The stress±strain curves exhibited stress maximum which depended on the strain rate. Increased strain rates
Fig. 6. Variation of grain size during isothermal annealing of the alloy in the states A and D at 5158C (l: grain axis in the direction of working, t: grain axis normal to the direction of working).
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Fig. 7. Variation of grain size during SPF of the alloy in the states A and D at initial strain rate 5 10 4 s 1 , and 5158C.
Fig. 8. Flow stress variation with strain at various initial strain rates for material A at 5008C.
constant cross-head velocity [25,26]. At constant true strain rate, the ¯ow stress is changed with strain in a different way than during elongation obtained by pulling with constant cross-head velocity (Fig. 9). The strain rate is, next to temperature, the main parameter which in¯uences the SPF of a material. The optimal strain rate was 5 10 4 s 1 at 5158C for the majority of test pieces (Fig. 2). Tensile tests with a ®xed initial strain rate which was changing with increasing strain and the tests with constant true strain rate in¯uenced the SPF of the alloy in different ways (Fig. 10). The elongations achieved at a constant strain rate were lower, the highest values of elongations were shifted to lower strain rates, when compared with the ®xed initial and changing strain rate. In a suitable way, thermomechanically treated alloy A exhibited good superplastic properties at working temperatures above 5008C. Increase of working temperature up to 5308C enabled the highest elongations at higher strain rates (Fig. 11). The temperature increase of 308C in that temperature interval enabled to achieve equal elongations at three times higher
Fig. 9. Flow stress variation with strain for material A which was tested with initial strain rate of 7:5 10 4 s 1 and constant strain rate of 7:5 10 4 s 1 at 5158C.
Fig. 10. The highest elongations of the alloy A at 5158C at fixed initial and constant strain rate.
Fig. 11. The higher elongations of alloy A in relationship to initial strain rates, and to work-in temperatures (": no failure).
strain rates. Essential is the result at the initial strain rate of 1:5 10 3 s 1 which shows that increased temperature enables the working of 7475 alloy technologically and commercially with more suitable strain rates.
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4. Discussion The basic mechanism for material ¯ow during the SPF of an alloy of the Al±Zn±Mg±Cu group is the sliding process on grain boundaries which includes grain rotation and exchange of sites [1,5,9]. The size, shape, and stability of grains are thus from the viewpoint of material the main parameters which in¯uence the superplasticity. A suitable thermomechanical treatment enables in the 7475 alloy to achieve very ®ne grains which are the basic condition for good workability. During the overageing of the alloy, after the hot rolling, solution annealing, and quenching, there are formed relatively coarse MgZn2 precipitates in the matrix, being of the size up to 1 mm. Matrix in the neighbourhood of precipitates is highly deformed during further rolling at temperatures below 2008C. Densely distributed, highly deformed regions are the preferential sites for formation of the recrystallisation nuclei which create ®ne-grained microstructure during the recrystallisation annealing [12]. Such an effective thermomechanical treatment is not easy to be performed in industrial conditions. In our research, the basic thermomechanical treatment was changed with the intention of ®nding to which degree simpli®ed procedures reduce superplastic properties of the alloy. The size of crystal grains in the tested alloy varied a great deal during the thermomechanical treatment procedures. The ®nest grains were found in sheet which was manufactured according to the original and the most demanding procedure A. Omitting or changing of single technological steps, which caused formation of coarse precipitates, also caused an increase of crystal grain size and their instability for dynamic growth. The highest elongations, strain rate sensitivity values, and ¯ow stresses during the SPF depended on the stability and size of crystal grains. At the beginning of SPF, the stress increased with strain. Deformation hardening in this region of forming was mainly the consequence of dynamic growth of grains [26]. The material in the state A was formed with the lowest stresses since there were the most stable grains. Flow stress was reduced above a certain strain. Stress reduction was the consequence of lower growth rate of grains, and mainly of reduced strain rate. In the test pieces, where the overageing step was limited or omitted, higher stresses were needed for forming. All the materials in various states showed low static growth of grains and therefor it did not in¯uence the superplastic properties. Related to the grain size, the best plasticity of materials and the highest strain rate sensitivity values were achieved. The highest elongation of over 1000% had the alloy in the state A. With the material in the state D where overageing coincided with slow cooling from the annealing temperature after hot rolling, the achieved elongations were up to 590%. These elongations are still acceptable for industrial practice if the simple process of thermomechanical treatment is taken into account.
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The in¯uence of strain rate on the ¯ow stress and plasticity of the alloy is mainly the consequence of its in¯uence on the SPF mechanism. At lower strain rates, the main accommodation process is represented by the diffusion of atoms along grain boundaries if the GBS mechanism prevails. Increased strain rate reduces the time available for diffusion. Accommodation process cannot anymore follow the process of forming, and therefore the true ¯ow stress is increased. During the SPF also the simultaneous dynamic grain growth occurs which also causes the increase of true stress. The reason is bigger crystal grains which cause longer paths of atoms in the accommodation mechanism of the GBS. In tensile tests with constant strain rates, the highest elongations at lower strain rates were obtained when compared with elongations where equal initial strain rate was applied. But this is only an apparent phenomenon since the initial strain rate of 5 10 4 s 1 in SPF is already at elongation of 100% reduced to 2:5 10 4 s 1 , while at elongation of 500% it has value of 8:3 10 5 s 1 , and at an elongation of 1000% even only 4:5 10 5 s 1. The results show that it is possible to achieve higher elongations at a certain initial strain rate, which later varies, than with the constant strain rate. In references, the most often cited temperature for the SPF of 7475 alloy is 5168C (9608F). This investigation showed that higher elongations of material were achieved with higher strain rates at 5308C, without the danger of incipient melting, as already cited [26]. 5. Conclusions 1. The 7475 alloy was thermomechanically treated in four various ways. The highest elongations of over 1000% were achieved with test pieces where the procedure of treatment consisted of solution annealing, quenching in water, and overageing. With the test pieces which were thermomechanically treated by the simplest procedure and which could be easily performed also in industrial conditions, the elongations of up to 590% were obtained. 2. The optimal strain rate for the alloy in all the states had the value of 5 10 4 s 1 at 5158C. 3. The highest elongations were achieved at the working temperature of 5308C. At that temperature, the highest elongations were achieved at higher strain rates than those used at 5158C. References [1] J.W. Edington, Microstructural aspects of superplasticity, Metall. Trans. A 13 (1982) 703±715. [2] N. Ridley, Superplastic microstructures, Mater. Sci. Technol. 6 (1990) 1145±1156. [3] H.C. Heikkenen, T.R. McNelly (Eds.), Superplasticity in Aerospace, The Metallurgical Society, Warrendale, PA, 1988.
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