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Superplastic forming technology of aircraft structures for Al–Li alloy and high-strength Al alloy
Journal of Materials Processing Technology 72 (1997) 183 – 187
Superplastic forming technology of aircraft structures for Al– Li alloy and high-strength Al alloy Xun Yuwei, Zhun Yiyuan, Mao Wenfeng, Cui Jainzhong PO Box 863, 100024 Beijing, People’s Republic of China Received 6 June 1996
Abstract Superplastic 2091 and 7475 sheets have been used to fabricate load-bearing aircraft structures, the results showing that a mass reduction of approximately 15% and a cost saving of 25% are obtained for the formed structures. This paper presents the manufacturing techniques of 2091 and 7475 parts, which have complex shape and a maximum true thickness strain of 0.92. The cavitation, post-SPF mechanical characteristics, SPF parameters, thickness distribution, heat treatment and surface treatment are discussed also. © 1997 Elsevier Science S.A. Keywords: Superplastic sheets; Al–Li alloy; Manufacturing techniques
1. Introduction
2. Sheet materials
A great interest in the superplastic forming of highstrength aluminium alloys and aluminium – lithium alloys has followed in the wake of superplastic titanium alloys, which have shown pay-offs in aircraft structures. The superplasticity of structural Al and Al–Li alloys has been investigated in detail previously. Presently, many kinds of superplastic sheets, such as 2090, 2091, 8090, 7475 and 1421, are available [1,2]. The Beijing Aeronautical Manufacturing Technology Research Institute (BAMTRI) has been pursuing the research and development of superplastic forming technology of Al–Li and high strength Al alloys for about 10 years and large amounts of superplastic Al alloys structures have been fabricated for aeronautical applications. This paper is concerned the application of the superplastic forming of high-strength aluminium alloy 7475 and aluminium – lithium alloy 2091 sheets, the microstructures, superplasticity, cavitation, post-SPF mechanical properties of the sheets and the manufacturing technology of the actual parts being dealt with in the paper.
High-strength aluminium alloy 7475 and aluminium– lithium alloy 2091 sheets were used in this study. The nominal composition of Alcoa 7475 and Al– 5.7Zn– 2.2Mg–1.5Cu–0.2Cr, with Fe B 0.12 wt.% and Si B0.1 wt.%. and the nominal composition of 2091 was Al– 2.20 Li–2.65 Cu–1.20 Mg–0.15Zr. Both of the superplastic sheets were produced from thick plate by thermomechanical treatment for superplasticity, but the 7475 sheets were completely recrystallized and the 2091 sheets were in the cold-rolled condition. Typical microstructure of 1.5 mm thick 7475 sheets and 1.2 mm thick 2091 sheets are shown in Figs. 1 and 2, these figures illustrate that the grain structure in 7475 sheets is fine, near-equiaxed and uniform from the surface of the sheet to its center, whilst the grain structure in 2091 sheets is elongated (the grain boundary is not visible). The measure results of uniaxial superplasticity are shown in Table 1. It is necessary for 2091 sheets to be held for 25–30 min at the superplastic forming temperature for noncomplete recrystallization. Fig. 3 shows the microstruc-
0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 4 - 0 1 3 6 ( 9 7 ) 0 0 1 4 8 - 9
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
184
Fig. 1. Microstructure of superplastic 7475 sheets.
Fig. 3. The microstructure of SP 2091 sheets after heating at 515°C for 25 min.
Fig. 2. Microstructure of superplastic 2091 sheets.
Fig. 4. Cavitation vs. superplastic strain curve for 2091 sheets for o= 8 × 10 − 4 s − 1. (A) Pb =0.0; (B) Pb =0.5; (C) Pb1.0; (D) Pp = 1.5; and (E) Pb =1.5 MPa.
ture of 2091 sheets that were heated at 515°C for 25 min.
3. Cavitation in superplastic alloy formed materials The application of superplastic deformation to the forming process of structural components could be restricted, if there is extensive cavitation, which later can cause a deleterious effect on the subsequent performance of the component during service. The back-pressure forming method is most effect and economic for aluminium and aluminium – lithium alloys. It has been suggested that the cavitation can be eliminated when the back pressure is high than 0.75se (se is the flow stress), but the usage of back pressure can result in much higher forming pressure for production parts of large size. The relationship between cavitation, compressive hydrostatic pressure and superplastic strain level is investigated under biaxial tension conditions. The cavitation measurement
is plotted as a function of the SPF strain for 2091, as shown in Fig. 4. Considering these results, the values of superplastic strain at which the cavitation begins can be obtained for different values of the various technical parameters. It can be seen that the effect of cavitation is minimized, or even eliminated, when the SPF strain less than 0.8 (true thickness strain, lnt0/t) by using a hydrostatic pressure of 2.0 MPa for 2091 sheets. Thus different back pressure can be used according to the maximum strain of the aircraft part in the manufacturing process. The superplastic strain before cavitation formation for 7475 is greater than it is for 2091. For both materials, the strain rate has effect on the cavity content when it ranges from 10 − 3 to 10 − 4 s − 1 and it has an effect on the size and distribution of the cavities. Metallgraphical examination results show that the cavities are often formed in at the center area of materials (see Fig. 5). Fig. 6 shows that the cavitation is related to the (Fe, Si) phase.
Table 1 Superplasticity of 2091 and 7475 alloy Material
T (°C)
o(s−1)
m
s (MPa)
EL(L) (%)
EL(T) (%)
2091 7475
510–520 512–517
8×10−4 4×10−4
0.5 0.7
2.7 – 3.47 2.5 – 3
5470 5570
5480 5430
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
Fig. 5. Cavitation in SPFed material (2091).
4. Mechanical testing Trough-shaped pans of different geometries, representing different magnitudes of maximum superplastic strain, were fabricated for determinating the superplastic parameters mentioned earlier. The superplastically formed parts were air-cooled and heat-treated to a peak age (T6) condition (7475: solution at 494°C, 15 min, water quenching, aged at 121°C, 24 h; 2091: solution at 530°C, 30 min, water quenching, aged at 175°C, 48 h). Specimens were subsequently excised for the determination of post-SPF material properties, most of the specimens being obtained from the (flat) bottom portion of the pans. The testing is in accordance with ASTME8, the tensile test results are shown in Fig. 7 and Fig. 8. The post-SPF material strength and elongation show a slight loss (less than 10%) over a range of SP strains, whilst Young’s modulus of the post-SPF material is independent of the superplastic deformation if the cavitation is minimized. The post-SPF properties investigation also suggests that, the compressive stability of 2091 and 7475 is higher than that of conventionally-manufactured sheets
Fig. 6. SEM examination (2091, Distance = 2.7).
185
Fig. 7. Mechanical properties vs. SPF strain curves for 2091, LT.
such as 2024 and 7075, whilst the fatigue strength of 2091 and 7475 (with a strain of 0.8) can meet the requirements.
5. The deformation and alloying of clad Al The aluminium–lithium alloy 2091 sheets are clad by pure aluminium, the clad layer being about 50 mm for 1.2 mm thick sheets. The clad layer deformed uniformly during SPF when the SP strain is less than 0.8. However, if the SP strain is higher than 0.8, the clading layer could be cracked and discontinuous. The EDX and SIMS analysis results show that the alloying elements (Cu, Mg, Li) diffuse to the clad layer during SPF [3].
6. Production of aircraft structures
6.1. Die-technology A die for aluminium-SPF must be gas-tight and be designed for higher pressure than a die for titanium-
Fig. 8. Mechanical properties vs. SPF strain curves for 2091, L.
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X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
Fig. 9. Aircraft structures: (a) 7475 alloy (A); (b) 2091 alloy (B).
SPF. A die made of medium carbon steel is cost effective and can meet the demands of production of superplastic forming structures. A gas-tight sealing between the two halves of the die is achieved by the convex. The die is machined to a surface roughness of Ra =1.6 mm.
6.2. Aircraft structures production Fig. 9 shows two examples of 7475 and 2091 superplastic-forming parts for fighter application, these parts being used in airplanes already. Part A was made of 7475, part B was made of 2091. For part A, a cost reduction of 35% and a mass reduction of 50% were achieved, and the number of piece-parts was reduced. For part B, a cost reduction of 25%, and a mass reduction of 15% were achieved. Both of the structures were superplastic formed under a back pressure and were cavitation free. Fig. 10 shows the post-SPF microstructure of the 2091 sheets. Because of the non-
complete crystallizing, the microstructure is non-uniform. Fig. 11 is the pressure versus time curve, which is calculated by computer and optimized by experiment.
6.3. Wall thickness The wall thickness distribution can be improved by optimizing the technical parameters, reverse blowing [4,5] and lubricant. Fig. 12 shows the reverse-blowing technique of part A. Lubricant is very important during SPF for aluminium–lithium alloys, BN and graphite being effective.
6.4. Surface treatment The surfaces of superplastic alloy formed 7475 parts are treated by sulphuric acid anodizing and those of 2091 parts by chromic acid anodizing, to achieve excellent corrosion resistance.
6.5. Producti6ity Hot open-die technology is used to raise the productivity of the superplastic forming of Al and Al–Li alloys structures, in which the parts are cooled by gas, but where distortion must be avoided.
7. Conclusions
Fig. 10. The microstructure of 2091 sheets after SPF (T6).
(1) The manufacturing development of Al and Al–Li SPF parts shows that economical and cost-effective production is possible, cost and weight reduction being especially attractive for aerospace applications.
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
187
Fig. 11. Pressure vs time profiles: (a) 7475 aircraft structure; (b) 2091 aircraft structure.
provide guiding processes for the production of aluminium and aluminium–lithium alloys SPF structures. References
Fig. 12. Reverse blowing of 7475 for the aircraft structure.
(2) The post-SPF mechanical properties can meet the requirements of aircraft structures. (3) The research results presented in this paper
.
[1] J. Wadsworth, T.G. Nieh, ICASM-91, vol. 78, Osaka, Japan, 1986, pp. 13 – 22. [2] M.C. Pandey, J. Wadsworth, et al., Mater. Sci. Eng. 78 (1986) 115 – 125. [3] Song Feilin, Mao Wenfeng, Zhu Yiyuan, in: Symposium on Progress of Advanced Materials in China. National Advanced Materials Committee of China, 1993, pp. 123 – 125. [4] D.B. Laycock, Superplastic Forming of Structural Alloys, The Metallurgical Society of AIME, San Diego, CA, 1982, pp. 251 – 271. [5] Zhu Yiyuan, Song Feilin, Aero. Manuf. Tech. 2 (1994) 20–33.
Superplastic forming technology of aircraft structures for Al– Li alloy and high-strength Al alloy Xun Yuwei, Zhun Yiyuan, Mao Wenfeng, Cui Jainzhong PO Box 863, 100024 Beijing, People’s Republic of China Received 6 June 1996
Abstract Superplastic 2091 and 7475 sheets have been used to fabricate load-bearing aircraft structures, the results showing that a mass reduction of approximately 15% and a cost saving of 25% are obtained for the formed structures. This paper presents the manufacturing techniques of 2091 and 7475 parts, which have complex shape and a maximum true thickness strain of 0.92. The cavitation, post-SPF mechanical characteristics, SPF parameters, thickness distribution, heat treatment and surface treatment are discussed also. © 1997 Elsevier Science S.A. Keywords: Superplastic sheets; Al–Li alloy; Manufacturing techniques
1. Introduction
2. Sheet materials
A great interest in the superplastic forming of highstrength aluminium alloys and aluminium – lithium alloys has followed in the wake of superplastic titanium alloys, which have shown pay-offs in aircraft structures. The superplasticity of structural Al and Al–Li alloys has been investigated in detail previously. Presently, many kinds of superplastic sheets, such as 2090, 2091, 8090, 7475 and 1421, are available [1,2]. The Beijing Aeronautical Manufacturing Technology Research Institute (BAMTRI) has been pursuing the research and development of superplastic forming technology of Al–Li and high strength Al alloys for about 10 years and large amounts of superplastic Al alloys structures have been fabricated for aeronautical applications. This paper is concerned the application of the superplastic forming of high-strength aluminium alloy 7475 and aluminium – lithium alloy 2091 sheets, the microstructures, superplasticity, cavitation, post-SPF mechanical properties of the sheets and the manufacturing technology of the actual parts being dealt with in the paper.
High-strength aluminium alloy 7475 and aluminium– lithium alloy 2091 sheets were used in this study. The nominal composition of Alcoa 7475 and Al– 5.7Zn– 2.2Mg–1.5Cu–0.2Cr, with Fe B 0.12 wt.% and Si B0.1 wt.%. and the nominal composition of 2091 was Al– 2.20 Li–2.65 Cu–1.20 Mg–0.15Zr. Both of the superplastic sheets were produced from thick plate by thermomechanical treatment for superplasticity, but the 7475 sheets were completely recrystallized and the 2091 sheets were in the cold-rolled condition. Typical microstructure of 1.5 mm thick 7475 sheets and 1.2 mm thick 2091 sheets are shown in Figs. 1 and 2, these figures illustrate that the grain structure in 7475 sheets is fine, near-equiaxed and uniform from the surface of the sheet to its center, whilst the grain structure in 2091 sheets is elongated (the grain boundary is not visible). The measure results of uniaxial superplasticity are shown in Table 1. It is necessary for 2091 sheets to be held for 25–30 min at the superplastic forming temperature for noncomplete recrystallization. Fig. 3 shows the microstruc-
0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 4 - 0 1 3 6 ( 9 7 ) 0 0 1 4 8 - 9
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
184
Fig. 1. Microstructure of superplastic 7475 sheets.
Fig. 3. The microstructure of SP 2091 sheets after heating at 515°C for 25 min.
Fig. 2. Microstructure of superplastic 2091 sheets.
Fig. 4. Cavitation vs. superplastic strain curve for 2091 sheets for o= 8 × 10 − 4 s − 1. (A) Pb =0.0; (B) Pb =0.5; (C) Pb1.0; (D) Pp = 1.5; and (E) Pb =1.5 MPa.
ture of 2091 sheets that were heated at 515°C for 25 min.
3. Cavitation in superplastic alloy formed materials The application of superplastic deformation to the forming process of structural components could be restricted, if there is extensive cavitation, which later can cause a deleterious effect on the subsequent performance of the component during service. The back-pressure forming method is most effect and economic for aluminium and aluminium – lithium alloys. It has been suggested that the cavitation can be eliminated when the back pressure is high than 0.75se (se is the flow stress), but the usage of back pressure can result in much higher forming pressure for production parts of large size. The relationship between cavitation, compressive hydrostatic pressure and superplastic strain level is investigated under biaxial tension conditions. The cavitation measurement
is plotted as a function of the SPF strain for 2091, as shown in Fig. 4. Considering these results, the values of superplastic strain at which the cavitation begins can be obtained for different values of the various technical parameters. It can be seen that the effect of cavitation is minimized, or even eliminated, when the SPF strain less than 0.8 (true thickness strain, lnt0/t) by using a hydrostatic pressure of 2.0 MPa for 2091 sheets. Thus different back pressure can be used according to the maximum strain of the aircraft part in the manufacturing process. The superplastic strain before cavitation formation for 7475 is greater than it is for 2091. For both materials, the strain rate has effect on the cavity content when it ranges from 10 − 3 to 10 − 4 s − 1 and it has an effect on the size and distribution of the cavities. Metallgraphical examination results show that the cavities are often formed in at the center area of materials (see Fig. 5). Fig. 6 shows that the cavitation is related to the (Fe, Si) phase.
Table 1 Superplasticity of 2091 and 7475 alloy Material
T (°C)
o(s−1)
m
s (MPa)
EL(L) (%)
EL(T) (%)
2091 7475
510–520 512–517
8×10−4 4×10−4
0.5 0.7
2.7 – 3.47 2.5 – 3
5470 5570
5480 5430
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
Fig. 5. Cavitation in SPFed material (2091).
4. Mechanical testing Trough-shaped pans of different geometries, representing different magnitudes of maximum superplastic strain, were fabricated for determinating the superplastic parameters mentioned earlier. The superplastically formed parts were air-cooled and heat-treated to a peak age (T6) condition (7475: solution at 494°C, 15 min, water quenching, aged at 121°C, 24 h; 2091: solution at 530°C, 30 min, water quenching, aged at 175°C, 48 h). Specimens were subsequently excised for the determination of post-SPF material properties, most of the specimens being obtained from the (flat) bottom portion of the pans. The testing is in accordance with ASTME8, the tensile test results are shown in Fig. 7 and Fig. 8. The post-SPF material strength and elongation show a slight loss (less than 10%) over a range of SP strains, whilst Young’s modulus of the post-SPF material is independent of the superplastic deformation if the cavitation is minimized. The post-SPF properties investigation also suggests that, the compressive stability of 2091 and 7475 is higher than that of conventionally-manufactured sheets
Fig. 6. SEM examination (2091, Distance = 2.7).
185
Fig. 7. Mechanical properties vs. SPF strain curves for 2091, LT.
such as 2024 and 7075, whilst the fatigue strength of 2091 and 7475 (with a strain of 0.8) can meet the requirements.
5. The deformation and alloying of clad Al The aluminium–lithium alloy 2091 sheets are clad by pure aluminium, the clad layer being about 50 mm for 1.2 mm thick sheets. The clad layer deformed uniformly during SPF when the SP strain is less than 0.8. However, if the SP strain is higher than 0.8, the clading layer could be cracked and discontinuous. The EDX and SIMS analysis results show that the alloying elements (Cu, Mg, Li) diffuse to the clad layer during SPF [3].
6. Production of aircraft structures
6.1. Die-technology A die for aluminium-SPF must be gas-tight and be designed for higher pressure than a die for titanium-
Fig. 8. Mechanical properties vs. SPF strain curves for 2091, L.
186
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
Fig. 9. Aircraft structures: (a) 7475 alloy (A); (b) 2091 alloy (B).
SPF. A die made of medium carbon steel is cost effective and can meet the demands of production of superplastic forming structures. A gas-tight sealing between the two halves of the die is achieved by the convex. The die is machined to a surface roughness of Ra =1.6 mm.
6.2. Aircraft structures production Fig. 9 shows two examples of 7475 and 2091 superplastic-forming parts for fighter application, these parts being used in airplanes already. Part A was made of 7475, part B was made of 2091. For part A, a cost reduction of 35% and a mass reduction of 50% were achieved, and the number of piece-parts was reduced. For part B, a cost reduction of 25%, and a mass reduction of 15% were achieved. Both of the structures were superplastic formed under a back pressure and were cavitation free. Fig. 10 shows the post-SPF microstructure of the 2091 sheets. Because of the non-
complete crystallizing, the microstructure is non-uniform. Fig. 11 is the pressure versus time curve, which is calculated by computer and optimized by experiment.
6.3. Wall thickness The wall thickness distribution can be improved by optimizing the technical parameters, reverse blowing [4,5] and lubricant. Fig. 12 shows the reverse-blowing technique of part A. Lubricant is very important during SPF for aluminium–lithium alloys, BN and graphite being effective.
6.4. Surface treatment The surfaces of superplastic alloy formed 7475 parts are treated by sulphuric acid anodizing and those of 2091 parts by chromic acid anodizing, to achieve excellent corrosion resistance.
6.5. Producti6ity Hot open-die technology is used to raise the productivity of the superplastic forming of Al and Al–Li alloys structures, in which the parts are cooled by gas, but where distortion must be avoided.
7. Conclusions
Fig. 10. The microstructure of 2091 sheets after SPF (T6).
(1) The manufacturing development of Al and Al–Li SPF parts shows that economical and cost-effective production is possible, cost and weight reduction being especially attractive for aerospace applications.
X. Yuwei et al. / Journal of Materials Processing Technology 72 (1997) 183–187
187
Fig. 11. Pressure vs time profiles: (a) 7475 aircraft structure; (b) 2091 aircraft structure.
provide guiding processes for the production of aluminium and aluminium–lithium alloys SPF structures. References
Fig. 12. Reverse blowing of 7475 for the aircraft structure.
(2) The post-SPF mechanical properties can meet the requirements of aircraft structures. (3) The research results presented in this paper
.
[1] J. Wadsworth, T.G. Nieh, ICASM-91, vol. 78, Osaka, Japan, 1986, pp. 13 – 22. [2] M.C. Pandey, J. Wadsworth, et al., Mater. Sci. Eng. 78 (1986) 115 – 125. [3] Song Feilin, Mao Wenfeng, Zhu Yiyuan, in: Symposium on Progress of Advanced Materials in China. National Advanced Materials Committee of China, 1993, pp. 123 – 125. [4] D.B. Laycock, Superplastic Forming of Structural Alloys, The Metallurgical Society of AIME, San Diego, CA, 1982, pp. 251 – 271. [5] Zhu Yiyuan, Song Feilin, Aero. Manuf. Tech. 2 (1994) 20–33.