DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY IN COMMERCIAL 5083 Al-Mg ALLOYS

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1 Pergamon Scripta Materialia, Vol. 40, No. 6, pp , 1999 Elsevier Science Ltd Copyright 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved /99/$ see front matter PII S (98) DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY IN COMMERCIAL 5083 Al-Mg ALLOYS I.C. Hsiao and J.C. Huang Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. (Received June 29, 1998) (Accepted October 21, 1998) Introduction Superplastic forming has been one of the forming techniques for aircraft industry. In developing superplastic aircraft-used aluminum alloys, two successful means have been applied. One was to modify the alloys by adding extra amount of particle-forming elements, such as the case of Al-Cu base Supral 150 with 0.5wt% of Zr added (Al-6wt%Cu-0.5%Zr) [1]. The other was to process the commercial alloys by a series of thermomechnical treatments (TMTs), such as the efforts made for the Al-Zn-Mg base 7075 [2] and Al-Li base 8090 [3] alloys. For all these relatively more expensive Al-Cu, Al-Zn-Mg, and Al-Li base alloys, the primary concern was on the applications in aircraft industry. Due to the consideration of fuel consumption and air pollution, aluminum vehicles may soon become popular. Superplastic forming of commercial lowpriced aluminum sheets could also be developed into one of the important fabrication means. There are at least three main factors that need to be taken into account: (1) the alloy itself is commercially widely available and cheap; (2) the forming rate is sufficiently high; and (3) the forming temperature is as low as possible. Higher forming rate to a strain rate greater than 10 2 s 1 would satisfy the current fabrication speed [4]; while lower forming temperature would save fabrication energy, prevent from severe grain growth, cavitation and solute-loss from surface layers, as well as maintain superior post-form properties [5]. Following these guidelines, the development of superplastic Al-Mg or Al-Mg-Si base alloys has attracted attention lately, including experimental alloys such as Al-3Mg and commercial alloys such as AA5052, 5083, 6061, 6011, etc. There have been numerous efforts in processing aluminum materials to exhibit high rate superplasticity (HRSP) and/or low temperature superplasticity (LTSP). Recent success in producing extrafine grained aluminum alloys using equal channel angular (ECA) extrusion to strain levels greater than 3.0 has demonstrated the possibility in processing Al-Cu-Zr or Al-Li-Cu-Mg-Zr alloys to possess HSRS and LTSP at C and s 1 [6]. These impressive results suggested the trial in using previous rolling-typed TMTs but to a higher rolling reduction for the Al-Mg base alloys. There have been numerous reports on the superplasticity in solution strengthening Al-Mg base alloys [7 17], as summarized in Table 1. One group [7 11] was aimed on the high temperature superplasticity (HTSP) originated from the solute drag creep typed mechanism. These materials were usually 7 20 m in grain size, with optimum superplastic conditions at C and 10 4 s 1 and optimum superplastic elongations in the neighborhood of 400%. 697

2 698 DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY Vol. 40, No. 6 TABLE 1 Summary of the Reports on HTSP or LTSP in the Al-Mg Base Alloys Alloy TMTs Grain size ( m) SP characteristics Year, ref. HTSP: 5083 Rolling C, s 1, 300% 1996, [7,8] 550 C, s 1, 150% 550 C, two-step, 470% 5083 Rolling C, s 1, 450% 1997, [9] 5083 Rolling C, s 1, 480% 1997, [10] 550 C, s 1, 380% 550 C, s 1, 180% 5083 Rolling C, s 1, 480% 1998, [11] 550 C, s 1, 380% LTSP: Al-10Mg-0.1Zr TMT C, s 1, 1100% 1993, [12] 300 C, s 1, 400% Al-3Mg ECA C, s 1, 170% 1993, [13] 130 C, s 1, 130% 130 C, s 1, 80% 130 C, s 1, 30% Al-5.3Mg, Al-7Mg Rolling 300 C, s 1, 160% 1997, [14] Al-11Mg 300 C, s 1, 120% 400 C, s 1, 150% 5056 (Al-4.8Mg) ECA C, s 1, 90% 1997, [15] 200 C, s 1, 110% 275 C, s 1, 185% Al-5.8Mg-0.3Sc Rolling C, s 1, 130% 1997, [16] 475 C, s 1, 170% 350 C, s 1, 80% Al-2.8Mg, Al-5.5Mg Rolling C, s 1, 84% 1998, [17] 200 C, s 1, 51% 300 C, s 1, 228% 300 C, s 1, 136% 300 C, s 1, 39% 342 C, s 1, 147% Al-3Mg-0.25Mn Rolling C, s 1, 192% 1998, [17] Al-3Mg-0.2Zr 400 C, s 1, 165% 400 C, s 1, 146% The other group [12 17] was focused on LTSP and/or HRSP. The materials contained much finer (sub)grain size to 1 m or below, expected to possess superplasticity at temperatures 300 C and strain rates 10 4 s 1. From Table 1, it can be seen that there have not been satisfactory results on LTSP in the commercial 5083 alloy. Kawazoe et al. [15] employed ECA to process the 5063 system (Al-4.8Mg- 0.07Mn), resulting in a grain size to 0.3 m, but the LTSP properties still did not exceed 200%. This was the same for the experimental alloy Al-3Mg [13]. By adding other elements, such as Sc [16], Mn or Zr [17], the HTSP and/or HRSP might have been improved, but the LTSP behavior was basically below 200%. On the other hand, successful results have been accomplished in an experimental alloy, the Al-10Mg-0.1Zr system [12] with an excess amount of Mg. This alloy, after subjected to TMTs, possessed 1 m grain size and showed an outstanding elongation of 1100% at 300 C and s 1 and 400% at 300 C and s 1. However, the trial in binary Al-7Mg or Al-11Mg alloys [14] was not as promising. Based on the current development situation, this study is intended to process the

3 Vol. 40, No. 6 DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY 699 Figure 1. TEM micrograph of the TMT processed 5083 thin sheet (a) before and (b) after superplastic loading at 250 C and s 1, showing the fine (sub)grains. low-priced commercial 5083 alloys using simple TMTs so as to develop LTSP at temperatures below 300 C and with superplastic elongations in excess of 200%. Experimental Methods The 5083 alloys were obtained from China Steel Corp., Taiwan, in the form of hot-rolled thick plates to 30 mm, with a composition of Al-4.7wt%Mg-0.7%Mn. The as-received (AR) thick plates possessed elongated grains measuring m and did not exhibit any LTSP. A simple TMT process was applied, including annealing at 500 C for 1 hr, followed by air cooling and a series of low temperature rolling ( C) to a final thickness of mm. The rolling reduction ratio and true strain received during TMTs varied from 90% 98% and , respectively. No static recrystallization heat treatment was carried out before tensile loading. Tensile tests were conducted using an Instron 1125 universal testing machine equipped with a three-zone furnace, with the loading direction parallel to the final rolling direction. The specimen gauge length was 8 mm. The microstructures were characterized using a Jeol 200CX transmission electron microscope (TEM). Results Figure 1 shows the TEM micrographs of the TMT processed sheet before and after tensile loading, taken from the rolling plane. The grain structures were not well-defined. The 3D (sub)grain size was around m, nearly equiaxed on the rolling plane and narrower along the longitudinal plane. After tensile loading to failure, the grains near the fracture end reached a size 1.5 m at temperatures 250 C and greater than 10 m at 300 C or above. Table 2 lists the room temperature TABLE 2 Room Temperature Tensile Properties of the 5083 Alloy Grain size ( m) As received (Cast hot rolling) Best TMT route

4 700 DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY Vol. 40, No. 6 Figure 2. Typical stress-strain curves of the AR plates and thin sheets processed by two different TMT routes. tensile properties for the AR and TMT processed 5083 alloys. The TMT processed sheets were in the as-worked condition, so that they exhibited high strength owing to work hardening. Typical stress-strain curves for the AR and TMT processed alloys at 250 C are shown in Fig. 2, where it can be seen that the AR materials shows continuous work hardening and the TMT processed ones reveals overall strain softening behavior. In Fig. 2, the stress-strain curves for the sheet materials subjected to different TMTs are also compared. If the thin sheet possessed poor LTSP characteristics, the stress usually declined rapidly after 0.1. On the other hand, for materials showed LTSP elongations higher than 200%, the flow stress exhibited initial hardening, followed by softening. The maximum stress typically occurred at 0.4. It was apparent that once the subgrains formed during TMTs transformed successfully into high angle grain boundaries during the critical initial straining over 0 0.5, the material could further deform smoothly to large strains. Figure 3 shows the typical tensile loaded specimens, which were failed through cavitation-induced fracture. This was contrary to the severe necking to a line for the AR specimens. The tensile test results at C of the AR and TMT processed sheets are presented in Table 3. At 250 C, the tensile elongation of the AR specimens was only 84% and the strain rate sensitivity (m) was varied within Consistent with the solute drag creep behavior [18,19], the tensile elongation of the AR specimens increased with increasing temperature, with an optimum superplastic temperature around 550 C (at which temperature m was 0.3). After appropriate TMTs, the thin sheets exhibited LTSP elongations to 400% at 250 C and s 1. Compatible LTSP behaviors were Figure 3. Tensile specimens before and after loading for the AR and TMT processed materials.

5 Vol. 40, No. 6 DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY 701 TABLE 3 Comparison of the LTSP Properties of the AR and TMT Processed 5083 Alloys As received (Cast hot rolling) Best TMT route TABLE 4 The LTSP Properties of the TMT Processes 5083 Thin Sheets at 250 C Best TMT route TABLE 5 Comparison of the LTSP Properties of the 5083 Thin Sheets Subjected to Different TMTs As received TMT, cold rolling, to 2 mm TMT, cold rolling, to 1 mm TMT, warm rolling, to 2 mm TMT, warm rolling, to 1 mm TABLE 6 Comparison of the LTSP Properties of the 5083 Thin Sheets Processed Using a Rolling Direction During TMTs Perpendicular ( ) or Parallel (F) tothe Original Rolling Direction in Hot Rolling TMT route, TMT route, F

6 702 DEVELOPMENT OF LOW TEMPERATURE SUPERPLASTICITY Vol. 40, No. 6 seen over the temperature range C (Table 3) and s 1 (Table 4). The m-values were mostly varied within , distinctly different from the case of the AR materials. The flow stress levels of the TMT processed thin sheets were always much lower than the stresses of the AR counterparts. For example, the maximum stress dropped from 133 MPa of the AR materials at 250 C and s 1 to 64 MPa of the TMT processed ones, suggesting the stress reduction effect in the fine-grained thin sheets from the activation of grain boundary sliding at low temperatures. Several TMT parameters would affect the LTSP characteristics, such as the rolling temperature and rolling reduction ratio, as depicted in Table 5. Warm rolling was still better than cold rolling, but the difference was not pronounced. With increasing rolling reduction, the LTSP elongations would usually increase. A higher rolling reduction ratio tended to not only refine more the grain structures but also increase the uniformity of (sub)grain size distributions. As the final sheet thickness became less than 2 mm, the difference in (sub)grain sizes in the near-surface and mid-plane regions became minimum. Meanwhile, changing the rolling direction resulted in only slight difference in LTSP (Table 6), possibly due to the fact that the resulting (sub)grains had all transformed into nearly equiaxed grain shape after severe straining during TMTs and superplastic loading, no matter what the previous rolling direction was. Finally, the heating rate of the furnace before tensile loading imposed some effects. A rapid heat rate (e.g. 15 min from room temperature to 250 C, or 15 C/min) resulted in lower LTSP elongations than a slow heating rate (45 min, or 5 C/min). This suggested that a minimum recovery time was needed before tensile loading so as to partially transform the dislocation and subgrain substructures. It also indirectly implied that a static heat treatment (static recovery or partial static recrystallization) for mim at C might be helpful for further dynamic recovery or dynamic recrystallization. But this step can be saved to apply until superplastic forming. Note that the strain level experienced during TMTs in this study was around 3, which was smaller but comparable to the case of ECA. The current study suggested another feasible and simple mean in processing fine-grained LTSP aluminum sheets. The material factory can utilize common rolling facility to produce large-scaled superplastic sheets for further forming. It does not need to redesign extrusion or pressing machine, it prevents from the troublesome water quenching after annealing, and it excludes the overaging heat treatment [5] during TMTs. Since most formed parts in automobiles require very small amounts of deformation, typically less than 100%, the cold rolled 5083 thin sheets might already satisfy some engineering needs. Summary The current study developed a simple rolling-type TMT to process the low-priced commercial 5083 to exhibit low temperature superplasticity at around 250 C and s 1, with an optimum tensile elongation to 400%. The TMT processed thin sheet contained (sub)grains measuring m. At temperatures lower than 300 C, the grains grew limitedly and maintained LTSP, with failure by cavitation coalescence. The flow stress of the LTSP specimens dropped to nearly one half as compared with the AR non-superplastic samples, and the strain rate sensitivity increased from of the AR specimens to of the LTSP ones. The effects from rolling temperature, rolling reduction ratio, and rolling direction on the LTSP characteristics were also examined. Acknowledgment The authors would like to gratefully acknowledge the sponsorship from National Science Council of ROC under the project no. NSC E

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