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MECHANICAL PROPERTIES OF 7075 ALUMINUM ALLOY SHEET FABRICATED BY TWIN ROLL CASTING AND ROLLING PROCESS

Hyoung-Wook Kim¹, Yun-Soo Lee¹, Min-Seok Kim¹, and Cha-Yong Lim^{1 - 1}Affiliation1, Korea Institute of Materials Science, Light Metals Department, Korea

Abstract. 4.4 mm thick 7075 strip was successfully fabricated by Twin Roll Casting (TRC) under an optimum process condition. The fabricated 7075 strips were annealed and rolled to 0.8mm thick sheets. The sheets consisted of fine grains with the mean grain size of 10um after annealing treatment at 500^oC for 1hour. The annealed 7075 sheets showed high strength of 370MPa and large elongation of over 20%. After aging at 180^oC for 30minumtes, tensile strength and yield strength of the 7075 sheets were 480MPa and 350MPa, respectively. The yield strength of the sheets increased up to 432MPa after bake hardening treatment by introducing pre-aging treatment.  Also, large elongation of 200% was obtained at relatively high strain rate of 5×10^-3s^-1 at 450^oC. The high elongation at ambient temperature and high temperature came from the fine recrystallized grain structure. The fabricated 7075 sheet had high strength and enough formability for automobile body application.

Introduction

Nowadays, light weighting of automobile body structure is big issue to reduce energy consumption and CO2 emission. A lot of light materials have been used to reduce the weight of automobile body structure [1,2] However, the weight is still high to satisfy the increasing demands for future vehicles like Electric Vehicle (EV) and Fuel Cell EV (FCEV). The 7075 sheet has been expected to be a possible material for weight reduction of automobile body structure, however, it can not be used for the porpose by its high materials cost. Twin roll casting have been used for low-cost fabrication process to make aluminum thin sheets with low strength. Recently, high strength Al-Zn-Mg-Cu alloy strip were fabricated by Twin Roll Casting (TRC) by optimizing the process condition.[3,4,5] The microstructure consisted of very fine dendrites and the segregation was remarkably reduced. 7075 alloy sheets with the thickness of below 1 mm were successfully fabricated by annealing and cold rolling. The annealed 7075 sheets showed high strength and large elongation. Also, large elongation of above 200% was obtained at high strain rate at 450^oC [6,7] In this paper, the mechanical properties of 7075 alloy sheet fabricated by twin roll casting and rolling were investigated, and the effect of microstructure on mechanical properties were discussed.

Experimental

Twin roll strip caster with Cu-Cr twin rolls was prepared to get a high cooling rate at the roll gap. Molten Al alloy was transferred to the roll gap through ceramic nozzle. The nozzle was set basically to keep the set back distance at 35mm. The melt was cooled by the rotating twin rolls and solidified to thin strip with the thickness of 4.4mm and the solidification was completed in the roll gap. Al-Zn-Cu-Mg alloys with same composition of commercial 7075 aluminum alloy were melted in an electric melting furnace and transferred to the rolls at 680^oC. The casting speed was changed from 3m/min to 7m/min by controlling the rotating speed of the rolls to reduce surface cracks and center segregation.

The cast strip was annealed at 400^oC for 1 hour, and rolled down to thin sheets with a thickness of 2.0mm at 250^oC. The rolled sheets were annealed again and roll down to 0.8mm thick sheets. It was solutionized at different temperatures of 450, 480 and 500^oC for 1 hour and quenched into water bath. Microstructures of the rolled sheets were observed by an optical microscope after the mechanical polishing and chemical etching. The tensile specimens of the rolled sheets were prepared according to ASTM E8M; the tensile properties of the annealed sheets were evaluated by an Instron 4201 tensile testing machine at ambient temperature. For the tensile testing at high temperature, samples with 5mm x 1mm in cross section and 10mm gauge length were machined from the cold rolled sample. Uni-axial tensile tests were conducted at initial strain rates of 0.001, 0.01, 0.1/sec and at different temperature of 300, 350, 400, 450^oC.

Result and Discussion

Microstructure of the strip. 7075 strip was sucessfully fabricated by twin roll casting under an optimized process condition for Al-Zn-Mg-Cu alloy.[1-3]  The strip thickness was 4.4 mm and the width of the strip was 150mm. The chemical composition of the strip is shown at Table 1. Figure 1 shows microstructures of 7075 strip at strip center and strip edge region. Although small porosities at the thickness center, the grain size and dendrite arm spacing is very fine and uniform through the strip thickness. And the microstructure at strip center is similar with at strip edge, it consist of very fine equiaxed grains through the whole thickness.

Table 1 Chemical composition of 7075 strip fabricated by twin roll casting.

Figure 1 Microstructure of 7075 alloy strip fabricated by Twin Roll Casting(TRC).

Properties of the annealed sheets.In order to improved mechanical property, the cast strip was rolled to 2 mm thick at 250^oC and annealed at 400^oC for 1hour. And the annealed sheets  were rolled again at ambient temperature to the thickness of 0.8mm.The sheets showed sound surface and no craks at the edge. The rolled sheets were annealed at diffrent temperature of 450^oC, 480^oC and 500^oC for 1hour and water quenched. Figure 2 is the microstructure of 7075 sheets annealed at different temperature. The grain size near subsurface is similar with the one near sheets center due to the homogeneous deformation during cold rolling. The microstructure consisted of recrystallized equi-axed grains through the whole thickness. The mean grain size of recrystallized grain at center is larger in the sheets annealed at higher temperature. Mechanical properties of the annealed sheets were evaluated by tensile test. The tensile properties of the annealed 7075 sheets are shown in Figure 3. Figure 3 (a) is the tensile properties of 7075 sheets annealed at 500^oC for 1hour. Tensile strength of the specimen is about 370MPa along the rolling direction with no relation with tensile direction. Meanwhile, tensile elongation was 25% at rolling direction and it slightly decreased to 21% at transverse direction (90^o to the rolling direction). 7075 aluminum alloys is age hardening alloy, so it can be expected additional increase of yield strength after paint bake hardening treatment(180^oC x 30min). As shown in Figure 3 (b), tensile strength and yield strength after bake hardening treatment were increased to 480MPa and 350MPa, respectively. However, the yield strength is lower than those of fully age hardening 7075 alloy sheets. 

Figure 2 Microstructure of 7075 alloy sheets annealed at different temperature;  a) 480^oC, b) 500^oC

Figure 3 Tensile properties of 7075 alloy sheets annealed at different temperature; (a) annealed at 500^oC for 1 hour, b) 500^oC for 1hour and aged at 180^oC for 30min.

Figure 4 Tensile properties of 7075 alloy sheets at different annealing condition.

Figure 5 precipitates of 7075 alloy sheets after bake hardening treatment (a) without pre-aging treatment (b) with pre-aging treatment.

In order to increase yield strength after bake hardening, pre-aging treatment was performed before bake-hardening. In case of pre-aging treatment at 120^oC for 30min after solution treatment at 500^oC for 1hour, the yield strength increased to 432MPa, it is 124MPa higher than one of the bake hardening sheets without pre-aging treatment as shown in Figure 4. The superior hardening properties of 7075 sheets after bake hardening treatment with pre-aging treatment was originated from the high density of fine ᶯ’ precipitate due to large number of GP zone after pre-aging as shown in Figure 5. The precipitation behavior of 7075 sheets during pre-aging and bake hardening were discussed well in previous paper. [8]

Tensile properties at high temperature. Hot tensile test was conducted to investigate formability of 7075 sheets at high temperature. In figure 2, 7075 sheets fabricated by twin roll casting and rolling process had very fine equi-axed grains with the grain size of below 10 um through the whole thickness. The fine grained microstructure can be expected for good formability at high temperature. The cold rolled 7075 sheets showed large tensile elongation of 200% at 450^oC and high strain rate of 0.05/sec as shown in Figure 6. The formation of fine grain in 7075 sheets fabricated by twin roll casting during hot tensile testing was discussed well the previous paper [7]. High fraction of fine particles with the mean size of 1um attributed the homogeneous recrystallized microstructure with fine grains induced by particle stimulated nucleation (PSN) [7]. The combination of fine grains and a sufficient volume fraction of high-angle grain boundaries were responsible for the large fracture elongation at high strain rate. It can be used for quick plastic forming materials to make automobile parts with very complex shape with reasonable cost.

Figure 6 Tensile fracture elongations of 7075 alloy sheets at elevated temperature.

Conclusions

7075 aluminum ally strip were fabricated by twin roll casting, it have good formability after annealing due to fine dendrite and grains through the whole thickness. Fine grained 7075 sheets with the grain size of below 10um were obtained after cold rolling and annealing, the 0.8mm thick sheet showed high strength and formability.  The tensile strength and elongation were 370MPa and 25%, respectively. The yield strength of 7075 sheet pre-aged at 120^oC for 30 minutes increased up to 432MPa after bake hardening treatment. In addition, large elongation of 200% was obtained at relatively high strain rate of 5×10^-3s^-1 at 450^oC. The high elongation at ambient temperature and high temperature were originated from the fully recrystallized fine grained structure. The 7075 sheets fabricated by twin roll casting and rolling have many merits in the view point of mechanical properties and materials cost for automobile body application, therefore, it can be expected to use for future vehicle structure to reduce its weight.

Acknowledgements

The authors are grateful for financial support from the Fundamental Research Program of Korea Institute of Materials Science (KIMS).

References

[1] W.S.Miller, L.Zhuang, J.Bottema, A.J.Wittebrood, P.De Smet, A.Haszler and A. Vieregge, Mater. Sci. Eng., A208 (2000), p37-49.

[2] J.Hirsch and H.I.Laukli, “Aluminum in innovative light-weight car design” (Paper presented at the 12th international conference on aluminum alloys, Yokohama, Japan, 5 September 2010), p46-53

[3] H.W.Kim, J.H.Cho, C.Y.Lim and S.B.Kang, “High strength Al-Zn-Mg-Cu aheets fabricated by twin roll casting”, (Paper presented at the Aluminum 2000, Milano, Italy, 16 May 2013)