Hole Expansion Characteristics of W-Tempered 7075 Aluminum Alloy Sheet in Comparison with Peak Aged T6 Tempered Alloy Sheet

Abstract

The hole expansion formability of W-tempered aluminum 7075 sheet, which is prepared by solution heat treatment and rapid cooling, is investigated comparatively with the peak aged T6 tempered alloy. The W temper heat treatment has been known to be a potential application to cold forming of high strength aluminum including 7075 alloy as an alternative to the warm or hot forming process. The hole expansion tests are designed with a conical punch and the holes are fabricated using wire-cut and punching. Basic mechanical properties and microstructure analyses are performed to study the effect of the strength and ductility in tension on the hole expansion ratios of specimens with different tempers and hole conditions. From the experimental study, the following conclusions are mainly reached. (1) The W-tempered sheets show much improved HER than T6 tempered sheets; i.e., 31 (T6) vs. 58% (W) for wire-cut hole and 19 (T6) vs. 57% (W) for punched hole. (2) The HER of W-tempered sheets show very similar HER values between wire-cut and punched hole specimens, which has not been commonly reported. (3) The initiation of cracks at hole edges is different depending on hole preparation; i.e., RD or TD (wire-cut T6 and wire-cut and punched W) vs. RD, DD, and TD (punched T6). (4) The KAM map validates the cause of lower HER of punched specimen attributes to earlier crack initiation by prior plastic deformation during punching, but the strengthening of shear affected zone has limited effect on HER. (5) The HERs of T6 and W tempered sheets are well correlated to the yield strength, ultimate tensile strength, and total elongation. However, the effect of post uniform elongation on HER is not correlated to the existing report.