Influence of natural aging on the formability of Al-Mg-Si alloy blanks

The formability of naturally-aged Al-Mg-Si alloy blanks (6xxx alloys) is considered as stable within 6 months after the solution heat treatment (SHT). After this period of time the blanks are usually classified as expired, since their formability may decrease, which affects the stability of the forming processes as well as the integrity of the formed components. Extending the period of stable formability is of utmost interest for the sheet metal forming industry; however, detailed knowledge about its time-dependent degradation caused by natural aging (NA) is still lacking. Therefore, the present work investigates the influence of NA on the formability of four 1-mm-thick Al-Mg-Si alloy blanks with different contents of Si and Mg and dry lubricants. The formability-related properties of the blanks were monitored over a total period of 18 months. A cross-shaped open tool and a complex-shaped closed tool representing geometrical features of typical car body panels were used for deep drawing. Moreover, plate-bending tests for characterizing the hemming behavior were performed. After 18 months of NA the yield strength of the Al-Mg-Si alloy blanks increased by about 20-25 %, but no significant decrease of uniform strain, fracture strain and anisotropy (r-value) was observed. The formability decreased neither at the low forming velocity of 10 mm/s, nor at the high forming velocity of 100 mm/s. Hence, the formability can be related to the r-value, since both were constant during NA. Moreover, a relation between the bendability and the strain hardening behavior (n-value) was detected, since both continuously decreased during NA.