Inner Surface Cracking of an Aluminum Alloy in Small-Radius Bending

Aluminum alloys, due to their low density, high strength to weight ratio and formability, are widely used in automotive components. At present, most of the sheet alloy being used is AA6111; an Al-Mg-Si alloy with addition of Cu. AA6111. These alloys contain micrometer sized inclusions and second phase particles, with good combination of strength and formability [1]. However, at the same time, the formability of AA6111 is also limited because of these micro-sized inclusions and second phase particles [2]. To improve the formability of sheet metal used as automotive body such as panels, a newer alloy AA6022 containing nano-sized strengthening precipitates and enhanced formability has been developed. A number of research works have been done on the precipitation sequences and phase development during aging of these alloys. Recently Miao and Laughlin have reported that the precipitation sequence in the AA6022 is in the following reaction: solid solution α → GP zones → β″ → β′ + lath-like precipitate ← β + Si [3, 4]. As to AA6111, the sequence of precipitation is believed to initiate with the metastable phases, β″ and β′ leading to the equilibrium β phase. The structure and composition of the β phase have been well established to be of the fluorite structure with a composition Mg2Si [5–7]. Recent works also report the presence of a quaternary phase, Q and its metastable precursor, Q′ in the precipitation sequence [8]. The aim of this report is to find the relationship between the microstructure and the failure of the hole expanded and small angle bended samples. We will report a finding of inner surface fracture during small-radius bending due to the tensile residual stress development in the inner surface.