Effect of material and process variability on the formability of aluminium alloys

Automotive parts are increasingly being manufactured to be lighter and stronger to minimise the environmental impact and to improve the crash performance of automobiles. The materials that are being used to achieve these aims tend to have lower formabilities compared to the traditionally used mild steel. This is particularly true for cold forming operations. As a consequence of the smaller forming window that is available, there is a greater need to understand the safety margins that are applied when manufacturing parts made from these materials. These safety margins are determined by estimations of the impact of material and process variabilities on formability as well as the attitude that is adopted towards risk. This study looked at the impact of material and process variabilities on the cold formability of two aluminium grades: AA6111-T4 and AA5754-O. The forming factors studied included changes to overall material properties, tool surface roughness, quantity of lubricant, tooling temperature and gauge. Because of the complexity of the forming process, the problem was reduced to a study of formability under plane strain stretch conditions. Particular emphasis was placed in quantifying the temperature of tooling during cold forming and understanding its effect on formability. It was found that the safety factor applied to AA5754-O can be lower than that used for AA6111-T4.