Effect of superimposed pressure on the fracture behaviour of aluminum automotive alloys

The tensile fracture of AA 5754, and AA 6111 alloys has been examined with respect to iron based inclusion content, heat treatment and stress state. For 5754 alloys, a transition in the appearance of failure from cup and cone to void sheeting was observed with increasing iron content. In these alloys, testing under superimposed hydrostatic pressure changes the fracture mode of the high iron alloy to ductile rupture in which the sample necked to a knife edge fracture. For 6111 alloys in the T4 condition, both low and high iron content alloys failed via void sheeting. When tested under pressure, the high iron alloy failed via ductile rupture. A transition in fracture mode from intergranular to void sheeting was observed for increasing iron contents in the 6111 T6 alloys. This is believed to be the result of the decrease in grain size caused by increasing iron contents. When tested under pressure, the failure of the low iron T6 alloy switched from an intergranular mode to void sheeting, due to the suppression of the intergranular fracture. No change in fracture mode with pressures up to 500 MPa was found for the high iron T6 alloy.