Effect of intermetallic compound layer on tensile strength of dissimilar friction-stir weld of a high strength Mg alloy and Al alloy

Abstract

The friction stir weldability of a fine-grained high strength AZ3 l B magnesium alloy to A5083 Al alloy was evaluated at various welding conditions, by using a tool with shoulder diameter of 15 mm, pin diameter of 5 mm and pin length of 3.9 mm. A square butt dissimilar joint without any defect was obtained at the condition of welding speed l00mm/min, tool rotating speed 500 rpm and offset 0 mm. Higher or lower welding speeds or rotating speeds led to either the formation of defect or lack of bonding in the joint. Defects occurred also in the case that the offset was not 0 mm, i.e. the insertion position of the probe was on either Mg side or Al side, when tool rotating speed was 500 rpm and welding speed was 100 mm/min. The maximum tensile strength of the dissimilar joints in the present study was about 115 MPa, lower than that of Al alloy base metal (about 308 MPa). Transmission electron microscopy showed that an intermetallic compound (IMC) layer, which consisted of Al12 Mg 17 and Al3Mg2, formed at the bonding interface of the joints, and it was found that the formation and growth of the IMC were controlled by the react diffusion of Mg and Al atoms, instead of the eutectic reaction. The present study demonstrated that the tensile strength of the dissimilar joints was mainly affected by the thickness of IMC layer and the mechanical interlock between magnesium and aluminum alloys. The tensile strength decreased remarkably with the increase in the thickness of IMC layer, which made the mechanical interlock weaker. © 2009 The Japan Institute of Metals.