Microstructure and Residual Stress Formation during Friction-Stir Welding of Semi-Solid Cast ZK60-RE Magnesium Alloy

Abstract

In this work, we report on the friction stir weldability of a semi-solid cast ZK60 alloy modified with 1.5 wt% mischmetall in the lap-joint configuration using a 120WV4 steel tool with concave shoulder and conical pin. The coarser solidification microstructure in the semi-solid cast ZK60-1.5%RE alloy requires low strain rates and increased heat input to produce lap-joints without inner defects. This was achieved with 250 rpm tool rotation and 50 mm/min welding speed. Friction stir welding results in a very fine grained microstructure in the stir zone probably due to dynamic recrystallization. In the thermomechanically affected zones dynamic recrystallization seems to occur within the solute enriched intergranular zones. The distribution of longitudinal residual stresses exhibit stress maxima at both thermomechanically affected zones. A compression peak is observed at the retreating side, whereas a tensile stress maximum occurs at the advancing side.

Microstructure and Residual Stresses in a Friction Stir Welded Butt Joint of as-cast ZK60 Alloy Containing Rare Earths

“…Allow compression occurs at the periphery of the shoulder at the advancing side. The main heat input during FSW is caused by the friction between the tool shoulder and the sample surface and by the material flow during welding 15,16 . Thus, the highest temperatures will be achieved at the edges of the shoulder due to the highest tangential speeds generated in this region.…”