Thermal process and material flow during dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys

Abstract

A computational fluid dynamics (CFD) model for double-sided friction stir spot welding (FSSW) between AZ31 and ZK60 magnesium alloys using adjustable pins is proposed in this paper. Multiple phase flow theories are combined to track the metal interface and phase distribution using FLUENT software. The heat transfer and material flow for the 5 stages of the friction stir welding process, including the pre-heating, plunging, welding, rising and post-heating stages, are presented. For further evaluation, the calculated flow and thermal responses are compared with experimental data, which overall showed good agreement. The material between the upper and lower pins is softened after the pre-heating process and is then driven by the plunging pin to form a keyhole below the upper tool and a bulge region in the bottom of the plates. The welding interface between the pins is bowl-shaped after plunging, and grows uneven during the welding stage due to the extensive plastic material flow. The keyhole is then fully eliminated after the rising of the pins. After welding, the region between the tools is heated to a maximum temperature of 670 K and the welding interface slightly fluctuates and a phase mixing phenomenon occurs, suggesting that AZ31 and ZK60 plates are well joined by using the adjustable pins.