Investigation on solidification cracks in pulsed laser spot welding of an AZ31 magnesium alloy

Abstract

A direction-dependent model of solidification crack initiation and propagation is established to analyze the influence of the melt pool shape on the distribution of solidification cracks in pulsed laser spot welding. The formation direction of the initial crack is related to the variation of the solidification parameters. When the aspect ratio of the spot welds with a default Rectangular pulse is greater than 0.3 and less than 0.8, an initial crack is formed in the 45° direction of the mushy zone, and then the crack propagates along the grain boundary. That is, the solidification cracks are distributed in the direction of 45° on both sides of the surface center of the welding cross-section. When the aspect ratio of the spot welds is less than 0.3 or greater than 0.8, an initial crack is formed in the 90° direction of the mushy zone, and then the solidification crack is vertically distributed at the center of the welding cross-section. The Step-Down-10ms-0.3P pulse can significantly reduce the solidification rate of the melt pool interface, preventing the formation of initial cracks in the sensitive directions of the spot welds with an aspect ratio less than 0.8, thereby eliminating solidification cracks.