Mitigating solidification cracking during the laser welding of extruded Al–Mg–Si alloys by tailoring the microstructure

Abstract

This study proposes a novel approach for mitigating the internal solidification cracking that occurs during the laser welding of extruded Al–Mg–Si alloys. The microstructures of the extruded alloy, such as the peripheral coarse grains (PCGs) distributed on the surface and the fibrous grain distributed internally, are closely related to these cracks. Therefore, to investigate the effect of the texture of the base metal on internal cracking, an as-extruded alloy with PCGs distributed on the surface and an alloy with the PCGs removed through surface machining were used as the base metal for overlap welding. Furthermore, welding was performed by a coaxial dual-beam laser with four different core-to-ring power ratios to determine the effect of this ratio on the formation of internal cracks. Then, cross-sections of the weld specimens were analysed to compare the cracks, pores, and microstructural texture, such as the grain size, morphology, and grain orientation. Machining away the PCGs on the surface resulted in the formation of columnar grains with low-angle grain boundaries, and it reduced the number of internal cracks that formed along the fusion line. Moreover, dual-beam laser welding followed by surface machining led to the formation of low-angle grain boundaries along the fusion line and grain refinement in the weld centre, thereby mitigating internal solidification cracks by tailoring the microstructure of the base and weld metal.