A fundamental study of laser welding of hot extruded powder metallurgy (P/M) AZ31B magnesium alloy

Laser welding characteristics of a hot extruded AZ31B magnesium alloy produced by rapid solidification powder metallurgy (P/M) were investigated. Extremely porous weld metals were formed under all the employed welding conditions although the base material possessed a fully dense microstructure without pores. Features of porosity morphology in the weld metals as well as X-ray transmission real-time observation results of the molten pool during welding implied that pores originated from the base material. Investigation of the chemical compositions of the base material revealed the presence of a small amount of oxygen and nitrogen. However, the high pressure applied in the initial production processes of the base material caused these gases to expand to large volumes once released when the material was melted during welding, resulting in the formation of large pores. The use of insert-layer of conventionally extruded AZ31B alloy did not improve the situation because a large amount of porosity was formed even when melting of the P/M alloy was minimized.

► Abnormal porosity was formed regardless of the employed welding parameters.
► Porosity was attributed to the expansion of gases entrapped in the base material.
► Welding with an insertion of extruded alloy could not suppress porosity formation.