Laser welding of die-cast AZ91D magnesium alloy

It is well known in welding of die-cast magnesium alloys that porosity is easily formed and degrades the performance of the welded joints. The porosity produced inevitably during laser deep-penetration welding has been studied through metallographic observation, gas content analyses and a high-speed X-ray transmission real-time imaging system. Porosity formation was chiefly attributed to the phenomena that the gases in the pre-existing pores in the base material were swelled to form bubbles during welding and these bubbles had few chances to escape from the weld molten pool because of the vigorous melt flows and the rapid solidification associated with high welding speeds. It was also observed that keyhole instability contributed to porosity formation by generating bubbles from a keyhole tip or a bulged keyhole. Moreover, the insertion of an extruded porosity-free alloy sheet between the die-cast alloy parts resulted in 50% reduction in the weld metal porosity.

► Many bubbles were formed behind and along the rear wall of the keyhole during welding.
► Bubbles were generated from gases in the base material and unstable keyhole.
► The vigorous melt flows and fast solidification prevented bubbles from escaping.
► Welding with an insertion of extruded alloy reduced the porosity.