Reducing the Porosity in Die-Cast Magnesium Alloys during Laser Welding

Weld porosity is a severe problem during laser beam welding of magnesium alloys. In this paper, gas content in the base metal of high-pressure die-cast AM50 magnesium alloy is measured by inert gas fusion analysis, and the high gas content, 63.4 mL/100 g (mostly hydrogen), is found to attribute to the high weld porosity during laser welding. Furthermore, effects of welding parameters on weld porosity are investigated, and a porosity prevention technique is realized by appropriately selecting laser power density and careful control of heat input. The control strategy of this technique is that the selected laser power density should be greater than 1.8 x 10(6) W/cm(2) such that the variation of weld porosity with heat input minimizes at a moderate heat input value where complete joint penetration is also ensured; however, the power density should be less than 4.1 x 10(6) W/cm(2) to avoid poor weld surface quality. Sound welds with low porosity (<5%), complete joint penetration, and good surface quality are obtained through this prevention technique, and the tensile strengths and elongations of these welded joints are also comparable to the base metal.