In this study, weldability of dissimilar metal butt joint of A6061 alloy and AZ31 Mg alloy sheets of 3mm in thickness was investigated using fiber laser welding.

First, laser welding without filler metal changing some welding conditions of laser power, welding speed and laser irradiation position, was performed. As a result, obtained direct butt joint without filler metal showed crackfree and full-penetrated weld bead with smooth surface within the limited range of laser power of 4～6kW and welding speed of 4～12 m/min at laser irradiation position just on the butt joint interface. However, joint tensile strength was 20～30 MPa irrespective of welding conditions due to brittle intermetallic compounds of Al₃Mg₂ and Al₁₂Mg₁₇ in the weld metal and the interface of weld metal and Mg alloy. Offsetting of laser beam irradiation position toward Al alloy side was beneficial to decrease the melting ratio of Mg alloy in the weld metal, but no remarkable increase of joint tensile strength was obtained due to comparably thick intermetallic compound layer as 12～25 micron meter along the fusion boundary of Mg alloy.

Next, to inhibit decreasing joint tensile strength by Al-Mg based intermetallic compounds, laser welding with Zn filler metal was performed. Zn filler metal sheet set at butt joint interface showed remarkable increase of joint tensile strength up to 133 MPa with adequate welding conditions by separating Al alloy and Mg alloy from melting mixture by forming an Zn-rich intermediate weld metal.