An Investigation Of Dissimilar Metal Laser Welding Between Magnesium And Alluminium Alloys

Magnesium alloy and aluminum alloy are widely used in aircraft and automobile industry. Because of their extensive and crossing application, it's necessary to join magnesium and aluminum alloys. The available joining methods are solid state welding at present, such as friction stir welding and diffusion welding. However these solid state welding methods are limited in application. When fusion welding methods like TIG, electron beam and resistance spot welding etc. join magnesium and aluminum alloys, intermetallic compounds (IMC) are formed in weld metal,which results in brittleness and hot cracks, hence degrades the weldability. This thesis involves an investigation of dissimilar metal laser welding between Magnesium alloy AZ31B and aluminum alloy 6061,in lap and butt joint forms respectively.The study on processing parameters of dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy indicated that the critical laser power density for deep penetration welding of AZ31B and 6061 with Slab CO₂ laser is higher than that of Nd:YAG laser. Critical laser power density for deep penetration welding of 6061 is higher than AZ31B for both lasers. The kind delivery and flow rate of shield gas influenced the welding process. The depth and width of weld increased with laser power increasing and decreased with welding speed increasing. Focus distance affected the depth, width and shape of welds greatly.It's difficult to avoid solidification cracks in center-line welded joint. Joint without cracks can be obtained by decreasing weld depth, but the strength of this kind of joint is low. There were no cracks in edge-line welded joint. It was observed that fusion line zone near Mg base metal is composed of IMC Al₃Mg₂ and Al₁₂Mg₁₇, the weld was mainly composed of Al solid solution and dispersed IMC Al₃Mg₂. Dissimilar metal butt joint was welded in the experimental conditions. There were no cracks in butt welded joint of AZ31B and 6061, too. Fusion line zone near Mg base metal was composed of IMC Al₃Mg₂ and Al₁₂Mg₁₇, while the weld was composed of Al solid solution, Mg solid solution and dispersed IMC Al₃Mg₂. The maximum tensile strength obtained is 31.84MPa. Rupture of edge-line welded joint and butt welded joint occurred in fusion line zone near Mg base metal, showing a combined cleavage and integrated fracture mode.