The ZK60 magnesium alloy plates were welded by laser beam welding (LBW) and the microstructures in the partially melted zone (PMZ) of welded joints were investigated. For the as-cast alloy, the eutectoid mixtures along grain boundaries (GBs) in the PMZ are liquefied during welding, and their re-solidified materials present hypoeutectic characters, which lead to more severe segregation of the Zn element along GBs, and thus enhance the cracking tendency of the PMZ. The main reasons for liquation cracking of PMZ are described as that the absence of liquid at the terminal stage of solidification leads to the occurrence of shrinkage cavities in PMZ, from which liquation cracking initiates, and propagates along the weakened GBs under the tensile stress originating from solidification shrinkage and thermal contraction. Lower heat input can reduce the cracking tendency, and the plastic processing such as rolling also contributes to the mitigation of PMZ liquation cracking by reducing the size of eutectoid phases and changing their distribution in the base metal.
Fine-grained ZK60 magnesium alloy sheets of 2.0 mm in thickness were successfully joined by laser beam welding （LBW）. The effects of welding parameters including laser power and welding speed on the microstructures and mechanical properties of the joints were investigated. A sound bead, with the ultimate tensile strength （UTS） of 300 MPa and elongation of 12.0%, up to 92.5% and 65% of those of the base metal, respectively, is obtained with the optimized welding parameters. No liquation cracking is visible in the partially melted zone （PMZ） owing to the inhibitory action of the fine dispersed precipitates and the fine-grained microstructure in the as-rolled magnesium alloy sheets. The fusion zone （FZ） is featured with the equiaxed dendritic grains of the average grain size about 8 μm, which are similar to those in the heat affected zone （HAZ）, and this contributes to the relatively high joint efficiency.
The characteristics of the welding molten pool of AZ91 magnesium alloy were studied and the welding interface model was built using metallographic observation and scanning electorn microscope(SEM) composition analysis.The welding area was divided into heat affected zone(HAZ),liquefaction zone(LZ),rich poly zone(RPZ),sparse zone(SZ) and weld zone(WZ).The analyses of the microstructure and composition of each region show that optimizing the welding process can improve mechanical properties of weld zone.While for LZ,its tensile properties can be strengthened only by improving the composition of the parent metal and the second phase distribution.And the way to improve the tensile properties of LZ,RPZ,and SZ is to give priority to improve the parent metal composition and the second phase distribution,improving welding technology as the complementary method.Furthermore,based on the results above and the analyses of the microstructure and composition of welding cracks,it is found that the tensile fracture is mainly caused by the stress cracking rather than composition crack.
