Research On Laser Welding Of Dissimilar Magnesium Alloys AZ31 And NZ30K

In order to research laser welding of dissimilar magnesium alloys NZ30K and AZ31, use a 15 kW CO₂ laser to join two pieces of dissimilar magnesium alloy sheets. The welding parameter was analyzed, the microstructure and the precipitated phase and weld element distribution as well as the hardness and tensile strength of the joint was studied. The research for the position of the protective gas nozzleshowed that when the gas nozzle is behind the laser beam in welding direction, the oxidation of the weld was in a lower degree. When the flow rate of the side assist gas was 25L/min, the weld formation was good. The flow rate of the back assist gas should be just enough to lift the molten metal and insulate the weld. Complete penetration of the weld was get at speed 3.5m/min, 8kW and 4m/min, 8.5 kW. The top and bottom weld width increased as the laser power increased and decreased as the laser speed increased. The optical microscope (OM) showed that the weld metal was a fusion of the two different alloys, which presented an incomplete mixture and interweaved in wave-form of each, equiaxed crystal and dendrite and lamellar were existed in the fusion zone. By X-ray diffraction (XRD) analysisα-Mg,Mg17Al12 and Mg12Nd phases were identified in the joint. The scanning electron microscope (SEM) showed regions of different chemical composition in the weld. The lamellar has a similar gradient as AZ31 and the fine dendrite has a similar gradient as NZ30K. An obvious concentration gradient of alloying elements crossing the joint was observed too. The hardness of the joint was not lower than that of the base metal, and the micro hardness of heat-affected zone of 10mm thickness joint was higher than that of the 4.5mm thickness joint. The highest tensile strength of the tensile species was 209MPa, broken at the base metal NZ30K. The tensile strength of the fusion zone and the heat-affected zone were both lower than that of the base metals. Pores were found in the weld, most of which were hydrogen pores. The crack problem was also found in weld, when heat input was too large the weld developed macro crack, the low-melting-point eutectic tend to be the crack source. The crack at grain boundary due to higher heat input was considered the reason for the decrease of the tensile strength.