Microstructure And Properties Of Magnesium Matrix Composites Joints Produced By Laser Welding Method

The laser welding of particle reinforced SiCp/ZC71 magnesium matrix composite with 1.5 thickness was carried out using a Nd:YAG laser welding experimental system. The microstructure of laser-welded seam of magnesium matrix composites was investigated by scanning electron microscope (SEM), energy dispersive analysis (EDS). The tensile test of the laser-welded samples was carried out to test tensile strength and elongation, and the hardness was tested by the MH-3 Micro-hardness tester.The results show that laser power, scan velocity were the main parameters affecting properties on the joint, when the power of the laser p=350W, scan velocity v=350 mm/min, frequency f=60Hz, the welded seams have penetrated completely and formed nicely and theσb of the joint was up to 80.6MPa.The microstructure of the welded seam indicate that due to the concentrated laser energy and rapidly heating and cooling, heat affected zone is not apparent. The micron-sized SiC particulates are homogeneously distributed in the welded seam and small white CuMgZn particle is finer. The much lower tensile strength of the seam compared to the tensile strength of the SiCp/ZC71 magnesium matrix composite substrate attributed to some solidification micron-sized pores (especially between SiC particles and magnesium alloy matrix) and defects(crack, pore),fracture occurred in the welded seam. The fracture morphology of welded joint indicated a brittle cleavage fracture. The wt% of Mg and Zn increased from welded seam top to welded seam bottom while The wt% of Si decreased. The microhardness of welded joints was higher than that of the base metal. The weld zone mainly consists ofα-Mg, SiC and CuMgZn.The test of Laser welding adding Al powder indicates that though Al powder can protect molten pool and refine grain, but the distribution of Al powder is heterogeneous. It is not helpful for mechanical properties of welded seam and add the complexity of welding technology.It's found that the large number of pores in the joint seriously impact the mechanical properties of the joint, the pores may be reduced by adjusting the parameters and eliminating the factors of gas, but they can't be entirety avoided Magnesium has higher hydrogen solubility in liquid than in solid. The difference in the solubility between the solid and liquid phase was major cause of the porosity of magnesium alloys during welding. Crack could be found in welds. Because low melting eutectic existed in SiCp/ZC71 magnesium matrix composite and a big thermal stress resulted from the physical chemistry properties difference between SiC particles and Magnesium. However, these defects can be avoided or minimized under the condition of appropriate process parameter and suitable treatment methods before and after welding.