Study On Fiber Laser And Laser-Arc Hybrid Welding Of AZ31B Wrought Magnesium Alloy

Magnesium and its alloys are the wide used metallic structural materials due to the particular advantages, for example the low density, high specific strength and excellent sound damping capabilities. However, some special physical and chemic characteristics such as low boiling temperature, high vapor pressure and big solidification shrinkage cause great challenges in the fusion welding of magnesium alloys. The main problems are the metallurgical defects, such as underfill, excess root penetration, solidification crack, porosity and large distortion of the welded joints, which decrease the joint mechanical properties. Then, this paper aims to improve the welding properties of AZ31B wrought magnesium alloys by fiber laser and laser-arc hybrid welding. The following are the main results obtained in present study.Fiber laser welding of AZ31B wrought magnesium alloy is carried out to reveal the effects of the welding parameters on bead shape, microstructure and mechanical properties of welded joints, and the optimal welding parameters are obtained.. The good welds with fine grains, large aspect ratio and very narrow joints can be obtained. As the heat input reduces to 48 J/mm or lower, the tensile specimens are fractured in the base metal, and the maximum real joining strength reaches 265MPa, which is 113% of base metal. Besides, the least ultimate tensile strength of the joints also reaches 227 MPa, which is 94.6% of base metal.Fiber laser-MIG hybrid welding of AZ31B wrought magnesium alloy is developed and the correlations of welding parameters, bead formation, weld microstructure and joint mechanical performance are studied in this article. The results show the main metallurgical problems such as underfill and undercut which occurs in laser welding are avoided. In comparison of laser welding, the weld penetration of hybrid welding increases by 10% under the same laser power. The largest ultimate tensile strength of hybrid welds reaches 104% of base metal. In addition, with the optimal welding parameters, the joining strength of fiber laser hybrid welds is higher than that of CO2 laser welds obtained in previous studies.The processing properties and joint characteristics of pure laser and laser-MIG hybrid welding are compared in detail to evaluate the advantages of the two processes. It can be found that the porosity and coarsened grain are the main reason reducing the joining strength. However, laser-MIG hybrid welding can reduce the porosity by the appropriate parameters. The porosity reduction mechanism is summarized as following. The strong arc pressure accelerates the metal ?ow in the molten pool and pushes down the molten pool to form a concave at pool surface. Then, the bubbles in the molten pool can escape to the air easier compared to pure laser welding. As a result, fiber laser-MIG welded magnesium alloy can get the lower porosity when the welding velocity is slow and the least area proportion of the pores to whole fracture surface is 0.1%. The results show that the fracture surface of the joints obtained by the two processes is is characterized by the ductile-brittle mixed fracture, which is composed of the dimple patterns but with some cleavages.