Study On Automatic Melting Welding Technology Of AZ31B Magnesium Alloy

Owing to the advantages of low density,higher specific strength,good casting performance magnesium alloys have referred to as the"21st century green engineering material",and been widely used in automobiles,electronics,high iron,aerospace and other fields.The magnesium alloy industry is one of the key supporting industries in China.Welding technology is the key to promote the development of magnesium alloy industry,and it is also the bottleneck technology,which restricts the development of magnesium alloy.Therefore,it is of great theoretical value and practical significance to carry out the research of magnesium alloy welding technology and to promote the industrialization of magnesium alloys.In this paper,based on the current problems of magnesium alloy welding technology,the AZ31B magnesium alloy was chose as the research object.This paper systematically studied the welding processes of AZ31B magnesium alloy by using argon arc welding?TIG?and laser beam welding?LBW?equipped on the six axis robot.And the microstructures,precipitates,texture evolution,and mechanical properties of the welded AZ31B alloy joints before/after heat treatment process have been discussion.1.The automatic arc welding process of AZ31B magnesium alloy was studied.By analyzing the welding parameters such as welding current,welding speed,wire feeding speed and shielding gas flow,the optimal welding process was obtained.The welded joints have no pores,inclusions,microcracks,and other welding defects.And the surface of the welding seam is smooth,uniform and regular.The TIG welding process resulted in coarse recrystallized grains in the heat affected zone?HAZ?and in epitaxial growth of the columnar grains in the fusion zone?FZ?.Substantial changes of texture between the base material?BM?and the FZ were detected.The{0002}basal plane in the BM was largely parallel to the sheet rolling plane,whereas the c-axis of the crystal lattice in the FZ inclined approximately 25°with respect to the welding direction?WD?.The maximum pole density increased from 9.45 in the BM to 12.9 in the FZ.In HAZ and FZ,a large number of precipitated phases were observed,and the electron spectroscopy?EDS?and X-ray diffraction?XRD?results confirmed that the precipitated phase was?-Mg₁₇Al₁₂.2.The low power pulse Nd:YAG laser equipped on a six-axis welding robot is used to weld the AZ31B alloy.Compared to TIG welding,the welding seam of laser welding is obviously narrow?only about 2 mm?.The transverse cross-sectional microstructure of the AZ31B alloy joint was composed of base metal?BM?,columnar grains zone?CGZ?,and equiaxed grains zone?EGZ?in the center of the fusion zone?FZ?.Some pores in the FZ were observed and believed to be related to the pre-existing pores in the BM.Especially,it is noted that many columnar grains observed in the transverse cross-section of the FZ were almost perpendicular to the solid-fusion boundary.Nevertheless,a large number of fine equiaxed grains formed on the top surface of the FZ and exhibited obvious random orientation.The different grain growth mechanisms between transverse cross-section and top surface of the weld bead have been empirically discussed.Owing to apparent grain coarsening,the lowest microhardness value?⁴3 HV?was detected in the center of the FZ,which was evidently lower than that in the BM?⁶8 HV?.3.The different post-weld heat treatment processes on the joints of AZ31B magnesium alloy were studied.The joints of AZ31B magnesium alloy were heat treated after welding at 260 ^oC-1 h,350 ^oC-20 min and 400 ^oC-1 h,respectively.The results showed that after the heat treatment of 260 ^oC-1 h,the number of the?-Mg₁₇Al₁₂2 precipitates were reduced and the grains of the fusion zone?FZ?and the base material?BM?slightly increased,compared to untreated specimen.After heat treated at 350 ^oC for 20 min,the?-Mg₁₇Al₁₂2 precipitates are sharply reduced,and the grain size in BM and FZ were significantly larger.After the treatment of 400 ^oC-1 h,the number and size of the precipitation phase were obviously increased,and the grains in BM and FZ were obviously coarsened.The results of tensile test show that after treatment of 260 ^oC-1 h and 350 ^oC-20 min,the tensile strength and elongation of the welded joints were significantly improved.