Study On The Weldability And Metallurgical Performance Of AZ31B Alloy

Magnesium alloy has a series of merits,especially as a light-quality engineering structure material,it attracts great attention.The processing technique of magnesium alloy also becomes the hotspot on the research. Because of the low melting point,the high heat condution and the thermal expansion coefficient and so on,the weld performance of magnesium alloy is bad.Aimed to solve the problems of the magnesium alloy joining,the research on the basic theory of its weld and metallurgy performance is still a key content.This paper traverses the weld and metallurgy performance of the magnesium alloy of its there common welding methods - the resistance spot welding SW,the tungsten inert-gas welding TIG and the melting inert-gas welding MIG.The weld performance of the resistance spot welding on the thickness of 2 mm AZ31B magnesium alloy was studied.The well welded joints of the resistance spot welding were obtained.The optimal weld processing parameters were weld current 4.85 kA,weld time 1.3 s,squeeze time 0.44 s, cool time 0.4 s,hold time 0.4 s,off time 0.4 s.The base metal,the heat affect zone and the nugget in the SW joints were clearly divided.The base metal and the nugget were segmented by the ringed plasticity zone of the HAZ.The mixed microstructure of dendrite and equiaxed grain in the nugget was found. The maximum of the shearing force of the SW welded joints reached to 2300 N.When welding the thickness of 8 mm AZ31B magnesium alloy by TIG, the same kind of matters and phases-α-Mg matrix and Mg₁₇Al₁₂precipitate was achieved in the weld metal zone and the base metal.But their contents were different;the intensity peak value of the base metal was higher than the weld metal zone.Along with the increasing of the weld current,the size of the crystal grain in the HAZ and weld metal zone became coarser;the quantity of the intermetallic compound-Mg₁₇Al₁₂became more.And Mg₁₇Al₁₂was distributed in the grain boundary and inner grain dispersedly. However,the changing extent of the weld metal zone was little than the HAZ. The tensile strength of the AZ31B magnesium alloy and its butt welding joints was measured,respectively 228.76 MPaå’Œ149.31 MPa,so the tensile strength of the welded joints was 65.27%of the base metal.Meanwhile,the micro-hardness of the HAZ was lower than the base metal,the least was 42 HK;but the micro-hardness of the weld metal zone was highest in order to the more intermetallic compound,reached 95 HK.The tendency of the weld hot crack of the AZ31B magnesium alloy under the TIG welding was bigger.The hot crack was discovered in the weld metal,which started from the upper surface of the weld metal zong.The main crack was intercrystalline extending,and there were some branches in both sides.The branches were mixed extending ways-intercrystalline and transcrystalline.The formation of the weld hot crack was mainly effected by the weld processing parameters.While the weld current was over 170 A,the hot crack was appeared in the weld specimen.When the weld current was less than 160 A,there were no any cracks.The weld metal zone of every specimen was observed after the Y-groove cracking test,the hot crack was found in the two specimens whose weld heat input was higher.The extending characteristic of the weld hot crack was similar with the upper;the main crack also belonged to the intercrystalline fracture;there were branches crack during the extending.But the difference was that the branches belonged to the transcrystalline extending.The formation mechanism of the hot crack was alike.There were also no any crack on the surface and inner the weld metal zone of the no-crack specimen after deposited 48 h.Therefore,there was little trend of the TIG welding cold crack for AZ31B magnesium alloy.When welded AZ31B magnesium alloy with MIG welding,the grains of the fusion drops were fine and uniform;there were splash and undercut;the molding of the weld bead was inferior to the TIG welding.The optimal weld processing parameters of the thickness of 10 mm AZ31B magnesium alloy were weld current 250 A;weld voltage 27 V;weld velocity 13.37 mm/s; shielding gas flux 25 L/min;wire extention 15 mm.Compared with TIG welding,the intermetalline compound was continurally distributed in the grain boundary and its quantity was increased,the crystal grain were finer; and the precipitate were presented as pieces inner grain.The grain boundary of the weld metal zone were remedied by the dispersed Mg₁₇Al₁₂,so the extending of the weld hot crack was limited,the crack was decreased.The maximum of the microstructure in the weld metal zone of MIG welding was 102 HK that was higher than the TIG welding.The electro-chemical property in the 3.5%NaCl neutral solution of the AZ31B magnesium alloy and its TIG and MIG weld metal was studied.The E_corrand I_corrof the base metal were -1.425 V and 1.093×10^-2A.The E_corrand I_corrof the TIG weld metal were -1.489 V and 1.598×10^-2A.The E_corrand I_corr of the MIG weld metal were -1.497 V and 1.645×10^-2A.Therefore,the corrosion resistance of the weld metal was worse than the base metal;and that of MIG welded joint was poorer than the TIG.