Study On CMT Welding Process Of Mg/Al Dissimilar Metals Based On The Control Of Energy Input Process

With the continuously increasing requirement of lightweight constructions,there are a large number of Mg/Al hybrid structures required to be welded in the industrial field.The problem of dissimilar welding of Mg/Al alloys has to be solved.Cold metal transfer?CMT?has the excellent characteristic of low energy input.With tightly controlled the energy input process,CMT is suited to control the formation of hard and brittle intermetallic compounds?IMCs?which are detrimental to the strength of Mg/Al dissimilar welded joint.However,the effects of energy input process on Mg/Al dissimilar CMT welding process are still unknown.In this paper,based on the study of process control of CMT welding process,the effects of energy input process on Mg/Al dissimilar CMT welding process were systematically investigated,and the relationships between the features of the Mg-Al IMCs interface layer and the joint strength were emphatically analyzed and discussed.For Mg/Al dissimilar CMT welded joints using wire ER4043 as filler metal,the hard brittle Mg-Al IMCs layers were formed in the weld interface near AZ31B substrate and consisted of three intermediate layers:Mg₂Al₃ layer,Mg₁₇Al₁₂ layer,and Mg₁₇Al₁₂+?-Mg solid solution eutectic layer.Based on the transmission electron microscopy?TEM?analysis results,the phase compositions of the Mg-Al IMCs layers were confirmed,the rules to distinguish the three intermediate layers according to element composition were established,and the relationships between crystal structure and strength of these three types of Mg-Al IMCs phase were discussed.There are significant differences among the energy input processes of the four modes of CMT welding process.During variable polarity CMT?VPCMT?or variable polarity CMT and pulse hybrid?VPCMT+P?welding process,the electrode negative?EN?CMT cycle provide a relative“cooling period”for the thermal process of weld metal in comparison with electrode positive?EP?CMT or pulse cycle.VPCMT or VPCMT+P welding process is suitable to weld Mg/Al dissimilar alloys because it can reduce the formation of hard and brittle IMCs.To study the effects of characteristic parameters on CMT welding process,the control principles of energy input process for CMT welding process were obtained.During the Mg/Al dissimilar VPCMT welding processes,with the decreased EP/EN ratio,the energy input decreased,and the thickness of the whole Mg-Al IMCs layer was gradually decreased,resulted in a significant increase in joint strength.With the increased EP/EN continuous number,there was no obviously change in joint strength for VPCMT welding process;however,for VPCMT+P welding process,the joint strength first increased and then decreased.In addition,Mg/Al dissimilar joint was effectively produced by modified CMT welding process using wire AZ31 as filler metal,and the low energy input process showed a significant pulsed characteristic.Thus,a kind of welding-brazing joint was obtained.It is noted that a diffusion Mg-Al IMCs interface layer formed in the weld interface near 6061 substrate.The diffusion Mg-Al IMCs interface layer had the similar microstructures with the Mg-Al interface layers using wire ER4043 as filler metal.The tensile strength of Mg/Al dissimilar CMT welded joint was closely related to the features of three intermediate layers.It was fairly dependent on the lowest strength of the three intermediate layers.To improve the joint,minimizing the thickness of IMCs intermediate layers to ensure each one is strong enough is essential,especially the Mg₂Al₃ intermediate layer.In conclusion,formation of the hard brittle Mg-Al IMCs layers can be controlled by the low energy input process of CMT welding process using different processing parameters.To reduce the thickness of Mg₂Al₃ intermediate layer to less than 10?m,the reliable Mg/Al dissimilar CMT welded joint would be obtained.