Research On Active Gas Assisted Ti/Fe CMT Welding Process And Mechanism

Titanium and its alloys have the advantages of high strength,small specific gravity and good corrosion resistance,And stainless steel has the advantages of low production and processing cost,high annual output,and good thermoelectric and mechanical properties.Titanium/steel dissimilar metal joints have the advantages of both titanium and steel,and the combination of the two into heterogeneous metal structures can fully exploit the performance and economic advantages of both metals.Adding active gas CO₂ to pure Ar gas can combine the characteristics of CMT welding with the advantages of Active welding.This paper first studied the effect of CO₂ on the weld forming,fracture mechanism and mechanical properties of TC4/304L dissimilar joint.The study shows that adding active gas CO₂ in the welding process can significantly improve the weld forming of TC4/304L dissimilar joint,increase the effective connection area of the filler metal on the base metal,and especially improve the spreading wetting of Cu Si₃ on 304L stainless steel.According to the analysis of mechanical properties,compared with the joints welded under pure Ar protective gas,the joints welded under Ar-CO₂ mixed gas have higher tensile strength.The tensile strength of TC4/304L dissimilar joint increased first and then decreased with the increase of CO₂ content in the mixed gas.The fracture location of the joint also varies with the heat input and the content of CO₂ in the gas mixture.The joint welded under pure Ar protection gas broke in the middle of the weld seam.When the CO₂ content is low and the heat input is low,the joint broke at the side of 304L/seam.When the heat input is high,the joint broke at the side of TC4/seam.At the same time,the hardness of the cross section of the joint was analyzed.Compared with the joint welded under pure Ar protective gas,the hardness of the cross section of the joint welded under Ar-CO₂ mixed gas was improved to a certain extent.Then the interface microstructure characteristics of TC4/304L dissimilar joints in different protective gas were analyzed.The joints were divided into three zones:TC4/seam interface,central weld zone and 304L/seam interface.The types,size and distribution of compounds changed greatly after adding CO₂ into the protective gas.On the interface of TC4/seam,the distribution of compounds was stratified,and the distribution of elements was more obvious.The thickness of the dense compound layer changed with the change of CO₂ content.The compound distribution is more uniform in the middle of the weld because CO₂ promotes the flow of the pool.The amount of Fe-Cu solid solution increased with the increase of base metal melting on the side of 304L.Finally,the influence mechanism of CO₂ on arc shape,welding heat cycle,molten pool flow and compound formation was analyzed.Under the action of CO₂,the welding arc contracts in accordance with the principle of minimum voltage,and the peak temperature of base metal increases.The flow direction of the bath was changed by changing the surface tension coefficient under the action of active gas CO₂.Furthermore,the distribution of elements in the molten pool is affected and the morphology and quantity of compounds are changed.