Study On Manufacturing Process And Properties Of AZ61 Magnesium Alloy CMT Welding Arc Additive Manufacturing

Magnesium alloys have the advantages of high specific strength,high modulus of elasticity,good heat dissipation,good shock absorption,strong impact load resistance and good electromagnetic shielding,etc.They have a broad application prospect in automotive,rail transit,aerospace and medical fields.Currently,the production of large size and complex structure of magnesium alloy components is an important trend for its application in order to further achieve the goal of lightweighting.Additive manufacturing has the advantage of design freedom.Forming complex structural parts can achieve the minimum resource waste and minimum energy use.Among them,cold metal transition welding arc additive manufacturing technology（CMT-WAAM）is the most suitable technology for magnesium alloy welding arc additive manufacturing due to low heat input and effective avoidance of oxidation and burnout during the additive manufacturing process.In this study,AZ61 magnesium alloy,one of the most widely used deformed magnesium alloys,is investigated for its CMT-WAAM process and performance.Firstly,the molding process of AZ61 magnesium alloy was studied,and the effects of deposition current,deposition speed,deposition angle,and preheating temperature on single pass single layer molding were analyzed.When the deposition current was130 A,the deposition speed was 0.4 m/min,deposition angle was less than 90°,and the preheating temperature was 200℃,the single-layer single-pass deposition process was stable,with slight spatter,no undercut phenomenon,smooth deposition channel,and better forming characteristics and microstructure.Secondly,the research on multilayer single-pass molding process was carried out,and the influence law of process parameters on macroscopic molding,microstructure and mechanical properties of molded parts was mainly investigated.When the deposition current was 130 A,the deposition speed was 0.4 m/min,deposition angle was 90°,and the interlayer residence time was 1 min,the forming effect of the formed parts was the best,with good surface flatness and high dimensional accuracy.The microstructure of the magnesium alloy consists ofα-Mg matrix and eutectic structure formed byα-Mg andβ-Mg₁₇Al₁₂.Under different process parameters,the morphology of eutectic structure changes between network,elongated and block.Under the process parameters of deposition current of 130 A,deposition speed of 0.4 m/min,deposition angle of 110°,and interlayer residence time of 1 min,the best tensile properties of the formed parts were obtained with tensile strength and elongation of 228 MPa and 22.1%in the horizontal direction and 224 MPa and 18.4%in the vertical direction,respectively.Moreover,under various process parameters,the tensile properties do not exhibit significant performance anisotropy in the horizontal and vertical directions.The multi-layer single-pass inclined thin-wall test was carried out under the optimal molding process parameters,and the molded parts with good surface flatness and high dimensional accuracy were successfully prepared.The inclined angle of 30°is the limit of the inclined angle,at which the tensile strengths in the horizontal and vertical directions were maintained at 96%and 93%of those in the untilted direction,respectively,which proved that the CMT-WAAM has the potential to manufacture complex magnesium alloy components.Finally,the T4 and T6 heat treatment processes of AZ61 magnesium alloy were investigated.T4 heat treatment was carried out by one-stage solution,and after the solution treatment,the microstructure composed of eutectic structureα-Mg andβ-Mg₁₇Al₁₂ in the microstructure was completely solid-soluble into theα-Mg matrix,at which time the diffusely distributed Al₈Mn₅ particle phase remained in the microstructure.Solution treatment for 2 hours has the best effect:in the horizontal direction,the tensile strength and elongation are 245 MPa and 20.9%,respectively;In the vertical direction,the tensile strength and elongation are 244 MPa and 25.3%,respectively.The tensile properties of AZ61 magnesium alloy molded parts after T6heat treatment are better than those of T4 heat treatment.When the solution time is 2hours and the aging time is 2 hours,the tensile properties of the molded parts are the best:in the horizontal direction,the tensile strength and elongation are 258 MPa and22.5%,respectively;In the vertical direction,the tensile strength and elongation are255 MPa and 20.4%.The T6 heat treatment of AZ61 magnesium alloy after additive manufacturing can further improve the performance.The strengthening mechanism is mainly based on precipitation strengthening.