Study On Microstructure And Mechanical Properties Of Magnesium Alloy Fabricated By Cold Metal Transfer Wire Arc Additive Manufacturing

Magnesium alloy is widely used in 3C,automobile,aerospace and other fields due to its excellent performance.The traditional forming methods for magnesium alloys mainly focused on casting and extrusion processes.However,when magnesium alloys are cast and formed,there are often defects such as coarse particles,shrinkage cavities,and slag inclusions,which reduce the mechanical properties of magnesium alloy parts.At the same time,it is difficult to rapidly form complex magnesium alloy parts by using extrusion forming because this process needs to be carried out at a relatively high temperature.Therefore,the traditional forming method of magnesium alloys has not been able to meet the needs of the integral forming of complex large-scale magnesium alloy parts,which has considerably limited the development of magnesium alloys.Therefore,a new forming method is urgently needed to produce complex magnesium alloy parts.Additive manufacturing is extremely suitable for forming complex structural components because of its layer-by-layer stacking characteristics,which provides a better choice for magnesium alloy forming.However,the use of the selective laser melting technology to form magnesium alloys is not only inefficient but also limited by the forming size.Furthermore,because of the high chemical reactivity of magnesium and the relatively high surface area of the powder,the risk of forming is high.Wire arc additive manufacturing uses wires to form structural components.This not only improves the forming efficiency but also considerably reduces the risk of forming.In this paper,AZ31 and Mg-Gd-Y-Zn-Zr alloy wires were taken as research objects,and the wires were deposited into complete deposited wall by wire arc additive manufacturing technology,and the microstructure evolution of magnesium alloy deposited wall was studied.Further,the mapping relationship between process parameters,microstructure,and mechanical properties was established to reveal the solidification behavior characteristics of the wire arc additive manufacturing process,and the strengthening mechanism of wire arc additive manufacturing of AZ31 and Mg-Gd-Y-Zn-Zr alloys was obtained.Based on this,the influence of Mg₂₄Y₅inside Mg-Gd-Y-Zn-Zr alloy on the nucleation of magnesium atoms in the matrix was studied by first-principles calculation method,and the interfacial bonding strength was calculated.In addition,the effects of different heat treatment mechanisms on the microstructure morphology,precipitated phase distribution,grain orientation and texture strength of Mg-Gd-Y-Zn-Zr deposition were analyzed.The main research results are as follows:（1）The microstructure and properties of the additive-substrate interface bonding zone and the additive zone of AZ31 magnesium alloy deposited walls were studied.Due to the repeated heating under the heat source during the additive manufacturing process,the longitudinal section of the additive zone had obvious non-uniform features.The grain size of the longitudinal section of the additive zone increased from 4.27μm to 9.51μm from the bottom zone to the top zone.The phases were composed ofβ-Mg₁₇Al₁₂phases distributed along the grain boundaries andη-Al₈Mn₅phases dispersed on theα-Mg matrix.The mechanical property anisotropy of the cross section was stronger because of its higher fiber texture strength.At the same time,the longitudinal section microhardness of the additive zone increased from 74.22 HV to 81.46 HV from the top zone to the bottom zone.The tensile strength of the WAAM AZ31 magnesium alloy in the vertical direction was 286.47MPa,and its elongation rate was 15.89%,which was considerably greater than the ultimate tensile strength of the cast and the extruded AZ31 magnesium alloy.（2）The microstructure and properties of the additive-substrate interface bonding zone and the additive zone of Mg-Gd-Y-Zn-Zr alloys deposited walls were studied.The microstructure of the deposited walls showed obvious layered characteristics.In each layer of deposited metal,the bottom zones had a higher cooling rate than the top ones,which led to the supersaturation of rare earth elements.The precipitated phase in the whole deposited walls gradually decreased from the bottom zone to the top zone due to the influence of multiple thermal cycles.Due to the thermal effect of the arc on the substrate zone,the precipitates on the surface part of the substrate zone transformed from the 18R-LPSO phase to the 14H-LPSO phase.The ultimate tensile strength（UTS）of the deposited walls along the travel direction was 227.27 MPa,and the elongation（EL）was 8.08%.Compared with the cast Mg-Gd-Y-Zn-Zr alloy,the deposited wall showed higher tensile properties.（3）The materials properties of Mg and Mg₂₄Y₅were calculated using first principles,and their energy band structures and density of states were analyzed.The interfacial binding work and energy of Mg/Mg₂₄Y₅interface were studied,the bonding and interfacial bonding strength were discussed,and nucleation analysis was performed.The bonding strength of the model interface is greater than the interfacial energy betweenα-Mg/Mg melts,but part of the interface energy can be released by introducing lattice mismatch,which made Mg₂₄Y₅become the nucleating base ofα-Mg.（4）The effect of solution aging treatment on the microstructure evolution and mechanical properties of Mg-Gd-Y-Zn-Zr alloy deposited wall fabricated by wire arc addition manufacturing was studied.With the increase of solution time,the grain boundaries become obvious,and there is a significant phenomenon of grain growth.The hardness of the deposited sample will decrease after the solution time exceeds 8h.With the increased of aging time,the number of newly precipitated fine granular phases increases significantly.When the aging treatment time reached 12h,lamellar LPSO phases appeared in the interior of the grains,and the fine granular phases distributed within the grains transform into LPSO phases.Under the heat treatment condition of 520℃×4 h+220℃×12h,the maximum hardness reached 84.36 HV.At this time,the internal grains of the deposited sample were the smallest,and there were more lamellar LPSO phases inside.（5）The effect of solution aging heat treatment on the grain orientation and texture of Mg-Gd-Y-Zn-Zr alloy deposited walls were studied.The heat treatment provides sufficient driving force for the grain boundary movement of the deposited walls to allow the grain to grow.After solution aging heat treatment,the low angle grain boundaries of the sample transform into high angle grain boundaries,indicating that a large number of dislocation recovery occurred to strengthen the texture composition of rare earth.The grains grew with preferred orientation,which made the grains with similar orientation incline towards a certain texture,resulting in more concentrated texture components,the texture strength was also higher.In addition,the heat treatment effectively reduced the dislocation and internal stress of the deposited walls,which had a certain impact on improving its plasticity.