The Investigation Of Microstructure And Mechanical Properties And Numerical Simulation Of Wire Arc Additive Manufacturing WE43 Magnesium Alloy

Magnesium alloy has the advantages of high specific stiffness,specific strength,thermal and electrical conductivity,damping and electromagnetic shielding,etc.It can save energy and reduce emissions,and bring significant lightweight benefits.It is widely used in aerospace,rail transit,military industry,3C and other fields.Casting technology and deformation processing are the traditional forming methods of magnesium alloy.Due to the strong oxidation tendency of magnesium itself,the quality of die-cast magnesium alloy components is low,which limits its application in key fields.Magnesium alloy has good deformation mechanical properties,but it is limited by the complexity,efficiency and cost of machining workpieces.At present,the proportion of engineering applications is still low.In the past 30 years,the development of additive manufacturing technology has attracted much attention,especially the wire arc additive manufacturing(WAAM)technology which can integrate and rapidly form large-size integrated metal components.WAAM technology based on arc cladding layer by layer has the advantages of fast metal filling speed,high efficiency,low additive cost,unlimited additive size,etc.It has obvious advantages in rapid prototyping of medium and large parts with low complexity.As a classic rare earth magnesium alloy,WE43 magnesium alloy has excellent thermal stability,high temperature creep and biocompatibility,and is widely used.In order to study the forming condition of the deposited workpiece,this paper explores the forming process parameters of WAAM WE43.In order to understand the influence of multiple thermal cycles on the microstructure and mechanical properties of WAAM part,observe its microstructure and test its mechanical properties.The whole WAAM process is simulated by finite element analysis to determine the evolution law of temperature field and stress field in each stage.In order to overcome the problem of insufficient performance of WAAM WE43 and improve its engineering application potential,the appropriate process parameters of different heat treatment methods were explored.In order to realize strengthening and analyze the mechanism,the microstructure and mechanical properties of WAAM WE43 sample after heat treatment were compared.The following research results are obtained:(1)WAAM WE43 magnesium alloy has remarkable microstructure characteristics and good mechanical properties.The prepared WAAM WE43 magnesium alloy has uniform microstructure,and the grain size is about 20.3 μm,but the microstructure is different in different areas.Further analysis proves that due to the nonlinear conduction and accumulation of heat input in the process of layer-by-layer stacking,the second phase in the non-equilibrium solidification structure of molten pool metal is promoted to dissolve back.The macro-feature of the dissolution is that the structure in the top circular arc area gradually changes to the middle stable area.The difference of microstructure makes the overall hardness distribution different.The average hardness of the single multi-layer parts is 73 HV,while the average hardness in the top arc area is 68 HV.The mechanical properties test results show that the tensile and compressive properties of WAAM WE43 samples are similar in horizontal and vertical directions,and there is no obvious anisotropy.Its tensile properties UTS(Ultimate tensile strength),YS(Yield strength)and EL(Elongation)are 235.3MPa,165.0 MPa and 16.7%respectively,which is better than that of the as-cast and rolled alloy,and its fracture is better.(2)The temperature field and stress field in WAAM WE43 process are simulated by finite element method.The simulation results can be well fitted with the actual measured temperature cycle curve,which indicates that the established single-channel single-layer and single-channel 15-layer model has a good effect.In the single-pass single-layer deposition process,the molten pool is gradually opened and then stabilized,the initial circular temperature field is gradually elongated into an ellipse,and then the heat is gradually transferred to the periphery of the substrate for rapid cooling,and the highest temperature of the stabilized molten pool is about 1050℃.The post-cooling stress is distributed in the deposition part and extends around the substrate,and the red highest stress area appears at the junction of the deposition part and the substrate.Besides,in the process of single-pass multilayer deposition,when the number of deposition layers is small,the temperature field spreads on the deposition part and the substrate.When the number of deposition layers increases and deposition height increases,temperature field gradually shrinks to the deposition part,peak temperature between layers increases from 1029℃ to 1283℃ due to the accumulation of heat.The influence range of electric arc is in the 3-layer deposition,and the 1-2 layer is in the semisolid state which is not completely melted.The stress cycle also shows the peak value and the lowest stress value of the corresponding times,and the stress value of the material in the melted state is 0 MPa.The distribution of stress values in different paths shows a trend of first increasing and then decreasing,which has certain symmetry.As the number of deposition layers increases,the maximum temperature of heat transferred to the substrate gradually decreases,resulting in the overall stress value decreasing.However,along the deposition direction,the stress protrusion will appear at the interface between the deposition part and the substrate,and the higher the deposition number,the more obvious this phenomenon will be.(3)WAAM WE43 magnesium alloy was treated by traditional post-heat treatment methods(including T4,T5 and T6).In samples treated by T4 condition,the lamellar and granular second phases distributed along grain boundaries or within grains can be effectively dissolved,the element distribution is more uniform,the stress concentration at grain boundaries is reduced.The plasticity of the deposited parts is improved,and the elongation is increased from 16.7% to 22.3%.Number of the second phase of samples after T5 treatment is the largest.Compared with the as-deposited state,the mechanical properties are improved but the plasticity is decreased.UTS,YS and EL can reach 308 MPa,210 MPa and 10.8%respectively,and the strength is anisotropic.After T6 treatment,the microstructure of samples has the largest precipitation of the second phase with aging,forming precipitation strengthening.Rockwell hardness increases from 73 HV to 105 HV,UTS,YS and EL can reach331 MPa,237 MPa and 9.3% respectively,and the strength is increased by nearly 31%compared with that of the deposited state.After heat treatment,WAAM WE43 has excellent performance,which is comparable to or superior to that of as-cast and SLM alloy,but its comprehensive mechanical properties are still low.Fracture morphology features: in T4 state,the cracks usually originate in the flocculent second phase or interface;in T5 state,the cracks originate in the reticular second phase with intergranular fracture characteristics;in T6 state,the cracks originate in the blocky second phase precipitated by aging,with intergranular and transgranular mixed fracture characteristics.