Process Optimization Of MM31 Magnesium Alloy For Wire Arc Additive Manufacturing With Weaving Path Strategy

With the advantages of high forming efficiency and low cost,wire arc additive manufacturing（WAAM）has become an important direction for additive manufacturing of large magnesium alloy parts.However,in practical applications,additive tasks with different widths and heights need to be handled,which requires different path strategies to achieve.The conventional path strategy is accomplished by overlapping multiple weld paths for area filling,which is prone to problems such as uneven surface or unfused.These problems can be solved by using weaving path strategy.The weaving path strategy is a way to make the arc in the arc additive manufacturing process use different paths of weaving action,which can increase the deposition rate,reduce the heat accumulation,and improve the surface flatness of the component.In this paper,a rare earth-modified Mg-3Al-1Zn magnesium alloy（MM31）is used as the filler material for arc additive manufacturing using cold metal transfer（CMT）technology based on the weaving path strategy,and the resulting MM31 magnesium alloy components are investigated.The effects of different process parameters on the formability,microstructure and mechanical properties of the components in the additive manufacturing process based on the weaving path strategy were investigated,and the mechanism of strengthening based on the weaving path strategy for WAAM magnesium alloys was revealed,and the relationship between process-microstructure-properties was established.The main research contents and conclusions are as follows:（1）The forming law of single-pass single-layer additive manufacturing components was investigated,and the influence law of process parameters on the evaluation methods of melting width,residual height and contact angle of single-pass deposited layers was optimized.When the heat input is in the range of 160-280 J/mm,the welding current is 60-120 A,the travel speed is 4-6 mm/s,the weaving frequency is 3 Hz,and the substrate preheating temperature is from room temperature to 200°C,a single-pass deposited layer without obvious defects,well formed,and suitable for subsequent single-pass multilayer additive manufacturing can be obtained.（2）The effects of various process parameters on the formed components for single-pass multi-layer additive manufacturing were investigated based on the good formation of single-pass single-layer additive manufacturing components.The effects of different process parameters on the macroscopic morphology,geometric characteristics,dimensional accuracy,microstructure and mechanical properties of the components were investigated to obtain single-pass multi-layer additive formed components with excellent performance.Among them,the magnitude of the deposition current directly determines the deposition rate,while it has almost no effect on the organization and properties;while travel speed directly determines the quality of the formed component,which has a great impact on the mechanical properties.When there are no high requirements for the final formed components,additive manufacturing can be carried out within a wide range of process parameters.The optimized parameters are:welding current of 120 A,travel speed of 8 mm/s,weaving amplitude and frequency of 4 mm and 3 Hz,respectively,and interlayer waiting time of 60 s.（3）The microstructures in different areas are composed ofα-Mg,Al₈Mn₅ phase together with Al₈Mn₄RE phase.The grain morphology consists mainly of equiaxed crystals and columnar crystals growing along the construction direction,and the overall structure shows a clear layered structure.The top area forms equiaxed crystal organization of uniform size due to the absence of subsequent remelting and thermal influence;the bottom area dissipates heat faster due to contact with the substrate and bench,producing columnar and equiaxed crystals of smaller size.Meanwhile,there are residual stresses in WAAM,and the distribution of residual stresses in different regions is different.The residual stresses in the bottom region are large and concentrated,while the residual stresses in the middle and top regions are less and mostly distributed along the grain boundaries;the residual stresses in the arc starting and arc extinguishing areas are higher.（4）The difference in microstructure causes the difference in mechanical properties in different areas and directions.The difference in mechanical properties in the different height areas is small,and the mechanical properties in the middle area are higher;the change in heat input and the difference in height in the arc starting area and the extinguished area lead to the change in microstructure and the generation of defects,resulting in the decrease in mechanical properties.The mechanical properties are anisotropic in the deposition direction and the building direction,and the mechanical properties along the building direction are better than those in the deposition direction.In addition,the anisotropy of tensile strength and elongation is smaller,while the yield strength is more different.The results showed that the mechanical properties of the intermediate area along the building direction were the best,with the tensile strength,yield strength and elongation of 193.8 MPa,72.0 MPa and 13.3%,respectively.