Basic Study On Wire-arc Additive Manufacturing Of Magnesium Alloy Fabricated By Cold Metal Transfer Heat Source

Magnesium alloy,as the most potential light metal structural material,has the advantages of high specific strength and specific stiffness,which contributes to the realization of lightweight in the industrial field.Magnesium alloy is sensitive to heat input process and has poor plastic deformation ability during the manufacturing process.Traditional casting and forging technology for magnesium alloy has many difficulties,which can not meet the needs of large size and complex structure of magnesium alloy components in the industrial field.Wire-arc additive manufacturing(WAAM)technology has the characteristics such as low cost and high efficiency.Cold metal transfer(CMT)welding arc is helpful to achieve the additive manufacturing of large-size magnesium alloy components due to the low heat input and high depostion rate.In this paper,the CMT heat source and the wire of AZ31 magnesium alloy were used to study the basic technology of WAAM for magnesium alloy.The bead on plate trials were carried out to study the weldability of CMT depostion for magnesium alloy under different welding parameters.The results show that the wettability and grain size of the weld seam both increased gradually with the increase of wire feeding speed(WFS)from 5 m/min to 8 m/min.When the WFS was5 m/min,the wettability and interface bonding of weld seam was not good,which was not suitable for the additive manufacturing.Single-pass multilayer forming experiments were carried out to investigate the macrostructure,microstructure and mechanical properties of Mg alloy WAAM thin-wall component.The results show that the thin-wall component has uniform surface without the overflowing and collapsing of the welding pool.The microstructure of the WAAM thin-wall component showed obvious layer characteristics.The deposited layer was mainly composed of columnar grains zone,equiaxed grains zone,and coarse equiaxed grains(heat-affected zone,HAZ)zone from the bottom to the top.The thin-wall component exhibited anisotropic tensile properties.The tensile strengths and elongations in the deposition direction and in the building direction were 225.7 MPa and 28.3%,188.3 MPa and 17.2%,respectively.The mechanical properties of the WAAM component are similar to those of cast and forged components.The band distribution of the HAZ is the main reason for the anisotropic tensile properties.The effects of different process parameters on the microstructure characteristics of WAAM thin-wall components for Mg alloy were analyzed.The results show that,due to the heat accumulation during the forming process,The WFS and interlayer cooling time(ICT)have significant effects on the microstructure characteristics of the center and edge areas of the deposition layers at different heights of the thin-wall components.For the 30 th layer,with the increase of WFS from 6 m/min to 8 m/min,the thickness and average grain size of the HAZ increased from 270 ?m to 632 ?m and from 92 ?m to 239 ?m,respectively.But the thickness of the columnar grain zone decreased slightly.With the increase of ICT from 30 s to 120 s,the thickness and grain size of HAZ did not change significantly,but the thickness of columnar grain zone increased from 0 ?m to 1200 ?m.These results show that the WFS has an obvious effect on the grain size of HAZ,but has slight effect on the thickness of columnar grain zone.ICT has slight effect on the grain size of HAZ,but has significant effect on the thickness of columnar grain zone.The finite element analysis of the thermal process of WAAM thin-wall components for magnesium alloy were performed using ABAQUS software.The results show that the difference of thermal cycle process at the different positions of components under different processing parameters was the main reason resulting in the obvious layer characteristics for microstructure.The WFS could significantly increase the peak temperature of the molten poolmaking the microstructure coarsening obviously,but has slight effect on the columnar crystal morphology.Extending the ICT can significantly increases the temperature gradient during solidification of molten pool and changes the morphology of columnar grains,but has slight effect on the grain size of deposited weld seam.In summary,the thermal accumulation effect of CMT WAAM for magnesium alloy is the fundamental reason for the difference of microstructure characteristics in different regions of the components.Controlling the thermal process is the key process to improve the quality of fabricated component.