Research On Forming Process And Microstructure Properties Of Arc Additive Manufacturing 321 Stainless Steel

Wire and arc additive manufacturing（WAAM）is an advanced manufacturing technology for layer-by-layer deposition of metal materials through program control,which can produce complex shaped parts with high precision in a short period of time,reducing the complex process of traditional machining,and has attracted more and more attention in the field of industrial manufacturing.Due to the high temperature gradient,high cooling rate and reheat treatment effect in the forming process of WAAM parts,this will make the structure of the parts coarse and unevenly distributed,resulting in poor mechanical properties and anisotropy and other problems.Therefore,this paper first studies the basic forming process of arc fuse additive manufacturing,and then characterizes and analyzes the microstructure and mechanical properties of WAAM multi-channel multi-layer parts,aiming at the problem of unevenness and anisotropy of mechanical properties of WAAM parts,and improves the microstructure and mechanical properties of the parts by performing interlayer ultrasonic impact treatment during the forming process.According to the forming characteristics of multi-layer additive manufacturing of arc fuses,the influence of forming process parameters on the forming quality of single layer and single layer multi-layer was studied.It is found that adding a certain proportion of CO₂ to pure Ar shielding gas will increase the fluidity of liquid metal in the weld and improve the forming quality of the weld,but the excessive CO₂ concentration will lead to surface oxidation,and 2.5%CO₂+97.5%Ar mixture is the best choice.Compared with CMT,CMT+P arc mode welding has higher heat input,better spreadability of single welds,larger aspect ratio and smaller contact angle,which is more conducive to additive manufacturing multi-pass lap forming.With the increase of the lap rate between the tunnels,the surface flatness of single-layer multi-channel parts shows a trend of first increasing and then decreasing,and the flatness is best when the lap rate is 40%.The larger the aspect ratio of a single-pass weld,the better the surface flatness of a single layer and multiple passes.After metallographic and SEM observations of different cross-sections of multi-channel multi-layer block parts,it is found that the overall microstructure of the parts is mainly composed of austenite andδferrite with different morphologies,and there is also a small amount of Cr₂₃C₆ precipitated phase.There are typical epitaxial growth austenitic columnar crystals in the product,and their growth direction is parallel to the forming direction（Z）.As the cooling rate from the bottom to the top decreases during the forming process,the structure becomes coarse,from fine cell-like dendrite to coarse austenitic columnar crystal with obvious growth direction,the spacing between secondary dendrite arms increases from 7.3μm to 15.2μm,and the decrease of cooling rate reduces the ferrite content from 6.61%to 5.18%.The uneven distribution of the overall microstructure of the parts also led to a decrease in the microhardness from the bottom(215.6HV_0.5)to the upper part(204.1HV_0.5).The tensile and impact tests of multi-channel multi-layer parts in different directions found that their tensile and impact properties were anisotropic in the horizontal（X and Y directions）and forming directions（Z directions）.For the tensile properties,the tensile strength and yield strength in the Y direction of the part were 82.5MPa and 61.7MPa higher than those in the Z direction,respectively,while the elongation after breaking was 7%lower.For impact performance,the impact toughness in the Z direction was 30.34 J/cm² and 23.27 J/cm² higher than that in the X and Y directions,respectively.Both tensile and impact fractures are distributed with a large number of ligament sockets,and their fracture forms are ductile fractures.The microstructure of ultrasonic shock and non-ultrasonic impact parts was characterized and analyzed,and the results showed that the parts would be recrystallized under the thermal effect of the subsequent deposited layer after interlaminar ultrasonic impact treatment,so that the grains could be refined.The average grain size decreased from 358.6μm to 173.8μm,and the average grain aspect ratio decreased from 3.14 to 1.93;the grain refinement caused by recrystallization blocked the epitaxial growth of columnar crystals,transformed the coarse columnar crystals concentrated in the forming direction into equiaxed grains with random orientation,and the texture of<100>concentrated in the forming direction was weakened after ultrasonic impact,the texture orientation was more random,the maximum texture strength was reduced from 21.88 to 7.83,and the uniformity of the microstructure of the parts was improved.After ultrasonic impact treatment,the grain refinement and microstructure homogenization of the parts improved the mechanical properties and improved their anisotropy.