Improving mechanical properties of austenitic stainless steel by the grain refinement in wire and arc additive manufacturing assisted with ultrasonic impact treatment

In this study, the ER321 stainless steel is fabricated by wire and arc additive manufacturing (WAAM) assisted with ultrasonic impact treatment (UIT). The mechanical properties and microstructure characterizations of ER321 stainless steel with and without UIT are investigated. It is found that grain structure of ER321 stainless steel is reduced to equiaxed dendrites (with UIT) from coarse columnar dendrites (without UIT). The UIT effectively refines grain sizes that decreases by -150% and homogenizes the grain structure of the deposition layers. Simultaneously, the UIT also facilitates the recrystallization that leads to the reduction of dislocation and texture densities. Furthermore, the improvement of grain structure enhances yield strength (-10.5%), ultimate tensile strength (-3.7%), microhardness (-12.5%) and elongation of ER321 stainless steel. The grain boundary strengthening is the main strengthening mechanism, which leads to the yield strength increment of ER321 stainless steel under UIT condition. A way to effectively control grain structure in additive manufacturingfabricated metal products using an UIT technique is provided in this work.