Effects of layer-by-layer ultrasonic impact treatment on microstructure and mechanical properties of 304 stainless steel manufactured by directed energy deposition

The layer-by-layer ultrasonic impact treatment (UIT) is applied during the directed energy deposition (DED) process of 304 stainless steel. The microstructure and mechanical properties of the samples with and without the UIT are analyzed. The results show that a transition from coarse columnar grains to fine equiaxed grains in the overall 304 stainless steel deposited layers is achieved. This is attributed to recrystallization. Due to the mechanical effect of UIT, intense plastic deformation and a large number of dislocations are generated on the deposited layer surface. Combined with the thermal action of subsequent deposition, the recrystallization is facilitated. After the UIT process, the average microhardness increases from 126.3 HV_0.2 to 171.3 HV_0.2. Simultaneously, the average yield strength increases from 368.9 MPa to 401.8 MPa, the average ultimate tensile strength increases from 589.7 MPa to 622.9 MPa, and the average elongation increases from 40.7% to 46.9%. This can be attributed to grain refinement strengthening, dislocation strengthening and the reduction in the size and number of pores. This study can offer an idea for effectively improving the mechanical properties of additive manufactured parts.