A gradient strengthening method of wire-arc additive manufacturing coupling with surface rolling and its application

The gradient distribution of properties, which satisfies the service requirements of critical components operating in extreme environments, plays a significant role in enhancing the durability of these components. Here, a new method of wire-arc additive manufacturing (WAAM) coupling with surface rolling was proposed to construct the gradient distributed properties. The present study focuses on a typical application of this method, wherein the surface of an exhaust valve in a marine diesel engine is processed to achieve a gradient property. The gradient strengthening mechanisms were investigated through numerical simulation and physical experiments. Simulation results demonstrated that the residual tensile stress resulting from WAAM was significantly reduced and mostly converted to compressive stress after surface rolling. The initial maximum residual tensile stress of 208.6 MPa, located at the surface of the deposited valve, was reduced to -167.8 MPa after surface rolling. The depth of residual compressive stress was extended 6-7 mm from the rolled surface to the deposited zone. Analysis of microstructures revealed that three distinct zones with different microstructural characteristics were generated through the proposed method, thereby forming the gradient microstructures. Specifically, the surface rolled zone was characterized as nanocrystallines (<1 mu m), refined grains (1-10 mu m), and deformed grains (>10 mu m), while the deposited zone and substrate corresponded to columnar grains and equiaxed grains, respectively. A wide gradient strengthening layer with an approximate thickness of 8 mm was achieved on the processed valve surface, exhibiting a hardness gradient from 314 MPa at the substrate to 566 MPa on the rolled surface. The gradient strengthening mechanisms were attributed to intrinsic material strengthening, grain boundary strengthening and dislocation strengthening.