Microstructure and mechanical properties of laser cladded Cr Ni alloy by hard turning (HT) and ultrasonic surface rolling (USR)

The CrNi alloy powder was deposited by laser cladding (LC) on the surface of 1045 and then processed by hard turning (HT) and combined HT with ultrasonic surface rolling (USR). USR was introduced to improve surface properties of LC layer by exerting severe plastic deformation. Microstructure, surface roughness, surface residual stress, and mechanical properties (microhardness, elastic modulus and surface fracture toughness) of the cladding layer after LC, HT and combined HT with USR were investigated by optical microscope, scanning electron microscopy, X-ray diffraction, roughness measuring instrument and nano-indenter. The results reveal that USR can neither change the element distribution on the surface of laser cladding layer nor induce the formation of new phase. After USR, the 〈111〉 texture on the surface is weakened. Fine particles layers with thickness of 2.5 μm and 2 μm are observed on the topmost surface of USR and HT samples, but not on the surface of LC samples. However, obvious grain refinement and change of grain morphology in the subsurface and interior of cladding layer are not observed in this work. The surface properties of the USR samples are greatly improved. Compared with HT samples, the surface roughness decreases by 88.5%, the surface microhardness increases by 34.6%, the surface residual stress changes from tension (449.6 MPa) to compression (−334 MPa), the surface elastic modulus changes little, and the surface fracture toughness increases by 106%. Nevertheless, USR has weak effect on the properties of y-z cross section. The elastic modulus of y-z cross section along the depth does not increase significantly and regularly. Under the experimental parameters, the hardened depth of y-z cross section is about 140 μm. Above results indicate that USR can improve the surface properties of laser cladding layer as a post-treatment process, which has a certain grain refinement and work hardening effect.