In this study, two innovative surface severe plastic deformation (SSPD) methods, namely abrasive waterjet peening (AWJP) and ultrasonic nanocrystal surface modification (UNSM), were applied to a 304 stainless steel to improve the mechanical behavior. The surface roughness, microstructure, residual stress, hardness, and tensile mechanical properties of the alloy after the two SSPD treatments were studied systematically. The results show that both the AWJP and UNSM treatments have greatly positive effects on the mechanical-properties improvements by successfully introducing a hardening layer. Especially the UNSM-processed specimen possesses the most outstanding comprehensive mechanical properties (high strength with the comparable ductility). The yield strength with the UNSM treatment is 443 MPa, corresponding to the 109% and 19% improvements, as compared to that of the base (212 MPa) and AWJP-treated specimens (372 MPa). The results can be attributed to a much thicker hardening layer (about 500 μm) and a better surface integrity with lower roughness (Ra: 0.10 μm) formed by the UNSM technique.

Abrasive waterjet peening (AWJP) as an important surface strengthening method can effectively improve surface properties. In this study, after multiple AWJP, the distribution of compressive residual stress and roughness on the surface of 18CrNiMo7-6 steel has been evaluated by an X-ray diffraction (XRD) method and a 3D surface topography system, respectively. Compared with the single AWJP, multiple AWJP can obviously increase the surface residual stresses (−1024 MPa to −1455 MPa) and the depth of maximum compressive residual stress (100 μm to 120 μm), as well as make the stress distribution more uniform. In terms of the surface roughness, multiple AWJP influences its uniform distribution and reduces the surface roughness (S_a = 0.69 μm), compared with a single AWJP (S_a = 2.96 μm), due to the smaller shot balls and a uniform deformation during multiple AWJP. In addition, we have studied the effects of multiple AWJP on the hardness of the surface layer. The results show that multiple AWJP increases the hardness by up to 15.9%, compared to the single AWJP. These studies provide useful insight into improving the surface properties of 18CrNiMo7-6 steel by multiple AWJP.

Due to the processing characteristics of laser cladding, the beads and joints of the laser cladding coatings have different grain sizes. We used the laser ultrasonic (LU) method to measure the distribution of the grain sizes of laser cladding coatings nondestructively. The surface acoustic wave amplitude was influenced by the grain size; hence, the amplitude varied for beads and joints because of their different attenuations. The spatial resolution of LU is higher than that of the traditional ultrasonic testing method, leading to a fringe distribution of the C-scan results in the scanning area. The LU results were verified through metallographic experiments. It was concluded that the LU method can be used to determine the grain sizes of coatings.

Ultrasonic nanocrystal surface modification (UNSM) was applied to hot-rolled Mg-Li alloys (LAE361 and LA106). The microstructure, mechanical properties, deformation mechanisms, and corrosion resistance properties of these alloys after UNSM treatment were systematically studied. Significant improvement in surface hardness and decrease in surface roughness were achieved by UNSM treatment. Meanwhile, the basal texture intensity of the Mg-Li alloys reduced significantly, and several deformation twins appeared on the surface layer. The α phase of the surface layer underwent twin deformation and basal plane slip. The fibre textures in the β phase of LA106 Mg-Li alloy changed from γ and η to α and ε, which mainly resulted in the dislocation slip. More importantly, UNSM treatment exhibited enhanced strength and improved plasticity of LAE361 and LA106 Mg-Li alloys. The corrosion current density of LAE361 Mg-Li alloy reduced approximately 29.3% by UNSM treatment, while it increased the corrosion current density of LA106 Mg-Li alloy by 189.7%. These studies show that the application of UNSM to improve the corrosion resistance of duplex phases of LA106 Mg-Li alloy needs further investigation.