Facing the long-term problem of poor corrosion resistance of magnesium alloys, it is considered that preparing coatings on the surface of magnesium alloy substrates is an effective means. Dense coatings can effectively isolate Mg alloys from external corrosive environments. Compared with coating preparation techniques, such as thermal spraying, which requires molten material, cold spraying avoids the melting and recrystallization of materials, high-temperature oxidation, stress cracking, and other problems. However, the surface of the coating generally has high roughness, which has an adverse effect on the anti-corrosion performance. Laser shock peening can effectively improve the surface morphology of the cold-sprayed coating and reduce the roughness of the coating surface. Simultaneously, problems, such as oxidation and material cracking, caused by thermal effect are avoided, maintaining the advantages of cold spraying. In addition, laser shock peening can improve the residual stress state of the coating surface and increase its microhardness.

In this paper, the mechanism of the effect of multiple laser shock peenings (LSPs) on the surface morphology of cold-sprayed pure aluminum coatings is discussed. A pure aluminum coating is prepared on the surface of a magnesium alloy by cold spraying. A nanosecond-laser transmitter is used to shock the surface of the cold-sprayed coating. The laser pulse energy is 4 J, spot diameter is 3 mm, and overlap rate is 50%. The effect of LSPs with different impact numbers on the surface morphology, roughness value, phase, residual stress state, and microhardness of the coatings are studied. The surface morphologies of the coatings before and after laser shock are measured using a confocal laser microscopy. The 3D and 2D profiles are used to describe the change in the coating surface morphology before and after laser shock, and the evolution process and mechanism of the coating surface morphology during laser shock are analyzed. The phase composition of the coating before and after laser shock is detected using an X-ray diffractometer. The effect of laser shock peening on the phase retention ability of a cold-sprayed pure aluminum coating is studied. The states of the residual stress on the coating surface before and after laser shock are measured using an X-ray stress tester. The effect of laser shock on the transformation of residual stress on the surface of the cold-spray coating is investigated. The hardnesses of the coating surface before and after laser shock are measured using a microhardness tester. The effects of different impact numbers on the microhardness of the coating surfaces are compared.

The original cold-sprayed coating without laser shock has a rough surface morphology. There are obvious micro-peak and micro-valley morphologies on the surface. After laser shock, the surface maintains good integrity, although the coating is not metallurgically bonded (Fig. 6). The sharply protruding micro-peak area on the surface suffers severe plastic deformation and sinks after being crushed by the laser shock wave. The micro-valley area is reduced under the plastic deformation and extrusion of the material. The original uneven surface morphology of the coating is flattened owing to the sinking of micro-peaks and micro-valleys . With an increase in the number of laser shocks, the degree of surface flatness gradually improves (Figs. 7 and 8). The line and surface roughness values of the coating surface continuously decrease (Fig. 9). The X-ray diffraction (XRD) patterns reflect the phases of the coating surface before and after laser shock. The results demonstrate that no high-temperature oxidation occurs on the coating before or after laser shock (Fig. 10). Owing to the high thermal expansion coefficient of pure aluminum, the volume of the particles is easily affected by heat, and there is low residual tensile stress on the surface after spraying. After laser shock, the residual stress on the coating surface gradually transforms from tensile to compressive (Fig. 11). Owing to the severe plastic deformation of the coating surface layer caused by laser shock, a deformed hardened layer forms on the surface layer, the surface microhardness gradually improves, and the ability of the material to resist plastic deformation improves. However, the ability of the subsequent impact to produce plastic deformation of the coating is attenuated; thus, the increase in microhardness gradually decreases with an increase in the number of impacts (Fig. 12).

In this study, the effect of laser shock peening on the surface morphology of cold-sprayed pure aluminum coatings is investigated. The specific results are as follows: 1) After laser shock peening, the coating surface maintains good integrity. The originally uneven coating surface is flattened by the rolling deformation of the laser shock wave, and the surface morphology is reshaped. The surface roughness value gradually decreases with an increase in the number of laser shocks. After three impacts, the surface and line roughness values decrease from 31.85 μm and 21.39 μm to 12.88 μm and 8.87 μm, respectively. 2) After laser shock peening, the coating does not exhibit serious oxidation phenomenon, and the material maintains the original powder characteristics. With an increase in the number of impacts, the residual stress on the coating surface transforms from tensile to compressive, the surface microhardness value gradually increases, and the maximum surface microhardness reaches 51.67 HV. 3) With an increase in the number of laser shocks, the surface roughness value decreases, together with the magnitude of each drop; the increase in the microhardness value demonstrates the same trend. The effect of the first laser shock is the most obvious; the effects of subsequent shocks continuously weaken, and the overall effect increases with the increase of the number of shocks.