4. Optimization of Substrate Performance When utilizing a laser to remove the oxide film or paint layer from an aluminum alloy surface, a controlled cleaning can be achieved by adjusting the laser process parameters, such as laser beam power, energy density, and scanning speed. Interestingly, optimizing the laser process parameters can efficiently remove oxide films or paint layers from aluminum alloy surfaces and improve the performance of the substrate. 4.1. Corrosion Resistance The corrosion resistance of the aluminum alloy surface is improved after the oxide film or paint layer is removed by using a laser, and the corrosion resistance of the aluminum alloy is better than that obtained via mechanical cleaning. By comparing the substrate corrosion resistance and the mechanisms after the mechanical and laser cleaning, the research found that the aluminum alloy corrosion resistance after laser cleaning is higher than that obtained after mechanical cleaning because the grain size did not change [66]. However, the aluminum alloy was thermally oxidized after the laser cleaning, the surface was remelted, and the initial MgAl2O4 on the aluminum alloy surface was decomposed to obtain a new nanostructured layer comprising MgO and Al2O3 [67], with making the element distribution uniform and the refining of the grains [68]. This result can be clearly seen from the statistics chart of the grain size of aluminum alloy substrate after the laser cleaning (Figure 7), the average grain size of the whole region was 7.98 μm, and the percentage of grains with less than 10 μm accounted for 65% of the total. The average grain size of the upper region was 7.30 μm, and 73% of the total number of grains were smaller than 10 μm in diameter. These results lead to the difference between the grain boundary and intragranular composition minimal, lowering the corrosion current density [69]. Zhu et al. [13] conducted electrochemical experiments and verified that the laser cleaning does not reduce the corrosion resistance of aluminum alloy surfaces, and that the improvement of corrosion performance is related to energy density. Figure 7. Statistics of grain size of aluminum alloy substrate after laser cleaning. (a) electron backscatter diffraction results; (b) entire region; (c) upper region [68].
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4.2. Welding Performance Weldability of aluminum alloy is increased by using the laser cleaning to remove the oxide films from the aluminum alloy surfaces. The researches found that the laser cleaning can remove most lubricants and contaminants attached to the surface, eliminate hydrogen and other gases generated by coatings, lubricants, and surface contaminants, and reduce the source of hydrogen generated by micropores during welding, so as to significantly improve the welding quality [8,62,70,71]. By comparing the weld seam porosity on an aluminum alloy surface after three pretreatments, Zhou et al. [62] found that after laser cleaning in air, the weld seam porosity of the aluminum alloy was reduced to 2.91% from the maximum value of 9.68% when the surface was untreated. After the laser cleaning in argon, the weld seam porosity was further reduced to just 1.59%. Haboudou et al. [70] also obtained similar results, as shown in Figure 8. With all surface preparations, the porosity rate tends to be lowered at both welding speeds, but laser cleaning has the most noticeable impact, practically completely suppressing pores in A356 (the residual porosity rate is less than 2%) and decreasing them by a factor of two on AA5083. Furthermore, Chen [72] found that after the laser cleaning, the oxide film on aluminum alloy surfaces formed a micro-nanostructure, and the welding depth initially decreased and then increased to a gradual level during welding, which was 23.4% higher than that of the uncleaned surface and 10.1% higher than that of the mechanically polished surface. Figure 8. Influence of surface preparation on the porosity rate with two different welding speeds at 4 kW in (a) A356 and (b) AA5083 [70]. 4.3. Adhesive Performance The paint layers are removed from aluminum alloy surfaces for maintenance and respraying, and the removal effect of the paint layer is the main factor to be considered. The adhesion performance of the resprayed paint layer is also an issue following laser cleaning. The study showed that the adhesion of the substrate coating is significantly improved after the laser cleaning, and it is superior to the
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mechanical grinding [68]. The primary reason is that the substrate surface roughness decreases after the laser cleaning, and, consequently, so does the coating shear force under the same load. Furthermore, the laser ablation treatment produces a new oxide layer with a higher surface activity on the aluminum alloy substrate [71,73], promoting the complete wetting of the adhesive and enhancing the bond strength. 4.4. Other Performances The laser cleaning improves the corrosion resistance, welding performance, adhesion performance, tensile and bending properties, surface microhardness, and friction and wear performance of the aluminum alloy [35,73,74,75,76]. Wang et al. [35] found that the surface microhardness of the 7075 aluminum alloy increased by 5.62–8.45% as a result of the shockwaves generated by plasma explosion and air and water explosion in the strip defect, as shown in Figure 9. Yuan et al. [76] used a pulsed laser to remove the oxide layer from the surface of an AA2024 aluminum alloy used for the aerospace industry. The results showed that the laser cleaning could improve the friction and wear performance of the AA2024 surface by altering its microstructure. Furthermore, the laser cleaning produced a hardened layer on the AA2024 surface and increased its microhardness. Figure 9. (a) Microhardness indentation and test point position; (b) change in microhardness values before and after laser cleaning [35].
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