Objective Aluminum alloy is widely used in aerospace, automobile manufacturing, and other fields. An oxide layer easily forms on the surface of aluminum alloy, which seriously affects the welding quality. As a result, there is an urgent need for high-quality removal technology. Compared with traditional cleaning methods, laser cleaning has obvious advantages in cleaning effect and process flexibility. Laser cleaning changes the surface morphology and roughness of aluminum alloy and seriously affects subsequent processing, such as welding and painting. At present, most researchers have only compared the differences in the surface morphology of aluminum alloy before and after laser cleaning. However, they have not systematically explained the reasons for the changes in morphology and roughness. Therefore, this article investigates aluminum alloy to explore the correlation between laser cleaning process parameters and surface morphology and roughness. A laser cleaning process window for the natural oxide layer on the surface of 6061 aluminum alloy was established. Based on this window, the process parameters were optimized to maximize surface roughness, which provides process guidance for adjusting the surface morphology of aluminum alloy.
Methods This article uses a pulsed laser with a maximum power of 100 W (YDFLP-100-LM1) for laser cleaning experiments. The laser wavelength is 1064 nm, and the spot diameter is 70 μm. In this article, the fixed pulse width is 100 ns and the repetition frequency is 100 kHz. This paper uses 6061 aluminum alloy with a thickness of 2 mm as the substrate. Single-factor experiments are used to study the influence of average power, scanning speed, and line spacing on the laser cleaning effect. After laser cleaning, the FEI Sirion 200 scanning electron microscope was used to characterize the surface morphology, the energy spectrometer was utilized to test the changes in surface element content, and the OLYMPUS DSX 510 three-dimensional microscope was employed to test the changes in surface roughness. Furthermore, this paper uses the response surface analysis method to optimize the process parameters within the established process window.
Results and Discussions When the average power is 15 W, only small craters are formed on the surface. Most areas do not have ablation craters of the same size. With an increase in power, apparent crater overlap morphology is gradually formed. When the power is increased to 75 W, the laser energy is too large, resulting in severe thermal ablation, and a large amount of metal splash destroys the lap morphology of the craters. As the power increases from 15 W to 75 W, the roughness increases from 0.608 μm to 1.636 μm. Consequently, the variation in aluminum alloy surface morphology with power can be divided into four stages (Fig.6). Since the scanning speed and line spacing change the positioning between adjacent laser spots, the changes in their surface topography exhibit similar laws. When the scanning speed is 2000 mm/s, and the line spacing is 0.02 mm, the surface is relatively flat, and the overlap marks of the craters are not noticeable. With increased scanning speed and line spacing, the crater morphology with regular arrangement can be observed with an increased degree of morphology fluctuation. As the scanning speed and line spacing increase, the roughness first increases and then decreases. As a result, the change in aluminum alloy surface morphology can be divided into three stages (Fig.10). Through the element content test, with the oxygen content below 2% as the indicator, the laser cleaning process window for the natural oxide layer on the surface of 6061 aluminum alloy is established as follows: the average power is between 30 W and 60 W, the scanning speed is between 3000 mm/s and 5000 mm/s, and the line spacing is between 0.03 mm and 0.05 mm. Based on the response surface analysis method, the function of the surface roughness change after laser cleaning is formulated. According to the analysis of variance results, average power, scanning speed, and line spacing influence roughness in the increasing order of significance. The optimal process parameters obtained theoretically include a laser power of 60 W, a scanning speed of 4950 mm/s, and a line spacing of 0.041 mm. The three verification experiments confirm that the results have high credibility.
Conclusions In this paper, the influence of laser cleaning on the surface morphology of 6061 aluminum alloy was studied. The average power affects the crater morphology, which in turn changes the surface roughness. As the laser power increases, the roughness gradually increases. The scanning speed and line spacing affect the lap morphology of adjacent craters, thereby changing the roughness. As the scanning speed and line spacing increase, the roughness first increases and then decreases. According to the results of the elemental energy spectrum test, it is found that the process parameter window for laser cleaning the natural oxide layer on the surface of 6061 aluminum alloy is as follows: the average power is between 30 W and 60 W, the scanning speed is between 3000 mm/s and 5000 mm/s, and the line spacing is between 0.03 mm and 0.05 mm. Based on this window, a function model for the surface roughness change of 6061 aluminum alloy was established. The process parameters are as follows when the surface roughness reaches the maximum: the average power is 60 W, the scanning speed is 4950 mm/s, and the line spacing is 0.041 mm. Elemental energy spectrum test results demonstrate that the relative content of surface oxygen is less than 1%, which meets industrial application requirements.
