Results and discussions Laser treatment is a process in which a pulsed laser beam instantly vaporizes or melts aluminum alloys, forming circular concave spots on the surface (Fig.2). When the pulse width and frequency are kept at 100 ns and 80 kHz, respectively, the circular spot size remains essentially unchanged with the number of laser treatment times. The entire area is gradually covered by circular spots, and only a thin oxide layer is detected around the spatter particles on the surface following laser treatment (Fig.4). The roughness of the substrate increases linearly with the number of laser treatments (Fig.5). When the number of laser treatments is five, the surface roughness is equivalent to that of sandblasting (Rˉ_a is approximately 4.3 μm). The wetting behaviors of the TL-W017 and TL-19A coatings on the aluminum alloy surface following laser treatment differ significantly. After laser treatment, the contact angle of the TL-W017 coating with higher viscosity is between 65° and 73°, which is similar to that of sandblasting treatment, indicating that there is no significant correlation between the contact angle and the number of laser treatments. A much smaller contact angle is obtained for the TL-19A paint, whose viscosity is lower after laser treatment than after sandblasting (Table 2). The adhesion grades of the TL-W017 and TL-19A coatings on the laser-treated aluminum alloy surface reach level 0, whereas that of the TL-37 coating reaches level 1, indicating excellent bonding properties (Fig.7).

A 6061 aluminum alloy coated with four-proof paint is a commonly used structural component in aerospace products. To improve the bonding strength between the coating and substrate, the aluminum alloy surface must be roughened. Conventional sandblasting causes significant deformation of thin-walled aluminum alloy products. Previous research has shown that laser treatment can improve surface roughness. However, our experience indicates that the deformation of thin-walled parts is significant when a relatively large pulse width and low pulse frequency are used during laser processing. Considering the aforementioned difficulty, the present research utilized a nanosecond laser to roughen the surface of the 6061 aluminum alloy using an appropriate energy input while reducing deformation. The effects of laser treatment on the microstructure and roughness of the aluminum alloy are evaluated, and the wetting behaviors and adhesion strengths of several typical paints are measured. The overall objective of this study is to verify the feasibility of laser roughening as an alternative to sandblasting for thin-walled parts.

A pulsed fiber laser with an average output power of 100 W is used, and the laser frequency and pulse width are controlled at 80 kHz and 100 ns, respectively. The number of laser surface treatments for the 6061 aluminum alloy is 1?10 times. For comparison, 100-mesh white corundum sand is blasted, where the pressure is 0.2 MPa during sandblasting. Surface metallography and 3D profiles are observed following surface treatment. Scanning electron microscopy (SEM) is used to examine the microstructure of the surface, and the elemental distribution is tested using an energy dispersive spectroscopy (EDS) module. The levels of roughness before and after laser treatment are measured. The surface phase compositions are determined using an X-ray diffractometer. Two typical paints, TL-W017 and TL-19A, invented by the institute of aerospace materials and technology, are selected to comparatively study their wetting behavior on a laser-roughened aluminum alloy.

In this study, laser treatment technology is used to roughen the surface of the 6061 aluminum alloy thin-walled parts and is compared with the conventional sandblasting process. The vaporization or melting of aluminum alloys is triggered by a pulsed laser beam that forms circular concave spots on the surface, and a thin oxide layer is generated on the surrounding splashing particles. The surface roughness increases linearly with the increase of laser treatment times, and when five times are reached, surface roughness is close to that of the sandblasting treatment (R_a is 4?5 μm). The surface contact angles are different for the three types of coatings and correlate with the laser treatment times. The adhesion between the three typical coatings and the 6061 aluminum alloy following laser roughening treatment is level 0 or 1, indicating excellent adhesion strength. Our work demonstrated that laser surface roughening is a promising replacement for sandblasting as a pretreatment process for organic coating spraying on aluminum alloys.