The 7075 aluminum alloy material is widely used in stress structural parts such as the spacer frame and wing beams of a large passenger aircraft. The surface of these structural parts is coated with a protective anodic oxidation film. However, with the increase of the service life of the passenger aircraft, this anodic oxidation layer is worn or partially peeled off. Therefore, it is necessary to remove the anodic oxidation layer on these structural parts to facilitate the damage detection or apply a new anodic oxidation layer. Laser cleaning can improve the cleaning quality of aircraft structural parts because of its friendly environment, wide application, high cleaning precision, and good cleaning effect. At the same time, the process parameters for laser cleaning have an important impact on the cleaning effect and surface quality. However there is little research on the process for cleaning the anodic oxidation film of the 7075 aluminum alloy by an Yb-doped pulsed fiber laser. Therefore, the influence of process parameters on the surface morphology, elemental content, and surface roughness of the 7075 aluminum alloy after cleaning is explored here. The process parameters for cleaning the anodic oxidation film on the sample surface are obtained without influencing the surface roughness, which provides process guidance for ensuring the safety of aircraft structural parts after laser cleaning.
The 7075 aluminum alloy plate coated with an anodic oxidation film is used here. First, the IPG Yb-doped pulsed fiber laser is used, and the anodic oxidation film on the 7075 aluminum alloy surface is cleaned under different average powers, scanning speeds, and pulse frequencies. Then, the surface morphology of the 7075 aluminum alloy after cleaning is observed by optical microscope, scanning electron microscope (SEM), and laser confocal microscope, and the elemental content of the sample surface is analyzed by X-ray energy spectrum (EDS). Finally, the effects of pulsed laser cleaning average power, scanning speed, and pulse frequency on the surface morphology, microstructure, roughness, and elemental content of the 7075 aluminum alloy are analyzed. At the same time, the removal mechanism of the anodic oxidation film on the 7075 aluminum alloy surface by laser cleaning is also studied.
With the increase of the average power, the energy density increases, the molten pool formed by single laser pulse cleaning deepens, and the cleaning depth of the oxidation film increases (Fig. 5). When the average power increases from 175 W to 225 W, the energy density increases and the cleaning depth of the oxidation film becomes deeper. When the average power is 250 W and the energy density reaches 4.44 J/cm2, laser cleaning can make the morphology of the sample surface more uniform (Fig. 6). Scanning speed influences the spot bonding rate. When the scanning speed is too small, the spot bonding area is too large, the energy is too dense, and the sample surface remelts seriously and produces secondary oxidation [Fig. 7(a)]. When the scanning speed is too high, the spot overlap rate is too small, only the oxidation film in the spot lap area is cleaned, and the residual oxidation film increases [Fig. 7(d)]. When the scanning speed is 2500 mm/s, the spot overlap rate is 33.3%, and the oxidation film can be cleaned with good surface quality [Fig. 8(c)]. Pulse frequency influences the spot overlap rate and the energy density. When the pulse frequency increases, the overlap rate increases, the energy density decreases, and the cleaning rate of the oxidation film on the sample surface decreases (Fig. 9). When the pulse frequency is 2.5 kHz, the energy density is 4.44 J/cm2, and the spot overlap rate is 33.3%. The laser energy distribution is uniform, no large ablative pit appears after cleaning, and no obvious oxide film remains on the sample surface [Fig. 10 (a)]. Combined with the microscopic morphology of the sample surface after laser cleaning and the EDS analysis, the laser cleaning of the 7075 anodic oxidation film has the gasification mechanism and the elastic vibration peeling mechanism (Fig. 12). When the average power is 250 W, the scanning speed is 2500 mm/s and the pulse frequency is 2.5 kHz, the oxygen content on the sample surface is nearly close to 0. Therefore, it is considered that the anodic oxidation film on the sample surface has been cleaned (Figs. 1315). When the average power is 250 W, the scanning speed is 2500 mm/s, and the pulse frequency is 2.5 kHz, the three-dimensional topography of the sample surface fluctuates a little, and the roughness of the sample surface reaches 0.450.6 μm. Compared with that of the original sample, the roughness of the sample surface does not change significantly (Fig. 19).
The effects of laser cleaning process parameters on the microstructure, element content and surface roughness of the 7075 aluminum alloy are studied. With the increase of the average power, the energy density increases gradually, and the cleaning rate increases. With the increase of the scanning speed, the spot overlap rate decreases gradually, and the cleaning rate first increases and then decreases. With the increase of the pulse frequency, the spot overlap rate increases, the energy density decreases, and the cleaning rate decreases. With the increase of the average power, the roughness of the sample surface decreases gradually. With the increase of the scanning speed, the roughness of the sample surface decreases first and then increases. With the increase of the pulse frequency, the surface roughness increases first and then decreases. When the average power P=250 W, the scanning speed v=2500 mm/s, and the pulse frequency F=2.5 kHz, the energy density is 4.44 J/cm2 and the spot overlap rate is 33.3%. The highest cleaning rate is 98.7% and the minimum surface roughness is 0.45 μm. The removal mechanism of anodic oxidation film on the 7075 aluminum alloy surface is mainly the gasification mechanism and the elastic vibration stripping mechanism.
