Using a flattened top laser to remove paint from metal

Laser cleaning is an environmentally friendly cleaning technique and has attracted much attention in the industry. Due to its superior characteristics, laser cleaning has proved to be very successful in the cleaning of polymers, rubber tire molds, large mirrors,magnetic head sliders,artworks and historical heritage pieces, and semiconductor chips with surface particles. However, the early application of laser cleaning techniques had a relatively low cleaning efficiency. Although there has been successful applications of laser cleaning to remove paint from a metal surface, the technique has its disadvantages.
For example the cleaning speed is relatively slow due to the small cleaned area, owing to small laser spot size. In addition, overexposure of the Gaussian laser pulses may result in substrate damage due to its high energy density beam.

Therefore, a more effective laser cleaning technique requires not only a high cleaning efficiency but also no damage to the substrate. Chen et al found that the cleaning efficiency of removing a paint coating from a substrate was dependent on the laser power and overlap ratio. Daurelio et al. demonstrated that the paint removing efficiency was related to the laser power and the paint type. However, utilization of a conventional Gaussian beam laser for cleaning has its own disadvantages because the substrate can easily be damaged especially in the centre of the beam spot while the paint at edge of the area cannot be removed, which reduces the efficiency. If a higher energy laser is used, more serious damage to the substrate is induced by a laser beam. Therefore it is necessary to investigate the laser cleaning techniques not only forimproving the cleaning efficiency but also to minimize the damage to critical surfaces in the cleaning process. Schweizer and Werner mentioned that a flatten top beam profile is better than a Gaussian beam during paint stripping of aircraft using a TEA CO2 laser. However they did not do experimental and theoretical studies on the influence of the beam profile in the work. Francois et al. investigated experi-mentally the influence of laser fluence, repetition rate and pulse duration effects in the laser cleaning paint process.

In this paper, we investigate the effect of flattened top laser cleaning and compare the efficiencies when using a flattened top laser and a Gaussian profile laser. In order to meet the requirements of industrial large surface cleaning applications, we numerically optimize the laser cleaning control method to enhance the coating cleaning rate and reduce the substrate damage in the two-layer coating-substrate system. In the experiment an efficient laser cleaning strategy is presented by studying the iron surface modifications induced by the irradiation of a laser in air.

2. Numerical simulations

2.1. Thermal conduction theory

The geometry coordination of laser irradiation on a coating-substrate system is schematically shown in
Fig. 1. The system includes two layers, a paint coating and a substrate, which can be analysed by the thermal stress in the layer. The coating is paint deposited on an iron substrate. The spatial mode of the laser beam is assumed to be a nearly flattened top distribution. The temperature distribution in the x and y direction is symmetrical, so we discuss only the x direction in the next analysis for simplicity. The thermal conductive equation is described as where , c, and k are the density, thermal capacity,
and thermal conductive coefficient respectively; T represents the temperature distribution at time t. In the coating layer and substrate the thermal equations are the same.

3. Experiment

3.1. Experimental setup

A schematic diagram of the experimental set-up for the laser cleaning system is depicted in Fig. 4. The
acousto–optic Q-switched Nd:YAG laser is used with a 1064-nm wavelength and 25-mJ pulse energy. The pulse duration is 200 ns–400 ns and the repetition rate can reach 3 kHz. In order to obtain a generally flattened top laser beam, in the optical path we lay lenses each with a spherical aberration, and then set an aperture to make the centre of the beam pass through. The lens system comprises lens with a negative spherical aberration. Different lenses with different coefficients may be selected to adjust the diameter of the laser spot Laser cleaning is carried out to remove the surface paint coating from an iron substrate. The iron substrate used in this experiment has a generally smooth surface with micro texture. By observing the changes of surface texture the influence of laser irradiation can be identified. In other words, the substrate damage level can be detected. The surface coating is painted with a thickness of about 50 m, which is sprayed evenly. The repetition rate is relatively high, so the scanning mirror apparatus is utilized to separate the laser spot. The coupon is fixed on the mechanical platform which can move in the X and Y directions on the 2D platform.

3.2. Experimental results

The cleaning results of Gaussian laser irradiation and nearly flattened laser irradiation are shown in
Fig. 5, respectively. The micrographs of the experimental results show that the surface of the coupons after laser irradiation is restructured with different levels. Figure 5(a) shows that the central area of the Gaussian laser spot is seriously damaged, which results from the unevenness of the laser profile. A concave in the centre of the laser spot can be obviously seen. These illustrate that near the centre of the laser spot, the temperature of substrate is higher than the melting point of iron, so a melting and re-solidifying ring can be observed. Meanwhile at the edge of the laser spot the coating layer is vaporized or vibrated. However, due to the uniform intensity central region of the flattened laser, the surface of the substrate is cleaned uniformly, so that there is no obvious substrate damage in Figs. 5(b)–5(d). There is no obvious vaporizing area in the central spot.

We also ran experiments using flattened laser spots with different diameters (0.4 mm, 0.7 mm, and
1.0 mm) separately. Comparing the three experimental results of the different flattened laser spots, different phenomena can be observed as shown in Fig. 5. Due to the difference in thermal expansion between the coating layer and the substrate, the heating and cooling rate of the substrate are faster than those of the coating layer. Hence the transient surface heating and cooling can provide a compressive shock force on the coating layer and remove the coating layer away from the substrate. In the first experiment of diameter 0.4 mm, the area near the spot centre is clean and is patterned into ring grooves. Meanwhile the substrate is melted and re-solidified. In the second experiment of diameter 0.7 mm, almost all of the substrate surface is covered with micro convexes. In the third experiment of diameter 1.0 mm, the micro structures of surface are not changed apparently. Moreover, the surface colours are also different in the three embodiments. In the first experiment the exposed substrate area near the centre of laser spot has clear colour. On the contrary, the colours of the latter two experiments are dark due to oxidation. This dark is a mixed colour of yellow Fe2O3 and black FeO and Fe3O4. In the first experiment of a flattened top laser, due to the fact that the melting iron is rapidly solidified, a ring structure can be recorded under the effect of stress and edge intensity peak. The ring structure is shown in Fig. 5(b). However, in the experiments of 0.7 mm diameter and 1.0 mm diameter, the ring structure does not appear. This can be explained as follows. In experiment of 0.7 mm diameter the laser power intensity is lower than that of the experiment of 0.4 mm diameter. The substrate can melt, but the melting deepness is very shallow, and the laser induced stress cannot form the ring structure. Only some re-solidified dots are left. Furthermore in the experiment of diameter 1.0 mm, the temperature of substrate iron is lower than the melting point. The micrograph shows that the surface of the iron substrate after laser irradiation is covered with a dark layer. The dark layer is an oxide layer which actually acts as a passivation protection layer.
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