Mechanism And Process Of Laser Cleaning Of Paint On Aluminum Alloy Surface

In aerospace,marine ships,high-speed rail and other fields,manufacturers are seeking for a greener method to remove paint.In this context,the paint removal method by laser cleaning gradually received attentions in industry.At present,the depainting methods from material surface are mainly chemical depainting and mechanical depainting,which have the disadvantages of polluting the environment,high labor intensity and endangering health of staff.Considering the existing problems of traditional paint removal methods,this thesis studies the laser cleaning technology to achieve a green,efficient and high-quality paint removal method.In this thesis,various colors of polyurethane paint on aluminum alloy surface were cleaned by laser cleaning.Blue and red polyurethane paints with representative cleaning phenomena were selected to carry out laser cleaning experiments,exploring the mechanisms between laser and paint in cleaning process,and analyze the impact on the properties of aluminum alloy substrate during laser cleaning.Firstly,two laser cleaning theoretical models were established,and the calculation methods of laser cleaning threshold of the two paints with different absorption characteristics were defined.The cleaning threshold of blue paint with high absorptivity is 0.71 J/cm²,and that of red paint with high transmittance is 1.08J/cm².The cleaning thickness of blue paint can be well predicted by heat transfer model.The average prediction error is 2.04μm.Through the calculation of laser energy distribution model,the removal thickness of the two paints can be predicted precisely.The average calculation errors of blue and red paints were 3.49μm and 1.06μm,respectively.It is proved that the energy distribution model can accurately predict the cleaning thickness for high transmissvity paint.A high-speed camera was used to observe the dynamic behavior of the two paints in laser cleaning process.It was found that the high absorptivity blue paint cleaning products included smoke and particles.The main cleaning mechanisms were ablation and vaporization,accompanied by phase blasting and plasma blasting,etc.The high transmittance red paint was splashed off in blocks.The main removal mechanism was the thermal stress caused by the increase of substrate temperature,which leaded to paint stripping,accompanied by plasma blasting effect.During the laser cleaning of red paint process,the spectrum of aluminum peak was more easily detected,which predicted aluminum alloy was more easily to be damaged.In laser cleaning process,the energy is constantly accumulated through the continuous action of multi photons,which can break the molecular bond.Meanwhile,the paint burns and releases energy,breaking the chemical bonds of polymer compounds and forming plasma.Through orthogonal test,the influence degree of each factor on cleaning results and optimized process parameter range were obtained.Through the single factor experiments,it was found that when the laser fluence was low or the laser head moved fast,residual paint appeared on the aluminum alloy substrate surface.When the laser fluence was high or the laser head moved slowly,the aluminum alloy substrate was easy to be damaged.When the laser fluence is 3.2 J/cm² and the laser head speed is 6mm/s,the paint on aluminum alloy surface can be completely removed without obvious damage to the substrate.Red paint cleaning process window is smaller than that of the blue paint because that the red paint has higher transmission rate.The laser energy can easily penetrate through the paint layer to the aluminum alloy,causing damage to the substrate surface.The surface morphology,composition and chemical bond of the substrate after laser cleaning were analyzed to reveal the substrate surface morphology evolution mechanism during laser cleaning.After cleaning at laser fluence of 4.8 J/cm²,6.4J/cm² and 8.0 J/cm²,the aluminum alloy substrate formed three surface feature morphologies due to the different melting time and melting thickness of substrate.The microstructure of the substrate surface layer before and after laser cleaning were explored.The substrate surface properties after cleaning were tested and analyzed.It was found that the substrate surface after laser cleaning at high laser fluence was reacted with oxygen in air to form a melting layer,which mainly contained oxides such as Mg O,Mg₂O₃ and Al₂O₃.Micron scratch tests were used to test the critical load between the substrate and the paint layer after laser cleaning.The critical load increased when laser fluence was higher than 4.8 J/cm².Nano indentation tests were used to study the behavior of substrate surface mechanical properties after laser cleaning.When the laser fluence was higher than 4.8 J/cm²,the microhardness of substrate surface decreased,and the wear resistance of the surface also decreased.This is due to the formation of melting layer and heat affected layer on the substrate surface under laser ablation.The heat affected layer is recrystallized,leading to the decrease of dislocation density,resulting in the decrease of surface hardness and wear resistance of substrate surface.Electrochemical impedance spectroscopy was used to study the corrosion behavior and corrosion law of the substrate surface after laser cleaning.For the aluminum alloy surface after cleaned at 3.2 J/cm² and 4.8 J/cm²,nano synapses were formed on the surface to enhance the hydrophobicity,thus increasing the corrosion resistance of the surface.