| Titanium alloy materials will undergo a series of thermal processing or heat treatment in the process of forming aerospace structural parts or components.The surface of titanium alloy is easy to combine with oxygen in high temperature environment to form an oxide layer.This oxide layer will seriously affect the subsequent processing and use of titanium alloy,so it must be cleaned up.The traditional cleaning methods mainly include mechanical grinding,sandblasting,high pressure water injection and pickling.These methods have many problems such as high labor intensity of workers,serious dust,waste of water resources,pollution of the environment and so on.It is difficult to meet the growing demand for green industrial cleaning.Laser cleaning technology,as a new cleaning technology,has the advantages of intelligent control,automatic efficiency,environmental protection and so on.In this paper,laser cleaning is proposed to remove the oxide layer generated in the high temperature environment of titanium alloy,so as to promote the application of laser cleaning technology in aviation industry.The main research work is as follows:The thickness,composition and phase of high temperature oxide layer on the surface of TC4 titanium alloy were detected.The particle size and properties of aerosol particles produced during laser cleaning were analyzed.The interaction mechanism between laser and oxide layer was clarified by macroscopic and high-speed photography,and based on the above analysis results,the establishment of finite element model of laser cleaning was guided.The effects of energy density,scanning speed and pulse width on the temperature field and the removal thickness of Ti O2 oxide layer were discussed by using the finite element model.The theoretical calculation of the cleaning threshold and complete removal threshold of Ti O2 oxide layer,the burning threshold of pure TC4 matrix and oxidized TC4 matrix were carried out.The main findings were that the scanning speed had no effect on the peak temperature,but had influence on the thermal equilibrium temperature and the time required to reach the thermal equilibrium state.The energy density and pulse width had effects on peak temperature,thermal equilibrium temperature and the time needed to reach thermal equilibrium.The removal thickness of oxide layer increased with the increase of energy density and pulse width and decrease of scanning speed.The theoretical calculation of the cleaning threshold and complete removal threshold of Ti O2 oxide layer,the burning threshold of pure TC4 matrix and oxidized TC4 matrix all increased with the increase of scanning speed,and decreased with the increase of pulse width.Under the laser parameters of pulse frequency of 600k Hz,scanning speed of 3m/s and pulse width of 5ns,the thresholds of cleaning and complete removal of Ti O2 oxide layer were 0.13J/cm2 and 2.32J/cm2,respectively.And the burning threshold of pure TC4 matrix and oxidized TC4 matrix were0.56J/cm2 and 2.88J/cm2,respectively.The results of numerical simulation were used as the guidance to carry out the process test.The effects of energy density,scanning speed and pulse width on the macroscopic cleaning quality of the samples were studied.The accuracy of the model was verified by comparing the actual removal thickness of the oxide layer with the theoretical removal thickness.The appropriate parameters were selected for process optimization,and the samples cleaned by laser were compared with those cleaned by traditional methods.The results showed that with the increase of energy density,the decrease of scanning speed and the increase of pulse width,the macroscopic cleaning quality of the sample was improved.The model was more accurate,thanks to the test measured oxide removal thickness was close to the simulation.After cleaning with appropriate laser parameters,Silvery white metal matrix was exposed on the sample surface,and the oxide was basically removed,and the oxygen content was greatly reduced.Compared with the traditional cleaning method,it was found that the flatness of laser cleaning samples was better than that of mechanical polishing and pickling samples.In addition,the microhardness of the laser cleaning samples was significantly higher than that of the mechanical polishing and pickling samples due to the thermal effect during the laser cleaning process. |
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