Research On Mechanism And Process For Laser Removal Of Titanium Alloy Oxide Film

Titanium alloys are widely used in industry for their excellent properties.However,its tendency to generate oxide film in the oxygenated natural environment will impair the processing quality of subsequent processes,which must be removed.The existing techniques of mechanical grinding and chemical treatment suffer from highly damaged,heavily contaminated and size limited.Laser removing,which has the advantages of high quality and efficiency,low pollution,low damage,and unlimited size and scene,has attracted widespread attention,making it the preferred solution for oxide film removal.This paper investigates the mechanism and process for laser removal of TA15 titanium alloy oxide film.Since the laser removing of oxide film inevitably changes the native morphology and leads to changes in the superficial microstructure.A study of the evolution of the superficial morphology by the laser removal process was therefore conducted.The effect of the change in superficial microstructure on wear resistance and corrosion resistance was analyzed.The effect of the laser removing of oxide film on the welding properties was investigated.The main work of this paper are as follows:（1）The laser energy density was used as a comprehensive parameter to elucidate the correlation law between experimental parameters and processing results.Numerical simulations and thermodynamic calculations were used to investigate the critical removal conditions of oxide film and the energy threshold for minimum oxide generation on the surface.Based on the results of numerical simulations,high-speed camera and process product,the existence of thermal,thermal-impact composite mechanism was revealed.Increasing laser energy density leads to its transformation.The threshold value is approximated as 14 J/cm².Oxide film removal changes from vaporization to thermal-force synergistic removal by vaporization+ejection,pushing and expansion fragmentation.（2）Elucidation of the evolution of superficial morphology during the laser removal process of oxide film.With 8.75 J/cm² and 35 J/cm²,uniform craters and stripes were formed respectively,but the superficial roughness increased.Through numerical simulation,it is found that in the former case,the surface of the molten pool is depressed by the vaporization pressure,and rapid solidification enabling the supercooled edges to continue to increase,eventually forming a crater;in the latter case,the heat accumulation of the latter leads to the fusion of multiple molten pools into a large-size molten pool,as well as the increase in flow velocity at the edge of the crater,the prolongation and the increase in size of the molten pool,resulting in a trapezoidal-triangular-tower evolutionary trend of the edge of the crater,which leads to a decrease in size and even disappearance,ultimately forming stripes.（3）During laser removal of oxide film,new oxide film will be generated on the titanium alloy in the oxygen environment and the rapid fusion and solidification also leads to micro-grain changes,resulting in improved wear resistance and corrosion resistance caused by the changed superficial microstructure.The composition of Ti O₂（R）（rutile type）+Ti₂O₃（≤17.5 J/cm²）has the best corrosion resistance,due to its lower corrosion tendency,more stable products,and less influence on acidity.The time required to corrode to the same depth,with 8.75 J/cm²,was 20.43 times that of untreated sample.The larger temperature gradient and solidification cooling rate（10⁸ K/s）of the molten pool,led to a grain refinement of the superficial micromelting layer.The fine-grain reinforcement reduces its coefficient of friction by more than 40%compared to the base materials.（4）The reasons for the suppression of metallurgical-type and keyhole-type pores in welds by the laser removal of oxide film.Laser removes the native oxide film,as well as generates less new oxide film,which reduces thermal decomposition of adsorbed moisture to suppress metallurgical-type pores.Laser forms homogeneous microforms with increased roughness and improves the superficial laser absorption rate.Consequently,the molten pool and keyhole stability are enhanced to reduce the keyhole-type pores.The porosity of metallurgical-type and keyhole-type type is 0.08%and 0.2%,respectively.Relative to the existing technology,the macro and microscopic morphology is uniform by laser removing,the oxygen content of the surface is all lower than 3%,the oxide film on the surface varies little,and its tensile strength and elongation are the highest,reaching 1130.47 MPa and 8.88%,which can advantageously replace the existing technology.