脉冲激光清洗过程中振镜扫描速度对TA15钛合金表面氧化层的影响

Effect of Scanning Speed of Galvanometer on Surface Oxide Layer of TA15 Titanium Alloy in Pulsed Laser Cleaning

采用脉冲激光清洗技术以不同的振镜扫描速度对TA15钛合金表面的氧化层和油污进行去除,分析了激光清洗后TA15钛合金的表面形貌以及清洗表面的成分变化和硬度变化。结果表明:随着振镜扫描速度从7000 mm·s^(-1)增大到10000 mm·s^(-1),清洗表面的钛元素含量呈现先增加再降低的趋势,而氧元素含量则是先降低再升高;当扫描速度为8000 mm·s^(-1)时,清洗表面的钛元素含量达到最高值,w(Ti)=79.47%,氧元素含量达到最低值,w(O)=8.62%;清洗表面的硬度随着扫描速度的减小而增大;激光清洗去除TA15氧化层的机制主要为气化机制和相爆炸机制。

Objective TA15titanium alloy is often used to manufacture aircraft partitions,wall plates,welding bearing frames,and aircraft engine parts.These parts often need to withstand high temperatures and various stresses.However,the oxide layer on the surface of such parts reduces its welding performance,electrical conductivity,and bonding with the coating;therefore,it is necessary to clean the oxide layer film regularly.Compared with the traditional cleaning methods,laser cleaning technology has the advantages of high precision,broad applicability,simple process,and clean and green.As a new type of laser,pulsed fiber laser has the advantages of high photoelectric conversion efficiency,good beam quality,and high reliability compared with other lasers.This can solve the problems of poor consistency and low efficiency for cleaning some parts of the existing aircraft.This laser cleaning breaks the technical bottleneck of the traditional aircraft cleaning process regarding the cleaning mechanism,which is conducive to improving aircraft re-service safety and service life.It provides the scientific basis and technical support for the popularization and application of laser cleaning technology in aviation and other major equipment.This also ensures the safety of equipment in service to promote the green and high-quality development of Chinese industries including aviation,marine equipment,and rail transit.Methods In this study,a TA15titanium alloy plate with oil stain and an oxide layer on the original surface was used.First,we used an IPG pulsed fiber laser and different scanning speeds of galvanometers to clean the original surface of TA15.Then,the surface morphologies of TA15before and after the laser cleaning were observed using an ultra-depth three-dimensional field microscope to determine the feasibility of laser cleaning the oil stain and oxide layer on the TA15surface.Then,a scanning electron microscope,an energy spectrum analyzer,and an X-ray diffractometer were used to observe and analyze the effect of different scanning speeds of galvanometers on the surface morphology,surface-element content,and surface composition of the cleaned TA15surface.Simultaneously,the removal mechanism of the oil stain and oxide layer on the TA15surface was studied.Finally,the hardness of the TA15surface before and after the cleaning was tested using Vickers hardness tester to explore the influence of laser cleaning on the TA15surface and the relationship between the scanning speed of the galvanometer and hardness during the laser cleaning.Results and Discussions After pulsed laser cleaning,the original black surface of TA15becomes white and bright.The scanning speed of the galvanometer has a certain influence on the cleaning effect[Fig.1(a)and Fig.4].When other laser cleaning process parameters remain unchanged,the overlap ratio of galvanometer spot increases with the decrease of scanning speed.This leads to an increase in the laser energy absorbed by the cleaning surface so that the porous and loose oxide layer on the TA15surface can be removed.Moreover,the surface cleaned with the small scanning speed of the galvanometer is smoother than the surface cleaned with the large scanning speed,and the removal effect of the oxide layer is also more obvious[Fig.1(b)and Fig.5].Combined with the principle of laser cleaning and the oxide debris observed on the surface of the TA15alloy,it can be inferred that gasification and phaseexplosion mechanisms are followed during the laser cleaning process for the oxide layer and oil stain on the TA15surface.Because the air and moisture contained in the porous structure of the original oxide layer explode under the irradiation of the laser,phase-explosion is caused in the cleaning surface.Additionally,some oil stain can be attached to the oxide layer debris generated by the phase-explosion and removed together(Fig.6).Due to the element content changes on the cleaning surface,as the scanning speed of the galvanometer increases from 7000mm·s^(-1)to10000mm·s^(-1),the Ti-element content on the TA15surface first increases and then decreases,while the O-element content first decreases and then increases(Fig.7).The oil stain and oxide layer on the TA15surface cannot be well cleaned when very high scanning speed is used.However,very low scanning speed can easily cause thermal oxidation of the cleaned surface and generate new Al2O3and TiO2layers that adhere to the cleaned surface(Fig.8).Through the analysis of the change of the hardness of the cleaned surface,it is concluded that the plasma and phase-explosions can cause the plastic deformation of the TA15surface and the refinement of the grain,which can produce the effect of laser shock strengthening and improve the surface hardness of TA15.With the decrease of scanning speed,the effect of laser shock strengthening is also improved(Fig.9).Conclusions The scanning speed of the galvanometer in pulsed laser cleaning is an important factor influencing the cleaning effect of oil stain and oxide layer on the TA15surface,and the removal mechanisms of the oil stain and oxide layer on the TA15surface are mainly gasification and phase-explosion mechanisms.When the scanning speed increases from 7000mm·s^(-1)to 10000mm·s^(-1),the surface cleaning effect of TA15is the best at the scanning speed of8000mm·s^(-1).At this time,the Ti-element content of the cleaned surface reaches the highest value of 79.47%,and the O-element content reaches the lowest value of 8.62%;the surface Al2O3is almost removed,and the TiO2content is low.Laser cleaning can strengthen the TA15surface by laser shock and improve the hardness of the cleaning surface.Reducing the scanning speed of the galvanometer will enhance the effect of laser shock strengthening and increase the hardness of the cleaned surface.