【作者】 张林；

【导师】 张维平；

【作者基本信息】 大连理工大学 ， 材料工程（专业学位）， 2022， 硕士

【摘要】 钛合金具有诸多优异的性能,在航空航天、石油开采、生物医疗及特种装甲等领域中得到广泛应用。然而其表面性能的不足同样也限制了其应用范围,使用激光熔覆技术在钛合金表面制备一层性能优异的表面涂层可有效改善其表面性能不足的缺陷。本文使用TC21钛合金作为基体,通过使用激光熔覆技术,设计并制备Ti-Al基复合粉末,成功制得一层性能优异的涂层,分别研究WC含量变化、激光工艺参数及CeO₂添加量对涂层组织与性能的影响。使用XRD、EPMA及SEM对熔覆层组织及成分进行分析;使用显微硬度计及磨损实验对熔覆层性能进行测量,综合组织、成分的特点及性能的变化讨论工艺参数及WC、CeO₂含量变化对涂层的影响。通过激光熔覆技术成功在钛合金表面制备Ti-Al/WC复合涂层,分析结果表明:添加WC后,Ti、Al、WC混合粉末在激光束的辐照作用下发生了原位反应,生成了具有增强作用的Ti-Al金属间化合物Ti Al、Ti3Al及碳化物Ti C、Ti2Al C。随WC含量的增加,原位生成的增强相的随之增多,熔覆层硬度及耐磨性得到显著提升。当WC含量为20wt.%时,熔覆层的耐磨性最佳,约为基体的4.2倍,硬度约为基体的1.75倍。为探究激光工艺参数的变化对熔覆层的影响,设计了9组不同激光功率与扫描速度搭配的方案。实验结果表明:在激光能量密度相近时,激光功率对熔覆层稀释率的作用要强于扫描速度。虽然稀释率由激光功率及扫描速度协同作用所控制,但是激光功率主要影响熔覆层的宽度及基体熔化区的深度,其作用远大于扫描速度;熔覆层高度主要受扫描速度控制,但其作用只稍大于激光功率,整体稀释率受激光功率的影响较大。结合稀释率的大小,发现激光功率为1000 W,扫描速度为4 mm/s时熔覆层质量最好。在对熔覆层组织的研究中发现,熔覆层组织一定程度上受激光能量密度的控制,激光能量密度决定了熔覆层的能量输入及熔池的存在时间,从而影响熔覆层温度梯度、冷却速度及过冷度等,进而改变熔覆层组织的形态及大小,一定程度上影响了熔覆层的力学性能。当激光功率为1200 W、扫描速度为5 mm/s时,耐磨性约为基体的3.51倍,硬度约为基体1.61倍,稀释率相对较低。CeO₂具有改善涂层质量,减少裂纹、气孔等缺陷的作用,在已有Ti-Al/WC复合涂层中添加不同含量CeO₂,研究CeO₂的变化对涂层的影响。实验表明:添加CeO₂后,熔覆层物相组成整体变化不大,检测出少量CeO₂,以白色颗粒状在组织中存在;CeO₂在晶界处钉扎,起到了细晶强化的作用,组织随CeO₂添加量的增加逐渐细化,整体更加均匀致密;通过对硬度进行测量,发现硬度随着CeO₂添加量的增加逐渐减低,虽然CeO₂一定程度上促进了晶粒的细化,但熔覆层中Ti C等增强相在CeO₂添加后形态发生了变化,对熔覆层的增强作用逐渐减弱,致使硬度不增反减;当含量为0.8 wt%时,熔覆层磨损量最低,约为0.0012 g,此时具有最佳的耐磨性能,这与其较低的摩擦系数有关。更多还原

【Abstract】 Titanium alloys have many excellent properties and are widely used in aerospace,petroleum exploration,biomedical and special armor fields.However,the deficiency of its surface performance also limits its application scope.Using laser cladding technology to prepare a layer of excellent surface coating on the surface of titanium alloy can effectively improve the deficiency of its surface performance.In this paper,TC21 titanium alloy was used as the matrix,and Ti-Al composite powder was designed and prepared by laser cladding technology.A layer of coating with excellent performance was successfully prepared.The effects of WC content change,laser process parameters and CeO₂ addition amount on the microstructure and properties of the coating were studied respectively.XRD,EPMA and SEM were used to analyze the microstructure and composition of the cladding layer.The properties of the cladding layer were measured by microhardness tester and wear test,and the effects of process parameters and WC and CeO₂ content changes on the coating were discussed based on the characteristics of microstructure,composition and properties.Ti-Al /WC composite coating was successfully prepared on the surface of titanium alloy by laser cladding technology.The analysis results show that Ti-Al /WC powders react in situ under laser beam irradiation after adding WC,and Ti-Al intermetallic compounds Ti Al and Ti3 Al and carbides Ti C and Ti2 Al C with enhanced effect are generated.With the increase of WC content,the reinforcement phase generated in situ increases,and the hardness and wear resistance of cladding layer are significantly improved.When WC content is 20 wt.%,the wear resistance and hardness of the cladding layer are about 4.2 times and 1.75 times that of the matrix.In order to investigate the influence of laser process parameters on the cladding layer,nine schemes with different laser power and scanning speed were designed.The experimental results show that the effect of laser power on the cladding dilution rate is stronger than that of scanning speed when the laser energy density is similar.Although the dilution rate is controlled by the synergistic effect of laser power and scanning speed,the laser power mainly affects the width of the cladding layer and the depth of the melting zone of the matrix,and its effect is far greater than that of scanning speed.The height of the cladding layer is mainly controlled by the scanning speed,but its effect is only slightly greater than the laser power,and the overall dilution rate is greatly affected by the laser power.Combined with the dilution rate,it is found that the quality of cladding layer is best when the laser power is 1000 W and the scanning speed is 4 mm/s.Found in the study of cladding layer of tissue,the cladding layer organization to some extent under the control of the laser energy density,laser energy density determines the energy input of the cladding layer and the existence of the molten pool,thus affecting the cladding layer temperature gradient,cooling rate and undercooling degree,and then change the configuration of cladding layer organization and size,to a certain extent affect the mechanical properties of cladding layer.When the laser power is 1200 W and the scanning speed is 5 mm/s,the wear resistance and hardness are about 3.51 times of the matrix,and the dilution rate is relatively low.CeO₂ can improve the quality of coating,reduce cracks,pores and other defects.Different content of CeO₂ was added to the existing Ti-Al/WC composite coating to study the influence of the change of CeO₂ on the coating.The experimental results show that after adding CeO₂,the overall phase composition of the cladding layer does not change much,and a small amount of CeO₂ is detected in the white granular structure.When CeO₂ was nailed at the grain boundary,it played the role of fine grain strengthening.The microstructure was gradually refined with the increase of CeO₂ addition level,and the overall structure became more uniform and compact.The hardness was measured,and it was found that the hardness decreased gradually with the increase of CeO₂.Although CeO₂ promoted the grain refinement to a certain extent,the morphology of Ti C and other enhanced phases in the cladding layer changed after CeO₂ was added,and the strengthening effect of Ti C on the cladding layer was gradually weakened,resulting in the decrease of hardness instead of increase.When the content is 0.8wt %,the wear amount of cladding layer is the lowest,about 0.0012 g,which has the best wear resistance,which is related to the lower friction coefficient.更多还原