摘要
加工效率低、表面完整性差以及严重的刀具磨损是微细铣削TiAl金属间化合物的主要原因。提出一种激光诱导可控氧化辅助微细铣削的复合加工方法,针对纳秒脉冲激光诱导氧化TiAl金属间化合物开展研究。主要分析了不同激光参数以及辅助气体对材料氧化行为的影响规律,揭示了TiAl材料的氧化机理以及氧化层与过渡层的形成规律与特征,为后续微细铣削加工中的参数选择提供了理论依据和数据支持。研究结果表明:TiAl材料在激光辐照下发生氧化反应并生成了疏松易去除的氧化物,生成的氧化层与过渡层随着激光能量密度的增大而变得越厚;随着激光扫描速度的加快,氧化层厚度逐渐减小,而激光扫描速度对过渡层厚度的影响不显著。此外,在相同的激光参数下,富氧环境对材料的氧化效果最佳。TiAl金属间化合物在优化的激光参数(激光能量密度为8.82 J/cm^(2)和扫描速度为1 mm/s)和富氧环境氛围下获得的氧化效果较优,生成的氧化层与过渡层厚度分别达到了66μm与22μm。
Objective Poor rigidity of micro milling tools and a high milling force are the main causes of low machining efficiency,poor surface integrity,and severe tool wear in micro milling Ti Al intermetallic alloys.In this study,an innovative hybrid machining process comprising laser-induced controllable oxidation assisted micro milling was proposed to address these problems.In the proposed process,a controllable oxidation reaction occurs in the cutting zone,and loose oxides,which are easy to cut,could be synthesized during the hybrid machining,thereby decreasing the milling force and achieving a mass removal rate.Subsequently,micro milling would be applied to the subsurface materials and high quality microstructures would be manufactured.Most importantly,in this study,nanosecond pulse laser-induced oxidation of Ti Al intermetallic alloys was studied,and the influence of laser machining parameters together with an assisted gas atmosphere on the oxidation behavior was investigated.The micro-zone oxidation mechanisms of workpiece materials under both laser irradiation and oxidizer were investigated in detail,and the forming mechanisms of loose oxidation were studied.A control strategy of loose oxidation was proposed;then,the oxidation behavior was adjusted subjectively.The results of this study will provide both theoretical and technical supports in micro milling of Ti Al intermetallic alloys.Methods Ti Al intermetallic alloys were used in this work(Fig.1).Laser-induced oxidation experiments were performed with high precision nanosecond(ns)pulsed laser equipment composed of a pulsed ytterbium fiber laser(YLP-F20,IPG Photonics Corporation)and CNC air floating platform.The laser spot diameter and pulse repetition frequency were fixed at 57μm and 20 k Hz,respectively.Laser-induced oxidation experiments were performed in a 99.5%pure oxygen-rich atmosphere and an injection velocity of 5 L/min.The laser energy density was varied from 6.86 J/cm2 to 11.76 J/cm2,and the laser scanning speed was 1 mm/s,3 mm/s,6 mm/s,and 12 mm/s(Table 3).The oxidation behavior in the atmosphere of air,argon(Ar),and nitrogen(N2)under the same laser parameters was studied.A scanning electron microscope(SEM,Hitachi S-4800)was used to observe the morphologies and cross-sections of both the oxide layer and sub-layer.The hardness of Ti Al alloys before and after laser-induced oxidation was measured with a Vickers diamond pyramid indenter(HVS-50)with a static load of 196 N and a loading time of 15 s.The phase compositions with the laser energy density after laser irradiation were detected by X-ray diffraction(XRD,Bruker D8).Cu-K(α)radiation with a scanning step of 0.02°and a sweep speed of 6(°)/min were used.Results and Discussions At the fixed laser pulse repetition frequency and laser spot diameter,the absorbed energy of the irradiated surface increased as the laser energy density increased.When the laser energy density was greater than the ablation threshold of the irradiated material,the oxidation reaction between the irradiated material and oxygen-rich atmosphere occurred,producing the titanium oxides.However,when the laser energy density was too high,the thermal effect accumulated on the surface of the irradiated material ablated the generated oxide(as shown in Fig.5).The varied laser energy density significantly influenced the topographies of the sub-layer.At low laser energy density,the subsurface was flat,and residual oxides as well as micro-cracks existed.At lower laser energy density,the oxide layer primarily included low valent titanium oxides,such as TiO_(2) and Ti_(2)O_(3),as well as Ti_(3)O_(5) and Al_(2)O_(3).As the laser energy density increased,stable and high valent titanium oxides were produced,and the phase compositions primarily consisted of anatase TiO_(2),rutile TiO_(2),and Al_(2)O_(3)(Fig.6).At high laser energy density,the subsurface had a recasting-layer and many tiny micro craters together with large cracks(Fig.7).In addition,the thickness of the oxide layer and sub-layer increased as the laser energy density increased(Fig.8).Moreover,the low laser scanning speed produced better oxidation results compared with the results produced under high scanning velocity at the fixed laser energy density and repetition frequency(Fig.9).It was noted that at lowscanning speed,the thickness of the oxide layer was better than that at high scanning speed(Fig.10).Furthermore,the irradiated material had better oxidation results under the oxygen-rich atmosphere,compared with other assisted gas atmospheres(Fig.11).Conclusions In this paper,the oxidation behavior of the irradiated material was studied under changing laser energy densities.All other laser parameters remained unchanged.In the oxygen-rich environment,the accumulated energy absorbed by Ti Al material increased gradually as the laser energy density increased,which further promoted the oxidation reaction.In addition,the thickness of the generated oxide layer gradually increased.However,when the laser energy density was more than 9.80 J/cm2,the produced oxides started to melt and a dense recast layer was formed.The heat-affected zone generated by thermal diffusion expanded rapidly and the thickness of sub-layer increased dramatically.At high laser energy density,the oxide layer was primarily composed of anatase TiO_(2),rutile TiO_(2),and Al_(2)O_(3).For the varied range of laser parameters,the oxidation result was better at a lower laser scanning speed.However,the laser scanning speed and assisted gas atmospheres other than the oxygen-rich environment had no effect on the thickness of the sub-layer.Overall,at laser energy density of 8.82 J/cm^(2) and laser scanning speed of 1 mm/s,as well as in an oxygen-rich environment,Ti Al intermetallic alloys had better oxidation results,where the thickness of the oxide layer and sub-layer was 66μm and 22μm,respectively.After laser irradiation,the hardness of the sub-layer(200 HV)was lower than that of the substrate(365 HV,Table 1),which indicated that the laserinduced oxidation can improve the micro machinability of Ti Al intermetallic alloys and promote the service life of micro end mills.
作者
赵国龙
夏宏军
李亮
王珉
何宁
Zhao Guolong;Xia Hongjun;Li Liang;Wang Min;He Ning(College of Mechanical and Electrical Engineering,Nanjing University of Aeronautics and Astronautics,Nanjing Jiangsu210016 China;Nanjing Institute of Measurement and Testing Technology,Nanjing Jiangsu210049 China.)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2021年第22期202-215,共14页
Chinese Journal of Lasers
作者简介
通信作者:赵国龙,zhaogl@nuaa.edu.cn。
