Ti6Al4V/NiTi异质功能材料原位梯度增材成形与界面组织演化(特邀)

In-Situ Gradient Additive Forming and Interfacial Microstructure Evolution of Ti6Al4V/NiTi Heterogeneous Functional Material(Invited)

面向航天领域极端复杂工况,Ti6Al4V/NiTi异质功能结构可在充分发挥高比强度、耐蚀性等材料性能优势的同时实现智能变形等功能需求。然而,两种金属过渡界面区具有较高的开裂倾向。笔者采用激光熔化同步输送异质合金粉末沉积成形工艺,在富氧环境下开展了组分梯度过渡的Ti6Al4V/NiTi合金的原位制备,并在此基础上通过等能量密度成形法和基板热管理,实现了Ti6Al4V/NiTi异质材料的一体化沉积成形,分析了过渡区界面组织的演化规律。扫描电镜和能谱分析结果表明:经组分梯度优化后,梯度层界面之间呈良好的冶金结合;随着NiTi组分逐渐增加,从Ti6Al4V区到NiTi区,相组成演变为α-Ti+β-Ti→α-Ti+NiTi_(2)→NiTi_(2)→NiTi_(2)+NiTi→NiTi→NiTi+Ni3Ti。梯度过渡区的显微硬度从Ti6Al4V区的343 HV±13 HV变化到NiTi区的275 HV±10 HV,40%Ti6Al4V+60%NiTi区域由于NiTi_(2)强化相的析出而具有最高的硬度值,硬度值为576 HV±5 HV。

Objective For the extreme complex working conditions in the aerospace field,Ti_(6)Al_(4)V/NiTi heterogeneous functional structure can give full play to the advantages of its high specific strength,corrosion resistance and other material properties while realizing the functional requirements such as intelligent deformation.However,the two alloys have significant differences in melting point,coefficient of thermal expansion,thermal conductivity and specific heat capacity,leading to the challenge in the highquality preparation of Ti_(6)Al_(4)V/NiTi alloy heterostructures.On the one hand,the brittle intermetallic compounds(such NiTi_(2),Ni3Ti and Al3Ti)generated during the forming process,induce a decrease in the interfacial bonding strength,bring on a potentially high cracking tendency during the forming process.On the other hand,cracks are sprouted in the laser deposited formed parts due to the high temperature gradient during the deposition process and the accumulation of thermal stresses caused by rapid solidification,thus restricting the metallurgical bonding between the interfaces of heterogeneous material structures.In this study,Ti_(6)Al_(4)V/NiTi heterogeneous materials were successfully prepared using insitu gradient additive technology for heterogeneous materials.We hope that this study will lay the foundation for the practical application of aerospaceoriented Ti_(6)Al_(4)V/NiTi heterogeneous functional materials on complex structural components.Methods Ti_(6)Al_(4)V and NiTi alloy powders were used in this study.Firstly,insitu preparation of 11 thinwalled Ti_(6)Al_(4)V/NiTi alloys with different mass fraction ratios was carried out in an oxygenenriched environment using insitu gradient additive technology for heterogeneous materials.Secondly,the microstructures and phase compositions of the composites with 11 compositional ratios were analyzed and characterized by energy dispersive Xray spectroscopy(EDS)and Xray diffraction(XRD).On this basis,actual characterization results of the elemental content of the 11 component ratios were compared with the compositional design results.Then,Ti6Al4V/NiTi heterogeneous materials were prepared by combining gradient transition composition design and substrate thermal management.And the metallurgical bonding properties between the interfaces of different gradient regions as well as the elemental species and contents were characterized by scanning electron microscope(SEM)observations and EDS analyses.Finally,microhardness tests were performed on the prepared Ti6Al4V/NiTi heterogeneous materials to characterize their mechanical properties.Results and Discussions For the 11 kinds of Ti6Al4V/NiTi alloys with different mass fraction ratios,the XRD analysis results show that the phase compositions from 100% Ti6Al4V to 100% NiTi are in the following order:αTi+βTi→αTi+NiTi_(2)→NiTi_(2)→NiTi_(2)+NiTi[see Fig.3(a)].With the increase of NiTi alloy powder content,the Ti elemental mass fraction changed from 90.7% to 46.5% and the Ni elemental mass fraction increased from 0.1% to 53.3%(Fig.5).The compositional design is in good agreement with the actual results.SEM and EDS analysis results show that the Ti6Al4V/NiTi heterogeneous materials prepared after component gradient optimization have good metallurgical bonding between the gradient layer interfaces(Table 2).With the gradual increase of NiTi component,the phase composition from Ti6Al4V zone to NiTi zone evolves asαTi+βTi→αTi+NiTi_(2)→NiTi_(2)→NiTi_(2)+NiTi→NiTi→NiTi+Ni3Ti(Table 2).The average microhardness in the gradient transition zone varied from 343 HV±13 HV in the Ti6Al4V zone to 275 HV±10 HV in the NiTi zone;whereas,the precipitation of NiTi_(2) reinforced phase resulted in the highest hardness value of 576 HV±5 HV in the 40% Ti6Al4V+60%NiTi zone(Fig.8).Conclusions In this study,preparation of Ti6Al4V/NiTi alloys with different mass fraction ratios was firstly carried out in an oxygenenrichedenvironment by employing an insitugradient additive technology for heterogeneous materials. Microstructure evolution and phase composition of the composites with 11 compositional ratios were also analyzed. EDS spectroscopy results show a good agreement between the compositional design and the actual characterization, thus proving the feasibility of the Ti_(6)Al_(4)V/NiTi heterogeneous alloy powder synchronous conveying method proposed in this paper. Then, the integrated deposition and forming of Ti_(6)Al_(4)V/NiTi heterogeneous materials with the optimized component gradient transitions was finally achieved by proposing an isoenergetic energy density forming method and thermal management of the substrate at 400 ℃ to reduce the content of brittle intermetallic compounds as well as to lower the thermal stresses. Metallographic observations show good metallurgical bonding between the interfaces in the different gradient regions. Thermal management of the substrate at 400 ℃ helps to reduce the cracking tendency of the Ti_(6)Al_(4)V/NiTi heterogeneous alloy. Our study shows that integrated deposition and forming of Ti_(6)Al_(4)V/NiTi heterogeneous materials can be carried out by rational gradient composition design combined with temperature regulation of the forming process. Purpose of this study is to lay a foundation for the practical application of Ti_(6)Al_(4)V/NiTi heterogeneous functional materials on complex structural parts.