For the extreme complex working conditions in the aerospace field, Ti6Al4V/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 high-quality preparation of Ti6Al4V/NiTi alloy heterostructures. On the one hand, the brittle intermetallic compounds (such NiTi₂, 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, Ti6Al4V/NiTi heterogeneous materials were successfully prepared using in-situ gradient additive technology for heterogeneous materials. We hope that this study will lay the foundation for the practical application of aerospace-oriented Ti6Al4V/NiTi heterogeneous functional materials on complex structural components.

Ti6Al4V and NiTi alloy powders were used in this study. Firstly, in-situ preparation of 11 thin-walled Ti6Al4V/NiTi alloys with different mass fraction ratios was carried out in an oxygen-enriched environment using in-situ 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 X-ray spectroscopy (EDS) and X-ray 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.

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: α-Ti+β-Ti→α-Ti+NiTi₂→NiTi₂→NiTi₂+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 as α-Ti+β-Ti→α-Ti+NiTi₂→NiTi₂→NiTi₂+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 NiTi₂ reinforced phase resulted in the highest hardness value of 576 HV±5 HV in the 40% Ti6Al4V+60% NiTi zone (Fig. 8).

In this study, preparation of Ti6Al4V/NiTi alloys with different mass fraction ratios was firstly carried out in an oxygen-enriched environment by employing an in-situ gradient 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 Ti6Al4V/NiTi heterogeneous alloy powder synchronous conveying method proposed in this paper. Then, the integrated deposition and forming of Ti6Al4V/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 Ti6Al4V/NiTi heterogeneous alloy. Our study shows that integrated deposition and forming of Ti6Al4V/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 Ti6Al4V/NiTi heterogeneous functional materials on complex structural parts.