NiTi形状记忆合金具有优异的形状记忆效应和超弹性，以及良好的生物相容性、耐磨性和力学性能等，在航空航天、生物医疗、汽车、工程建筑等领域具有广泛的应用前景。增材制造技术赋予了制备复杂可定制化NiTi合金构件极大的自由度，对于拓宽NiTi合金的应用场景具有重要意义。增材制造NiTi合金领域可分为两个研究方向：一是使用预合金化的原材料进行增材制造，目前存在的关键问题是难以表现优异的拉伸超弹性；二是使用单质原材料在增材制造过程中原位合成NiTi合金，目前存在的关键问题是难以制备具有典型功能特性且成分可调控的NiTi合金。本文提出采用电子束熔丝增材制造技术（EBAM）针对上述两个研究方向存在的关键问题展开研究。 以预合金化原材料制备NiTi合金时，首先通过适当提高原材料中的碳含量，将主体第二相由粗大Ti2Ni相转为细小TiC相，显著改善塑性，延伸率获得成倍提升。随后研究了EBAM关键工艺参数对组织和性能的影响，表明在较大工艺参数范围内均能获得良好的成形质量；不同工艺参数制备的样品均表现出了较高的致密度和平行于高度方向的柱状晶；相变温度对工艺参数十分敏感，不同工艺参数下的Ni元素烧损量不同是影响相变温度的关键因素；打印态的NiTi合金无法表现良好的超弹性。通过热处理引入弥散分布的纳米Ni4Ti3相以强化基体，实现了优异的拉伸超弹性：在6%的拉伸超弹性循环测试中，回复率可稳定在90%左右。 以纯Ni丝和纯Ti丝为原材料原位合成NiTi合金时，首先通过综合调控电子束能量密度、“丝-丝-束”空间位置、电子束扫描方式和熔滴过渡距离，建立了连续液桥熔滴过渡模式，实现了连续、稳定、无飞溅的沉积过程，保证了良好的成形质量和Ni、Ti元素的充分混合。在此基础上，通过多层沉积制备了富钛NiTi合金，其在远离基板的区域成分均匀，具备典型的相变行为和压缩记忆效应，在6%的压缩形状记忆效应测试下可表现63%的回复率。最后，通过改变双丝送丝速度配比，实现了不同成分NiTi合金的原位合成；所制备的近等原子比NiTi合金，延伸率可达10%左右，并首次实现了原位制备NiTi合金的拉伸形状记忆效应：在6%的拉伸形状记忆效应测试下可表现70%的形状回复率。
NiTi shape memory alloys possess excellent shape memory effect and superelasticity, together with good biocompatibility, wear resistance and mechanical properties, which have wide application prospect in fields of aerospace, biomedical, automobile, engineering construction and so on. Additive manufacturing technology gives a great freedom to fabricate complex and customizable NiTi alloy structures, which is of great significance for broadening the application scenarios of NiTi alloys. There are two research directions in the field of additive manufacturing of NiTi alloys: The first is using the pre-alloyed raw materials for additive manufacturing, in which the current key problem is that it is difficult to achieve excellent tensile superelasticity. The second is to in-situ synthesize NiTi alloys with pure Ni and Ti as raw materials in additive manufacturing, in which the current key problem is that it is difficult to fabricate NiTi alloys with controllable composition and typical functional properties. In this thesis, the technology of electron beam wire-feed additive manufacturing (EBAM) is employed to study the above key problems in two research directions. For EBAM of NiTi alloys with pre-alloyed raw materials, the carbon content of raw material is appropriately increased at first. Thereby, the second phase particles in the as-built NiTi alloys are converted from coarse Ti2Ni phase to fine TiC phase, which significantly improve the ductility and double the elongation. Then, the effect of EBAM key process parameters on microstructure and properties is studied. Results show that good forming quality can be obtained over a large range of process parameters. The samples prepared with different process parameters all exhibit high density and columnar grains parallel to the building direction. The phase transformation temperatures are very sensitive to the variation of process parameters, and the different amount of Ni-evaporation under different process parameters is the key influencing factor. The as-built NiTi alloys cannot exhibit good superelasticity. By further introducing the dispersively distributed nano-Ni4Ti3 precipitates by heat treatment to strengthen the matrix, the excellent tensile superelasticity can be achieved, with a stable recovery rate of 90% around during the cyclic tensile superelastic test to a constant strain of 6%. For in-situ synthesizing of NiTi alloys by EBAM with pure Ni wire and pure Ti wire, the continuous liquid bridge metal transfer mode is established at first, by integrated regulation of electron beam energy density, "wire-wire-beam spatial position", electron beam scanning mode and droplet transfer distance. Thereby, a continuous, stable and splash-free deposition process is realized, which further ensures good forming quality and sufficient mixing of Ni and Ti elements. Based on this, a Ti-rich NiTi alloy part is prepared by multi-layer deposition, which has uniform chemical composition away from the substrate, and exhibits typical phase transformation behavior and compressive shape memory effect. It shows a recovery rate of 63% under the 6% compressive shape memory effect test. Finally, by controlling the wire feeding speeds of Ni wire and Ti wire separately, the in-situ synthesized NiTi alloys with different chemical compositions are obtained. The in-situ synthesized near equiatomic NiTi alloys can exhibit an elongation of 10% around and good tensile shape memory effect, with a recovery rate of 70% under the 6% tensile shape memory effect test, which has not been achieved before.
