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Introduction
Shape memory alloy is a new type of functional material with shape memory effect and superelasticity. NiTi shape memory alloy with near equal atomic ratio exhibits excellent shape memory effect and superelasticity [1], good biocompatibility [2, 3], corrosion resistance [4] and low elastic modulus [5], which has been widely used in aerospace, micro-electromechanical systems and biomedical fields [6, 7]. Due to the high ductility and work hardening of NiTi alloy, it is easy to cause serious tool wear and poor surface finish [8–11]. NiTi alloy parts prepared by traditional processes are usually simple in shape, mostly wires, bars and plates [12], which seriously limits the development of NiTi alloys. Additive manufacturing technology is based on CAD data, and the original materials (powder and wire) are accumulated layer by layer through layered control, which avoids the dilemma of traditional processing technology and provides a new idea for the forming of NiTi alloy [13–15]. With the introduction of the concept of 4D printing, the shape, performance and function of intelligent components can be controlled in time and space dimensions, which further promotes the research of additive manufacturing NiTi alloy [16].
At present, the research on additive manufacturing of NiTi alloy mainly focuses on laser-based powder bed fusion (L-PBF) [17–19], electron beam-based powder bed fusion (EB-PBF) [20, 21] and two direct energy deposition processes: directed energy deposition-arc (DED-Arc) [22, 23] and blown powder deposition (BPD) [24–27]. L-PBF has a wide range of applications, high manufacturing accuracy, good surface quality, dimensional accuracy of up to 0.05 mm, and surface roughness Ra of only 5–20 μm [28, 29], which is mainly used for the manufacture of small-sized NiTi components. Recently, L-PBF, EB-PBF, and powder-type directed energy deposition (DED) manufacturing techniques have largely advanced the manufacturing of Ti alloys [30, 31]. L-PBF also offers advantages by providing a more flexible and cost-effective method for in-situ alloying of multi-materials [32–34]. Furthermore, NiTi shape memory alloys produced by L-PBF have shown significant improvement in both mechanical and shape memory properties [35, 36].
DED-Arc takes the arc as the heat source and forms by continuously melting wires layer by layer, the deposition methods mainly include gas metal arc welding (GMAW), gas tungsten arc welding (GTAW) and plasma arc welding (PAW). DED-Arc-formed components have...