Selective Laser Melting Of NiTi Shape Memory Alloy

Near equi-atomic NiTi shape memory alloy,which shows excellent shape memory effect and superelasticity,making it being an smart material which integrates sensing and driving.With its excellent functional properties,NiTi alloy is one of the most promising functional materials in many fields,such as aerospace,medical devices,automotive,among others.However,due to its high strength,high reactivity and high work harding,most of the NiTi alloy fabricated by conventional methods are small parts with simple geometries,which limits the performance improvement and application promotion of NiTi alloys.Additive manufacturing provides an effective way to fabricate the complex structures or large size NiTi parts.Selective laser melting(SLM)is a commonly method to fabricated NiTi alloy,because it provides a better geometrical accuracy and surface finish.In previous studies,it was found that the mechanical properties of SLM-fabricated NiTi alloys under compression mode were comparable to the NiTi alloys fabricated through conventional processing techniques.However,under the tensile mode,NiTi alloys fabricated by SLM are normally very brittle(tensile strain<7%),which limits the full applicability of NiTi alloys.The study of tensile properties of NiTi alloys is important because NiTi alloys are mostly used under tensile state.Moreover,tensile property of NiTi is more sensitive to defects(e.g.pores,microcracks,etc.)produced by SLM,which leads to a lower fracture strain of NiTi alloy when under tensile mode.Therefore,fabricating NiTi alloy with good mechanical properties is very meaningful.Since the functional properties of NiTi alloy are based on the transformation between martensite and austenite,the transformation temperature is the key factor affecting the practical application of NiTi alloy.However,there are many factors could affect the transformation temperature of NiTi alloy and it is very sensitive to the change of Ni-Ti ratio,which leads to the difficulty of modifying the transformation temperature.It is difficult to accurately control the Ni-Ti ratio in a small range when fabricated the NiTi alloy through the traditional processing techniques.Therefore,it is necessary to find an effective method to modify the phase transformation temperature of NiTi alloy.In this study,we found that SLM can not only produce the NiTi alloy with complex structures,but also provide an effective way to modify the Ni-Ti ratio of NiTi alloy.Process parameters vary widely during fabricating,for example,laser power changes from 60W to 200W,hatch spacing changes from 40 to 110 ?m and scanning speed changes from 400/s to 1200 mm/s.It is worth mentioning that although a wide range of SLM process parameters will lead to defects in the interior of some samples,the overall samples still maintain good mechanical properties.Moreover,based on the study of the effect of SLM process parameters on the transformation temperature of NiTi alloy,we found the transformation temperature changes monotonically with the change of SLM process parameters,which provides a method to modify the transformation temperature during the SLM process.NiTi alloy with good mechanical properties and tailorable transformation temperature was successfully fabricated by SLM with the wide range of process parameters.This provides a theoretical and technical basis for fabricating the NiTi alloy.