NiTi shape memory alloy (SMA) has shown great promise in micro components for bio-medical and aerospace applications due to its unique functional properties, i.e. shape memory effect and pseudoelasticity. However, the high ductility and work-hardening effect of NiTi lead to poor machinability. Additive manufacturing (AM), with unrivalled capability of fabricating complicated structures, has been used to fabricate NiTi components. To meet the increasing demand of product miniaturization, micro selective laser melting (μ-SLM) equipped with finer laser beam has been developed to improve product precision. This work studies the fabrication of NiTi SMA parts by μ-SLM for the first time. The effects of process parameters and post heat treatment on defects, microstructure and thermal/mechanical behaviors of μ-SLM NiTi were analyzed by means of optic microscopy (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), high resolution X-ray diffractometer (HRXRD), differential scanning calorimetry (DSC) and uniaxial compressive tests. Firstly, for the product quality, a process window for “Scanning speed (V)– Hatch spacing (H)” was determined by parametric experiments to produce crack-free and low-porosity NiTi parts. OM observations show that the porosity of as-printed NiTi part is more sensitive to V than to H. Hatch angle of 45° and 60° are more preferable than A90 due to higher sample availability and reduced risk of crack by residual stress. Secondly, for the microstructure, the effect of hatch angle on grain structure and texture of μ-SLM NiTi were studied. Columnar grains accompanied with equiaxed grains are observed, while the crystallographic texture is weak because of the frequent change of heat flux and grain growth direction by rotation of laser scanning direction. The matric phase B2 NiTi is detected in as-printed NiTi at room temperature accompanied with impurity phases TiC~1-x~N~x~ and Ti~4~Ni~2~O~x~, while post heat treatment introduces a number of Ti~2~Ni and Ni~4~Ti~3~ secondary phases. Thirdly, for the thermal behavior, the transformation peak of as-printed and post heat-treated NiTi parts is hardly detectable, resulting from microstructure inhomogeneity, as well as the Ti-rich impurity phases TiC~1-x~N~x~/Ti~4~Ni~2~O~x~ and Ti~2~Ni precipitates, which depress martensite transformation. Varying hatch angle and hatch spacing cannot visualize the transformation peaks. Aging treatment after high temperature annealing activates small R phase transformation peak, resulting from the stress field around the coherent Ni~4~Ti~3~ precipitates. Finally, the effect of hatch spacing, hatch angle and post heat-treatment on mechanical properties was studied. Hatch spacing and hatch angle have less influence on compressive strength and ductility of μ-SLM NiTi than post heat treatment. The compressive behavior of as-printed μ-SLM NiTi consists of three stages, representing elastic deformation of austenite, inhomogeneous stress-induced martensite transformation and elastic deformation of martensite, and plastic deformation of martensite, respectively. The compressive ductility reduces drastically after post heat-treatment because of the brittle Ti2Ni phase precipitating along the grain boundaries.


Micro Selective Laser Melting of Niti Shape Memory Alloy: Effects of Process Parameter and Post Heat Treatment

Division / Sponsor: 
Manufacturing Engineering Division (MED)

International Manufacturing Science and Engineering Conference - Paper Proceedings: https://asmedigitalcollection.asme.org/MSEC/MSEC2020/volume/84256

JSME 2020 Conference on Leading Edge Manufacturing/Materials and Processing - Paper Proceedings: https://asmedigitalcollection.asme.org/lemp/LEMP2020/volume/83624

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