Microstructure and mechanical property of additively manufactured NiTi alloys: A comparison between selective laser melting and directed energy deposition

Abstract

NiTi shape memory alloy (SMA) with nominal composition of Ni 50.8 at% and Ti 49.2 at% was additively manufactured (AM) by selective laser melting (SLM) and laser directed energy deposition (DED) for a comparison study, with emphasis on its phase composition, microstructure, mechanical property and deformation mechanism. The results show that the yield strength and ductility obtained by SLM are 100 MPa and 8%, respectively, which are remarkably different from DED result with 700 MPa and 2%. The load path of SLM sample presents shape memory effect, corresponding to martensite phase detected by XRD; while the load path of DED presents pseudo-elasticity with austenite phase. In SLM sample, fine grain and hole provide a uniform deformation during tensile test, resulting in a better elongation. Furthermore, the nonequilibrium solidification was studied by a temperature field simulation to understand the difference of the two 3D printing methods. Both temperature gradient G and growth rate R determine the microstructure and phase in the SLM sample and DED sample, which leads to similar grain morphologies because of similar G/R. While higher G×R of SLM leads to a finer grain size in SLM sample, providing enough driving force for martensite transition and subsequently changing texture compared to DED sample.

Keywords

Ni50.8Ti49.2 shape memory alloy / additive manufacturing / selective laser melting / laser directed energy deposition / mechanical properties

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