Laser processing of net-shape NiTi shape memory alloy

Abstract

Fabrication of bulk NiTi alloy parts by conventional processing contains undesirable intermetallic phases in varying proportions depending on the composition and requires postfabrication heat treatments, which necessitate development of new processing routes. We have fabricated net-shape samples, with and without designed porosity, from equiatomic NiTi alloy powders using laser engineered net shaping (LENS), a commercially available rapid prototyping technique. Laser-processed NiTi samples retained the desired phases of initial powder and showed higher hardness due to the retention of high-temperature cubic B2 phase. Thermally induced stresses and defects, due to high cooling rate in laser processing, increased the reverse transformation temperatures of laser-processed samples. LENS-processed samples also showed high compressive strengths (plastic flow stress) in the range of 890 to 1,050 MPa due to finer grain size. This article discusses the influence of laser parameters on microstructure, mechanical properties, and phase transformation behavior of fully dense NiTi parts produced via LENS. © The Minerals, Metals & Materials Society and ASM International 2007.