Directed energy deposition combined with interlayer remelting for improving NiTi wear resistance by grain refinement

Abstract

Directed energy deposition of NiTi coatings has received much attention, however, its performance is difficult to meet the increasing requirements in the engineering field. In order to refine the grains to improve wear resistance, a novel interlayer remelting method for fabricating NiTi alloys by directed energy deposition is proposed in this study. The influence of laser interlayer remelting parameters on lattice structure, phase transformation behavior, microstructure, crystallographic evolution, and microhardness was thoroughly analysed in comparison. Good formability is demonstrated after interlayer remelting, while the high energy density leads to microstructure refinement and eutectic regions are observed between dendrites. The grain size decreases with increasing remelting speed, which is due to the faster cooling rate having a greater degree of subcooling with more nucleation points. The faster interlayer remelting speed shows a higher microhardness (586 HV_0.2) with higher wear resistance, which is attributed to its finer grain size and significant grain boundary strengthening. Reciprocating sliding wear experiments show that the wear resistance is improved after interlayer remelting, especially at a load of 30 N, the specific wear rate is 1.35 * 10⁻⁴mm³/Nm, which is only 47 % of that of the untreated sample. This study proposes a technique to achieve synergistic enhancement of microhardness and sliding wear properties of reinforced coatings, which provides a theoretical basis for the application of NiTi coatings in engineering and extends the field of additive manufacturing.