Controlling microstructure evolution and phase transformation behavior in additive manufacturing of nitinol shape memory alloys by tuning hatch distance

Jia-Ning Zhu,Vera Popovich,Vitaliy Bliznuk,Marcel Hermans,Roumen Petrov,Evgenii Borisov,Xiaohui Liang,Anatoly Popovich,Richard Huizenga

doi:10.1007/s10853-022-07007-z

Abstract

Laser powder bed fusion (L-PBF), categorized as additive manufacturing technique, has a capability to fabricate NiTi (Nitinol) shape memory alloys with tailorable functional properties and complex geometries. An important processing parameter, hatch distance (h), is often related to macroscale structural defects; however, its role on controlling the microstructure and functional properties is usually underestimated in L-PBF of NiTi. In this work, equiatomic NiTi (50.0 at% Ni) parts were fabricated with various hatch distances to tailor the microstructure and their shape memory characteristics. Contrary to what is observed in Ni-rich NiTi alloys, in this work, we demonstrate that phase transformation temperatures of L-PBF equiatomic NiTi do not decrease proportionally with hatch distance but rather relate to a critical hatch distance value. This critical value (120 μm) is derived from the synergistic effect of thermal stress and in situ reheating. Below this value, epitaxial grain growth and in situ recrystallization are enhanced, while above, irregular grains are formed and dislocations induced by thermal stresses decrease. However, the critical value found herein is characterized by high dislocation density and fine grain size, resulting in a superior thermal cyclic stability. The proposed finite element model is proven to be an effective tool to understand and predict the effect of hatch distance on grain morphology and dislocation density evolutions in L-PBF NiTi SMAs. In the present study, we provide a comprehensive understanding for in situ controlling L-PBF NiTi microstructure and functional characteristics, which contributes to designing 4-dimensional shape memory alloys.

Highlights

Due to the reversible martensitic phase transformation, shape memory effect (SME) and pseudo-elasticity are manifested in NiTi (Nitinol) shape memory alloys (SMAs) [1, 2]
Based on the results from the single laser track finite element modeling (FEM) simulations, overlapping zones slightly decrease with increasing hatch distances (Fig. 11a and d) and there will be no overlap when the hatch distance is larger than 140 lm (Fig. 11a), which is consistent with our previous work [12]
The present work investigated the effect of hatch distance on microstructure, phase transformation and thermal cyclic stability in NiTi alloys fabricated via laser powder bed fusion

Summary

Introduction

Due to the reversible martensitic phase transformation, shape memory effect (SME) and pseudo-elasticity are manifested in NiTi (Nitinol) shape memory alloys (SMAs) [1, 2]. For near equiatomic NiTi (50 at% Ni) alloys, the reversible martensitic phase transformation can be accomplished by the one-step phase transformation, i.e., the phase transformation between the high-temperature stable austenite (cubic B2-type crystal structure) and the low-temperature stable martensite (monoclinic B19-type crystal structure) [1, 3, 4]. Owing to their remarkable properties, NiTi SMAs are widely used in fields, such as actuators [5], dampers [6], sensors [7] and medical devices [8, 9]. For the fabrication of L-PBF NiTi parts, main processing parameters include laser power (P, W), scanning velocity (v, mm s-1), layer thickness (t, lm) and hatch distance (h, mm). The hatch distance was mainly selected as a fixed value to prevent structural defect formation (such as lack of fusion and surface roughness) [26–28], and its role in microstructural evolution, phase transformation behavior and functional properties were underestimated and ignored

Methods

Results

Discussion

Conclusion