Comparative study on crystallographic orientation, precipitation, phase transformation and mechanical response of Ni-rich NiTi alloy fabricated by WAAM at elevated substrate heating temperatures

Abstract

In this investigation, a Ni-rich NiTi alloy was in-situ deposited with different substrate heating temperatures and the evolution of crystallographic orientation, precipitation, phase transformation, and mechanical responses were evaluated. The experimental results indicated that with the increment of substrate heating temperature from 150 °C to 350 °C, the average B2 grain size and the high angle grain boundaries (HAGBs) gradually increased from 53.44 μm to 85.38 μm and 53.6%–62.4%, respectively. The crystallographic texture exhibited a dominant, strong (001) orientation with comparatively weak (111) and (101) orientations in all conditions and the intensity of {100}<001> increased slightly as the substrate heating temperature increased. Moreover, Ni₄Ti₃ precipitates with an inhomogeneous size distribution were identified within the B2 NiTi matrix. Increasing the substrate heating temperature coarsened the Ni₄Ti₃ precipitates. All the phase transformation temperatures increased when the substrate heating temperature increased, indicating that the martensitic transformation is more likely to occur. As the substrate heating temperature increased from 150 °C to 350 °C, the yield stress and ultimate tensile stress decreased from 683.9 to 513.1 MPa and 855.2 to 743.8 MPa, respectively, and the ductility decreased from 6.90% to 6.13%. In addition, a remarkable ε_ir, poor recovery ratio and a broad stress hysteresis were obtained during the initial deformation of the cyclic loading-unloading tension. The highest recoverable strain (ε_re), recovery ratio and elastic energy storage efficiency (ƞ) were obtained in samples processed with the lowest substrate heating temperature. These findings provide useful references concerning process optimization in fabricating Ni-rich NiTi components by WAAM with acceptable microstructure and mechanical properties.

Introduction

NiTi alloys possess unique characteristics, such as shape memory effect, superelasticity behavior, good biocompatibility, and excellent corrosion resistance, as such, NiTi is a highly promising material across extensive fields of engineering, such as automotive, aerospace, and biological engineering [[1], [2], [3]]. Nevertheless, the inherent poor machinability limited the development of geometrically complex components for conventionally processed NiTi alloys [4]. Recently, additive manufacturing (AM) of NiTi alloys has gained attention due to cost savings and the potential to build more accurate geometric shapes [5]. Most of the relevant studies on AM NiTi are devoted to the evolution of microstructure, phase transformation, and mechanical response, as a function of process optimization, spatial dependence, or post-treatments [[6], [7], [8]]. It is worth noting that the functional properties of NiTi are extremely sensitive to the martensitic transformation, which relies on the chemical constitution, second-phase precipitation, texture, and defects [5,7]. Therefore, AM process parameters require careful control as these will define the microstructural characteristics (grain information, texture, precipitates, etc.) and martensitic transformation temperature, which control the mechanical responses of NiTi alloys [7,[9], [10], [11]]. A higher energy input will generate a superheated melt pool to liquefy more material, and thus a prolonged duration for solidification is required. A lower cooling rate caused by a higher heat input will bring about the generation of coarser grains and Ni-rich precipitates [12]. Further, the grain morphology will change from a columnar shape to an axial appearance with an increment of laser scanning speed. A higher scanning velocity was also reported to cause a decrease in the martensitic transformation due to a higher cooling rate reducing the fraction of Ni-rich precipitates [13]. These precipitates were revealed to contribute to increasing the transformation temperatures by reducing the local Ni constituent, inducing incoherency stresses, and acting as martensite nucleation sites [14].

However, through extensive literature research, scarcely any investigations focused on the effects of substrate preheating temperature on the material properties of AM-processed NiTi alloys. The substrate preheating temperature is one of the most significant process parameters during AM deposition that can influence the microstructure and properties of the as-printed component. Besides, preheating the substrate could effectively control the residual stress through decreasing the cooling rate and temperature gradient, and can also reduce defects, such as solidification cracking [15]. In other alloy systems, the importance of substrate heating on mechanical properties can readily be established. Li et al. [16] revealed that the texture, phase and nano hardness of selective laser melting (SLM) processed TiAl alloys could be tailored with optimal substrate preheating temperatures during the deposition. Ding et al. [15] investigated the effects of substrate preheating temperature on the 12CrNi₂ steel processed by laser cladding and found that the surface residual stress reduced at elevated substrate preheating temperatures. Wu et al. [17] proved that the substrate preheating temperature should be controlled within a certain range for the geometric accuracy and minimization of surface oxidation during the wire arc additive manufacturing (WAAM) deposition process of Ti6Al4V alloys, even through the microstructure and mechanical properties were not significantly affected. Li et al. [18] concluded that the interface bonding degree and the solidification microstructure were strongly affected by the substrate temperature during the SLM deposition of AlNiYCoLa metallic glass. According to the studies conducted by Shen et al. [19], a sufficient substrate heating temperature was necessary during the WAAM deposition of the first few layers of Fe₃Al based iron aluminide alloys due to the high thermal conductivity. Furthermore, an optimal combination of strength and plasticity could be achieved by adjusting the substrate temperature. From the above discussion, it is clear that the substrate temperature is a significant factor for controlling the shape, microstructure and mechanical properties of materials fabricated by AM methods.

The fabrication of Ni-rich NiTi alloys using WAAM method either through in-situ alloying or feeding dedicated NiTi wire has been proved to be feasible [[20], [21], [22]]. For the current research, the influence of substrate heating temperature on the microstructure evolution, texture, precipitation, phase transformation and mechanical response of WAAM-processed Ni-rich NiTi alloy was comprehensively studied. This research will provide guidelines for the optimum process parameters for the fabrication of NiTi components using WAAM.