Laser powder bed fusion (LPBF) of NiTi alloy using elemental powders: the influence of remelting on printability and microstructure

Abstract

Purpose The purpose of this paper is to investigate the effect of remelting each layer on the homogeneity of nickel-titanium (NiTi) parts fabricated from elemental nickel and titanium powders using laser powder bed fusion (LPBF). In addition, the influence of manufacturing parameters and different melting strategies, including multiple cycles of remelting, on printability and macro defects, such as pore and crack formation, have been investigated. Design/methodology/approach An LPBF process was used to manufacture NiTi alloy from elementally blended powders and was evaluated with the use of a remelting scanning strategy to improve the homogeneity of fabricated specimens. Furthermore, both single melt and up to two remeltings were used. Findings The results indicate that remelting can be beneficial for density improvement as well as chemical and phase composition homogenization. Backscattered electron mode in scanning electron microscope showed a reduction in the presence of unmixed Ni and Ti elemental powders in response to increasing the number of remelts. The microhardness values of NiTi parts for the different numbers of melts studied were similar and ranged from 487 to 495 HV. Nevertheless, it was observed that measurement error decreases as the number of remelts increases, suggesting an increase in chemical and phase composition homogeneity. However, X-ray diffraction analysis revealed the presence of multiple phases regardless of the number of melt runs. Originality/value For the first time, to the best of the authors’ knowledge, elementally blended NiTi powders were fabricated via LPBF using remelting scanning strategies.

Synthesis and Characterization of Innovative Fe–Cr–Mn Ferritic Stainless Steel by Laser Powder Bed Fusion

“…[ 27 ] Remelting with the ED of 33 J mm −3 was applied in the LPBF process of NiTi and it was found to eliminate the cracks effectively in the case of lower laser power (25 W) and scanning speed (1000 mm s −1 ). [ 28 ] Considering the melting behavior is quite critical for the crack elimination mechanism during rescanning, it should be further investigated based on the in situ melt pool monitoring under different LPBF parameters.…”

“…The crack elimination with laser rescanning was studied for direct energy deposition process (DED) [23][24][25][26] but rarely reported for LPBF. [27,28] Ferritic stainless steels are not so frequently produced as austenitic stainless steel using LPBF. The major challenges lie in the relatively low ductility of the ferritic phase compared to the austenitic structure.…”

“…The crack elimination with laser rescanning was studied for direct energy deposition process (DED) [ 23–26 ] but rarely reported for LPBF. [ 27,28 ]…”

The Effect of Microstructural Defects on High-Cycle Fatigue of Ti Grade 2 Manufactured by PBF-LB and Hydrostatic Extrusion

“…These laser scanning parameters gave a summarized energy density of approximately 130 J/mm 3 , and they were used to create fully dense titanium parts based on our previous studies [28,29]. A detailed explanation of the optimization methodology for the fabrication of titanium parts using the PBF-LB process is provided in [30]. The laser pattern utilized in this study involved a 5 × 5 mm checkerboard zone with alternating laser scanning and a 90 • rotation for every opposite checkerboard zone, as described in more detail in [31].…”

Mechanical Properties of Ti Grade 2 Manufactured Using Laser Beam Powder Bed Fusion (PBF-LB) with Checkerboard Laser Scanning and In Situ Oxygen Strengthening

“…For all fabricated samples, the layer thickness was 25 µm, the energy density used for powder consolidation was 120 J/mm 3 (15 µm point distance and 40 µs exposure time resulting in a scanning speed of 375 mm/s, hatch distance of 40 µm and laser power 45 W). This set of parameters was optimized on 100 cubic samples of 125 mm 3 (5 × 5 × 5 mm) according to the procedure described in our previously reported research [32]. In brief, from a set of 100 different laser powers, scanning speeds, distances between scanning vectors and distances between exposure points and exposure times, we have chosen one set of parameters where the theoretical density of the fabricated samples was an average of 99.9% of the theoretical density of Ti grade 2.…”