On the Transformation Behavior of NiTi Shape-Memory Alloy Produced by SLM

Abstract

Selective laser melting has been applied as a production technique of nickel titanium (NiTi) parts. In this study, the scanning parameters and atmosphere control used during production were varied to assess the effects on the final component transformation criteria. Two production runs were completed: one in a high (*1800 ppm O 2 ) and one in a low-oxygen (*220 ppm O 2 ) environment. Further solution treatment was applied to analyze precipitation effects. It was found that the transformation temperature varies greatly even at identical energy densities highlighting the need for further in-depth investigations. In this respect, it was observed that oxidation was the dominating factor, increased with higher laser power adapted to higher scanning velocity. Once the atmospheric oxygen content was lowered from 1800 to about 220 ppm, a much smaller variation of transformation temperatures was obtained. In addition to oxidation, other contributing factors, such as nickel depletion (via evaporation during processing) as well as thermal stresses and textures, are further discussed and/or postulated. These results demonstrated the importance of processing and material conditions such as O 2 content, powder composition, and laser scanning parameters. These parameters should be precisely controlled to reach desired transformation criteria for functional components made by SLM.

A Short Review on the Microstructure, Transformation Behavior and Functional Properties of NiTi Shape Memory Alloys Fabricated by Selective Laser Melting

“…[ 24 , 29 ]. It is worth mentioning that various optimized energy densities for NiTi alloys have been reported in literature, from 55 to 300 J/mm 3 [ 24 , 29 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ]. An even higher energy density of 595 J/mm 3 was used by Ma et al [ 42 ], by reducing the hatch spacing.…”

3D printing of shape memory alloy based smart structures

“…In a separate study performed by Speirs et al, they reported that the oxygen content present during the fabrication process had a strong effect on the transformation characteristics of the SLM NiTi samples [35]. as the scanning speed increased [35]. Unquestionably, more studies are required to validate these contradictory findings.…”

“…According to the information shown in [16,21,25,28] and the effect of process parameters [7,20,21,33,35] on the transformation characteristics, shape memory responses and mechanical properties of NiTi.…”

“…These two phases are pure NiTi (austenitic or martensitic phase) and precipitates (either In order to achieve full melting of the powder particles in a single scan, the amount of linear energy density directed to the powder bed would be relatively high. However, the high laser absorptivity and low heat conductivity of the powder particles as illustrated in reported, where huge deviations in the transformation temperatures could be observed [16,21,25,28,35]. Therefore, for the future research, homogenous NiTi powder and repetitive scanning approach can be utilised together.…”

“…3.1 presented the potential for controlling the chemical composition and transformation temperatures of SLM NiTi via the alteration of energy density. On the other hand, Dadbakhsh et al[7] and Speirs et al[35] demonstrated the possibility of influencing the transformation characteristics by varying the different SLM process parameters.In the research by Dadbakhsh et al, they came up with various combinations of parameters with similar magnitudes of energy density[7]. These sets of parameters were categorized as low laser parameters (LP) and high laser parameters (HP).…”