Laser Powder Bed Fusion of NiTiHf High-Temperature Shape Memory Alloy: Effect of Process Parameters on the Thermomechanical Behavior

Mohammadreza Nematollahi,Michael J Mills,Mohammad Elahinia,Othmane Benafan,Haluk Karaca,S Ehsan Saghaian,Guher P Toker,Alejandro Hinojos,Keyvan Safaei

doi:10.3390/met10111522

Abstract

Laser powder bed fusion has been widely investigated for shape memory alloys, primarily NiTi alloys, with the goal of tailoring microstructures and producing complex geometries. However, processing high temperature shape memory alloys (HTSMAs) remains unknown. In our previous study, we showed that it is possible to manufacture NiTiHf HTSMA, as one of the most viable alloys in the aerospace industry, using SLM and investigated the effect of parameters on defect formation. The current study elucidates the effect of process parameters (PPs) on the functionality of this alloy. Shape memory properties and the microstructure of additively manufactured Ni-rich NiTiHf alloys were characterized across a wide range of PPs (laser power, scanning speed, and hatch spacing) and correlated with energy density. The optimum laser parameters for defect-free and functional samples were found to be in the range of approximately 60–100 J/mm3. Below an energy density of 60 J/mm3, porosity formation due to lack-of-fusion is the limiting factor. Samples fabricated with energy densities of 60–100 J/mm3 showed comparable thermomechanical behavior in comparison with the starting as-cast material, and samples fabricated with higher energy densities (>100 J/mm3) showed very high transformation temperatures but poor thermomechanical behavior. Poor properties for samples with higher energies were mainly attributed to the excessive Ni loss and resultant change in the chemical composition of the matrix, as well as the formation of cracks and porosities. Although energy density was found to be an important factor, the outcome of this study suggests that each of the PPs should be selected carefully. A maximum actuation strain of 1.67% at 400 MPa was obtained for the sample with power, scan speed, and hatch space of 100 W, 400 mm/s, and 140 µm, respectively, while 1.5% actuation strain was obtained for the starting as-cast ingot. These results can serve as a guideline for future studies on optimizing PPs for fabricating functional HTSMAs.

Highlights

High-temperature shape memory alloys (HTSMAs) are a subgroup of shape memory alloys (SMAs) with a martensitic transformation temperature (TT) above 100 ◦ C
This study focuses on the thermomechanical properties of several selective laser melting (SLM)-fabricated NiTiHf specimens with multiple process parameters (PPs) and the effects of SLM PPs on the microstructure and properties
This study provides generalwindow guidelines to select the optimum of energy density properties of and PPs to tailor the shape memory properties of SLM NiTiHf parts

Summary

Introduction

High-temperature shape memory alloys (HTSMAs) are a subgroup of shape memory alloys (SMAs) with a martensitic transformation temperature (TT) above 100 ◦ C. Examples of the potential implementation of HTSMAs in aeronautics include actuators for the deployment mechanism of protective shrouds and seals for booster ring segments of rockets [2], variable geometry chevrons for jet noise reduction [3], adaptive components for gas turbines [4], and more recently, the development of NiTiHf/Zr torque tubes for shape-morphing airplanes [5]. NiTiPd alloys with Pd less than 35 at.% showed high irrecoverable strain during thermal cycling under load. They displayed other favorable characteristics, such as good work output, low hysteresis, and a reasonable shape memory effect (SME) under some loading conditions [10]

Methods

Discussion

Conclusion