Control of mechanical and shape memory characteristics in martensitic NiTi by setting L-PBF parameters and build orientation

Abstract

The current work encompasses a study of as-fabricated martensitic NiTi samples printed via Laser Powder Bed Fusion (L-PBF) in the horizontal and vertical build orientations, with systematic variations in the L-PBF process parameters. The sample densities, compositions, thermal characteristics, and mechanical responses were analysed and correlated with L-PBF parameters. Mechanical responses measured and analysed included microhardness, impact energies, and elastic modulus. The process parameters were found to have a significant effect on the resulting properties. The variation in physical properties and mechanical responses between each sample of the vertical build were less compared to those measured for the horizontal built samples. The L-PBF samples were compressed to 6% engineering strain (within elastic limit), and their shape memory strains were thermally recovered. Digital Image Correlation (DIC) based full-field strain analysis was performed during compression to measure local strain fields which revealed in detail the macroscopic effects during detwinning. The shape memory capability of the L-PBF samples was quantised based on strain recoveries attained after thermal recovery. Microstructural variations in submicron scale were also analysed in as-fabricated, post-compression and post-recovery stages, showing the sample response during different deformation mechanisms.

Keywords

• Laser powder bed fusion (L-PBF)
• Digital image correlation (DIC)
• Nitinol
• Shape memory alloys
• Phase transformation