Investigation of the microstructure and phase evolution across multi-material Ni50.83Ti49.17-AISI 316L alloy interface fabricated using laser powder bed fusion (L-PBF)

Abstract

This study evaluates the phase and microstructural evolution of additively manufactured (AM) Nickel Titanium (NiTi) alloy, across the interface with 316L stainless steel build plate, in order to understand the processing parameter (input power, layer thickness and scan speed), composition, and microstructure interrelationships necessary to achieve excellent multi-material bonding between NiTi and 316L. The effect of the process parameters utilised was characterised using the Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD), X-ray diffraction (XRD), and Energy-dispersive X-ray spectroscopy (EDX). SEM/EBSD results demonstrated, for the first time, that the microstructure and phase close to the interface was complex and comprised martensite, austenite and Fe phases, sequentially arranged in a layered sandwich pattern across the build direction. This complexity was necessary for excellent bonding. The L-PBF process parameters influenced the diffusion behaviour and the concentration of elements found at the interface. The diffusion rate of Fe and Ti across the NiTi-316L interface was 3.05×10-6m2/s and 3.27×10-8m2/s, respectively, representing a 93.27-fold increase. The observed microstructural and phase evolution is related to the generated interface chemistry and the thermomechanical history related strain resulting from the L-PBF process.