Abstract
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•Laser-powder bed fusion (L-PBF) fabrication of functionally graded nickel-titanium (NiTi)-AISI 316L material demonstrated.•Excellent bonding between NiTi and AISI 316L alloys controlled via complex bands of multi-material layers.•Phase and microstructural evolution controlled through the L-PBF settings.•Multi-material phase and microstructure formation/evolution mechanisms elucidated.
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.