Speaker
Description
Additive manufacturing (AM) generates complex multi-scale microstructures in metallic components, differing from conventional microstructures in terms of grain structure, cell substructure, elemental distribution, and residual stress. It is therefore important to analyse AM microstructures across multiple length scales using correlative microscopy techniques to establish comprehensive understanding of the microstructural evolution. This will unlock a greater level of microstructure and property control for future applications of AM.
We investigate 17-4 precipitate hardening stainless steel, a high-strength multi-purpose alloy with applications in the aerospace and petrochemical industries. Samples were fabricated by laser powder bed fusion using a variety of scanning patterns and differing laser powers. We utilise scanning electron microscopy, electron backscattered diffraction, neutron scattering analysis, electron probe microanalysis, and atom probe tomography to characterize the microstructure across different length scales. Results show clear variation in sample texture, residual stress, phase fraction and distribution, and compositional variation as a function of the processing parameters. The main phase of the as-printed microstructure is δ-ferrite with a low volume fraction of austenite, which is mainly observed at melt pool boundaries. The total fraction of austenite increases as the time between adjacent passes of the laser decreases. In the as-printed state, samples are shown to be highly stressed, suggesting heat treatments must be applied before any practical implementation in engineering applications. Higher laser power leads to an increase in ferrite grain size and intensity of <100> texture along the build direction due to the enhanced effect of epitaxial growth.