Development of an In-Situ Alloyed Microstructure in Laser Additive Manufacturing

Abstract

Additive Manufacturing (AM) processes are gaining prominence in industry as they can build parts to near-net-shape with minimal postprocessing. Metal laser AM techniques, such as Selective Laser Melting (SLM), offer rapid cooling rates on the order of 10^5-10^6 K/s. This is due to a highly-focused laser heating a microscopic volume in an otherwise lower-temperature environment. Hence, metal laser AM can manufacture novel, out-of-equilibrium microstructures that cannot be produced in near-net-shapes with other processes. It is desirable to optimize feedstocks for metal AM processes to leverage their advantages. One option of optimizing feedstocks is through in-situ alloying, or by using elemental powders. Elemental powders homogenize over the course of multiple laser passes, or intrinsic heat treatments. However, rapid cooling rates prevent the homogenization of a layer when first printed. To investigate the homogenization process, this thesis used synchrotron X-ray Diffraction (sXRD) to track the phase transformations during the SLM of a 14-layer single wall (single-hatch, multilayered) of Ti-1Al-8V-5Fe (Ti-185) from elemental Ti, Fe and an alloyed AlV powders, capturing frames at 250 Hz. Infrared imaging was performed simultaneously on the surface at 1603.5 Hz to observe the temperature changes at the surface. Post-mortem electron microscopy was performed on cross-sections of the wall perpendicular to the scanning direction to observe the changes in the microstructure with respect to the build direction. Specifically, Electron Dispersive X-Ray Spectroscopy and Electron Backscatter Diffraction were performed to observe the alloying elemental distribution and microstructure of the wall with respect to the build direction. The research performed found that in the melted zone, phase transformation times below 50 ms yielded a partially-alloyed microstructure, with regions concentrated and dilute in alloying elements. Partial mixing was diffusion-induced by laser beam heat and the exothermic heat of mixing of Ti-185 from its constituent elements. Further diffusion during reheating cycles yielded an alloyed microstructure.