Crystallization in additive manufacturing of metallic glass

Sammanfattning: Metallic glasses are non-crystalline metals that are obtained by rapid cooling of the melt to bypass crystallization. The amorphous atomic structure shows enhanced properties relative to the crystalline counterpart. For example, enhanced mechanical properties, improved corrosion resistance, as well as excellent soft magnetic properties. The unique properties of metallic glasses make them promising for a wide range of applications, e.g. spring materials,structural components, electrical motors, and biomedical implants. One drawback is the cooling rate required for glass formation, which limits the thickness of cast components to only a few millimeters. As a solution, additive manufacturing (AM) shows promising potential to produce large-scale metallic glass components. In AM, the solidification process is short and confined to a small volume that is repetitively added. Despite the highheating and cooling rates in AM, control of crystallization is still an issue and a complete understanding of the interplay between the thermal process and crystallization is missing. This thesis presents numerical simulations and experimental analyses related to the formation and growth of crystals in a Zr-based bulk metallic glass. The aim is to provide a better understanding of crystallization in metallic glasses during non-isothermal processing, with special emphasis on AM by laser powder bed fusion (LPBF). The experimental investigations involved in-situ small-angle neutron scattering measurements of nucleation and growth of crystals in a Zr-based metallic glass processed by LPBF and suction casting. It is concluded that crystals form at a higher rate in the material processed by LPBF as a result of the increased oxygen content. Further, the crystallization mechanisms were identified as rapid nucleation followed by diffusion-controlled growth in both materials. The numerical simulations are based on phase-field and classical nucleation and growth theory, which were developed to study the nucleation, growth, and dissolution of crystals in metallic glasses. The models have been used to predict time-temperature-transformation and continuous-heating/cooling-transformation diagrams, but also to simulate the crystallization process during LPBF by utilizing thermal finite element simulations of the laser-material interaction. The simulation results demonstrate several important aspects of crystallization in the LPBF process, such as the effect of rapid heating and cooling on the nucleation rate, the importance of the growth mode during cyclic reheating as well as the resulting gradients in particle size and density arising from localized laser processing. In particular, the results emphasize that numerical models that track the evolution of the particle size distribution are well suited for modeling crystallization in LPBF processing of metallic glass.