Comprehensive Semi-Analytical and Experimental Optimization of Additively Manufactured Zr-Based Bulk Metallic Glass: Insights into Nano- and Global-Scale Relaxation and Crystallization

Abstract

Laser powder bed fusion (LPBF) of Zr-based bulk metallic glasses (BMGs) has recently attracted attention due to its capacity for microstructure control, the potential for custom-tailored properties, and its versatile applicability across various industries. Achieving a balance between relative density, relaxation, and crystallinity is crucial for the 3D printing of amorphous components. However, prior findings have not revealed an exclusive optimization concerning low fractions of crystallinity (<5 vol.%) and atomic-scale effects, e.g. the degree of relaxation of the amorphous structure during the LPBF process. This study employs a systematic experimental approach, complemented by semi-analytical modeling, to comprehensively understand the nano- and macro-scale amorphous structure and the associated mechanical behavior. Differential scanning calorimetry along with flexural stress-strain measurements disclose that the highest relaxation and crystallization enthalpies, which are obtained for samples printed with lower heat input, are not in correlation with optimum mechanical properties. Instead, samples printed with optimum heat input (ΔH=44) reveal the maximum density range (~99.95%) and laboratory-XRD amorphous structure, leading to the highest flexural strength (~2080 MPa), elastic deformation (~5.5%), and the lowest Young’s modulus (~70 GPa). In this case, the optimum crystallinity (0.02-0.2 vol.%) and relaxation enthalpy (45-55 J/g) are calculated from the modeling and experimental results. Computed heating and cooling rates reveal elevated rates corresponding to increased heat input at designated locations under laser irradiation; nevertheless, heat-affected zones (HAZs) are subjected to diminished rates. This behavior not only corroborates the complex thermal history occurring during LPBF but also explains crystal formation in HAZs that experience lower heating and cooling rates.

Keywords: Bulk metallic glass (BMG), Laser powder bed fusion (LPBF), Semi-analytical simulation, relaxation, crystallization