Deformation Rate-dependent Behavior of AlSi10Mg Alloy 3D-Printed via Laser Powder Bed Fusion

Abstract

Strain rate sensitivity, deformation rate-controlling mechanisms, and work hardening behavior of AlSi10Mg alloy, 3D-printed via laser powder bed fusion, were comprehensively assessed as a function of strain, strain rate and stress state, considering the as-built optimized microstructure. Accordingly, in the first step, the process parameters were optimized for as-built samples printed with a new specialty spherical powder before further assessment of mechanical performance of 3D-printed samples; i.e., different powder attributes were compared with the common machine’s default powder with non-spherical particle morphology. A novel 4-step process parameter optimization method was developed for the first time. The proposed approach involves the effective use of prevalent energy density/enthalpy models complemented by an in-depth microstructural and mechanical characterization. Using a formulation derived from functioning characteristics of a pulsed-wave laser system, a reliable volumetric energy density (VED) model is developed, from which a trend in VED-porosity relationship is identified. This 4-step optimization method resulted in the selection of “SO1”, sample with superior mechanical properties (ultimate tensile strength of ~461 MPa and elongation to fracture of ~9.8%). This optimized as-built sample was used for all further microstructural and mechanical analysis. A strong non-linear strain rate sensitivity behavior was observed after room temperature tensile and compressive deformation at different constant strain rates (quasi-static range, 10-4-10-1 s-1, and medium-dynamic regime, 1-5 s-1). For both stress states, except for a negative strain rate sensitivity in 10-2-10-1 s-1 range, a positive trend was observed. This negative strain rate sensitivity was correlated with the deformation-induced microstructural evolution, i.e., manifested as a gradual transition in the dominant dislocation-precipitate interaction mechanism from shearing to looping, along with the disappearance of dislocation-solute interaction in the form of dynamic strain aging. The precise measurement of activation volume evolution with flow stress further confirmed the transition from shearing to looping in the 10-2-10-1 s-1 range. Shear band formation was observed for the first time after compressive deformation above ~0.05-0.07 strains, with higher intensity at higher strain rates (e.g., 5 s-1). Using the Kocks-Mecking model, the unique work hardening behavior of as-built samples were correlated to the evolution of extra-fine, non-equilibrium as-built microstructure during deformation.