A physically-based process-structure-property model for additively manufactured materials

Abstract

Successful application of additive manufacturing (AM) across a broad range of industries requires stable and predictable performance of AM materials. However, the microstructure and properties of AM parts vary widely based on different process parameters. Modelling the interdependent relationship based on comprehensive understanding of process, structure and property helps reduce the uncertainties and leads to enormous time and cost savings and efficiency for process design and selection of manufacturing parameters, compared to the current trial-and-error approach. This thesis presents the development of a physically-based model for process-structure-property (PSP) control of AM. A key process-structure motivation is to capture the effect of the layer-by-layer AM process and quantify microstructure variables which control mechanical behaviour, such as different solidification morphologies, phase fractions and grain size. Thus, a finite element (FE) model of the AM process for AM Ti-6Al-4V is developed for the prediction of thermal history and spatial distributions of temperature. Based on the relationships between FE predicted thermal history, thermal gradient and key manufacturing parameters such as laser power and scanning speed, an AM process map for different solidification morphologies, including columnar-to-equiaxed transition, is developed. Phase transformation kinetics for the non-isothermal steps is adopted and implemented within a stand-alone code based on the FE predicted thermal histories of sample material points. The structure-property component is a physically-based Taylor-type model with microstructure-sensitivity, including prediction of yield strength, ultimate tensile strength, uniform elongation and flow stress (strain hardening), for AM Ti-6Al-4V. The interdependent effects of solutes, grain size, phase volume fraction and dislocation density are explicitly included. Solid-state phase transformation and dislocation density evolution are incorporated to simulate the effects of different phase at high temperature. Predictions are validated by comparison with measured tensile test data for (i) effects of additive manufacturing process conditions (such as build orientation and sample size) on tensile properties, based on the microstructure attributes inherited from the process, and (ii) the effect of temperature on tensile stress-strain response across a broad range of temperatures. In addition, a computational multi-scale homogenization based model is developed to study the orientation-dependent plastic deformation of stainless steel 316L (SS316L) with tailored single-crystal-like structure fabricated by laser beam powder bed fusion (PBF-LB). The polycrystal model, based on the visco-plastic self-consistent (VPSC) formulation, considers deformation by slip and twinning, adapting a dislocation-based approach with latent hardening, including slip-slip, slip-twin, twin-slip and twin-twin interactions, to account for the orientation-dependent strain hardening behaviour. Tensile responses along the <100>, <110> and <111> crystallographic directions were studied and compared to experiments, which show considerably different strength ratios in different orientations from a standard face-centered cubic (FCC) alloy. It is shown that the orientation-dependent superior strength-ductility behaviour of textured PBF-LB SS316L can be attributed to the beneficial effects of deformation twinning and the cellular sub-grain structure. The developed process-structure-property model is a building block towards an AM-design tool for industry, which not only can be used for the process parameter optimization, but also for the tailoring of microstructure and subsequent mechanical properties.