Brief Paper: Long-Range Mass Transport Mechanisms in Solid State Sintering for Additive Manufacturing

Abstract Recent advancements in additive manufacturing (AM) such as binder jetting, vat photopolymerization and direct ink writing have made it possible to fabricate complex ceramic and metal structures ranging from simple geometry to complex lattice structures. The postprocessing step of sintering is crucial across numerous AM processes to attain the targeted mechanical properties and achieve precise resolution of the final geometry. Incorrect sintering process parameters and sintering schedule can lead to various defects within the sintered part such as shape distortion and high porosity which essentially lead to damage. Recent studies have shown that nonhomogeneous temperature in the green body arising from non-uniform heating and high temperature ramping is the main contributing factor in causing shape distortion and subsequent cracking. According to more studies, temperature gradients activate long-range mass transport mechanisms such as particle squeezing and biased diffusion which accumulate in the grain-scale and contribute to shape distortion of the printed part in the part-scale. Hence, it is of great interest to explore the mechanisms underlying shape distortion because of its potential in creating models for accurate prediction. Such models would allow control of the degree of distortion of parts to either preserve the initial shape or for applications in 4D printing, which will allow parts to change shape in response to external stimuli. To investigate the long-range mass transport mechanisms at the grain scale, a mesoscopic approach using phase-field modelling is applied in this study because of its ability to handle grain boundaries, grain morphology and incorporate mass diffusion mechanisms involved in the sintering process. The long-range mass transport mechanisms are studied by solving the Cahn-Hillard and Allen-Cahn Equations with rigid body motion for the sintering of particles in a representative volume element (RVE) of a part-scale geometry exposed to an initial temperature gradient. The mobility coefficients are assumed to be deformed Arrhenius type functions of temperature to capture the effects of sintering activation and temperature. The degree of distortion of RVEs will be investigated for initial temperature gradients and temperature ramp rate to study the mechanisms underlying shape distortion that reflect in the part-scale. This study aims to examine the influence of long-range mass transport under different particle configurations to provide further insights into the mechanisms of shape distortion in the sintering process, whether particle squeezing or biased diffusion or both is the mode of long-range mass transport laying foundations for the development of better predictions and design of the sintering process for digital twins.