Sintering Mechanisms in Metal Extrusion-Based Sintering-Assisted Additive Manufacturing: State-of-the-Art and Perspectives

Extrusion-based sintering-assisted additive manufacturing (ES-AM) enables the fabrication of intricate metal structures, spanning from simple geometries to complex lattice structures. Sintering plays a vital role in metal densification that requires effective design and optimization of sintering processes for high-quality sintered parts. Notably, sintering behaviors in ES-AM differ from those in traditional methods, primarily due to the heterogeneous distribution of particles and pores induced by the anisotropic fabrication nature of additive manufacturing (AM). This review offers an overview of sintering processes and mechanisms fundamental to ES-AM. Theories governing solid-state sintering and liquid-phase sintering are summarized to advance a thorough comprehension of the associated sintering mechanisms. Computational studies on sintering processes at different length scales are also discussed, including atomic-level molecular dynamics, microlevel simulations (Monte Carlo, phase field, and discrete element method), and macroscopic continuum models. The distinctive anisotropic sintering behaviors in the ES-AM process are further elucidated across multiple levels. Ultimately, future directions for ES-AM, encompassing materials, sintering process, and sintering mechanisms, are outlined to guide research endeavors in this field. This review summarizes multiscale sintering behaviors in both traditional manufacturing and AM, contributing to a deeper understanding of sintering mechanisms and paving the way for innovations in the next generation of manufacturing.