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Abstract 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. 

The mechanical properties of bimetallic composites are significantly influenced by their interfacial morphologies. This study delves into the impact of various heat treatment conditions on the microstructure and mechanical attributes of steel/nickel bimetallic (17-4PH/IN625) components produced through extrusion-based sintering-assisted additive manufacturing (ES-AM). The bimetallic composites were annealed at 1150°C for 1, 4, and 8 h, followed by an aging treatment at 482°C for samples annealed for 8 h. After annealing, microstructural heterogeneities, including variations in grain size and elemental distribution within the transition zone close to the interface, were observed. It was found that in the diffusion transition zone between the two alloy layers, the diffusion of iron (Fe) and nickel (Ni) elements increased with longer holding times, as corroborated by microhardness tests and quantified through theoretical parabolic diffusion law. The transition zone exhibited two distinct areas: an Fe-predominant zone and a Ni-predominant zone, with the latter containing oxides and molybdenum (Mo)- and niobium (Nb)-rich precipitates. No new phases emerged post-heat treatment; however, shifts in peak due to stress relaxation and the emergence of precipitates were identified through X-ray diffraction (XRD) observations. Microhardness within the transition zone increased following heat treatment, peaking at 186 HV1.0 after a 4-h annealing. The optimal heat treatment condition was identified as 1150°C for 4 h, which facilitated the development of uniform microstructures and improved bonding strength. This study demonstrates an enhanced interfacial bonding strength in 17-4PH and IN625 bimetallic parts manufactured through ES-AM, suggesting their wide-ranging potential applications in industry.