Solidification in metal additive manufacturing: challenges, solutions, and opportunities

The physics of alloy solidification during additive manufacturing (AM) in methods such as laser powder bed fusion (LPBF), electron beam powder bed fusion (EPBF), and laser directed energy deposition (LDED) is distinct due to the combination of (a) the rapid solidification conditions that often prevail in AM, (b) adjoining scan tracks that result in the overlap of the adjacent melt pools, and (c) layer-wise fabrication that causes the pre-deposited layer to influence the subsequent layer’s microstructural evolution. The complex interplay between these and each alloy’s distinct solidification characteristics results in a wide spectrum of hierarchical microstructures that span multiple length scales, with diverse grain morphologies and non-equilibrium phases. Consequently, a detailed understanding of the solidification phenomena that occur during LPBF, EPBF, and LDED is necessary for controlling the microstructural evolution, which ensures repeatable and predictable mechanical response of the built part and, hence, structural reliability of it in service. Keeping this in view, substantial efforts have been made to develop a detailed understanding of the solidification during AM of alloys, which are summarised in this review. From the local interfacial equilibrium applicable to a range of rapid solidification conditions to non-equilibrium conditions that prevail during ultra-fast solidification are reviewed. Numerical efforts ranging from the atomic scale to the macro-scale have been reviewed to highlight the phenomena of dislocation evolution, grain growth, and phase formation during solidification. Specific challenges, such as solidification cracking in non-weldable alloys and porosity-cracking dilemmas, are discussed. Unique opportunities for tailoring microstructures, such as in-situ alloying, are presented.