On the Mechanism of Mushy Layer Formation during DropletBased Processing

Droplel-based deposition was analyzed numerically by establishing a framework whereby the behavior of individual droplets was taken into consideration. The objective of this study was to establish the numerical formulation necessary to describe the thermal environment of a collection of individual droplets, and thereby enable the prediction of the conditions that lead to the formation of a mushy (solid/liquid) layer on the deposited materialʹs surface. The present results reveal that in the initial stage of deposition, the continuous mushy layer does not exist and the isolated mushy zones are confined to the scale of a single deformed droplet (a.k.a. splat). This is a consequence of the fast cooling, which leads to the complete solidification of individual droplets prior to arrival of the following ones. However, the temperature at the deposited materialʹs surface increases with its thickening because of an increase of internal conduction resistance, and it ultimately exceeds the solidus temperature at a certain distance above the substrate, thus resulting in the formation of the mushy layer. The factors that influence the formation of the mushy layer include deposition rate, liquid fraction, droplet size, and heat transfer coefficient at the deposited material and substrate interface. The temporal variation of interfacial heat transfer coefficient in the initial stage has a limited effect on the initially deposited material: however, it has no discernible influence on the cooling behavior of the deposited material once a critical thickness is attained.