On the mechanism of mushy layer formation during droplet-based processing

Droplet-based deposition was analyzed numerically by establishing a framewo rk whereby the behavior of individual droplets was taken into consideration . The objective of this study was to establish the numerical formulation ne cessary to describe the thermal environment of a collection of individual d roplets, and thereby enable the prediction of the conditions that lead to t he formation of a mushy (solid/liquid) layer on the deposited material's su rface. 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 solidificati on of individual droplets prior to arrival of the following ones. However, the temperature at the deposited material's surface increases with its thic kening because of an increase of internal conduction resistance, and it ult imately exceeds the solidus temperature at a certain distance above the sub strate, thus resulting in the formation of the mushy layer. The factors tha t influence the formation of the mushy layer include deposition rate, liqui d fraction, droplet size, and heat transfer coefficient at the deposited ma terial and substrate interface. The temporal variation of interfacial heat transfer coefficient in the initial stage has a limited effect on the initi ally deposited material; however, it has no discernible influence on the co oling behavior of the deposited material once a critical thickness is attai ned.