FLUID-FLOW, HEAT-TRANSFER, AND SOLIDIFICATION OF MOLTEN-METAL DROPLETS IMPINGING ON SUBSTRATES - COMPARISON OF NUMERICAL AND EXPERIMENTAL RESULTS

A mathematical representation has been developed, and computed results are presented describing the spreading and solidification of droplets impacting onto a solid substrate. This impingement is of major practical interest in plasma spraying and spray forming operations. Experiments in which molten metal drops were made to impinge onto a substrate were used to test the model. High-speed videography was used to record the spreading process, which typically took a few milliseconds for the experimental conditions employed. A comparison was made of the theoretical predictions with the experimental measurements; these were found to be in very good agreement, suggesting that the theoretical treatment of the model is sound. These calculations permit the prediction of the time and extent of the spreading process, the solidification rate, and the effect of process parameters, such as droplet size, droplet velocity, superheat, and material properties, provided that a value of the thermal contact coefficient is known. The most important finding of the modeling work is that for large droplets (approximately 5-mm diameter) with low impinging velocities (approximately 2 m/s), spreading and solidification appear to take place at comparable rates; in contrast, for small (approximately 100-mum diameter) particles impacting at a high velocity (approximately 100 m/s), the time scale for spreading appears to be shorter than the time scale for solidification (within the range of parameters of this study.)