AN ANALYSIS OF FREEZE-UP PHENOMENA DURING GAS ATOMIZATION OF METALS

A numerical model is developed to describe the flow and heat transfer behavior of molten metals during flow in the delivery tube in gas atom ization and spray deposition. Numerical simulations for Al, Cu, Mg, Ni . Ti and W melt are conducted to investigate the influence of processi ng parameters and material properties on the minimum melt superheat th at is necessary to prevent the tube from premature solidification duri ng delivery of the molten metal prior to atomization. Processing maps are developed to provide direct insight into the complex relationship among the minimum melt superheat, processing parameters and material p roperties. A quantitative correlation is obtained by means of a regres sion analysis of the numerical results, which Facilitates application of the numerical model. The calculated results demonstrate that For th e materials studied, the minimum melt superheat ranges from 0.005T(m) to 0.19T(m), depending on processing parameters and material propertie s. The dependence can be expressed using a correlation derived from th e regression analysis such as [GRAPHICS] Increasing the overpressure c an effectively decrease the minimum melt superheat, especially for a l arge tube-length: diameter ratio and for materials possessing low dens ities. This effect diminishes with increasing overpressure. The minimu m melt superheat can also be decreased by reducing the tube length: di ameter ratio, by selecting a smooth delivery tube with low thermal con ductivity and thick tube wall, and/or by enhancing the ambient gas tem perature. Materials with high thermal conductivity, high thermal capac ity and/or large density allow a small melt superheat to prevent the d elivery tube from freeze-up, while materials with high melting tempera ture and/or high viscosity require a large melt superheat.