MODELING OF DROPLET DYNAMIC AND THERMAL HISTORIES DURING SPRAY FORMING .3. ANALYSIS OF SPRAY SOLID FRACTION

The complex equations governing droplet dynamic and thermal behaviour during spray forming have been analysed to give an approximate expression for the spray solid fraction as a function of the process parameters of the form [GRAPHICS] where upsilon(gi) is the initial gas velocity at atomisation, M is the molten alloy mass flow rate, Delta T is the molten alloy super heat above the liquidus, and z is the axial distance. This expression provides an extremely convenient relationship to indicate the variation of spray solid fraction during spray forming, which is a key variable controlling final as sprayed billet microstructure. Predicted N-2 atomised Al-4 wt%Cu spray solid fractions based upon the analytical expression have been compared with equivalent results obtained from a detailed computer model described previously in Parts I and II under a wide range of conditions of upsilon(gi), M, Delta T and z and gave good agreement. Best-fit values of alpha(1), alpha(2) and alpha(3) were 1.9 x 10(-1),-1.4 x 10(-3) and 1.3 x 10(-3) respectively, in good agreement with predicted values.