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 express ion for the spray solid fraction as a function of the process paramete rs of the form [GRAPHICS] where upsilon(gi) is the initial gas velocit y 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 distan ce. This expression provides an extremely convenient relationship to i ndicate the variation of spray solid fraction during spray forming, wh ich is a key variable controlling final as sprayed billet microstructu re. 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 a nd z and gave good agreement. Best-fit values of alpha(1), alpha(2) an d alpha(3) were 1.9 x 10(-1),-1.4 x 10(-3) and 1.3 x 10(-3) respective ly, in good agreement with predicted values.