DIRECT MEASUREMENT OF SPRAYFORM TEMPERATURES AND THE EFFECT OF LIQUIDFRACTION ON MICROSTRUCTURE

Surface temperatures and the corresponding liquid fractions in a spray deposited commercial 2000 series Al alloy have been measured accurate ly by plunging thermocouples into sprayforms during growth. Two colour pyrometry was shown to be reasonably accurate for measuring surface t emperature but only under constant, or preferably zero, overspray dens ity. Yield, porosity and grain size have been shown to be strong funct ions of the top surface temperature and liquid fraction in the sprayfo rm during spray deposition, over a wide range of liquid fraction. The final grain size cannot be predicted from the liquid fraction using co nventional coarsening theories, because they do not take into account variations in nucleation density and are not applicable at low liquid fractions. Rapid variation of liquid fraction with top surface tempera ture during spray deposition implies that to maintain optimum conditio ns requires control of the temperature to within at least +/- 2 degree s C. (C) 1998 Acta Metallurgica Inc.