ANALYSIS OF SOLIDIFICATION IN SPRAY ATOMIZED AND CODEPOSITED METAL-MATRIX COMPOSITES .2. REINFORCEMENT INJECTION AND DEPOSITION

The influence of the injection of reinforcing particles (for the produ ction of metal matrix composites and of the droplets-td-substrate heat transfer on the resulting microstructural uniformity of spray atomize d and codeposited composite material is analyzed. The reinforcement pa rticles injection velocity has to be limited between an upper and a lo wer critical values, in order to ensure entrapment into the matrix dro plets in flight. The thermal history of the injected droplets during t he deposition stage is calculated with the assumption that the in-flig ht solidifying droplets reach the substrate while containing still at least 20% liquid volume fraction, in order to avoid porosity of the de posited material. The substrate to pouring-tube orifice distance where that condition is achieved depends strongly on the atomization pressu re and the convective heat transfer coefficient of the substrate. It i s demonstrated that ''tailoring'' the microstructures and the reinforc ement volume percent in the deposited material is feasible. The critic al process parameters: the atomization pressure, the melt flow rate, t he substrate to pouring-tube orifice distance, the reinforcement parti cles injection location and rate can all be adequately chosen in order to obtain any desired microstructure, grain size, reinforcement volum e percent, with the additional benefit, if wanted, of rapid solidifica tion processing.