Hot pressing densification of Al (Al–Cu) short Al2O3 fibre mixtures

Metal matrix composites (MMCs) are made by axial hot pressing (at constant temperature) of mixtures of Al (or Al–Cu) powders and short alumina ceramic fibres. The external pressure P was applied in two stages; in stage 1, pressure was increased at a fixed rate up to P max, then in stage 2, the pressure was maintained at Pmax for different time intervals. For Al based materials during stage 1, a graph of relative density D versus P gave a good fit according to Konopicky's model P = (2σy/3) ln[1/(1 – D)]; this allowed the yield stress (σy) to be estimated. The MMC gave increased values of σy compared with the matrix alone. Similar behaviour was found for materials based on mixtures of Al and Cu (to give about Al–4·5%Cu) at 450°C, that is for temperatures lower than 548°C (eutectic in the Al–Cu binary diagram). For higher temperatures (i.e. Al–Cu based compositions at 606–623°C), the Konopicky model was followed only at P values higher than about 15 MPa; for lower pressures, it also operated a liquid phase mechanism. It is proposed that at 15 MPa, the Cu diffuses from the liquid into the Al, forming an Al–Cu solid solution of increased σy. For stage 2, the densification data is well described by the power law creep densification model, with a best fitting exponent of n = 2, a value typical of superplastic materials.