Hot pressing densification of Al (Al–Cu) short Al 2 O 3 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 P_max 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 = (2σ_y/3) ln[1/(1 – D)]; this allowed the yield stress (σ_y) to be estimated. The MMC gave increased values of σ_y 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 σ_y. 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.