Technical note - 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, pre ssure was increased at a fixed rate up to P-max then in stage 2, the pressu re was maintained at P-max for different time intervals. For Al based mater ials during stage 1, a graph of relative density D versus P gave a good fit according to Konopicky's model P=(2 sigma(y)/3)ln[1/(1-D)]; this allowed t he yield stress (alpha(y)) to be estimated. The MMC gave increased values o f sigma(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 d egrees C, that is for temperatures lower than 548 degrees C (eutectic in th e Al-Cu binary diagram). For higher temperatures (i.e. AI-Cu based composit ions at 606-623 degrees C), the konopicky model was followed only at P valu es 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 l iquid into the Al, forming an Al-Cu solid solution of increased sigma(y). F or 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 typica l of superplastic materials.