Metal composites forged to save weight

Metal matrix composites can now be forged by large amounts, sufficient to make aircraft wheels. Tom Shelley reports

Eureka Metal matrix composites, long stuck in the laboratory, are increasingly being used in commercial products, and can now be successfully forged to shape by large amounts. Dunlop Aerospace recently revealed that they are able to forge aircraft wheels and brake components out of silicon carbide particle reinforced aluminium metal matrix composites. The process is far from easy, since a very high degree of deformation is required to forge a wheel, and the presence of ceramic particles makes this difficult. If the metal matrix does not have sufficient time to be able to flow round the particles during the forming process, the billet cracks. Dunlop has been working with HDA forgings, now Mettis Aerospace in Redditch. Rather than use impacts as in traditional forging, the process appears to involve relatively slow movements in a 12,000 tonne press. Neither Dunlop nor Mettis Aerospace were keen to explain exactly how they had managed to forge their metal matrix composites, so we asked Professor Peter Degischer at the Vienna University of Technology Institute of Materials Science and Testing. He replied, "More than ten years ago I was involved in forging particulate reinforced connecting rods, helicopter parts, and brake discs based on the Duralcan AA6061/Al2O3/15-20p aluminium oxide reinforced aluminium material. Forging of MMCs requires more care than unreinforced aluminum in avoiding tensile strains. Superplastic behaviour could be exploited, but is not necessary. It can be done by skilled forging. Forge de Bologne in France forges silicon carbide reinforced helicopter blade sleeves for the Eurocopter France N4 and EC120 helicopters. Its use reduces rotating mass by nearly 14 kg over titanium. Fatigue strength is 270 MPa at 107 cycles compared to 155-180 MPa for 2024-T4 and 7075-T6. "Nowadays, deformation simulations are executed to optimise the shape of the starting material for a given die and to plan the forging steps. We can produce flow-curves to determine the forging temperature window and are currently involved in developing the forging parameters for a particulate reinforced titanium alloy." The advantage of particulates in metal matrix composites is that they resist deformation, allowing use of thinner and lighter sections. Aircraft designers constantly look for ways to reduce weight, in order to increase the ratio of revenue generating load to the weight of the aircraft. Reduced rotating mass also decreases loads on braking systems as well as aiding acceleration. Forged MMC wheels, however, do not yet seem to have made it into the high performance end of the automotive sector, although there is no reason why this should not happen in time. The fundamental material costs are less than for carbon fibre. Pointers It is now economically possible to forge wheels and other components requiring high amounts of deformation from metal matrix composites Commercial uses are presently restricted to aerospace, but there is no fundamental reason why high deformation forging of MMCs cannot find their Dunlop Aerospace Mettis Aerospace Assessment of Metal Matrix Composites for Innovations thematik network