Forming metal foam structures by cold-gas spraying

A foamed metal structure formed by introducing metal particles 14 and foaming agent particles 15 into a propellant gas 11, projecting the particles 14-16 and gas 11 onto a substrate 19 with the particles 14-16 having sufficient velocity to form a deposit of pressure welded metal particles 14 containing the foaming agent particles 15, and heating 24 at least the deposit 18 to thereby activate expansion of the foaming agent while softening the metal particles 14. Projection may be carried out using a supersonic nozzle 13 presenting a substantially rectangular jet cross-section profile with the gas being preheated to 300-500 0 F and pressurised to at least 350 psi. The metal particles 14 may be aluminium, alloys thereof, magnesium, alloys thereof, zinc or bronze. Reinforcing particles 16 (e.g. Si, SiC, TiC, SiO2, graphite) may be incorporated in the deposit 18. The foaming agent may be titanium hydride, calcium carbonate or thermally decomposable carbonates, nitrates or sulphates. The gas 11 may be nitrogen, air, helium or mixtures thereof.