Formation of TiAl intermetallics by heat treatment of cold-sprayed precursor deposits

Cold spray is a rapidly developing coating and manufacturing technology in which metal particles are deposited at velocities of over 700 ms−1 without significant melting. The severe plastic deformation of the particles on impact produces a deposit that is very dense, with low oxide content and no thermally induced tensile stresses. In this study, solid shapes of precursor material were made by the cold spray process using titanium and aluminium powder mixtures and then converted to titanium aluminide with suitable heat treatments. Such heat-treated material has been characterised using differential scanning calorimetry to trace the kinetics of the phase transformation, X-ray diffraction to determine the phases present and scanning electron microscopy with microprobe analysis to observe the microstructure and measure the chemical composition. It was found that the first intermetallic phase to form with heat treatment was TiAl3. The desirable set of intermetallic phases (TiAl3, r-TiAl2, TiAl and Ti3Al) was obtained by varying heat treatment conditions. While the porosity in the as-sprayed precursor deposits was about zero, significant porosity developed during heat treatment and the sources for this are discussed.