Microwave activated combustion synthesis of porous Al–Ti structures for composite reinforcing

A research on porous Al–Ti materials produced by combustion synthesis is presented. Some of the manufactured preforms could be infiltrated with liquid aluminium alloy to obtain locally reinforced castings. The synthesis progress was analysed, as well as a structure formation, its phase composition, homogeneity degree and type of porosity. Cylindrical green compacts were prepared of Al and Ti powders with various stoichiometric ratios. Synthesis was performed in a specially designed microwave reactor, by placing a compact in a waveguide, in intensely focused field. Temperatures recorded by a pyrometer showed that the synthesis propagation accelerated at the moment of Al3Ti creation. The highest synthesis temperature was reached by the specimens containing 45–67% Al (all percentages atomic). Microscopic examinations revealed rounded grains, sometimes with cores of solid solution Ti(Al). The grains were basically composed of AlTi3 that changed to AlTi at the edge. The grain structure was similar in almost all kinds of the specimens, especially in those with higher Al content. Around those grains, envelopes of AlTi and Al2Ti were formed. Chemical composition of the matrix was approximately equivalent to stoichiometric ratio of the initial powder mixture. XRD investigations confirmed multiphase structure of the obtained compacts and a slight effect of the Ti powder granularity. Some of the specimens revealed regular structure with interconnected porosity. Those materials containing 75, 67 and 55% Al were used for producing the aluminium alloy-based composites.