Numerical study of the second ignition for combustion synthesizing Ni-Al compounds

The propagation of a combustion front during Ni-Al combustion synthesis is often extinguished halfway through the reaction, due to the lower exothermic heat of the metallic reactions. To facilitate the complete propagation of the combustion front, the reaction is always ignited again during the experimental demonstration. The position and time of the second ignition have been found to influence the subsequent temperature profiles. In this numerical study, the different second-ignition positions in the combusted region, the reacting region, and the preheating region, as well as the different second-ignition times before and after the stop of the first combustion front, are chosen to study the effect of the second ignition. The stable propagation is only observed as the reaction is ignited again in the reacting region. When the reaction is ignited a second time in the combusted region or the preheating region, part of the specimens cannot be completely synthesized due to the low combustion temperature. In addition, the combustion temperature may be significantly enhanced in the other area and results in a heterogeneous microstructure. Delay of the second ignition time is also found to increase the initial propagation velocity of the new combustion front. From the results generated in this study, a process map of the second ignition is established. The process map provides the appropriate second-ignition conditions to propagate the combustion front completely and achieve a product of homogeneous microstructure.