Ignition and combustion of Al·Mg alloy powders prepared by different techniques

Alloys of aluminum and magnesium have long been explored as potential reactive materials and replacements of pure aluminum powders in energetic formulations. It has been recently shown that mechanical alloying (MA) can be used to prepare a range of Al·Mg powders with different compositions and particle sizes. Conventionally, such alloys are prepared by melt processing; however, no direct comparisons of combustion characteristics of such alloys prepared using different methods are available. This work is aimed at comparing the oxidation, ignition, and combustion characteristics for MA and cast-alloyed (CA) Al·Mg powders with similar bulk compositions and particle sizes. Particle size distributions are measured using low-angle laser light scattering. Electron microscopy and X-ray diffraction are used to examine particle morphology and phase makeup, respectively. Thermal analysis is used to identify the low-temperature reactions that could affect ignition. Ignition behavior of the two powders is studied using a heated filament ignition apparatus. Constant volume explosion experiments are performed to compare effectiveness and rate of combustion of the aerosolized alloy particles. A laser ignition setup is used to characterize combustion rates and temperatures for individual alloy particles. Low-temperature exothermic features were observed for the MA powder but not for the CA powder in thermo-analytical experiments. MA powders had slightly lower ignition temperatures than CA powders. Aerosol combustion experiments showed a substantial increase in both the maximum pressure and rate of pressure rise for the MA powders as compared to the CA powders. In single particle laser ignition experiments, MA particles ignited more readily than CA particles. MA powders burned in a staged sequence, with the first stage dominated by combustion of Mg and the second stage primarily representing combustion of Al. No similar staged combustion behavior was observed for the CA powders, which generated very short emission pulses with a relatively low brightness, and thus might not have burned completely. It is proposed that the difference in the structure and morphology between the MA and CA particles results in different ignition and combustion scenarios.