(742f) Ignition and Combustion of Al∙Mg Alloy Powders Prepared By Different Techniques

Aluminum is a high combustion enthalpy metal capable of boosting the energy density of energetic formulations used in propellants, explosives, and pyrotechnics.  A number of approaches have been investigated that shorten aluminum ignition delay, increase burn rate, and decrease the tendency of aluminum droplets to agglomerate.  Aluminum-based materials, including its alloys are of interest as alternatives to pure aluminum powders used as fuel additives in energetic formulations.  In the past, relatively limited studies were performed comparing combustion characteristics of pure Al and Al∙Mg alloys.  It has been recently suggested, in particular, that Al∙Mg powders prepared by mechanical alloying offer reduced ignition delays and increased combustion temperatures.  Alloyed powders with similar compositions can also be prepared by melt processing.  There have been no direct comparisons of combustion characteristics of such alloys prepared using different methods.  This work is aimed at comparing the  oxidation, ignition, and combustion characteristics between two powders of Al·Mg with similar bulk compositions: one produced via mechanical alloying, and another, produced via grinding of a cast alloy.  These comparisons are expected to clarify which processes and materials characteristics govern ignition and combustion for Al∙Mg alloys.  In particular, the processes to be considered as affecting ignition include formation and oxidation of metastable intermetallic phases upon material heating, oxidation at the surface grain boundaries, and selective low-temperature evaporation and oxidation of magnesium. 

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.  A study of ignition behaviors of the two powders relied on a heated filament ignition apparatus.  It provides information on ignition temperature trends as a function of heating rate.  Thermal stability and temperature-dependent phase transformations are also studied using differential scanning calorimetry and thermogravimetric analysis in both oxidative and inert environments to observe reactions with and in the absence of O₂, respectively.  The mechanically alloyed material exhibits low-temperature weakly exothermic processes, which are not detected for the melt cast alloy. 

Constant volume explosion experiments (used to study aerosol combustion behavior) were performed for similarly sized mechanically-alloyed and grinded cast alloy powders.  A higher maximum pressure and a shorter ignition delay were observed for the mechanically alloyed powder.  The rate of pressure rise for the mechanically alloyed powder was almost twice that of its grinded cast alloy counterpart.  The mechanically-alloyed powder has a broader size distribution than the commercial powder, which may contribute to its enhanced performance.