Modifying the ignition, combustion and agglomeration characteristics of composite propellants via Al-Mg alloy additives

The performance of propellants, explosives and pyrotechnics is known to benefit from the use of metallic alloys as fuel additives. Previous studies have shown that switching from pure Al to Al-Mg alloy can improve the agglomeration and combustion features of composite propellants. However, the optimal ratio of Al and Mg has yet to be explored. We experimentally examine the effects on ignition, combustion and agglomeration arising from substituting pure Al particles with four different Al-Mg alloys whose Mg content ranges from 10 to 60%. Using thermogravimetric−differential scanning calorimetry, we find that switching to Al-Mg alloy can enable oxidation to occur earlier and can increase the oxidation rate and the enthalpy of thermal reaction, which is found to be positively correlated with the Mg content. Using laser ignition measurements, we find that the addition of Mg can promote the ignition and combustion of Al, with (Al/Mg)_0.5 exhibiting the most balanced combination of a high combustion intensity, a short ignition delay, and a high mass burning rate. Through high-pressure measurements taken at 1 to 7 MPa, we find that switching to Al-Mg alloy can increase the propellant burning rate (by nearly a factor of two) and the pressure exponent (from 0.23 to 0.43). We also find that the burning rate and pressure exponent are both highest for the propellant containing (Al/Mg)_0.1, but that both quantities decrease with further increases in Mg content. High-speed microscopic surface imaging at 1 MPa reveals that increasing the Mg content can increase the overall combustion intensity of the propellant and decrease the diameter of spherical agglomerates, consistent with the observed size distribution of the condensed combustion products. From these measurements, it can be concluded that (Al/Mg)_0.5 provides the optimal balance of ignition, combustion and agglomeration characteristics in the formulation of composite propellants containing Al-Mg alloy.