Highly tuned gas atomization for controlled preparation of coarse powder

While close-coupled gas atomization has been demonstrated for highly controlled production of fine powders in many alloy systems, defense applications for Mg powder demand that such an atomization system also be capable of producing a narrow standard deviation for coarse Mg powders. To develop this capability, a series of 5 gas atomization experiments were conducted with Al, as a less hazardous surrogate for Mg, over a range of a very low atomization gas pressures. The optimum result produced an average particle diameter of about 460 gm with a standard deviation of 1.53 using Ar atomization gas, sufficiently close to the 500 gm target size. These experiments achieved the process uniformity needed for this narrow standard deviation by stabilizing melt filming with an expanded discrete jet, close-coupled atomization nozzle and a slotted trumpet bell pour tube. Initial analysis of the size results indicated that decreased atomization gas velocity, acting within the acceleration wave model, was the key controlling variable in this low-pressure regime. No consistent influence of gas/melt mass flow ratio was detected in the data. Experimental observation of the atomization spray images also indicated that a practical lower limit to atomization gas pressure is about 69 kPa for achieving atomization process uniformity with the method of this study.