Determining the role of surfaces and interfaces in the powder metallurgy processing of aluminum alloy powders

The current options for solid-state consolidation processing of powder-based advanced aluminum alloys have been very limited and complicated, resulting in a segregation of the applications primarily to the aerospace segment. Throughout any consolidation sequence for aluminum powders, the oxide and/or hydroxide films on the typical powder surfaces can interfere with densification and interparticle bonding. In fact, the consolidation sequence for many low-strength aluminum alloy powder metallurgy parts involves transient liquid-phase sintering to massively disrupt the powder surfaces, producing improved bonding but introducing a coarsened resolidification microstructure. Although preventing aluminum oxide formation is nearly impossible during aluminum powder production, the gas atomization reaction synthesis (GARS) method - an advanced powder production technique - can modify the oxide coating and enable improved consolidation processing. This report compares the effects of the GARS process and other representative atomization processes on the surface structure and properties of aluminum powders and on their ability to sinter. The powders were characterized with transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, quadrapole mass spectroscopy and with a new ultrasonic method for in situ sensing of the evolution of sintering. In general, a marked reduction in the surface film thickness and in the level of chemisorbed moisture and moisture-borne impurities was observed in the GARS powders. This change in powder surface characteristics also was effective in promoting sintering processes in the GARS powders, as monitored by our new technique. An initial direct comparison of explosivity for the different types of aluminum powder revealed that the GARS powder also had a reduced hazard level. All of these findings indicate that the GARS approach to aluminum powder production may enable mass-produced lightweight powder metallurgy parts from advanced aluminum alloys with simple consolidation processing techniques. Copyright (C) 2001 John Wiley & Sons, Ltd.