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-base d advanced aluminum alloys have been very limited and complicated, resultin g in a segregation of the applications primarily to the aerospace segment. Throughout any consolidation sequence for aluminum powders, the oxide and/o r hydroxide films on the typical powder surfaces can interfere with densifi cation and interparticle bonding. In fact, the consolidation sequence for m any 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 microstructur e. 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 coa ting 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 a bility to sinter. The powders were characterized with transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, qua drapole mass spectroscopy and with a new ultrasonic method for in situ sens ing of the evolution of sintering. In general, a marked reduction in the su rface film thickness and in the level of chemisorbed moisture and moisture- borne impurities was observed in the GARS powders. This change in powder su rface characteristics also was effective in promoting sintering processes i n the GARS powders, as monitored by our new technique. An initial direct co mparison of explosivity for the different types of aluminum powder revealed that the GARS powder also had a reduced hazard level. All of these finding s indicate that the GARS approach to aluminum powder production may enable mass-produced lightweight powder metallurgy parts from advanced aluminum al loys with simple consolidation processing techniques. Copyright (C) 2001 Jo hn Wiley & Sons, Ltd.