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

Abstract

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.

Consolidation and Forging Methods for a Cryomilled Al Alloy

“…The compact must be deformed by shear stresses to break up the Al oxide/hydroxide layers of the prior particle boundary (PPB) structure and obtain sufficient tensile ductility. [4] For example, the tensile ductility of a cryomilled Al 5083 billet consolidated only by hot isostatic pressing (HIP) is generally very low, despite the achievement of nearly full density. [5] A combination of HIP and extrusion has often been used in the past to produce cryomilled Al alloy rod or bar with high strength.…”

The Effect of Particle Shape on the Sintering of Aluminum

“…Similar observations on commercially atomized powders have been made elsewhere. [13] Additional, measured characteristics of the as-received aluminum powders are collected in Table II, which also gives the sintered densities of the Al-4Sn alloy fabricated from these powders. The irregular powders have a consistently higher sintered density (88 to 91 pct) than the spherical powders (65 to 73 pct).…”

Research on Semisolid Microstructural Evolution of 2024 Aluminum Alloy Prepared by Powder Thixoforming

“…However, the coarsening does not operate during the 10-30 min period. Alumina layers have already formed on the surfaces of the alloy powders [23]. The great difference in the coefficient of thermal expansion between Aluminum and alumina will generate stress on the oxide shell during heating, leading to a dynamic oxide breakage and repair process if there is any oxygen in the atmosphere [24].…”