Mechanisms of densification and bonding in ultrasonic consolidation of aluminum powder

Abstract

Conventional sintering-based powder consolidation processes produce powder metallurgy (PM) parts, but with persistent residual porosity, while pressure-assisted full-density powder consolidation processes require long exposure of powder to high temperature. An ideal full-density powder consolidation process may be defined as one in which both full densification and metallurgical bonding are achieved rapidly, economically, and without affecting the microstructure of the starting powder. Ultrasonic powder consolidation (UPC) is an innovative powder consolidation process developed at the Advanced Materials Processing Laboratory (AMPL) that can produce full-density, metallurgically integrated materials within a few seconds at low to moderate temperatures. However, the fundamental mechanisms of the rapid densification and particle bonding in UPC have yet to be fully understood in terms of the material behavior in high strain-rate severe plastic deformation (SPD). The present work was conducted to understand the microstructural evolution during the UPC of an aluminum powder in relation to the effects of crystal defects, particularly excess vacancies, on the shape change of powder particles that must occur for the densification and bonding to result. The material transfer required for particle shape change was found to occur by both plastic deformation and liquid flow. The rapid plastic deformation of particles involves dynamic recovery/recrystallization (DRV and CDRX) which is boosted in the presence of excess vacancies that would increase dislocation climb rate by many orders of magnitude. The densification by liquid flow occurs as liquid created at powder particle junctions is pumped to fill the interstices at particle triple junctions. The liquid-transferred material at the triple junctions exhibits a layered amorphous and nanocrystalline structure, reflecting the repetitive transfer of supercooled liquid produced at the particle junctions. With the unique ability of rapid powder consolidation at low temperatures, UPC provides a novel means of materials processing for various advanced materials. The findings from the present work will also lead to translational ultrasonic materials processing (UMP) technologies. --Author's abstract