Solid-state sintering of aluminum powders is a key method for producing products of complex shape and a given microstructure. The process includes several stages: the formation of interparticle contacts, their growth due to diffusion of the substance, compaction of the structure and, finally, the removal of residual porosity. The speed and efficiency of sintering are determined by the temperature, holding time, size and shape of the powder particles, and the presence of alloying elements.
Aluminum matrix composites (AMCs) offer the possibility of combining the advantages of aluminum (lightness, corrosion resistance) with improved mechanical properties achieved by introducing reinforcing particles such as silicon carbide (SiC), aluminum oxide (Al2O3) or carbon nanotubes (CNTs). Solid-state sintering is a promising method for consolidating AMCs, ensuring uniform distribution of reinforcing elements in the matrix.
Instead of the traditional liquid phase sintering, a vacuum solid phase sintering process was investigated. Both free powders and hot forged quasi-isostatic samples were studied, made from commercial inert gas atomized aluminum powder (CIGA) and high purity aluminum powder. The latter was obtained by the gas atomization reaction synthesis (GARS) method, which allows obtaining a spherical powder with a minimal oxide film on the surface.
After 100 hours of vacuum sintering at 525 °C, SEM analysis showed that GARS Al powder exhibited significantly more advanced sintering stages compared to CIGA Al powder. The tensile test results of the forged samples revealed that despite the lower tensile strength (95 MPa versus 147 MPa), the ductility of GARS Al samples was higher than that of CIGA Al samples.
A model powder-based composite system reinforced with quasicrystalline Al-Cu–Fe particles was created by forging, where the same powder production methods as in the monometallic case were compared. Analysis of the cleavage surface revealed signs of improved adhesion between the matrix and reinforcing particles in the composite created from GARS powder, which is confirmed by interdiffusion in the alloy layer and is consistent with previously obtained tensile strength data. Overall, the results indicate the potential for using high-purity aluminum powders with a thin oxide film to simplify existing technologies for consolidating aluminum powders.
However, sintering of KAM is associated with a number of difficulties. Differences in the coefficients of thermal expansion between aluminum and reinforcing particles can lead to thermal stresses and deterioration of mechanical properties. In addition, reactions at the interface between the matrix and the reinforcement can lead to the formation of undesirable phases.
To overcome these problems, various methods are used, such as optimization of sintering parameters, application of protective atmospheres, modification of the surface of reinforcing particles and use of special additives. These approaches allow obtaining KAM with high density, improved mechanical properties and minimal defect content.
Author: F Tang,I. E. Anderson,SB Biner
Institute: 222 Development Metals, Ames Laboratory, Iowa State University, Ames, IA 50011-3020, USA