Document Type : Research Paper
Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran.
One of the main challenges in processing of metal matrix nanocomposites through the powder metallurgy method is achieving a dense compact with minimum internal porosity. Pores act as stress risers and deteriorate the mechanical properties of nano-materials. In the present investigation, powder mixtures of commercially pure Al (CP-Al) and 5252 Al alloy reinforced with nanometric SiC particles (0-7 wt.%) were produced by in situ powder metallurgy (IPM) method. These powders were consolidated through cold compaction, sintering and hot extrusion processes and subjected to density measurements, microstructural studies and thermal analysis. Microstructural studies showed that SiC nanoparticles formed a continuous network around the CP-Al powders, restricting effective densification during the cold compaction stage. This network was also shown to prevent metal-to-metal contact during sintering, especially at higher SiC contents. Therefore, a remarkable decrease in the sintered relative density was observed with increasing SiC contents in the CP-Al/SiC compacts. However, in the 5252 Al/SiC composite powders, the SiC nanoparticles embedded within the alloy matrix during the IPM process. As a result, a more homogeneous SiC particle distribution was attained. This led to enhanced cold densification and improved sinterability compared with those of CP-Al/SiC powder mixture. Besides, the presence of Mg in the 5252 alloy matrix was effective in reducing the oxide film covering the Al particles. The differential scanning calorimetry (DSC) revealed the formation of liquid phase during the sintering of 5252 Al/SiC powder compacts. As a result, mass transfer promoted through the liquid phase sintering enhancing densification. However, improved densification was obtained after hot extrusion of the nano-SiC reinforced composites. Results showed that the pressure required for extrusion increased with increasing SiC content. This was attributed to the enhanced redundant work induced by SiC particles.