Ultrafast fabrication of high-density Al–12Si compacts with gradient structure by electro-discharge sintering

Abstract

Inert gas-atomized Al–12Si powder was used as a model to study the gradient structure in high-density Al–12Si compacts fabricated by electro-discharge compaction. The gradient structure was characterized by a gradual change of hardness in the radial direction. Detailed characterization of the microstructure confirmed that the gradual changes of composition and microstructure in the radial direction were responsible for the hardness gradient. As the initial transition resistance in the powder column was positively correlated to the distribution of the pressure field, the preferred path of current flow was predetermined by the pressure field. Consequently, the gradient structure was developed by the combined effects of inhomogeneous Joule heating and the pressure field. This process was explained in a simple and clear manner using parallel circuit theory. Unlike for high-density Al–12Si compacts fabricated by spark plasma sintering, the ultrafast processing time of electro-discharge compaction and the concentration of high energy in the powder column led to retention of the fine microstructure of the original powder in the electro-discharge compacts to a great extent, with metallurgical bonding only among particle boundaries. As a result, the hardness was almost twice that of the spark plasma sintered compacts and cast Al–12Si alloy. Thus, this approach is a promising high-throughput powder metallurgy method for the fabrication of small-scale functionally gradient materials.