Sintering densification process of powder metallurgy aluminum alloy

In this study, an Al-Mg-Si-Cu alloy was prepared by conventional press-and-sinter powder metallurgy techniques using pure Al powder, Cu element powder, and binary Al-Mg and Al-Si powders to investigate the processes of atom diffusion and microstructure evolution. The relative density of the sintering samples exceeded 98%. It is found that the sintering densification process can be approximately divided into three stages. In the first stage (from room temperature to 460℃), after the Al-Mg eutectic liquid phase formed at 450℃, the Mg atoms in the liquid diffuses into Al and Al-Si particles and reacts with Al₂O₃ at the metal/oxide interface to form an Al-Mg-O compound. Meanwhile, the interdiffusion between Al and Cu leads to the formation of Al₂Cu compounds. In the second stage (from 460 to 560℃), the micro-channels or small holes between the grain boundaries are rapidly filled by Al-Cu and Al-Si eutectic liquids, which leads to a significant increase of density. In this stage, the densification mechanisms are particle rearrangement controlled by the capillary driving force and contact flattening dominated by solution-reprecipitation. In the last stage (from 560 to 600℃), the residual large holes are finally filled by the liquid because of the enhancement of wettability and grain growth with the increase of sintering temperature. In this stage, the densification mechanism is mainly pore-filling, resulting from the grain growth. The sample is fully dense through this stage. In addition, MgAl₂O₄ and MgAlCuO compounds are found in the grain boundary region; thus, it can be speculated that the mechanism of oxide film disruption is related to the alloy composition. Furthermore, because of the good wettability between Al-Cu liquid and Al, the surface of Al particles is quickly wetted by the liquid under the capillary driving force; therefore, no AlN is found in the grain boundary region in this research.