Effect of Holding Time on Densification, Microstructure and Selected Properties of Spark Plasma Sintered AA7075-BC Composites.

The paper presents the effect of the holding time, varying between 1 min 15 s and 10 min, on the microstructure evolution and development of selected properties of spark plasma sintered AA7075-based composites reinforced with 3, 5 and 10 wt% sub-micro BC powder. The sintering temperature and the compaction pressure were 500 °C and 80 MPa, respectively. Composites with a near full density of 96-97% were obtained. Microstructure studies were performed employing the techniques of light microscopy and scanning electron microscopy, along with an analysis of the chemical composition in micro-areas. Additionally, the phase composition was investigated by means of X-ray diffraction. In addition, hardness and flexural strength tests were performed. It was found that the holding time did not significantly influence the microstructures of the examined materials nor the hardness or flexural strength. The sintered composites had a fine-grained microstructure with a strengthening phase located at the grain boundaries. As a result of the spark plasma sintering process, fine precipitates of intermetallic phases were also observed in the aluminum grains, suggesting partial supersaturation, which occurred during fast cooling.