Effects of sintering techniques and parameters on microstructure and mechanical properties of Al-15Si-2,5Cu-0.5Mg compacts and Al-15Si-2,5Cu-0.5Mg/B4C composites

Abstract

In this study, Al-Si compacts and Al-Si/B4C composites were produced from hypereutectic Al-Si alloy powders by cold pressing/conventional sintering and cold pressing/microwave sintering techniques. The effects of sintering temperature, sintering time and B4C particle addition on microstructural properties, density, hardness and transverse rupture strength were investigated. The phases and intermetallic compounds formed in the microstructure were determined by X-ray diffraction. Densities of the samples were measured by Archimedes’ technique. Macro hardness measurements were carried out with Brinell hardness test. Three-point bending tests were applied to the samples according to ASTM-B528–16. Elemental Al regions, master alloy (Al-Si-Cu-Mg) regions, primary Si particles and Cu- and Mg-rich secondary phases were determined in the microstructure. It was determined that the Cu- and Mg-rich secondary phases are θ(CuAl2), β(Mg2Si), γ(Al2CuMg) phases. An increase in porosity was determined in conventional sintered samples compared to green Al-Si specimens. In addition, it was observed that the amount of porosity increases, grain size and primary Si grains coarse with increasing sintering temperature in conventional sintering. The change in the amount of pores of the samples produced by microwave sintering without holding time at the sintering temperature is not as obvious as in conventional sintering. Similar microstructures were obtained in the samples at increasing sintering temperatures in microwave sintering. In the samples produced by conventional sintering, the hardness value decreased with increasing sintering temperature. Addition of 5 wt% B4C caused an increase in hardness at all sintering temperatures. The addition of 10 and 15 wt% B4C caused a decrease in hardness. In microwave sintering, the general trend in hardness change was a decrease with increasing B4C particle addition and increasing sintering time. Transverse rupture strength values close to and similar to each other in conventional sintered samples were obtained at different sintering temperatures. Transverse rupture strength values increased with the addition of 5 wt% B4C particles, and decreased with the addition of 10 and 15 wt% B4C particles.