Tensile and Fracture Properties of Cast and Forged Composite Synthesized by Addition of Al-Si Alloy to Magnesium

Abstract

Cast Mg-Al-Si composites synthesized by addition of Al-Si alloy containing 10, 15, and 20 wt pct of Si, in molten magnesium, to generate particles of Mg 2 Si by reaction between silicon and magnesium during stir casting has opened up the possibility to control the size of these particles. The microstructure of the cast composite consists of relatively dark polyhedral phase of Mg 2 Si and bright phase of b-Al 12 Mg 17 along the boundary between dendrites of a-Mg solid solution. After hot forging at 350°C, the microstructure has changed to relatively smaller sizes of b-Al 12 Mg 17 and Mg 2 Si particles apart from larger grains surrounded by smaller grains due to dynamic recovery and recrystallization. Some of the Mg 2 Si particles crack during forging. In both the cast and forged composite, the Brinell hardness increases rapidly with increasing volume fraction of Mg 2 Si, but the hardness is higher in forged composites by about 100 BHN. Yield strength in cast composites improves over that of the cast alloy, but there is a marginal increase in yield strength with increasing Mg 2 Si content. In forged composites, there is significant improvement in yield strength with increasing Mg 2 Si particles and also over those observed in their cast counterpart. In cast composites, ultimate tensile strength (UTS) decreases with increasing Mg 2 Si content possibly due to increased casting defects such as porosity and segregation, which increases with increasing Mg 2 Si content and may counteract the strengthening effect of Mg 2 Si content. However, in forged composite, UTS increases with increasing Mg 2 Si content until 5.25 vol pct due to elimination of segregation and lowering of porosity, but at higher Mg 2 Si content of 7 vol pct, UTS decreases, possibly due to extensive cracking of Mg 2 Si particles. On forging, the ductility decreases in forged alloy and composites possibly due to the remaining strain and the forged microstructure. The initiation fracture toughness, J IC , decreases drastically in cast composites from that of Mg-9 wt pct. alloy designated as MA alloy due to the presence Mg 2 Si particles. Thereafter, J IC does not appear to be very sensitive to the increasing presence of Mg 2 Si particles. There is drastic reduction of J IC on forging of the alloy, which was attributed to the remaining strain and forged microstructure, and it is further lowered in the composites because of cracking of Mg 2 Si particles. The ratio of the tearing modulus to the elastic modulus in cast composites shows a lower ratio, which decreases with increasing Mg 2 Si content. The ratio decreases comparatively more on forging of cast MA alloy than those observed in forged composites.