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 Mg2Si 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 Mg2Si and bright phase of β-Al12Mg17 along the boundary between dendrites of α-Mg solid solution. After hot forging at 350 °C, the microstructure has changed to relatively smaller sizes of β-Al12Mg17 and Mg2Si particles apart from larger grains surrounded by smaller grains due to dynamic recovery and recrystallization. Some of the Mg2Si particles crack during forging. In both the cast and forged composite, the Brinell hardness increases rapidly with increasing volume fraction of Mg2Si, 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 Mg2Si content. In forged composites, there is significant improvement in yield strength with increasing Mg2Si particles and also over those observed in their cast counterpart. In cast composites, ultimate tensile strength (UTS) decreases with increasing Mg2Si content possibly due to increased casting defects such as porosity and segregation, which increases with increasing Mg2Si content and may counteract the strengthening effect of Mg2Si content. However, in forged composite, UTS increases with increasing Mg2Si content until 5.25 vol pct due to elimination of segregation and lowering of porosity, but at higher Mg2Si content of 7 vol pct, UTS decreases, possibly due to extensive cracking of Mg2Si 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, JIC, decreases drastically in cast composites from that of Mg-9 wt pct. alloy designated as MA alloy due to the presence Mg2Si particles. Thereafter, JIC does not appear to be very sensitive to the increasing presence of Mg2Si particles. There is drastic reduction of JIC 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 Mg2Si particles. The ratio of the tearing modulus to the elastic modulus in cast composites shows a lower ratio, which decreases with increasing Mg2Si content. The ratio decreases comparatively more on forging of cast MA alloy than those observed in forged composites.