Effect Of Cobalt Addition On Iron-bearing Phases In Hypereutectic Al-Si Alloys

Hypereutectic Al-Si alloy is a kind of very promising materials. It has the following characteristics:low coefficient of thermal expansion (CTE), excellent abrasion resistance and corrosion resistance, admirable dimensional stability to heating, relatively low density and eminent heat conductivity. Due to this it has a wide range of applications in manufacturing pistons. In the practical production, there are always some impurities containing iron in the aluminum alloys. And the morphology of the iron-bearing phases in Al-Si alloys is usually shown as long needles. That could lead to bad effect on the mechanical properties of alloys significantly. However, the addition of Fe could enhance the high-temperature mechanical properties somewhat. To reduce the harmful effect of iron and improve high-temperature tensile strength, a study was conducted aiming at this problem. The study mainly focused on changing the iron-bearing intermetallic compounds in hypereutectic Al-Si alloys with cobalt addition. It could provide theoretical foundation for developing new hypereutectic Al-Si alloy with admirable performance.In this thesis the variation and mechanism of iron containing phases with Co addition in hypereutectic Al-Si alloys was studied. All the research and experiments were conducted in Al-20Si-2Cu-0.4Mg-1Ni-0.7Fe alloy and Al-20Si-2Cu-0.4Mg-1Ni-1.0Fe alloy. The optimal Co/Fe ratio and impacting mechanism could be investigated by changing the amount of Co addition. The results indicated that the tensile strength of alloy with0.7%Fe achieved the maximum (266MPa) when the Co/Fe ratio is1.3:1(Co content is0.91%) and was lifted by23MPa (about9.5percent) compared with tensile strength in alloy with no Co addition (243MPa). The long needle iron-bearing phases gradually disappeared and turned to block shape phases when Co added. The optimal Co/Fe ratio in alloy with1.0%Fe content is also1.3:1(Co content is1.3percent). The tensile strength on this condition is296MPa,10.4percent better than that in alloy with no Co.The effect of Co on high-temperature (350℃) mechanical properties in hypereutectic Al-Si alloys was also investigated. The alloy with Co addition had better high temperature performance that that without Co. The tensile strength of alloy with0.7%Fe and1.0%Fe is115MPa and122MPa respectively when Co/Fe ratio is1:1.In the dissertation synthetic impact of ultrasonic vibration and Co on microstructure of hypereutectic Al-Si alloys was discussed. The aluminum melt was vibrated from710℃in120seconds. The vibration power is1200W. The experiment results showed that tensile strength of alloys after ultrasonic vibration significantly boosted. Tensile strength of as-cast alloy with0.7%Fe reached the peak (184MPa) when the Co/Fe ratio is1:1and about15percent better compared with that in alloy with no ultrasonic vibration. The addition of Co had no influence on tensile strength in as-cast alloys with1.0%Fe. The greatest tensile strength is201MPa when vibration.