Control of morphology Al-Fe-Si phase in Al-Si-Cu hypoeutectic alloy

Aluminum-Silicon (Al-Si) alloys are one of the most versatile aluminum alloys. Iron is considered one of the most harmful elements in Al-Si diecasting alloy because its presence leads to the precipitation of many AlFeSi intermetallic phases and unacceptable mechanical properties, such as reduction in ductility. Thus controlling the fraction and morphology of AlFeSi phase, especially the β-AlFeSi phase is an important way to improve the ductility of Al-Si die casting alloys. In this dissertation, thermodynamics calculation of AlFeSi phase formation and fraction change during solidification process were conducted using Thermo-Calc software. A series of experiments, including adjusting cooling rates, addition of alloying elements, and heat treatment were conducted and effective methods to eliminate the detrimental effect of β-AlFeSi phase were examined. Thermodynamics calculations, provided many worthwhile findings, including that the formation temperature and fraction of α-AlFeSi and β-AlFeSi are mainly affected by the content of Mn and Fe elements; that both elements can effectively increase the formation temperature of both AlFeSi phases; that addition of Mn can reduce the amount of more detrimental β-AlFeSi, while Fe addition facilitates the formation of both α-AlFeSi and β-AlFeSi; that Mg addition can reduce the formation temperature of β-AlFeSi, which means it can reduce the size of β-AlFeSi; that other elements do not have a significant effect on the formation and fraction of α-AlFeSi, β-AlFeSi, fcc α-Al and Si phase. Thus the findings show that an effective method for reducing β-AlFeSi phase is to reduce Fe content, control the amount of Mn content, and increase Mg content. With low Mn and Fe, β-AlFeSi phase is a stable phase and can be formed with low cooling rates. Thus increasing cooling rates will effectively reduce the amount of β-AlFeSi phase. With the addition of Mn or Fe, α-AlFeSi is more stable and high cooling rates can facilitate the formation of β-AlFeSi phase in reverse. The reason lies in high content Mn and Fe increase the stability of α-AlFeSi phase. Thus increasing cooling rate method should not be used for high Fe, high Mn Al-Si alloy. Addition of Ca and Sr is shown to result in a reduction of the β-AlFeSi phase. They have the effect of fragmentation by the rejection of Si at the β-phase/Al matrix interface. On the other hand, with addition of Ca and Sr, the introduced Al2CaSi2/Al2SrSi2 can be the substrate of α-AlFeSi. From the experiment results, the Sr and Ca addition should be controlled between 0.05 and 0.1%. Addition of K seems to have the most beneficial effect on β-AlFeSi phase modification. K2O is formed to be the nucleation substrate of α-AlFeSi and, because of the solution ability of K in aluminum, no introduced intermetallics formed. It is proved that 1%K addition can effectively modify β-AlFeSi phase. The study also shows that the high-temperature short-time heat treatment can lead to an effective reduction of the size the fraction of β-AlFeSi phase. The reduction of β-AlFeSi phase is because of not only the decomposition of β-AlFeSi but also the transformation of β-AlFeSi to α-AlFeSi for A380 alloy. Heat treatment at 525 °C for 1 hour is enough to eliminate all the β-AlFeSi. However, the treatment temperature cannon be to high to avoid other detrimental effect. In summary, the effective way to reduce the detrimental effect of β-AlFeSi include the increase of the cooling rate, control of the content Fe and Mn, addition of Sr/Ca/K element, and short time high temperature heat treatment