The Precipitation Behavior of ω–Al7Cu2Fe and Its Influence on the Phase Precipitation of AlSi10.5Mg0.35Fe0.2Cux Alloy in Heat Treatment Process
摘要
Fe element in aluminum alloy could combine with the strengthening elements as Cu and Mg to form iron-rich phases (e.g., π–Al8FeMg3Si6 or ω–Al7Cu2Fe), and consequently reduce the amount of them to precipitate strengthening phase in the heat treatment process. For cast Al–Si alloys containing Cu and Fe, ω–Al7Cu2Fe being a non-ambient temperature phase theoretically exists at the temperature above 228 °C and is relatively less studied. In this work, the precipitation behavior and effect of ω–Al7Cu2Fe on the precipitation of strengthening phase in heat treatment process of AlSi10.5Mg0.35Fe0.2Cu0.6 have been studied. It is found that the ω–Al7Cu2Fe phase in this alloy precipitates through peritectoid reaction and decomposes by its inverse reaction. The precipitation of ω–Al7Cu2Fe begins on the external surface of β–AlFeSi phase, accompanied by Cu diffusing inward to β–AlFeSi and Si diffusing outward from β–AlFeSi. The ω–Al7Cu2Fe phase is easy to be retained to the ambient temperature under the non-equilibrium cooling process, even the common air cooling condition. The undecomposed ω–Al7Cu2Fe is a serious obstacle to the precipitation of the θ′–Al2Cu in aging process. The Cu elements occupied by the undecomposed ω–Al7Cu2Fe phase must first undergo decomposition before they can form the θ′–Al2Cu or θ–Al2Cu phase with Al. The incomplete precipitation of the Al7Cu2Fe phase induced by non-equilibrium cooling will result in the supersaturation of Cu in α–Al matrix exceeding 0.17 wt pct (theoretical value), and the part of Cu exceeding 0.17 wt pct cannot bypass the precipitation of Al7Cu2Fe first and directly precipitate as θ′–Al2Cu or θ–Al2Cu in the aging process.