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Heat treatment has important influence on the microstructure and mechanical properties of Al-Si alloys.The most common used heat treatment method for these alloys is solution treatment followed by age-hardening.This paper investigates the microstructural evolution of a direct chill (DC) cast Al-15.5Si-4Cu-1Mg-1Ni-0.5Cr alloy after solution treated at 500,510,520 and 530℃,respectively for different times.The major phases observed in the as-cast alloy are a-aluminum dendrite,primary Si particle,eutectic Si,Al7Cu4Ni,Al5Cu2Mg8Si6,Al15(Cr,Fe,Ni,Cu)4Si2and Al2Cu.The Al2Cu phase dissolves completely after being solution treated for 2 h at 500℃,while the eutectic Si,Al5Cu2Mg8Si6 and Al15(Cr,Fe,Ni,Cu)4Si2 phases are insoluble.In addition,the Al7Cu4Ni phase is substituted by the Al3CuNi phase.The a-aluminum dendrite network disappears when the solution temperature is increased to 530℃.Incipient melting of the Al2Cu-rich eutectic mixture occurrs at 520℃,and melting of the Al5Cu2Mg8Si6 and Al3CuNi phases is observed at a solution temperature of 530℃.The void formation of the structure and deterioration of the mechanical properties are found in samples solution treated at 530℃.