Effects of Cu content including 1.5%

2.5%

3.6%

4.5% on polynary eutectic copper-rich phases in cast Al-7Si-xCu-0.5Mg alloys and the dissolution characteristics of the phases were investigated by differential scanning calorimerty （DSC） and back-scattered SEM-EDX analysis with variation of solution temperature and time. The results indicate that quaternary or ternary eutectic reaction occurs in all tested alloys at 510 ℃ and 525 ℃ under the condition of un-equilibrium solidification

in which polynary eutectic copper-rich phase Al2Cu phase is created. Both ternary eutectic reaction of the alloy with 1.5% and 2.5% Cu content and quaternary eutectic reaction of the alloy containing 3.5% Cu at 510 ℃ are mestable eutectic reaction

in which mestable eutectic Cu-rich phase can be fully dissolved into the matrix by primary solid solution treatment at 500～505 ℃ to some time

the melting temperature of the alloy was increased to 542 ℃

525 ℃ and 521 ℃

respectively. In addition

the eutectic reaction of alloy containing 3.5% Cu and containing 4.5%Cu at 510 ℃ and 525 ℃ is a nature process in equilibrium solidification. The results show that the solution hardening effects of the Al-7Si-xCu-0.5Mg cast alloy containing Cu in range of 1.5% to 3.5% can be strengthened by increasing temperature of secondary solution treatment in two-step-solution treatment

while the alloy containing 4.5% Cu should be treated at less than 510℃ to prevent the melting of the eutectic phase with low smelting point in the alloys.