On the microstructure and solution treatment of hot tearing resistant semi-solid 206 alloy

The microstructure and solution treatment aspects of experimental semi-solid 206 cast alloy containing a high Si level of 1.2%, introduced to improve the alloy hot tearing resistance, were studied. Microstructure observations showed that the Cu-rich Al2Cu phase precipitated in both blocky and lamellar forms. The Al15(FeMnCu)3Si2 phase was observed to be the predominant Fe-rich intermetallic phase in the microstructure, whereas the needle-like Al7Cu2Fe phase was rarely detected. The differential scanning calorimetry (DSC) analysis revealed that the melting of the lamellar Al2Cu phase occurred at 511.4 °C and the melting point of the blocky Al2Cu phase was depressed to 518.6 °C compared to the common 206 alloys. The precipitation of the low melting point lamellar Al2Cu phase and the temperature depression of the blocky Al2Cu phase necessitated redesigning the solution heat treatment. Several solution treatment conditions were examined through DSC analysis along with an evaluation of the respective microstructures and tensile properties. The two-step solution treatment involving a first step at 500 °C for 6 h followed by a second step at 515 °C for 8 h was found to be the optimum solution treatment to avoid harmful incipient melting and to attain the highest mechanical properties.