Effects of solution treatment and aging process on microstructure refining of semi-solid slurry of wrought aluminum alloy 7A09

A new method was exploited using solution treatment and aging process as a pretreatment in preparing semi-solid slurry with fine microstructure before isothermal treatment of wrought aluminum alloy 7A09. Parameters of pretreatment were optimized by orthogonal experiment design and proper precursor was prepared. The evolution of microstructure of semi-solid slurry during isothermal treatment was analyzed and the mechanism of microstructure refining was discussed. The result of orthogonal experiment design shows that the optimum parameters are 462 °C for solution temperature, 40 min for solution time, 132 °C for aging temperature and 14 h for aging time. Microstructure of isothermal treatment is fine, homogenous, with globular solid grains and a solid fraction between 50% and 70%, which is qualified for later semi-solid forming process. Mechanism of microstructure evolution includes the agglomeration of α-phase and Ostwald ripening. Precipitations prepared by solution and aging treatment prevent the grains from coarsening and promote the grain ripening to globular shape.