Microstructural Evolution During Homogenization of Direct-Chill-Cast Al–Zn–Mg–Cu Alloy

We investigated the microstructural evolution during homogenization of a large direct-chill (DC)-cast AA7050 billet. The as-cast billet consists of Al/η-Mg (Zn, Cu, Al)₂ eutectic, Mg₂Si, and Fe intermetallic compounds and exhibits higher constituent phase fraction and solute segregation and larger microstructures in the center region. As the homogenization temperature increased, the secondary phase dissolution rate and solute homogenization rate increased. The center region with larger grain size and dendrite arm spacing exhibited a sluggish rate of phase dissolution and solute homogenization. At 475 °C, a transformation from η into S phase with an orientation relationship was observed in the Al/η-Mg (Zn, Cu, Al)₂ eutectic. The η into S phase transformation did not occur at homogenization temperatures of 485 and 500 °C. Instead, an incipient melting of the Al/η eutectic occurred, resulting in formation of a rosette microstructure composed of fine Al/η eutectic and θ-Al₂Cu phases during quenching. The incipient melting region transforms to an Al matrix or porosity as alloying elements diffuse into the adjacent Al matrix. The hardness of the homogenized billets increased due to dissolution of the constituent phase but decreased as porosity due to the incipient melting becoming severe. The optimal homogenization process for the DC-cast AA7050 billet is discussed in terms of phase dissolution, solute homogenization, and incipient melting.