Abstract: Microstructure evolution of 7075 alloy from dendrite to non-dendrite during semi-solid isothermal heat treatment was studied and the difference of the phase constituents between as-cast and semi-solid microstructure was also investigated. The results show that the formation of non-dendritic structures has three stages: the disappearance of branching characteristics， flattening grain boundaries， arcuating grain boundary and grain growth. The disappearance of branching characteristics is caused by the melt of the low-melting-temperature phases located in the secondary dendrite arm: the agglomeration of part liquid phase wrapped inside the solid particles， some liquid phase flowed from the dendrite channels to the grain boundaries and the other liquid phase lapped together to form new solid particles. The phase constituents of both as-cast and semi-solid microstructures in 7075 alloys are all α-Al， η(MgZn₂) and θ(Al₂Cu).

Key words: semi-solid, isothermal heat treatment, 7075 alloy, non dendrites, microstructure