Transformation of structure and properties of the Al–Zn–Mg–Cu alloy under complex processing. Part I. Microstructure
摘要
The structure formation of 7075 alloy (Al–Zn–Mg–Cu system) with a maximum allowable zinc content (6.1 mass%) and a crystallization range of 162 °C was studied during the complex technological process “twin-roll casting–hot rolling–heat treatment”. It is shown that the structure of the cast strip during twin-roll casting is non-dendritic, with dispersed intermetallic precipitates present within the grains. At the grain boundaries, non-equilibrium eutectics are formed. Hot rolling with a deformation degree of ε = 64 to 8% causes the orientation of the primary solid solution (Al) crystals in the rolling direction and a reduction in their size in the plane perpendicular to the strip surface. The volume fraction and size of intermetallics decrease monotonically due to the mechanical effects of rolling and the partial dissolution of strengthening phases in the solid solution (Al). During heat treatment, the size and shape parameters of dendritic cells (Al) and intermetallics decrease due to recrystallization and spheroidization processes. After quenching and natural aging, the supersaturation of the solid solution increases due to the dissolution of the strengthening phases located at the grain boundaries (Al), the iron-containing intermetallics formed during crystallization, and the preservation of spheroidization of secondary intermetallics. During artificial aging, the quantity and size of eutectic and secondary intermetallics increase, but their shape parameter decreases. This structural transformation differs from that observed in traditional processing technology, with smaller grain size, higher dispersion, and a more uniform distribution of intermetallic phases, as well as the absence of macrosegregations of cast heterogeneities. This is primarily due to the hereditary influence of high cooling rate from the liquid state (v approx. 103 °C/s) at the initial stage of the complex process.