Influencing factors of the coarsening behaviors for 7075 aluminum alloy in the semi-solid state

Abstract

The kinetics of microstructural coarsening for the semi-solid wrought 7075 aluminum alloy was determined. The variation of the coarsening rate constant K with the increasing liquid fractions and the corresponding coarsening mechanisms were determined for the recrystallization and partial remelting-processed sample. The effect of plastic pre-deformation on the value K was considered for equal channel angular pressing-based stain-induced melting activation-processed sample. A succinct review of the attempts to understand the various parameters involved in grain growth in this study and some similar literature was also provided. The results show that the rate of grain growth depends on the liquid content, temperatures, alloy composition and processing routes. The volume fraction of liquid influences both the liquid–solid interfacial area and the mean diffusion distance. The actual coarsening rate constant is the summation of independent solid and liquid contribution to the atoms diffusion. Three different coarsening mechanisms, viz. coalescence, inhibited Ostwald ripening and classical Ostwald ripening, are dominant for the elevated liquid fractions, respectively. A greater strain in the solid state or shearing rate in the liquid state usually leads to a lower coarsening rate for the alloys in the semi-solid state due to the facilitated nucleation–growth rate ratio. Further, the wrought aluminum alloys exhibit lower coarsening rate than the cast aluminum alloys due to the inhibited coarsening process by the intermetallic precipitates.