The Thermomechanical Responses of Thin-Strip Cast AA6005 Sheets

Abstract

This thesis focuses on understanding the effects of the high cooling rate provided by Thin Strip (TS) casting on the as-cast microstructure and heat treatment processes of age-hardenable AA6005 alloy. The TS as-cast microstructure showed a reduced grain size (37.8 vs. 66.0µm), finer secondary dendritic arm spacing (4.6 vs. 9.8µm), and improved solute supersaturation (matrix Si content of 0.51 vs. 0.30wt%) compared to a conventional Direct Chill (DC) cast material. Cold-rolled DC and TS sheets exhibited similar aging responses to T4 and T6 tempers. Upon direct artificial aging (TSH), the as-rolled TS sheets showed a larger precipitate volume fraction (primarily β′) and higher yield strength than the DC sheets (285 vs. 195MPa). This was attributed to the higher Mg/Si supersaturation in the matrix. The TS-TSH samples exhibited comparable yield strengths to that of the T6 temper, though at a reduced ductility (6 vs. 12%elongation). The reduced volume fraction of the hardening precipitates (vs. the T6 temper) was compensated for by the remaining cold work for the TS sheet at the TSH temper. The TSH conditions were optimized based on mechanical testing, with the peak aging conditions determined to be near 160°C-4hrs, 180°C-1hr, and 200°C-15min. At each temperature, aging beyond the peak aging time still resulted in an increase in the precipitate volume fraction. The concurrent drop in the yield strength could be explained by the offsetting effect of the recovery of the residual cold work. A model was developed to describe the precipitation and recovery process during TSH aging, correlating the precipitate and dislocation evolution to the strengthening mechanisms. The model predicts a TSH peak aging time of 100min at 180°C, which aligns with the experimental results. Through XRD measurements, an unconventional dislocation density evolution during TSH aging were observed, which can be explained by the formation of β′ precipitates on dislocations, as well as the interaction between precipitates and the aluminum matrix. This research contributes to the field by providing experimental data on as-cast AA6005 materials, demonstrating the potential for cost-effective and energy-efficient fabrication routes, and developing a yield strength model for the TSH aging process.