Yield strength modelling via precipitation/recovery kinetics in rapidly solidified thin-strip cast AA6005 AlMgSi sheets subjected to cold rolling and direct aging

Abstract

A comprehensive and experimentally verified yield strength model of AA6005 Al–Mg–Si sheets rapidly solidified via a novel Thin Strip (TS) casting route is developed and presented. The cast sheets were cold-rolled and directly aged via a heat-treatment process (termed TSH) that effectively utilizes the increased solute supersaturation of matrix due to the high casting cooling rates experienced. The TSH conditions were optimized based on the hardness and tensile data, with the peak aging conditions found to be 160°C-4 hrs, 180°C-1 hr and 200°C-15 min. The TEM analysis revealed that, due to the presence of prior cold work, β′ precipitates formed during TSH, i.e., as opposed to the β" precipitates that form upon a typical T6 aging treatment. Also, while the total precipitate volume fraction was comparable among the three peak aging conditions, the sample aged at 200 °C exhibited a lower peak aging strength (300 MPa vs. ∼315 MPa in the other two conditions), i.e., due to a higher dislocation recovery rate and therefore a lower strengthening contribution from the remaining cold work. Interestingly, 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 remaining cold work. Therefore, thanks to the sufficiently high solute supersaturation of the matrix during TSH, together with the remaining cold work (as well as the formation of β′), peak aging at 180 °C is achieved after only 30 min to 1 h, i.e., as opposed to generally 4–6 h of aging needed for a traditional T6 aging. This gives the low-cost, less energy-intensive TS-cast sheets also a significantly faster response to the paint-baking process (typically performed at near 180 °C for around 30 min), thus allowing for a higher strength achieved via a typical automotive body fabrication route. Finally, a robust model of precipitation/recovery kinetics, incorporating precipitate aspect ratio evolution, was developed to simulate the yield strength during TSH. The model predicts a peak-aging time of ∼100 min, aligning well with the observed 60–120 min interval measured from the tensile test results.