Correlation between quenching rate, mechanical properties and microstructure in thick sections of Al-Mg-Si(-Cu) alloys

Thermal treatment is key for the mechanical behavior of 6000 series aluminum alloys. Numerous studies have therefore been devoted to the impact of aging treatment parameters like time and temperature. However the influence of quenching rate is rather poorly documented while it could be of primary interest for applications requiring thick sections. Using a series of Al-Mg-Si(-Cu) specimens quenched in various fluids, we propose to monitor and describe the quench properly and study the impact of the quenching rate on the microstructure and mechanical properties obtained after aging. Acknowledging the critical quench rate to be about 10 degrees C s^-1, quenching was performed in various quenching fluids (water, air and oil) on 10 cm side 6061 alloy cubes. The quench rate was evaluated by thermocouples placed at several locations and by numerical modeling. For each temper, the microstructure and mechanical behavior has been studied at local and macro scales to highlight some correlations. The combination of transmission electron microscopy imaging and related image analysis, chemical and crystallographic mapping, microhardness maps and tensile tests has pointed out many micro structural differences at nanoscale and inhomogeneous mechanical behavior we strived to explain and correlate. (authors)