Abstract: The effect of Retrogression and re-ageing treatments on the microstructure and properties of a new type low frequency electromagnetic casting ultra-high-strength aluminum alloy were investigated using hardness and electric conductivity tests, DSC analysis and TEM observation. It is found that after ageing at 120℃ for 24h, the studied alloy can gain high hardness and strength. After pre-ageing at 120℃ for 24h, retrogression at 180℃ for 60min, and then re-ageing at 120℃ for 24h, the tensile strength, yield strength, elongation and electric conductivity(IACS) of the studied alloy are 721MPa, 700MPa, 8.1% and 34.5% respectively. The TEM observation shows that the property changes of the alloy during RRA are closely related to the microstructural evolution. In the early stage of retrogression, the re-dissolution of GP zone and η′ leads to the decrease of hardness, while precipitation of η′ and η makes hardness increase again to a peak value. Then, the transition of η′→η and coarsening of η lead to the decrease of hardness. Precipitation of η′ after reageing increases the hardness, strength and electric conductivity of the studied alloy.
Effect of RRA on microstructure and properties of new type ultra high strength aluminum alloy
Abstract:
The effect of Retrogression and re-ageing treatments on the microstructure and properties of a new type low frequency electromagnetic casting ultra-high-strength aluminum alloy were investigated using hardness and electric conductivity tests, DSC analysis and TEM observation. It is found that after ageing at 120 ℃ for 24 h, the studied alloy can gain high hardness and strength. After pre-ageing at 120 ℃ for 24 h, retrogression at 180 ℃ for 60 min, and then re-ageing at 120 ℃ for 24 h, the tensile strength, yield strength, elongation and electric conductivity (IACS) of the studied alloy are 721 MPa, 700 MPa, 8.1% and 34.5% respectively. The TEM observation shows that the property changes of the alloy during RRA are closely related to the microstructural evolution. In the early stage of retrogression, the re-dissolution of GP zone and η′ leads to the decrease of hardness, while precipitation of η′ and η makes hardness increase again to a peak value. Then, the transition of η′→η and coarsening of η lead to the decrease of hardness. Precipitation of η′ after reageing increases the hardness, strength and electric conductivity of the studied alloy.