Thick Al-Zn-Mg-Cu alloy products are important lightweight structural materials in the modern aerospace industry due to the advantages of high strength, resistance to stress corrosion cracking and toughness. In the aging process, a large amount of nanoprecipitation is formed by decomposing the supersaturated solid solution during quenching, which satisfies the required performance. However, the quenching process will produce significant residual stress, which will reduce the performance of the product during subsequent machining and maintenance. When significant residual stress occurs, up to 95% of the raw materials can be wasted due to deformation or cracking caused by the residual stress. Therefore, in order to meet the practical application requirements, the residual stress in the industrial Al-Zn-Mg-Cu alloy must be controlled at an extremely low level.
The team of Jiang Fulin from Hunan University proposed a novel method of cladding and quenching. The high-temperature inorganic adhesive and aluminum foil are coated on the surface of Al-Zn-Mg-Cu alloy, which can directly reduce the residual stress caused by quenching. The mechanical properties, electrical conductivity, and microstructure of the aging alloy were studied. Related papers were published in Journal of Materials Research and Technology with the title "Reduced residual stress and retained properties in Al-Zn-Mg-Cu alloys using a novel cladding quenching process".
Paper link:
https://www.sciencedirect.com/science/article/pii/S223878542031245X
Generally, due to high quenching sensitivity, low residual stress and high mechanical properties cannot be achieved in Al-Zn-Mg-Cu alloys at the same time. For heat-treatable aluminum alloys, the time-temperature-performance (TTP) curve can be used as an important reference for optimizing heat treatment. The TTP curve can predict the critical temperature range based on the duration of quenching, within which the cooling rate must be high enough to prevent high temperature precipitation. The TTP curve represents 99.5% of the maximum achievable Vickers hardness of the 7150 aluminum alloy. The area within the C curve is the area where rapid high-temperature precipitation occurs, or the supersaturated solid solution cannot be solidified well during the quenching process. The results show that during the quenching of samples CWQ and CQ0.6, precipitation of supersaturated solid solution will not occur, and the samples have obtained better mechanical properties after artificial aging.
In this work, the quenched samples (CQ and CWQ) retained most of the supersaturated solid solution during quenching due to very few high temperature precipitations. In aging samples, TEM images show similar nano-precipitation characteristics within the grains, so the strengthening effects of solid solution strengthening and precipitation strengthening should be similar. In the CWQ alloy, the observed dislocations can produce a slight strengthening effect and accelerate the formation of the precipitation process. Therefore, under the same aging treatment, the hardness and strength of the CWQ sample is slightly higher than that of the CQ sample. In addition, the increase in elongation of the CQ 0.6 sample may be due to the discontinuous precipitation of grain boundaries.
Studies have shown that the residual stress decreases with the increase of the cladding thickness and is almost eliminated at the cladding thickness of 0.6 mm. After the coating thickness of 0.4-0.6mm is quenched, the aging alloy samples maintain excellent mechanical properties and electrical conductivity. The reduction of the residual stress is related to adjusting the difference between the cooling curve of the 7150 aluminum alloy surface and the core, and eliminating the surface compressive stress during the cladding quenching process.
In summary: the dislocation density of the aging alloy sample using the cladding quenching process is lower than that of the alloy sample using the conventional quenching process, the nano-precipitation characteristics within the grains are similar, and the discontinuous precipitation of the grain boundaries is more , Which corresponds to maintaining mechanical properties and improving conductivity. The study further studies finite element analysis and time-temperature-performance curves, and discusses in-depth mechanisms for reducing residual stress and maintaining performance, thereby providing a theoretical basis for further research.