Effect Of Thermo-mechanical Treatment On The Microstructure And Properties Of 2024 Aluminum Alloy

The 2024 aluminum alloy has the advantages of high strength,good ductility,lightweight,and excellent specific strength,and can be widely used in aerospace(e.g.for aircraft skins and rocket liquid hydrogen tanks).In recent years,more attention has been paid to the aerospace field in the country,and further requirements for the comprehensive properties and processing technology of aluminum alloys have been put forward.Thermo-mechanical treatment is one of the effective methods to improve the properties of aluminum alloys.This study aims to further improve the strength-ductility combination and the processing technology of the 2024 aluminum alloy by cold rolling and subsequent aging.The samples were cold rolled to 20-90% in thickness reduction which is much beyond the strain level used in the traditional thermomechanical treatment of this alloy.Micro-hardness measurements and tensile testing were applied to evaluate the mechanical properties.Differential scanning calorimetry(DSC)and transmission electron microscopy(TEM)techniques were used to analyze the microstructure of cold-rolled and aged samples.The following conclusions were reached:(1)During cold rolling from low to high strain,the microstructure evolution is characterized by the formation of a dislocation structure.At low strain,a large number of dislocation tangles are formed.With increasing strain,the grains are gradually subdivided by the geometrically necessary boundaries(GNBs)and incidental dislocation boundaries(IDBs)formed within the grains.At strains higher than 70%,the microstructure gradually transforms into a lamellar structure with GNBs parallel to the rolling direction.(2)With increase strain,the strength of the alloy first increases rapidly followed by a slow increase,while the ductility decreases monotonically.The tensile stress-strain curves show that the alloy has significant work hardening stage II where the work hardening rate is high.With further increasing strain,the work hardening rate gradually decreases to a low and constant level.(3)The cold-rolled and aged samples show high strength and good ductility at peak aging when the aging temperature is below 150 °C.The yield strength is increased by1.5-2 times than that after traditional T6 treatment while the uniform elongation are6.5-7.5%.The sample 90% cold-rolled and peak-aged at 150 °C sample exhibits the best combination of strength and ductility: a yield strength of 660 MPa and uniform elongation of 7.4%.(4)The aging temperature and rolling strain affect strongly the aging behavior.The higher the aging temperature,the more sufficient the precipitation process.Dispersed precipitates are formed in the 20% cold-rolled and peak-aged at temperatures below150 °C,while a similar aging treatment results in little precipitation in the 90%cold-rolled and aged sample.