Effect Of Aging Treatment On Mechanical Properties And Corrosion Behavior Of 6061 Aluminum Alloy

The 6XXX series aluminum alloys exhibit high specific strength,low density,exceptional impact resistance,and corrosion resistance,making frequently employed in automobile crash beams,battery trays,and other applications.However,it is important to note that different components have distinct requirements concerning strength,press formability,and corrosion resistance.Improper heat treatment processes may diminish strength or aggravate corrosion tendencies,consequently leading to structural part failures due to inadequate strength or subpar corrosion resistance.Consequently,conducting a systematic investigation into the influence of heat treatment processes on the mechanical properties and corrosion behavior of the 6061 aluminum alloy is crucial.This study can provide some valuable theoretical guidance and experimental references for enhancing the heat treatment process,corrosion design,and ensuring the secure performance of aluminum alloys in automotive applications.The changes of mechanical properties of 6061 aluminum alloy are analyzed by uniaxial tensile test and hardness test.By considering the intracrystalline desolvation sequence,the combination of aging parameters with optimal mechanical properties is obtained.The results indicate that the alloy’s strength stabilizes after a natural aging period of 96 hours.During this period,the yield strength notably increases compared to the solid solution state,accompanied by a pronounced phenomenon known as the PortevinLe Chatelier(PLC)effect observed in the stress-strain curve.Furthermore,through peak aging at 180℃for 8 hours,achieving the alloy’s optimal mechanical properties,resulting in a yield strength 1.48 times higher than that attained through natural aging for 96 hours.Conversely,over-aging at 180℃ for 10 hours led to a significant reduction in the alloy’s strength due to the coarsening of the precipitated phase.Moreover,analysis of extreme differences highlights the significant impact of the double-stage aging temperature on the mechanical properties.Based on investigations,the recommended double-stage aging process for optimal results is 170℃ for 2 hours followed by 200℃ for 2 hours.The effects of different aging processes on the microstructure and fatigue life of 6061 aluminum alloy are investigated.The impact of different aging conditions on grain size,fracture morphology characteristics,and the alloy’s service life is examined.Furthermore,the underlying causes for the changes in microstructure and mechanical properties induced by aging are explained from a microscopic morphology perspective.The results reveal that the aging process notably affects grain size and uniformity.After natural aging,the grain size is small and uniform,whereas over-aging leads to significant grain coarsening and reduced uniformity.Additionally,the double-stage aging process results in grain boundary discontinuity due to the depletion of Cu atoms caused by low-temperature aging.Tensile tests conducted after various aging stages exhibit ductile fracture and ductile-brittle composite fracture modes.Furthermore,fatigue fracture analysis reveals a mixed fracture mode characterized by destructive fracture and ductile fracture.The effects of different aging processes on the electrochemical corrosion resistance and intergranular corrosion susceptibility of 6061 aluminum alloy are investigated.The results indicate that,in a 3.5%NaCl solution,the polarization resistance and Warburg impedance gradually decrease,while the self-corrosion current increases and the self-corrosion potential shifts negatively to the left in the order of natural aging(NA),double-stage aging(DA),peak aging(PA),and over aging(OA).The results suggest that the coarsening of the precipitated phase contributes to the deterioration of the alloy’s corrosion resistance.Mott-Schottky curves demonstrate that the passivation film on the alloy’s surface,after different aging stages,exhibits semiconducting behavior.The coarsening and shedding of the discrete phase introduce point defects that accelerate the breakdown of the passivation film,resulting in a highly corrosionsusceptible surface.The intergranular corrosion susceptibility of the 6061 aluminum alloy follows the order:NA<DA<OA<PA,with no intergranular corrosion tendency observed after natural aging,where pitting corrosion prevails.The double-stage aging process significantly reduces the intergranular corrosion tendency,thus representing a substantial improvement compared to the peak aging state.In an alkaline environment(pH=10),pitting behavior primarily occurs in the different aging states of the 6061 aluminum alloy.While pitting damage has a minor impact on the material’s tensile properties,the presence of etch pit defects originating from crack sources significantly reduces the fatigue life of the alloy.The fatigue life follows the order:NA>DA>PA>OA.Under alkaline conditions,the fracture behavior of the 6061 aluminum alloy predominantly exhibits a combination of crystal and mixed fracture modes(destructive fracture+ductile fracture).Fatigue cracks mainly initiate at sites where corrosion induces defects in the material,such as corrosion pits or pitting pits resulting from secondary phase shedding.These fracture modes encompass cleavage,ductile fracture,and intergranular fracture.Analysis of the electrochemical impedance spectrum(EIS)and Tafel polarization curves demonstrates that the corrosion resistance magnitude of the 6061 aluminum alloy in alkaline conditions follows the order:NA>DA>PA>OA.