| In this paper,commercial 7055 aluminum alloy bar was used as the experimental material and was treated with multi-direction forging and solid solution.The microstructure and comprehensive properties of 7055 aluminum alloy under nonisothermal aging process were studied by transmission electron microscopy(TEM),differential calorimetry(DSC),X-ray diffraction(XRD)combined with a series of property tests,.At the same time,through the thermal kinetics analysis of precipitation,the composite non-isothermal aging with the combination of heating and cooling was designed and improved,which significantly improved the mechanical properties of the alloy,greatly shortened the aging time and improved the aging efficiency.The main research contents and conclusions of this paper are shown as follows:1)the comprehensive properties of alloy after heating aging reach the level of T74 aging,but there is still a certain gap for mechanical properties compared with T6 aging.The nucleation of precipitates can be finished quickly in the heating environment,and a large number of fine-sized ? 'phases are precipitated in a short time.These ? 'phases begin to coarse and change to ? phase in later aging stage,which leads to the decrease of mechanical properties of the alloy.In the non-isothermal aging process,coarsened grain boundary precipitates are distributed intermittently at grain boundaries and the width of grain boundary precipitation-free zone(PFZ)is relatively large.Compared with the T6 aging process,the electrical conductivity of the material is greatly improved.The precipitation sequence in the alloy is SSS?GP zone ?? 'phase ?? phase.2)the heating aging process still includes nucleation,growth and coarsening.Compared with traditional aging treatment,the nucleation rate of precipitates increases rapidly with the increase of temperature and decreases rapidly after reaching the peak value.Under high temperature environment,the diffusion rate of solute atoms increases exponentially with temperature,which greatly accelerates the growth of precipitates and leads to the premature occurrence of precipitates coarse.It is difficult for the new phase to obtain a stable enough growth environment.The volume fraction of precipitates is restricted.The volume fraction of precipitates under heating ageing is lower than that in T6 aging condition3)the size of precipitates in grains is small but the size distribution range is large.The aging strengthening mechanism includes modulus strengthening,co-lattice strain strengthening and Orowan strengthening.Compared with the more uniform size distribution and higher volume fraction of intragranular precipitates under T6 aging,the strengthening effects of precipitates under this condition can be further improved.4)with the help of the analysis of thermal kinetics of precipitation,a composite nonisothermal aging process with rapid temperature rise to a lower temperature was designed.Compared with the heating aging process,the comprehensive properties of the alloy under this process were further improved.The mechanical properties of the alloy are equal to or even higher than that of T6 aging under the condition in case of the electrical conductivity remains unchanged.The H40-175?-C20 alloy shows the best comprehensive properties: tensile strength is 723MPa,yield strength is 660 MPa,elongation is 11.5%,and electrical conductivity reached to 34.4% IACS.While the aging process took 5.625h,which greatly shortened the aging time.5)a large number of fine ? 'phase precipitates rapidly in the composite aging and heating stage,and these fine precipitates grow up rapidly in the cooling process,while the continuous decrease of aging temperature can inhibit the coarsening of precipitates well.Meanwhile,the low temperature environment in later aging period creates a more favorable environment for the precipitation of newly formed phase,which promotes the generation of secondary precipitation.The precipitation sequence of this process is as follows: SSS?GP zone ??' phase ?GP region ??' phase. |
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