| 7B04 alloy belongs to the aging strengthening alloy,and has good mechanical properties,which is one of the important structural materials in the aerospace field.In recent years,the application of electric field,electrostatic field,magnetic field and other fields in metal heat treatment process has provided a new direction for improving the process of high-strength aluminum alloy.This paper attempts to study effects of DC and AC current field treatment on microstructure and properties of 7B04 alloy during homogenization,solution and aging,to explore the feasibility of current processing technology as a kind of new heat treatment method.The major results of this study are drawn as follows:(1)There is a large number of non-matrix phases between the grain boundary and dendrite in the as-cast 7B04 alloy,resulting in the low hardness and the high conductivity of the as-cast 7B04 alloy.After as-cast 7B04 alloy is homogenized,the area fraction of secondary phases is significantly decreased,and the hardness and conductivity of as-cast alloy decrease.Elevating temperature and applying electric current treatment would further promote dissolution of second phases into matrix,bringing the solid solubility of alloy elements in matrix increasing,so that the hardness increases and the conductivity decreases.The second phase area fraction of the alloy was 1.14%,the hardness was 157.5HV and the conductivity was 28.7%IACS when as-cast 7B04 alloy is homogenized with D.C electric field treatment.The sequence of magnitude of electric current processing function is:D.C current>AC current>without current.(2)Elevating homogenization temperature and applying electric current filed can accelerate the transformation from white T phase to gray phase and then to dark gray phase.The gray phase becomes coarse and the deviation in compositions between the gray and dark gray phases reduces when the temperature is>440?.At the same time,the order of the promotion of current to phase transformation is:D.C current>A.C current>without current.A diffusion layer of Cu is observed at the neighboring matrix of T phase at 440? and becomes significant with applying D.C current field.(3)Elevating solution temperature and applying AC or DC current feild can promote the dissolution of the second phase of 7B04 alloy,and increase the degree of supersaturation of the alloy.After the as-quenched alloy age at 120?/2h,the hardness increases with temperature increased and current applied increase,while the conductivity decreases.The effect of DC current on the solution is more than the AC current.The optimum solution temperature of the alloy is 470?.(4)With aging temperature increased,the hardening rate is accelerated until it reaches at any temperature,and the time to peak strength is shortened,but the peak strength decreases.After solid solution treatment for the 7B04 extrusion alloy,the peak strength can not achieve until 48h for aging at 100?.Aging at 120? and 140?,the time to peak hardness are 24h and 12h,respectively,and the peak tensile strength are 715MPa and 700MPa.Through diffraction and DSC analysis,strengthening phases of the peak aging are G.P.zone which completely coherent with the matrix and small amounts of semi coherent ?' phase.(6)Elevating solution temperature and applying AC or DC current treatment can promote the dissolution of the second phase of 7B04 alloy,and increase the degree of supersaturation of the alloy.After the as-quenched alloy age at 120?/2h,the hardness increases with temperature increased and current applied increase,while the conductivity decreases.The effect of DC current on the solution is more than the AC current.The optimum solution temperature of the alloy is 470?.(5)The effect of the aging strengthening of the alloy is not the same as that of applying the DC current at the solution stage or at the aging stage.Applying DC current at the solution stage can shorten the time to peak strength and increase the peak strength and hardness.For the latter case,the peak strength can be increased but the time to peak strength is not shortened for the aging at 120? The aging time to peak strength can be shortened,but the intensity can be decreased compared with the conventional condition for the aging at 140?.Applying DC current in the both solution and aging stage,the effect of aging strengthening is best,not only the time of the alloy reaching peak aging is shortened,but also the peak aging strength is increased.The current treatment in the solution and aging process is more favorable for the formation of the tough nest,and can increase the depth of the tough nest and bring the uniform distribution of the tough nest,which makes the alloy elongation higher than the conventional condition.(6)DSC study showed that the precipitation sequence of 7B04 alloy:?(SSS)?G.P zone??'??(MgZn2).Applying DC current during solid solution and aging treatment can promote subsequent ?' and ? phase transformation process.Applying D.C current during solution current can make the temperature of ?' and ? phase exothermic peaks decreased about 3?,make the maximum transition rate increased by 4.4%and 35.1%respectively,and make the time of accomplish the transformation reduced by 18.9%and 11.8%respectively.For the aging stage,applying current is different from the front situation,it makes the peak temperature of exothermic peaks increased about 1?5?,but the maximum transition rate increase by 9.6%and 16.2%,and the time of accomplish the transformation reduces by 20%and 11.8%. |
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