| This paper is part work of study on the heat treatments of A1 alloys for building with electric current,which is a project in 973 program.In present work,the tensile test,conductivity test,hardness test,optical microscopy,scanning electron microscopy,energy dispersive spectrum analysis and X-ray diffraction analysis were used to investigate the effects of direct and alternating current on 7050 alloy homogenization microstructure and mechanical properties and phase transformation during homogenization process;the influence of direct and alternating current on 7050 alloy solution and aging treatment microstructure and mechanical properties,this paperfurther probes into the function mechanism of current field on phase transformation and aging precipitation during 7050 alloy homogenization process.With current heat treatment,the stability of the temperature field and temperature distribution show that:when the current intensity is 1500A,the temperature of center of specimen stability is 428 ? can't reach the preset temperature of 470 ?;when the current intensity is 1500A and fan frequency is 10Hz,specimen can't reach the presettemperature of 470 ?;when the current intensity is 2000A and fan frequency is 5Hz,specimen reach the preset temperature of 470 ? in 945s.When cooling water is used in copper clamp of both sides,the temperature difference between the core and surface of specimen reduces to 41 ?,temperature field has better symmetry;after the auxiliary heating,the core temperature of specimen is same as the surface temperature,when using blowing air during the current heating process,and becomes less than ± 5?when using the auxiliary heating.,which satisfy the experiment condition of current homogenization.During the homogenization process of 7050 alloy,the T phase was transformed into S phase.By comparision the secondary electron and backscattered electron images,three kinds of zones can be seen within T phase during the homogenization process.An elemental diffusion cell network was clearly observed within T phase which is consisted of coarse and white cell wall and light gray cell interior.The composition range of cell wall is(wt%)12-19 Mg,18-34 Al,22-28 Cu and 30-42 Zn.Cell interior has a composition range of(wt%)10-18Mg,24-32 Al,23-30 Cu and 17-23 Zn.Comparing with the composition of T phase in as-cast 7050 alloy,besides having same Mg and Cu contents,cell wall has higher Zn and while cell interior has lower Zn.With increasing temperature,the cell network weakens gradually within bright white,gray and dark gray phases and the amount and the size of gray and dark gray phases increase when the temperature is less than 420 ?.When the temperature is higher than 440 ?,the cell network nearly disappears within dark gray phase,the amount and the size of gray and dark gray phases increase and the difference in contrasts between gray and dark gray phases decreases.The zone having Zn content higher than 10 wt%is called T-base phase in the present work.The appearance of the elemental diffusion cell network demonstrates that Zn in the cell interior is diffusing out to the cell wall,and the cell interior is at the transitive stage of the phase transformation from T to S.With the progress of homogenization,Zn content within T phase continues to reduce,the light gray cell interior changes to the gray phase and then to the dark gray phase.The network fades when the transformation from the cell interior to the gray phase happens,and disappears when the transformation from the gray phase to the dark gray phase occurs.With increasing temperature,the amount and the size of the gray and dark gray phases increase;the difference in the composition between the two phases reduces.The composition of the dark gray phase finally approaches the equilibrium S phase at 450 ?.The gray phase contains Zn higher than 10wt%,and is still T-based phase when the temperature is less than 420 ?,and changes to S-based phase when the temperature is higher than 440 ?.The existence of the elemental diffusion cell network indicates the progress of phase transformation from T to S,and it disappears when the phase transformation finishes.With the application of direct and alternating electric current,the amount of white cell networks in unequilibrium eutectic were higher and more obvious than those in alloy homogenized without DC current at the same temperature.The DC and AC current obviously decreased the apparent activation of the transformation from T phase to S phase and accelerated the phase transformation from T phase to S phase in 7050 alloy during homogenization.During the solution treatment stage,applying current could improve the alloy supersaturation and promote the separating of phase ?' during the aging stage,shortening the application time until the alloy reaches aging peak.The alloy reaches aging peak when solution at 470? with 500A DC and AC current and aging at 120?for 2h.During the aging state,it is the application current that accelerates the separating of ?' and shortens the application time until the alloy reaches aging peak.The alloy reaches aging peak when solution at 470? and aging at 120 ? for 12h with 500A DC and AC current.During both the solution treatment stage and the aging stage,the time of reaching aging peak is shortened and alloy performance is improved.The alloy reaches aging peak when solution at 470 ? and aging at 120 ? for 6h with 500A DC current.Comparing with conventional solution and aging treatment without current,when the current is applied,the activation energy of ?' phase separating decreases that applying current promotes separating of the ?' phase.The activation energy of ?' phase separating is lower than that of application AC after applying DC.After applying current,the activation energy of ?' phase separating during solution treatment is lower than that during aging treatment.The activation energy is relative low when current is applied during solution and aging treatment,and reaches the lowest when the DC is applied. |
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