| 7N01 aluminum alloy is a kind of typical Al-Zn-Mg(7×××)series aluminum alloy with high toughness and strength.It is widely used in the manufacture of structural parts such as traction beams and bolster beams for high-speed trains.Extrusion can improve the plasticity of the alloy,which is very suitable for 7xxx series aluminum alloy and other difficult-to-form materials.Aging treatment can improve the alloy properties of Al-Zn-Mg extruded profiles.However,mechanism on the microstructure and mechanical properties of Al-Zn-Mg alloys under the aging process are not fully investigated,and most researches stay in the traditional isothermal aging stage.In this paper,Al-Zn-Mg alloy was studied.The three-blade helical extrusion profile was obtained through the extrusion experiment,and the microstructure evolution of the extrusion during the isothermal(single-stage)and non-isothermal aging process was investigated.Meanwhile,the mechanical properties,electrical conductivity and corrosion properties were analyzed during different aging processes,and the relationship between the precipitation phase and performance of Al-Zn-Mg alloy extrusions was clarified.Finally,a reasonable non-isothermal aging process for the alloy was obtained.The main work and conclusions of this paper are as follows:(1)The evolution of the microstructure(including grain morphology and second-phase particles)during hot extrusion,subsequent solid solution(470? for 160 min)and aging process were analyzed.The structure of the Al-Zn-Mg cast alloy was equiaxed grain,still with dendrite segregation phenomena after homogenization treatment.The extrusion effect was obviously,particularly such as the dendrite segregation of the squeezed alloy was reduced in the spiral surface.The grains were elongated along the extrusion direction,and some with small angle grain boundaries were distributed.There was recrystallization phenomenon at the grain boundary,and the size of the original crystal grain and the recrystallized crystal grain were quite different.There were many rod-like and point-like ?-AleMnSi phases in the alloy,which were brittle compounds mainly distributed at the grain boundaries.The brittle compounds would reduce the mechanical properties of the alloy.(2)The isothermal(single-stage)aging test was carried out on the profile after solution treatment and keeping for 3 days.It was found that the alloy reached peak hardness and strength at the isothermal(single-stage)aging of 120-39 h,and the mechanical properties gradually stabilized in the later aging period.The experimental results showed that the electrical conductivity of the alloy generally increases with the increase of the aging time,which indirectly indicated that the isothermal(single-stage)aging was beneficial to improve the resistance of the alloy to stress corrosion.The precipitation process of the alloy crystal was Supersaturated solid solution(SSS)?GP? zones?Metastable ?,and ? phase has not been formed.Rod-shaped ? phases were continuously distributed on the grain boundaries.(3)Non-isothermal aging test was carried out on the profile after solution treatment and storage for 3 days.The mechanical properties of the alloy at different maximum temperatures(Tmax)and starting temperature(Ts)were compared.It was found that if the maximum temperature(Tmx)was low,precipitation was insufficient.Under this condition the cooling stage could improve the mechanical properties.If the maximum temperature(Tmax)was too high,it would be detrimental to the mechanical properties.In addition,the fracture mode of the alloy in the drawing process was also discussed.The fracture mechanism of the fracture gradually changed from breaking along the sliding surface to the internal necking during the non-isothermal aging.The microstructure evolution of precipitates in the grain and grain boundaries of the alloy under different conditions was compared.It was found that the grain boundary precipitation phases were semi-continuous,arranged in two or more rows on the grain boundary.At this condition,there was no obvious PFZ(precipitation free zone)in the alloy.This structure was conducive to improving the corrosion resistance of the alloy.The mechanical properties of the alloy decreased with the increase of starting temperature,and increased slightly at Ts=200?.The electrical conductivity of the alloy decreased with the increase of the starting temperature. |
PDF Full Text Request