喷射沉积态Al-Zn-Mg-Cu合金的高温变形行为及组织演变

Deformation Characteristics and Microstructure Evolution of Spray-Deposited Al-Zn-Mg-Cu Alloy

利用物理模拟技术对喷射沉积态Al-Zn-Mg-Cu合金在高温变形参数(350~450℃,0.001~20.000 s^(-1))下的热变形特征进行了研究。结合金相显微镜(OM)、扫描电镜(SEM)以及透射电镜(TEM)等微观多种表征手段系统分析了材料在变形过程中的微观组织演化过程,尤其是其第二相的演变规律。实验结果表明:变形温度和应变速率的变化对该合金所受的变形抗力影响显著,其抗力值随变形温度的上升或应变速率的下降而下降,并呈现出动态回复(DRV)的特征;基于动态材料模型理论(DMM)构筑了Al-Zn-Mg-Cu合金的热加工图,并结合不同变形区域条件下的微观组织进行验证。明确了该合金的流变失稳区域,主要位于高温(>420℃)高应变速率(>1.000 s^(-1))附近,并随应变的增加失稳区域有所扩大。合金的热加工窗口为390~450℃,0.001 s^(-1);原始态合金中存在着100 nm左右的η-MgZn_(2)相,随着变形温度的升高,η-MgZn_(2)相粗化趋势显著,形变诱导析出了大量纳米级的共格Al_(3)Zr相,有效起到钉扎位错迁移的作用。

7055 aluminum alloy is a typical kind of Al-Zn-Mg-Cu alloy,which is widely used in the high-end equipment manufacturing fields such as the aerospace industry and defense industry.7055 aluminum alloy possesses a series of excellent properties,such as high specific strength,specific stiffness,toughness and great corrosion resistance,which renders itself a title of the best comprehensive property aluminum alloy.In the manufacturing process of such aluminum alloy,hot forging,hot rolling and other hot processing can enhance the material properties through deformation strengthening,recrystallization and other behaviors.However,the determination of improper hot processing parameters can easily cause the material to fail,generating cracks,poles and adiabatic shear band.In addition,such kinds of aluminum alloys possess abundant second phases,which attributes to the high strength of the alloy.In the process of hot processing,the evolution of the second phase particles will have a potential impact on the subsequent microstructure transformation.As a result,the determination of reasonable hot processing parameters is of paramount importance to improve the material final working and service performance.The aim of this paper is to precisely detect the flow stress behavior of the 7055 Al-Zn-Mg-Cu alloy,determining the best hot deformation domain and investigating the microstructure evolution law of such alloy,especially the evolution of the second phase during hot deformation.Thus,the successful investigation of this paper can be useful to both the industry manufacturing and scientific studies.In this paper,a special kind of Al-Zn-Mg-Cu aluminum alloys with spray forming state was prepared.And a series of experiments were carried out to systematically investigate the high-temperature deformation behavior and microstructure evolution of such Al-Zn-Mg-Cu aluminum alloys.The raw material was machined into a round bar shape with a diameter of 10 mm and a height of 15 mm.The hot compression test was carried out using a Gleeble-3500 thermal simulator.Before compression,graphite foils were stuck to both ends of the sample to minimize the influence of friction on the experiment and prevent the bulging effect.The sample was heated with a rate of 10℃·s^(-1)to the preset temperature(350~450℃,25℃intervals),and then it was held at this temperature for 30 s before hot compression.After that,the specimens were compressed with a strain rate range of 0.001~20.000 s^(-1).After the test,the material was cut from the central part of the cylinder and polished with 400#~2000#emery paper.After polishing,it was corroded with Keller reagent for 20~30 s.Zeiss optical microscope and Phenom XL SEM were used for the microstructure characterization.Finally,after thinning the sample,Fei Tecnai TEM was used to investigate the evolution of the second phase structure.The experimental results showed that:the changes of deformation parameters,such as deformation temperature and strain rate,would have significant effects on the flow stress of the alloy.The flow stress value decreased with the increase of deformation temperature or the decrease of strain rate.The flow stress curves also presented the characteristics of dynamic recovery(DRV).Attention should be paid that the flow stress values abnormally decreased at high strain rate(5.000,20.000 s^(-1))deformation conditions.It could be attributed to the appearance of the adiabatic heating phenomenon that the cumulative energy was difficult to dissipate due to the adiabatic container.Thus,the actual temperature of the specimen was higher than the preset one due to the adiabatic phenomenon,and cause the flow stress to decline.Moreover,on the basis of the dynamic material model(DMM)theory,the hot processing map of the investigated AlZn-Mg-Cu alloy was constructed to study the optimum deformation domain.The instability region of this alloy was clarified,mainly located near the high strain rate(>1.000 s^(-1))at high temperature(>420℃),and the instability region enlarged with the increase of the true strain.The hot processing domain of such kind of alloy was determined to be 390~450℃and 0.001 s^(-1).The optical microstructure analysis showed that the strain rate had a significant influence on the dynamic recrystallization(DRX)evolution.At 425℃and 0.001 s^(-1),the DRX grain fraction came to maximum,showing great machinable properties and going well with the prediction of the hot processing map.Bone-like second phase was detected in the Al-Zn-Mg-Cu alloy and was then verified to beη-MgZn_(2)phase.And it tended to segregate between the grain boundaries.With the increase of deformation temperature,the coarsening ofη-MgZn_(2)phase tended to be more significant.The rectangle AlCuMg phase was also detected with the size of 1~2μm under non-equilibrium solidification.In addition,the deformation progress induced a large number of nano-scale Al_(3)Zr precipitation phases to take place,which effectively pinned the dislocation migration,causing the flow stress to increase.The number of the nano-scaleη-MgZn_(2)phase in the crystal was greatly reduced through redissolving into the aluminum matrix at high temperatures.