Static softening behaviors of 7055 alloy during the interval time of multi-pass hot compression

Multipass plain strain compression test of 7055 alloy was carried out on Gleeble 1500D thermomechanical simulator to study the effect of interval time on static softening behavior between two passes. Microstructural features of the alloy deformed with delay times varying from 0 to 180 s after achieving a reduction of ,-~52 % in the 13 stages was investigated through TEM and EBSD observations. The 14th pass of peak stresses after different delay times were gained. The peak stress decreases with the interstage delay time increasing, but the decreasing trend is gradually slower. Static recovery, metadynamic recrystallization, and/or static recrystallization can be found in the alloy during two passes. The recovery and recrystallization degree increases with longer interstage delay time. The static recovery is the main softening mechanism. Subgrain coalescence and subgrain growth together with particle-stimulated nucleation are the main nucleation mechanisms for static recrystallization.

Effect of deformation temperature on microstructure and mechanical properties of 7055 aluminum alloy after heat treatment

The multi-pass hot compression test of 7055 aluminum alloy was performed at different temperatures and then the samples were heat treated by T6 heat treatment.The compressed samples were analyzed by OM and TEM.The results reveal that the average aspect ratio of the grains in the specimens compressed first decreases and then increases,the dislocation density decreases and subgrain diameter increases with the increase of deformation temperature.The effects of deformation temperature on the microstructure and mechanical properties of 7055 aluminum alloy after heat treatment were investigated by means of OM and mechanical property test.The results indicate that the deformation temperature significantly influences microstructure and mechanical property of 7055 aluminum alloy.The volume fraction of recrystallization grains presents a ＂fall-rise＂ pattern with the deformation temperature rising.The mechanical properties get better when the volume fraction of recrystallization grains decreases.Moreover,the volume fraction of recrystallization grain has a minimum value,appropriately 45%,and the sample exhibits the highest strength and elongation at the deformation temperature of 400 ℃.

Effect of T7951 secondary aging treatment on crack propagation behavior of 7055 aluminum alloy

The crack propagation rates of T6 peak aging and T7951 secondary aging 7055 aluminium alloys were tested under stress ratios (R) of 0.6, 0.05 and ?1, respectively. The microstructures and fracture surfaces were analyzed by TEM and SEM. The results reveal that the crack propagation rate is affected by the stress ratio and microstructure such as the distribution, dimension and volume fraction of matrix precipitates, grain boundary precipitates and precipitate free zone. For both heat-treated specimens, crack propagation rate increases with the improvement of R when it is a positive value while crack propagation rate at R=?1 is much similar to that at R=0.06. The crack growth rates exhibit no obvious difference in lower stress intensity factor range (ΔK), while the difference starts to be obvious when ΔK exceeds certain value. The fracture analysis testifies a better fracture toughness for 7055-T7951 with a smaller striation space in Paris region.

TEM STUDY ON MICROSTRUCTURAL DEVELOPMENT OF SINGLE-AGEING 7055 ALUMINUM ALLOYS

Mierostruetural development of a commercial 7055 （Al-Zn-Mg-Cu）alloy is studied by transmission electron mieroseope（TEM） during the process of single-ageing for up to 48 h. It is observed that Guinier-Preston （GP） zones are formed on { 111} planes when the sample is aged for a short time and grows up gradually with increase of ageing time. η＇ phase is formsed after ageing for 4 h at 120℃, having the orientation relationship with the matrix as[0 0 0 1]η＇//[1 1^- 1]Al and （1 0 1^- 1）η＇//（1 1 0）Al.η phase starts to occur after 24 h ageing and has an orientation relationship with matrix as [1^- 1 0 0]η//[1 1 0]Al and （0 0 0 1）η// （1 1 1）Al. Since the density of both η＇ phase and η phase particles is much lower than that of GP zone on aged alloy, GP zones are important to control the properties of the alloy.

Optimization of deformation parameters of dynamic recrystallization for 7055 aluminum alloy by cellular automaton

In order to simulate the microstructure evolution during hot compressive deformation,models of dynamic recrystallization(DRX)by cellular automaton(CA)method for7055aluminum alloy were established.The hot compression tests were conducted toobtain material constants,and models of dislocation density,nucleation rate and recrystallized grain growth were fitted by leastsquare method.The effects of strain,strain rate,deformation temperature and initial grain size on microstructure variation werestudied.The results show that the DRX plays a vital role in grain refinement in hot deformation.Large strain,high temperature andsmall strain rate are beneficial to grain refinement.The stable size of recrystallized grain is not concerned with initial grain size,butdepends on strain rate and temperature.Kinetic characteristic of DRX process was analyzed.By comparison of simulated andexperimental flow stress–strain curves and metallographs,it is found that the established CA models can accurately predict themicrostructure evolution of7055aluminum alloy during hot compressive deformation.

Modified constitutive model and workability of 7055 aluminium alloy in hot plastic compression

To obtain flow behavior and workability of 7055 aluminium alloy during hot deformation,hot compression tests at different temperatures and strain rates are conducted.True stress?strain curves of 7055 aluminium alloy under different conditions are obtained and the flow stress increases with ascending strain rate and descending temperature.For Arrhenius constitutive equation,each material parameter is set as a constant,which will bring forth large error for predicting flow behavior.In this work,material parameters are fitted as a function of temperature or strain rate based on experimental results and a modified constitutive equation is established for more accurate prediction of flow behavior of 7055 aluminium alloy.The effects of temperature and strain rate on power dissipation and instability are analyzed to establish a processing map of 7055 aluminium alloy.The dominant deformation mechanism for microstructure evolution at different deformation conditions can be determined and high efficiency of power dissipation may be achieved from power dissipation map.Meanwhile,proper processing parameters to avoid flow instability can be easily acquired in instability map.According to the processing map,optimized processing parameters of 7055 aluminium alloy are temperature of 673?723 K and strain rate of 0.01?0.4 s^?1,during which its efficiency of power dissipation is over 30%.Finite element method(FEM)is used to obtain optimized parameter in hot rolling process on the basis of processing map.

Effects of precipitates on fatigue crack growth rate of AA 7055 aluminum alloy

The effects of precipitates on the fatigue crack growth rate of AA 7055 Al alloy subjected to different ageing treatments were investigated using transmission electron microscope and fatigue crack growth testing.The results show that the T77 treated samples exhibit the lowest crack growth rate,while the crack growth rate of over-aged samples is the highest.In terms of the model based on the reversibility of dislocation motion within the plastic zone close to the crack tip,the improved crack growth resistance is attributed to many precipitates that are coherent with Al matrix in the under-aged and T77 treated samples.When the precipitate is coherent with the Al matrix,the larger the precipitate is,the slower the fatigue crack grows.The effects of grain boundary precipitates and precipitate free zone on the fatigue crack growth resistance are less significant than those of precipitates within grains of the alloy.

Simulation of low proportion of dynamic recrystallization in 7055 aluminum alloy

The hot deformation and dynamic recrystallization(DRX)behaviors of 7055 aluminum alloy were studied at temperatures of 390−470℃ and strain rates of 0.01−1 s^(−1).A low DRX fraction between 1% and 13% was observed by using EBSD technique.A modified JMAK-type DRX model was proposed for such low DRX fraction problems.The model was used together with commercial FEM software DEFORM-3D to simulate the hot compression of 7055 aluminum alloy.There was a good agreement between experimental and predicted DRX fractions and grain size with an average absolute relative error(AARE)of 13.7% and 6.3%,respectively.In order to further verify the validity of the proposed model,the model was also used to simulate DRX in industrial hot rolling of 7055 aluminum alloys.The results showed that the distribution of DRX fraction was inhomogeneous,and agreed with experimental observations.

Prediction on hot deformation behavior of spray-formed 7055 aluminum alloy via phenomenological models

Hot compression tests in the temperature range of 340-450 ℃ and strain rate range of 0.001-1 s^-1 of spray-formed 7055 aluminum alloy were carried out to study its hot deformation behavior. Three phenomenological models including Johnson-Cook, modified Fields-Backofen and Arrhenius-type were introduced to predict the flow stresses during the compression process. And then, a comparative predictability of the phenomenological models was estimated in terms of the relative errors, correlation coefficient（R）, and average absolute relative error（AARE）. The results indicate that Johnson-Cook model and modified Fields-Backofen model cannot well predict the hot deformation behavior due to the large deviation in the process of line regression fitting. Arrhenius-type model obtains the best fit through combining the effect of strain rate and temperature.

Influence of quench transfer time on microstructure and mechanical properties of 7055 aluminum alloy

The influence of quench transfer time on the microstructure and mechanical properties of 7055 aluminum alloy with and without zirconium was investigated by tensile properties test,optical microscopy,scanning electron microscopy and transmission electron microscopy.For the Zr-free alloy,the strength increases to the highest value at 20 s with transfer time,and then decreases slightly.The elongation decreases slowly with transfer time within 20 s,and more rapidly after 20 s.For the Zr-containing alloy,prolonging transfer time within 20 s results in slight decrease in the strength and elongation,and rapid drop of which is observed after 20 s.For the Zr-free alloy,prolonging transfer time can increase the percentage of intergranular fracture,which is mainly caused by wide grain boundary precipitate free zone.The failure mode of the Zr-containing alloy is modified from the predominant transgranular void growth and intergranular fracture to transgranular shear and intergranular fracture with increase in the transfer time,which is attributed to the wider grain boundary precipitate free zone and coarse equilibrium η phases in the matrix.

Effects of single and multi-stage solid solution treatments on microstructure and properties of as-extruded AA7055 helical profile

The effects of single-stage solution treatment(SST),enhanced solution treatment(EST),high-temperature pre-precipitation(HTPP)and multi-stage solution treatment(MST)on the microstructure,mechanical properties and corrosion resistance of the as-extruded 7055 aluminium alloy(AA7055)helical profile were investigated using differential scanning calorimetry(DSC),optical microscopy(OM),scanning electron microscopy(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscopy(TEM).It was observed that EST and MST could promote the dissolution of the second-phase particles compared with the traditional SST,and the intergranular phases were distinctly discontinuously distributed after HTPP and MST.There was obvious difference in the main texture type and texture strength for the alloy after different solid solution treatments.HTPP could improve the corrosion resistance of the alloy by regulating the intergranular phases,but the mechanical properties were severely weakened.While the good corrosion resistance of the alloy could be obtained by MST without obvious strength loss.As a result,the MST is an ideal solid solution treatment scheme for AA7055.

Relationship of particle stimulated nucleation,recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

The variations of coarse intermetallic particles in hot-extruded 7055 aluminum alloys with 0.041 wt%Fe and 0.024 wt%Si increasing to 0.272 wt%Fe and 0.134 wt%Si were investigated.The particle stimulated nucleation(PSN)behaviors for different kind of coarse particles were detailly analyzed by EBSD.Moreover,the effect of PSN responding to Fe and Si contents on recrystallization and tensile properties of 7055 alloys was evaluated.With increasing Fe and Si contents,the size and number density of coarseη/S particles are reduced,while the number densities of coarse Al7Cu2 Fe and Mg2Si particles are both increased and the coarse Al7Cu2 Fe particles transform from rod-like to irregular.More PSN recrystallized grains with predominant orientations deviated from the extruded fiber textures are stimulated by the irregular Al7Cu2 Fe and Mg2Si particles,because a higher degree of local non-uniform deformation is produced.The rod-like Al7Cu2 Fe particles cause the greatest degree of local non-uniform deformation owing to the largest aspect ratio,but the shape also restricts the area of particle deformation zone(PDZ)resulting in fewer PSN recrystallized grains.The irregularη/S particles give rise to the lowest degree of local non-uniform deformation and fewest PSN recrystallized grains with the major orientations close to the extruded fiber textures.Consequently,despite the number and size of coarseη/S particles are reduced,the proportion of high angle grain boundaries(HAGBs)is increased and the extruded fiber textures are weakened with Fe and Si contents increasing,because of the increased Al7Cu2 Fe and Mg2Si particles.The strength is slightly declined by the weakened<111>//ED(extrusion direction)fiber texture,while the elongation is reduced for a larger number of coarse particles and more HAGBs with higher Fe and Si contents.

Effects of Trace Amount Modified SiC Nanoparticles and Electromagnetic Stirring (EMS) on Microstructure and Mechanical Properties of 7055 Aluminum Alloy

The effects of trace amount modified SiC nanoparticles and electromagnetic stirring(EMS)on mierostructures and mechanical properties of 7055 aluminum alloy are investigated experimentally.The result shows that the original developed coarse dendrite and columnar with obvious orientation microstructure turns into homogeneous equiaxed microstructure with the application of trace amount modified SiC nanoparticles and EMS.A minimum grain size is 96 μm and has been observed when the addition of modified SiC nanoparticles is 0.05%and the current of EMS is 100A.The ultimate tensile strength is increased by 15.8%and the elongation is improved by 50%compared to those without modified SiC nanoparticles and EMS.Moreover,the compound effects of trace amount modified SiC nanoparticles and EMS is discussed to explain the mechanisms of grain refinement and mechanical properties on 7055 aluminum alloy.

Precipitates and corrosion resistance of an Al-Zn-Mg-Cu-Zr plate with different percentage reduction per passes

7055 aluminum alloy plates with the same size were rolled by two processes： small percentage reduction per pass （PRPP） and large percentage reduction per pass, respectively. Meanwhile, the effect of PRPP on the precipitates and corrosion resistance of 7055 aluminum alloy plate was investigated. The mechanisms were analyzed and discussed by optical microscopy （OM）, transmission electron microscopy （TEM）, high-resolution transmission electron microscopy （HRTEM） and electron back-scattered diffraction （EBSD） technique. Large PRPP can improve the corrosion resistance. For the plate rolled by small PRPP, the main precipitate is guinier-preston （GP） zone and continuous grain boundary precipitates （GBPs）, while, for the plate rolled by large PRPP, the main precipitates are the GP zone and η precipitate, and the GBPs are discontinuous.