Fatigue crack propagation of 7050 aluminum alloy FSW joints after surface peening

The surface composite modification of the 7050 aluminum alloy friction stir-welded joints was performed by shot peening(SP)/multiple rotation rolling(MRR)and MRR/SP,and the fatigue performance of the nugget zone(NZ)was investigated.The results demonstrated that the fatigue life of SP/MRR samples is longer than that of MRR/SP.On the plane 150μm below the surface.The grains with high angle grain boundary account for 71.5%and 34.3%for MRR/SP and SP/MRR samples,respectively.The crack propagation path of the MRR/SP is transgranular and intergranular,and it is intergranular for the MRR/SP.Multitudinous fatigue striations and some voids appeared at the fracture during the stable crack propagation stage.However,fatigue striations for SP/MRR are with smaller spacing,fewer holes,and smaller size under SP/MRR compared with fatigue fracture of MRR/SP.The differences in fatigue properties and fracture characteristics of the NZ are related to the microstructure after the two combined surface modifications.

Effects of silicon content on microstructure and stress corrosion cracking resistance of 7050 aluminum alloy

Evolution of microstructure and stress corrosion cracking （SCC） susceptibility of 7050 aluminum alloy with 0.094%, 0.134% and 0.261% Si （mass fraction） in T7651 condition have been investigated. The results show that the area fraction of Mg2Si increases from 0.16% to 1,48% and the size becomes coarser, while the area fraction of the other coarse phases including Al2CuMg, Mg（Al,Cu,Zn）2 and A17Cu2Fe decreases from 2.42% to 0.78% with Si content increasing from 0.094% to 0.261%. The tensile strength and elongation of 7050-T7651 alloys is decreased with the increase of Si content by slow strain rate test （SSRT） in ambient air. However, electrical conductivity is improved and SCC susceptibility is reduced with the increase of Si content by SSRT in corrosion environment with 3.5% NaCl solution.

Exfoliation corrosion behavior of 7050-T6 aluminum alloy treated with various quench transfer time

The exfoliation corrosion （EFC） behavior of 7050-T6 aluminum alloy treated with various quench transfer time after solution heat treatment was investigated by standard EFC immersion tests, strength loss measurements after EFC tests and electrochemical impedance spectroscope （EIS） technique. The results showed that EFC resistance of the alloy decreased with increasing quench transfer time. Backscattered electron scanning electron microscope （SEM） together with transmission electron microscope （TEM） observations revealed that the coverage ratio and microstructure of precipitates at grain boundary area are the most important factors which influence the EFC susceptibility of the alloy, while precipitate-free zone （PFZ） near grain boundary has no or only a minor effect on it. In addition, galvanostatic measurements of the alloy present a good correlation between EFC resistance and transients in potential. The cumulated number of transients in potential can be used to evaluate EFC resistance of the alloy.

Effect of ultrasonic power and casting speed on solidification structure of 7050 aluminum alloy ingot in ultrasonic field

With the experiment and finite element simulation, the influences of power ultrasonic on the solidification structure of 7050 aluminum alloy ingot in semi-continuous casting were researched, and the effects of casting speed on solidification structure in ultrasonic field were also analyzed. The experiment and simulation results show that the solidification structure of the ingot is homogeneously distributed, and its grain size is obviously refined at ultrasonic power of 240 W. The average grain sizes, which can be seen from the Leica microscope, are less than 100 μm. When the casting speed is 45-50 mm/min, the best grain refinement is obtained.

Effect of different quenching processes following solid-solution treatment on properties and precipitation behaviors of 7050 alloy

The effects of quenching in different ways following solid-solution treatment on properties and precipitation behaviors of7050 alloy were investigated by transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, selected areaelectron diffraction （SAED）, hardness and electrical conductivity tests. Results show that after quenching in different ways, electricalconductivity of the alloy decreases rapidly in the first 48 h of natural aging. The electrical conductivity of 7050 alloy in natural agingstate is determined by the size and density of GP zones, and the size of GP zones is the main factor. After natural aging for 70 d, thesize of GP zones is 1.8-2.6 nm in matrix of the immersion quenched sample and it is 1.4-1.8 nm in matrix of both water mist andforced air quenched samples. After natural and artificial peak aging, the hardness of the water mist quenched sample is HV 193.6 andits electrical conductivity is 30.5% （IACS） which are both higher than those of the immersion quenched sample. Therefore, watermist quenching is an ideal quenching method for 7050 alloy sheets after solid-solution treatment.

Effects of retrogression heating rate on microstructures and mechanical properties of aluminum alloy 7050

The effects of the retrogression heating rate(340℃/min,57℃/min,4.3℃/min)on the microstructures and mechanical properties of aluminum alloy 7050 were investigated by means of hardness measurement,tensile properties testing,differential scanning calorimetry(DSC)and transmission electron microscopy(TEM).The results show that the retrogression heating rate significantly affects the microstructures and mechanical properties of the alloys treated by retrogression and re-aging(RRA)process, and it is found that the medium rate(57℃/min)leads to the highest mechanical properties.The strengthening phases in the matrix are mainly the fine dispersed η′precipitates and GP zones,and the grain boundary precipitates are coarse and discontinuous η phases.

Correspondence between Susceptibility to SCC of 7050 Aluminum Alloy and Passive Film-induced Stress at Various pH Values

The passive film-induced stress and the susceptibility to SCC of 7050 aluminum alloy in 3.5%sodium chloride solution at various pH values were investigated by slow strain rate testing（SSRT） and flowing stress differential method.The results showed that the passive film-induced stress and the susceptibility to SCC decreased with increasing pH values when pH≤7,while they increased with increasing pH values when pH7.However,the corrosion type was interpreted as exfoliation corrosion when pH=l and 14,and there was no film formed on the surface of the specimens.The whole variation plots of film-induced stress and the SCC susceptibility with pH values were both presented as a valley shape.The symbol and amount of the film-induced stress were related to the compositions of the passive film,which were analyzed using X-ray photoelectron spectroscopy（XPS）.

Effect of strain on DRX of 7050 aluminum alloy during extrusion at high temperature

Extrusion experiments with different filling contents at the temperature of 440 ℃ were conducted for solution treated 7050 aluminum alloy.The microstructure of typical regions with different filler contents were described qualitatively and characterized quantitatively by electron backscattered diffraction technique(EBSD) and TEM.Fraction of deformed structure decrease continuously since ε=0.72.Second phase particles distribute dispersively and interact with dislocations when ε=0.66,which delays the occurrence of DRX.As the strain increases,particles dissolve into the matrix,and their number reduces rapidly.The influence of particles on dislocation accumulation is impaired,so dislocations accumulate on the subgrain boundaries continuously,and lead to the transformation of subgrain boundaries into high angle grain boundaries(HAGBs).The continuous dynamic recrystallization(CDRX) occurrs at the strain of 0.81～0.85.At ε=0.92,CDRX convert rapidly with the increase of HAGBs fraction.But the size of CDRX grains is deduced to be minimum 3.3 μm by theoretical analysis,and it is obtained from experimental result that the maximal size of CDRX is 5.3 μm.

Residual Stress Relaxation of Thin‑walled Long Stringer Made of Aluminum Alloy 7050‑T7451 under Transportation Vibration

Thin-walled long stringer made of aluminum alloy 7050-T7451 is prone to deformation during transportation,so a research of residual stress relaxation was launched in this paper.The transport resonance stress of long stringer was analyzed based on the power spectral density of road transport acceleration.The residual stress relaxation experiment of aluminum alloy 7050-T7451 under different equivalent stress levels was designed and carried out.According to the amount of residual stress relaxation in the experiment,an analytical model was established with the equivalent stress level coefficient.The deflection range of long stringer was evaluated under different damping ratios.The results show that when the equivalent stress exceeds 0.8σ0.2,the residual stress relaxation of the thin-walled samples occurs.The residual stress relaxation increases linearly with the equivalent stress,which is logarithmically related to the loading cycle.The deformation caused by residual stress relaxation of the long stringer is proportional to the square of the length and the bending moment caused by stress rebalance,and inversely proportional to the moment of inertia of the structure.As the damping ratio decreases from 0.03 to 0.01,the total deflection of the long stringer increases from 0 to above 1.55 mm.

Influence of two-step aging on structure and stress corrosion sensitivity of friction stir welded 7050-T7451 aluminum alloys

3 mm thick 7050-T7451 aluminum alloy joint was obtained by friction stir welding, and the two-step aging treatment was performed at 121 ℃ × 5 h + 163 ℃× 27 h after welding. Microstructure, hardness profiles and stress corrosion sensitivity of the joint were measured and studied. The results indicate that through the two-step aging, the grain size is coarsened, the age-hardening precipitates and PFZ become wider at the same time, which results in the discontinuous grain boundary;the microhardness of the FSW joints decreased, but the heat-affected zone significantly narrowed, which increased the uniformity of the microhardness of the FSW joints;and the two-stage aging effectively reduced the stress corrosion sensitivity of the FSW joints. The joints with aging treatment were not broken after 60 days,however all the joints without aging treatment were broken within 1 day.

Micro porosity and its effect on fatigue performance of 7050 aluminum thick plates

Micro porosity in aluminum alloys may contribute to fatigue life degradation, which can largely limit the application of alloys. Therefore, the fatigue life of a commercial 7050-T7451 thick plate and an experimental plate with different porosities was compared in this study. The X-ray computed tomography(XCT) was utilized to characterize the size, number density and spatial distribution of porosity inside various samples, and the fracture surface of fatigued specimens was compared by using scanning electron microscope(SEM). The results showed that the fatigue cracks prefer to initiate from constituent particles in the commercial alloy. Whereas the micro porosity is the predominant site for crack nucleation and subsequent failure in the experimental one. The presence of micro porosity in experimental7050-T7451 thick plate may reduce the fatigue life by an order of magnitude or more compared with the defect-free alloy. The pores close to sample surface are the main fatigue crack initiation site, among which larger and deeper pore leads to a shorter fatigue life. The crack initiation is also affected by the pore geometry and direction. Besides, the overall porosity inside the bulk can affect the crack propagation during fatigue tests.

Milling surface roughness for 7050 aluminum alloy cavity influenced by nozzle position of nanofluid minimum quantity lubrication

In nanofluid minimum quantity lubrication(NMQL)milling of aviation aluminum alloy,it is the bottleneck problem to adjust the position parameters(target distance,incidence angle,and elevation angle)of the nozzle to improve the surface roughness of milling,which has large and uncontrollable errors.In this paper,the influence law of milling cutter speed,helical angle,and cavity shape on the flow field around the milling cutter was studied,and the optimal nozzle profile parameters were obtained.Using 7050 aluminum alloy as the workpiece material,the milling experiment of the NMQL cavity was conducted by utilizing cottonseed oil-based Al2 O3 nanofluid.Results show that the high velocity of the surrounding air flow field and the strong gas barrier could be attributed to high rotating velocities of the milling cutter.The incidence angle of the nozzle was consistent with the helical angle of the milling cutter,the target distance was appropriate at 25–30 mm,and the elevation angle was suitable at 60°–65°.The range and variance analyses of the signal-to-noise ratio of milling force and roughness were performed,and the chip morphology was observed and analyzed.The results show that the optimal combination of nozzle position parameters was the target distance of 30 mm,the incidence angle of 35°,and the elevation angle of 60°.Among these parameters,target distance had the largest impact on cutting performance with a contribution rate of more than 55%,followed by incidence angle and elevation contribution rate.Analysis by orthogonal experiment revealed that the nozzle position parameters were appropriate,and Ra(0.087 lm)was reduced by 30.4%from the maximum value(0.125 lm).Moreover,Rsm(0.05 mm)was minimum,which was 36%lower than that of the seventh group(Rsm=0.078 mm).

Effects of dry/wet ratio and pre-immersion on stress corrosion cracking of 7050-T7451 aluminum alloy under wet-dry cyclic conditions

The stress corrosion cracking(SCC) behavior and mechanism of 7050-T7451 aluminum alloy under wet-dry cyclic conditions were investigated. Slow strain rate tests(SSRTs) and electrochemical tests were used to study the effects of dry/wet ratio(DWR) and pre-immersion on SCC.Fracture and side surface characterizations were observed by scanning electron microscopy(SEM).The results demonstrate that SCC susceptibility decreases with an increase of the DWR. With an increase of the pre-immersion time, both continuous pre-immersion(CP) and wet-dry cyclic preimmersion(WDP) samples are more sensitive to SCC, and the cracking mode in the SCC fracture region is intergranular. Furthermore, the effect of WDP on SCC is greater than that of CP when the total time immersed in solution before an SSRT is the same with each other. In fact, each single wetdry cycle can be divided into three processes with respect to the change of solution on samples’ surface. Volatilization of water on the surface results in an increase in solute concentration, thus accelerating corrosion.

Experimental and numerical investigations on fatigue behavior of aluminum alloy 7050-T7451 single lap four-bolted joints

The fatigue behavior of aluminum alloy 7050-T7451 single lap four-bolted joints was studied by high- frequency fatigue test and finite element （FE） methods. The fatigue test results showed that a better enhancement of fatigue life was achieved for the joints with highlocked bolts by employing the combinations of cold expansion, interference fit, and clamping force. The fractography revealed that fatigue cracks propagated tortuously; more fatigue micro-cliffs, tearing ridges, lamellar structure were observed, and fatigue striation spacing was simultaneously reduced. The evaluation of residual stress conducted by FE methods confirmed the experimental results and locations of fatigue crack initiation. The extension of fatigue lives can be attributed to the evolution of fatigue damage and effect of beneficial compressive residual stresses around the hole, resulting in the delay of crack initiation, crack deflection, and plasticityinduced crack closure.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Study on the reaming process of aluminum alloy 7050-T7451 under different cooling conditions

Aluminum alloy 7050 is widely used in the aeronautical industries.However,owing to their highly ductile property,chips created during high-speed machining cannot be naturally broken,and long continuous chips are unavoidably formed,impacting the machining stability and quality of the parts.Because a smaller cutting allowance is required compared with conventional machining operations,the behavior of the chips during reaming operation may be more complex and different from those determined in previous investigations.Therefore,studying the characteristics of chip formation and hole quality during the reaming process is essential to improve the machinability of aluminum alloy 7050.In this study,three different cooling conditions were applied to reaming aluminum alloy 7050-T7451 with polycrystalline diamond(PCD)reamers.The finite element models(FEMs)were established to simulate the chip formation.The macro-and micro-morphologies of chips under the three cooling conditions were compared to analyze the chip behaviors.The diameter,surface roughness,and micro-morphologies of the reamed holes were also analyzed to evaluate the hole quality.The results showed that the chip morphology was strongly influenced by the cutting parameters and cooling strategies.It was found that the desired chip morphologies satisfactory geometrical accuracy and surface quality during the reaming of aluminum alloy 7050-T7451 could be achieved using internal cooling at a spindle speed of 8000 r/min and a feed rate of 0.0l mm/z.This study also demonstrates the feasibility of an internal cooling strategy for breaking chips when reaming aluminum alloy 7050-T7451,which opens new possibilities for improving the chip-snarling that occurs during hole machining.