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.

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.

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.

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.

TEM study on microstructures and properties of 7050 aluminum alloy during thermal exposure

The microstructures of 7050 aluminum alloy under different thermal exposure conditions were investigated by means of transmission electron microscopy(TEM),high resolution electron microscopy(HREM)and tensile test.Guinier preston(GP)zone andη′phase are the main precipitates in original 7050 alloy.The orientation relationship betweenη′and matrix is Al [0001]η′//[111] andA l (1010)η′//(110).When the alloy is exposed at different temperatures for 500 h,with the thermal exposure temperature increasing,it can be seen under TEM that the precipitates become larger and the width of precipitate free zones(PFZ)becomes larger. The higher temperature the alloy is exposed at,the more the strength is reduced.Both GP zones andη′precipitates getting coarser and the PFZ getting wider should be responsible for the strength decline and elongation rise of 7050 alloy during thermal exposure.

Process of friction-stir welding high-strength aluminum alloy and mechanical properties of joint

The process of friction-stir welding 2A12CZ alloy has been studied. And strength and elongation tests have been performed, which demonstrated that the opportunity existed to manipulate friction-stir welding parameters in order to improve a range of material properties. The results showed that the joint strength and elongation arrived at their maximums (331 MPa and 4%) at 37.5 mm/min and 300 rpm. As welding parameters changing, joint tensile strength and elongation had similar development. Hardness measurement indicated that the weld was softened. However, there was considerable difference in softening degree for different joint zone. The weld top had lower hardness and wider softening zone than other zone of the weld. And softening zone at advancing side was wider than that at retreating side.

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.

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.

High-strength aluminum alloys hollow billet prepared by two-phase zone continuous casting

The two-phase zone continuous casting(TZCC)technique was used to continuously cast high-strength aluminum alloy hollow billets,and a verified 3D model of TZCC was used to simulate the flow and temperature fields at casting speeds of 2-6 mm·min^(-1).Hollow billets under the same conditions were prepared,and their macro/microstructures were analyzed by an optical microscope and a scanning electron microscope.During the TZCC process,a circular fluid flow appears in front of the mushy zone,and the induction heated stepped mold and convective heat transfer result in a curved solidification front with depressed region near the inner wall and a vertical temperature gradient.The deflection of the solidification front decreases and the average cooling rate in the mushy zone increases with increasing casting speed.Experimental results for a 2D12 alloy show that hot tearing periodically appears in the hollow billet accompanied by macrosegregation near the inner wall at casting speeds of 2 and 4 mm·min^(-1),while macroscopic defects of hot tearing and macrosegregation weaken and the average size of columnar crystals in the hollow billets decreases with further increasing casting speed.2D12 aluminum alloy hollow billets with no macroscopic defects,the finest columnar crystals,and excellent mechanical properties were prepared by TZCC at a casting speed of 6 mm·min^(-1),which is beneficial for the further plastic forming process.

Recent progress in corrosion protection of friction stir welded high-strength aluminum alloy joints

Friction stir welding （FSW） has been widely used in many industries, with which high-strength aluminum alloys can be well joined. However, the corrosion resistance of FSW high-strength Al alloy joints is relatively poor, which limits their industrial applications. The joints shall be protected against corrosion. In this review, therefore, the current status and development of corrosion protection for FSW high-strength Al alloy joints are presented. Particular emphasis has been given to different protection methods ： lowering heat input, post-weld heat treatment, surface modification and spray coatings. Finally, opportunities are identified for further research and development in corrosion protection of FSW high-strength Al alloy joints.

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.

Constituents evolution of LFEC 7050 aluminum alloy during processing

The d 120 mm ingots of 7050 aluminum alloy were made by low frequency electromagnetic casting(LFEC) and conventional DC casting process,respectively. After homogenization treatment the ingots were extruded to rods and the solution and aging treatment were carried out for the rods. Constituents evolution during processing and effects of LFEC on constituents and remnant constituents were studied. The results show that 7050 aluminum alloy mainly contains Al-Zn-Mg-Cu type and Al-Cu-Fe type constituents. Al-Zn-Mg-Cu type constituents dissolve during homogenization,while Al-Cu-Fe type constituents could not dissolve. After homogenization treatment,the main remnant constituent is Al7Cu2Fe which crushes and orients along the extrusion direction after extrusion. Compared with DC process,by the process of LFEC,the constituents or remnant constituents are smaller in size and less in content. The LFEC process shows significant improvement in elongation by LFEC in both as-cast state and final state.

Microstructure Distribution Characteristics of High-Strength Aluminum Alloy Thin-Walled Tubes during Multi-Passes Hot Power Backward Spinning Process

The microstructure of the thin-walled tubes with high-strength aluminum alloy determines their final forming quality and performance. This type of tube can be manufactured by multi-pass hot power backward spinning process as it can eliminate casting defects, refine microstructure and improve the plasticity of the tube. To analyze the microstructure distribution characteristics of the tube during the spinning process, a 3D coupled thermo-mechanical FE model coupled with the microstructure evolution model of the process was established under the ABAQUS environment. The microstructure evolution characteristics and laws of the tube for the whole spinning process were analyzed. The results show that the dynamic recrystallization is mainly produced in the spinning deformation zone and root area of the tube. In the first pass, the dynamic recrystallization phenomenon is not obvious in the tube. With the pass increasing, the trend of dynamic recrystallization volume percentage gradually increases and extends from the outer surface of the tube to the inner surface. The fine-grained area shows the states of concentration, dispersion, and re-concentration as the pass number increases. .

Influence of pre-stretching on quench sensitive effect of high-strength Al-Zn-Mg-Cu-Zr alloy sheet

The influence of pre-stretching on quench sensitive effect of high strength Al-Zn-Mg-Cu-Zr alloy AA 7085 sheet was investigated by tensile testing at room temperature,transmission electron microscopy(TEM)and differential scanning calorimetry(DSC).The water-cooled and aged alloy exhibits higher strength than the air-cooled and aged alloy;2.5%pre-stretching of tensile deformation exerts little effect on strength of water-cooled and aged alloy but increases that of air-cooled and aged one,and therefore the yield strength reduction rate due to slow quenching decreases from about 3.8%to about 1.0%,reducing quench sensitive effect.For the air-cooled alloy,pre-stretching increases the sizes ofη'strengthening precipitates but also increases their quantity and the ratio of diameter to thickness,resulting in enhanced strengthening and higher strength after aging.The reason has been discussed based on microstructure examination by TEM and DSC.

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.

7050-T7451铝合金铣削参数多指标优化研究

通过Spike切削力测量设备及在线监控系统,采用单因素试验,研究硬质合金刀具铣削7050-T7451铝合金时,主轴转速、铣削宽度、铣削深度、进给速度以及顺、逆铣加工方式分别对刀具所受弯矩、表面粗糙度及功率的影响。运用正交试验法,通过S/N比分析和方差分析,获得7050-T7451铝合金的最优铣削参数。研究表明:顺铣更适用于7050-T7451铝合金的铣削加工;刀具所受弯矩随着主轴转速的增加而逐渐减小,最终趋于平稳,并随着铣削宽度、铣削深度及进给速度的增加而增加;表面粗糙度随主轴转速的增加呈先减小后增加;各铣削参数与加工功率均呈正相关关系;为获得较低加工弯矩和功率、较好表面质量,应取较高的主轴转速、较低的进给速度和铣削宽度,7050-T7451铝合金的最优铣削参数水平为A_(3)B_(1)C_(2)D_(1),即n=6000r/min,a_(e)=3.0mm,a_(p)=1.6mm,v_(f)=1500mm/min。

激光冲击和深冷处理对7050铝合金残余应力和力学性能的影响

研究了不同激光冲击处理后进行深冷处理以及补充低温回火处理对7050铝合金残余应力和力学性能的影响及其规律。结果表明,激光冲击能够显著提高7050铝合金的表面压应力至-260 MPa,而增加深冷-低温回火处理后表面压应力减小约23%。但合理调整激光冲击工艺参数,能够有效弥补深冷-补充低温回火处理对表面压应力的降低,其中,当激光冲击次数为3次时,增加深冷-补充低温回火后表面压应力可达-230 MPa。此外,与单一激光冲击处理相比,激光冲击-深冷-补充低温回火处理后铝合金基体、表面之间的硬度得到均匀化。深冷处理能够促使激光冲击后铝合金试样表面变形加剧,同时晶粒得到进一步细化并且亚晶界增多,使7050铝合金的强度提高。

钛元素对再生7050铝合金细化作用的研究

试验研究了Ti含量及加Ti后熔体在炉内停留时间对再生7050铝合金铸锭晶粒细化的影响。结果表明,以再生铝为原料配制的7050铝合金添加Ti的质量分数达到0.035%时,晶粒细化效果最佳,树枝晶基本消失。当Ti质量分数达到0.036%时,铝液在炉内停留不超过2 h时,晶粒有粗化倾向,但整体变化不大;当停留时间达到或超过3 h时,晶粒粗化明显。再生7050铝合金铝液添加Ti后,保温时间宜不超过2 h,宜快速冷却进行铸造。