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.

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.

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。

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.

工艺参数对7050铝合金铣削表面质量的影响

为了探究7050铝合金铣削表面质量的关键影响因素,寻找最优工艺参数,对7050铝合金在浇注冷却条件下进行了正交试验,建立了表面粗糙度与切削深度、主轴转速之间的回归模型,分析了加工过程中刀具-工件相对接触状态不同时所产生的刀痕差异。结果表明:切削深度对表面粗糙度的影响最大,主轴转速其次,进给量的影响最小;刀痕的表现形式与刀具-工件相对接触状态存在直接联系,刀具切入工件位置主要受切削深度的影响产生单向圆弧状刀痕,刀具完全进入铣削工件后则会形成交错的圆弧状刀痕,在刀具离开工件位置主要受进给量影响产生较浅刀痕。

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。

不同冷速下7050铝合金的晶粒细化研究

采用V型模具、光学显微镜和扫描电镜等研究不同冷速下7050合金的细化机理。结果表明随着冷却速度从1.8℃/s增加到54.3℃/s,7050铝的晶粒度从75.5μm降低至45.6μm;随着冷却速度的增加,第二相中T相(AlZnMgCu)和S相(Al_(2)CuMg)的平均尺寸逐渐减小,其分布更加细小弥散,该两种相可以作为铝晶粒的形核质点,促进7050合金的晶粒细化。

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。

航空用高品质7050铝合金大规格扁锭熔铸技术研究

7050铝合金具有密度低、强度高、热加工性能优异等优点,在航空航天结构件上应用广泛。但在7050铝合金大规格铸锭半连续铸造过程中,存在热裂、成形困难、严重显微疏松和氧化夹杂等问题。本研究开发了大规格扁锭半连续铸造过程中合金熔体冶金质量控制技术,系统考察了精炼工艺、晶粒细化技术对大规格扁锭品质的影响规律,确定了大规格扁锭铸造工艺。结果表明,优化工艺制度能严控熔体内氢和夹杂物含量,采用Al-Ti-C晶粒细化工艺制度将晶粒度控制在1~1.5级,高质量的熔体质量缓解了合金凝固收缩应变,极高比例地解决了扁锭开裂问题。最终,形成了一套适用于大规格7050合金扁锭的熔铸工艺制度。

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.

基于响应面法和BP神经网络的7050铝合金腐蚀疲劳寿命预测及对比

对7050铝合金依次进行腐蚀和疲劳试验,获得了腐蚀后合金的疲劳寿命;分别利用响应面法和BP神经网络得到腐蚀时间、NaCl溶液浓度、加载频率、最大应力与腐蚀疲劳寿命之间的映射关系,并对该合金进行腐蚀疲劳寿命预测,比较了两种模型的预测误差。结果表明:在不同载荷条件下,两种模型的可靠性均较好,响应面模型和BP神经网络模型预测得到的腐蚀后合金的对数疲劳寿命与试验值的均方根误差分别为0.0710,0.0683,决定系数分别为0.9519,0.9980;BP神经网络模型的预测精度优于响应面模型。

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.

冷拉变形对7050铝合金丝材组织与性能的影响

对7050铝合金丝材经冷拉变形后过时效态以及冷加工变形态的组织与性能进行了研究。结果表明,随着冷变形量的增加,晶粒拉长,屈服强度、抗拉强度和剪切强度增大。与冷拉态相比较,过时效态丝材的这些指标增加不明显;晶粒越均匀细小,合金丝材冷镦时越不易发生开裂。

晶粒细化剂对7050铝合金铸锭晶粒组织的影响

对分别采用AlTi3C0.15和AlTi5B1细化的不同规格7050铝合金铸锭进行低倍组织观察,并利用光学显微镜和金相分析软件对各铸锭不同位置进行显微组织分析和晶粒尺寸测量。结果表明,采用AlTi3C0.15细化的铸锭,其低倍晶粒度明显大于采用AlTi5B1细化的铸锭;采用AlTi3C0.15细化的铸锭,其3/8厚度位置的晶粒尺寸大于1/2厚度位置,采用AlTi5B1细化的铸锭其3/8厚度位置的晶粒尺寸小于1/2厚度位置。

变形温度对多向锻造7050铝合金组织与力学性能的影响

对均匀化后的7050铝合金铸锭进行不同变形温度(390、420、450℃)的多向锻造实验。通过光学显微镜、扫描电镜、电子背散射衍射、透射电镜和室温拉伸性能测试,研究变形温度对锻造态、固溶态和时效态7050铝合金显微组织和力学性能的影响。结果表明:多向锻造对合金中粗大的残余结晶相起到一定的消减效果,残余相的面积分数随变形温度升高而降低。多向锻造后,合金芯部基本为粗大的原始晶粒,随变形温度升高动态再结晶程度增大;固溶处理后,随变形温度升高残余结晶相的面积分数降低,亚晶含量增加;时效处理后,随变形温度升高纳米级析出相的含量增加。时效态合金的强度随变形温度升高而提高,变形温度为450℃时,合金综合力学性能最好,其屈服强度为496.7 MPa,抗拉强度为555.1 MPa,伸长率为8.1%。

腐蚀条件对7050航空铝合金断口形貌及失效机理的影响

为研究腐蚀条件对7050航空铝合金失效形式的影响,分析不同腐蚀时间和腐蚀溶液浓度对7050航空铝合金试样断口宏观及微观形貌、能谱、金相组织的影响,并进一步分析其失效机理。结果表明:在3.5%NaCl溶液和5%NaCl溶液的腐蚀环境下,7050航空铝合金试样中的Al元素含量会随着腐蚀时间的增加而逐渐降低,而5%NaCl溶液中腐蚀的试样含有更多的O元素;腐蚀时间和腐蚀液浓度的增加会使得7050航空铝合金损伤机理发生转变,还会使试样产生更多的由腐蚀产物堆积的局部腐蚀点和深色狭长的被腐蚀晶界。