Microstructure and Low-Cycle Fatigue of a Friction-Stir-Welded 6061 Aluminum Alloy

Strain-controlled low-cycle fatigue (LCF) tests and microstructural evaluation were performed on a friction-stir-welded 6061Al-T651 alloy with varying welding parameters. Friction stir welding (FSW) resulted in fine recrystallized grains with uniformly distributed dispersoids and dissolution of primary strengthening precipitates β″ in the nugget zone (NZ). Two low-hardness zones (LHZs) appeared in the heat-affected zone (HAZ) adjacent to the border between the thermomechanically-affected zone (TMAZ) and HAZ, with the width decreasing with increasing welding speed. No obvious effect of the rotational rate on the LHZs was observed. Cyclic hardening of the friction-stir-welded joints was appreciably stronger than that of base metal (BM), and it also exhibited a two-stage character where cyclic hardening of the friction-stir-welded 6061Al-T651 alloy at higher strain amplitudes was initially stronger followed by an almost linear increase of cyclic stress amplitudes on the semilog scale. Fatigue life, cyclic yield strength, cyclic strain hardening exponent, and cyclic strength coefficient all increased with increasing welding speed, but were nearly independent of the rotational rate. Most friction-stir-welded joints failed along the LHZs and exhibited a shear fracture mode. Fatigue crack initiation was observed to occur from the specimen surface, and crack propagation was mainly characterized by the characteristic fatigue striations. Some distinctive tiremark patterns arising from the interaction between the hard dispersoids/inclusions and the relatively soft matrix in the LHZ under cyclic loading were observed to be present in-between the fatigue striations.     

Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

Friction-stir processing (FSP) of twin-roll cast (TRC) Al-Mg-Sc alloy resulted into ultrafine-grained microstructure. The alloy was processed in as-received and aged (563 K [290 °C], 22 hours) conditions and at three different tool rotation rates: 800, 400, and 325 rpm. The microstructural features were characterized using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The grain size varied from 0.89 μm to 0.39 μm depending on the processing and initial thermo-mechanical conditions of the alloy. The TRC alloy processed at 325 rpm in aged condition had all the grains less than 1 μm, and 95 pct of grains had high-angle grain boundaries (HAGBs). In all the cases, the fraction of HAGBs were more than 80 pct. The variation of misorientation angle distribution was similar to the theoretical MacKenzie distribution for cubic crystal materials. Grain size analysis at different sections and locations on the transverse section of the dynamically recrystallized zone showed a homogeneous and equiaxed microstructure. The average dispersoid (Al₃(Sc,Zr)) size was ~8.0 nm in diameter obtained using high-resolution TEM. Grain size reduction was observed with increase in Zener–Hollomon parameter. It was shown that under the current microstructural and deformation conditions, dynamic recrystallization via particle-stimulated nucleation might not be possible during FSP.     

Influencing Factors of Preparation of Al-Mg-Sc Master Alloy

Through the experiment, the reduced temperature, the containing scandium quantity of the fused salt, the agitated intensity and the restoring time that influence the rate of receives of scandium, and determined the technological conditions which uses the method of hot Al-Mg restoring anhydrous the chloride of scandium to prepare the Al-Mg-Sc master alloy. The best experimental technological conditions are that the master alloy can contains 8.0% of scandium while the restoring temperature is at 900℃, the restoring time is 20min and the agitated time is a half minute. When the method of the fused salt covered with bell, agitated and pressed the argon is used in the expansion tests, the agitated number is 4 times and the average receives rate of Sc is 90.0%.     

Al-Mg-Sc合金的原子键强与力学性能

运用固体经验电子理论(EET)，对掺微量Sc的Al—Mg合金的价电子结构进行计算。结果表明：在合金熔体中Sc与Al原子存在强的结合倾向，易形成粗大的Al3Sc颗粒，起非匀质形核细化晶粒作用；在合金凝固过程中，形成Al—Sc偏聚区，析出细小弥散的Al3Sc颗粒，增强了合金基体和晶界的共价键络，使合金的硬度、强度和晶粒热稳定性都得到提高。     

Microstructure and Cyclic Deformation Behavior of a Friction-Stir-Welded 7075 Al Alloy

Microstructural changes and cyclic deformation characteristics of friction-stir-welded 7075 Al alloy were evaluated. Friction stir welding (FSW) resulted in significant grain refinement and dissolution of η′ (Mg(Zn,Al,Cu)₂) precipitates in the nugget zone (NZ), but Mg₃Cr₂Al₁₈ dispersoids remained nearly unchanged. In the thermomechanically affected zone (TMAZ), a high density of dislocations was observed and some dislocations were pinned, exhibiting a characteristic Orowan mechanism of dislocation bowing. Two low-hardness zones (LHZs) between the TMAZ and the heat-affected zone (HAZ) were observed, with the width decreasing with increasing welding speed. Cyclic hardening and fatigue life increased with increasing welding speed from 100 to 400 mm/min, but were only weakly dependent on the rotational rate between 800 and 1200 rpm. The cyclic hardening of the friction-stir-welded joints exhibiting a two-stage character was significantly stronger than that of the base metal (BM) and the energy dissipated per cycle decreased with decreasing strain amplitude and increasing number of cycles. Fatigue failure occurred in the LHZs at a lower welding speed and in the NZ at a higher welding speed. Fatigue cracks initiated from the specimen surface or near-surface defects in the friction-stir-welded joints, and the initiation site exhibited characteristic intergranular cracking. Crack propagation was characterized by typical fatigue striations along with secondary cracks.     

Al-Mg-Sc合金的拉伸性能和显微组织研究

研究了Al-5.63Mg-0.13Sc(wt%)合金经过250℃和300℃退火后室温的拉伸性能和显微组织特征；分析了Sc对合金组织与拉伸性能的影响。结果表明：退火后拉伸强度先迅降低，但随着退火时间延长，拉伸性能稳定保持不变。透射电镜观察表明，在Al-Mg-Sc合金中，Sc以细小的Al3Sc粒子的形式在晶内及亚结构上析出，它强烈地钉扎或阻碍位错的运动，稳定亚结构。Ml-Mg合金添加Sc能使该合金在应变硬化和稳定化处理后具有良好的机械性能。     

Ultrafine-Grain Structure Formation in an Al-Mg-Sc Alloy During Warm ECAP

Microstructural evolution taking place during equal-channel angular pressing was studied in a commercial Al-6Mg-0.3Sc alloy at 523 K (250 °C) (~0.5T _m). The structural changes are mainly associated with development of microshear bands (MSBs) that are continuously formed by strain accumulation and microstructural heterogeneities in each pass, which result in fragmentation of coarse original grains. New ultrafine grains (UFGs) with moderate-to-high angle boundary misorientations are concurrently evolved in the interiors of MSBs accompanied by rigid body rotation at medium-to-large strains. Such strain-induced grain refinement process occurs very slowly and incompletely in the present heavily alloyed Al alloy, leading to formation of a mixed microstructure, i.e., the UFGs in colony and some weakly misoriented fragments of original grains. The microstructure evolved at ε ≈ 12 is characterized by a bimodal crystallite size distribution with two peaks at d ₁ ≈ 0.2 to 0.3 μm and d ₂ ≈ 0.6 to 0.7 μm, and the fraction of high angle boundaries of about 0.35 ± 0.05. The main factors promoting dynamic formation of UFGs and the effects of thermal processes on it during severe plastic deformation are discussed in detail.     

The Work Hardening Rate of an Aged and Recovered Al-Mg-Sc Alloy

The combined role of recovery and Al₃Sc precipitates on the work hardening of an Al-Mg-Sc alloy was explored in this work. Application of a single internal state variable model consistent with previous work on this alloy in its as-aged state suggests a similar influence of nonshearable precipitates and subgrain boundaries on the work hardening rate. These results are discussed in relation to previous models developed to describe the same alloy in the as-aged state as well as the work hardening of recovered binary Al-Mg alloys.     

共电沉积制备铝镁钪合金的工艺参数研究

研究了共电沉积法生产铝镁钪合金过程,采用连续脉冲—示波器法监控电解过程中的分解电压、电流强度等参数的计时变化。结果显示,实际分解电压随电流强度的增强而变大,加入0.5%MgO-0.5%Sc_2O_3可使分解电压平均减小0.3V;实际分解电压随着极距的增大而变大,后期增势渐缓,而升高温度可降低实际分解电压;根据实际分解电压随着电解时间的变化判断出加料周期为23min;在720℃、极距3cm、电流强度3A时,电解2h成功制备出Al-2.71%Mg-0.63%Sc合金。     

Al-Mg-Sc合金中组元活度及活度相互作用系数

以 Miedema二元合金生成热模型及 Toop方程为基础 ,利用元素的基本性质 ,计算了 1 0 73K下 Al-Mg- Sc三元合金溶液中组元的活度及组元间活度相互作用系数。结果表明 ,lnγ0Mg=- 0 .0 0 54;εMg Mg=0 .0 0 2 2 ;εSc Mg=εMg Sc=0 .0 2 8;ρMg Mg=- 0 .0 0 1 3;ρSc Mg=0 .0 1 5;ρMg Sc Mg =- 0 .0 75。     

微量锆对Al-Mg-Sc合金力学性能与显微组织的影响

制备了成分分别为Ａｌ５Ｍｇ（０．１０～０．３０）Ｓｃ和Ａｌ５Ｍｇ（０．１０～０．３０）Ｓｃ（０．０５～０１５）Ｚｒ的两种合金，测试了其不同状态下的拉伸力学性能σｂ、σ０２和δ，采用金相显微镜、透射电镜及扫描电镜观察分析了其不同状态下的显微组织结构。结果发现，微量锆的添加明显提高了ＡｌＭｇＳｃ合金的强度，细化了合金铸锭组织的晶粒尺寸，抑制了合金形变组织的再结晶。另外，对有关的作用机理进行了探讨研究。     

Effect of Processing Parameters on Plastic Flow and Defect Formation in Friction-Stir-Welded Aluminum Alloy

The effect of processing parameters on material flow and defect formation during friction stir welding (FSW) was investigated on 6.0-mm-thick 2014Al-T6 rolled plates with an artificially thickened oxide layer on the butt surface as the marker material. It was found that the “S” line in the stir zone (SZ) rotated with the pin and stayed on the retreating side (RS) and advancing side (AS) at low and high heat inputs, respectively. When the tool rotation rate was extremely low, the oxide layer under the pin moved to the RS first and then to the AS perpendicular to the welding direction, rather than rotating with the pin. The material flow was driven by the shear stresses produced by the forces at the pin–workpiece interface. With increases of the rotation rate, the depth of the shoulder-affected zone (SAZ) first decreased and then increased due to the decreasing shoulder friction force and increasing heat input. Insufficient material flow appeared in the whole of the SZ at low rotation rates and in the bottom of the SZ at high rotation rates, resulting in the formation of the “S” line. The extremely inadequate material flow is the reason for the lack of penetration and the kissing bonds in the bottom of the SZ at extremely low and low rotation rates, respectively.     

Effect of Alclad Layer on Material Flow and Defect Formation in Friction-Stir-Welded 2024 Aluminum Alloy

The effect of the Alclad layer on material flow and defect formation during friction-stir welding (FSW) of 6.5-mm-thick 2024Al-T351 alloy plates was investigated. To characterize the material flow during FSW, different cross sections of the keyhole and “stop-action weld” were made for metallographic observations. It was found that the top Alclad assembled at the shoulder/workpiece interface, thereby weakening the material flow in the shoulder-driven zone and favoring the formation of void defect at high traveling speeds. The bottom Alclad layer extended into the weld at excess material flow state, which could be avoided at balanced material flow state. A conceptual model of material flow was proposed to describe the formation of the weld. It was indicated that a perfect FSW joint of Alclad 2024Al alloy without defect could be obtained at an optimum FSW condition.     

X-Ray and Neutron Diffraction Measurements of Dislocation Density and Subgrain Size in a Friction-Stir-Welded Aluminum Alloy

The dislocation density and subgrain size were determined in the base material and friction-stir welds of 6061-T6 aluminum alloy. High-resolution X-ray diffraction measurement was performed in the base material. The result of the line profile analysis of the X-ray diffraction peak shows that the dislocation density is about 4.5 × 10¹⁴ m⁻² and the subgrain size is about 200 nm. Meanwhile, neutron diffraction measurements have been performed to observe the diffraction peaks during friction-stir welding (FSW). The deep penetration capability of the neutron enables us to measure the peaks from the midplane of the Al plate underneath the tool shoulder of the friction-stir welds. The peak broadening analysis result using the Williamson–Hall method shows the dislocation density of about 3.2 × 10¹⁵ m⁻² and subgrain size of about 160 nm. The significant increase of the dislocation density is likely due to the severe plastic deformation during FSW. This study provides an insight into understanding the transient behavior of the microstructure under severe thermomechanical deformation.     

均匀化热处理对Al-Mg-Sc铝合金铸锭微观组织和性能的影响

采用DSC差热试验方法和电镜能谱仪等检测手段,对不同温度均匀化退火后合金铸态组织和硬度进行观察和分析。结果表明:Al-Mg-Sc合金铸锭中的初生相主要为分布在晶粒内部的Al3(Sc,Zr,Ti)相与分布在晶界处的Al3Mg2相。采用较高温度470℃均匀化退火时,合金的加热应保持较低的加热速率,或者采用二级以上均匀化热处理工艺。在470℃均匀化退火时,在加热2h内硬度下降明显,保温时间延长,硬度下降变得缓慢。     

Microstructure and Strengthening Mechanisms in an Ultrafine Grained Al-Mg-Sc Alloy Produced by Powder Metallurgy

Additions of Sc to an Al-Mg matrix were investigated, paying particular attention to the influence of Al₃Sc precipitates and other dispersoids, as well as grain size, on mechanical behavior. Prior studies have shown that Sc significantly increases the strength of coarse-grained Al-Mg alloys. Prompted by these findings, we hypothesized that it would be of fundamental and technological interest to study the behavior of Sc additions to an ultrafine-grained (UFG) microstructure (e.g., 100’s nm). Accordingly, we investigated the microstructural evolution and mechanical behavior of a cryomilled ultrafine grained Al-5Mg-0.4Sc (wt pct) and compared the results to those of an equivalent fine-grained material (FG) produced by powder metallurgy. Experimental materials were consolidated by hot isostatic pressing (HIP’ing) followed by extrusion or dual mode dynamic forging. Under identical processing conditions, UFG materials generate large Al₃Sc precipitates with an average diameter of 154 nm and spaced approximately 1 to 3 μm apart, while precipitates in the FG materials have a diameter of 24 nm and are spaced 50 to 200 nm apart. The strengthening mechanisms are calculated for all materials and it is determined that the greatest strengthening contributions for the UFG and FG materials are Mg-O/N dispersion strengthening and precipitate strengthening, respectively.     

GH150合金叶片的疲劳性能

研究了高压压气机叶片用Ni基高温合金GH150的疲劳性能.试验方式分别为轴向拉伸和旋转弯曲疲劳.试验温度为500℃和600℃,试验按置信度90%和存活率50%进行.轴向拉伸和旋转弯曲疲劳性能试验Kt=1,应力状态分别为r=0.1和r=～1.用升降法求得500℃轴向拉伸疲劳107疲劳极限为481.8MPa,600℃疲劳极限为502MPa;500℃旋转弯曲疲劳107疲劳极限为418.57MPa,600℃为435.71MPa.轴向拉伸和旋转弯曲疲劳S-N曲线显示,随载荷应力的增加,疲劳循环次数呈减小趋势;在低应力长寿命区600℃的疲劳性能较500℃稍优.疲劳试样断口形貌照片显示,断口的源区、扩展区和最后瞬断区特征清晰;扩展区有明显的疲劳条带;瞬断区为韧性断裂.     

Process Optimization for Friction-Stir-Welded Martensitic Steel

Advanced high-strength M190 steel sheets were joined by friction-stir welding under different tool rotational and traversing speeds. The optical microstructure of the joints exhibited complete martensite and partial martensite at the weld nugget depending on the cooling rate during welding. The first heat-affected zone outside of the weld nugget revealed ferrite-pearlite phase aggregate, and the second heat-affected zone showed a tempered martensitic structure. The interplay of process variables in terms of peak temperature and cooling rate was studied to observe their effect on joint efficiency under shear testing. The peak hardness at weld nugget was close to the parent alloy at an intermediate cooling rate of 294 to 313 K/s. The lowest hardness was observed at the first heat-affected zone for all welded joints. Joint efficiency was dependent on relative quantity of ferrite-pearlite at first heat-affected zone. In that respect, the intermediate temperature to the tune of ~1193 K to 1273 K (~920 °C to 1000 °C) at the weld nugget was found to be beneficial for obtaining an adequate quantity of pearlite at the first heat-affected zone to provide joint efficiency of more than 50 pct of that of parent alloy.