7050-T7451铝合金搅拌摩擦焊组织及性能分析

在不同的焊接工艺参数下对8mm厚的7050-T7451锚合金板进行 了搅拌摩擦焊接实验通过对焊接接头组织的分析,发现焊核区晶粒为明显的细化再结晶等轴晶;热机影响区晶粒沿流线方向拉长且有细小沉淀相在晶界上析出;热影响区的组织发生了晶粒粗化.沿焊缝横截面的显微硬度分布呈高-低-高-低-高的趋势,硬度最低处位于后退侧的热影响区.实验结果表明,在旋转速度为375r/min、焊接速度为100mm/min时,可以获得较好的焊缝组织织,抗拉强度达到了 452MPa.     

双级时效对7050-T7451铝合金搅拌摩擦焊组织及应力腐蚀敏感性的影响

采用搅拌摩擦焊方法对3 mm厚7050-T7451铝合金进行焊接,为改善接头应力腐蚀敏感性,焊后进行121 ℃ × 5 h + 163 ℃ × 27 h双级时效处理. 通过对微观组织、显微硬度以及应力腐蚀敏感性的分析,研究双级时效对焊接接头性能的影响. 结果表明,双级时效后晶粒发生粗化,晶界内析出相和周边无沉淀析出带(PFZ)变宽,导致在热影响区和热力影响区出现大量不连续晶界;接头热影响区的显微硬度有所下降,但范围明显变窄,接头组织的均一性得到改善;时效处理后的接头在进行应力腐蚀试验 60天后仍未发生断裂,而未经时效处理的接头在1天内全部发生断裂,说明双级时效有效降低了焊接接头的应力腐蚀敏感性.     

7050-T7451铝合金静轴肩搅拌摩擦焊接头组织与性能研究

采用SSFSW技术成功实现了7050-T7451高强铝合金的焊接。对该接头的外观形貌、显微组织、硬度分布及力学性能分别进行了试验研究。结果表明,与常规FSW相比,SSFSW的接头成形美观,表面光滑,焊缝无减薄现象;焊缝组织结构也有明显的不同,热影响区范围明显窄小,前进侧TMAZ只有60 μm;接头硬度呈典型的“W”形分布,最低硬度出现在靠近焊核的热影响区附近,显微硬度为128 HV;接头的抗拉强度为487 MPa,达到了母材的91%,力学性能良好。断裂发生在热影响区,为微孔聚集型断裂。     

7050-T7451铝合金搅拌摩擦焊接头低周疲劳性能研究

对航空用5mm厚铝合金7050-T7451搅拌摩擦焊接头的低周疲劳性能进行了研究。在合适的焊接工艺参数及良好的焊接接头的基础上,进行了焊接接头的低周疲劳实验,得到了低周疲劳寿命表达式和应变-寿命曲线。用扫描电镜观察了疲劳断口的微观形貌并分析了疲劳断裂机理。结果表明：裂纹在接头底部启裂,沿前进侧热机影响区与焊核区的交界处扩展至断裂;试样在高应变下循环硬化和软化现象不明显;较低应变时有循环软化倾向。     

7050-T7451铝合金的搅拌摩擦焊接试验分析

在不同焊接参数下进行了7050-T7451铝合金的搅拌摩擦焊接试验,对接头显微组织进行了光学和TEM分析,并测试了接头的抗拉强度和硬度分布.焊接工艺参数通过影响接头微观组织和焊接缺陷来影响接头的力学性能,在转速800r/min和焊速200mm/min的情况下,接头的抗拉强度最高达到母材强度的88%.焊接热输入较高时,接头的拉伸断裂出现在热影响区,而热输入较低时,焊缝底部出现未焊合,接头从此处首先发生开裂.结果表明,焊核区发生了动态再结晶和沉淀相溶解;热影响区发生了沉淀相粗化,晶间出现无沉淀带.     

铝合金的搅拌摩擦焊

在详细介绍搅拌摩擦焊原理，特点的基础上，针对铝合金的搅拌摩擦焊特点，性能以及工业应用进行了阐述，并且对搅拌摩擦焊在中国市场的发展和应用作了简略介绍和预测。     

7050-T7451铝合金搅拌摩擦焊接头组织和性能与疲劳断裂

文中主要对7050-T7451铝合金搅拌摩擦焊焊接接头组织性能和疲劳断裂进行研究,对焊件进行金相组织、硬度、疲劳试验,并根据试验结果进行分析。试验结果表明,焊核区和热力影响区组织有明显的分界,整个接头表面和截面的不同区域的组织有明显的不同,焊核区经历高温热循环,并且受到强烈的搅拌作用,发生了显著的动态再结晶,组织均匀细小,没有明显的方向性。热力影响区组织在搅拌摩擦焊接过程中主要受到热循环作用的影响,同时一定程度上受到了机械外力的作用,产生了较小的塑性变形,但储能不够,最终未出现动态再结晶。热影响受到热循环的作用,组织比母材要粗大,但没有发生明显塑性变形,仍具有母材纤维组织的特点。硬度试验的结果表明,母材区和焊核区的硬度比较大,而热影响区的硬度偏低,热力影响区的硬度在它们之间。通过对疲劳试验分析,应力水平为200 MPa时,试样在前进侧热影响区发生断裂,断口的表面较为粗糙。应力水平为250 MPa时,试样在焊核区发生断裂,断口表面较为平整。应力水平为300 MPa时,试样在母材处发生断裂,断口表面十分平整。创新点： 通过搅拌摩擦焊工艺形成的7050-T7451铝合金焊接接头,成形良好,技术优势突出,适用于航天航空等多个重要领域。     

搅拌摩擦焊焊缝缺陷的超声相控阵检测技术

采用超声相控阵技术对不同厚度铝合金搅拌摩擦焊焊缝的缺陷进行了探测,通过改变相控阵的扫描角度、聚焦深度、焦点尺寸等参数对焊缝内部缺陷进行了准确定量和图像再现,并对缺陷信号标记部位进行了解剖、观察了缺陷的宏观形貌.结果表明,探头检测灵敏度高,易于快速分辨焊缝内部缺陷,结合不同的扫查方式直观给出缺陷三向视图,可准确定位缺陷的位置、尺寸、分布;缺陷形状直接影响回波的形状和峰值,小孔洞为单一尖锐波峰,峰值高大;包铝层和疏松由多簇波峰构成,S显示疏松亮点比包铝层密集且分布面较大.     

搅拌摩擦焊焊缝缺陷的超声相控阵检测技术

采用超声相控阵技术对不同厚度铝合金搅拌摩擦焊焊缝的缺陷进行了探测,通过改变相控阵的扫描角度、聚焦深度、焦点尺寸等参数对焊缝内部缺陷进行了准确定量和图像再现,并对缺陷信号标记部位进行了解剖、观察了缺陷的宏观形貌.结果表明,探头检测灵敏度高,易于快速分辨焊缝内部缺陷,结合不同的扫查方式直观给出缺陷三向视图,可准确定位缺陷的位置、尺寸、分布;缺陷形状直接影响回波的形状和峰值,小孔洞为单一尖锐波峰,峰值高大;包铝层和疏松由多簇波峰构成,S显示疏松亮点比包铝层密集且分布面较大.     

7050铝合金搅拌摩擦焊动态再结晶组织影响因素

进行了不同旋转频率和焊接速度下的7050铝合金搅拌摩擦焊试验,研究了搅拌头旋转频率和焊接速度对焊核区晶粒尺寸的影响．为进一步分析焊接参数影响焊核区晶粒尺寸的机理,进行了不同应变速率和变形温度下的等温压缩试验．分析了变形参数对动态再结晶的影响规律．结果表明,焊核区晶粒尺寸随搅拌头旋转频率的变化不大,随焊接速度的增加而减小．在发生完全动态再结晶的范围之内,再结晶晶粒尺寸随着lnZ值的增大而减小．焊接参数对z参数具有不同的影响规律,进而影响焊核区晶粒尺寸．     

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 °C× 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 microhardnes...     

Fatigue lives of friction stir spot welds in aluminum 6061-T6 sheets

The fatigue lives of friction stir spot welds in aluminum 6061-T6 lap-shear specimens under cyclic loading conditions are investigated in this paper. The paths of fatigue cracks near friction stir spot welds in lap-shear specimens are first examined. The experimental observations suggest that under cyclic loading conditions, the fatigue crack is initiated near the possible original notch tip in the stir zone and propagates along the circumference of the nugget, then through the sheet thickness and finally grows in the width direction to cause final fracture. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of friction stir spot welds. The global and local stress intensity factors are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.     

Torque,temperature and hardening precipitation evolution in dissimilar friction stir welds between 6061-T6 and 2014-T6 aluminum alloys

Dissimilar friction stir butt welds between the 2014-T6 and the 6061-T6 Al alloys were performed with various sets of welding parameters including a lateral shift of the tool from the initial separation between the plates to be welded and by placing the alloys, either on the advancing, or on the retreating side of the weld. Torque and temperature measurements during welding as well as macrographies and hardness profiles measurements were performed after welding. It was found that the welding torque, the temperature, the metal flow and the welds’ hardness profile depend on the proportion of each alloy included in the stirred zone. Those results are attributed to the difference between the softening temperatures of both alloys. The 6061 alloy's HAZ is the weak link in all dissimilar welds. The evolution of the hardening precipitation, the hardness and the local yield strength in the 6061 alloy are therefore modeled. The model reasonably well predicts the experimentally measured hardness of both similar and dissimilar welds. It also explains the influence of the alloys placement or tool lateral shift on the welds hardness by their influence on the precipitate radius and volume fraction.     

Microstructural investigation of friction stir welded 7050-T651 aluminium

The grain structure, dislocation density and second phase particles in various regions including the dynamically recrystallized zone (DXZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) of a friction stir weld aluminum alloy 7050-T651 were investigated and compared with the unaffected base metal. The various regions were studied in detail to better understand the microstructural evolution during friction stir welding (FSW). The microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Using the combination of structural characteristics observed in each weld region, a new dynamic recrystallization model has been proposed. The precipitation phenomena in different weld regions are also discussed.     

Themo-mechanical and microstructural modeling of friction stir welding of 6111-T4 aluminum alloys

Plastic deformation and thermal history as well as microstructure evolution of friction stir welded 6111-T4 aluminum alloys were numerically simulated. Material and heat flow during friction stir welding were calculated considering the momentum balance equation and energy balance equation under the steady state condition. Based on the calculated temperature history, the coupled nucleation, growth, and coarsening of precipitates were simulated using microstructural modeling, as proposed by Myhr et al. [7,8]. Finally, the distribution of precipitates was used to calculate the mechanical properties of the weld zone, particularly the yield stress, based on the dislocation theory. The results compared well with the measurements, suggesting that the method can be applicable to predict yield stress.     

Coating residual stress effects on fatigue performance of 7050-T7451 aluminum alloy

The tendency of the aircraft industry is to enhance customer value by improving performance and reducing environmental impact. In view of availability, aluminum alloys have a historically tendency to faster insertion due to their lower manufacturing and operated production infrastructure. In landing gear components, wear and corrosion control of many components is accomplished by surface treatments of chrome electroplating on steel or anodizing of aluminum. One of the most interesting environmentally safer and cleaner alternatives for the replacement of hard chrome plating or anodizing is tungsten carbide thermal spray coating, applied by the high velocity oxy fuel (HVOF) process. However, it was observed that residual stresses originating from these coatings reduce the fatigue strength of a component.An effective process as shot peening treatment, considered to improve the fatigue strength, pushes the crack sources beneath the surface in most of medium and high cycle cases, due to the compressive residual stress field induced. The objective of this research is to evaluate a tungsten carbide cobalt (WC-Co) coating applied by the high velocity oxy fuel (HVOF) process, used to replace anodizing. Anodic films were grown on 7050-T7451 aluminum alloy by sulfuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on axial fatigue strength of anodic films grown on the aluminum alloy surface is to degrade the stress-life performance of the base material. Three groups of specimens were prepared and tested in axial fatigue to obtain S-N curves: base material, base material coated by HVOF and base material shot peened and coated.Experimental results revealed increase in the fatigue strength of Al 7050-T7451 alloy associated with the WC 17% Co coating. On the other hand, a reduction in fatigue life occurred in the shot peened and coated condition. Scanning electron microscopy technique and optical microscopy were used to observe crack origin sites, thickness and coating/substrate adhesion.     

A look in the corrosion of aluminum alloy friction stir welds

Aluminum alloy friction stir welds exhibit corrosion susceptibility, where sensitization of the microstructure occurs. The response of the microstructure to the welding is intense, and intergranular corrosion, mainly located along the nugget’s heat-affected region, is promoted by coarsening of the grain boundary precipitates. Short-term post-weld heat treatments, with temperatures similar to the welding temperatures, modify the microstructure and reduce the corrosion. An increase in the corrosion resistance may also be reached by modifying the microchemistry during welding.     

Microstructural evolution ahead of the tool in aluminum friction stir welds

The microstructural evolution around the tool during friction stir-welding of aluminum is described. The rotating tool induces a gradual rotation of the crystal lattice with a concomitant development of elongated subgrains. Additional small subgrains form within this structure by continuous dynamic recrystallization, gradually increasing in misorientation, eventually developing high-angle (>15°) grain boundaries. The development of texture and similarities to friction stir welds in titanium alloys are also discussed.     

Rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys

The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum alloys, which are influenced by the welding parameters. When the joints are free of welding defects, the AA2017-T351 joints are ruptured in the weld nugget adjacent to the thermo-mechanically affected zone on the advancing side and the rupture surfaces appear as oval contours of the weld nugget, while the AA6061-T6 joints are ruptured in the heat affected zone on the retreating side and the rupture surfaces are inclined at a certain degree to the bottom surfaces of the joints. When welding defects are present in the joints, the AA2017-T351 joints are ruptured in the weld center, while the AA6061-T6 joints are ruptured on the retreating side near the weld center. The rupture locations of the joints are dependent on the internal structures of the joints and can be explained through them.     

Material flow and microstructure in the friction stir butt welds of the same and dissimilar aluminum alloys

The material flow and microstructural evolution in the friction stir welds of a 6061-Al alloy to itself and of a 6061-Al alloy to 2024-Al alloy plates of 12.7 mm in thickness were studied under different welding conditions. The results showed that plastic deformation, flow, and mechanical mixing of the material exhibit distinct asymmetry characteristics at both sides of the same and dissimilar welds. The microstructure in dissimilar 6061-Al/2024-Al welds is significantly different from that in the welds of a 6061-Al alloy to itself. Vortex-like structures featured by the concentric flow lines for a weld of 6061-Al alloy to itself, and alternative lamellae with different alloy constituents for a weld of 6061-Al to 2024-Al alloy, are attributed to the stirring action of the threaded tool, in situ extrusion, and traverse motion along the welding direction. The mutual mixing in the dissimilar metal welds is intimate and far from complete. However, the bonding between the two Al-alloys is clearly complete. Three different regions in the nugget zone of dissimilar 6061-Al/2024-Al welds are classified by the mechanically mixed region (MMR) characterized by the relatively dispersed particles of different alloy constituents, the stirring-induced plastic flow region (SPFR) consisting of alternative vortex-like lamellae of the two Al-alloys, and the unmixed region (UMR) consisting of fine equiaxed grains of the 6061-Al alloy. Within all of these three regions, the material is able to withstand a very high degree of plastic deformation due to the presence of dynamic recovery or recrystallization of the microstructure. The degree of material mixing, the thickness of the deformed Al-alloy lamellae, and the material flow patterns depend on the related positions in the nugget zone and the processing parameters. Distinct fluctuations of hardness are found to correspond to the microstructural changes throughout the nugget zone of dissimilar welds.