Comparison of 2A12 aluminium alloy joint by ultrasonic assisted friction stir welding and friction stir welding

Ultrasonic assisted friction stir welding （UAFSW） is a recent modification of conventional friction stir welding, which adds ultrasonic energy directly into the friction stir welding area by the pin. In this study, 2A12 aluminum alloy was welded by this process and conventional, respectively. The tensile tests, microstructure and fracture surface of FSW joint and UAFSW joint were analyzed. The research results show that the surface forming texture of ultrasonic assisted friction stir welding joint, compared with conventional, is finer and more uniform, showing metallic matte color. The grains are much finer in weld nugget zone, thermo-mechanically affected zone and heat-affected zone; S-phase particles size is much smaller and distribution is more homogeneous in the matrix. The tensile strength of UAFSW joint is 94. 13% of base metal, and the elongation is 11.77%. The tensile strength of FSW joint is 83.15% of base metal, and the elongation is 8.81%. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints.     

Exploring material flow in friction stir welding using stacked structure of 2024 and 6061 aluminum alloys

An experimental technique based on stacked structures was developed to observe the material flow behavior of the friction stir welding （FSW） process. Analysis of section views along different directions revealed important new details of the material flow in FSW process. In this work, a general flow model of FSW was constructed based on the analysis of different static section views of stacked structure weld. The formation of onion rings was found to be a geometric effect due to layered deposition attd the extrusion occurred at the interface between flow arm （FA） and stirring zone （SZ）.     

Mechanical properties of 2024-T4 aluminium alloy joints in ultrasonic vibration enhanced friction stir welding

Ultrasonic vibration enhanced friction stir welding （UVeFSW） is a recent modification of conventional friction stir welding （FSW）, which transmits ultrasonic vibration directly into the localized area of the workpiece near and ahead of the rotating tool. In this study, a high strength aluminium alloy （2024-T4） was welded by this process and conventional FSW, respectively. Then tensile tests, microhardness tests and fracture surface analysis were performed successively on the welding samples. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints. The microhardness of the stir zone also increases.     

Numerical analysis of reverse dual-rotation friction stir welding process

A three-dimensional model of reverse dual-rotation friction stir welding （RDR-FSW） is developed to conduct the numerical simulation of heat generation and material flow during the process. The reverse rotation of the assisted shoulder and the tool pin is considered to model the heat generation rate. The predicted temperature difference between the advancing side and the retreating side in RDR-FSW is less than that in conventional FSW. There are two reverse flows during the RDR-FSW which is beneficial to the uniformity of the temperature profile. Due to the reverse rotation effects of the assisted shoulder, the predicted shape and size of thermal-mechanically affected zone （TMAZ） based on the iso-viscosity line are decreased greatly compared to the conventional FSW. It lays solid foundation for optimizing the process parameters in RDR-FSW.     

Effect of rotary direction and speed on mechanical properties in friction stir spot welding of A6061 sheets e

Friction stir spot welding of A6061 sheets was conducted using a tool with thread pin. The hook geometries, hook formation and mechanical properties of the joints welded with different rotary directions and speeds were investigated. The results show that the hook in the joint welded in clockwise was curved upwards and that in anticlockwise was curved downwards. The hook formation was related to the plastic material flow in the joint. With increasing the rotary speed in clockwise direction, the hook moved upwards and far way from the center of the keyhole, resulting in an increase in the effective weld width and a decrease in the effective sheet thickness. Three types of fractuces were observed and they were affected by the hook geometries. The tensile shear load increased firstly and then decreased when the rotary speed increased in clockwise direction, which was related to the hook geometries.     

搅拌摩擦焊温度场最新研究进展

早期的搅拌摩擦焊温度场模型只考虑了摩擦产热,而忽略了塑性变形产热。在刚结束的第五届搅拌摩擦焊国际会议上,有许多学者就塑性变形产热对其温度场的影响进行了研究,本文对此作了概括地介绍分析。     

搅拌摩擦焊温度场最新研究进展

早期的搅拌摩擦焊温度场模型只考虑了摩擦产热，而忽略了塑性变形产热。在刚结束的第五届搅拌摩擦焊国际会议上，有许多学者就塑性变形产热对其温度场的影响进行了研究，本文对此作了概括地介绍分析。     

Temperature measurement problems in welding processes

This study examined development of the stir zone and plastic flow of the material during friction stir spot joining of aluminium alloys. The development of the stir zone was discussed through the observation of macro sections of the joints produced at various process times. The friction stir spot joining trial with insert of Au foil to lap surface and the dissimilar lap friction stir spot joining showed that the movement of the lap surface was attributed to the downward plastic flow from the upper plate to the lower plate around the probe. Additionally, it was clarified that the direction of the plastic flow was roughly the same as the direction of the tool rotation. The dissimilar butt friction stir spot joining suggested that the plastic flow occurred not only within the stir zone but also in a larger area below the tool shoulder. The present study revealed that the threads on the probe surface were important for producing the plastic flow of thickness direction during friction stir spot joining.     

Friction Stir Welding of Discontinuously Reinforced Aluminum Matrix Composites:A Review

Friction stir welding(FSW) is considered a promising welding technique for joining the aluminum matrix composites(AMCs) to avoid the drawbacks of the fusion welding. High joint efficiencies of 60%–100% could be obtained in the FSW joints of AMCs. However, due to the existence of hard reinforcing particles in the AMCs, the wearing of welding tool during FSW is an unavoidable problem. Moreover, the low ductility of the AMCs limits the welding process window. As the hard materials such as Ferro-Titanit alloy, cermet, and WC/Co were applied to produce the welding tools,the wearing of the tools was significantly reduced and the sound joints could be achieved at high welding speed for the AMCs with low reinforcement volume fraction. In this article, current state of understanding and development of welding tool wearing and FSW parameters of AMCs are viewed. Furthermore, the factors affecting the microstructure and mechanical properties of the joints are evaluated in detail.     

Heterogeneous structure-induced strength and ductility synergy of α-brass subjected to rapid cooling friction stir welding

The 2 mm-thick α-brass plates were successfully joined using conventional friction stir welding (CFSW) with air cooling and rapid cooling friction stir welding (RCFSW) with liquid CO₂ cooling. The microstructure and mechanical properties of the two welds were carefully investigated by electron back-scattered diffraction and transmission electron microscopy. The stir zone of CFSW exhibited homogeneous equiaxed grains, while the stir zone of RCFSW showed a heterogeneous grain structure, i.e. ultrafine grains containing massive dislocations and nano twins. Compared with the CFSW, yield strength and ultimate tensile strength of RCFSW were increased by 31% and 24%, respectively. The enhanced yield strength and improved strain hardening capacity were attributed to grain boundary strengthening and dislocation strengthening. Furthermore, good ductility was achieved due to the released stress concentration of the nano twins caused by the plastic deformation.     

搅拌摩擦加工研究进展

搅拌摩擦加工(FSP),是一种新型的材料塑性变形加工方法,它是在搅拌摩擦焊(FSW)的基础上提出的。从发明至今,研究者已经成功将FSP用于铸造金属微观组织细化、超塑性材料的制备、材料表面改性以及各种复合材料的制备中。搅拌摩擦加工工艺与搅拌摩擦焊接工艺基本相同,工艺参数对搅拌摩擦加工材料质量有很大的影响。综述了搅拌摩擦加工近年来的研究进展,主要包括不添加增强相的FSP和添加增强相的FSP两大类。其中不添加增强相的FSP主要有铸造金属微观组织细化和超塑性材料制备,添加增强相的FSP主要有材料表面改性和复合材料制备。搅拌摩擦加工制备复合材料根据添加相是否与基体反应生成增强相,又分为非原位合成法制备复合材料与原位合成法制备复合材料。文中对以上内容分别进行了总结与评述,最后指出了FSP今后发展应用的方向。     

Structural evolution and superplastic formability of friction stir welded AA 2095 sheets

Friction stir welding was used to join superplastic AA 2095 sheets. The effect of welding rate on the grain size distribution and grain boundary misorientations in the stir zone was investigated. The superplastic behavior of the weld nugget parallel to the welding direction was also characterized at 495 °C and strain rates from 10⁻⁴s⁻¹ to 10⁻²s⁻¹. Increasing the welding rate during friction stir welding augmented the formation of a fine-equiaxed high-angle grain boundary structure within the stir zone. Increasing intensity of plastic straining during friction stir welding resulted in enhanced properties during subsequent superplastic formation. The maximum strain-to-failure was obtained for the weld made at a tool speed of 1000 rpm and a weld rate of 4.2 mm/s when tested at a superplastic forming strain rate of 10⁻³s⁻¹.     

An integrated model for analysing the effects of ultrasonic vibration on tool torque and thermal processes in friction stir welding

An integrated model of ultrasonic vibration enhanced friction stir welding (UVeFSW) is developed by integrating the thermal-fluid model with the ultrasonic field model and tool torque model. The tool torque and the heat generation rate at tool/workpiece contact interfaces are coupled with the interfacial temperature, strain rate and ultrasonic energy density. The model is used in quantitatively analysing the effects of ultrasonic vibration on tool torque and thermal processes in friction stir welding (FSW). The results show that ultrasonic vibration reduces the flow stress, which results in a decreasing of tool torque, interfacial heat generation rate and interfacial temperature. The complicated interaction of ultrasonic energy with the thermal processes in FSW leads to a gentle thermal gradient and an enhanced plastic material flow in UVeFSW. The model is validated by a comparison of the calculated thermal cycles and tool torque at various welding parameters with the experimentally measured ones.     

微观组织对镁合金FSW焊缝应变硬化行为的影响

采用传统搅拌摩擦焊和冷源辅助搅拌摩擦焊对3 mm厚的AZ31B镁合金进行焊接. 利用电子背散射衍射、透射电子显微镜和静拉伸试验研究焊缝区的微观组织对力学性能的影响. 结果表明,液态二氧化碳不仅降低焊接峰值温度,还提高焊后冷却速度. 焊缝峰值温度的降低为激活{10-12}孪生行为创造了有利条件. {10-12}孪晶可降低基面织构的强度,也可进一步分割晶粒,起到细化晶粒的作用. 焊后冷却速度的提高使焊接过程中产生的大量位错保留在晶粒内部. 因此冷源辅助搅拌摩擦焊缝表现为具有大量{10-12}孪晶和位错的细晶结构. 在拉伸过程中,细晶强化和位错强化为主要强化机制. 孪晶界面可有效吸收和分解变形时产生的位错,从而协调应变和减小应力集中,使焊缝具有合理的应变硬化行为和强塑性匹配.     

铝合金搅拌摩擦焊的研究现状与展望

在介绍搅拌摩擦焊技术的基本原理及影响因素的基础上,综述了铝合金搅拌摩擦焊在接头金属塑性流动、显微组织以及性能等方面的国内外研究现状.研究现状表明,搅拌头形状和工艺参数对接头组织和性能具有重要影响,在合适的工艺参数下可获得综合性能良好的铝合金搅拌摩擦焊接头,并取得了一定规模的应用.此外,指出了搅拌摩擦焊技术的不足和接头性...     

基于摩擦加工技术的镁合金表面改性研究现状

鉴于当前高性能镁合金的应用需求,亟待提高镁合金的表面硬度、摩擦磨损性以及耐蚀性等表面性能。新型固态加工技术——搅拌摩擦加工以及摩擦堆焊能够实现材料的大塑性变形,在镁合金表面微观组织结构改性、表面复合材料化以及金属焊敷层制备等方面得到了成功的应用。在介绍搅拌摩擦加工以及摩擦堆焊技术特性的基础上,分别从工艺手段、组织演变以及性能改善等方面综述了摩擦加工技术用于镁合金表面改性的研究现状。国内外研究结果显示,搅拌摩擦加工可有效细化镁合金表面晶粒,破碎粗大第二相,导入增强粒子,实现表面复合化,进而显著提高镁合金的硬度、耐磨性以及耐蚀性。摩擦堆焊技术可在镁合金表面成功制备无稀释、结合完整性高、均匀细化的金属焊敷层,有效改善镁合金表面硬度及耐磨性。通过对用于镁合金表面改性的摩擦加工技术研究现状的总结,展望了镁合金搅拌摩擦加工以及摩擦堆焊的发展前景,提出了需要进一步研究的方向。     

Effect of temperature field on flashes formation of continuous drive friction welding

On basis of the finite element software DEFORM, the 2D coupled thermo-mechanical model of continuous drive friction welding of ring parts is established. The temperature and the flashes during the welding process are investigated. The results of numerical simulation show that the temperature of friction surface is higher than that of other region and the peak temperature increases with the increase of welding time. During the process of friction stage, no flash appears because of the low temperature and the small axial friction pressure. At the forging stage, the flashes appear, whose dimensions and bending degree increase with the increase of welding temperature. Moreover, with the increase of rotational velocity and axial forging pressure, the dimensions and the bending degree of flashes of continuous drive friction welding increase.     

Effect of Zener-Hollomon Parameter on Microstructure and Mechanical Properties of Copper Subjected to Friction Stir Welding

In this work, the influence of the Zener-Hollomon (Z) parameter on the microstructure and mechanical properties of copper subjected to friction stir welding (FSW) was investigated. Liquid N₂ cooling was conducted to control the cooling rate after the FSW. The obtained results demonstrate that the Z parameter was dependent on the tool rotation rate during the FSW, i.e., a higher tool rotating rate resulted in a lower Z parameter. The grain size in the stir zone decreased with the increase in the Z parameter. The relationship between the yield strength and the Z parameter is established as σ_0.2 = σ₀ + kZⁿ. This relationship exhibited two different plots under the conditions of air cooling and liquid N₂ cooling. Even at a similar Z parameter, a significant yield strength difference occurred because massive dislocations, which were caused by the prevention of the post-annealing effect, were maintained in the stir zone. This study suggests that the influence of the post-annealing effect should not be neglected when analyzing the relationship between the Z parameter, microstructure, and mechanical properties.     

Finite element analysis of the effect of micro-pore defect on linear friction welding of medium carbon steel

Micro-pore is a very common material defect. In the present paper, the temperature fields of medium carbon steel joints with and without micro-pore defect during linear friction welding （LFW） were investigated by using finite element method. The effect of micro-pore defect on the axial shortening of joints during LFW was examined. The x- and y-direction displacements of micro-pore during the LFW process were also studied. In addition, the shape of micro-pore after LFW was observed. The heat conducted from the weld inteace to the specimen interior. The fluctuation range of the temperature curves for the joint with micro-pore is larger than that without micro-pore. Position of micro-pore changes with the change of the friction time. The circular shape of micro-pore becomes oval after welding.     

焊后塑性变形对搅拌摩擦焊AZ31镁合金耐腐蚀的影响

设计了AZ31镁合金的母材(BM)、搅拌摩擦焊接头(FSW)、焊后横向拉伸5%(FSW+T)和焊后拉伸加退火(FSW+T+A)等4组试样。通过对比4组试样在质量分数为3.5%的NaCl溶液中的耐腐蚀性能,探索焊后塑性变形对接头耐腐蚀性能的影响。结果发现,AZ31镁合金的耐腐蚀性能在搅拌摩擦焊接后显著改善,但焊后横向拉伸将显著降低接头的耐腐蚀性能,而拉伸后退火处理使接头耐腐蚀性能增强。4组试样的腐蚀速率由大到小为:BM>FSW+T>FSW≈FSW+T+A。焊后横向拉伸在AZ31镁合金焊接接头中引入拉伸孪晶及位错滑移,两者对接头的耐腐蚀性能产生不利影响,但位错滑移所起的作用更显著。