Internal defect and process parameter analysis during friction stir welding of Al 6061 sheets

Welding parameters like welding speed, rotation speed, plunge depth, shoulder diameter etc., influence the weld zone properties, microstructure of friction stir welds, and forming behavior of welded sheets in a synergistic fashion. The main aims of the present work are to (1) analyze the effect of welding speed, rotation speed, plunge depth, and shoulder diameter on the formation of internal defects during friction stir welding (FSW), (2) study the effect on axial force and torque during welding, (c) optimize the welding parameters for producing internal defect-free welds, and (d) propose and validate a simple criterion to identify defect-free weld formation. The base material used for FSW throughout the work is Al 6061T6 having a thickness value of 2.1 mm. Only butt welding of sheets is aimed in the present work. It is observed from the present analysis that higher welding speed, higher rotation speed, and higher plunge depth are preferred for producing a weld without internal defects. All the shoulder diameters used for FSW in the present work produced defect-free welds. The axial force and torque are not constant and a large variation is seen with respect to FSW parameters that produced defective welds. In the case of defect-free weld formation, the axial force and torque are relatively constant. A simple criterion, (?τ/?p)_defective?>?(?τ/?p)_{defect free} and (?F/?p)_defective?>?(?F/?p)_{defect free}, is proposed with this observation for identifying the onset of defect-free weld formation. Here F is axial force, τ is torque, and p is welding speed or tool rotation speed or plunge depth. The same criterion is validated with respect to Al 5xxx base material. Even in this case, the axial force and torque remained constant while producing defect-free welds.     

搅拌摩擦焊下压力控制系统的开发及在模拟非刚性环境下的验证试验

搅拌摩擦焊接(Friction stir welding, FSW)是材料固态连接新技术,但FSW在焊接过程中一般会对工件施加较大的下压力,焊接设备和被焊工件在下压力的作用下均可能产生变形,使得常规FSW中设定的下压量这一关键参数偏离预期值,无法保证焊接工艺的稳定性。为了解决这一问题,开发一套下压力反馈控制系统,通过调节搅拌头对工件的下压量来调节下压力,使焊接过程中下压力保持稳定。该系统使用一台计算机作为顶层控制器,根据压力传感器反馈的实时下压力调节FSW设备Z轴的进给。使用该系统在悬空的钢板上焊接6082-T6铝合金平板对接焊缝,焊接过程中工件在下压力的作用下产生的弯曲变形高达0.931 mm,但所得的焊缝成形良好,沿焊缝方向不同位置的接头的横截面形貌基本一致,其横向拉伸应力应变曲线高度重合,接头的平均抗拉强度为222.8 MPa。结果表明,工件在下压力作用下产生变形的条件下,下压力控制的FSW系统仍能保证工艺稳定性。     

Material removal model of ultrasonic elliptical vibration-assisted chemical mechanical polishing for hard and brittle materials

Friction stir welding (FSW) has emerged as an attractive process for fabricating aerospace vehicles. Current FSW state-of-the-art uses large machines that are not portable. However, there is a growing need for fabrication and repair operations associated with in-space manufacturing. This need stems from a desire for prolonged missions and travel beyond low-earth orbit. To address this need, research and development is presented regarding two enabling technologies. The first is a self-adjusting and aligning (SAA) FSW tool that drastically reduces the axial force that has historically been quite large. The SAA-FSW tool is a bobbin style tool that floats freely, without any external actuators, along its vertical axis to adjust and align with the workpiece’s position and orientation. Successful butt welding of 1/8 in. (3.175 mm) thick aluminum 1100 was achieved in conjunction with a drastic reduction and near elimination of the axial process force. Along with the SAA-FSW, an innovative in-process monitor technique is presented in which a magnetoelastic force rate-of-change sensor is employed. The sensor consists of a magnetized FSW tool that is used to induce a voltage in a coil surrounding the tool when changes to the process forces occur. The sensor was able to detect 1/16 in. (1.5875 mm) diameter voids. It is concluded that these technologies could be applied toward the development of a portable FSW machine for use in space.     

Study on governing parameters of thermal history during underwater friction stir welding

Underwater friction stir welding (FSW) could widely extend the submarine applications of solid-state welding methods. Since, in the case of underwater FSW, the temperature field exhibits profound effects on the acquired weld properties, studying the corresponding governing parameters is of high priority. With this end in view, in order to explicate the heat generated by the FSW tool, the applied forces on the FSW tool, as the unknown parameters in the heat generation equation, are obtained. Subsequently, the heat transfer of the surrounding fluid, which dictates the heat transfer through the workpiece is investigated. The results reveal that upon comparison to FSW in air medium, both translational and axial forces considerably increase leading to greater heat generated by the underwater FSW tool. However, the peak temperature in each point during underwater welding declines dramatically (40 %) compared to the in-air welding, which can be attributed to the extreme boiling heat transfer of water on both the workpiece and FSW tool. This behavior may be the main reason for the acquired mechanical properties of the underwater-welded AA7075-T6 plates as a precipitating hardening alloy. The mentioned heat transfer is non-uniform over the workpiece and comprises different types including nucleation and transition boiling as well as free convection. Furthermore, the study of the mechanical characteristics revealed that underwater welding leads to joints with more strength and lower ductility compared to those obtained by in-air welding.     

超声振动强化搅拌摩擦焊的热力行为及微观组织特征

为了利用超声振动降低搅拌摩擦焊过程中金属材料的屈服应力和流动应力,研发超声振动强化搅拌摩擦焊试验装置,开展6061-T6铝合金的焊接工艺试验。采用实时采集焊机电参数并将其转化成力矩和力的方法,测试超声振动作用下搅拌摩擦焊的焊接载荷,利用热电偶测试施加超声时的焊接热循环,通过体视显微镜和金相显微镜分别观测焊缝截面尺寸和接头微观组织,并与相同参数下常规搅拌摩擦焊的情况进行比对。研究结果表明,超声振动能够显著降低焊接轴向压力和搅拌头转矩,增大焊缝横截面尺寸,细化和均匀焊核区和热力影响区的晶粒组织。热循环的测量结果显示,超声振动的施加略微降低了测量点的焊接热循环峰值温度。分析认为,超声振动与搅拌头附近的塑性变形材料相互作用,降低了金属材料的屈服应力和流变应力,进而改变了原有的温度场,从而产生了优异的工艺效果。     

Friction stir welding characteristics of 2219-T6 aluminum alloy assisted by external non-rotational shoulder

Friction stir welding (FSW) of 2219-T6 aluminum alloy assisted by external non-rotational shoulder was carried out, and effects of the welding speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of welding speeds from 50 to 300 mm/min. The microstructural deformation and weld formation were dominated by the rotating tool pin and subsize concave shoulder but the non-rotational shoulder exerted very little effects for all joints. Compared with the weld obtained by conventional FSW, less intense stirring effects in FSW assisted by external non-rotational shoulder can only generate a narrower thermomechanically affected zone, whose width decreased with increasing of the welding speed. Microstructures and Vickers hardness distributions showed that this new welding process is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone. The maximum tensile strength was up to 69 % of the base material.     

Optimization of friction stir welding process of AA7075 aluminum alloy to achieve desirable mechanical properties using ANFIS models and simulated annealing algorithm

Current work deals with experimental investigation, modeling, and optimization of friction stir welding process (FSW) to reach desirable mechanical properties of aluminum 7075 plates. Main factors of process were tool pin profile, tool rotary speed, welding speed, and welding axial force. Also, main responses were tensile strength, yield strength, and hardness of welded zone. Four factors and five levels of central composite design have been utilized to minimize the number of experimental observations. Then, adaptive neuro-fuzzy inference systems (ANFIS) have been used to generate mapping relationship between process factors and main response using experimental observations. Afterward, the developed models were applied as objective function to select optimal parameters, in which the process reaches to its desirable mechanical properties by using the simulated annealing algorithm. Results indicated that the tool with square pin profile, rotary speed of 1,400 RPM, welding speed of 1.75 mm/s, and axial force of 7.5 KN resulted in desirable mechanical properties in both cases of single response and multi-response optimization. Also, these solutions have been verified by confirmation tests and FSW process physical behavior. These verifications indicated that both ANFIS model and simulated annealing algorithm are appropriate tools for modeling and optimization of process.     

A cutting force prediction dynamic model for side milling of ceramic matrix composites C/SiC based on rotary ultrasonic machining

The relevance, importance and presence of industrial robots in manufacturing have increased over the years, with applications in diverse new and nontraditional manufacturing processes. This paper presents the complete concept and design of a novel friction stir welding (FSW) robotic platform for welding polymeric materials. It was conceived to have a number of advantages over common FSW machines: it is more flexible, cheaper, easier and faster to setup and easier to programme. The platform is composed by three major groups of hardware: a robotic manipulator, a FSW tool and a system that links the manipulator wrist to the FSW tool (support of the FSW tool). This system is also responsible for supporting a force/torque (F/T) sensor and a servo motor that transmits motion to the tool. During the process, a hybrid force/motion control system adjusts the robot trajectories to keep a given contact force between the tool and the welding surface. The platform is tested and optimized in the process of welding acrylonitrile butadiene styrene (ABS) plates. Experimental tests proved the versatility and validity of the proposed solution.     

Design of tool system for the external nonrotational shoulder assisted friction stir welding and its experimental validations on 2219-T6 aluminum alloy

The nonrotational shoulder assisted friction stir welding (NRSA-FSW) is still in the feasibility study stage. To reveal details in the tool system designing and highlight advantages of this novel technology, the tool system for the NRSA-FSW was designed and utilized to weld high-strength aluminum alloy 2219-T6 for validations. Compared with the joints welded by the friction stir welding (FSW) without assistance of the nonrotational shoulder (NRS), the effect of the NRS on the weld formation and mechanical properties was illustrated. At a constant welding speed 100 mm/min, defect-free joints can only be obtained at the tool rotation speed 800 rpm by the FSW without assistance of the NRS, but the NRSA-FSW can produce defect-free joints in a wider range of tool rotation speeds 600–900 rpm. The NRS prevented all plasticized materials from escaping from the stirring zone, thus the weld nugget zone transformed from the basin-type formation to the spherical formation with increasing of the stirring effect when the tool rotation speed increased gradually. For joints welded by these two FSW processes, both the tensile strength and the elongation showed nearly the same trend with the tool rotation speed, but the NRSA-FSW can produce joints with the maximum tensile strength in a wider range. Compared with the maximum joint efficiency 71.2 % of the FSW without assistance of the NRS, the maximum tensile strength obtained by the NRSA-FSW also reached 69.0 % of the base material. All tensile specimens machined from defect-free joints fractured at the weakest region with minimum Vicker’s hardness; while for those joints with cavity defects, the fracture occurred at the defect location.     

铝合金液冷冷板窄台阶搭接搅拌摩擦焊工艺研究

文中研究了铝合金液冷冷板窄台阶搭接搅拌摩擦焊工艺。冷板基底材料为6063 铝合金,盖板材料为3A21 铝合金。设计了窄搭接搅拌头,减小了轴肩宽度和焊接压力,增加了接头焊接时材料的流动性,针对不同焊深窄台阶冷板进行了窄搭接焊接工艺试验。研究表明,通过优化搅拌头形貌尺寸和工艺参数能够实现4-2（焊缝深度–台阶宽度,mm）、6-4 和9-6 的窄搭接搅拌摩擦焊焊接,焊接过程中进行定位预焊能有效避免产生焊缝S 型曲线,前进侧为6063 或3A21 时均能形成良好的焊缝。     

Stability of the friction stir welding process in presence of workpiece mating variations

This paper explores common process variations encountered in friction stir welding (FSW) and the limits to which acceptable joint strength is maintained while welding with a robotic FSW system. Part fit-up and mating variations are common in manufacturing, yet the limits to which a friction stir welding process can weld without major process adjustment are unclear. The effects on joint strength and mechanical properties of several of the most common mating variations (i.e., faying surface gap, misalignment, mismatch, etc.) are experimentally determined as individual effects as well as among common welding parameters. Experimental results on 5-mm-thick aluminum alloy 5083-H111 show that ultimate tensile strength, yield strength, and elongation begin to decrease from nominal weld conditions when either the tool offset distance from weld centerline or gap in abutted plates exceeds 25% of the average pin diameter (6?mm). In addition, vertical plate mismatch and lack of penetration can be tolerated up to 2.5% and 10%, respectively, before adverse effects on mechanical properties are observed. The work also indicates that of all the mating variations tested in this study, tool misalignment, followed by travel angle, has the most significant effect on the measured joint strength. Process stability testing has shown that the FSW process is able to endure part fit-up and mating variations within a defined tolerance, giving the practitioner an awareness of how well stock workpiece tolerances must be controlled before joint strength is adversely effected.     

Optimal design of 3-DOF PKM module for friction stir welding

Friction stir welding (FSW) has recently emerged as a solid-joining technology for high-strength aluminum alloys and light metal welding. The large axial force to be maintained between the welding tool and workpiece is the primary requirement of FSW process, which has also been a great obstacle to the design and application of FSW in manufacturing. Further complicating the issue is the need to perform FSW over three-dimensional contours, which requires a mechanism dexterous enough to set the stir pin used in welding to track a predefined trajectory with prescribed poses. Apart from the position specification, the design of a dexterous mechanism to pose the orientation of stir pin is a great challenge. This paper proposed the application of 3-PRS (P, R, and S standing for prismatic, revolute, and spherical joint, respectively) parallel mechanism as a welding tool head and employed it to form a five-axis welding machine tool for FSW. In order to accommodate the orientation capability requirements, the kinematic feature has been analyzed. With the dimensionless treatment of the Jacobian matrix of 3-PRS manipulator, a global condition index is proposed to estimate the kinematic dexterity in the whole orientation workspace. Finally, by means of an optimal design method and performance atlas, optimal designs of the 3-PRS parallel mechanism is carried out. A preferable set of optimized geometric parameters are obtained to achieve a compact and dexterous design, and the optimization results are used in development of a prototype machine for FSW.     

Effects of welding speed on microstructures and mechanical properties of AA2219-T6 welded by the reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel FSW technology in which the tool pin and the assisted shoulder rotates reversely, thus it has the capability to obtain appropriate welding conditions through adjusting the rotating tool pin and surrounding assisted shoulder independently. In the present study, a RDR-FSW tool system was designed and successfully applied to weld high strength aluminum alloy 2219-T6, and the effects of welding speed on microstructures and mechanical properties were investigated in detail. At a constant rotation speed of 800 rpm for both the rotating tool pin and the reversely rotating assisted shoulder, defect-free joints were obtained at welding speeds ranging from 50 to 150 mm/min, while a cavity defect appeared at the three-phase confluction on the advancing side when the welding speed increased to 200 mm/min. With increasing of the welding speed, the width of the softened region decreased, but the minimum microhardness value increased gradually. When compared with the joints welded by the conventional FSW, there is only a minor variation of the Vickers hardness across the stirring zone in the joint welded by the RDR-FSW. The maximum tensile strength 328 MPa (73.7 % of the base material) was obtained at the welding speed of 150 mm/min, while the elongation reached its maximum 7.0 % (60.9 % of the base material) at the welding speed of 100 mm/min. All defect-free joints were fractured at the weakest region with the minimum Vickers hardness, while for the joint with cavity defects the fracture occurred at the defect location. The tensile fracture was in the ductile fracture mode.     

Friction stir butt welding thin aluminum alloy sheets

0.8-mm-thick alclad 2024-T4 aluminum alloy sheets were friction stir butt welded. A 15-mm diameter shoulder tool was used to guarantee sufficient heat input during welding. A 0.08-mm shoulder plunge depth was adopted to reduce sheet thickness reduction. Sound joints were obtained at rotating speeds from 400 to 1000 rpm and welding speeds from 50 to 150 mm/min. A thickness reduction of 6% was achieved at 1000 rpm and 50 mm/min. Secondary phases firstly precipitated at the black lines in the stir zone (SZ). The hardness of the SZ showed a decrease about 6% compared with the base metal. A maximum tensile strength of 399.5 MPa and an elongation of 5.6% were achieved at 1000 rpm and 150 mm/min. The fracture morphologies showed typical ductile fracture mode.     

Effect of laser post-treatment on Al2O3-TiB2-TiN composite coating with free hBN

Friction stir welding (FSW) is a solid-state joining process for many materials including aluminum alloys, steel, titanium, and plastics. However, attendant process variability such as uneven workpiece flatness and workpiece temperature changes is known to lead to joints with inconsistent material properties. The weld quality can be improved by monitoring and controlling the spindle power. This work presents a multi-level adaptive fuzzy control approach for maintaining constant FSW power. Implementation of the control scheme is conducted on a vertical milling machine configured for FSW using an open architecture controller. Experimental results show that the multi-level fuzzy technique dramatically reduces power variations in the presence of significant process disturbances.     

Predictions of the optimized friction stir spot welding process parameters for joining AA2024 aluminum alloy using RSM

The friction stir spot welding process (FSSW) is a variant of the linear friction stir welding process in which the material is being welded without bulk melting. The FSSW parameters such as tool rotational speed, plunge rate, plunge depth, and dwell time play a major role in determining the strength of the joints. A central composite rotatable design with four factors and five levels was chosen to minimize the number of experimental conditions. An empirical relationship was established to predict the tensile shear fracture load of friction stir spot-welded AA2024 aluminum alloy by incorporating independently controllable FSSW process parameters. Response surface methodology (RSM) was applied to optimize the FSSW parameters to attain maximum lap shear strength of the spot weld.     

Predictions of the optimized friction stir welding process parameters for joining AA7075-T6 aluminum alloy using preheating system

The scope of this investigation is to evaluate the effect of welding parameters on the mechanical properties and microstructural features of 3-mm-thick AA7075-T6 aluminum alloy subjected to gas heating system as a preheating source during friction stir welding. Toward this end, a gas heating system was designed to heat up the weld seam just ahead of rotating tool to soften the material before being stirred. Three welding parameters, five levels, and a central composite design (CCD) have been used to minimize the number of experimental conditions. The joining parameters such as tool rotational speed, welding speed, and shoulder diameter have a significant influence on determining the mechanical properties of the welded joints. It was found that using preheating system mostly can result in higher total heat input into the weld joint and effectively reduces the formation of defects when unsuitable process parameters were used. Also, an attempt has been made to establish the mathematical model to predict the tensile strength and microhardness of the joints. The optimal welding conditions to maximize the final responses were investigated and reported. The results show that the joint fabricated at a rotational speed of 1,050 rpm, welding speed of 100 mm/min, and shoulder diameter of 14 mm exhibited higher mechanical properties compared to other joints.     

Effect of tool pin profile on friction stir butt welding of high-zinc brass (CuZn40)

In this experimental study, the effects of tool pin profiles (chamfered taper, single-threaded taper, three-flute, threaded cylinder, threaded taper, spline, and hexahedron) in friction stir welding of high-zinc brasses were explored through mechanical and micro-structural examinations. To evaluate the effect of temperature in friction stir welding, the temperature was measured by embedding thermocouples within the fixture body. Furthermore, in order to evaluate the effects of the tool shape, the main FSW parameters (rotational speed, travel speed and plunge force) were maintained constant. Mechanical tests (hardness, tensile and bending) and micro-structural examinations were performed to study the properties of welded samples with regard to temperature measurement. The results indicated that suitable tools can generate enough heat below the shoulder due to further materials stirring. Moreover, studies of a hexahedron sample revealed that accumulated defects near the weld were one reason for mechanical weakening with regard to a lower heat generated.     

Influences of tool pin profile and axial force on the formation of friction stir processing zone in AA6061 aluminium alloy

AA6061 aluminium alloy (Al-Mg-Si alloy) has gathered wide acceptance in the fabrication of light weight structures requiring a high strength-to-weight ratio and good corrosion resistance. Compared to the fusion welding processes that are routinely used for joining structural aluminium alloys, the friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a non-consumable tool to generate frictional heat in the abutting surfaces. The welding parameters such as tool rotational speed, welding speed, axial force etc., and the tool pin profile plays a major role in deciding the weld quality. In this investigation an attempt has been made to understand the effect of axial force and tool pin profiles on FSP zone formation in AA6061 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular and square) have been used to fabricate the joints at three different axial force levels. The formation of FSP zone has been analysed macroscopically. Tensile properties of the joints have been evaluated and correlated with the FSP zone formation. From this investigation it is found that the square tool pin profile produces mechanically sound and metallurgically defect free welds compared to other tool pin profiles.     

Study of ultrasonic vibrations’ effect on friction stir welding

In this paper, effect of ultrasonic vibrations on friction stir welding (FSW) is studied. Ultrasonic vibrations were employed on the tool in pin direction (perpendicular to the welding direction). To do this study, a vibration tool was designed by Abaqus software in a way to have a longitudinal frequency about 20 kHz and was then manufactured and assembled with an ultrasonic transducer and was controlled using an ultrasonic generator to oscillate ultrasonically with a peak-to-peak amplitude of 10 μm. After preparation of experimental setup, some experiments were performed on AA6061-T6 as a work material, and the effect of ultrasonic vibrations on force, temperature, tensile strength, and hardness was investigated in FSW. Based on the achieved results, ultrasonic vibrations can decrease force and increase temperature in FSW.