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.     

搅拌摩擦焊工具的研究现状

搅拌焊工具技术是搅拌摩擦焊工艺最重要的因素。搅拌焊工具主要由肩部和焊针组成，焊接薄板时，旋转的肩部和工件之间摩擦产生的热量是主要热源，而随着板厚的增加，更多的热量必须靠旋转的焊针和工件摩擦产生。焊接工具的主要作用是保证连接区材料产生足够的塑性变形，并控制焊针周围塑性体的流动，形成优质的焊接接头。     

Finite element modeling for maximum temperature in friction stir welding and its validation

This paper presents a new scheduling method for a flexible manufacturing system (FMS) in a Petri net framework. Petri nets can concisely model multiple lot sizes for each job, the strict precedence constraint, multiple kinds of resources, and concurrent activities. To decrease the likelihood of rejecting the critical markings, our algorithm adopts an improved checking method for previous generated marking. To reduce the computation complexity, an elaborate scheme is applied, which performs A* search locally and backtracking search globally in the reachability graph of the Petri net. Some numerical experiments are carried out to demonstrate usefulness of the algorithm.     

Ultrasonic thermometry for friction stir spot welding

This investigation presents the feasibility of ultrasonic temperature measurement of friction stir spot welding (FSSW). FSSW is an automated solid state joining process. Thermal profiles of the weld zone are crucial for implementing informed process changes to improve weld quality. Ultrasonics present a novel and non-invasive method of monitoring changes in temperature.Ultrasonic time of flight (TOF) measurement method is used to calculate the temperature of Al 6061 as it is heated. Comparisons of the ultrasonic temperature calculations with thermocouple readings confirm the accuracy of the ultrasonic system. The ultrasonic signal is then recorded during spot welding and processed. The results show that ultrasonic technology is a feasible method of monitoring the heating and cooling profiles of the weld zone during welding. The paper also discusses challenges presented by the system as well as recommendations for its future implementation in the friction stir welding manufacturing industry.     

Numerical analysis of heat generation and temperature field in reverse dual-rotation friction stir welding

A 3D model is developed for numerical analysis for heat generation, temperature field, and material flow in reverse dual-rotation friction stir welding (RDR-FSW) process. The reverse rotation of the assisted shoulder and the tool pin is considered to determine the heat generation rate. Friction heat, plastic deformation heat, and their partition coefficients are analyzed. Due to the tool pin and assisted shoulder being separated and reversely rotated independently, the temperature difference between the advancing and retreating sides is weakened. The reverse material flow is beneficial to the uniformity of both the temperature and microstructure at the advancing and retreating sides. The calculated temperature profiles agree well with the corresponding experimentally measured values.     

Solid mechanics-based Eulerian model of friction stir welding

A new Eulerian model is established based on solid mechanics. With comparisons to the experimental data and the ALE model, the current model is validated. The power and the heat generations from pin side surface, pin tip surface, and shoulder contact surface in different rotating speeds are studied. Results indicate that the ratio of the heat input powers from the pin and the shoulder keeps constant in different rotating speeds. The velocity of the material flow around the welding tool and the slipping velocity are both increased with the increase of the rotating speed. The increase of the slipping velocity is the main reason for the increase of the heat input with the increase of the rotating speed. The torque from the shoulder contact surface is the major part of the total torque. The contribution to the total torque from the pin tip surface is the smallest.     

Using TRIZ methods in friction stir welding design

Welding is an extremely important joining method in the manufacturing process. For the last few years, the friction stir welding (FSW) method has significantly increased the quality of a weld. However, FSW has a slightly short research and application progress. The related applied experiences are not prevalent. Therefore, FSW has a lack of reference information on related welding applied design, such as fixture, joining, and integrated design. This article intends to combine innovative design methods in the application of FSW design. Additionally, this article establishes the applied design mode of FSW through case analysis to assist engineers or design personnel who are not familiar with the FSW process. This will help to decrease trial and error or failure risks in the welding process of fixture design. Encountered welding difficulties are thus solved after being guided by the theory of inventive problem solving (TRIZ) design method. By combining traditional TRIZ design methods, this article also refers to other TRIZ methods proposed by some scholars because work will often encounter various engineering challenges. This article hopes to provide welding design personnel with innovative design ideas under research and for practical application.     

Effect of shoulder size on the temperature rise and the material deformation in friction stir welding

A fully coupled thermo-mechanical model is adopted to study the effect of shoulder size on the temperature distributions and the material deformations in friction stir welding. Numerical results indicate that the maximum temperature can be increased with the increase of the shoulder diameter. The stirring zone can be enlarged by the increase of the shoulder size. With consideration of the recrystallization formula, it is found that the temperature variation is the main factor for controlling the grain growth near the welding line. But, when the strain and the strain rate become smaller near the border of the stirring zone, the recrystallization process is dominated by the material deformations instead of the temperature rise.     

Material behaviors and mechanical features in friction stir welding process

This paper presents the 3D material flows and mechanical features under different process parameters by using the finite element method based on solid mechanics. Experimental results are also given to study the effect of process parameters on joining properties of the friction stir welds. Numerical results indicate that the tangent flow constitutes the major part in the material flow. The shoulder can accelerate the material flow on the top half of the friction stir weld. The distribution of the equivalent plastic strain can correlate well with the microstructure zones. Increasing the angular velocity of the pin, the material in the nugget zone can be more fully mixed, which improves the joining quality of the two welding plates. The increase of speeds, including the rotational speed and the translational speed, can both accelerate the material flow, especially in front of the pin on the retreating side where the fastest material flow occurs. The contact pressure on the pin-plate interface is decreased with the increase of the angular velocity. An erratum to this article can be found at     

A fully coupled thermo-mechanical model of friction stir welding

This paper presents a new developed fully coupled thermo-mechanical model of the friction stir welding process. Results indicate that the rotation of the shoulder can accelerate the material flow behavior near the top surface. The material deformation and the temperature field can have relations with the microstructural evolution. The texture of the appearance of the friction stir welds can correlate well with the equivalent plastic strain distributions on the top surface. The temperature field in the friction stir welding process is approximately symmetric to the welding line. The material flows in different thicknesses are different. The shoulder can have a significant effect on material behaviors on the top surface, but this effect is greatly weakened when the material gets closer to the bottom surface of the welding plate.     

Mode I failure modeling of friction stir welding joints

This paper analyzes mechanical response by finite element method up to the decohesion failure in fracture mode I for joints of friction stir welding (FSW) of an aluminum alloy. It first describes experimental investigations on specimens with FSW embedded, subjected to uniform traction and local punch tests used to characterize local elastic and plastic material parameters. The heterogeneity of the mechanical properties induced by the FSW process is taken into account for the elastic-plastic finite element simulation. The growing damage and the opening failure of the welding zone are described by the adoption of a cohesive interface model with specific mechanical properties.     

Torque control of friction stir welding for manufacturing and automation

Friction stir welding (FSW) is a solid-state welding process that utilizes a rotating tool to plastically deform and forge together the parent materials of a workpiece. The process involves plunging the rotating tool that consists of a shoulder and a pin into the workpiece and then traversing it along the intended weld seam. The welding process requires a large axial force to be maintained on the tool. Axial force control has been used in robotic FSW processes to compensate for the compliant nature of robots. Without force control, welding flaws would continuously emerge as the robot repositioned its linkages to traverse the tool along the intended weld seam. Insufficient plunge depth would result and cause the welding flaws as the robot’s linkages yielded from the resulting force in the welding environment. The research present in this paper investigates the use of torque instead of force to control the FSW process. To perform this research, a torque controller was implemented on a retrofitted Milwaukee Model K milling machine. The closed loop proportional, integral plus derivative control architecture was tuned using the Ziegler–Nichols method. Welding experiments were conducted by butt welding 0.25 in. (6.35 mm)?×?1.5 in. (38.1 mm)?×?8 in. (203.2 mm) samples of aluminum 6061 with a 0.25 in. (6.35 mm) threaded tool. The results indicate that controlling torque produces an acceptable weld process that adapts to the changing surface conditions of the workpiece. For this experiment, the torque was able to be controlled with standard deviation of 0.231 N-m. In addition, the torque controller was able to adjust the tool’s plunge depth in reaction to 1 mm step and ramp disturbances in the workpiece’s surface. It is shown that torque control is equivalent to weld power control and causes a uniform amount of energy per unit length to be deposited along the weld seam. It is concluded that the feedback signal of torque provides a better indicator of tool depth into the workpiece than axial force. Torque is more sensitive to tool depth than axial force. Thus, it is concluded that torque control is better suited for keeping a friction stir welding tool properly engaged with the workpiece for application to robotics, automation, and manufacturing.     

Theoretical and experimental investigation into friction stir welding of AA 5086

This research investigates the relationship between the microstructures of thermomechanically affected zone (TMAZ) and heat input in friction stir welding (FSW) of 5086 aluminum alloy. First, welding heat input has been predicted using a three-dimensional finite element analysis; then, welding experiments have been carried out on annealed and work-hardened conditions to study the developed microstructures and the mechanical properties of the welded metal. The results show that the temperature field in the FSW process is asymmetrically distributed with respect to the welding line. Also, both experimental and predicted data illustrates that peak temperatures are higher on the advancing side than the retreating side. In addition, the microstructures are strongly affected by the heat input, while the grain size within the TMAZ decreases with decreasing heat input per unit length during FSW.     

Load bearing capacity of tool pin during friction stir welding

Although friction stir welding (FSW) is now widely used for the welding of aluminum and other soft alloys, premature tool failure limits its application to hard alloys such as steels and titanium alloys. The tool pin, the weakest component of the tool, experiences severe stresses at high temperatures due to both bending moment and torsion. It is shown that the optimum tool pin geometry can be determined from its load bearing capacity for a given set of welding variables and tool and work-piece materials. The traverse force and torque during friction stir welding are computed using a three-dimensional heat transfer and viscoplastic material flow model considering temperature and strain rate-dependent flow stress of the work-piece material. These computed values are used to determine the maximum shear stress experienced by the tool pin due to bending moment and torsion for various welding variables and tool pin dimensions. It is shown that a tool pin with smaller length and larger diameter will be able to sustain more stress than a longer pin with smaller diameter. The proposed methodology is used to explain the failure and deformation of the tool pin in independent experiments for the welding of both L80 steel and AA7075 alloy. The results demonstrate that the short tool life in a typical FSW of steels is contributed by low values of factor of safety in an environment of high temperature and severe stress.     

SIMOTION D435在搅拌摩擦焊接机中的应用

针对目前我国搅拌摩擦焊设备的自主研发水平不高,大部分只能焊接一维直线或者无法恒压力控制的问题,设计了一套既能焊接二维曲线又可恒压力控制的搅拌摩擦焊设备.通过对搅拌摩擦焊接的焊接原理和控制要素的分析,在综合比较了各种控制方案后,选用了以SIMOTION D435为核心的控制方案;在对所需功能进行分析的基础上,控制程序采用模块式的编程方法,综合运用LAD语言和MCC语言,实现了搅拌摩擦焊接时的二维轨迹插补和压力PID控制,并对压力的控制精度和进给轴的定位精度进行了实验分析.研究结果表明,该系统运行稳定,压力控制精度高,适合于铝合金、镁合金等轻合金的二维平面的搅拌摩擦焊接.