Bobbin Tool Friction Stir Welding Research Articles

Numerical analysis of thermal differential distribution in Bobbin tool friction stir welding of 7075-AZ31B dissimilar alloy

The theoretical model of bobbin tool friction stir welding (BT-FSW) structure and dynamic process is established. Because of the great differences in properties of different materials, the mathematical relationship between material stress and deformation will be expressed based on 3D transient heat conduction equation and J-C constitutive model. The BT-FSW process of 7075 and AZ31B was analyzed by finite element method, and the field distribution, thermal cycle curve and stress distribution characteristics of different welding areas of the welded material were simulated. To predict and control the non-equilibrium distribution of welding temperature and stress field, the influences of different Rotating speed, welding speed, offset and other variables on the BT-FSW process of 7075/AZ31B aluminum magnesium dissimilar alloy were studied comprehensively. Finally, the reliability of the model is proved by fitting the experimental and numerical analysis results. The research results show that the temperature of BT-FSW presents an “hourglass” symmetrical distribution, and the stress distribution on both sides of the plate is uniform when the aluminum alloy is placed on the advancing side, the stirring tool is offset to the aluminum alloy by 2mm, and the rotating speed and welding speed are 700r/min and 180mm/min, respectively.

Inhibition study of curcumin extract’s effect on dissimilar aluminium joint

Aluminium welded joints are offering greater interest to researchers owing to the replacement of heavy steel structures and reduction in the weight of the components used in the automobile and marine environments. In this study AA6061 and AA8011 have been welded by using the bobbin tool friction stir welding method and by varying the process parameters with the samples being subjected to corrosion environments. The corrosive nature of the welded alloys in the absence and presence of inhibitors (curcumin) has been examined by electrochemical methods and compared with raw samples. The ratio has been observed between 0.075 and 5.42 A cm-2. The results reveal that corrosion control tendency has been improved by the AA6061 and AA8011 aluminium alloy joint in the presence of curcumin extract.

Thermal Cycling, Microstructure, and Mechanical Properties of Al-Mg-Si-Cu Alloy Bobbin Tool Friction Stir Welded Joints Based on Thermal Index

The two main process parameters of Bobbin tool friction stir welding (BT-FSW) are ω (rotational speed) and v (traverse speed). Both of these factors have a significant effect on heat input, microstructure, and mechanical properties. At present, most studies on friction stir welding adopt the control variable method to study the thermal cycling during the welding process and the mechanical properties of joints, and there are few studies on changing the two process parameters at the same time, because it can be difficult to assess the correlation between heat input and mechanical properties when changing both factors at the same time. In this study, the w/v ratio is defined as the thermal index, which is a characteristic value of heat input. The study uses ABAQUS 6.5 software to establish a BT-FSW CEL (coupled Eulerian–Lagrangian) thermal coupling model. This model explores the relationship between joint thermal cycles, microstructure, and mechanical properties for different w and v values with the same w/v ratio. The results show that increasing rotational and traverse speeds under the same w/v ratio leads to an increase in the peak temperature of the nugget zone (NZ). However, the peak temperature of the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) remained almost constant. Joint strength was highest at a rotational speed of 750 r/min and a traverse speed of 650 mm/min, with a yield strength of 227 MPa. As rotational and traverse speeds increased, the recrystallized grain content of the NZ showed an increasing trend followed by a decreasing trend. The recrystallized grain content of the advancing side thermo-mechanically affected zone (AS-TMAZ) and retreating side thermo-mechanically affected zone (RS-TMAZ) showed a decreasing trend. Joint hardness had a “W” shaped distribution, with the highest average hardness value found in the NZ.

Effects of different friction stir welding processes on residual stress and deformation of Ti62A alloy joints

Residual stress induced by welding easily causes deformation and even fracture failure in welded structures. In this paper, a three-dimensional thermal-mechanical coupling finite element model of friction stir welding (FSW) was developed. The effects of different welding processes, namely conventional friction stir welding (C-FSW), stationary shoulder friction stir welding (SS-FSW), and bobbin tool friction stir welding (BT-FSW), on the temperature, residual stress, and deformation of welded joints were investigated by employing a heat source model and a force model to the welding analysis. The results reveal that a discernible disparity in temperature gradient is observed in the region ahead of the tool and behind it among various welding processes. The peak values of longitudinal residual stress along the surface and bottom paths of SS-FSW are reduced by 35.7% and 34.1%, respectively. In addition, the deformation at the initial end of the weld is slightly larger than that at the final end along the welding direction in three welding processes. Compared with other FSW processes, SS-FSW exhibits the smallest deformation, which is advantageous for improving assembly accuracy and enhancing the load-bearing capacity of weldments. The above results provide an important theoretical reference for the FSW process optimization of Ti62A alloy.

Study on mechanical properties of 2219 aluminum alloy bobbin tool friction stir welding

The Bobbin tool friction stir welding (BT-FSW) process experiment was performed on 2219 aluminum alloy with different butt clearances and misalignments. The maximum allowable clearance and misalignment amount for BT-FSW were obtained, and the weld formation, mechanical properties and microstructure under different parameters were compared and analysed. The results show that the face and back sides of the weld joint surface are perfect and there are no defect inside with the butt clearance under 2 mm and the misalignment under 1.5 mm. During the increase of butt clearance and misalignment, the tensile strength of the weld will be decreased gradually. In comparison, the impact of misalignment on weld strength is lower than that of the butt clearance. The tensile strength from both of them will reach more than 60% of the base metal strength. The lowest hardness value occurs in the thermal-mechanical influence zone, which is 79.1HV, about 70% of the base metal hardness value. The microstructure characteristics are similar to those of conventional friction stir welding. The macroscopic morphology of the welded joint is typical ‘dumbbell type’, and there are many ‘dimples’ in the tensile fracture, which is a typical ductile fracture. A large number of dispersed phase particles (Al2Cu) were found in the dimples, which is the main reason for the decrease in mechanical strength and hardness compared with the substrate.

An in-situ defect detection method for aluminum alloy bobbin tool friction stir welding assisted by lateral force

This paper proposes a force-driven method for in-situ defect detection in aluminum alloy bobbin tool friction stir welding (BTFSW). The variations of the three component forces in BTFSW, including traverse force (Fx), lateral force (Fy) and plunge force (Fz) are studied. The mean value of Fy exhibits the highest correlation with tunnel defects. Numerical simulation results show that higher stress on the retreating side due to insufficient material flow and severe material jam results in an abnormal increase of Fy for defective joints, revealing the response mechanism of Fy to tunnel defects. The proposed method offers a promising approach for real-time detecting tunnel defects in aluminum alloy BTFSW.

Influence of Tool–Base Metal Interference on the Performance of an Aluminium–Magnesium Alloy Joined via Bobbin Tool Friction Stir Welding

Bobbin tool friction stir welding (BTFSW) is a variant of the FSW process which uses the special two-shoulder tool that forms the top and bottom of a weld surface. As such, a significant simplification of the welding setup is achieved. One of the dominant parameters of the BTFSW process is the interference between the tool shoulder pinch gap and the weld metal thickness. In this research, the influence of interference of the square pin tool with convex shoulders on process temperature, microstructure, tensile, impact, and bend performance were studied, and appropriate correlations were devised. The base metal was an aluminum–magnesium alloy in which the interference varied in the range of 0.1 to 0.5 mm. Wormhole defects and irregularities were found in all specimens except in the specimen welded with 0.4 mm interference. An optimal interference of 0.4 mm resulted in the best mechanical properties, which, in terms of tensile strength and reduction of area, were similar to the base metal. Furthermore, the impact strength was significantly higher, which was attributed to the grain refinement effect in the nugget zone.

Micro-hardness evaluation of the bobbin tool-friction stir welded AA6063 using regression-based machine learning

This research work focuses on the prediction of micro-hardness in Bobbin tool friction stir welding (BT-FSW) of AA6063 plates using machine learning (ML) models. The objectives are to determine the effect of process parameters on micro-hardness and to compare the performance of various ML techniques. The dataset obtained from experimental runs is subjected to classical supervised learning techniques such as LR, SVR, DTR, KNN, as well as ensemble techniques including Bagging, Random Forest, Extra Trees, AdaBoost, Gradient Boosting, and Extreme Gradient Boosting. The results show that ML models effectively predict micro-hardness, with ensemble methods outperforming individual techniques. The tool rotational speed is identified as the most influential parameter affecting micro-hardness.

Thermal Effect of Bobbin Tool Friction Stir Welding on the Mechanical Behavior of High Density Polyethylene Sheets: Experimental Study

Bobbin friction stir welding (BT-FSW) is a variant of the conventional friction stir welding (C-FSW). This method has been applied of welding high density polyethylene (HDPE) plates; where a rotating symmetrical tool causes a fully penetrated bond, it can weld the upper and lower surface of the work-piece in the same pass. BT-FSW process involves complex heat generation and HDPE flow, which directly affects on the weld area and on mechanical properties of welded joint. Heat generation and material flow during BT-FSW are significantly affected by the tool design features, process parameters and mechanical behavior of work piece materials. Studying the temperature of polyethylene sheets welded by BT-FSW can help in analyzing the mechanism of weld formation and also can provide theoretical guidance for the tool design, process parameter selection and even new process development. In the unique work described in this paper, the 11.4 mm-thick HDPE plates were welded successfully by bobbin-tool friction stir welding. Measurements of the material temperatures were performed by thermocouples which are placed near and at the weld seam. The weld quality was determined in terms of no defects in the stir zone and the tensile strength of the joint. It was found that considerable melting occurred between the rotating shoulders and on the trailing side of the rotating pin. Movement of the molten material by the rotating tool created a very black band in the stir zone. Thermocouples measurements indicated that the temperatures were higher on the advancing side (AS) compared to those on the retreating side (RS). Tensile tests and hardness measurements were performed on welded and seamless sheet samples. The results were analyzed to compare the mechanical properties. To demonstrate the variation in micromechanical properties between welded and seamless sheet samples, micro hardness (HV) testing was used to explain the difference. The HV of the HDPE plates weld by BT-FSW, were relatively symmetrical with respect to the parting line. The maximum hardness levels were reached in the weld bead at around 66 HV in the welded nugget; there was a rise in the level of hardness, in particular at retreating side (RS) and at advancing side (AS) where the value reached 68.60 HV.

Study on temperature field, microstructure, and properties of T2 pure copper by bobbin tool friction stir welding

Study on temperature field, microstructure, and properties of T2 pure copper by bobbin tool friction stir welding

Friction Stir Welding of Aluminum in the Aerospace Industry: The Current Progress and State-of-the-Art Review.

The use of the friction stir welding (FSW) process as a relatively new solid-state welding technology in the aerospace industry has pushed forward several developments in different related aspects of this strategic industry. In terms of the FSW process itself, due to the geometric limitations involved in the conventional FSW process, many variants have been required over time to suit the different types of geometries and structures, which has resulted in the development of numerous variants such as refill friction stir spot welding (RFSSW), stationary shoulder friction stir welding (SSFSW), and bobbin tool friction stir welding (BTFSW). In terms of FSW machines, significant development has occurred in the new design and adaptation of the existing machining equipment through the use of their structures or the new and specially designed FSW heads. In terms of the most used materials in the aerospace industry, there has been development of new high strength-to-weight ratios such as the 3rd generation aluminum-lithium alloys that have become successfully weldable by FSW with fewer welding defects and a significant improvement in the weld quality and geometric accuracy. The purpose of this article is to summarize the state of knowledge regarding the application of the FSW process to join materials used in the aerospace industry and to identify gaps in the state of the art. This work describes the fundamental techniques and tools necessary to make soundly welded joints. Typical applications of FSW processes are surveyed, including friction stir spot welding, RFSSW, SSFSW, BTFSW, and underwater FSW. Conclusions and suggestions for future development are proposed.

Friction Stir Welding of Non-Heat Treatable Al Alloys: Challenges and Improvements Opportunities

Friction stir welding (FSW) is an effective solid-state joining process that has the potential to overcome common problems correlated with conventional fusion welding processes. FSW is used for the joining of metallic materials, in particular Al alloys (non-heat-treatable and heat-treatable). The heat produced by the friction between the rotating tool and the workpiece material generates a softened region near the FSW tool. Although the heat input plays a crucial role in producing a defect-free weld metal, it is a serious concern in the FSW of work-hardened non-heat-treatable Al alloys. In this group of alloys, the mechanical properties, including hardness, tensile properties, and fatigue life, are adversely affected by the softening effect because of grain growth and reduced dislocation density. Considering this challenge, work-hardened Al alloys have been limited in their industrial use, which includes aerospace, shipbuilding, automotive, and railway industries. The current comprehensive review presents the various approaches of available studies for improving the quality of FSW joints and expanding their use. First, the optimization of welding parameters, including the tool rotational and traverse speeds, tool design, plunge depth, and the tilt angle is discussed. Second, the incorporation of reinforcement particles and then underwater FSW are stated as other effective strategies to strengthen the joint. Finally, some supplementary techniques containing surface modification, bobbin tool FSW, copper backing, and double-sided FSW in relation to strain-hardened Al alloys are considered.

Effect of Shoulder Fillet Radius on Welds in Bobbin Tool Friction Stir Welding of A1050

In this study, five bobbin tools with different shoulder fillet radii were employed for the bobbin tool friction stir welding (BT-FSW) of A1050-O sheets to systematically evaluate the effects of shoulder fillet radius on the welding defect formation, flash formation, weld thickness, grain size of the stir zone, and tensile properties. The quality classifications of the joints' appearance were summarized as process windows, and the appropriate welding condition range for each shoulder fillet radius was clarified. It was observed that an increase in the shoulder fillet radius decreased the welding defects and flash formation; however, it increased the minimum thickness of the weld except when the shoulder fillet radius was 0.5 mm. The grain size of the stir zone increased with increasing shoulder fillet radius from 0.5 mm to 6 mm. The ultimate tensile strength (UTS) of the stir zone decreased with increasing shoulder fillet radius from 0.5 mm to 1 mm, increased from 1 mm to 3 mm, and remained constant from 3 mm to 6 mm. The results indicate that a shoulder fillet radius larger than 3 mm is effective in decreasing flash formation and maintaining a constant weld thickness.

Study on the relationship between welding force and defects in bobbin tool friction stir welding

In this paper, the characteristics of traverse force (Fx), lateral force (Fy), and axial force (Fz) in bobbin tool friction stir welding (BTFSW) and their relationship with welding defects were studied. Fx, Fy, and Fz showed periodic oscillations and had sinusoidal waveforms in defect-free joints. Defect formation strongly influenced the variation of welding force, and Fy increased most noticeably in defective joints. The average value of Fy determined whether defects occurred in the weld. The material flow behavior during defect formation was analyzed and it was demonstrated that an increase in Fy was due to severe material deposition on the retreating side (RS), which significantly increased the extrusion force between the pin and the workpiece. Due to the good correlation between welding force and defects, a decision tree model based on the force features in the time and frequency domain was built to predict the welding defects. The accuracy of this model in predicting defects reached 95.3 %. Furthermore, the prediction results showed that Fy was the most sensitive force to the generation of defects, and the average value of Fy was the key feature in predicting whether defects occurred.

Enhancing joint strength of bobbin tool friction stir welded Al–Mg–Si alloy via post-weld aging process

For the precipitation-strengthened aluminum alloys such as Al–Mg–Si alloys, the friction stir welded (FSW) joints produced with T6-treated plates usually exhibit obvious softening in the heat-affected zones (HAZ), significantly reducing the joint strength. When bobbin tool FSW (BT-FSW) is used, severer softening of the HAZ occurred compared to conventional FSW. It is therefore crucial to achieve precipitate morphology control in the HAZ for BT-FSW through original plate state selecting, welding parameter optimizing, and post-weld artificial aging (PWAA). Compared to the conventional welding process (plates welded directly under T6 condition, T6-BT), in the present work, a post-weld aging process was proposed: the plates under T4 condition were BT-FSWed using optimizing parameters (T4-BT), and then subjected to PWAA (T4-BT-PWAA). The intrinsic microstructural differences between the T6-BT and T4-BT-PWAA joints were revealed. More attentions were paid on the microstructural evolution in the lowest hardness zone (LHZ) during PWAA for the T4-BT joint. The LHZ of the T6-BT joint exhibited an over-aged state characterized by coarsened β′ phase and the hardness in this zone had very limited enhancing space when subjected to post-weld peak aging. For the T4-BT joint, the LHZ was characterized by a low density of precipitates and presented excellent re-precipitating ability of β'' phase during PWAA, with the ultimate tensile strength increased by ∼11.4% compared to its T6 counterpart, which is very beneficial for engineering applications.

Microstructure and corrosion behavior of bobbin tool friction stir welded 2219 aluminum alloy

This study employed the bobbin tool friction stir welding (BT-FSW) technique to weld 2219 aluminum alloy with 6 mm thickness. The microstructure investigation revealed that the weld nugget zone (WNZ) has a small number of coarse θ phases, whereas the heat-affected zone (HAZ) exhibits colossal amount of θ' phases. The impedance analysis demonstrated that the corrosion resistance of the WNZ is better than that of other zones. Also, the salt spray corrosion test showed that the WNZ has the best corrosion resistance, whereas the base metal (BM) has the worst corrosion resistance. Therefore, the poor corrosion resistance of the BM can be attributed to a high content of the precipitates.

Effect of Tool Design on the Mechanical Properties of Bobbin Friction Stir Welded High-Density Polyethylene Sheets: Experimental Study

Welding polymers by the friction stir welding (FSW) technique is one assembly process among several known assembly techniques which consists in welding two materials without filler material. The FSW process is based on the generation of heat due to friction and material deformation under an axial force. Among the main aspects affecting material flow, the choice of welding tool geometry has become of great interest to improve the welds quality. The main objective of this work is the welding of polymers using the FSW technique. A new method of welding HDPE (high density polyethylene) plates, called BT-FSW (bobbin tool friction stir welding) was developed. Standard rectangular shape intended for the distribution of natural gas has been successfully welded by BT-FSW. Tensile tests and hardness measurements were carried out on samples cut from the welded sheets and the results were analyzed to compare the mechanical characteristics of the plates welded by the BT-FSW and conventional FSW (C-FSW) processes. The results of the comparative studies on the micro-hardness characteristics and mechanical properties of the two welding processes indicate that welding using the bobbin tool can significantly reduce hardness and improve both weld formation and mechanical properties of joints. This study showed that the design of the welding tool has a big impact on the weld strength. An improvement in the mechanical properties of the specimens welded by BT-FSW was observed to give a better welding quality for the polymers studied.

Microstructure and Mechanical Properties Analysis of Al/Cu Dissimilar Alloys Joining by Using Conventional and Bobbin Tool Friction Stir Welding

The feasibility of producing welding joints between 6061-T6 aluminum and pure copper sheets of 6 mm thickness by conventional friction stir welding (CFSW) and bobbin tool friction stir welding (BTFSW) by using a slot-groove configuration at the joining surface was investigated. The microstructure of the welded samples was examined by using an optical microscope and X-ray diffraction. Furthermore, the mechanical properties of the weld samples are compared based on the results of the tensile test, hardness measurement, and fractography test. The slot-groove configuration resulted in the presence of a bulk-sized Al block on the Cu side. The microscopic observations revealed the dispersion of fine Cu particles in the stir zone. The presence of intermetallic compounds (IMCs) CuAl2, which are hard and brittle, lowered the strength of the weld joints. The strength of the weld joints produced with BTFSW was superior to that of the C-FSW. The maximum hardness values of 214 HV and 211 HV are reported at the stir zone for BTFSW and CFSW, respectively. The fracture location of all the joints was at the intersection of the stir zone and the thermomechanically affected zone was on the Cu side.

Influence of Tool and Welding Parameters on the Risk of Wormhole Defect in Aluminum Magnesium Alloy Welded by Bobbin Tool FSW

Bobbin tool friction stir welding (BTFSW) utilizes a special tool that possesses two shoulders interconnected by a pin instead of one: the top shoulder and the pin in the conventional FSW tool. This greatly simplifies the kinematics in the otherwise complicated setup of FSW since the bottom shoulder forms the bottom surface of the weld, without the need for a backing plate. Moreover, the tool enters the base metal sideways and travels, forming the joint in a straight line while rotating, without the need for downward and upward motion at the beginning and end of the process. This paper presents a study on the BTFSW tool geometry and parameters on the risk of wormhole defect formation in the AA5005 aluminum–magnesium alloy and the wormhole effect on mechanical properties. It was shown that higher stress imposed by the tool geometry on the joint has a significant influence on heating, an effect similar to the increased rotational speed. Optimal kinematic and geometrical tool properties are required to avoid wormhole defects. Although weld tensile strengths were lower (between ~111 and 115 MPa) compared with a base metal (137 MPa), the ductile fracture was obtained. Furthermore, all welds had a higher impact strength (between ~20.7 and 27.8 J) compared with the base material (~18.5 J); it was found that the wormhole defect only marginally influences the mechanical properties of welds.

Bobbin Tool Friction Stir Welding of Aluminum: Parameters Optimization Using Taguchi Experimental Design.

This work aims to optimize the performance evaluation characteristics such as the temperature at the weld center of the lap joint (Tw), the tensile shear load (TSL), and the hardness using an experimental design experiment for bobbin tool friction stir welding (BT-FSW) of AA1050 lap joints. BT-FSW is characterized by a fully penetrated pin and double-sided shoulder that promote symmetrical solid-state welds. This study contributes to improving the quality of 10 mm thick lap joints and addressing challenges to obtaining a sound weld deprived of any defects. Taguchi L9 orthogonal array (OA) experimental design was performed. Three different pin shapes (cylindrical, triangular, and square) and three levels of welding travel speeds of 200, 400, and 600 mm/min were selected as input controllable process parameters at a constant tool rotation speed of 600 rpm. A travel speed of 200 mm/min with square pin geometry significantly improves the TSL of the joint up to 6491 N. However, the hardness characteristic is optimized by using 600 mm/min travel speed and a cylindrical tool pin. The minimum temperature in the weld joint can be obtained using 600 mm/min or more with triangular pin geometry. From ANOVA results, it was seen that the BT-FSW of AA 1050 thick lap joints performance in terms of TLS and Tw were greatly influenced by travel speed; however, the tool shape influences the hardness more. For the validation of the models, BT-FSW experiments have been carried out for AA1050 using the applied processing parameters. Furthermore, regression models were developed to predict the Tw, TSL, and hardness. The calculated performance properties from the mathematical models were in an acceptable range compared to the measured experimental values.