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.     

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.     

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.     

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.     

Analyzing dissimilar friction stir welding of AA5754/AA7075

AA5754/AA7075 was butt-welded by friction stir welding, and the joint of each weld case was identified by ultimate tensile strength, percentage of elongation, and hardness. Moreover, the significance of each parameter was investigated, and a mathematical relation was constructed by regression analysis. A defect-free joint was achieved in the case of a weld produced with 1000 rpm of tool rotational speed, 80 mm/min of welding speed, and an 22-mm tool shoulder diameter. Most of the failures are located at the bottom of the pin and side of AA7075. The ultimate tensile strength (UTS) decreases with increasing welding speed (WS) or increasing tool rotational speed (TRS). Hardness distribution in the weld zones varied dependent on the nugget zone formation affected by TRS and WS. The present study also investigated the significance and contribution of each parameter on the UTS by analysis of variance (ANOVA). From the results of ANOVA, the conclusion reached is that the all the parameters have a great influence on UTS. The contributions are 41.41 % for WS, 17.58 % for diameter, and 13.28 % for TRS. Moreover, a full quadratic model was constructed between the parameters and the UTS value. The results show that the variation from the predicted values was between 0.41 % and 10.36 %. The strength of the model was analyzed by R-Sq. The achieved R-Sq is 0.892, which means that there is a strong relation between predicted and actual values.     

Statistical analysis to predict grain size and hardness of the weld nugget of friction-stir-welded AA6061-T6 aluminium alloy joints

AA6061 aluminum alloy has gathered wide acceptance in the fabrication of light weight structures requiring high strength-to-weight ratio and good corrosion resistance. 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 FSW process and tool parameters play a major role in deciding the joint strength. Joint strength is influenced by grain size and hardness of the weld nugget region. Hence, in this investigation an attempt was made to develop empirical relationships to predict grain size and hardness of weld nugget of friction-stir-welded AA6061 aluminium alloy joints. The empirical relationships are developed by response surface methodology incorporating FSW tool and process parameters. A linear regression relationship was also established between grain size and hardness of the weld nugget of FSW joints.     

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.     

Microstructures and mechanical properties of friction stir welded dissimilar steel-copper joints

Welding dissimilar metals by fusion welding is challenging. It results in welding defects. Friction stir welding (FSW) as a solid-state joining method can overcome these problems. In this study, 304L stainless steel was joined to copper by FSW. The optimal values of the welding parameters traverse speed, rotational speed, and tilt angle were obtained through Response surface methodology (RSM). Under optimal welding conditions, the effects of welding pass number on the microstructures and mechanical properties of the welded joints were investigated. Results indicated that appropriate values of FSW parameters could be obtained by RSM and grain size refinement during FSW mainly affected the hardness in the weld regions. Furthermore, the heat from the FSW tool increased the grain size in the Heat-affected zones (HAZs), especially on the copper side. Therefore, the strength and ductility decreased as the welding pass number increased because of grain size enhancement in the HAZs as the welding pass number increased.     

Study on microstructure and mechanical properties of dissimilar steel joint developed using friction stir welding

In the present study, microstructure and mechanical properties of dissimilar weld of structural steel and ferritic stainless steel (FSS) plates of thickness 3 mm were investigated. The plates were butt welded by friction stir welding and defect-free welds were produced at a traverse speed of 20 mm/min and rotational speed of 508 rpm using a tungsten carbide tool. The weld joint consisted of alternate bands of both steels resembling an onion ring pattern. In the weld joint, six distinct regions were found including both the base metals. The stir zone of structural steel revealed refined grain structure of ferrite, pearlite, and martensite whereas in ferritic stainless side, highly refined ferritic grains with grain boundary martensite was observed and also confirmed by x-ray diffraction (XRD). The hardness of the weld joint varies from 186 to 572 HV. This scatter of hardness in stir zone is due to the presence of metal from both sides. The ultimate tensile and yield strengths of the transverse weld specimens was higher than the structural steel base metal whereas lower than the ferritic stainless steel, having fracture from structural steel side.     

Study of laser butt welding of SUS301L stainless steel and welding joint analysis

This paper describes a study on laser butt welding of 4 and 2 mm SUS301L stainless steel and a detailed analysis of welding joints. The gap tolerance of butt joint was also studied with optimized process parameters. The electrolytic etching in 10 % oxalate solution was used to test the intergranular corrosion of the 4 mm SUS301L welded joint. Fatigue property of the 2 mm SUS301L welded joint was tested under the conditional cycle times of 1?×?10⁷. Using optical microscopy, the changes of metallurgical microstructure in the weld zone of 4 mm SUS301L were also studied. It has been found that laser butt welding of 4 mm SUS301L is able to achieve sound metallurgical morphology and high strength weld joint when the butt gap is within certain tolerance. The weld joint also has good resistance to intergranular corrosion and has a fatigue limit of 310 MPa.     

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.     

Research on the mechanism of flash line defect in coining

In the present article, the effect of friction stir welding (FSW) parameters on the weldability and the characteristics of dissimilar weld of aluminum alloys, called AA2024-T4 and AA7075-O are investigated. A number of FSW experiments are carried out to obtain high-quality welds by adjusting the rotational and welding speeds. The weldability and blending of two materials are evaluated by using the macrostructural analysis to observe whether making a notch in a threaded cylindrical tool will lead to a better blend rather than the threaded taper tool or it will have no effects. The mechanical properties of the welds are studied through microhardness distribution and tensile tests. Furthermore, the microstructure analysis is performed to study the influence of the pin profile and the rotational speed on the grain size. Moreover, in the present study, one of the most major goals is to obtain high-quality welds by spending as little expenditure as possible. Therefore, it prevents using complicated and insupportable high welding speed equipments.     

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.     

Effect of welding speed on microstructure and mechanical properties of friction-stir-welded aluminum

The weld properties remain an area of uncertainty with respect to the effect of different speeds of friction stir welding (FSW). For this purpose, hardened steel tool of FSW was used, which consists of the shoulder and pin. The shoulder of the tool not only provides additional heat generated by friction but also prevents plasticized material to escape. In the present investigation, aluminum welds were made at various welding speed using the FSW technique. The welds were characterized for mechanical properties and microstructural investigation. It is observed that good correlation exists between the mechanical properties and welding speeds. The best mechanical properties were obtained at lower welding speed.     

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.     

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.     

Polyethylene friction stir welding parameter optimization and temperature characterization

The use of friction stir welding (FSW) to join thermoplastics has proven to produce strong welds with good surface quality when compared to conventional welding methods. In this study, a Teflon stationary shoulder was developed to weld 3-mm-thick plates of high molecular weight polyethylene in butt-joint configuration. Different sets of welding parameters were chosen and tested to evaluate their effect on the weld strength. Also, in order to increase joint performance, the temperature generated during welding was measured. For that purpose, thermocouples were located underneath of the weld nugget surface to measure the generated frictional heat for different tool diameters and parameters. Tool diameter and rotational and welding speeds are the most influential parameters regarding the welding temperature; however, all the input parameters had statistically significant effect on the weld quality. Unlike FSW in metals, using this tool, the heat is generated mainly by surface contact of the rotating probe and copper sleeve than the base material. The strongest welded joint was able to withstand 97% of the force that is necessary to fracture the base material, without using an external heating source.     

Modeling of multiple characteristics of an arc weld joint

Process parameters modeling have always been one of the key aspects in development of an adaptive control of arc welding process. The welding process parameters are inherently nonlinear, time-delayed, and interdependent, and their on-time adjustment highly influences a sound weld bead formation and process monitoring. During the welding process, parameters control is the primary goal to leads a quality welding. Moreover, the final weld joint behavior, i.e., residual stress, welding strength, and micro-crack formation are generally observed after cooling of the weld product. Thus, it has always been a difficult task to control mechanical properties of a final weld joint. To obtain the best mechanical properties, the final weld joint characteristics needed to be controlled and predicted during the process itself by precise adjustment of the process parameters. The paper presents a neuro-fuzzy modeling approach to provide adaptive control for the automatic process parameter adjustment. Three input parameters wire feed speed, welding gap, and torch speed are modeled with welding current output, providing control over weld bead formation during the welding. The same input process parameters are also modeled to predict final weld joint characteristics, i.e., dilution ratio, hardness of weld bead, hardness of fused zone, and bead width. In order to ascertain the effectiveness of the neuro-fuzzy modeling approach, multiple regression models were also developed to compare the performances.     

A study on mechanical characteristics of the friction stir welded A6005-T5 extrusion

In this paper, A6005-T5 extruded aluminum alloy sheets which are used for floor, roof or wall panels of railroad vehicles were welded by the friction stir welding (FSW) and gas metal arc welding (GMAW) techniques. The mechanical characteristics including the tensile strength, micro-hardness and fatigue strength of the FSW joint were compared to those of the base metal and GMAW joints. In order to determine the relationship between the welding variables of FSW and the mechanical characteristics of the joint, the response function was derived using the least square method and the sensitivity analysis was performed. The rotational speed, welding speed and tilting angle of the welding tool were chosen as design variables. On the basis of the Plackett-Burman design table, eight different FSW experiments were done, and then the effects of design variables on the mechanical characteristics of the FSW joint were analyzed. The result showed that the welding speed has a most significant effect on the tensile and fatigue strength. In the case of the micro-hardness, the effect of the tilting angle was the biggest.     

Multiresponse optimization of laser welding of stainless steels in a constrained fillet joint configuration using RSM

This paper presents experimental design approach to process parameter optimization for CW Nd/YAG laser welding of ferritic/austenitic stainless steels in a constrained fillet configuration. To determine the optimal welding parameters, response surface methodology was used to develop a set of mathematical models relating the welding parameters to each of the weld characteristics. The quality criteria considered to determine the optimal settings were the maximization of weld resistance length and shearing force, and the minimization of weld radial penetration. Laser power, welding speed, and incident angle are the factors that affect the weld bead characteristics significantly. A rapid decrease in weld shape factor and increase in shearing force with the line energy input in the range of 15–17 kJ/m depicts the establishment of a keyhole regime. A focused beam with laser power and welding speed respectively in the range of 860–875 W and 3.4–4.0 m/min and an incident angle of around 12° were identified as the optimal set of laser welding parameters to obtain stronger and better welds.