Bobbin Tool Friction Stir Welding Research Articles

Investigation on the Effects of Welding Speed on Bobbin Tool Friction Stir Welding of 2219 Aluminum Alloy

The microstructure and mechanical properties of bobbin tool friction stir welded joints for 2219-T87 aluminum alloy using different welding speeds were investigated based on the experimental results together with a thermo-mechanical model developed. The stir zone created in the joint shows a dumbbell shape. As welding speed increases, both the width and grain size of stir zone decrease. The symmetrical hardness distribution forms across the joint thickness, while the asymmetric hardness is measured along the width of the stir zone. The hardness of the stir zone on the retreating side is lower than that on the advancing side. The hardness profile presents a W-shape, with the minimum values measured increasing with welding speed. The tensile residual stresses increase gradually towards the center of the joint from the outer surface, presenting a symmetrical feature as well. As welding speed increases, both the tensile strength and elongation show a maximum value 70% of joint efficiency. Tensile fracture occurs in the stir zone and shows a symmetrical surface with three different layer fracture modes identified. In layers I and III, the fracture surface presents 45° direction, while fracture propagates along the S-line in layer II.

Effect of Tool Rotational Speed on the Microstructure and Mechanical Properties of Bobbin Tool Friction Stir Welded 6082-T6 Aluminum Alloy

Samples of 6082-T6 aluminum alloy were welded by bobbin tool friction stir welding at different rotational speeds. The thermal cycles, microstructure, microhardness, and tensile properties of the specimens were investigated. The results show that the maximum temperature at the joint increases first and then decreases with increasing rotational speed, and the maximum temperature is 509 °C at 1000 r/min. The macromorphology of the cross-section of the joint is rectangular, and an ‘’S” line and gray-white texture can be observed. The stirred zone had much smaller equiaxed recrystallized grains. With increasing welding speed, the average grain size in the stirred zone region decreases. The microhardness distribution of the cross-section of all joints is W-shaped. When the rotational speed increases, the hardness of the heat-affected zone decreases gradually, and the hardness of the stirred zone increases. At 600 r/min, the strength is the lowest. The fracture location is between the stirred zone and the thermomechanically affected zone. When the rotational speed is increased, the fracture location is entirely located in the heat affected zone, and the fracture surface is dimple-like; the strength significantly increases and reaches a maximum at 800 r/min.

Effects of postweld aging on the microstructure and properties of bobbin tool friction stir-welded 6082-T6 aluminum alloy

Samples of 6082-T6 aluminum alloy were subjected to bobbin tool friction stir welding (BT-FSW), and the joints were treated by postweld natural aging (PWNA) and postweld artificial aging (PWAA). The microstructure, microhardness, and tensile properties of the aged and as-welded specimens were investigated. Transmission electron microscopy (TEM) observations revealed that a large number of Guinier-Preston (GP) zones precipitated in the form of a network on the stir zone (SZ) after PWNA for 60 d, and a large number of β″ phases precipitated in the matrix for after PWAA for 6 h. As the aging time increased, the microhardness of the SZ and the thermomechanically affected zone (TMAZ) increased significantly, and the hardness of the SZ after PWAA for 6 h was close to that of the base metal (BM). With increasing PWNA time, the strength and strain increased slightly. When the PWAA time increased, the strength clearly increased, with a maximum value of 279.9 MPa after 6 h, while the strain decreased.

Influence of pin geometry on mechanical properties of 5A05-H112 aluminum alloy during bobbin-tool friction stir welding

In the work described in this paper, the 10 mm-thick 5A05-H112 aluminum alloy plates were welded successfully by bobbin-tool friction stir welding with two pins of the same length but different geometries. Results show that better mechanical properties of weld joints can be obtained when a pin with no plane is used with the same weld parameters. The shape of the pin influences the flow behavior of the plastic metal and could change weld contour. The macro-appearance is ‘dumbbell’-shaped when a pin with three planes is used, while the macro-appearance is ‘drum’-shaped when a pin with no plane is used. Using a pin with no plane, the tensile strength of the weld joint is 95% of the base metal and liquid phase formation was found in the local area of the axial shoulder. The weld joint is broken between the heat-affected zone and thermo-mechanical affected zone of the retreating side, the joint exhibits ductility fracture.

Influence of Cu/Li ratio on the microstructure evolution of bobbin-tool friction stir welded Al–Cu–Li alloys

Two modern aluminum lithium alloys were welded by semi-stationary bobbin tool friction stir welding. The influence of the Cu/Li ratio on precipitation phenomena under process heat impact was investigated by comparing the response of low Cu/Li alloy 2196-T8 and high Cu/Li alloy 2060-T8. Identical process parameters with a weld pitch of one rotation per mm were used to conduct flawless weldments. The thermal history and microstructural features were studied and correlated to the resulting mechanical properties of the welds. Analysis of microstructure using differential scanning calorimetry and high energy X-ray diffraction technique showed significant differences in the precipitation sequence of the base metal and in the welded samples of the two alloys of interest. A low Cu/Li ratio led to a higher softening resulting in a reduction of 43% of base metal yield strength while the high Cu/Li ratio alloy AA 2060 could demonstrate more thermal stability (38% reduction). Severe dissolution of the T1 precipitate and presence of equilibrium phases were confirmed for the stirred zone of both alloys. The heat affected zone suffered dissolution and overaging reactions leading to a mechanically unfavorable microstructure. The low Cu/Li alloy 2196 developed a higher process temperatures and exhibited a more evolved precipitation sequence.

Influence of Traverse Speed in Self-Reacting FSW of AA6061-T6

Influence of Traverse Speed in Self-Reacting FSW of AA6061-T6

Effects of shoulder fillet radius on thickness distribution, microstructure, and mechanical properties in bobbin tool friction stir welding

Effects of shoulder fillet radius on thickness distribution, microstructure, and mechanical properties in bobbin tool friction stir welding

An Analytical Model of Heat Generation for Friction Stir Welding Using Bobbin Tool Design

A mathematical model for heat generation for bobbin tool friction stir welding (BT-FSW) is proposed. The model applies on bobbin tool with Cylindrical pin to weld AA1050 alloy. The BT-FSW joints are welded at various travel speeds (200, 400, 600, 800, and 1000 mm/min) and constant rotation speed of 600 rpm. In the proposed model, only the heat generated by friction is considered. The proposed mathematical model was validated with measured results. The BT-FSW heat generation increase with increasing bobbin rotation speed, friction coefficient, shoulder radius, and pin radius. On the other hand, the heat generation decrease with increasing bobbin tool travel speed.

Numerical simulation and experimental investigation of band patterns in bobbin tool friction stir welding of aluminum alloy

A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the formation mechanism of band pattern (BP) during bobbin tool friction stir welding (BTFSW) process. The Johnson-Cook constitutive model which combines the strain rate and temperature effects in the material flow stress was used. The workpiece was modeled by the Eulerian formulation, while the tool was characterized by the Lagrangian formulation. The numerical analysis results agree well with experimental observations. The band structures with various sizes were discovered in the BP zone. Both the experimental and numerical analysis results indicate the BP zone forms on the advancing side of the joint. This is because the converged material flow is produced by both shoulders while being driven by the probe. The peak temperature, along the thickness of the BP zone, increased at the beginning followed by a decrease further away from the upper surface. The peak temperature obtained near the lower surface is slightly higher than that near the upper surface.

Development of a welding platform and tool for the study of weld and process parameters, during continuous friction stir welding of AA6082-T6 sheets

Bobbin tool friction stir welding is a relatively novel technology whose application, despite its benefits, is still limited due to unfamiliarity presented by less published literature. Advantages associated with the bobbin-tool technique lie imbedded in the resultant double-sided processed zone, of somewhat rectangular cross section, along the joint line. Currently, the joint integrity benefits are overshadowed by high setup costs associated with slightly more complex tool and platform designs of bobbin-tool friction stir welding. To largely exploit and optimise the technique, there is need for an in-depth understanding of interaction between welding parameters and response variables. This comprehension could assist with process optimisation, reproducibility, automation and possibly process economic feasibility. This paper proposes design considerations with regards to development of a continuous solid-state welding platform and tool, for instrumentation of process output variables. As an instance, upon tool and platform development, calibration and verification, data acquisition of weld forces developed during bobbin-tool friction stir welding, as a function of process time, can then be implemented to enable analysis. Feasibility of the proposed methodology is then left for evaluation in future work. Thus, analysis of weld forces can be facilitated by the design and development of an instrumented fixture and a bobbin friction stir welding tool, in joining AA6082-T6 aluminium plates.

A comparative research on bobbin tool and conventional friction stir welding of Al-Mg-Si alloy plates

In our work, Al-Mg-Si (6061Al -T4) plates of 6 mm thickness were subjected to bobbin tool friction stir welding (BT-FSW) and conventional friction stir welding (C-FSW) for comparison. How the welding process affects the microstructure and mechanical properties was investigated at various rotation rates and welding speeds. The results showed that butt FSW joints with high quality could be produced at the selected parameters. The joint strength rose with enhanced welding speed and was nearly independent of the rotation rate in both the welding processes. The strength of the joints produced with BT-FSW reached the same level as that of the C-FSW. The maximum joint strength of 229 MPa was 93% of the base material (BM), which is superior to the reported results of 60–80% for the T4 condition. The fracture position of most of the joints was in the heat-affected zone (HAZ), which is the lowest hardness zone.

Influence of Cu/Li Ratio on the Welding Forces and Mechanical Properties of Two Bobbin Tool Friction Stir Welded Al-Cu-Li Alloys

Increasing space activities produce a high number of space objects and lead to increasing collision risks which urges leading industries to look for future removal strategies. Material substitution in order to raise demisability during atmospheric reentry is a possible solution. Modern aluminum lithium alloys are under consideration to replace high-temperature melting materials like titanium. Further, friction stir welding was proposed as suitable joining technology in order to avoid high heat inputs during manufacturing. In this work, two modern Al-Li-Cu alloys, AA 2060 and AA 2196, in peak-aged temper were welded using the stationary shoulder variant of bobbin tool friction stir welding. Identical process parameters led to defect-free welds in both alloys. The macrostructural and microstructural features are shown and analyzed. The welds were mechanically tested to an efficiency of 78 and 70% of the base metal ultimate tensile strength for AA 2060 T8 and AA 2196 T8, respectively. The process forces as well as the thermal cycle experienced by the workpiece material were used to explain the mechanical performance. The difference in composition regarding the Cu/Li ratio of the alloys was taken into account when the mechanical properties were correlated with the thermally affected microstructure of the weldments.

Microstructure evolution in the conventional single side and bobbin tool friction stir welding of thick rolled 7085-T7452 aluminum alloy

The microstructure evolution during the conventional single side friction stir welding (SS-FSW) and the bobbin tool FSW (BB-FSW) of 7085-T7452 alloy thick plate was extensively studied by using the state-of-the-art materials characterization techniques. The Electron backscattering diffraction results reveal that a significant grain refinement and a larger percentage of high angle grain boundaries (HAGBs) exist in the weld nugget zone (WNZ) and the thermal-mechanical affected zone (TMAZ) compared with the base material. Meanwhile, more obviously inhomogeneous refined grains and HAGBs in WNZ along the thickness direction and a larger percentage of low angle grain boundaries (LAGBs) were observed in SS-FSW joint than that in BB-FSW joint. The change of the average grain size is the result of friction heat and strain rate of plastic deformation combined with a certain degree of recrystallization. Moreover, the recrystallization is mainly controlled by the strain rate during the FSW process. The presence of Cube and Cube ND after welding shows that the discontinuously dynamic recrystallization (DDRX) is a major factor in the final microstructural response. The research and findings help the understanding of the mechanism of microstructure homogenization and further optimizing the welding process for application.

Microstructural Characteristics and Mechanical Properties of Water Cooling Bobbin-Tool Friction Stir Welded 6063-T6 Aluminum Alloy

In this study, a novel welding method called water cooling bobbin-tool friction stir welding (WBT-FSW) was developed. 4 mm-thick 6063-T6 aluminum alloy sheets were successfully jointed by WBT-FSW. Comparative studies on macro/microstructural characteristics and mechanical properties of the WBT-FSW and conventional bobbin-tool friction stir welding (BT-FSW) joints were carried out. The results indicated that the water mist cooling can significantly decrease the welding temperature and improve both the weld formation and the mechanical properties of the joint. The tensile strength of the WBT-FSW joint was 11.4% higher than that of BT-FSW joint.

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410MPa, 82% of the base material, at a rotational speed of 300rpm and welding speed of 60mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.

Improving weld formability by a novel dual-rotation bobbin tool friction stir welding

A novel dual-rotation bobbin tool friction stir welding (DBT-FSW) was developed, in which the upper shoulder (US) and lower shoulder (LS) have different rotational speeds. This process was tried to weld 3.2mm thick aluminum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formability. Experimental results show that compared to conventional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process.

Process optimization in the self-reacting friction stir welding of aluminum 6061-T6

Self-reacting friction stir welding (SR-FSW), also called bobbin-tool friction stir welding (BT-FSW), is a solid state welding process similar to friction stir welding (FSW) except that the tool has two opposing shoulders instead of the shoulder and a backing plate found in FSW. The tool configuration results in greater heat input and a symmetrical weld macrostructure. A significant amount of information has been published in the literature concerning traditional FSW while little has been published about SR-FSW. An optimization experiment was performed using a factorial design to evaluate the effect of process parameters on the weld temperature, surface and internal quality, and mechanical properties of self-reacting friction stir welded aluminum alloy 6061-T6 butt joints. The parameters evaluated were tool rotational speed, traverse speed, and tool plunge force. A correlation between weld temperature, defect formation (specifically galling and void formation), and mechanical properties was found. Optimum parameters were determined for the welding of 8-mm-thick 6061-T6 plate.

Microstructural characteristics and mechanical properties of bobbin-tool friction stir welded 2024–T3 aluminum alloy

Cold-rolled 2024–T3 sheet alloy was subjected to bobbin-tool friction stir welding (BTFSW). The microstructural characteristics and mechanical properties of the nugget zone in the as-welded state were investigated. The results show that the equiaxed grain size of BTFSW 2024–T3 alloy decreases from 7.6 to 2.8 μm as the welding speed is increased from 80 to 120 mm/min; in addition, fine grains are generated in the nugget zone and the size distribution is non-uniform. All Al2CuMg (S′) precipitates dissolve into the Al matrix, whereas Mn-rich phases confirmed as T phases (Al20Cu2Mn3, Al6Mn, or Al3Mn) remain unchanged. The optimized parameters for BTFSW are verified as the rotation speed of 350 r/min and the travel speed of 100 mm/min. The variations in precipitation and dislocation play more important roles than grain size in the nugget zone with respect to influencing the mechanical properties during the BTFSW process. After the BTFSW process, the fracture mode of base material 2024–T3 alloy transforms from ductile rupture to ductile-brittle mixed fracture.

Empirical modeling for the effects of welding factors on tensile properties of bobbin tool friction stir-welded 2219-T87 aluminum alloy

For the joints of bobbin tool friction stir welding (BT-FSW), welding factors such as welding speed, tool rotation speed, and shoulder pinching gap have significant effects on the ultimate tensile strength (UTS) and tensile elongation (TE). This work developed empirical models to describe the relationships between the welding factors and the tensile properties of bobbin tool friction stir-welded 2219-T87 aluminum alloy. Based on the models, the factor effects can be analyzed quantitatively and graphically, and the UTS and TE of the joints can be predicted and optimized effectively. Firstly, experiments were carried out according to a three-factor and five-level central composite design (CCD). Secondly, empirical models were developed by fitting experimental data points. Adequacies of the models were checked by the analysis of variance (ANOVA). Thirdly, the analysis and optimization processes were conducted. Based on the expressions and plots of the developed models, the individual and interactive effects of the welding factors were investigated. By the multiple-desirability function and three-dimensional response surface methodology (RSM), the optimum welding factors were calculated from the models. The validation trial has been carried out, and the result shows that the models are adequate for predicting and optimizing the BT-FSW factor effects.

Study on the characteristics of the torque vibration and the detection of the gap

Bobbin tool friction stir welding (BTFSW) is a new solid-state welding technology that has been developed successfully in many joining applications, such as thick plate and hollow component welding. The existence of the gap can possibly cause an unstable welding, and the variety of the torque is closely related to the existence of the gap. Compared with the conventional friction stir welding (FSW), BTFSW has the different tool and welding process, especially some new feature of the periodic vibration signal arises when welding two sheets in the presence of gaps. Compared with vibration-accelerated signal, torque signal is capable of reflecting the vibration of the welding process. The aim of this study was to develop an effective algorithm to detect gaps in BTFSW operations. Experimental studies were conducted to determine the features of welding process parameters and the gap. Time-frequency (TF) analysis provides a display of subtle torque features, shows the corresponding duration of low frequency signal clearly. Followed by a low-pass filter, gauss wavelet transform (WT) algorithm would improve the detection and identification of the gap presence. Several experimental studies with a variety of process parameters and proposed algorithm was shown to be able to reliably detect the presence of gaps in BTFSW operations for traverse speeds below 200 mm/min and gap sizes above 0.3 mm. This gauss WT algorithm could become a useful noninvasive tool for welding quality assessment in practice.