Bobbin Friction Stir Welding Research Articles

Correction: Tamadon, A.; et al. Internal Material Flow Layers in AA6082-T6 Butt-Joints during Bobbin Friction Stir Welding. Metals 2019, 9, 1059

In the original publication [...]

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.

Microstructure and tensile behavior of a Bobbin friction stir welded magnesium alloy

In bobbin friction stir welding (BFSW), frictional heat input is simultaneously applied to the top and bottom surface of a workpiece by the rubbing action of the tool shoulders. Consequently, a relatively homogeneous microstructure is obtained in BFSW welds compared to the conventional FSW. In the current study, we performed BFSW bead-on-plate experiments on Mg alloy AZ31B with the goal to study evolution of microstructure and resulting tensile behavior of the welded material. We used three different combinations of intense process parameters which were intentionally chosen to be outside of the norm for BFSW i.e., tool rotational speeds >1500 rpm and constant translational speed of 10 mm/min. The BFS weldments showed the classic heat affected zone (HAZ), a small but discernible thermomechanical affected zone (TMAZ) and a large, near rectangular stir zone (SZ). The SZ consisted of a bimodal microstructure similar to the base material but some degree of grain refinement was also observed, which was more prominent on the retreating side of SZ. BFS welded specimen showed maximum of 67% and 48% joint efficiency (ratio of weld UTS to base material UTS) at room temperature and 200 °C, respectively. Fracture of specimens occurred in TMAZ/SZ interface and SZ, in room and elevated temperature tested specimens, respectively. Large number of twins were observed near fracture surface of the room temperature tensile tested specimen while coalescence of microcracks led to fracture at elevated temperature.

Особенности процессов структурообразования в соединениях сплава Д16, полученных сваркой трением с перемешиванием с инструментом типа «bobbin tool»

Introduction. One of friction stir welding types is the bobbin friction stir welding (BFSW) process, which allows to obtain welded joints in various configurations without using a substrate and axial embedding force, as well as to reduce heat loss and temperature gradient across the welded material thickness. This makes the BFSW process effective for welding aluminum alloys, which properties are determined by their structural-phase state. According to research data, the temperature and strain rate of the welded material have some value intervals in which strong defect-free joints are formed. At the same time, much less attention has been paid to the mechanisms of structure formation in the BFSW process. Therefore, to solve the problem of obtaining defect-free and strong welded joints by BFSW, an extended understanding of the basic mechanisms of structure formation in the welding process is required. The aim of this work is to research the mechanisms of structure formation in welded joint of AA2024 alloy obtained by bobbin tool friction stir welding with variation of the welding speed. Results and discussion. Weld formation conditions during BFSW process are determined by heat input into a welded material, its fragmentation and plastic flow around the welding tool, which depend on the ratio of tool rotation speed and tool travel speed. Mechanisms of joint formation are based on a combination of equally important processes of adhesive interaction in “tool-material” system and extrusion of metal into the region behind the welding tool. Combined with heat dissipation conditions and the configuration of the “tool-material” system, this leads to material extrusion from a welded joint and its decompaction. This results in formation of extended defects. Increasing in tool travel speed reduce the specific heat input, but in case of extended joints welding an amount of heat released in joint increases because of specific heat removal conditions. As a result, the conditions of adhesion interaction and extrusion processes change, which leads either to the growth of existing defects or to the formation of new ones. Taking into account the complexity of mechanisms of structure formation in joint obtained by BFSW, an obtaining of defect-free joints implies a necessary usage of various nondestructive testing methods in combination with an adaptive control of technological parameters directly in course of a welding process.

Internal Flow Behaviour and Microstructural Evolution of the Bobbin-FSW Welds: Thermomechanical Comparison between 1xxx and 3xxx Aluminium Grades

Abstract The influences of processing parameters and tool feature on the microstructure of AA1100 and AA3003 aluminium alloys were investigated using bobbin friction stir welding (BFSW). The research includes flow visualization and microstructural evolution of the weld texture using the metallographic measurement method. Results indicated that the operational parameters of the welding (e.g. feed rate, rotating speed) and the geometry of the tool can directly affect the flow patterns of the weld structure. The microscopic details revealed by the optical and electron microscope imply the dynamic recrystallization including grain refinement and precipitation mechanisms within the stirring zone of the weld region. The microscopic observations for the weld samples show a better performance of the fully-featured tool (tri-flat threaded pin and scrolled shoulders) compared to the simple tool without inscribed surface features. The fully-featured tool resulted in a more uniform thermomechanical plastic deformation within the weld structure along with the precipitation hardening and the homogeneity of the microstructure.

3D-Printed Tool Shoulder Design for the Analogue Modelling of Bobbin Friction Stir Weld Joint Quality

Abstract A variety of tool shoulder designs comprising three families i.e. blade, spiral and circular shaped scrolls, were produced to improve the material flow and restrictions to avoid the tunnel void. The bobbin tools were manufactured by 3D printing additive manufacturing technology using solid filament. The butt weld joint was produced by each tool using plasticine as the workpiece material. The apparent surface features and bi-colour cross-sections provided a physical flow comparison among the shoulder designs. For the bobbin friction stir welding (BFSW), the tool shoulder with a three-spiral design produced the most stability with the best combination of the flow patterns on surface and cross-sections. The circular family tools showed a suitable intermixing on the surface pattern, while the blade scrolls showed better flow features within the cross-sections. The flow-driven effect of the shoulder features of the bobbin-tool design (inscribed grooves) was replicated by the 3D-printed tools and the analogue modelling of the weld samples. Similar flow patterns were achieved by dissimilar aluminium-copper weld, validating the accuracy of the analogue plasticine for the flow visualization of the bobbin friction stir welding.

EBSD Characterization of Bobbin Friction Stir Welding of AA6082-T6 Aluminium Alloy

Abstract Electron Backscatter Diffraction (EBSD) was used to determine microstructural evolution in AA6082-T6 welds processed by the Bobbin Friction Stir Welding (BFSW). This revealed details of grain-boundaries in different regions of the weld microstructure. Different polycrystalline transformations were observed through the weld texture. The Stirring Zone (SZ) underwent severe grain fragmentation and a uniform Dynamic Recrystallisation (DRX). The transition region experienced stored strain which changed the grain size and morphology via sub-grain-boundary transformations. Other observations were of micro-cracks, the presence of oxidization, and the presence of strain hardening associated with precipitates. Flow-arms in welds are caused by DRX processes including shear, and low and high angle grain boundaries. Welding variables affect internal flow which affects microstructural integrity. The shear deformation induced by the pin causes a non-uniform thermal and strain gradient across the weld region, leading to formation of mixed state transformation of grain morphologies through the polycrystalline structure. The grain boundary mapping represents the differences in DRX mechanism I different regions of the weld, elucidates by the consequences of the thermomechanical nature of the weld. The EBSD micrographs indicated that the localised stored strain at the boundary regions of the weld (e.g. flow-arms) has a more distinct effect in emergence of thermomechanical nonuniformities within the DRX microstructure.

Evolution of grain structure, texture and mechanical properties of a Mg–Zn–Zr alloy in bobbin friction stir welding

The evolutions of grain structure, microtexture and mechanical properties during bobbin friction stir welding of Mg-5.89 wt%Zn-0.61 wt%Zr alloy were investigated. The electron backscattered diffraction analyses revealed that the grain structure evolution was a complicated process dictated by continuous dynamic recrystallization but also involved geometrized effects of strain and {10–12} twinning. Local texture varied significantly from the parent metal to the weld nugget. With increased access to the weld nugget, the initial <0001>||ND texture gradually reoriented towards <0001>||TD and finally transformed into a near-<0001>||WD texture. Arising of the near-<0001>||WD texture was associated with the extensive continuous dynamic recrystallization and the reorientations of the recrystallized grains under the effect of shearing deformation introduced by the rotating stir probe. Microhardness distribution exhibited weak grain size dependence. Transverse tensile elongation of the joint was lower than that of the parent metal owing to strain localization in thermo-mechanically affect zone (TMAZ)/weld nugget zone (WNZ) interface region. Decomposition of β1' precipitates and texture evolutions in WNZ and TMAZ, as well as dislocations concentration and grain size mutation existed in the interface region corporately resulted in the inhomogeneous plastic deformation.

Characterization of Dissimilar Al-Cu BFSW Welds; Interfacial Microstructure, Flow Mechanism and Intermetallics Formation

Abstract The purpose of this study is to elucidate the flow features of the dissimilar Al-Cu welded plates. The welding method used is Bobbin Friction Stir Welding (BFSW), and the joint is between two dissimilar materials, aluminium alloy (AA6082-T6) and pure copper. Weld samples were cut from along the weld line, and the cross-sections were polished and observed under an optical microscope (OM). Particular regions of interest were examined under a scanning electron microscope (SEM) and analysed with Energy Dispersive X-ray Spectroscopy (EDS) using the AZtec software from Oxford Instruments. The results and images attained were compared to other similar studies. The reason for fracture was mainly attributed to the welding parameters used; a higher rotational speed may be required to achieve a successful BFSW between these two materials. The impact of welding parameters on the Al-Cu flow bonding and evolution of the intermetallic compounds were identified by studying the interfacial microstructure at the location of the tool action. The work makes an original contribution to identifying the solid-phase hybrid bonding in Al-Cu joints to improve the understanding of the flow behaviours during the BFSW welding process. The microstructural evolution of the dissimilar weld has made it possible to develop a physical model proposed for the flow failure mechanism.

Analogue Modelling of Flow Patterns in Bobbin Friction Stir Welding by the Dark-Field/Bright-Field Illumination Method

Abstract The flow-inducing effect of the bobbin-tool features (tri-flat pin and scrolled shoulder) were replicated by a simple analogue model for aluminium welds by layered plasticine samples. Flow patterns of the weld zone were clarified by a typical stereomicroscopy instrument assisted by dark-field/bright-field illumination. The effects of the pin features, specifically threads and flats in centre of bond zone and scrolled shoulder in sides of stirred zone, were identified. This study shows that internal flow features for BFSW welds is transferable from the friction stir welding process to the functional metal forming processes such where the shearing can extensively affect the microstructure. The similarity between the flow pattern of the provided aluminium samples and the plasticine analogue can validate the accuracy of the flow model presented in this work.

Structural Anatomy of Tunnel Void Defect in Bobbin Friction Stir Welding, Elucidated by the Analogue Modelling

The potential position for tunnel defect within the structure of bobbin-tool friction stir welds was studied by analogue modelling. The welding process was simulated on layered plasticine slabs instead, compared to the aluminum plates. Observations in the modelled structure showed a high possibility for a continuous channelled discontinuity, like a tunnel-shaped void defect, in the entry zone of the weld, which mirrors the metal welding. The anatomy of tunnel defect in the entry zone was explained according to the mechanics of material during the plastic deformation process.

AFM Characterization of Stir-Induced Micro-Flow Features within the AA6082-T6 BFSW Welds

Bobbin Friction Stir Welding (BFSW) is a thermomechanical process containing severe plastic deformation by mechanical stirring and Dynamic Recrystallization (DRX) during recooling. Here we report the three-dimensional characteristics of the micro-flow patterns within the aluminium weld structure. The Surface topography observations by Atomic Force Microscopy (AFM) show the stirred-induced microstructural evolution where the rearrangement of dislocations at the sub-grain scale, and the subsequent High- and Low-Angle Grain Boundaries (HAGBs, LAGBs) exhibit specific alterations in grain size and morphology of the weld texture. The dislocations interaction in different regions of the weld structure also was observed in correlation to the thermomechanical behaviour of the BFSW process. These micro-flow observations within the weld breadth give a new insight into the thermomechanical characteristics of the FSW process during the stirring action where the plastic flow has a key role in the formation of the weld region distinct from the base metal.

Texture Evolution in AA6082-T6 BFSW Welds: Optical Microscopy and EBSD Characterisation.

One of the difficulties with bobbin friction stir welding (BFSW) has been the visualisation of microstructure, particularly grain boundaries, and this is especially problematic for materials with fine grain structure, such as AA6082-T6 aluminium as here. Welds of this material were examined using optical microscopy (OM) and electron backscatter diffraction (EBSD). Results show that the grain structures that form depend on a complex set of factors. The motion of the pin and shoulder features transports material around the weld, which induces shear. The shear deformation around the pin is non-uniform with a thermal and strain gradient across the weld, and hence the dynamic recrystallisation (DRX) processes are also variable, giving a range of observed polycrystalline and grain boundary structures. Partial DRX was observed at both hourglass boundaries, and full DRX at mid-stirring zone. The grain boundary mapping showed the formation of low-angle grain boundaries (LAGBs) at regions of high shear as a consequence of thermomechanical nature of the process.

Internal Material Flow Layers in AA6082-T6 Butt-Joints during Bobbin Friction Stir Welding

Bobbin friction stir welding with a double-sided tool configuration produces a symmetrical solid-state joint. However, control of the process parameters to achieve defect-free welds is difficult. The internal flow features of the AA6082-T6 butt-joints in bobbin friction stir welding were evaluated using a set of developed reagents and optical microscopy. The key findings are that the dark curved patterns (conventionally called 'flow-arms'), are actually oxidation layers at the advancing side, and at the retreating side are elongated grains with a high-density of accumulation of sub-grain boundaries due to dynamic recrystallization. A model of discontinuous flow within the weld is proposed, based on the microscopic observations. It is inferred that the internal flow is characterized by packets of material ('flow patches') being transported around the pin. At the retreating side they experience high localized shearing at their mutual boundaries, as evidenced in high density of sub-grain boundaries. Flow patches at the advancing side are stacked on each other and exposed to oxidization.

Influence of Bobbin tool design on Quality of welds made by FSW of Aluminium Alloys

Abstract Friction Stir Welding (FSW) an Eco-friendly process developed by The Welding Institute (TWI), UK, in 1991. Since its inception, sufficient developments have taken place in this area which we call as conventional or standard friction stir welding (CFSW). But the area of double shoulder or self-reacting tool is very lean in the literature and has a potential to explore it further. There is a need to understand the causality to study the effect of tool design parameters on weld quality. The heat developed in case of Bobbin Friction Stir Welding (BFSW) is different than CFSW due to two shoulders; and hence care must be taken in designing the tool. In general, FSW considered to be defect free solid state joining process, but it is not always true. Root canal, surface tunnel, joint line remnant, voids are some of the defects observed. The present paper will throw some light in the direction of development of bobbin tools, so that an idea will be conceived regarding design requirements and limitations in the fabrication of fixed gap bobbin tools. The tools developed were validated by actual experimental investigations and characterization for different aluminium alloys. A square pin fixed gap bobbin tool was found to be suitable for defect free welding of Al alloys.

Experimental Study the Effect of Tool Design on the Mechanical Properties of Bobbin Friction Stir Welded 6061-T6 Aluminum Alloy

اللحام بالخلط الاحتكاكي بأداة تشبه البكرة (bobbin) يكون مختلف عن اللحام المتعارف علية للطريقة نفسها, حيث يتم لحام قطعة العمل من الوجهين الاعلى و الاسفل في للشوط نفسها . يتضمن هذا الأسلوب لحام اللوحين من دون الانصهار. في هذا العمل، تم التحقق من تأثير تصميم الأداة على الخواص الميكانيكية لوصلات اللحام لسبيكة الالمنيوم نوع (6061-T6) و بسمك (6,25 ملم) والملحومة بطريقة اللحام الخلطي الاحتكاكي نوع (bobbin)،و كذلك تحديد أفضل تصميم لأداة اللحام نوع (bobbin). خمسة أشكال مختلفة لوتد أداة اللحام (أسطواني مستقيم مسنن, أسطواني مستقيم ذو 3 أوجه، أسطواني مستقيم ذو 4 أوجه، أسطواني مستقيم مسنن ذو 3 أوجه و أسطواني مستقيم مسنن ذو 4 أوجه) وبأبعاد مختلفة للأداة ( الدبوس و الأكتاف ) استخدمت لأجراء عملية اللحام. أتجاه عملية اللحام كان عموديا على أتجاه الدرفلة لألواح الألمنيوم. اختبارات الشد والانحناء تم اجراؤها لأختيار التصميم الأفضل لأداة اللحام نوع (bobbin) والتي تعطي افضل الخواص الميكانيكية لمنطقة اللحام. الأداة ذات الاسطواني المستقيم ذو 4 أوجه، بقطر للوتد (8 mm) والاكتاف (24 mm) أعطت افضل مقاومة شد (193 ميكا باسكال) وأستطالة (6.1%) مع أقصى قوة انحناء (5.7 كيلو نيوتن) وأفضل كفاءة لحام (65.4%) نسبة الى مقاومة الشد.

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.

Experimental Study the Effect of Tool Design on the Mechanical Properties of Bobbin Friction Stir Welded 6061-T6 Aluminum Alloy

Bobbin friction stir welding (BFSW) is a variant of the conventional friction stir welding (CFSW); it can weld the upper and lower surface of the work-piece in the same pass. This technique involves the bonding of materials without melting. In this work, the influence of tool design on the mechanical properties of welding joints of 6061-T6 aluminum alloy with 6.25 mm thickness produced by FSW bobbin tools was investigated and the best bobbin tool design was determined. Five different probe shapes (threaded straight cylindrical, straight cylindrical with 3 flat surfaces, straight cylindrical with 4 flat surfaces, threaded straight cylindrical with 3 flat surface and threaded straight cylindrical with 4 flat surfaces) with various dimensions of the tool (shoulders and pin) were used to create the welding joints. The direction of the welding process was perpendicular to the rolling direction for aluminum plates. Tensile and bending tests were performed to select the right design of the bobbin tools, which gave superior mechanical properties of the welded zone. The tool of straight cylindrical with four flats, 8 mm probe and 24 mm shoulders diameter gave better tensile strength (193 MPa), elongation (6.1%), bending force (5.7 KN), and welding efficiency (65.4%) according to tensile strength.

Thermomechanical Grain Refinement in AA6082-T6 Thin Plates under Bobbin Friction Stir Welding

Bobbin friction stir welding (BFSW), with its fully penetrated pin and double-sided shoulder, can provide high rates of heat generation. This produces solid-state thermo-mechanical grain refinement. In this paper, the microstructure evolution of the welded joints of AA6082-T6 obtained using BFSW process was investigated with a focus on grain refinement. Two sheets of the AA6082-T6 alloy were butt-welded with a fixed-gap bobbin tool. The microstructure at a mid-weld transverse cross-section was evaluated using optical microscopy and electron backscatter diffraction (EBSD). Significant grain refinement was observed, with a decrease in grain size from 100 μm in directional columnar grain morphology of the base metal, to an ultrafine size—less than 10 μm—for the equiaxed grains in the stirring zone. The EBSD results showed that with BFSW processing, secondary phase precipitation patterns were produced that are distinct from the primary artificial age-hardening precipitates created by the T6 tempering cycle. The severe plastic deformation and heat generation appear to accelerate dynamic recrystallization and precipitation during the BFSW process. The microstructural studies confirmed that the BFSW process can provide a highly efficient thermodynamically activated grain refinement in the solid-state without requiring additional processes such as heat treatment or external means of grain refinement.

Sustainability of Welding Process through Bobbin Friction Stir Welding

Welding process is in high demand, which required a competitive technology to be adopted. This is important for sustaining the needs of the joining industries without ignoring the impact of the process to the environment. Friction stir welding (FSW) is stated to be benefitting the environment through low energy consumption, which cannot be achieved through traditional arc welding. However, this is not well documented, especially for bobbin friction stir welding (BFSW). Therefore, an investigation is conducted by measuring current consumption of the machine during the BFSW process. From the measurement, different phases of BFSW welding process and its electrical demand are presented. It is found that in general total energy in BFSW is about 130kW inclusive of all identified process phases. The phase that utilise for joint formation is in weld phase that used the highest total energy of 120kWs. The recorded total energy is still far below the traditional welding technology and the conventional friction stir welding (CFSW) energy demand. This indicates that BFSW technology with its vast benefit able to sustain the joining technology in near future.