1 Introduction

In the early 1980s, aluminum emerged as a promising candidate for advanced structural applications due to its exceptional properties and cost efficiency compared to conventional materials [1]. However, fabricating aluminum using conventional fusion welding processes posed significant challenges due to severe defects in welded structures, resulting in frequent joint failures [2]. Addressing this welding dilemma led to the development of friction stir welding (FSW) by The Welding Institute (TWI) in 1991 [3]. FSW, known for its environmentally friendly, energy-efficient, solid-state welding nature, initially found extensive use in welding nonferrous materials [4]. Over time, its application has expanded to encompass steels, Ti alloys, Ni-based superalloys, ferrous metals, composites, and polymers [5].

Since the inception of FSW, researchers have diligently explored various facets, ranging from microstructural analysis and mechanical properties to practical applications. Among the myriad challenges in FSW, the joint configuration stands as a pivotal concern, as the overall quality of the welding structure hinges significantly upon it. Ideally, a welding joint configuration should not only be easy to prepare but also cost-effective in its manufacturing process. Despite its profound influence on joint quality, the literature on joint configuration in FSW remains relatively limited compared to other domains within this field. To address this gap, current research endeavors revolve around tailoring existing basic joint configurations, such as the simple square butt (SSB) and lap configurations. These modifications aim to yield more robust and defect-free joints, offering superior benefits compared to those achieved through basic joint configurations.

For instance, Cam et al. [6] introduced a modified butt joint configuration for joining Al-6 Mg to Ti-6Al-4 V alloy, achieving high-quality joints while preventing aluminum melting and reducing tool shoulder attrition. Their results demonstrated excellent joint formability and observed a significant strength equivalent to 92% of the parent alloy. Likewise, Li et al. [7] utilized a unique design for a butt joint configuration, joining Ti-6Al-4 V titanium alloy with 5A06 aluminum alloy, showcasing enhanced formability in joint macrostructures and improved material flow characteristics. This led to a subsequent increase in joint strength. In a separate study, Zhang et al. [8] investigated the impact of employing a tooth-shaped joint configuration technique to join AA6061 with commercially pure copper. The result was an enhancement in surface morphology and material flow patterns conducive to lamellar flow, ultimately achieving a ductile joint by minimizing brittle fracture regions. Their subsequent work aimed at altering different joint configurations to achieve overall mechanical improvements [9]. Additionally, Jayabalakrishnan et al. [10] implemented a new joint geometry to join AA 6061 and steel, effectively overcoming distortion effects while joining thin sheets. Similarly, Goel et al. [11] reported on the analysis of joining AA6063-T6 by FSW in a scarf joint configuration. These studies collectively contribute to the evolving landscape of weld joint configurations, though further details and works on weld joint configurations are available elsewhere [12].

Upon reviewing existing literature, it is evident that the proposed joint configurations in FSW have certain limitations that affect their widespread application. These limitations encompass various critical aspects: firstly, configurations tailored for dissimilar plate joining might not be suitable for similar materials. Secondly, designs intended for different plate thicknesses might not effectively apply to plates of identical thicknesses. Thirdly, certain configurations have complex shapes, resulting in costly manufacturing processes. These limitations have driven the need to develop a joint configuration that is easy to manufacture and adaptable across material types and thicknesses and ultimately enhances joint mechanical properties. Alternatively, this new configuration seeks to overcome limitations observed in conventional setups. In previous research [13,14,15,16], a comparison between the SSB and the double-butt-lap (DBL) joint configurations was demonstrated. The studies encompassed similar materials (Al-Al) as well as dissimilar materials (Al–Mg), consistently indicating superior mechanical properties in DBL-configured weld joints over SSB. Notably, these studies utilized a consistent DBL joint geometry across investigations. It was consistently observed that the optimal result was achieved when the upper butt (UB) of the DBL joint configuration extended towards the retreating side (RS), indicating the advantage of an RS-oriented UB offset. Placing the UB in the AS beneath the shoulder aims to optimize heat utilization. This positioning capitalizes on the higher heat and shear stress on the advancing side (AS) compared to the RS, ensuring better plasticization under the shoulder, primarily contributed by the AS [17]. By minimizing the volume of material on the RS plate under the shoulder at the RS in the DBL joint configuration, the goal is to maximize the additional heat and shearing action at the AS. Given that the DBL joint configuration is relatively new, it requires exploration in various directions to propose optimum joint geometries tailored for different materials based on their unique types, properties, and thicknesses. DBL stands as a novel technique in FSW, specifically targeting the joining of plates with equal thickness.

2 Experimental details

Figure 1 illustrates a modified DBL joint design tailored to achieve the optimal joint geometry for welding 6-mm-thick AA6061-T6 plates, aiming for superior mechanical properties across various joint configurations. This innovative DBL joint configuration enhances the contact surface within the nugget zone (NZ) compared to the conventional SSB joint used in FSW. The design is straightforward and can be easily prepared using a conventional milling machine, much like the SSB joint. It involves three distinct faying surfaces positioned relative to the weld centerline, setting it apart from the standard SSB type. By combining two SSBs (upper butt (UB) and lower butt (LB)) with an overlap, the weld centerline aligns with this overlap’s midpoint. The overlap width is kept smaller than the average tool pin diameter to ensure the entire joint falls within the active influence of the pin region. The UB to LB may vary based on factors like material properties, plate thickness, tool pin type, and prior FSW experience with the specific material. During this study, three distinct DBL joint configurations having UB to LB ratios of 1:2, 1:1, and 2:1 were examined to identify the most effective DBL joint geometry, particularly emphasizing an elongated UB on the RS side. To ensure the joint configurations remain within the active influence of a 3-mm tool pin tip diameter during welding, a fixed overlap length (LL) of 2 mm is maintained.

Fig. 1
figure 1

Schematic representation of SSB and DBL joint configuration and the proposed design adopted for study in the study

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A 3 T FSW setup from ETA Technologies Bangalore was utilized for welding the material. Table 1 contains detailed information on the chemical composition and mechanical properties of the plates. Before welding, plates of the specified geometry underwent meticulous preparation and cleaning to eliminate surface contaminants. In this study, the varying weld joint configurations were considered the primary variable, while all other machine parameters remained constant to ensure simplicity and ease of understanding. For the welding process, a left-hand taper threaded pin made of H13 tool steel was employed (as depicted in Fig. 2). A tool shank height of 70 mm, shoulder height of 30 mm, and a tool pin height of 5.8 mm were used in this experiment. This tool featured a shoulder diameter of 18 mm, a pin root diameter of 6 mm, a pin tip diameter of 3 mm, and a shoulder concavity of 3°. The pin had a thread pitch of 1.38 mm at a depth of 1.2 mm and boasted a hardness of 56 HRC. The rotation of the left-hand tool was in a clockwise (CW) direction to enhance material flow downward. The tool design aimed to generate increased heat in the weld zone due to its specific shape.

Table 1 Chemical composition and mechanical properties of AA6061-T6 plate
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Fig. 2
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Taper threaded tool used during the experiment

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A singular set of welding parameters was chosen: a tool rotational speed (TRS) of 1100 rpm, a tool traverse speed (TTS) of 1.25 mm/s, and a tool tilt angle (TTA) of 2°. These parameters were selected based on extensive literature surveys, trial runs on the AA6061-T6 using a conventional square butt joint configuration with the aforementioned tool, and the researcher’s hands-on experience. The primary objective of this study is to determine the optimal DBL joint geometry to achieve the desired joint properties. Therefore, different joint configurations and their geometries (refer to Fig. 1) are considered the main variable process parameters in this investigation. All variations and comparisons conducted in this study are presented by focusing on the discussion of the effects of changes in joint configuration geometry. Three sets of 100-mm-length runs were conducted for each welding, maintaining a fixed dwell time of 5 s and a plunging rate of 20 mm/min to a depth of 0.1 mm. Strain gauge-based load cells were employed to measure longitudinal force (X-force), vertical force (Z-force), and spindle torque. This measurement was conducted through a PLC system that integrated with IPC and NI LabView software, facilitating data storage, acquisition, and retrieval. The peak temperature is measured by using a 15 Gauge AWG K-type chromel–alumel thermocouple, capable of measuring temperatures up to 800 ℃, which was carefully inserted into 3-mm-diameter holes drilled at specific positions on the AS of the plates, both in the X and Y directions. These thermocouples were connected to a PC-based data acquisition system (DAQ) with 40 channels (Agilent Technologies, Model: 34980A) for the purpose of recording the temperature reached during the welding process. Precise care was taken to ensure proper contact between the thermocouple tips and the weld plate. The thermocouples were strategically placed on the AS of the weld plate, chosen due to the higher heat generation in AS as compared to the RS. These thermal profiles were constructed until the point at which the temperature equaled room temperature after the gradual cooling process. Samples were cut tangentially to the welding, meticulously polished, and etched for macroscopic observation under an optical microscope. Moreover, for each joint type, three sets of tensile test specimens were prepared following ASTM-E8 standards along the transverse direction of welding. These specimens were tested using an INSTRON 8801 UTM at a strain rate of 0.5 mm/min. Fractographic analysis of the required tensile fracture samples was conducted at the same resolution using a scanning electron microscope (SEM). Furthermore, three-point face bend tests were performed on the samples using an INSTRON 8801 UTM to assess the joint’s flexural strength.

3 Results and discussion

This section aims to validate the applicability of different joint geometry for DBL configuration and SSB within the realm of FSW. As this study aimed to find the best DBL joint geometry to find the better joining aspects of 6-mm-thick AA6061-T6 plates, the analysis is deliberately simplified, focusing on fundamental aspects. The study is limited to analyzing the forces, torque involved, macrostructure, and mechanical properties of joints across different configurations. Further critical studies may be pursued following the successful assessment of these foundational joint analyses with the current configuration.

3.1 Torque and force analysis

The importance of spindle torque and forces in FSW cannot be overstated, as they directly influence the overall weld quality [32]. These forces are primarily generated by the resistance encountered during the flow of softened material and the movement of the tool along the workpiece. They play a critical role in shaping the weld material, with spindle torque varying throughout the welding process depending on factors such as the material’s softened state and the tool’s forging action. The vertical force (Z-force) applied by the tool aids in consolidating the plasticized material behind it, with higher Z-force levels generating more frictional heat [33]. Additionally, the tool pin experiences a resistive force (X-force) from the movement of deformed material across it as the tool traverses the workpiece. This study focuses on examining the influence of varying double-butt-lap (DBL) joint geometries on spindle torque, Z-force, and X-force by analyzing the evolution of these forces throughout the welding process [33]. It is important to note that the data presented here were exclusively measured during the actual welding operation, excluding plunge and dwell times. This focused approach aims to elucidate the specific impact of DBL joint geometry variations on the forces and torque experienced during welding.

Figure 1 illustrates S0 as the representation of the SSB joint preparation. Additionally, three distinct DBL joint geometries labeled as S1, S2, and S3 were introduced by altering the UB to LB ratio while maintaining a consistent LL dimension and UB offset to the RS across all cases. Figure 3 compares the forces and spindle torque involved in the process for different DBL and SSB joint configurations. For instance, in S1, both the Z-force (Fig. 3a) and spindle torque (Fig. 3b) show a significant increase and subsequently decrease as the thickness of the UB increases (from S1 to S3). In S1, with a UB thickness of 2 mm, the effective mating length of the abutting surfaces has increased from 6 to 8 mm (UB2 + LL2 + LB2). This expanded mating surface dissipates more heat into the joint. The same trend is observed in S2 and S3. However, in S1, the thicker LB in the RS plate, positioned away from the shoulder, leads to increased torque and Z-force to deform the RS material. As the volume of material at the UB placed at the AS under the shoulder increases from S1 to S3 (and vice versa for the RS), heat utilization in the AS maximizes, reducing the need for material deformation in the RS. This explains the decreasing spindle torque. Additionally, reducing the LB thickness (from S1 to S3) diminishes the necessity for higher vertical forces to consolidate the deformed material under the shoulder, given the reduced material available surrounding the pin bottom side. The increased material in the AS benefits from higher heat generation and shear stresses, further optimizing the joint’s behavior. In addition to it, a V shape or dip in Z-force is observed for S3 in Fig. 3a. Initially, as the rotating tool stirs and mixes the material, there is a reduction in force as the softened material flows more easily, diminishing workpiece friction. Subsequently, as material flow and consolidation occur behind the tool, the force steadily increases, reflecting the compression and formation of a solid joint. These fluctuations in force are primarily influenced by stirring and mixing dynamics, workpiece friction, and the material’s flow and consolidation behavior during the FSW process.

Fig. 3
figure 3

Comparison of force and spindle torque between different DBL and SSB joint configurations: a Z-force plot; b spindle torque plot; c X-force plot; d comparison of average forces and spindle torque

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The trends in X-forces (Fig. 3c) show relatively minor changes; however, an overall decrease in X-force is evident across all DBL-configured joints. This decrease can be attributed to the offset joint line around the pin. Furthermore, there is a noticeable decrease in X-force fluctuation from S1 to S3. Changes in X-force primarily arise from fluctuations in the heat generated and the resulting flow stress in both the AS and RS regions caused by the pin [18, 19]. The reduction in thickness of the material in the bottom portion, extended towards the AS, accounts for the diminishing X-force fluctuation from S1 to S3. Due to the differences in heat generation at the AS and RS and also the stresses, the force required to flow the deformed material from AS to the RS is different. As a result, fluctuation in X-force is present. Hence, the variation of the magnitude of X-force and its distribution for a different level of applied configuration can be observed. It has been observed that the flow of X-force distribution is highly unsteady in S1 configurations. This may be due to the expanded mating surface dissipating more heat into the join. Figure 3d presents a comparative analysis of the overall averages of spindle torque and process forces (Z-force and X-force). This comparison aids in better comprehending the discussion provided earlier and offers a clearer insight into the impact of varying joint geometry configurations on spindle torque and process forces.

3.2 Weld bead appearance and macrostructure analysis of the joint

Figure 4 displays the appearance of the weld bead and the corresponding macrostructure of the transverse cross-section for various designs of the weld joint line. The macrographs reveal defect-free cross-sections in both the SSB- and DBL-configured joints, underscoring the weld parameter effectiveness and the DBL joint configuration’s capability to produce flawless welds. Yet, notable differences emerge in the appearance and material flow within the NZ of these joint configurations and their proposed design variations. The interplay of the tool’s linear and rotational movements significantly influences material flow, along with forces and torque variations. Considering the use of a left-handed threaded pin rotating clockwise, a downward material flow was observed in all welds (S0, S1, S2, and S3). Maelstrom flow (MF) [18] was evident across all welds, with a more pronounced presence in S1, S2, and S3. MF occurs when material flows downward from the top of the pin, coinciding with material deformation around the pin’s surface. As discussed earlier, increased heat dissipation into the abutting surface enlarges the mating surface, facilitating easier deformation of material at the UB, which then transfers to the AS. This results in robust material flow within the AS, driving material downward. Moreover, an outward flow around the pin’s bottom region expands the pin-influenced area in DBL designs. As the overlap position shifts downward, the shoulder-influenced area increases from S0 to S3, allowing the shoulder surface frictional heat to impact a larger area above the overlap. The onion ring observed in S1, S2, and S3 results from heightened thermomechanical activity, indicating the occurrence of multiple material circulations deformed around the tool pin. These rings signify differences in grain size, texture, precipitate distribution, and dislocation density, which may influence mechanical properties such as strength, toughness, and fatigue performance. Additionally, the NZ area expands from S0 to S3, indicating higher heat input in S1, S2, and S3 compared to S0. Consequently, the pin-influenced area in S0 is smaller, yielding a taper-shaped pin region compared to the broader region in DBL configurations due to increased heat input. The boundary of the nugget below the shoulder flow arm appears more vertical in S1, S2, and S3 compared to S0, indicating wider nuggets and higher heat input conditions. This observation aligns with findings in prior research [20]. The consistent depth observed in both the shoulder- and pin-influenced areas across S0, S1, S2, and S3 is credited to the harmonized interplay between the tool shoulder and the flow influenced by the threaded pin [15]. Table 2 illustrates the impact of altering joint geometry design on different regions within the weld bead structure. The data clearly shows a progressive increase in the total weld bead area from S0 to S3. The rise in temperature could be linked to extra heat generated, impacted by the joint geometry and the FSW process.

Fig. 4
figure 4

Weld bead appearance and respective macrostructure for different weld joint configurations

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Table 2 Measurement of the different regions in the weld bead structure
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This additional heat likely encouraged more material under the tool to actively participate in the welding process. Table 2 demonstrates noticeable variations in the shoulder-influenced area, pin-influenced area, shoulder-influenced depth, and pin-influenced depth (measured in millimeters), respectively. In all DBL scenarios, a greater area undergoes influence from the welding process compared to that of the SSB configurations.

4 Microstructure analysis

The microstructure developed significantly influences the mechanical properties of welded joints. Therefore, analyzing the microstructure of the material is crucial to ascertaining the performance of the welded joint under specific conditions. The optical micrographs displayed in Fig. 5 portray the variation of grain size in the NZ of the welded joints. The micrographs highlight the formation of very fine, equiaxed grains within all the joints, a result attributed to dynamic recrystallization occurring during FSW. When substantial plastic deformation meets elevated temperatures, it initiates dynamic recrystallization, leading to the refinement of grains in the aluminum alloy AA6061. This phenomenon of dynamic recrystallization is a widely recognized occurrence in FSW [21].

Fig. 5
figure 5

Microstructure of nugget zone at three different points (top of the nugget, near the overlap area, and bottom of the nugget) for the samples welded with different joint configurations

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The average grain size measured in the top, middle, and bottom of the NZ for the various samples is presented in Fig. 5. The refining of the grains can be seen from the top towards the bottom of the NZ. The grains in the shoulder-influenced region are comparatively bigger than the grains in the pin-influenced region. This difference arises because the tool’s shoulder makes complete contact with the NZ during the thermomechanical cycle, producing more heat due to the rotating shoulder. The pin, on the other hand, directly interfaces with the back plate, functioning as a heat sink. This direct contact leads to a rapid cooling process for the weldment, preventing grain coarsening at the bottom side. Simultaneously, the top side of the NZ experiences a comparatively slower cooling rate than the bottom side.

Figure 6 shows a plot between the grain distribution and the various weld configurations. A measured average grain size can be seen in the order S1 > S2 > S3 > S0. This can be correlated with the shoulder-influenced depth. In FSW, approximately 85% of heat generation originates from the tool shoulder, with the remaining 15% attributed to the tool pin [22]. As the shoulder-influenced depth increases, the heat generation also increases, leading to an increase in the grain size. The results of the SSB joint (S0 sample) depicted an average grain size of 11.76 µm, 10.56 µm, and 9.42 µm at the top, middle, and bottom of the NZ, respectively. A deviation of grain size > 2 µm can be seen in the NZ of the S0 sample. On the other hand, the DBL joints (S1, S2, and S3 samples) revealed grain size deviations of < 1.0 µm. The results significantly indicate that the DBL configuration is capable of producing joints with more microstructural homogeneity. Processes that produce homogeneous microstructures often yield more consistent material properties. This consistency in manufacturing leads to products with reliable structural integrity and performance, reducing the likelihood of unexpected failures.

Fig. 6
figure 6

Comparison of grain size distribution from top to bottom of the nugget zone for different welding geometry design configurations

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4.1 Peak temperature, grain size, and hardness relationship

Figure 7 displays a comparison plot of peak temperature, average grain size, and average hardness for the various joint configurations. As already discussed earlier in Section 3.1, among the various configurations, the S1 configuration is seen to have exhibited the highest Z-force and spindle torque. A thicker LB in the RS causes increased torque and vertical force to deform RS material away from the shoulder. Yet, with the expansion of material volume beneath the shoulder at the UB from S1 to S3 (and conversely for RS), the shoulder optimizes heat usage, diminishing deformation in the RS material and subsequently lowering spindle torque. Additionally, reducing LB thickness from S1 to S3 lessens the need for higher vertical forces to consolidate deformed material under the shoulder due to the reduced material surrounding the pin’s bottom side. The rise in both torque and vertical force contributes to increased heat input, potentially leading to higher peak temperatures and longer cooling times [1, 23]. Under these circumstances, the prolonged exposure to heat affects the nugget. As the temperature of metals or alloys increases, there is a tendency for grains to grow. Elevated temperatures provide greater energy to the atoms in the material, allowing them to move more freely. This enhanced mobility promotes grain boundary migration and coarsening, leading to grain growth. In such conditions, aluminum alloys become more vulnerable to experiencing grain growth, leading to an observed increase in grain size. This finding can be justified by Fig. 5.

Fig. 7
figure 7

Comparative study of average heat input, average grain size, and average hardness in the nugget zone for different joint configurations

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The effect of welding peak temperature change with variation in joint configuration geometry in the microstructure is significant. Higher welding peak temperatures (438 °C, S1) typically lead to increased material softening, larger grain sizes, and more uniform precipitate distribution. Conversely, lower welding peak temperatures (405 °C, S3) result in finer grains and potentially more heterogeneous precipitate distribution. These variations in microstructure directly influence mechanical properties such as hardness, tensile property, and other mechanical properties of the welded joint. Therefore, controlling welding peak temperature through process parameter and joint configuration geometry adjustment is crucial for achieving desired microstructural characteristics and mechanical properties in FSW aluminum alloy joints.

The hardness is seen to have followed a decreasing trend in the order S0 > S3 > S2 > S1. The maximum microhardness of 80.2 HV is exhibited by the S0 joint, while the minimum recorded hardness of 72.4 HV is displayed by the S1 sample. This decreasing hardness can be justified by the Hall-Petch relationship. This empirical relationship demonstrates that as the grain size decreases, the microhardness increases. This is due to the grain boundaries acting as barriers to dislocation movement, which impedes plastic deformation and strengthens the material. When grains are smaller, there are more grain boundaries per unit volume, effectively providing more barriers to dislocation movement and resulting in higher hardness.

4.2 Mechanical property analysis

4.2.1 Tensile strength

The transverse tensile properties, including ultimate tensile strength (UTS), percentage of elongation, and joint efficiency, are evaluated for AA6061-T6 joints created through FSW using various configurations. Figure 8 illustrates a comparative analysis of tensile properties across various joint configurations. Joint efficiency, calculated using Eq. 1 (ηj = UTSjoint/UTSBM × 100), is plotted to depict the differences. In Fig. 8a, distinct behavior is observed in the tensile plots of weld joints formed with different configurations. Figure 8b showcases the average values of UTS, % elongation, and joint efficiency for welding configurations concerning the SSB-configured joint. Both figures underline the evident influence of joint configuration on the tensile properties. Specifically, UTS values stand at 227.2 MPa, 237.28 MPa, 244.8 MPa, and 247.48 MPa; % elongation at 12.55, 13.01, 14.13, and 14.02; and joint efficiency at 73.76, 77.0, 79.48, and 83.76 for S0, S1, S2, and S3, respectively. A noticeable trend emerges: a decrease in LB height (and vice versa for UB) corresponds to an increase in UTS for the DBL-configured joint. Similar trends appear in % elongation, with S3 exhibiting a slight reduction compared to S2, yet remaining comparable to S0. Additionally, Fig. 8b highlights that joint efficiency in S2 and S3 surpasses that of S0, which in turn exceeds S1.

Fig. 8
figure 8

Comparison of the a tensile strength plot for different types of welding and b UTS, % elongation, and joint efficiency of different DBL-configured welded joints with SSB-configured welded sample

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4.2.2 Fractography analysis

Figure 9 illustrates both side and top views of fractured samples, displaying enlarged sections of the fractures alongside corresponding SEM fractographic morphology. The examination of these samples and their magnified views showcases the varying fracture locations and surface morphologies. The SEM images, or fractographs, highlight diverse fracture modes across different samples. These observations regarding the tensile failure of the samples are detailed in Table 3, outlining fracture location, type, and surface morphology as depicted in Fig. 9 for clarity and comprehension.

Fig. 9
figure 9

Tensile fractured specimen with an enlarged view of fracture zone (AS) of different samples with fractographic observation

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Table 3 Variation in fracture type for the welding formed with a different joint configuration
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4.2.3 Bend test analysis

Table 4 presents the bend test values for each joint configuration. The bending angle notably increases in the DBL configurations, potentially due to robust metallurgical bonding in these joints. Macroscopic images distinctly display conspicuous onion rings on the underside of S1, S2, and S3, suggesting enhanced material flow in the lower weld region, potentially leading to greater bending angles in DBL joints. S3 exhibits the highest flexural stress, indicating stronger bonding in the lower weld area, enhancing overall strength. Consistent with earlier findings, S3 also demonstrates the highest tensile strength. Root bend testing was conducted for the identification of potential root defects. The absence of cracks or openings upon bending in all specimens confirms the formation of sound joints in both configurations.

Table 4 Root bend testing data
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5 Conclusion

The study’s objective is to identify the most effective joint geometry within the recently introduced double-butt-lap (DBL) configuration for welding AA6061-T6 plates using FSW. This involved creating four weld joints—three employing various DBL geometries and one with a standard square butt setup. The examination focused on force and torque, joint macrostructure, and mechanical properties, revealing several key findings:

  1. a)

    Initial rise and subsequent decline in spindle torque and Z-force with varying UB thickness in DBL configurations compared to the standard SSB. Overall reduction in X-force, notably less fluctuation when altering UB thickness and position

  2. b)

    Increased weld area and shoulder- and pin-influenced areas in DBL configurations versus the standard joint

  3. c)

    Qualitative shift to finer, more uniform grain structures in DBL, indicating enhanced mechanical robustness

  4. d)

    Qualitative correlation between peak temperatures, grain size, and hardness across different joint configurations, emphasizing the need for optimized DBL geometry

  5. e)

    A significant 83.76% of joint efficiency achieved in the DBL configuration with a 2:1 UB to LB ratio compared to other joints

  6. f)

    Bend tests displaying higher flexural strength in S1 and S3 DBL-configured joints

Overall, the findings strongly advocate for using the DBL configuration (2:1 UB to LB ratio) to enhance welding strength. However, broader investigations with diverse parameters and materials are warranted for a comprehensive understanding of this technique’s versatility.