Optimization of FSW parameters on bio-inspired jigsaw suture patterns to improve the tensile strength of dissimilar thermoplastics

This study aimed to enhance the Tensile Strength (TSFSW) of dissimilar thermoplastic joints by utilizing a bio-inspired jig saw suture and optimizing the Friction Stir Welding (FSW) limits are Traverse Speed (TS) and Plunge Depth (PD) and Rotational Speed (RS) at three varied levels. Statistical analysis, response surface methodology (RSM), and experimental validation were involved in achieving the research objectives. The outcomes showed that the TS and PD parameters had a higher significance on Tensile Strength compared to RS. The RSM prediction results were validated through experiments, achieving an extreme Tensile Strength of 11.1 MPa with a low error percentage. The best values of the FSW limits were found to be Rotational Speed (RS) of 1200 rpm, Plunge Depth (PD) of 0.37 mm, and Traverse Speed (TS) of 49.39 mm min−1. The formulated mathematical model with regression co-efficient R2 of 0.96 and RSM proved effective in predicting the optimal FSW parameters and achieving superior TSFSW. These findings prove that combination design can be reliably applied to optimise with a 95% confidence interval. The optical microscope and SEM morphological results in this study make congruently accurate predictions for the joint of the tensile fracture zone. These findings contribute to the advanced FSW techniques for dissimilar thermoplastic joints, providing insights for industrial applications requiring strong and reliable joints.


Introduction
FSW has been applied to polymer materials in the laboratory to improve tool performance, optimize welding methods, and fabricate composite materials.It has the potential to produce defect-free joints and high-quality composite materials in the polymer industry.however, more research is needed to fully understand the materials' mixing and flow, microstructure, and properties [1].3D printed thermoplastic parts were successfully joined using FSW, with optimized process parameters to improve weld strength(WS) and geometric qualities [2].Using a stationary hot shoe, FSW of ABS sheets was performed by varying the RS, TS, and tool shoe temperature.A full factorial design of 33 experiment was used to study the effect of these parameters' effect on the welds' tensile strength [3].Tool temperature was the primary factor affecting tensile strength, and the most dominant parameter affecting elongation was RS.The welds with the highest elongation had no crazes on the fracture surface, while the welds with the minimum elongation had a fracturing surface with corrugations and crazes [4].A new technique called i-FSW is introduced for joining thermoplastic plates using induction heating.The technique was demonstrated using high-density polyethylene plates, and the optimal settings for achieving supreme joint strength were determined by keeping temperature of 45 °C in tool pin and a RS of 2000 rpm [5].Friction stir spot welding (FSSW) with a consumable tool has been studied to join similar or dissimilar thermoplastics.The tensile properties of the joints made using this process were comparable to those of virgin acrylonitrile butadiene styrene, making it suitable for maintenance and repair applications [6].The results of different FSW limits on the properties of polycarbonate sheets were thoroughly investigated.Under specific processing circumstances, the maximum tensile, flexural, and impact strengths were reached, and these factors also had an impact on the stir zone's size and structure.Divergent lines were visible on the surface, as revealed by fractographic examinations [7].The joint efficiency of FSW nylon-6 sheets was optimized by a RSM based regression model and the particle swarm optimization method.The supreme FSW joint efficiency of 49.68% was achieved using a square pin [8].The optimal parameters for avoiding defects in the polypropylene joints were found to be a threaded pin profile, a FSW speed of 40-50 mm min −1 , and a RS of 2250-1500 rpm [9].By applying pressure and heat with a pinless tool, a new FSW method is anticipated for attaching AA6082-T6 aluminium sheets to self-reinforced polypropylene.The joints were formed by changing hole diameter , pitch, and their mechanical resistance was evaluated [10].The consequence of dwell time, plunge rate of the tool, and RS on the forces and torque while FSSW of polycarbonate pieces with minimum thickness were analysed.The dwell duration had a nominal effect on generating forces but greatly influenced the temperature of material, weld area size, and mechanical characterstics of the weld joint [11].The goal of the study is to improve the mechanical properties of friction stir-welded polymethyl methacrylate joints by optimizing the process variables.The exterme TSFSW was attained at a tool RS of 1000 rpm and feed of 40 mm min −1 [12].FSW was used to join aluminium and thermoplastic sheets.The process produced mechanical locking between the materials and did not require special surface preparation.The impact of several parameters, such as RS, TS, and distance to backing were studied, and optimal values were identified [13].Friction stir welding was used to join 3D printed thermoplastic composites made of Al metal particulate reinforced with ABS and PA6.The process was optimized for mechanical, morphological, and thermal properties using a consumable tool at 1400 rpm, feed of 50 mm min −1 , and 4 mm PD [14].In this investigation, the influence of incorporating multi-walled carbon nanotubes (MWCNTs) on the structure of dissimilar thermoplastics FSW joints was explored.The results demonstrated that the introduction of MWCNTs effectively decreased the presence of defects within the joints, highlighting the promising potential of these nanotubes in enhancing the dissimilar joint quality and performance [15].RSM to optimize the flexural strength of thermoplastic joints made with friction stir welding.The WS was influenced by RS, TS, and the temperature of the tool shoe.The ANOVA and RSM showed that higher RS and lower TS resulted in stronger weld joints with fewer defects [16].The optimal mechanical attributes of the PMMA-PC welded joint were achieved at RS and TS of 2100 rpm and 8 mm min −1 and by maintaining the heater temperature of 120 °C.At these conditions, the welded joint had a strength upto to 98% of the PC material and a superior hardness value compared to PC material [17].Submerged FSW of dissimilar thermoplastics and the consequence of various process factors on the microstructure and TSFSW of the joints were studied.The researchers found that adding MWCNTs to the ABS/High-Density Poly-ethylene HDPE joint improved the TSFSW of the joint [18].The Yield Strength (YS) of the welded pipes was analyzed using Taguchi's approach, which examines the impact of several parameters of FSW like tool offset, RS and TS.The result showed that RS was the high impact creating parameter on YS, succeeded by tool offset and TS [19].
Friction stir welding's (FSW) impact on Cu-brass joint properties was explored through various tool rotational speeds.While higher speeds degraded properties, lower speeds (1000 rpm) showed improved microstructure and mechanical strength.Corrosion resistance and heat distribution were also investigated [20].This study investigates friction welding's impact on enhancing intermetallic surface morphology in Al7075-SiC metal matrix composites.Varying forging pressure and rotational speed improved mechanical properties, and artificial neural networks efficiently predicted outcomes [21].Bio-inspired interlocking structures are those that mimic the mechanically interlocking structures found in nature, such as those found in certain plant and animal tissues.These structures are characterized by their ability to lock together firmly and provide interfacial solid bonds.They can be divided into two primary categories: regulable interlocks, which provide tunable and reversible attachment, and static interlocks, which improve interfacial strength [22].The utilization of fractal design significantly enhanced the load-withstanding ability of the Koch fractal structures with interlocks, as discovered by the researchers.However, the researchers observed that deficiencies, such as gaps between rounded tips and interlocked pieces, had a notable impact on the mechanical responses of these structures [23].
To increase the reliability and strength of the metal-polymer joints in biomedical and dental prostheses by developing a mechanical interlocking technique that utilizes optimized AM characteristics on the surface of the metal.The process involved an optimization method and combining it with the mesh adaptive direct search algorithm and Finite Element Analysis (FEA) to identify the best magnitudes of the interlocking properties and tested through tensile experiments, it was identified that it increases the metal-polymer interface strength by 85% and significantly improved the overall performance of the prostheses [24].A new type of suture material was developed that utilizes jigsaw-like morphologies to create a bistable system that can lock into dual stable positions.AM and design exploration were used to fine-tune the architecture of the tabs to tailor the mechanical response of the material.The resulting materials exhibited up to ten times increased toughness compared to the base polymer.These materials showcased desirable characteristics, including significant, re-manufacturability, damage tolerance and reversible deformations.These materials have potential applications in the development of new architectured material [25].The utilization of carbon fiber reinforced PLA (Poly Lactic Acid) and bio-inspired interlock sutures led to enhanced strength and rigidity in 3D-printed components.By employing PSO optimization, the optimal printing parameters were predicted and tested, confirming that the combination of carbon fiber reinforced with PLA and spline-shaped interlock sutures enhances the Bending Strength (BS) of the AM components compared to PLA [26].This investigation explores a bionic approach inspired by natural sutured architectures, revealing that incorporating sinusoidal centre-lines in interlocking interfaces enhances the load-carrying capacity and toughness of joints, offering insights for robust engineering design [27].The effect of spline interlock angles on tensile strength was explored and the research reveals that the interlock angles play a crucial role in sutures and sutured materials.The optimization reveals that an interlock angle of 35 degrees offers the best pullout performance, enhancing the tensile strength and mechanical properties of inherently brittle materials [28].The study investigates the influence of nano silica addition and rectangular weave geometry on the mechanical and microstructural properties of friction stir welded nylon 6-6 thermoplastic.Optimal conditions of 1500 rpm, 2 mm step size, and 2 wt.% nano silica improve tensile strength, microhardness, and overall weld quality, offering the potential for efficient thermoplastic joining [29].
In this investigation, FSW was utilized to join thermoplastic materials, with a focus on optimizing the welding factors to maximize weld quality.The parameters studied included tool RS, TS, and axial force, and diverse tool profiles with differnet shapes such as square , cylinder and triangular shape with threaded pin were used.The weld excellence was estimated through TSFSW testing and microstructural investigation.It was found that proper control of the welding factors was essential in achieving strong, high-quality welds [30].This research explored the effect of several FSSW factors on the WS of HDPE sheets.The Taguchi method, an orthogonal array, and the SN ratio and ANOVA were used to decide the most significant factors and the best combination of welding factors for maximizing WS.The efficiency of the strategy for determining the factors that had the most effects on WS was supported by experimental data [31].This research focused on the influence of FSW factors on the WS of thermoplastic materials, such as HDPE and PE sheets.The Taguchi method was employed as a statistical DoE to recognise the optimal welding factors, and the SN and ANOVA were used to analyse the results.The experiments were organised by an orthogonal array, and the results were confirmed through further testing.The ultimate goal of the research was to improve the strength of the welds in order to increase the use and performance of thermoplastic joints [32].Effect of several input weld factors on the TSFSW of the welded samples using diverse pin profiles.The temperature and load-displacement behaviour of the welds were also considered to realise the welding behaviour of PLA [33].Taguchi method to optimize FSW process factors in order to increases the lap weld joints Tensile-Shear Strength (TSS) in composite PE sheets reinforced with carbon fibers.The results exhibited that WS, RS, and tilt angle had a vital impact on lap WS, and that an optimal TSS of 6.06 MPa was attained with a WS of 25 mm min −1 , a RS of 1250 rpm, and a tilt angle of 1 degree [34].FSSW to join polycarbonate (PC) sheets of 3 mm thickness.An artificial neural network (ANN) was also formulated to forecast the mechanical behaviour of the weld joints.The results of the study provide insights into the potential utilization of FSSW in the joining of thermoplastic materials [35].
The literature survey revealed that there is no established interlock pattern for joining dissimilar materials in friction stir welding (FSW), and no previous research has investigated how interlock patterns affect the strength of welded joints.This study focuses on implementing a jig saw interlock suture in FSW of dissimilar thermoplastics to determine its impact on the strength of the weld joints.

Methodology
A 3D model of a spline interlock suture was created using Fusion 360 software.This model was then used to fabricate 16 samples using desktop FDM with specific parameters such as an infill % of 30 and a wall thickness of 1.5 mm.The samples were then welded using FSW using a round pin tool according to a DoE.The TSFSW of all samples was tested, and the results were used to develop a regression model.The model was further optimized using RSM to predict the best FSW factor values for maximizing the TSFSW of the bio-inspired interlock sutures.The predicted results were then experimentally validated.

Materials and setup
3.1.Materials FDM commonly uses thermoplastic filaments as raw material to convert 3D models into physical components.PETG (Polyethylene terephthalate glycol) was used for the sample fabrication in this research due to its strength, durability, and temperature tolerance.

Specimen fabrication
The specimen fabrication setup for the FSW process includes the Work-Bee CNC router 1010 machine with a work volume of (1145 ×1125 × 510 mm), maximum spindle RPM of 30000 and a customized fixture.The fixture for FSW is designed to securely hold the samples while welding in a singular-pass square butt welding configuration.In this study, dissimilar thermoplastic materials PLA (Polylactic Acid) and PETG material properties as showed in table 1 were subjected to FSW using a single tool named Tapered Cylindrical pin, as depicted in figure 1. Figure 2 shows the specimen with dimensions of 5 mm thickness, 100 mm length, and 40 mm width were fabricated with a jigsaw-like spline pattern created using FDM process.To improve the TSFSW for dissimilar thermoplastics PLA and PETG novel approach was proposed, which involved the implementation of a jig saw suture interlock pattern consisting of a radius of 2.5 mm with an angle of 35 degrees [27,28].

FSW of dissimilar thermoplastic
A specialized fixture was designed to hold both PLA and PETG samples firmly in place and maintain precise alignment between the male and female spline patterns to weld the dissimilar thermoplastics.A custom circular tool was used to execute the welding process.The novel spline interlock pattern allowed for uniform distribution of both materials in the welded region, reducing the concentration of stress points and enhancing the strength of the joint.This research focused on investigating the spline interlock pattern's effect on the FSW process's tensile strength for dissimilar thermoplastics PLA and PETG.The Fabricated samples are shown in figure 3.

Tensile test
To measure the TSFSW of welded joints, a total of 16 samples were subjected to a gradual load using a Universal Testing Machine (UTM) with a capability of 100 kN (AILM 100 KN).The load was applied at a 1 mm min −1 rate until the specimens fractured.

Design of experiment
To attain the maximum TSFSW in the 3D-printed bio-inspired jigsaw suture pattern, significant process parameters of FSW, such as RS, TS, and PD were considered through a literature survey.The process parameters for the jigsaw suture interface varied at levels −1, 0, and +1.To avoid insufficient melting of PETG and void formation in PLA during FSW of dissimilar thermoplastics, the rotational speed (RS) is maintained between 800 and 1200 rpm.This provides sufficient heat generation for effective PETG melting and minimizes the risk of excessive heat leading to voids in PLA.TS of 30 mm min −1 to 50 mm min −1 allows for an appropriate interaction time between the tool and the samples.This ensures that both materials are adequately softened and mixed, facilitating good intermolecular bonding.It helps prevent incomplete melting or over-melting, which can compromise the quality of the weld joint.
Similarly, the plunge depth in FSW is typically between 0.2 mm and 1 mm.This range ensures controlled material mixing and heat generation, minimizing the risk of void formation while maintaining a reliable weld of PLA and PETG.The table 2 range values of selected process parameters are identified through experiments.The DOE utilised a Central Composite Design, which included a 2 3 -factorial points, 2 * 3 axial points and two central points.This Design Expert software resulted in a total of 16 experiments.For the spline interlock jig saw suture interface, a total of 16 specimens were fabricated following the DOE, as displayed in table 3.

Regression model
A mathematical model was developed to forecast the TSFSW for dissimilar thermoplastic 3D-printed Jigsaw sutures.The model takes into account the process parameters of RS, TS, and PD.The input and ouput factors of the FSW process are illustrated in figure 4. The model allows for accurate estimation of the TS based on these process parameters.
To predict the maximum TS FSW , a quadratic model is developed for the selected FSW factors RS, TS and PD is written as shown in equation (1) Where RS, TS, and PD are functions of response Y and are represented as The developed quadratic model of dissimilar thermoplastic with a jigsaw suture interface was given in equation (3) The developed model was assessed for its adequacy and demonstrated a high correlation with an elevated regression coefficient R 2 of 0.96.This indicates a strong relationship between the variables in the formulated model.Additionally, the F-value (16.46) of the model indicates its significance as showed in table 4. The  The comparison chart depicted in figure 5 clearly demonstrates that the experimental and predicted values exhibit a similar pattern.This confirms that the formulated model possesses the capability to accurately predict TSFSW, as it aligns well with the observed experimental values.

Result and discussion
This research aimed to improve the TS (FSW) of dissimilar thermoplastic joints by incorporating a bio-inspired jig saw suture and optimizing the FSW parameters.The optimization was successfully done by employing RSM techniques.The maximum TS (FSW) value predicted by the optimization technique was evaluated through experiments to validate its accuracy [36].

RSM results
RSM was utilized to determine the optimal FSW input variables for achieving enhanced tensile properties in the FSW joints of dissimilar thermoplastic bio-inspired jigsaw sutures.The predicted optimum FSW parameter values to maximize the TS(FSW) obtained through Design Expert software are presented in table 5.  Figure 6 depicts the desirability ramp function graph, which provides insights into the optimization process for maximizing the TS (FSW) in the FSW of dissimilar thermoplastic joints.The graph showcases the predicted optimum values of the Rotational Speed (RS) at 1200 rpm, Traverse Speed (TS) at 49.39 mm min −1 , and Plunge Depth (PD) at 0.37 mm, as predicted by Response Surface Methodology (RSM).The desirability ramp function graph combines the optimized parameter values RS, TS and PD using the desirability approach.The graph in figure 6 visually shows how close the predicted parameter values are to the desired targets for maximizing Tensile Strength.The ramps on the graph represent how each parameter's desirability changes and the final desirability value provides an overall measure of how well the parameters align with the optimization goal.The ramps on the graph represent the optimization results and highlight the achieved output response Tensile Strength of 11.299 MPa.This graph provides a visual representation of the optimization process, indicating the optimal parameter values predicted by RSM for maximizing the TS in FSW.

Influence of RS and TS on TS(FSW)
The relatively lower F-value of Rotational Speed (RS) compared to other factors, such as TS and PD, suggests that RS has a less significant influence on the TSFSW joints of dissimilar thermoplastics.When the jigsaw suture is present, at a maximum rotational speed of 1200 rpm, greater friction arises where the tool shoulder and base plate contact.This enhanced friction leads to optimal temperature as an input in the weldment region, causing the thermoplastics (PLA and PETG) to reach a plastic state.During this stage, the stirring action of the selected tool promotes the easy flow of material around the tool pin, resulting in a uniform distribution.This uniform distribution contributes to a better-plasticized state of dissimilar thermoplastic materials, allowing the tool to travel easily near the trailing edge of the sample.Although the 3-dimensional surface graph in figure 7 indicates a slight increase in TSFSW with a slight increase in RS, the lower F-value of RS suggests that its influence on TS is relatively less significant compared to other parameters.The F-value represents the statistical significance of a parameter, and in this case, a lower F-value for RS indicates that other factors, such as TS and PD, have a more dominant influence on the overall TSFSW joints.

Influence of TS and PD on TS(FSW)
The statistical analysis reveals that the TS and PD have a higher significance on the TSFSW joints compared to Rotational Speed (RS).This is evident from the higher F-values of 24.62 for TS and 28.41 for PD.These findings indicate that TS and PD are more influential in determining Tensile Strength.The surface plot analysis shown in figure 8 provides further insights into the relationship between TS and Tensile Strength.It clearly demonstrates  that increasing TS directly corresponds to an increase in Tensile Strength.This can be attributed to lower TS settings resulting in more friction and longer stirring times, leading to the formation of voids and reduced Tensile Strength.Conversely, higher Traverse Speed (TS) settings reduce stirring time, promoting a more uniform distribution of materials and consequently enhancing Tensile Strength.PD also significantly affects Tensile Strength, as indicated by its higher F-value.The surface plot illustrates that increasing PD initially enhances Tensile Strength.This is because greater PD allows for deeper penetration of the FSW tool, facilitating better material mixing and bonding.However, beyond a certain point (0.6 mm in this case), further increases in PD can have a negative impact on Tensile Strength.Excessive PD may also create voids in the weldment zone and reduce the thickness of the weld interface,  ultimately diminishing Tensile Strength.Maintaining an optimal combination of these parameters is crucial for achieving superior Tensile Strength in dissimilar thermoplastic joints.

Influence of RS and PD on TS(FSW)
The surface plot showed in figure 9 illustrations that the RS, although less significant than other parameters, influences the Tensile Strength to some extent.It is increasing RS results in increased friction and generates more heat among the tool shoulder and base plate.This improved heat input optimizes the plasticization of dissimilar thermoplastic materials within the weldment zone of the jigsaw interlock suture.The enhanced plasticization facilitates the smooth flow and uniform distribution of the materials around the tool pin.As a result, better bonding between the materials is achieved, ultimately contributing to improved Tensile Strength.
In contrast, PD is more prominent in influencing Tensile Strength, as indicated by its higher significance.Increasing PD initially enhances the Tensile Strength by enabling more extensive material mixing and improved bonding.The increased plunge depth allows for better penetration of the FSW tool, leading to a thorough blending of dissimilar thermoplastic materials.Consequently, the joints exhibit improved mechanical properties and higher Tensile Strength.However, it is crucial to note that excessively high PD can negatively affect Tensile Strength.Beyond a certain threshold, typically around 0.6 mm, excessive PD can result in the formation of blowholes in the weldment zone.These blowholes weaken the joint structure and reduce the overall Tensile Strength.
Additionally, excessive PD may lead to a reduction in the thickness of the weld interface, further compromising the joint's mechanical integrity.Optimizing both RS and PD parameters is essential for achieving maximum Tensile Strength in FSW joints.Adjusting RS influences the plasticization and material flow, while controlling PD ensures proper material mixing and bonding.By setting the RS to a maximum of 1200 rpm and the PD to 0.37 mm demonstrates the ability to achieve robust and dependable joints with exceptional Tensile Strength (TS).

Optical microscope and SEM analysis
Figure 10(a) presents an optical microscope image demonstrating the successful mixing of PLA and PETG in the FSW joint, providing visual confirmation of a high-quality weld.This visual evidence solidifies the conclusion that the weld has achieved a favourable level of integrity and material blending.To further validate the mechanical performance, figure 10(b) displays an image from the cross-section of the fracture zone obtained during the tensile test.The presence of clear evidence of pull-out signifies a robust tensile strength, reinforcing the notion that the weld has attained satisfactory mechanical properties.In addition, figure 10(c) Showcases the top surface of the FSW joints captured using scanning electron microscopy (SEM), revealing a uniform and wellblended composition of the dissimilar thermoplastics PLA and PETG.This SEM image offers a higher level of detail, confirming the homogeneous mixture of the materials in the joint.Furthermore, figure 10(d) Exhibits the fractured surface of the Friction Stir Welded samples under the optimized parameters, also captured through SEM.The occurrence of distinct dimples on the fracture surface signifies a ductile fracture mode, indicating high tensile strength properties.The presence of microvoids observed in the SEM image (d) indicates a pull-out mechanism, which can contribute to increased tensile strength.These voids act as stress concentrators, causing energy dissipation and hindering crack propagation, thus enhancing the overall mechanical performance.The identification of cleavage facets in the image suggests a brittle nature, indicating potential areas of weakness.However, it is essential to note that the presence of cleavage facets does not necessarily indicate a decrease in tensile strength.In fact, these facets can help absorb and distribute stress, leading to improved fracture resistance and higher energy absorption capabilities.The detection of oxide precipitation spots in the SEM image reveals the formation of oxides on the fracture surface.While oxide precipitation can indicate a chemical reaction or environmental exposure, its effect on tensile strength may vary.In some cases, oxides can act as strengthening agents by reinforcing the material's structure.However, excessive oxide formation may lead to localized weakening and reduce the overall tensile strength.These figures, obtained from both optical microscopy and SEM, collectively provide strong evidence supporting the successful mixing and favourable mechanical performance of the FSW joints.

Evaluation of RSM result with experiment result
The best FSW factors obtained from RSM are presented in table 6. Experimental validation of these parameters resulted in a maximum TSFSW of 11.1 MPa.Furthermore, less than 1% of the experimental data deviate from RSM predictions, indicating high accuracy.These findings strongly support the effectiveness of the formulated  model and RSM in predicting the optimal FSW process parameter values for achieving superior Tensile Strength in the jigsaw suture interfaced dissimilar thermoplastic joints.

Conclusion
This research focused on enhancing the tensile strength of dissimilar thermoplastic joints by utilising a bioinspired jig saw suture and optimising Friction Stir Welding (FSW) parameters.
• The bio-inspired Jig saw suture interface played a crucial role in improving the tensile strength of the joints.The Jig saw suture facilitated enhanced material flow, bonding, and distribution during the FSW process.
• By creating an optimal heat input and promoting plasticization of the thermoplastic materials, the Jig saw suture interface enabled better mixing and blending of the materials.This resulted in a more uniform distribution of dissimilar thermoplastic materials and ultimately contributed to increased tensile strength in the joints.
• In addition to the Jig saw suture, the optimization of FSW parameters also played a significant role in enhancing the tensile strength.The statistical analysis revealed that the Traverse Speed (TS) and Plunge Depth (PD) parameters had a higher significance on the tensile strength compared to the Rotational Speed (RS).
• Increasing the Traverse Speed (TS) led to improved tensile strength by reducing stirring time and promoting a more uniform distribution of materials.On the other hand, the Plunge Depth (PD) influenced tensile strength by facilitating better material mixing and bonding.However, excessive PD beyond a certain point had a negative impact on tensile strength due to the creation of voids and reduced thickness in the weld interface.
• Through the utilization of Response Surface Methodology (RSM), the optimum values of the FSW parameters were predicted.The experiments conducted to evaluate the predicted FSW parameters resulted in achieving a maximum tensile strength of 11.1 MPa, with an error percentage of less than 1% compared to the RSM predicted result.This validates the accuracy and reliability of the developed mathematical model and demonstrates the effectiveness of RSM in predicting the optimum FSW process parameter values.
In summary, in conjunction with the optimized FSW parameters, the bio-inspired jigsaw suture interface successfully enhanced the tensile strength of dissimilar thermoplastic joints.This research contributes to advancing the understanding and application of FSW techniques in various industries and applications, offering improved mechanical properties and performance in joint structures.

Figure 2 .
Figure 2. Schematic of spline suture interfaces and geometric parameters.

Table 1 .
PETG and PLA material properties.

Figure 5 .
Figure 5.Comparison of Experimental value and Predicted value of TS (FSW).

Figure 7 .
Figure 7. Surface Plot of RS and TS on TS(FSW).

Figure 8 .
Figure 8. Surface Plot of TS and PD on TS(FSW).

Figure 9 .
Figure 9. Surface Plot of RS and PD on TS(FSW).

Figure 10 .
Figure 10.An optical microscopic image showcasing the (a) Top surface of the FSW joint, (b) Revealing the fracture surface after conducting a tensile test SEM image depicting, (c) the top surface of the FSW joint, (d) The fracture surface after the tensile test.

Table 2 .
FSW parameters and levels.

Table 3 .
Experimental, Predicted TS (FSW) value and Error (%) derived from the regression model.
S.No RS (Rpm) TS (mm/min) PD (mm) Experimental -TS FSW (Mpa) Predicted-TS FSW (Mpa) Error (%) P-Values of the model terms, including TS, PD, RS * TS, RS * PD, TS * PD, RS2, TS2, and PD2, were found to be less than 0.0500, further confirming their significance.The effectiveness of the formulated model was confirmed by comparing it with experimental values.The Experimental and predicted TS (FSW) values with average error difference in equation (4) clearly demonstrate the model's ability to predict the response accurately.