Effect of tool geometry variations on weld bead structure and mechanical properties of friction stir welding of dissimilar material Acrylonitrile Butadiene Styrene and polystyrene sheets thermoplastics

Abstract

Acrylonitrile Butadiene Styrene (ABS) and Polystyrene Sheets (PS) are versatile thermoplastics that are likely to be used in engineering thermoplastics commonly employed in automotive interior components, consumer electronics housings, household and medical products, and lightweight packaging structures. ABS is durable and resistant to impact, while PS is stiff. The present study investigates the effect of different tool pin geometries and process parameters on the weld bead structure and mechanical properties of friction stir-welded (FSWed) dissimilar thermoplastics, ABS, and PS. Both tool pin profiles, namely Threaded cylindrical (TC) and threaded triangular (TT), were made with the same shoulder diameters (21 mm) and pin lengths (3.7 mm) and joint under varying process parameters, including traverse speeds (9, 18, 27 mm/min) and rotating speeds (800, 900, 1100 rpm) is used to join ABS & PS. With a maximum UTS of 24.5 MPa, Shore D hardness of 83.6, and elongation of 5% at 900 rpm and 18 mm/min, the TC tool continuously outperformed the TT tool. Because TT tools required less heat and churning, the weld appeared smoother but had lower mechanical strength (higher strength was found at 18.98 MPa at 900 rpm and 27 mm/min). The findings demonstrate that TC geometry offers better material flow and bonding, making it more suitable for thermoplastic joints with high strength.

Introduction

In an effort to minimize product weight, the usage of lightweight materials, such as reinforced polymer composites and thermoplastics that has been growing. ¹ Thermoplastics are used extensively in many industrial and technical applications because of their superior stress-to-weight ratio. ² As a result, new connecting techniques for both different and similar polymers have been developed. ³ Due to their vastly different mechanical and metallurgical characteristics, joining dissimilar materials using any welding technique is never easy. However, joints made of diverse materials are being used more and more in many industries because of their technical and commercial advantages. ⁴ The solid-state nature of FSW prevents melting and significantly reduces thermal degradation, making it a more suitable joining technique for ABS and PS plates than advanced welding methods.^5,6 Although laser welding is quick and precise, it can cause localised overheating in heat-sensitive polymers, potentially degrading them. ⁷ Although vibration welding creates robust joints, it can only be used on flat surfaces and with specific geometries. Overheating the material during friction welding could result in flash generation and other possible flaws. ⁸ While FSSW works well for spot joints, it is not as strong or continuous as full-length welds. On the other hand, FSW is ideal for thermoplastics like ABS and PS, as it provides improved heat control, reduced porosity, and superior joint strength, which was invented at TWI in 1991. Among these significant variables (RS, TS, tilt angle, plunge depth, & materials), tool pin shape influences joint quality.

Various researchers have joined different types of polymers using FSW under varying process parameters. Some of these studies have been included. Goswami et al. ⁹ investigated the influence of tool geometry variations on the mechanical and microstructural properties of dissimilar FSWed of thermoplastic polymers. Aval et al. ¹⁰ found that the geometry of the tool significantly affects heat input and microstructure; the concave shoulder with grooved probe produced superior stir zones, with finer grains on the AA 6061 side. Arif et al. ¹¹ found that FSW of ABS achieved 61% joint efficiency, with defects mainly on the retreating side. Weld strength peaked at 19.4 MPa, and the nugget zone exhibited the highest hardness, indicating effective consolidation under optimal parameters. FSLW of ABS and PC using a double-pin tool showed that increasing rotational speed enhanced weld strength (up to 19.38 MPa), improved material flow, and refined PC morphology, indicating promising potential for industrial dissimilar polymer joining [Yan et al. ¹² ]. Kumar et al. ¹³ revealed that incorporating MWCNTs in FSW of ABS–PS enhanced joint strength by 39.16%, with optimal performance at RS of 900 rpm and TS of 0.1 mm/s. MWCNTs improved weld integrity, demonstrating their potential for strengthening dissimilar thermoplastic joints. Guden et al. ¹⁴ found that optimal FSW parameters for AM-fabricated ABS M30 were 800 rpm, 10 mm/min, and a 16 mm shoulder. Weld quality improved with counter clockwise rotation, while high speeds and clockwise rotation caused tunnel defects and reduced weld density. Derazkola et al. ¹⁵ observed that revolution pitch significantly affects polypropylene joint quality by altering strain rate, viscosity, and mixing patterns. Optimal revolution pitch ensures robust joints by minimizing defects and controlling heat, enabling consistent mechanical properties despite varying tool velocities. Kumar et al. ¹⁶ used a hybrid, threaded taper, and triangular tool pin to join the Al-Li alloy and observed that the hybrid tool generates higher heat and lower waviness, and found higher joint efficiency (78.44%) and tensile strength (418.98 MPa) than other tools. ¹⁷ Dundar et al. ¹⁸ used Iosipescu shear experiments at different loading rates to examine how strain rate affected the shear yield behavior of ABS. Results showed that higher strain rates significantly increased shear yield strength. Finite element simulations using an elastic-viscoplastic model validated the experimental outcomes, confirming strong agreement with observed material behavior. Sin et al. ¹⁹ found that ramp cyclic thermal ageing causes less degradation in ABS copolymers than constant thermal ageing, preserving impact and tensile strength better and extending service life under varying thermal conditions. Yoon et al ²⁰ show that BfGN &ABS_1 nanocomposite offer significantly enhanced tensile strength, modulus, and toughness due to excellent dispersion and polymer compatibility. Khan et al. ²¹ observed that FSW effectively joins ABS and PC polymers, achieving maximum joint efficiencies of 52.71% (at RS of 1200 rpm and a TS of 10 mm/min.) and 54% (RS of 800 rpm and a TS of 40 mm/min), respectively. Optimal parameters enhance heat distribution, though increased traverse speed may reduce tensile strength or create voids.

This study investigates the effect of tool pin geometry TC and TT on joint quality in the context of dissimilar thermoplastic welding between ABS and PS using FSW. This work provides new insights and perspectives on joining two different thermoplastics with distinct mechanical properties, whereas the majority of FSW research has focused on similar thermoplastic or metallic joints. The study identifies ideal circumstances for producing high-strength, flawless polymer joints by methodically analyzing the combined effect of tool pin geometry and RS & TS on mechanical performance and microstructural features.

Experimental Procedures

In this study, Acrylonitrile Butadiene Styrene (ABS) and polystyrene (PS) sheets having dimension 120 mm (Length) x 60 mm (width) x 4 mm (thickness) were used as base materials to join using 3-axis CNC milling machine (model: VM-760), for FSW at three different tool (as shown in Figure 1) at RS (800, 900, and 1100 rpm) and TS (9, 18, and 27 mm/min). The mechanical properties of ABS & PS thermoplastics are tabulated in Table 1. PS exhibits higher stiffness than ABS due to its higher Young’s modulus, but it fails in a predominantly linear-elastic and brittle manner. The elongation values reported in Table 1 correspond to engineering strain. ABS, due to its ductile nature, shows a nonlinear stress–strain response with a distinct yield point, which has now been included in the table as suggested. The ABS was positioned on the advancing side and PS on the retreating side, is fixed on the bed, and the position of material has been fixed on the basis of their respective melting points. The tool pin geometry of TC & TT is used to join dissimilar thermoplastic ABS & PS in FSW.

Table 1.

Mechanical properties of ABS & PS.

Material	Density (g/cm3)	Yield Stress (MPa)	Young’s modulus (GPa)	Tensile Strength (MPa)	Elongation at Break (%)	Flexural strength [MPa]	Hardness (Shore D)
ABS	1.04 – 1.06	35-45	1.8-2.4	30.63	10 – 50	65 – 100	80
PS	1.04 – 1.06	38-55	3.0-3.5	30 – 46	1 – 3	60-90	75 – 80

Figure 1.

Different tool geometries used in joining material (a) Original view (b) Schematic diagram.

The process parameters are selected on a trial-and-error experimental basis. More than 40 experimental trials were conducted to determine the suitable ranges for both RS and TS, based on visual inspection of the weld quality. The findings indicated that when the RS was lower than 800 rpm, no sound joints formed due to insufficient heat generation during welding. Along the joint line, material removal was also observed in some instances. Conversely, at RS exceeding 1100 rpm, the base material melted, leading to its flow out of the weld seam. In these situations, the stationary shoulder failed to efficiently control the melted material. Therefore, the preferred range for RS during FSW of ABS and PS was established between 900 rpm and 1100 rpm. defective joints were also formed when the TS was below 9 mm/min, as well as at speeds higher than 27 mm/min, due to inadequate welding heat. Thus, the optimal range for TS was identified as between 9 mm/min and 27 mm/min. Within these suitable ranges, three levels of RS and TS were incorporated into the experimental design. By employing a full factorial technique, a total of 18 experiments were conducted for this study.

Figure 2 demonstrates the extensive experimental process used in this study for FSW of ABS and PS and extracting samples from the joint. In order to prepare the thermoplastic sheets for welding, the procedure starts with the first sizing of the raw materials, as seen in (a). As shown in (b), the sheets are then machined to the required dimensions, producing a consistent finish. For precision machining, a CNC machine is used in (c), guaranteeing surface uniformity and dimensional accuracy. To ensure correct alignment during the welding operation, the prepared sheets are then firmly secured inside a specially made fixture (d). The experimental setup for the actual welding is shown in (e), and the FSW method is used. After welding, the welded specimen is visually inspected to determine the joint integrity, as indicated in (f). The mechanical performance of the weld, namely its tensile strength, is then assessed by extracting and testing tensile test specimens, as shown in (g).

Figure 2.

Experimental procedure of ABS & PS.

A Universal Tensile Machine (UTM) (INSTRON, UTM Model 5969) applied load of 50 KN, as illustrated in Figure 3, was used to perform tensile testing on the FSWed samples by ASTM D 638. Three identical standard tensile test specimens were removed from the center of each welded sample in order to verify the accuracy of the tensile test machine results.

Figure 3.

Tensile testing specimen for testing ²² .

Results and Discussions

Weld Bead Structure Analysis

Microstructural analysis of the dissimilar ABS/PS welds revealed the presence of a distinctly defined stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected region on both sides of the joint. The SZ is characterized by intensive polymer intermixing and exhibits a smeared, vortex-like flow pattern typical of thermoplastic friction stir welding (FSW). The TMAZ exhibited partially deformed molecular chains with evident shear-induced orientation but without complete melting. The width of the FSW-affected region was approximately 5–7 mm from the tool centerline, contingent upon the tool geometry and heat input. Under the optimized parameters (900 rpm, 18 mm/min), the SZ was void-free, demonstrating uniform consolidation and continuous material mixing, which correlated with the highest tensile performance observed. Conversely, both low heat input (e.g., 800 rpm and 27 mm/min) and excessive heat input (e.g., 1100 rpm and 9 mm/min) resulted in microstructural irregularities such as discontinuous flow patterns, localized porosity, and insufficient interchain diffusion. The TT tool profile produced a more disturbed SZ with inconsistent material flow and partial bond formation across all parameter sets, accounting for its inferior weld morphology and mechanical performance compared with those of the TC tool. Figure 4 (a)-(i) presents the FSWed surface morphological and comparative visual assessment of dissimilar FSW between ABS and PS sheets, performed using a TC tool pin profile at varying rotational (800, 900, and 1100 rpm) and traverse speeds (9, 18, and 27 mm/min). However, the weld bead profile found from using the TT tool pin profile to join ABS & PS sheet is shown in Figure 4 (j)-(r). At lower rotational speed (800 rpm), the weld seam at 9 mm/min (Figure 4(a)) appears relatively rough with excessive flash formation, suggesting localized overheating and turbulent material flow. Increasing the traverse speed to 18 mm/min (Figure 4 (b)) reduces the flash but results in insufficient mixing. In comparison, a further increase to 27 mm/min (Figure 4(c)) exhibits signs of incomplete fusion and weak interfacial bonding. The optimal weld surface morphology is observed at 900 rpm and 18 mm/min (Figure 4(e)), where the joint shows a uniform, well-formed surface with minimal defects. This setting also corresponds to the highest mechanical strength recorded in the study, likely due to the balanced heat input and effective plasticization of the polymer sheets. The welds at 900 rpm with 9 mm/min (Figure 4(d)) and 27 mm/min (Figure 4(f)) exhibit either excess heat accumulation or insufficient material flow, respectively, resulting in inferior weld quality. At the highest RS of 1100 rpm, and TS of 9 mm/min (Figure 4 (g)) and 18 mm/min (Figure 4(h)) display excessive flash and surface irregularities, indicative of overheating and possible thermal degradation of the base materials. The 27 mm/min setting (Figure 4(i)) results in a visibly uneven and porous seam, reflecting inadequate interfacial bonding.

Figure 4.

Weld bead structure of friction stir welded samples at different RS & TS using TC & TT.

At a RS of 800 rpm, the seam at 9 mm/min (Figure 4 (j)) shows moderate surface continuity but lacks the compact structure seen in the TC tool counterpart. As the traverse speed increases to 18 mm/min (Figure 4 (k)), material flow appears erratic with evident surface defects and voids. The weld at 800–27 mm/min (Figure 4 (l)) shows a highly irregular and porous seam, indicating inadequate stirring and insufficient heat generation for proper fusion. The welds fabricated at 900 rpm present similar trends. At 9 mm/min (Figure 4 (m)), excessive flash and surface waviness are observed, while 18 mm/min (Figure 4 (n)) results in poor consolidation with visible discontinuities. The weld at 27 mm/min (Figure 4 (o)) demonstrates marginal improvement in visual regularity but still lacks the smooth, consolidated profile achieved with the TC tool. For the 1100 rpm condition, excessive tool speed led to significant surface defects. The weld at 9 mm/min (Figure 4 (p)) shows over-stirring with degraded appearance and inconsistent flow. The sample at 18 mm/min (Figure 4 (q)) exhibits substantial roughness and irregular material mixing. In comparison, the 27 mm/min condition (Figure 4 (r)) produces a highly porous, defect-laden seam with limited bonding integrity. It has been observed that the TT tool consistently performed more destructively than the TC tool profile, exhibiting uneven material flow, poor surface smoothness, and partial fusion across all parameter values.

SEM Analysis of the Weld Bead Structure of FSWed Samples

Figure 5 presents SEM images illustrating the weld bead structure of friction stir welded ABS and PS thermoplastics using TC and TT tools under varying process parameters, including rotational speed (RS) and traverse speed (TS). The micrographs reveal distinct morphological changes, indicating adequate material mixing and joint quality at optimized conditions. Moderate plastic flow, partial bonding, and localized deformation are seen in image (a) at 800-9, indicating beginning-stage fusion. More fractured and rough morphology is seen in image (b) at 800-18, suggesting better material mixing as a result of faster tool traverse speed. The 900-9 specimen in (d) exhibits a consistently smooth surface, demonstrating effective stirring and enhanced interface bonding. A solid, uniform surface with few voids at 900-18 (e) denotes ideal heat input and plasticization. With (e) being the most refined, these four TC parameters 800-9, 800-18, 900-9, and 900-18 present favourable weld morphology. Conversely, at higher parameters, (c), (f), (g), (h), and (i) show indications of increasing voids, fibre pull-out, and uneven mixing, which are indicative of either overheating or inadequate heat distribution.

Figure 5.

SEM analysis of weld bead structure used different TC & TT and RS and TS.

Similar findings are observed with the TT tool at 800-9 (j), 900-9 (m), and 900-18 (n). The consistent weld contact in (j) illustrates steady material flow and heat management. The efficacy of the threaded tool in augmenting polymer stirring under these conditions is demonstrated by image (m), which displays fine dispersion and full intermixing. This pattern is further supported by image (n), which shows better bonding properties due to the surface’s apparent smoothness and consistency compared to its TC counterpart. The thread shape probably aided the consistent channeling of heat throughout the weld zone.

The Figure 5 shows that the TT tool profile has an irregular surface morphology, which negatively affects joint quality. In contrast, the TC tool profile provides a more refined and continuous weld bead surface due to its consistent stirring efficiency and stable heat input.

Tensile Analysis of FSWed Samples

Figure 6 shows the graph drawn between the UTS and process parameters (RS & TS) of friction stir welded samples using TC and TT pin profiles. To obtain accurate and statistically reliable measurements of the joining strength, three samples were evaluated for each experimental setup, ensuring precision and reducing variability in the results. The findings demonstrate that both welding conditions and tool shape significantly influence the UTS. Using the TC tool at an RS of 900 rpm and a TS of 18 mm/min, the highest UTS of 24.6 MPa was obtained out of all trials (as discussed in Table 2). This may be due to the TC profile’s favourable stirring action and even heat distribution, which improves material mixing and joint integrity. On the other hand, the TT tool produced a lower UTS of 20.5 MPa under the same conditions, demonstrating the impact of pin shape on weld quality. It has been observed that the TC tool performed better than the TT profile. By minimizing voids and internal flaws, the cylindrical design enables a more symmetrical and steady material flow. Even though its corners effectively encourage plastic flow, the triangle design frequently produces uneven heat input and unreliable bonding, particularly at lower rotational speeds or greater traversal speeds. Sahu et al. ²³ published similar results, showing that cylindrical tools provided better weld strength in the FSW of polyethene than polygonal profiles. When paired with a high traverse speed (800–27 mm/min), both tool geometries produced comparatively lower UTS values at low rotational speeds (800 rpm). Insufficient heat generation is the cause of this, which results in inadequate joint formation and poor material softening. As a result of excessive thermal input leading to material degradation or flash formation, excessively high rotational speeds (1100 rpm) resulted in decreased mechanical performance, especially at low traverse speeds. These findings are consistent with the thermal sensitivity of polymeric materials (Mubarak et al., ²⁴ Notably, there was a considerable drop in UTS at 900–27 mm/min and 1100–9 mm/min, indicating that the TT tool’s performance varied significantly. This bolsters the notion that properly selected parameters are necessary for both effective heat input and fault minimization. The thermal cycle and defect generation in thermoplastic FSW are greatly influenced by tool pin shape, which impacts joint performance, according to studies by Singh et al. ²⁵ Overall, the best condition, as determined by this study, was 900 rpm and 18 mm/min by TC tool, highlighting the significance of coordinated tool design and parameter adjustment. The superior UTS attained here is on par with or marginally greater than those found in previous research on similar, dissimilar thermoplastic junctions (such as PP-PS or ABS-HDPE), which generally varied between 17 and 23 MPa under comparable circumstances.

Figure 6.

Effect of tool pin geometry and process parameters on tensile strength.

Table 2.

Mechanical properties of FSW welded samples at different process parameters.

Sr No	Pin Shape	TS (mm/min)	RS (rpm)	UTS [Mpa]	UTS [%]	Elongation [%]	e/eabs [%]	H [Shore D]	H/Habs [%]	Standard deviations in H [ Shore D]
1	TC	9	800	20.5	67.77	5	242.72	77.8	101.04	3.89
2	TC	18	800	19.1	63.14	2.7	131.07	78.7	102.21	3.935
3	TC	27	800	20	66.12	3.5	169.90	79.4	103.12	3.97
4	TC	9	900	21.3	70.41	2.5	121.36	78	101.30	3.9
5	TC	18	900	24.5	80.99	2.08	100.97	83.6	108.57	3.91
6	TC	27	900	15.1	49.92	2.5	121.36	78.2	101.56	4.18
7	TC	9	1100	14.88	49.19	3.3	160.19	76.7	99.61	3.835
8	TC	18	1100	14.22	47.01	4	194.17	77.7	100.91	3.885
9	TC	27	1100	18.32	60.56	2.24	108.74	76.4	99.22	3.82
10	TT	9	800	18.98	62.74	2.95	143.20	76.8	99.74	3.84
11	TT	18	800	16.02	52.96	2.41	116.99	77.2	100.26	3.86
12	TT	27	800	17.3	57.19	1.9	92.23	76.2	98.96	3.81
13	TT	9	900	18.7	61.82	2.19	106.31	76.7	99.61	3.835
14	TT	18	900	18.5	61.16	2.87	139.32	78.6	102.08	3.93
15	TT	27	900	16.8	55.54	3.87	187.86	79.4	103.12	3.97
16	TT	9	1100	17.5	57.85	2.1	101.94	76.1	98.83	3.805
17	TT	18	1100	17.77	58.74	2.5	121.36	76.2	98.96	3.81
18	TT	27	1100	17.99	59.47	2.9	140.78	76.5	99.35	3.825

Figure 7 shows that the elongation (%) of dissimilar FSWed ABS and PS sheets is affected by TC and TT at different rotational (800–1100 rpm) and traverse speeds (9–27 mm/min). The TC profile produced higher elongation values across the majority of parameters, reaching a maximum of 5% at 800 rpm and 9 mm/min. It indicates that the material flow was uniform and heat generation was stable, resulting in superior ductility. However, because of inefficient stirring and an uneven distribution of heat, the TT profile typically produced less elongation, especially at low RS and high TS. However, the TT tool demonstrated better elongation (∼3.8%) at 900 rpm and 27 mm/min, indicating that more intense material shearing caused by its sharp edges may promote bonding at higher traverse speeds. The TT tool was unable to generate welds that were as strong or of the same quality as those made with the TC tool, which supported the idea that tool geometry is crucial for both mechanical performance and weld formation. The best weld quality was observed at 900 rpm and 18 mm/min with the TC tool, a condition that was not effectively replicated using the TT profile. These findings are consistent with earlier research, which demonstrated that non-axisymmetric pins, such as TT, lead to irregular flow and reduced joint integrity. ²⁶ In contrast, cylindrical pins improved ductility in thermoplastics by uniformly plasticising them. ²⁷

Figure 7.

Typical variation on the Elongation (%) with different tool geometries, RS, and TS.

Hardness Analysis of FSWed Samples Joined at Different Tool Pins and RS & TS

Vickers hardness testing was conducted using a standard Vickers hardness testing apparatus with a load capacity ranging from 10 gf to 100 kgf, equipped with an automatic loading system and a dwell time of 10–15 s. A diamond pyramid indenter with a square base, an included angle of 136° between opposite faces, and a tip radius below 0.5 µm was employed to ensure accurate and consistent impressions. Hardness values were obtained using an integrated optical microscope, and the Vickers Hardness Number (HV) was calculated from the measured diagonal lengths. The hardness of the FSW joints was evaluated using Vickers microhardness testing to characterize localized mechanical properties across the stir zone. A 10-mm-wide transverse section was extracted from the weld region, and the sample surface was ground and polished following standard metallographic procedures to achieve a smooth, defect-free finish. A 50-kgf load with an appropriate dwell time was applied for each indentation. Due to the small size of the Vickers indenter relative to the weld width, the method provides highly localized hardness measurements within the microstructurally heterogeneous weld zone. Hardness indents were taken along the centerline from PS → weld → ABS, beginning from the PS side, through the stir zone, and continuing into the ABS side, with uniform spacing between indentations. Figure 8 presented the bar chart of the Shore D hardness of FSWed joints fabricated using two distinct tool pin geometries, TC and TT, under various combinations of RS and TS. The error bars signify the standard deviation across repeated trials, providing insight into the reproducibility and consistency of the welding outcomes. Across all tested parameters, the TC tool profile consistently produced higher hardness values than the TT profile, suggesting more effective stirring action and material consolidation due to its symmetrical and continuous geometry. The highest hardness was achieved using the TC tool at 900 rpm and 18 mm/min, yielding a Shore D value of 83.6, which stands out from all other parameter sets. This peak performance is attributed to a synergistic effect of moderate heat generation and optimal material flow that avoids thermal degradation and promotes superior interfacial bonding between ABS and PS.

Figure 8.

Effect of varying process parameters (RS -TS, and Tool pin profile on hardness).

In contrast, the TT profile displayed relatively lower and more uniform hardness values, ranging from 76.1 to 79.4 Shore D. While the TT tool at 900 rpm and 27 mm/min also exhibited its maximum hardness of 79.4, it still fell short of the corresponding TC value, reinforcing the influence of tool geometry on joint properties. Interestingly, at higher rotational speeds (1100 rpm), a decline in hardness was observed for both tools, particularly under lower traverse speeds. This trend likely results from excessive heat input leading to localized material softening or degradation, as thermoplastics are sensitive to overheating. At 1100-9, the hardness values for TC and TT were 76.7 and 76.1 Shore D, respectively, indicating inadequate joint integrity. These results corroborate earlier studies by Raturi et al., ²⁸ which highlighted how tool pin geometry influences the distribution of hardness and microstructural refinement in polymer welds. Additionally, Singh et al. ²⁹ found that cylindrical tools provide more uniform weld zones in thermoplastics due to continuous temperature cycles, which is compatible with the increased hardness associated with the TC profile.

Conclusions

The present study demonstrates that the mechanical performance and weld integrity of dissimilar friction stir-welded thermoplastics ABS and PS with varying tool pin profiles are significantly influenced by tool pin shape. The experiment revealed the following observation, which is discussed below:

⁃ Among the two geometries investigated, the Threaded Cylindrical (TC) tool pin demonstrated superior performance across all evaluated parameters.

⁃ The Threaded Cylindrical (TC) tool produced the highest UTS of 24.5 MPa at 900 rpm and 18 mm/min, outperforming all other parameter combinations.

⁃ Shore D hardness peaked at 83.6 for the TC tool under the same optimal condition (900-18), indicating better resistance to surface indentation.

⁃ The maximum elongation of 5% was observed with the TC tool at 800 rpm and 9 mm/min, highlighting improved ductility in the weld joint.

⁃ The TT tool reached its best UTS of 18.98 MPa at 900 rpm and 18 mm/min, which is approximately 22.5% lower than that of the TC tool at its optimal.

⁃ TT tool welds were visually smoother but suffered from reduced mechanical strength due to insufficient stirring action and lower heat input.