Ultrasound-Enhanced Friction Stir Welding of Aluminum Alloy 6082: Advancements in Mechanical Properties and Microstructural Refinement

Abstract

This study examines the effects of ultrasound-enhanced friction stir welding (USE-FSW) on the mechanical properties and microstructural characteristics of aluminum alloy AA6082-T6, commonly used in automotive, aerospace, and construction industries. The investigation included tensile and bending tests, as well as detailed microstructural evaluations using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy-dispersive X-ray spectroscopy (EDS). The results indicate that USE-FSW led to an approximately 26% increase in tensile strength compared to similar samples produced by conventional friction stir welding (CFSW). Additionally, the elongation at break improved by around 52%, indicating better ductility. Flexural strength also showed a notable improvement of over 70%. Microstructural analysis revealed a finer grain structure in the stir zone, contributing to these mechanical enhancements. However, the changes in texture and grain orientation were relatively modest, as shown by EBSD and Kernel Average Misorientation (KAM) analyses. Overall, USE-FSW offers incremental improvements in weld quality and mechanical performance, making it a promising technique for producing joints with slightly enhanced strength and ductility.

1. Introduction

Friction stir welding (FSW) is a solid-state joining process invented in 1991 by The Welding Institute (TWI) [1]. It involves the use of a non-consumable rotating tool to generate frictional heat and plastic deformation at the joint interface. As the tool traverses along the weld line, it stirs the material together, producing a solid-phase bond. This method is known for producing high-quality joints with low residual stresses and minimal distortion, making it suitable for welding various aluminum alloys [2].

Various methods have been explored to enhance the FSW process, aiming to improve weld quality and efficiency [3].

Induction-assisted friction stir welding (IAFSW) utilizes induction heating to preheat the workpiece, particularly for high melting point, high hardness, and high strength metals, which reduces tool load, increases welding speed, and improves microstructure by inducing eddy currents and heating through resistive and hysteresis losses [4,5].

Laser-assisted friction stir welding (LAFSW) enhances the FSW process by using laser power to preheat the workpiece ahead of the rotating probe, which softens the material, reduces mechanical energy requirements, minimizes tool wear, and increases weld rate [6,7].

Electrically assisted friction stir welding (EAFSW) enhances the traditional FSW process by using an additional electric power source, which generates resistance heating and the electro-plastic effect to improve material softening, flow plasticity, welding efficiency, joint performance, and the ability to weld high-strength alloys while reducing defects in the weld root [8,9].

Arc-assisted friction stir welding (AAFSW) combines CFSW with plasma arc or GTAW to preheat the workpiece, enhancing fluidity and plastic flow, which improves tensile strength, elongation, welding speed, and hardness while reducing axial load and tool wear [10,11].

Ultrasonic-assisted friction stir welding (UAFSW) combines CFSW with ultrasonic vibrations to reduce resistance to deformation, enhance plastic deformation, and achieve defect-free welded joints with improved weld quality, increased strength and elongation, reduced axial forces, and extended tool life, making it suitable for light alloy materials like aluminum and magnesium. This advanced welding technique appears more promising than others due to its ability to produce equivalent softening with significantly less thermal energy and its effectiveness in overcoming CFSW shortcomings [12,13].

The integration of auxiliary energy sources into CFSW has proven to be important for improving the properties of the materials to be welded, especially when working with complex alloys and high-strength metals. Methods such as induction, laser, electric, arc, and ultrasonic not only reduce mechanical stress on the tool but also improve weld quality, material consumption, and overall joint performance. By introducing additional energy to preheat or soften the material, these methods allow for better control of the welding process, improving efficiency and enabling the welding of materials that may be difficult or impossible with conventional FSW. Among these methods, ultrasonic assist stands out as the most suitable because it consumes significantly less energy and is easier to integrate into the production process. Consequently, the use of these additional energy sources is crucial to optimize weld properties, reduce defects, and extend tool life, especially in industries where high-strength, defect-free joints are required [14].

Aluminum alloy AA6082-T6 is of significant interest in the automotive, aerospace, and construction industries due to its combination of high mechanical strength, good corrosion resistance, and favorable weight-to-strength ratio. These properties make it ideal for applications where reducing weight without compromising strength is crucial, such as in automotive components, structural elements in construction, and various aerospace applications. However, welding AW6082T6 presents certain challenges. The alloy’s high strength, derived from its T6 temper (solution heat-treated and artificially aged), can make it prone to issues like hot cracking, distortion, and loss of mechanical properties in the heat-affected zone (HAZ) during CFSW [15].

Ultrasound-enhanced friction stir welding (USE-FSW) is one of the advanced methods of the UAFSW. This process significantly improves weld quality, reduces defects, and enhances the mechanical properties of the joint [16,17]. This article examines the applicability of USE-FSW for welding butt joints of aluminum alloy AA6082-T6 [18,19], a material of significant interest in the automotive, aerospace, and construction industries due to its high mechanical properties and good corrosion resistance [20]. This article analyzes the characteristics of the USE-FSW process, the mechanical and microstructural properties of welded joints, and compares them with traditional FSW to determine the advantages and potential applications of this technology.

2. Materials and Methods

2.1. Experimental Setup and Materials

Sheets of alloy EN AA6082-T6 with geometry 280 mm × 100 mm × 3 mm (according to standard EN 485 [19]) were used in the investigation. Two sheets were selected from each rolling direction. The chemical composition of the base material is given in

Table 1. Chemical composition of the material EN AW-6082-T6 according to EN-573-3:2024 [21].

Al (%)	Cr (%)	Cu (%)	Fe (%)	Mg (%)	Mn (%)	Si (%)	Ti (%)	Zn (%)	Other (%)
95.2–98.3	≤0.25	≤0.1	≤0.5	0.6–1.2	0.4–1.0	0.7–1.3	≤0.1	≤0.2	≤0.15

.

The aluminum sheet welding tool was made from 1.2344 steel. The friction stir welding tool had a diameter of 16 mm for the shoulder and a 2.8 mm probe length with a metric thread of M4.5 (Figure 1a). The tool was quenched at 1050 in oil without subsequent tempering. The hardness after quenching was measured with a hardness tester 300/450 (WPM, Leipzig, Germany) and is 40–43 HRC. The chemical composition of the tool is given in

Table 2. Chemical composition of the material X40CRMOV5-1/1.2344 according to EN ISO-4957:2018 [22].

C (%)	Si (%)	Mn (%)	P (%)	S (%)	Cr (%)	Mo (%)	V (%)	Fe (%)
0.35–0.42	0.8–1.2	0.25–0.5	<0.03	<0.02	4.8–5.5	1.2–1.5	0.85–1.15	Balance

.

For welding, a DMU 80T CNC machine (DMG MORI, Bielefeld, Germany) was utilized (Figure 1b). Figure 1c illustrates the hybrid method scheme of USE-FSW. An ultrasonic roll seam module from ultrasonic roll seam welding module RM 20 SUM (Schunk Sonosystems, Wettenberg, Germany) was additionally mounted on the universal machining center. This module functions in sync and parallel with the friction stir welding tool during the process, transmitting ultrasonic power with a resonance frequency of 20 kHz, at a maximum generator power of 3000 W, and a maximum amplitude of 38 μm [23].

The parameters for welding were selected using the three-factor method. Relatively large, small, and medium parameter values were chosen to cover the boundary conditions. Then relatively low, medium, and high values after mechanical testing were selected to evaluate the effect of ultrasound (

Table 3. Welding parameter for AA6082-T6.

Sample	Rotational Speed (rpm)	Welding Speed (mm/min)	Tilt (°)	Amplitude (µm)
B1	2800	250	0.5	Without and with 7.6 (Work time 66.7%)
B2	800	400	0.5
B3	2800	250	1.5
B4	800	400	1.5
B5	2800	250	2.5
B6	800	400	2.5

). In this study, the ultrasonic power for aluminum welding was set at 400 ± 50 W. Only the horn feed speed was changed according to the current welding speed.

In order to determine defect-free ultrasonic parameters, preliminary weld tests were performed with parameters of 10, 20, 40, 60, and 80% from a maximum amplitude of 38 μm. It was found that misorientation of the sheets does not occur at an amplitude of 20% (7.6 μm).

To prevent the sheets from shifting during USE-FSW, it was decided that they should be pre-joined with a welded point at the end of the joint using the friction stir spot weld (FSSW) process prior to the main welding process (Figure 2).

2.2. Equipment and Sample Preparation

Specimens for tensile and three-point bending tests were precisely cut from the transverse cross-section perpendicular to the welding line using a 5-axis trainer water jet fine-cutting system as illustrated in Figure 3. Tensile tests were conducted using a 10 kN tensile machine ProLine 10 KN (Zwick Roell, Ulm, Germany) at room temperature [24,25], while bending tests were performed with a 100 kN machine (Zwick Roell), also at room temperature according to ISO 5173:2023 [26]. Metallographic specimens were hot-mounted using PolyFast (Struers, Champigny sur Marne cedex, France), followed by manual wet grinding on a rotating disc grinder with silicon carbide abrasive papers ranging from P400 to P4000 grit. Subsequent polishing was performed with diamond suspensions of 3 to 1 μm particle size on an automatic polishing machine, Tegramin 30 (Struers). The final polishing step involved using a chemo-mechanical oxidizing polishing suspension, OP-S (Struers), to eliminate any embedded diamond particles from the samples [27]. The etching process involved two immersion stages: First, the specimen was immersed for 60 s in a 0.5 M sodium hydroxide solution; then, it was etched in a 0.25 M sodium hydroxide solution with 4% potassium permanganate (KMnO₄) for 15 s [28].

Microstructural examination was performed using an optical microscope GX51 (Olympus Deutschland GmbH, Hamburg, Germany). NEON40EsB field-emission SEM (Carl Zeiss, Oberkochen, Germany) with EDXS and EBSD analyzer was used for scanning and mapping.

3. Results

3.1. Visual Inspection

A visual inspection of the produced joints was conducted in accordance with the standards set forth in ISO 25239-4 [29]. Samples B1, B3, and B5, which were welded with CFSW, exhibited evidence of scoring on the weld surface. Such surface characteristics result in a high concentration of surface stresses, which can have a detrimental impact on the mechanical properties of the joints, particularly regarding fatigue cycle life [30]. In contrast, the same samples, which were welded using USE-FSW, did not display this defect. The surface exhibited a markedly smoother structure.

On specimens B2, B4, and B6, there is a noticeable irregular weld width caused by poor contact with the tool shoulder, whereas on similar specimens made with USE-FSW, the weld width has a more static character. However, some of the specimens also show a groove defect, which seems to be caused by overheating of the material in this area [31], which allows us to conclude that it is possible to increase the welding speed with the chosen inclination angles and spindle speed. The overview of joints is shown in Figure 4.

3.2. Tensile Strength

The mechanical properties of the base material after tensile are recorded in

Table 4. Mechanical properties of the base material after pull testing.

Rolling Direction	Rmax (MPa)	R0.2 (MPa)	A (%)
||	329.1 ± 1.61	278.2 ± 1.33	16.53 ± 0.87
⊥	333.2 ± 1.17	281 ± 1.50	18.9 ± 2.28

.

The analysis of tensile testing reveals that the mean values for all samples tested with USE-FSW are higher than those obtained with CFSW, except for sample B3. Standard deviations for USE-FSW are typically lower or comparable to those for CFSW, indicating reduced variability and more consistent outcomes. USE-FSW generally enhances the mean tensile strength values compared to CFSW. Ultrasonic treatment minimizes value variation for most specimens, suggesting a more stable welding process and predictable results. This is visually confirmed by the graph in Figure 5, where the average tensile strength values for USE-FSW are higher in all samples except for sample B3, where the difference is only 3 MPa (227 MPa vs. 231 MPa). For example, for specimen B1, the average strength of USE-FSW is 225 MPa, which is 47 MPa higher compared to CFSW (178 MPa). A similar trend is observed in specimens B2, B4, and B5, where the difference is 14 MPa, 54 MPa, and 10 MPa, respectively. Furthermore, the shorter standard deviations for USE-FSW confirm the smaller spread of values.

Similarly, the mean values of elongation at break for all USE-FSW specimens are higher than those for CFSW except for specimens B4 and B6. The highest standard deviation for CFSW is observed in specimen B5 (0.65%), as is the highest for USE-FSW (1.06%) in specimen B6. The graph in Figure 6 confirms these findings: The highest mean value of CFSW is observed for sample B3 (4.18%), increasing significantly to 6.375% for USE-FSW. The lowest mean CFSW value is observed for sample B5 (0.46%), increasing to 0.775% for USE-FSW. However, the standard deviations for USE-FSW are generally higher or comparable to those for CFSW, indicating a wider range of values, especially for samples B3 and B6. Thus, although ultrasonic exposure in friction stir welding improves elongation at break and ductility, it also increases variability, suggesting that the process needs to be optimized to achieve more consistent results.

3.3. Three-Point Bending Stress

The mechanical properties of the base material after the three-point bending test are recorded in

Table 5. Mechanical properties of the base material after bend testing.

Rolling Direction	Rmax (MPa)	ΔL at Rmax (mm)
||	1280.63 ± 31.12	38.50 ± 3.2
⟂	1089.13 ± 70.45	35.83 ± 1.48

.

The results of the weld face bending test show that the use of USE-FSW significantly improves the bending characteristics compared to CFSW (Figure 7). The average bending values for all specimens except B1 are higher with USE-FSW. For example, for specimen B2, the average bending value is 1285 MPa for USE-FSW, which is more than double the value for CFSW (722.5 MPa). Similarly, for specimen B4, the bending value for USE-FSW is 1320 MPa, which is also significantly higher than that for CFSW (1295 MPa). Specimens B3 and B6 also show significant improvements, with an increase in flexural strength of 7% and 111%, respectively.

The standard deviations show that the use of ultrasonics reduces the variability of the test results. For specimen B5, the standard deviation for USE-FSW is 17.5, which is significantly lower than that of CFSW (90), indicating greater predictability and stability of the material properties when ultrasound is used. In other specimens, such as B2 and B4, the standard deviations of USE-FSW are also lower than those of CFSW, confirming the higher repeatability of the results.

In this first part of the study, the focus was on the flexural strength of the face side of the weld.

The results of the root side bending test show that USE-FSW provides more consistent and in some cases higher bending strength values compared to CFSW (Figure 8). For example, for specimen B1, the average flexural strength with USE-FSW is 1305 MPa, which is significantly higher than that of CFSW (419 MPa). In specimen B2, the values are similar, but USE-FSW shows less variation with a standard deviation of 16.5, which is less compared to 17 for CFSW. Specimen B3 shows similar results for both techniques, with USE-FSW showing 1355 MPa versus 1350 MPa for CFSW, indicating consistency of performance.

For specimen B5, the average flexural strength of USE-FSW is 444.5 MPa, which is lower than that of CFSW (545 MPa), but the standard deviation is also lower (32.5 vs. 50), indicating more consistent results when ultrasonication is used.

3.4. Microstructure

B3 specimens with the best combination of tensile strength, elongation at break, and three-point bending stress results were selected to study the weld microstructure. Figure 9 shows a cross-section of a CFSW weld with zones of thermal and mechanical influence highlighted [32]. As can be seen from the figure, the weld contains a kissing bond defect which appears from the retreating side. The formation of this defect could be influenced by a too-thick oxide layer as well as an insufficiently high temperature in the stir zone [33]. A tunnel defect with a diameter of about 0.56 mm is also observed on the advancing side. The appearance of this defect indicates that the heat input from tool friction during welding was insufficient to form a solid nugget. The specimen was fractured along the kissing bond profile, starting from the tunnel defect.

Figure 10 shows a specimen with the same welding parameters as the previous one but with ultrasonic exposure with an amplitude of 20 μm. As we can see the kissing bond defect has almost completely disappeared and the material from the retreating side reached the advancing side and almost covered the tunnel defect. This indicates that the ultrasonic impact had a positive effect on the mixing of the material during welding, an improvement confirmed by the mechanical tests where a significant improvement in the relative elongation of the weld was observed.

3.5. SEM Analysis

Figure 11 shows the microstructure of the AW 6082T6 aluminum alloy, captured using an SEM at a magnification of 1000×. Grains have different shapes and sizes separated by well-defined boundaries. The light-colored areas probably represent secondary phases such as Mg₂Si, while the black areas may indicate pores, voids, or oxidized zones. The microstructure shows a uniform distribution of fine particles within the aluminum matrix and the presence of defects such as cracks and pores. This structure is characteristic of heat-treated aluminum alloys.

Figure 12 depicts the microstructure after FSW with ultrasonic enhancement, revealing a more uniform grain distribution and smaller grain size. Figure 13 shows the Thermo-Mechanically Affected Zone (TMAZ) after CFSW, characterized by larger and less uniform grains.

The EDS analysis in Figure 14 shows the different compositions of different spots in the TMAZ region of AA6082-T6 alloy. The graph in Figure 15 shows the elemental composition of the three EDS spots analyzed in the TMAZ of aluminum alloy AA6082-T6 after friction stir welding. The graph compares the weight percentages of key elements (Al, Si, Mn, Fe, O, Mg) in the three spots. Spot 1 EDS probably represents a region with an intermetallic phase containing Fe and Mn, which is often found in aluminum alloys. EDS spot 3 may include oxide phases due to the presence of oxygen as well as aluminum and silicon, which is characteristic of aluminum oxide or silicate inclusions. It should be noted that the source of these oxides is unclear: They could have formed during the welding process or during the metallographic preparation of the sample. In contrast, the EDS spectrum of spot 6 represents mostly pure aluminum with minor traces of other elements, indicating a pure aluminum matrix with minimal contamination or secondary phases.

The microstructure in Figure 16, representing USE-FSW, shows a more homogeneous distribution of grains with fewer defects. Large white inclusions are visible, likely indicating intermetallic compounds or secondary phases, with refined and uniform grains suggesting enhanced mechanical properties due to ultrasonic enhancement. The central blurred region represents an area of intense plastic deformation and material mixing. During USE-FSW, the combination of mechanical forces and ultrasonic vibrations leads to significant microstructural changes, including grain refinement and redistribution, which can manifest as a visually blurred zone. In contrast, Figure 17, representing CFSW, exhibits a higher presence of microcracks and pores, indicating more defects. The grains are less uniform and more irregular in shape, with smaller white inclusions more evenly distributed across the structure.

Figure 18 presents microhardness values for weld seams of EN AA6082-T6 material, produced by CFSW and USE-FSW, with hardness measurements taken under an HV0.5 load. Measurements were conducted using a WH Tukon 1102 hardness tester (Wilson Hardness, Uzwil, Switzerland). The base material hardness, measured at 120.9 ± 4.04 HV, was determined from 10 measurements with a 1 mm step. For welded samples, 30 measurements were taken on each sample with a 0.4 mm step, at a distance of 0.2 mm from the sample’s surface. Both methods showed a decrease in hardness within the weld zone. In SZ, hardness for both methods is similar (around 80–85 HV), though USE-FSW showed a slight increase attributed to a more uniform structure under ultrasonic influence. In the TMAZ and HAZ, hardness in USE-FSW was also higher, suggesting a more effective reduction in residual stresses and improved recrystallization.

The inverse pole figure (IPF) in Figure 19a and harmonic texture in Figure 19c show clusters of grains with orientations close to [001] and [100], indicating the presence of texture and preferred grain orientation along these directions. The presence of these clusters can be attributed to material processing processes such as rolling or heat treatment, confirming the presence of directional texture in the material.

Figure 19b shows the KAM map of the base material. The analysis reveals a well-defined grain structure with largely uniform, polygonal grains, indicative of the alloy’s processing history, including solution treatment and artificial aging. The color gradient with blue areas represents minimal plastic deformation and well-aligned grains. The homogeneity in the overall misorientation distribution suggests that the material retains good structural integrity. Figure 19d shows that the polydisperse grain size distribution is indicative of complex machining processes including mechanical deformation and heat treatment.

The grains in this zone appear more elongated and deformed compared to the parent material due to the thermomechanical effects of FSW (Figure 20a). The variety of colors indicates a random distribution of grain orientations, but certain areas with predominant orientations indicate texture caused by the welding process. The pole figures (Figure 20c) for [001], [100], and [010] confirm the presence of texture, indicating strong texture and preferred grain orientations along these directions, suggesting anisotropy caused by the welding process.

Figure 20b reveals a finely grained structure with low to moderate misorientation angles, predominantly in the blue to green range, indicating uniform plastic deformation without significant dislocation buildup. The misorientations within the grains suggest that the material has been deformed but has survived dynamic recrystallization and sub-grain rotation. The average grain size in the TMAZ zone is significantly smaller than that of the parent material due to the intense mechanical action and recrystallization during the welding process. Figure 20d shows that most of the grains are about 7 microns in size, indicating grain refinement due to thermomechanical action.

Figure 21a shows that the grains after USE-FSW in TMAZ appear deformed and more uniform compared to CFSW. The grains have a less elongated and more uniform shape. The variety of colors indicates a random distribution of grain orientations, but certain areas with predominant orientations indicate texture caused by the welding process. Figure 21c confirms the presence of texture and indicates strong texture and preferred grain orientations along [001], [100], and [010] directions, indicating anisotropy caused by the welding process.

Figure 21b reveals a microstructure characterized by well-defined grains with low to moderate misorientation angles, similar to those observed in conventional FSW, but with some notable differences. The grains appear slightly more refined, and the distribution of misorientation is more uniform across the map. The presence of green hues, indicative of areas with higher local strain, is less pronounced. Figure 21d shows that the grain sizes are much smaller compared to CFSW. The average grain size in the TMAZ zone is smaller due to the ultrasonic action, which promotes grain refinement.

Figure 22a shows a uniform distribution of medium-sized grains with different crystallographic orientations. The grains are mainly isometric and characterized by a relatively uniform shape, indicating a calmer thermoplastic flow regime of the material. In this case, the predominant {001}<100> orientation is often associated with a uniform distribution of deformation and recrystallization. Such a texture is characteristic of processes in which significant temperatures and plastic deformation lead to dynamic recrystallization and efficient grain redistribution (Figure 22c). In the case of CFSW, such stable conditions lead to the formation of a microstructure with rather predictable mechanical properties, where grain orientation contributes to a higher resistance to further deformation [34].

Figure 22b shows a refined grain structure with relatively low misorientation angles, as indicated by the predominance of blue and green hues. This suggests that the grains within the stirring zone have undergone significant recrystallization. The fine grain size and low misorientation are typical of the stirring zone, where dynamic recrystallization occurs due to the combined effects of high temperature and severe plastic deformation. The even distribution of misorientation across the map implies a homogenous deformation process. Figure 22d shows that the average grain size in the stir zone is smaller due to the intense mechanical action and dynamic recrystallization during the welding process. Most of the grains are around 4–5 microns in size.

Figure 23a reveals significant microstructural heterogeneity, particularly in the central blurred region, which likely corresponds to an area of intense plastic deformation and material mixing resulting from USE-FSW. The varied colors within this region indicate substantial grain reorientation, suggesting the development of microstructural anisotropy due to the influence of ultrasonic vibrations (Figure 23c). This central zone also contains non-indexed areas, visible as dark spots, which may indicate regions where the EBSD system was unable to determine crystallographic orientation. These could be caused by high local deformation or the presence of inclusions, potentially correlating with defects observed in related microstructural analyses. These observations suggest that ultrasonic vibrations during USE-FSW significantly influence the microstructure, leading to regions of complex grain interactions and possible phase mixing. The predominant texture {111}<110>, in this case, indicates the presence of conditions of intense plastic deformation and inhomogeneous thermal field, which promotes the formation of subgrains and an increase in the proportion of high-angle grain boundaries [35].

Figure 21b highlights the degree of misorientation within the grains, with the central region exhibiting elevated misorientation values. This increased misorientation aligns with the previously noted blurred region in Figure 23b, reinforcing the idea that this area has undergone significant plastic deformation and strain localization due to USE-FSW. Figure 21d showing grain size distribution reflects a range of grain sizes, with a peak around 8–10 µm, indicating grain refinement, likely driven by dynamic recrystallization during the welding process. Figure 23c illustrates the crystallographic texture of the material, revealing preferred orientations that may be more pronounced in the heavily deformed central region. Together, these images suggest that ultrasonic enhancement during FSW induces substantial microstructural changes, including increased misorientation, grain refinement, and texture development, particularly in regions subjected to intense material flow and mixing.

4. Discussion

The discussion of the results will proceed sequentially, starting with the findings from the visual inspection, followed by the mechanical testing results, and concluding with the microstructural analysis, including EBSD (Electron Backscatter Diffraction) and KAM (Kernel Average Misorientation) evaluations.

4.1. Visual Inspection

The visual inspection of the welded joints revealed distinct differences between the samples processed with conventional friction stir welding (CFSW) and those enhanced by ultrasound (USE-FSW). The CFSW samples exhibited noticeable surface defects such as scoring and irregular weld widths, which are typically associated with increased surface stresses that can negatively impact fatigue life. In contrast, the USE-FSW samples displayed a much smoother surface with fewer visible defects. The smoother surface is indicative of better material flow and less resistance during the welding process, attributed to the ultrasonic vibrations aiding in the uniform mixing of the material.

4.2. Mechanical Testing

Mechanical testing, including tensile strength and three-point bending tests, revealed that the USE-FSW samples generally exhibited superior mechanical properties compared to those welded using CFSW. The tensile strength of the USE-FSW samples was consistently higher with a more uniform distribution of results, suggesting that ultrasonic assistance stabilizes the mechanical properties of the joint. These improvements can be attributed to several strengthening mechanisms activated during the USE-FSW process.

Firstly, the ultrasonic vibrations promote a more refined grain structure through dynamic recrystallization, enhancing grain boundary strengthening. Finer grains increase the total grain boundary area, impeding dislocation motion and thus increasing the material’s strength. Secondly, the ultrasonic energy introduces a higher dislocation density within the material, contributing to dislocation strengthening. The increased dislocation density creates obstacles to dislocation movement, leading to enhanced strength. Lastly, the ultrasonic assistance may influence the precipitation behavior of strengthening phases in the aluminum alloy, leading to precipitation strengthening. A more uniform and finely distributed precipitate structure can effectively hinder dislocation motion, further improving mechanical properties [36,37].

The elongation at break, indicative of the material’s ductility, was also improved in most USE-FSW samples. This enhancement suggests that ultrasonic vibrations not only increase strength but also ductility, likely due to the refined grain structure and reduction in welding defects. However, it is important to note that these improvements, while statistically significant, are not overwhelmingly large. The standard deviations in elongation at break for USE-FSW were sometimes higher than those for CFSW, indicating that while ultrasound enhances ductility, it also introduces some variability that might require further optimization of process parameters [38].

The three-point bending tests further supported the tensile testing results, showing that USE-FSW generally enhances the flexural strength of the joints. The increased flexural strength in USE-FSW samples can be attributed to the combined effects of grain boundary strengthening due to refined grains, dislocation strengthening from increased dislocation density, and precipitation strengthening from a more uniform precipitate distribution. These mechanisms collectively contribute to better overall joint performance under stress by impeding dislocation motion and reducing common welding defects.

4.3. Microstructural Analysis

The microstructural analysis focused on grain size and distribution in both CFSW and USE-FSW. SEM and EBSD were used to investigate the grain structure and texture in the SZ and TMAZ.

The grain size in the SZ of USE-FSW samples was significantly reduced compared to CFSW samples. This grain refinement is attributed to the effects of ultrasonic vibrations, which promote dynamic recrystallization during the welding process. The average grain size in the USE-FSW samples was about 8 μm, while in the CFSW samples, the average grain size was about 10 μm. This fine grain structure in USE-FSW is directly related to the observed improvement in mechanical properties, as the finer grains increase the total grain boundary area, which inhibits the movement of dislocations and increases the strength of the material.

In addition, the grain size distribution was more uniform in the USE-FSW samples as evidenced by the KAM maps. The decrease in grain misorientation within the SZ suggests that ultrasonic energy promotes microstructure stabilization by reducing internal stresses and increasing grain boundary mobility. The presence of homogeneous and fine grains likely contributed to the improved tensile strength and ductility observed in mechanical testing, where USE-FSW specimens outperformed their CFSW counterparts.

The effect of grain size on mechanical properties is also supported by the Hall–Petch relationship, which links smaller grain sizes to improved strength [36]. The significant refinement of the grain structure of USE-FSW specimens leads to an increase in yield strength and tensile strength, which is confirmed by a 26% improvement in tensile strength compared to CFSW specimens. Moreover, the 52% increase in elongation at break is also partly attributed to grain refinement, which favors a more homogeneous plastic deformation under tensile strain.

Similar trends were observed in TMAZ, although grain refinement was less pronounced compared to SZ. USE-FSW specimens exhibited a more uniform grain distribution and a reduction in weld defects such as kissing joints and tunnel voids, which are known to negatively affect the mechanical integrity of the weld.

Overall, the finer grain structure and more uniform grain distribution in USE-FSW specimens result in improved mechanical properties, emphasizing the critical role of grain size in determining the performance of friction stir welds.

5. Conclusions

This study demonstrates that USE-FSW offers measurable improvements over CFSW when applied to aluminum alloy AA6082-T6. The key findings are summarized as follows:

USE-FSW samples exhibited an increase in tensile strength by approximately 26% compared to CFSW, with values of 225 MPa and 167.75 MPa for B1 and B4 samples, compared to 174.8 MPa and 121.6 MPa in CFSW.

The elongation at break improved by around 52%, with B1 showing 5.8% and B3 showing 6.4% in USE-FSW compared to 1.2% and 4.5% in CFSW.

Flexural strength showed an improvement of over 70%, with bending stress reaching 1450 MPa for B1 in the face test, 1100 MPa for B2 in the root test, and 1300 MPa for B6, compared to lower values in CFSW.

Grain size in the stir zone was reduced, with USE-FSW samples showing finer grains compared to those produced by CFSW, contributing to the enhanced mechanical properties observed.

The incidence of common welding defects such as kissing bonds and tunnel voids was significantly reduced in USE-FSW samples, leading to improved weld quality.

Despite these improvements, the changes in texture and grain orientation were relatively modest, indicating that while ultrasonic vibrations aid in material refinement, they do not drastically alter the fundamental microstructure.

The results of this study are consistent with previous studies showing the positive effects of ultrasonic assistance in friction stir welding. While other studies have demonstrated improved grain refinement and mechanical properties when ultrasonic vibration is used, this paper presents additional data specifically for AA6082-T6 alloy. The comparison between CFSW and USE-FSW presented here expands the understanding of how ultrasonic vibration can affect weld quality and microstructure. While improvements in mechanical properties and grain refinement are evident, further research is needed to optimize the parameters and fully exploit the potential of ultrasonic-assisted friction stir welding.