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Article

Mechanical and Microstructural Properties of High-Speed Friction Stir Welding of AA 7020 Aluminum Alloy Using Multi-Pin Tool

by
Ramin Delir Nazarlou
*,
Samita Salim
,
Michael Wiegand
,
Christian Wolf
and
Stefan Böhm
Department for Cutting and Joining Manufacturing Processes, University of Kassel, 34125 Kassel, Germany
*
Author to whom correspondence should be addressed.
Metals 2025, 15(5), 511; https://doi.org/10.3390/met15050511
Submission received: 26 March 2025 / Revised: 28 April 2025 / Accepted: 29 April 2025 / Published: 30 April 2025
(This article belongs to the Section Welding and Joining)
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Abstract

High-speed friction stir welding (HSFSW) has emerged as a promising technique for improving the manufacturing efficiency of aluminum alloy structures by enabling faster welding while maintaining the quality of welded joints. This study investigates the mechanical properties and microstructural characteristics of AA 7020-T651 aluminum alloy joints welded using a novel multi-pin tool at high feed rates ranging from 2500 to 6000 mm/min under a constant rotational speed of 4000 rpm. Defect-free welds were successfully fabricated, as confirmed by metallographic analysis and micro-computed tomography (µ-CT). The multi-pin tool facilitated consistent material flow and heat distribution, which contributed to reliable joint formation across all feed rates. At the highest feed rate, the tensile strength reached 76% of the base material. A consistent softening in the nugget zone (NZ) was observed, and electron backscatter diffraction (EBSD) analysis showed a more than 70% grain size reduction in this zone, averaging ~3 µm, due to dynamic recrystallization. These findings underscore the suitability of HSFSW with multi-pin tools for high-speed industrial applications, offering enhanced productivity without compromising structural integrity.

Graphical Abstract

1. Introduction

Aluminum alloys are considered a viable alternative to steel construction in the transportation industry due to their lightweight nature [1]. Heat-treatable 7xxx aluminum alloys are ideally suited for this purpose due to their low density, good corrosion resistance, and excellent mechanical properties. Welding of precipitation-strengthening (2xxx, 6xxx, and 7xxx series) aluminum alloys by fusion welding is extremely difficult due to the formation of dendritic microstructure, micro-cracks, and porosity that compromise the structural integrity of the joints [2]. Hence, they are classified as non-weld-able aluminum alloys by means of fusion welding processes [3]. Fusion welding can be successfully applied to certain aluminum alloys, including 6xxx series alloys such as AA6061, particularly in applications like bicycle frames. However, for many precipitation-strengthened grades, especially in the 2xxx and 7xxx series, fusion welding can present challenges such as hot cracking, porosity, and reduction in mechanical properties [4]. These challenges often motivate the use of alternative solid-state processes like friction stir welding (FSW), particularly in applications requiring high joint integrity and minimal distortion. FSW is a superior alternative to traditional fusion welding procedures to join these alloys, as this process occurs in the solid state and thus avoids the appearance of liquid phases. Therefore, the problems associated with molten aluminum can be easily avoided and simplify the manufacturing procedures [5,6].
FSW is a process that was developed at The Welding Institute (TWI) [7] in which a non-consumable rotating welding tool softens work pieces to be joined through friction and plastic work, resulting in weld joints. Due to severe heat and mechanical strain during FSW, the welded zone undergoes significant metallurgical changes in the weld nugget zone (NZ) and adjacent areas [8]. The thermal cycle generated during FSW does not lead to the melting of the base metal (BM) but is sufficient to alter the mechanical properties in the welded joint. Hence, it can prevent solidification cracking and produce high-quality joints when optimized parameters are employed. The other advantages of FSW compared to fusion welding include better corrosion resistance and tensile strength. Also, FSW typically results in welds with favorable visual appearance, along with low distortion and residual stress—advantages that are especially pronounced when compared to some conventional fusion welding techniques [8,9,10].
The FSW tool, which consists of a shoulder and a pin, is one of the important parameters of FSW, and there have been notable developments in FSW tool design to improve the weld quality and process effectiveness [11,12]. Research on various pin geometries including square, triangular, circular, triflute, and tapered has already been carried out [6,13]. A multi-pin tool is a relatively novel concept in FSW in which more than one pin is used, arranged eccentrically in the FSW tool [14,15]. Previous reports showed that a single-pin tool has drawbacks such as insufficient material mixing and heat generation that can lead to defect formation such as voids, tunnels, and incomplete root welds [9,10]. It has been shown in earlier research that using the multi-pin tool (two pins) in EN AW 5083-H111 and EN AW 7020-T651 alloy improves the material stirring in the nugget zone, resulting in a gap bridge ability of up to 50% of the sheet thickness in the butt joint and broken oxide lines (lazy S defect) in the stirred zone [14]. Furthermore, previous reports indicate that the presence of this kind of lazy S defect at the root of the weld can initiate tensile fracture and propagate along this line [16,17]. Additionally, the usage of two-pin tools improves corrosion resistance and induces a greater degree of plastic deformation which also improves the mechanical properties of the joint [18].
Feed rate and tool rotation speed are two additional key parameters in FSW which must be specifically adapted to the respective application. They both control the material flow and heat input during the process [8]. Furthermore, the feed rate has been reported to be one of the major parameters in controlling the cooling rate after welding [19,20]. At a high feed rate, the rotating tool proceeds through the work piece faster after joining the material generating a high cooling rate and finer grain size [21]. Nonetheless, if the generated heat is insufficient because of an excessive feed rate and lower rotational speed, the material mixing is incomplete and causes many types of defects such as tunnels and internal voids [8]. In precipitation-strengthening Al alloys, a key aspect is the dissolution and coarsening of precipitates and the extent of microstructure distribution, which are influenced by the thermal cycle and material flow behavior, significantly affecting the final mechanical properties of welded joints. To achieve high-strength friction stir-welded joints in 7xxx aluminum alloys, it is essential to maximize the dissolution process while limiting the coarsening process [21]. This can be accomplished by increasing the rotational speed in which particle dissolution can be facilitated as well as by increasing the transverse speed to minimize the coarsening process [22]. In addition to friction stir welding, explosion welding (EXW) is another solid-state joining process that has shown significant success in bonding metallic materials, especially those that are difficult to weld using conventional fusion techniques. EXW operates by accelerating one metal plate toward another using controlled explosive charges, creating a high-velocity impact that produces a metallurgical bond through extreme plastic deformation and interfacial jetting. This technique is particularly well suited for joining dissimilar metals or for producing multi-layered clad materials with excellent mechanical and corrosion-resistant properties [23]. It is widely used in industries such as petrochemical, aerospace, and marine applications, where high-performance bimetallic structures are required.
However, while EXW offers excellent bonding strength and reliability, its operational nature—based on shock wave propagation and rapid energy discharge—makes it more appropriate for batch production or cladding applications, rather than continuous or automated welding of structural components. It also requires specialized safety protocols, explosive handling expertise, and post-weld processing, which can limit its practical implementation in standard manufacturing lines.
On the other hand, friction stir welding (FSW), and particularly high-speed FSW (HSFSW) using advanced tool designs like multi-pin configurations, provides a more controlled, continuous, and scalable solution for industrial applications involving similar alloys such as AA 7020. FSW enables precise heat and material flow control, minimal distortion, and high repeatability—qualities essential for high-volume structural manufacturing. For these reasons, the current study focuses on the application and performance evaluation of HSFSW using a multi-pin tool, targeting process optimization for lightweight structural components while maintaining weld integrity and productivity.
Over the last 30 years, a lot of studies have been conducted on FSW of different grades of Al alloys, including assessments of joint characteristics, process parameter optimization, and process-specific material flow descriptions [24,25,26,27,28]. However, from an industrial point of view, FSW still faces some limitations because of its comparatively low feed rate. An enhanced feed rate in FSW is advantageous in terms of industrial productivity. When the welding parameters are selected in an industrial setting, the goal is to maximize the feed rate while maintaining adequate weld quality without any defects. However, achieving high speeds in the FSW process without impairing the mechanical properties is a major challenge.
Previous studies have attempted to investigate HSFSW of heat-treated aluminum alloys. Firstly, Rodrigues et al. [29] used two widely used structural aluminum alloys, EN AW 5083-H111 and EN AW 6082-T6. The 5083 alloy was capable of being welded at a maximum feed rate of 500 mm/min and rotation rate of 500 rpm for 4 mm sheet thickness, whereas for 6082-T6 alloy the maximum feed rate achievable was 1100 mm/min at a 1300 rpm rotation rate. Beyond these feed rates, all of the samples showed defects in the form of surface flaws or internal voids. A few years later, researchers found that at a 2500 mm/min feed rate and 2100 rpm rotation speed, fine and evenly dispersed grains were formed with a 2 mm-thick EN AW 6082-T6 alloy, which also increased the overall hardness in the nugget zone [30]. In another investigation by Xu [31], approximately 71% tensile strength of the base material could be obtained using the FSW of 3 mm-thick 6063-T6 alloy at a feed rate of 3000 mm/min and rotation rate of 3000 rpm. Recently, an improvement in HSFSW in AA 6063-T6 aluminum alloy was demonstrated in another study [32] in which the optical micrograph of the weld cross-section demonstrated complete weld penetration in the butt joint at a feed rate up to 4500 mm/min and 3500 rpm rotation speed. However, the joint efficiency dropped to 65% from 72% when the feed rate was increased from 4000 mm/min to 4500 mm/min. Microscopic examination of their tensile samples revealed that the weld surface was over-plunged or undercut, which was caused by the high plunge force employed to counteract the reduced tool rotation (3500 rpm). Bernard et al. [33] compared the thermal cycles and microstructural development of 5182-H111 alloy at low (200 mm/min) and high (1500 mm/min) speeds. The processes showed decreased flow stress, and the microstructures generated in the thermomechanical affected zone (TMAZ) and stirred zone are remarkably distinct at the two feed rates, with a substantial conduction component in low-speed welding and a high strain rate component in high-speed welding. In another investigation [34], aluminum tailor-welded blanks (TWBs) using AA 5182-O alloy were successfully produced with dissimilar sheet thicknesses (1.2 mm and 2 mm) at a 3000 mm/min feed rate. This enabled the development of high-volume aluminum TWBs, resulting in significant weight and cost reductions.
It must be emphasized that all of the previous investigations into HSFSW mentioned earlier focused mainly on 5xxx or 6xxx Al alloys, and there is relatively fewer literature addressing HSFSW of 7xxx alloys. Zhang et al. [21] reported that HSFSW on 2 mm-thick EN AW 7075-T6 alloy sheets at a welding and rotation speed of 2900 mm/min and 1950 rpm yielded small voids at the root of the stir zone in the advancing site. Additionally, the drop in hardness in the heat-affected zone (HAZ) was less severe at higher feed rates associated with low heat input and quick cooling rates.
Figure 1 provides a comparative summary of welding parameters reported in previous HSFSW studies on various aluminum alloy grades, represented by green squares [5,16,17,21,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. The parameter range investigated in the present study defined by the applied feed rates and constant rotational speed is denoted by red triangles. As mentioned earlier, HSFSW investigations were limited to feed rates up to 4500 mm/min and were exclusively conducted using conventional single-pin tools. In contrast, this study extends the process envelope by employing a multi-pin tool at feed rates up to 6000 mm/min, emphasizing its novelty and potential industrial relevance.
To the best of the knowledge of the authors, no prior work has been reported on the successful production of defect-free welds at feed rates as high as 6000 mm/min using a multi-pin tool on EN AW 7020 alloy. Hence, this study presents a novel approach with the potential to improve joint properties, as a multi-pin tool exhibits better material mixing ability in the stir zone, thereby minimizing the likelihood of forming defects at high speed [14,46]. EN AW 7020 alloy was selected because of its high strength-to-weight ratio, making it frequently used in light structures such as the storage tanks of space rockets, military portable bridges, and bicycle frames [5,6,24,47], and a high output volume in these industries can be accomplished by welding at high feed rates. To achieve this, feed rates ranging from 2500 to 6000 mm/min were applied at a fixed rotating speed of 4000 rpm. The microstructural and mechanical properties of the joints were investigated with optical microscopy, EBSD, and tensile test and Vickers hardness measurements. Additionally, the electrical resistivity was measured on the cross-sections of the welded samples to assess the microstructural changes and identify potential defects within the weld zone.

2. Materials and Methods

In the present investigation, 2 mm-thick AA 7020 aluminum alloy sheets (supplied by BIKAR METALLE GmbH, Bad Berleburg, Germany) were used in as-received T651 condition. Sheets with dimensions of 200 mm × 50 mm were utilized for the welding process. The nominal chemical compositions and mechanical properties of the sheet material according to [48] are listed in Table 1.
FSW was adopted in butt joint configuration and in the rolling direction on position control mode via a five-axis FSW machine (PTG Heavy Industries, Elland, UK) capable of producing 60 kN load force along the vertical axis, maximum torque of 160 Nm, 6000 mm/min feed rate, and 4000 rpm rotational speed. The welding tool was made from H13 tool steel and was heat-treated to the final 52 HRC hardness with 12 mm shoulder diameter and 3 mm diameter tapered threaded multi-pin (2-pin) geometry. The pin length and pin depth were 1.48 mm and 1.64 mm, respectively. The parameters were selected based on prior empirical knowledge and experience with the multi-pin tool. The tool shape and CAD model of the employed tool are given in Figure 2.
In order to achieve HSFSW, a fixed rotational speed of 4000 rpm was employed at a 2° tilt angle, with eight different feed rates ranging from 2500 mm/min to 6000 mm/min, increasing in increments of 500 mm/min to explore the effect of gradually increasing welding speed.
Optical microscopy (DM2700, Leica Microsystems GmbH, Wetzlar, Germany) was used to monitor the internal and surface defects on the welded samples. The cross-sections of specimens were prepared using standard metallographic technique based on [49] consisting of grinding with silicone-carbide paper up to 2500 grits, followed by mechanical polishing to obtain a mirror-like surface finish. Afterward, the samples were etched with Keller’s reagent.
Mechanical properties analysis was performed on a universal tensile testing machine (Zwick/Roell Z100 GmbH & Co. KG, Ulm, Germany) according [50], with a positioning accuracy of 2 mm and a load cell capacity of 30 kN. The tensile specimens had a gauge length of 50 mm and a reduced section width of 20 mm, with the weld positioned at the center of the gauge length. Specimens were extracted transversely, perpendicular to the welding direction, and taken from the three different positions along the weld length near the start, middle, and end to capture potential variations in mechanical properties. Three tensile tests were conducted for each sample to ensure measurement accuracy and repeatability. Testing was performed at a strain rate of 0.0067 s⁻1, with tensile force and strain recorded at 10 mm intervals. Moreover, the Vickers micro-hardness was measured using an automated hardness-testing machine (KB30, KB Prüftechnik GmbH, Hochdorf-Assenheim, Germany) along the centerline of the transverse weld section in three lines (Figure 3a) with a load of 0.980 N (HV 0.1). Furthermore, an electrical resistivity test was conducted on the cross-sections of the welded samples utilizing a four-wire Kelvin measurement using MR5-200C system (Schuetz Messtechnik GmbH, Lahr, Germany) with 0.1 Ω for R < 200 µΩ resolution. The four-wire Kelvin method was employed due to its superior accuracy in measuring low electrical resistances, which is essential for evaluating the subtle microstructural variations within the welded zone. This technique effectively eliminates the influence of contact and lead resistances, enabling precise determination of the true resistance of the material. Electrical resistance was measured by recording the voltage drop between the inner sensing probes while a constant current of 51 A was applied for 5 s. All measurements were conducted at room temperature, and consistent contact pressure between the measurement tips and the specimen surface was maintained. Figure 3b shows a schematic of the electrical resistance measurements in welded samples.
Additionally, μ-CT (Xradia Versa 520, Carl Zeiss AG, Oberkochen, Germany) was adopted in isometric view, and two perpendicular sections for a 3D characterization were analyzed with regard to possible internal defects (Figure 3c). Moreover, different zones generated during the HSFSW process are illustrated in Figure 3d, including the BM, HAZ, TMAZ, and NZ. Finally, scanning electron microscopy (SEM) (Zeiss REM Ultra Plus, Carl Zeiss AG) equipped with electron backscatter diffraction (EBSD) (Symmetry, Oxford Instruments plc, Abingdon, UK) at an accelerating voltage of 20 kV using a CCD-based EBSD detector (Bruker e-flash) was employed for grain size characterization. Following the microstructure analysis, ATEX software version 4.14 was used to calculated the grain size [51]. Prior to the EBSD examination, the surface of selected samples was prepared for 2 h using vibropolishing in a colloidal silica solution.

3. Results and Discussion

The first step for defect detection and weld performance analysis of the welded zone is investigating the cross-section using light microscopy. It is clearly visible in Figure 4 that despite using a shorter pin length and high-speed feed rates, material flow during welding is sufficient to produce complete thorough welds in each case without any defects, especially at the root of the welded zone. When welding with a multi-pin tool, an increased volume of material is stirred both horizontally and vertically because a large, activated diameter is created by the two pins when rotating inside the material. The multi-pin tool design enhances not only the plasticization of the material but also contributes to increased heat generation, owing to the greater contact area and frictional interaction compared to a conventional single-pin tool [18]. This elevated heat input facilitates improved material flow, thereby minimizing the risk of defect formation, such as voids and tunnels within the weld zone [52]. Previous studies have shown that tool geometries with extended surface engagement can significantly influence thermal conditions and result in higher weld integrity [53,54,55].
Previous research on FSW via multi-pin tool also confirms the improvement of material mixing, which leads to enhanced material flow [14]. Moreover, multi-pin tools allow for effective welding with shorter pin lengths, which is crucial for tool wear during welding [14].
The heat generated during the HSFSW process significantly influenced the microstructure within the weld zone, resulting in a noticeable reduction in hardness due to thermal exposure and plastic deformation [56]. Figure 5 presents the micro-hardness distributions across the transverse sections of the welds, illustrating the characteristic softening in key regions. The BM exhibited the highest hardness at 120 HV, while reductions were observed in the NZ, TMAZ, and HAZ, as expected from the thermal gradients and strain fields associated with the FSW process.
In 7xxx series aluminum alloys, mechanical strength is predominantly achieved through precipitation hardening. During friction stir welding, the elevated temperatures in the NZ cause the dissolution of these strengthening precipitates. Upon cooling, partial re-precipitation may occur depending on the thermal cycle and cooling rate. At higher feed rates, the exposure time is shorter, reducing the extent of precipitate coarsening. This phenomenon likely contributes to the slightly increased hardness observed in the NZ at higher feed rates (Figure 5, 4000–6000 mm/min). Additionally, higher feed rates may also induce greater plastic deformation, suggesting a contribution from strain hardening. Although a clear monotonic increase in hardness was not observed across the entire feed rate range, the peak hardness at 4000 mm/min, along with the finest grain size at the same condition, indicates an optimal combination of thermal and mechanical effects at this specific feed rate. The slight drop in minimum hardness observed at 4500 mm/min may be attributed to changes in heat input and reduced plasticization, as the process approaches the upper limit of effective material stirring.
It is reasonable to consider that the observed trend results from the combined effects of finer precipitate distributions and localized strain hardening, as also suggested in related literature [21]. The measured minimum hardness values support this interpretation: samples at 2500 and 3000 mm/min showed minimum values of 81 HV, while at 3500 mm/min it was slightly lower (80 HV). The highest minimum hardness was observed at 4000 mm/min (86 HV), followed by a modest decrease at 4500 (83 HV), 5000 (82 HV), 5500 (81 HV), and 6000 mm/min (82 HV). Despite this slight decline beyond 4000 mm/min, the hardness values at higher feed rates, including 6000 mm/min, remain within an acceptable range, indicating that sound welds can still be achieved with HSFSW. These findings suggest that 4000 mm/min represents a favorable processing condition, and that overall, high-speed processing using a multi-pin tool can maintain or even enhance localized hardness in the weld zone.
In the HAZ, although the thermal exposure was insufficient to fully dissolve the precipitates, it was adequate to promote their growth, contributing to the observed softening in this region [21]. The use of a multi-pin tool proved effective in promoting consistent heat input and material flow, which resulted in uniform weld microstructures and relatively consistent hardness profiles across all feed rates. This uniformity suggests that the selected process parameters provided stable conditions for reliable weld formation and performance [57,58].
Figure 6 illustrates the mechanical behavior of the welded samples under quasi-static tensile loading. It was found that the ultimate tensile strength (UTS) of the joints was lower than the BM (350 MPa), and the UTS of all samples were almost insensitive to a large variation in feed rates. The results indicate a robust and adaptable welding process that can maintain mechanical qualities across a wide range of feed rates. The constant rotational speed likely resulted in comparable heat input across all samples, leading to a similar microstructural evolution in the weld zone. As a result, the mechanical properties remained stable across the different feed rates. This stability suggests that the indirect welding parameters, including heat input and material flow, were effectively under control due to the implementation of the multi-pin. Also, this homogeneity of mechanical properties implies that the welding procedure using a multi-pin in HSFSW is reliable.
It has been established that material plasticization and material transfer processes have an impact on electrical resistivity due to the severe plastic deformation and dynamic recrystallization processes under high temperatures and pressures, which induce microstructural inhomogeneity [59]. The mentioned processes can lead to alterations in crystallographic orientations and dislocations within the welded joint. Additionally, the presence of pores and micro-cracks can change the electrical resistivity of the welded zone [60]. In this investigation, all welds were free from any kind of defects as shown in Figure 4. Hence, the change in electrical resistivity of all samples is very low compared to the BM. Table 2 shows the results of electrical resistivity measurement, affirming the discussed homogeneity of the welded zone. As seen, in all samples, the electrical resistivity of the welded areas is remarkably close to the base material. Only a maximum of 15% increase in electrical resistivity was observed in welded samples compared to the BM.
Moreover, to comprehensively evaluate the internal integrity and homogeneity of the welded joints, a non-destructive µ-CT analysis was performed. This technique was chosen for its high sensitivity in detecting internal discontinuities such as micro-voids, pores, and other subsurface imperfections that may not be visible through conventional optical or surface-based methods. Figure 7 represents the µ-CT scans of selected samples welded at three representative feed rates, 2500, 4000, and 6000 mm/min, corresponding to the lowest, mid-range, and highest parameters used in this study. Scans were conducted across three sectional planes (as illustrated in Figure 3), enabling a comprehensive volumetric assessment of each weldment.
The analysis revealed no internal defects or discontinuities in any of the examined sections, regardless of feed rate. This consistent absence of flaws strongly confirms the structural soundness of the welds and underscores the effectiveness of the multi-pin tool in promoting uniform plastic deformation and material consolidation throughout the weld zone. The results validate that even at the highest applied feed rate of 6000 mm/min, sufficient heat generation and material flow were achieved to prevent the formation of voids or micro-cracks, thus ensuring weld quality. These findings highlight the suitability of the multi-pin HSFSW technique for high-speed industrial applications where both productivity and joint reliability are critical.
The grain size of the welded zone in the BM and nugget zone was measured through fine EBSD scans to illustrate further the reliability and homogeneity of using the multi-pin tool in HSFSW. It is observed in Figure 8 that there are distinct differences in the size and shape of the grains in BM and NZ. The grains of the BM are elongated in shape in the rolling direction, and their average size is found to be 11.31 µm (Figure 8, BM). However, this microstructure of BM is substantially affected by the extensive plastic deformation during FSW. Fine equiaxed grains are found in NZ, having average grain sizes of 3.09, 2.47, and 3.01 µm at the feed rates of 2500 mm/min, 4000 mm/min, and 6000 mm/min, respectively (Figure 8, 2500, 4000, 6000 mm/min). Thus, a more than 70% reduction in grain size has occurred, which can be explained by dynamic recrystallization [16,21,41]. The finer grain size is found to be correlated with high feed rate [21,30]. At high-speed FSW, NZ experiences lower heat input per unit length at a constant rotation rate, and a short time is available for grain growth after recrystallization. As a result, smaller-sized grains are found in NZ [21,45]. During conventional FSW, in which a single pin is used, the pin causes stirring action inside the material during the welding process. Nonetheless, as two pins are used in this investigation, more material is stirred, and subsequent plastic deformation in the NZ occurs. The grain sizes in the NZ at three different feed rates are very close to each other. These uniform and fine microstructural characteristics assure the reliability and repeatability of HSFSW. Furthermore, the results proved that HSFSW using the multi-pin tool is highly suitable for industrial applications where consistent mechanical properties are critical, offering increased productivity without sacrificing microstructural integrity.

4. Conclusions

HSFSW was employed using a multi-pin tool in 2 mm-thick AA 7020-T651 alloy at feed rates ranging from 2500 mm/min to 6000 mm/min with a constant rotational speed of 4000 rpm to investigate the influence of welding speed on weld quality and performance. This study presents one of the few experimental investigations into high-speed FSW on a 7xxx series alloy using a multi-pin tool, with feed rates up to 6000 mm/min pushing the upper limit of process speed while maintaining weld quality. The following conclusions can be drawn from this study:
  • All welds were produced without any defects, indicating the process stability and effectiveness of the multi-pin tool. The tensile strength of all welded joints exceeded 70% of the base material strength, with a maximum of 266 MPa (76%) achieved at a feed rate of 6000 mm/min.
  • The stir zone exhibited significant grain refinement due to dynamic recrystallization, with the smallest average grain size (2.47 µm) observed at 4000 mm/min, suggesting optimal thermal and mechanical conditions at this speed.
  • Hardness measurements revealed localized softening in the NZ and HAZ, as expected for friction stir-welded 7xxx series alloys. A peak in minimum hardness was recorded at 4000 mm/min (86 HV), with only a modest decline observed at higher feed rates.
  • Even at the maximum feed rate of 6000 mm/min, the minimum hardness remained within an acceptable range, confirming that defect-free and mechanically reliable joints can be achieved under high-speed conditions.
  • The multi-pin tool enhanced material flow and heat distribution, contributing to consistent weld quality across all feed rates.
  • Overall, 4000 mm/min represents the most favorable condition in terms of mechanical strength and microstructural refinement, while the results at higher feed rates confirm the potential for increased manufacturing efficiency without compromising weld integrity.
These findings demonstrate the scientific and practical potential of using a multi-pin tool for high-speed FSW of 7xxx series aluminum alloys, offering a pathway toward industrial-scale implementation without sacrificing weld quality, even at process speeds up to 6000 mm/min.

Author Contributions

R.D.N.: conceptualization, methodology, investigation, formal analysis, validation, visualization, writing—original draft, and writing—review and editing. S.S.: conceptualization, methodology, investigation, formal analysis, validation, visualization, writing—original draft, and writing—review and editing. M.W.: conceptualization, methodology, investigation, formal analysis, validation, visualization, writing—original draft, and writing—review and editing. C.W.: conceptualization, methodology, investigation, formal analysis, validation, visualization, writing—original draft, and writing—review and editing. S.B.: conceptualization, methodology, investigation, formal analysis, validation, visualization, writing—original draft, and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that there exists no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

The following abbreviations are used in this manuscript:
FSWFriction stir welding
HSFSWHigh-speed friction stir welding
BMBase material
HAZHeat-affected zone
TMAZThermomechanical affected zone
NZNugget zone
µ-CTMicro-computed tomography
EBSDElectron backscatter diffraction
TWIThe Welding Institute
TWBTailored welded blanks
UTSUltimate tensile strength

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Figure 1. Welding parameter (feed rate and rotational speed) distribution of previous research findings and current investigation.
Figure 1. Welding parameter (feed rate and rotational speed) distribution of previous research findings and current investigation.
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Figure 2. (a) FSW tool CAD model and tool shape. (b) Pin CAD model and geometry.
Figure 2. (a) FSW tool CAD model and tool shape. (b) Pin CAD model and geometry.
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Figure 3. Schematics of (a) hardness mapping measurements (Rows I, II, and III correspond to the following hardness measurements.), (b) electrical resistance measurements, (c) μ-CT characterization view, and (d) different welding zones in HSFSW.
Figure 3. Schematics of (a) hardness mapping measurements (Rows I, II, and III correspond to the following hardness measurements.), (b) electrical resistance measurements, (c) μ-CT characterization view, and (d) different welding zones in HSFSW.
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Figure 4. Macroscopic cross-section of welded samples.
Figure 4. Macroscopic cross-section of welded samples.
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Figure 5. Micro-hardness distribution map of welded samples.
Figure 5. Micro-hardness distribution map of welded samples.
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Figure 6. The ultimate tensile strength (UTS), yield strength (YS), deformation rate, and %elongation of the welded samples. The numbers in the X-axis represent the feed rate in mm/min unit.
Figure 6. The ultimate tensile strength (UTS), yield strength (YS), deformation rate, and %elongation of the welded samples. The numbers in the X-axis represent the feed rate in mm/min unit.
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Figure 7. Three-dimensional images of welded samples obtained by µ-CT analysis.
Figure 7. Three-dimensional images of welded samples obtained by µ-CT analysis.
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Figure 8. Analysis of grain size evolution in the nugget zone of BM and welded samples.
Figure 8. Analysis of grain size evolution in the nugget zone of BM and welded samples.
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Table 1. Chemical composition and mechanical properties of 7020-T651 aluminum alloy in wt.%.
Table 1. Chemical composition and mechanical properties of 7020-T651 aluminum alloy in wt.%.
ElementSiFeCuMnMgCrZnAl
wt. (%)0.350.40.20.05–0.51–1.40.10–0.354–5Bal.
PropertyTensile Strength (MPa)Yield Strength (MPa)Elongation (%)Hardness (HV)Modulus of Elasticity (GPa)Density (g/cm3)
Value3202406120702.78
Table 2. Electrical resistivity measurements of base material and welded samples.
Table 2. Electrical resistivity measurements of base material and welded samples.
Sample No.Base Material2500 mm/min3000 mm/min3500 mm/min4000 mm/min4500 mm/min5000 mm/min5500 mm/min6000 mm/min
Electrical
Resistivity (µΩ.mm2/mm)		42.6649.3348.5649.248.2448.4248.1848.347.76
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MDPI and ACS Style

Delir Nazarlou, R.; Salim, S.; Wiegand, M.; Wolf, C.; Böhm, S. Mechanical and Microstructural Properties of High-Speed Friction Stir Welding of AA 7020 Aluminum Alloy Using Multi-Pin Tool. Metals 2025, 15, 511. https://doi.org/10.3390/met15050511

AMA Style

Delir Nazarlou R, Salim S, Wiegand M, Wolf C, Böhm S. Mechanical and Microstructural Properties of High-Speed Friction Stir Welding of AA 7020 Aluminum Alloy Using Multi-Pin Tool. Metals. 2025; 15(5):511. https://doi.org/10.3390/met15050511

Chicago/Turabian Style

Delir Nazarlou, Ramin, Samita Salim, Michael Wiegand, Christian Wolf, and Stefan Böhm. 2025. "Mechanical and Microstructural Properties of High-Speed Friction Stir Welding of AA 7020 Aluminum Alloy Using Multi-Pin Tool" Metals 15, no. 5: 511. https://doi.org/10.3390/met15050511

APA Style

Delir Nazarlou, R., Salim, S., Wiegand, M., Wolf, C., & Böhm, S. (2025). Mechanical and Microstructural Properties of High-Speed Friction Stir Welding of AA 7020 Aluminum Alloy Using Multi-Pin Tool. Metals, 15(5), 511. https://doi.org/10.3390/met15050511

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