
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12993-3
10.1016/j.heliyon.2024.e36962
e36962
Research Article
The effect of pin diameter, tool penetration depth and plate arrangement on mechanical and metallurgical properties of 6061-T6 and 5052-T32 aluminum dissimilar joints in friction stir spot welding (FSSW)
Feizollahi Vahid
Gerami Yousef
Saki Ahmad
Adelzadeh Behrooz
Zamani Mahmood
Ghobeiti Hasab Mehdi
Heidary Moghadam Ali alheidarym@yahoo.com
⁎
Department of Materials Engineering, Materials & Energy Research Center, Dezful Branch, Islamic Azad University, Dezful, Iran
⁎ Corresponding author. alheidarym@yahoo.com
03 9 2024
15 9 2024
03 9 2024
10 17 e3696229 6 2024
2 8 2024
26 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
In solid-state welding, the basis of joining is the combination of materials in the weld area using the tool. The tool pin is a key parameter that can facilitate tool penetration into the parts and result in better material mixing. On the other hand, the presence of the end cavity of the process can be a factor in reducing the mechanical properties of the weld. To address these two conflicting issues, this study examined the effect of pin thickness on the mechanical and metallurgical properties of aluminum 6061 and 5052 plates. The variable parameters included pin diameters (4 mm, 8 mm, and no pin) and three tool penetration depths (0.5 mm, 1.5 mm, and 2.5 mm). The impact of changing the arrangement of the plates on each other was also investigated regarding the properties and quality. Experimental results showed that decreasing the pin diameter and increasing the tool penetration depth improved the weld breakage force. The failure modes of all welds were button pull-out and ductile failure. Using a pinless tool increased microhardness in the weld nugget area and led to more uniform microhardness variations. Placing the aluminum 6061 alloy on top of the arrangement improved shear tensile strength and microhardness of the joints. The microstructure in the heat-affected zone showed an increase in grain size, which was accompanied by a decrease in microhardness. The microstructure in the stirred zone was dense and compacted, and with increasing tool penetration depth and decreasing pin diameter, the grain size became finer.

Keywords

Pin diameter
Tool penetration depth
Microhardness
Fracture strength
Microstructure
Nugget zone
End-hole
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pmc1 Introduction

Aluminum alloys are increasingly used in various industries, including aerospace and automotive. Aluminum alloys have a high strength-to-weight ratio, suitable tensile strength, desirable mechanical properties, and good corrosion resistance, making them a preferred choice for use in various structures (Hanapiah et al [1,2]). However, the challenges in the weldability of aluminum alloys have limited their use. Welding dissimilar aluminum alloys using fusion welding is relatively difficult and challenging due to their different mechanical properties, physical properties, and melting temperatures. Joining dissimilar materials can offer significant advantages in terms of reducing environmental pollution, lower costs, design flexibility, and reduced structural weight (Feizollahi and Heidary Moghadam [3,4] and Feizollahi et al. [2]). On the other hand, welds produced by fusion welding methods may not meet the desired mechanical and metallurgical properties for aluminum structures. The extreme heating and cooling rates during fusion welding processes can lead to weld defects such as porosity and cavities in the produced joints. In some cases, significant expenses are incurred to protect the weld pool, which can greatly increase welding costs. Therefore, solid-state welding can be one of the best options for joining aluminum alloys [5]. Friction stir spot welding (FSSW) was developed by The Welding Institute (TWI) in 2003 and offers various advantages such as low thermal distortion, high mechanical properties, uniform macrostructure and microstructure of the weld, and high welding efficiency. This welding process is mainly used in industries such as shipbuilding, automotive, and aerospace (Sekhar et al. [4] and Feizollahi and Heidary Moghadam [3]). The advantages of this process include defect-free and sound welds, no need for powder and shielding gas, low part distortion, no environmental pollution, and no waste. Since this process joins parts in a semi-solid state, it prevents full metal melting and the formation of extreme heating and cooling rates. Despite all the advantages it offers, this process also has some drawbacks, the most significant being the remaining pinhole at the end of the process. The most important parameters affecting this process include tool rotational speed, dwell time, tool penetration depth, pin and shoulder geometry, and the characteristics of the sheets. Feizollahi and Heidary Moghadam [3], Hanapiah et al [1].

The friction stir spot welding process consists of three different stages: plunging, stirring, and retracting. In this process, a rotating tool is positioned on the parts, and the friction generated between the tool and the sheet leads to the production of heat and the softening of the parts (Fig. 1(a)). In the next stage, the tool is pressed into the parts and penetrates to a certain depth. The rotational action of the tool causes severe plastic deformation and mixing of the softened materials (Fig. 1(b)). Finally, the tool is lifted off the sheets, leaving behind the pinhole where the tool was seated on the parts (Fig. 1(c)).) Feizollahi and Heidary Moghadam [3], and Feizollahi and Heidary Moghadam [6]).Fig. 1 Schematic of the friction stir welding process and its different stages: (a) Phase one - Tool plunging stage, (b) Phase two - Mixing and stirring stage, and (c) Phase three - Tool retracting phase.

Fig. 1

Piccini and Svoboda [7] demonstrated that for all analyzed conditions, microhardness decreases from the base metal to the stir zone and reaches a minimum value in the heat-affected zone for both configurations. Abbas et al. [8] showed that the tool pin profile is the most effective parameter on mechanical properties compared to other welding parameters.

Tier et al. [9] have shown that in the joining of 5052-T32 aluminum alloy plates, three distinct weld zones have been observed, including the primary bond, partial bond, and secondary bond. Andalib et al. [10] demonstrated that by controlling process parameters, point welds with approximately 37 % higher strength than conventional friction stir spot welds can be obtained. Saju and Narayanan [11] demonstrated that the maximum edge shear strength of welds was higher compared to joints made on similar material combinations in other studies. Friction stir samples exhibited the characteristics of an inverted W-shaped hardness profile on the cross-section surface. Li et al. [12] demonstrated that the keyhole defect can be eliminated using a modified welding process. The fracture mode of all joints was in the form of shear fracture, and the fracture morphology also exhibited a ductile fracture mode. Ibrahim and Yapici [13] showed that the keyhole at the end of the weld can be eliminated by introducing an intermediate layer in the center of the weld zone between the plates, resulting in a smooth weld zone. The weld performance in terms of bond strength improved. Shen et al. [14] demonstrated that using a modified tool, flawless welds can be produced, and metallurgical bonding, material mixing, and mechanical locking at the weld joint interface improved. Yazdi et al. [15] found that pin-less welds were stronger due to the larger effective cross-sectional area and absence of a keyhole. This partial improvement was attributed to a larger stirring zone and more severe stirring resulting from sharper-grooved features. Kumar et al. [16] demonstrated that the minimum hardness occurs at the boundary of the thermo-mechanical affected zone and the heat-affected zone. Three fracture modes were observed in tensile/shear tests, including surface fracture, interfacial fracture, and nugget pull-out fracture. Patel et al. [17] showed that the remaining keyhole at the end of friction stir spot welding in dissimilar joints is effectively repaired by optimizing the geometric features of the tool pin. Excellent material mixing occurs in the repaired region from the keyhole. Suryanarayanan and Sridhar [18] demonstrated that under optimal welding conditions, both the plate position parameter and tool specifications significantly affect the load-bearing capacity of the weld joint. Paidar et al. [[19], [20]] showed that tensile strength, hardness distribution, and fracture morphology of joints are improved by the flow behavior resulting from the change in the position of aluminum plates. Hamzah et al. [21] showed that optimal shear strength is achieved with nugget pull-out fracture instead of shear fracture upon failure. Optimal weld performance with pinless tools was achieved when the tool did not penetrate the upper plate. Paidar et al. [null] demonstrated that specimens welded with a modified process can lead to improved mechanical properties of joints due to the elimination of keyholes, better material flow between components, and suitable microstructural evolution. Ferreira et al. [22] showed that by optimizing welding process parameters, joints with high edge shear strength can be produced between aluminum alloys AA5754-AA6061-T6. The results indicated that the tool plunge depth had a significant effect on the tensile shear strength of the welds. Suryanarayanan and Sridhar [23] used pinless tools in joining dissimilar aluminum plates and prevented the formation of end-hole defects. Results showed that tool characteristics were the most influential parameters affecting joint strength. Gao et al. [24] demonstrated that optimizing the friction stir welding process by performing pre-hole drilling can increase the tensile/shear strength of the welded sample by approximately 50 % compared to conventional welding processes due to strong metallurgical bonding and proper mixing between base alloys. Zhang et al. [25] showed that increasing the tool plunge depth leads to the growth of recrystallized grains in the stir zone. Tensile-shear loading improved due to the creation of finer grains and strengthening of Al2CuMg precipitates in the weld. Kumar et al. (2020) investigated the effect of friction stir spot welding parameters on the mechanical properties and microstructure of dissimilar joints. The results indicated that the maximum weld hardness occurs in the stir zone. Memon et al. [26] showed that the material position has no effect on the fracture mode of joints produced by modified friction stir spot welding. Tiwan et al. [27] demonstrated that the height of the weld hook and the width of the bonded area are significantly dependent on the characteristics of the tool pin. The pin shape has a noticeable impact on the weld strength. Mehrez et al. [28] showed that welded joints are improved by eliminating keyhole defects, leading to the formation of high-strength connections. The input heat associated with the second stage of the modified friction stir spot welding process is responsible for this. Mertinger et al. [29] demonstrated that despite the visual heterogeneities in the stir zone where complete mechanical mixing occurs, the microstructure of dissimilar aluminum joints exhibits composite maps and alignment of grains and material flow lines, indicating excellent weld quality. Venukumar et al. [30] investigated the joining of dissimilar aluminum alloys AA5052-T32 to AA6061-T6-T6. To control heat conduction and consequently grain coarsening, AA6061-T6-T6 was held as the base plate on the backing plate during joining.

El-Sayed Seleman et al. [31] showed that flawless and high-quality joints in dissimilar aluminum joining are produced using a higher shoulder-to-pin ratio. Lewise et al. [32] demonstrated that in dissimilar aluminum alloy joints, a uniform material flow distribution was achieved in the weld zone, leading to a significant improvement in microhardness and tensile strength of the joints. Balamurugan et al. [33] showed that optimizing the characteristics of the pin can develop a flawless stir zone and enhancing grain size and microstructure formation can result in stronger welded joints. Lewise et al. [34] demonstrated that tool pin penetration and frictional heat can influence the microstructure, and the distribution of grain structure affects the microhardness and tensile properties of dissimilar joints. Dahi et al. [35] have used two-way FSSW technique including rotating anvil and pinless tool for joining aluminum plates. This approach not only increases the tensile strength of the joints, but also reduces possible defects such as keyholes. Mallieswaran et al. [36] demonstrated that the weld metal and the heat-affected zone play a very important role in the operational capacity of welds, and that weld characteristics can be improved by quenching and artificial aging [44]. Mallieswaran et al. [36] stated that nickel shot-peened welds exhibit greater strength compared to conventional welds produced by friction stir welding. Additionally, the shot-peened welds showed a 7 % higher strength compared to welds without heat treatment [45]. Mallieswaran et al. [37] demonstrated that a developed mathematical relationship can be used to predict the tensile strength of dissimilar joints of AA1100/AA6061 aluminum alloys with a 95 % confidence level [46]. Mallieswaran et al. (2020) developed an empirical relationship to predict the tensile strength of dissimilar aluminum joints to achieve maximum joint strength [47]. Balamoorgan et al. (2024) investigated the joining of dissimilar AA5052-H32 and AA6061-T6 plates using friction stir welding [48]. Balamoorgan et al. (2023), showed that welds produced at intermediate tool speeds have the maximum tensile strength, hardness, and elongation compared to other joints [49]. Balamurugan and Jayakumar [38] demonstrated that welding AA5052-H32 and AA6061-T6 aluminum alloys with a scandium (Sc) strip can improve material flow and also reduce the brittleness of the joint [50]. Kumar et al. (2023) demonstrated that adding a scandium interlayer with a thickness of 1.5 mm results in the highest ultimate strength of the joints, with this increase in strength attributed to the formation of fewer precipitates [51].

Aluminum alloy 5052-T32 possesses high corrosion resistance, high strength-to-weight ratio, and excellent tensile strength, making it suitable for marine industries and chemical applications. However, its elastical nature makes machining difficult. Additionally, it cannot be heat-treated for strengthening; only cold working, rolling, or forging can strengthen it. Aluminum alloy 6061-T6 has suitable hardness, desirable tensile strength, and good workability. Other characteristics include high strength-to-weight ratio, low density, lightness, and high thermal conductivity. Due to its high strength, permanent deformation of aluminum 6061-T6 is more challenging, but it can be strengthened through heat treatment. By combining aluminum alloys 6061-T6 and 5052-T32, one can benefit from the advantages of both, achieving corrosion resistance, good machinability, while also providing desirable strength and thermal conductivity.

The primary process of friction stir welding involves the application of frictional heat and mixing of sheet materials. The tool and pin characteristics are among the most influential factors affecting material mixing quality. The pin facilitates tool penetration between sheets and, upon penetration into the components, its rotational movement leads to better material mixing and compression. However, the crucial point is the pin's remaining location on the sheets, which can reduce the mechanical properties of the joints. Conversely, the absence of a pin can hinder tool penetration into the components and proper material mixing. Finding the optimal conditions in these conflicting situations requires determining the best conditions to enhance the mechanical and metallurgical properties of the joints and evaluating the changes in various weld zones under different tool pin conditions. Therefore, this research compares the conditions created in three different tool pin states, including large pin, small pin, and pinless state. Additionally, the tool penetration depth, one of the most significant parameters affecting weld strength, is examined. The effectiveness of altering the arrangement of aluminum alloy 6061-T6 and 5052-T32 sheets in friction stir welding with fewer point disturbances has been less explored and requires further investigation.

2 Materials and methods

Pages of dissimilar aluminum alloys AA6061-T6-T6 and AA5052-T32-T6 with a thickness of 4 mm have been selected. Before the welding process, the sheets underwent quantum analysis, and the percentage of their chemical compositions was determined. Table 1, Table 2 display the elements present in the chemical composition, corresponding to aluminum sheets AA6061-T6 and AA5052-T32, respectively, based on ASTM E1251-11 standard.Table 1 Illustrates the elements and their weight percentages in the chemical composition of aluminum alloy AA6061-T6 according to ASTM E1251-11 standard.

Table 1Elements	Al	Ni	Cr	Mg	Mn	Cu	Fe	Si	Zn	Ti	other	
Weight percentage of elements	Base	0.0051	0.187	0.986	0.0311	0.231	0.352	0.596	0.0069	0.0221	0.08	

Table 2 Presents the elements and their weight values in the chemical composition of aluminum alloy AA5052-T32 according to ASTM E1251-11 standard.

Table 2Elements	Al	Zn	Cr	Mg	Mn	Cu	Fe	Si	Ti	other	
Weight percentage of elements	Base	0.2	0.35	2.7	0.5	0.1	0.6	0.5	0.1	0.15	

Rectangular aluminum sheets with dimensions of 150 × 50 were cut. In the next step, the parts were subjected to grinding to remove any contamination and unevenness from their surface. For this purpose, various sandpapers ranging from coarse to fine (60, 100, 240, 400, 600, 800, 1000, 1200, and 1500) were used. The variable parameters of the process included pin thickness, tool penetration depth, and changes in the arrangement of aluminum sheets on top of each other. Three pin diameter states—small, large, and pinless—were considered for the welding process. Three tool penetration depths—low, medium, and high—were set at 0.5, 1.5, and 2.5 mm, respectively. The arrangement of the sheets on top of each other was adjusted in two states: aligned and overlapping. In the aligned state, the sheets were completely placed on top of each other, intended for examining the microhardness and microstructure of the welds (see Fig. 2 (a). In the overlapping state, the sheets were placed in an overlap configuration. An overlap area in the shape of a square with dimensions of 50 × 50 was formed, where the intersection of the square diagonals was the location of the tool pin placement on the sheets. Overlap welds were considered for tensile shear testing and joint failure analysis (see Fig. 2 (b). To investigate the influence of sheet arrangement on mechanical and microstructural properties, in both configurations, an aluminum sheet AA5052-T32 was placed on top of the sheets once, followed by placing an aluminum sheet AA6061 -T6 on top of the configuration, and welding was performed.Fig. 2 Shows the arrangement of plates for welding: (a) aligned arrangement to perform microhardness test and microstructure examination and (b) overlap arrangement for shear tensile test.

Fig. 2

Table 3 shows three variable parameters and their respective values considered.Table 3 Three variable parameters and their respective values considered.Other welding process.

Table 3	Tool penetration depth (mm)	Tool pin diameter (mm)	Plate Arrangement	
1	0.5	4	Aluminum AA6061-T6-T6 on Top	
2	1.5	8	Aluminum AA5052-T32-T6 on Top	
3	2.5	Pinless	–	

Parameters have been kept constant, including the tool shoulder thickness (24 mm), tool dwell time (5 s), and plate thickness (4 mm). The welding tools consisted of three H13 steel tools, each equipped with 4-mm and 8-mm pins at their ends, while the third tool had no pin. The welding tools were 120 mm long with a 24-mm tool shoulder diameter, featuring conically machined pins at the ends of two tools. The length of both pins was 4 mm, with the smaller tool having a pin thickness of 4 mm and the larger tool having an 8-mm pin thickness. To create a confined and suitable nugget zone, the tool shoulder was machined to a concavity of 3°, as the creation of this concave angle is crucial in preventing material spread beneath the pins. Fig. 3 depicts the designed tools along with their schematic diagrams.Fig. 3 Depicts the tools used for welding process as follows: (a) Tool with large pin (8-mm pin diameter), (b) Tool with small pin (4-mm pin diameter), and (c) Tool without pin.

Fig. 3

After final machining of the dimensions and geometry of the pins, heat treatment was performed to enhance the strength of the tools. For this purpose, the tools were placed inside a furnace at a temperature of 1000 °C for 1 h, and immediately quenched in water upon completion of the hour. The friction stir welding process was carried out using the FP4MK-F milling machine manufactured by Tabriz Machine Tool Manufacturing Company. The used milling machine was fully automatic, enabling precise adjustment of variable parameters on the workpieces. Fig. 4 illustrates the welded samples under different parameter conditions.Fig. 4 Welded samples under different parameter conditions: (a) Samples produced in two different arrangements: aligned state and overlap; and (b) samples produced in two conditions: pinless and pin-equipped.

Fig. 4

The conducted experiments included investigating the mechanical properties of the joints, such as H.V microhardness and tensile shear testing, as well as examining various regions of the produced welds through microscopy for microstructural analysis. The STM-150 tensile testing machine was used to perform the tensile testing of the components. The crosshead speed of the machine was set to 5 m/min, and a force/displacement graph was obtained from the machine output. Fig. 5 illustrates the tensile shear testing process.Fig. 5 Universal tensile testing machine SANTAM STM-150.

Fig. 5

Microhardness testing was conducted using an INNOVATEST microhardness testing machine. For this purpose, the specimens were first cut from the weld nugget center using a SPECIMEN CUTTER SQ80 cutter. In the next step, the specimens were heated and fully polished using grinding. The abrasive papers used included 120, 180, 240, 400, 600, 1000, and 1500 grit papers. Finally, the specimen surfaces were polished in three stages using diamond paste. The sizes of diamond pastes used in the first to third polishing stages were 0.25 μm, 1 μm, and 6 μm, respectively. Subsequently, a 19-point profile was taken along a straight line. In this profile, points started from the base metal aluminum 6061-T6 and after passing through the heat-affected zone, entered the welded area. Finally, after passing through the weld area, they entered the aluminum 5052-T32 side and terminated in its base metal (Fig. 6). The applied force in the microhardness test was 0.025 N, applied for a duration of 10 s.Fig. 6 Prepared sample for H.V microhardness testing and profile taken on the sample.

Fig. 6

The metallurgical examination of the welds was conducted through optical microscopy. Initially, the specimens were cut from the weld center and then subjected to grinding and polishing operations. The specimens were then immersed in a chlorine solution for 10 s. Finally, various regions of the welds were photographed using an HUVITZ optical microscope model HR3-TRF-P for microstructural analysis. By employing X-ray diffraction testing, the elements present in the weld and their percentage composition were identified. The temperature field was measured using the Testo 845 laser thermocouple. Four different points in the weld nugget area were measured, and the average of the obtained temperatures was calculated.

3 Discussion

3.1 Microstructure

Fig. 7 illustrates the macrostructure of different weld regions when using two pin diameters of 4 and 8 mm.Fig. 7 Macrostructure of different weld regions when using different tool pin diameters: a) 4-mm pin diameter, and b) 8-mm pin diameter.

Fig. 7

Four different regions have been obtained in the welded joints, including the base metal zone, the heat-affected zone, the thermo-mechanically affected zone, and the nugget zone (the stir zone). Additionally, an intermetallic layer was formed at the joint interface, resulting from the interactions between two aluminum sheets, which can influence the mechanical properties of the weld. Fig. 8 depicts the microstructure of the formed regions in the joining of aluminum alloy plates 6061-T6 and 5052-T32.Fig. 8 Microstructure of the formed regions in joining aluminum alloy plates 6061-T6 and 5052-T32.

Fig. 8

Fig. 9 illustrates the base metal microstructure in aluminum alloys 6061-T6 and 5052-T32 under different parameter conditions.Fig. 9 Base metal microstructure in aluminum alloys: a) Aluminum 5052-T32/4-mm pin diameter/2.5 mm penetration depth, b) Aluminum 5052-T32/8-mm pin diameter/2.5 mm penetration depth, c) Aluminum 6061-T6/4-mm pin diameter/2.5 mm penetration depth, and d) Aluminum 6061-T6/8-mm pin diameter/2.5 mm penetration depth.

Fig. 9

The obtained microstructures in the base metal of the produced joints under different parameter conditions are similar to each other. In both aluminum alloys 5052-T32 and 6061-T6, the base metal microstructure consists of alpha solid solution. Some dark-colored areas scattered in the base metal background are insoluble phases, mainly composed of iron compounds. The base metal microstructure is essentially the same as the initial microstructure of the aluminum alloy, indicating that the welding process interactions in this area have not significantly affected it. Changes in penetration depth and tool pin properties have had no significant effect on the microstructural variations in this region.

Fig. 10 illustrates the microstructure of the heat-affected zone under different parameter conditions.Fig. 10 Microstructure of the heat-affected zone in aluminum alloys: a) Aluminum 5052-T32/4-mm pin diameter/0.5 mm penetration depth, b) Aluminum 5052-T32/4-mm pin diameter/2.5 mm penetration depth, c) Aluminum 5052-T32/no pin/2.5 mm penetration depth, d) Aluminum 6061-T6/4-mm pin diameter/2.5 mm penetration depth, e) Aluminum 6061-T6/8-mm pin diameter/2.5 mm penetration depth, and f) Aluminum 6061-T6/no pin/2.5 mm penetration depth.

Fig. 10

The heat-affected zone is an area affected only by the heat generated during the welding process, and no mechanical work of the tool is involved in this zone. By comparing Fig. 10 (a) and 10 (b), it is evident that increasing the tool penetration depth leads to coarser grains in the heat-affected zone. The rotational movement of the tool on the workpieces results in the formation of softened frictional heat, which, when transferred to this area, causes grain growth and an increase in grain size. With the tool moving deeper, the engagement time between the tool and the plates increases, leading to more heat generation and transfer to various weld areas. Therefore, it is observed that grain size significantly increases due to tool penetration depth. Comparing s 10 (c) and 10 (d) reveals that increasing the tool pin diameter from 4 to 8 mm also results in larger grain size in the heat-affected zone. It seems that the larger tool pin diameter leads to better material mixing and more engagement between the tool and the plates in the welded area. In these conditions, more frictional heat is generated compared to smaller pin diameter, transferring more heat to the heat-affected area and causing larger grain coarsening in this state. In Fig. 10 (f), it can be seen that removing the pin leads to further grain coarsening. The tool pin facilitates tool penetration into the plates, and its removal requires more force for penetration into the parts. In these conditions, severe friction occurs between the tool shoulder and the plates, transferring more heat to the heat-affected area. The welding heat was measured using a laser thermocouple, which measured temperatures of 455, 480, and 525 °C for 4-mm pin, 8-mm pin, and no-pin tools, respectively. Comparing s 10 (c) and 10 (f) reveals that placing the aluminum 6061-T6 plate on top of the assembly results in increased grain size in the heat-affected zone. Aluminum alloy 6061-T6 has a melting point range of 580–645 °C, while the melting point of aluminum alloy 5052-T32 is more limited, ranging from 605 to 650 °C. The lower melting temperature and wider melting point range of aluminum 6061-T6 lead to more significant effects of the welding process temperature, which is created in the range between 400 and 580 °C, on the change in grain size in the heat-affected area. Therefore, grains in this aluminum plate are more affected by the process temperature and react to it more.

Fig. 11 illustrates the microstructure of the thermo-mechanically affected zone under different parameter conditions.Fig. 11 Microstructure of the thermo-mechanically affected zone in aluminum alloys: a) Aluminum 6061-T6/4-mm pin diameter/0.5 mm penetration depth, b) Aluminum 6061-T6/4-mm pin diameter/2.5 mm penetration depth, c) Aluminum 5052-T32/4-mm pin diameter/2.5 mm penetration depth, d) Aluminum 5052-T32/8-mm pin diameter/2.5 mm penetration depth, and e) Aluminum 5052-T32/no pin/2.5 mm penetration depth.

Fig. 11

The thermo-mechanically affected zone is a very thin intermediate area formed at the boundary between the thermo-mechanically affected zone and the nugget zone. This zone essentially possesses characteristics of both its adjacent zones. In fact, the thermo-mechanically affected zone is somewhat influenced by the mechanical work of the tool and also by the frictional heat of the welding process in this area.

By comparing Fig. 11 (a) and 11 (b), it was evident that increasing the tool penetration depth results in a clearer formation of the thermo-mechanically affected zone. The increased tool penetration depth and welding heat caused a more significant reduction in voids, and this parameter greatly affects the thermo-mechanically affected zone. The change in the arrangement of aluminum plates slightly altered the thermo-mechanically affected zone. The thermo-mechanically affected zone in aluminum alloy 5052-T32 is somewhat narrower and smaller than in aluminum alloy 6061-T6. It appears that the wider melting range of aluminum 6061-T6 and its faster transition to the mushy phase cause this zone to be more affected by the welding frictional heat, resulting in a greater reduction in void density in this area, hence widening the zone compared to aluminum alloy 5052-T32.

Comparison of Fig. 11 (c) and 11 (d), and 11(e) indicates that increasing the pin diameter from 4 to 8 mm had little effect on the thickness of the thermo-mechanically affected zone, but when using a pinless tool and increasing the temperature of the frictional process, the thickness of this zone slightly increased. It seems that increasing the surface interactions between the tool shoulder and the plates by removing the pin, as well as the uniformity of the nugget zone, led to better dissolution of deposits and a reduction in voids in this area, making the thermo-mechanically affected zone more uniform.

Fig. 12 illustrates the microstructure of the nugget zone in produced joints under different parameter conditions.Fig. 12 Microstructure of the intermetallic zone in aluminum alloys: a) Aluminum 5052-T32/4-mm pin diameter/0.5 mm penetration depth, b) Aluminum 6061-T6/4-mm pin diameter/0.5 mm penetration depth, c) Aluminum 6061-T6/8-mm pin diameter/0.5 mm penetration depth, d) Aluminum 6061-T6/8-mm pin diameter/2.5 mm penetration depth, e) Aluminum 5052-T32/4-mm pin diameter/2.5 mm penetration depth, f) Aluminum 5052-T32/8-mm pin diameter/2.5 mm penetration depth, and g) Aluminum 5052-T32/no pin/2.5 mm penetration depth.

Fig. 12

The intermetallic zone is influenced by two important factors: the frictional heat of the welding process and the mechanical action of the tool. Frictional heat is generated by the rotational movement of the tool, creating friction between the workpiece and the tool, resulting in the softening of aluminum plates. Additionally, the pressure exerted by the tool shoulder on this area, combined with the tool's movement outwards from the plates, causes the materials to mix with each other due to the rotational movement of the tool. The plastic deformation in this area is very severe, and the pressure applied by the tool shoulder compresses the materials together. The intermetallic zone has a formed structure consisting of solid solution alpha grains, and there is no phase change or microstructural alteration in it. The intermetallic compounds are dispersed throughout it, which, due to the tool's mixing action and the densification of the structure in this area, have become denser. A compact and fine-grained area has formed in this zone, which is much wider in aluminum alloy 6061-T6 than in aluminum alloy 5052-T32 (Fig. 12 (a) and 12 (b)). The earlier arrival of aluminum alloy 6061-T6 at the mushy phase temperature and its larger melting range have caused the materials to compress more in these conditions due to the tool's mixing action, resulting in a larger zone of fine-grained and intermetallic fusion. Therefore, it is recommended that to achieve a wider intermetallic zone, aluminum alloy 6061-T6 should be placed at the top of the arrangement, as its wider melting range can provide better results than aluminum alloy 5052-T32. The comparison between Fig. 12, Fig. 12 (d) shows that increasing the pin diameter results in a greater fine-graining of the intermetallic zone and its expansion. The tool pin has been able to mix and agitate the materials more effectively, causing more severe plastic deformation in these conditions compared to when a smaller pin is used. In the intermetallic zone, dynamic recrystallization and structure compression phenomena have occurred, which have improved with an increase in the pin diameter. The comparison between Fig. 12, Fig. 12 (g) indicates that using a larger tool pin diameter significantly expands the intermetallic zone compared to when a smaller pin is used. It appears that the larger diameter of the tool pin and the increased material mixing in the intermetallic zone lead to a finer-grained nugget zone and also expand the area of the intermetallic fusion. With the increasing size of the tool pin, it is observed that the plastic deformation created in the processing area becomes much more severe, resulting in better material mixing. Removing the pin has widened the intermetallic zone again. It seems that removing the pin has made it more difficult for the tool to enter the parts, which has increased the mechanical work of the tool shoulder and the pressure exerted on the intermetallic zone in these conditions. A wider and denser intermetallic zone has been created compared to the two previous states, and the microstructure has become more uniform due to the absence of a pin on the tool. Fig. 13 illustrates the microstructure of the intermetallic layer in the produced joint under different parameter conditions.Fig. 13 Microstructure of the intermetallic layer in welded joints: a) Aluminum 6061-T6/4-mm pin diameter/0.5 mm penetration depth, b) Aluminum 6061-T6/4-mm pin diameter/2.5 mm penetration depth, c) Aluminum 5052-T32/4-mm pin diameter/2.5 mm penetration depth, and d) Aluminum 5052-T32/no pin/2.5 mm penetration depth.

Fig. 13

The intermetallic zone is essentially a common boundary in the joining area of aluminum plates formed as a result of the interactions during the welding process. This area plays a very important role in the quality and strength of the formed connection between the plates, and in many cases, changes in welding parameters and conditions can significantly affect its thickness. By comparing Fig. 13 (a) and 13 (b), it is evident that at shallow tool penetration depths, the thickness of the intermetallic layer is high, and the plates exhibit some porosity and discontinuity. It seems that at lower tool penetration depths, the mechanical effect of the tool shoulder on the bonding area between the two plates is reduced, and the bonded layer is formed heterogeneously. However, increasing the tool penetration depth results in a reduction in the thickness of the intermetallic layer, which is due to proper material mixing and improved mechanical action of the tool shoulder on the fusion zone. By observing s 13 (a) and 13 (c), it is clear that when aluminum 6061-T6 is placed at the top of the arrangement, the thickness of the intermetallic layer between the metals increases. This is probably because aluminum alloy 6061-T6 melts at a lower temperature and has a wider melting range compared to aluminum 5052-T32. Therefore, it participates in the welding process for a longer period, resulting in a stronger and thicker intermetallic layer under these conditions. Holding the tool on the welding area for a longer period can lead to improved interactions between the plates, resulting in an increase in the thickness of the intermetallic layer between the plates. By comparing Fig. 13 (c) and 13 (d), and 13 (e), it is evident that reducing the diameter of the tool pin has continuously reduced the thickness of the intermetallic layer, so that when using a pinless tool, the thickness of this layer, which significantly affects the mechanical properties of the weld, has reached its minimum value. The tool pin, due to its stirring effect, leads to more material engagement in the fusion zone, and due to the increased heterogeneous interactions between the aluminum alloy plates in these conditions, the contribution of the plates to the joint line between the parts increases. Increasing the diameter of the tool results in more material mixing and a decrease in the pressure exerted by the tool shoulder on the plates, and due to the more heterogeneous nature of the fusion zone between the plates, the heterogeneous interactions between the parts increase, leading to a visible increase in the thickness of the intermetallic layer. By removing the pin, the mechanical pressure of the tool and its effect on the bonding area between the two plates increase, and the more uniform nature of the fusion zone leads to more uniform interactions in the area between the two plates, resulting in a thinner intermetallic layer.

The welding temperature in the outer region of the weld nugget was approximately 490 °C at the highest tool rotational speed. Peng et al. (2021) showed that the maximum welding temperature in the heat-affected zone (HAZ) ranged between 358 °C and 376 °C, which led to the dissolution and development of coarse grain boundary phases and precipitate-free zones. The peak temperature in the thermo-mechanically affected zone (TMAZ) ranged from 376 °C to 401 °C, resulting in complex precipitation behavior [52].

Ji et al. [39] showed that effective bonding interfaces were formed between the upper aluminum sheet and the lower copper sheet, which included the Cu/Cu interface and the Al/Cu interface. The Cu/Cu interface, free from kissing bond defects, and the Al/Cu interface, with thin layers of AlCu and Al2Cu intermetallic compounds, were beneficial for enhancing the tensile shear strength of the joint [53].

Yan et al. (2013) used thermocouples in three different configurations on the welding samples to measure temperatures. The results showed that the experimental thermal profiles were in agreement with the profiles calculated by the ANSYS model [54].

Similarly, Chada and Kumar [40] demonstrated that in the nugget region of the workpiece, a deformed microstructure was observed, consisting of refined grains co-aligned with distributed precipitates.

Similarly, Chu et al. [41] showed that the depth of tool penetration had the most significant effect on the tensile shear strength of aluminum joints and optimal resistance was achieved in the parameters leading to the failure of welded joints.

Similar findings were obtained by Sharma et al [42]. They illustrated that ring-like microstructures were formed in the nugget zone, with grains affected by heat being coarsened, while grains in the thermomechanically affected zone were deformed and elongated. Moreover, the shape of grains in the nugget zone indicated a co-aligned grain structure.

Similarly, Paidar et al. [null] demonstrated that severe plastic deformation during the process created more imperfections, thus providing more nucleation sites for deposits. Secondary phases in dissimilar aluminum alloy joints were fragmented and scattered uniformly in the nugget zone.

Additionally, Paidar et al. [null] showed that microstructure (debris scattering, void density, and interlocking) could also improve with changes in plate arrangement and optimal conditions selection.

Similar results were obtained in the research of Feizollahi et al. [2]. They showed that the microstructure of the heat-affected area increases in size and a compact and dense structure is obtained in the agitation area.

3.2 Tensile and shear strength of the weld

Fig. 14 illustrates the values of tensile shear forces in joints produced by varying different process parameters.Fig. 14 Displays the values of tensile shear forces in welded joints at different tool pin diameters and penetration depths: (a) positioning aluminum alloy 5052-T32 on top and (b) positioning aluminum alloy 6061-T6 on top.

Fig. 14

It is evident from Fig. 14, Fig. 14 (b) that the joint breaking force continuously increases with the increase in tool penetration depth. The tensile shear strength of the joints is minimum when using a tool penetration depth of 0.5, although the obtained strength under these conditions is still desirable and suitable. The shallow tool penetration depth results in a limited area and extent of the affected zone, thus reducing the interaction between the materials of the two sheets. In these conditions, the generated frictional heat is low, causing a reduction in material volume softening and plasticizing. As the tool penetrates into the sheets, softened material from the bottom sheet flows towards the top sheet. With increasing tool penetration depth, the volume of flowing material increases, and the material mixing in these conditions is significantly improved. At maximum penetration depths, the mechanical pressure from the tool shoulder and the frictional heat of the welding process reach their maximum values. The higher heat generated leads to more material softening and intensifies plastic deformation. The rotational motion of the tool ensures proper mixing of softened materials, resulting in a very strong bond between the components. In such conditions, the tensile shear forces of the joints increase significantly due to the proper mixing of materials, reaching their peak values at a penetration depth of 2.5 mm.

Additionally, Fig. 14 indicates that reducing the tool pin diameter results in an increasing trend in the joint breaking force. Although increasing the pin diameter may facilitate tool entry into the sheets and improve the welding disturbance in the nugget zone, a larger pin diameter causes some of the nugget zone area to be lost due to the end hole of the process, occupying more material volume in this area. Furthermore, it is observed that the unintended hole formed during the use of a larger pin becomes a stress concentration point, leading to a lower resistance of the joint against tensile shear forces. As previously mentioned, when using a larger pin diameter, the intermetallic layer formed between the sheets becomes thicker, which accompanies a decrease in tensile shear strength of the joints. It is also evident that with a smaller pin diameter, the length and energy of the joint breaking increase compared to joints produced with a larger pin diameter. In these conditions, a more uniform mechanical pressure is applied to the nugget zone by the tool shoulder, and the unintended hole that can serve as a stress concentration point and reduce the tensile shear strength of the joint is absent. Due to increased friction between the tool shoulder and the nugget zone, along with the increased frictional heat of the weld, plastic deformation is improved, resulting in a better mixing of materials between the two aluminum sheets. In these conditions, it is observed that material flow from the lower nugget zone towards the top of the weld increases, and the height of the weld's sidewalls increases.

By comparing the values of tensile shear forces, it was evident that when positioning the aluminum alloy 6061-T6 on top of the sheets, the tensile strength obtained increased. As mentioned earlier, the melting temperature of aluminum 6061-T6 is much lower than that of aluminum 5052-T32, and its softening range is also wider than aluminum 5052-T32. Due to the lower melting point of aluminum 6061-T6, this alloy reaches the plastic state more rapidly and also allows for a greater volume of softened material to enter the process temperature. Because of the wider softening range of aluminum alloy 6061-T6, it is observed that when this alloy is placed on top, the duration of interaction of softened materials in the nugget zone increases, resulting in better material mixing compared to other conditions. Additionally, during the welding operation, the nugget zone formed when aluminum 6061-T6 was placed on top was somewhat wider than when aluminum 5052-T32 was positioned on top. The primary elements in the chemical composition of aluminum 6061-T6 are magnesium and silicon. In fact, magnesium enhances the strength of the alloy, and silicon is the main factor in lowering the melting point of this alloy. Magnesium and silicon do not have the ability to undergo separate heat treatments, but when combined and alloyed, these two elements have acceptable heat treatability. Unlike aluminum 6061-T6, aluminum 5052-T32 only utilizes magnesium as an element in its structure and therefore does not have heat treatability. Alloy 5052-T32 has moderate mechanical properties, and therefore, when positioned on top, the final tensile shear force has decreased slightly.

Similar results were obtained in the study by Andalib et al. [10]. It was observed that the tool penetration depth in both welding stages and re-filling plays a key role in achieving a high-strength welded joint.

Similarly, Paidar et al. (2017) showed that the maximum shear strength of the edge is achieved with pinless tools. However, when using pin tools, contrary to this study, they showed that the shear strength of the lap decreases with increasing tool penetration depth but increases with pin diameter. For pinless tools, the formation of an ambiguous hook is evident, and cracking begins at the common joint.

Chada and Kumar [40] also showed that the position of the materials plays a crucial role in achieving better microstructures and higher mechanical properties. In contrast to the results of this study, in all cases, the softer material was positioned on top, and the movement of the tool in the softer material caused the heat generated during the process to first melt the softer material and then move the molten material towards the harder material.

Similarly, Memon et al. [26] demonstrated that the material's position significantly affects the peak temperature. Higher temperatures are achieved in joints when harder materials are positioned as the top sheet.

Yazdi et al. [15] also showed that forming a larger nugget zone and a higher volume of displaced metal due to a higher penetration depth, more input heat, and more severe stirring caused by a longer probe tool can increase the average tensile shear strength.

Similarly, Piccini et al. (2015) showed that failure loads increase with tool penetration depth for both aluminum sheet positions, but when the aluminum 6063 sheet was the top material, it increased more.

Shen et al. [14] also showed that when using shallow tool penetration depth, despite the presence of a mechanical lock, the metallurgical bond does not form completely through the weld center. With increasing tool penetration depth, more stirring occurred, and the metallurgical bond formed at the weld center with some voids at the weld boundary.

Furthermore, Paidar et al. [[19], [20]] showed that material positioning affects the flow behavior of dissimilar aluminum alloy joints due to inequalities in properties (flow stress). The weld hook path resulting from the flow near the probe tip changes with sheet arrangement.

Fig. 15 illustrates the failure modes of welded specimens after performing tensile shear tests, based on various process parameters.Fig. 15 Depicts the failure modes of welded joints under different parametric conditions.

Fig. 15

Similarly, Kumar et al. [43] determined that the position of the plate is the most influential welding parameter in determining the tensile properties of the weld. In addition, the joint made with AA6101-T6 provides sufficient weld reaction temperature, plastic flow and better tensile properties.

By comparing the failure of the welded joints, it is evident that in all joints, the failure occurred in the form of button pull-out. This indicates that the strength of the welded joints has been in an upper state under all conditions and has demonstrated high resistance against the applied tensile shear forces during testing. When using low tool penetration depth, the confined softened zone is relatively small, and gradually with increasing tool penetration depth, this softened zone becomes wider, indicating increased weld strength against tensile shear loading. Moreover, with an increase in the tool pin diameter and due to the larger remaining hole margin, the softened zone on the plates has become wider. When using pinless tools, it is observed that the softened zone actually extends to the entire shoulder area of the tool, indicating a lack of concentration of tensile stresses in the central region of the tool and high weld strength. Conversely, it is observed that the length of the welded area has increased due to the absence of the tool pin. In pin-containing tools, the pin penetration outside the plates affects the length of the welded joint, causing a reduction in the length of the connected area. However, in pinless tools, due to the penetration and influence of the tool shoulder on the parts, the length of the welded area is equal to the diameter of the tool shoulder, and therefore, it is observed that during weld failure, the entire tool shoulder area separates from the plate, indicating very high tensile shear forces. Additionally, when placing the aluminum alloy 6061-T6 plate on top of the stack, it is also observed that the surrounding area of the separated zone has relatively increased compared to the conditions where aluminum 5052-T32 was on top.

The most influential parameter in changing the failure modes of the welds has been the diameter of the tool pin, followed by the tool penetration depth and the alteration in the arrangement of the plates on top of each other.

Fig. 16 presents scanning electron microscope s of the failure modes of the joints.Fig. 16 Failure modes of welded joints using scanning electron microscopy.

Fig. 16

The obtained s demonstrate that the failure mode of the joints under different tool pin conditions has been in a flexible shape, and with a decrease in the pin diameter, the pull-out and weld resistance against applied loads have increased. Increasing the tool penetration depth and placing the aluminum alloy 6061-T6 plate on top of the stack have also led to an increase in weld strength and the resistance of the welded area against the applied forces during tensile shear testing.

Similarly, Ekinci and Imak [44] showed that according to the weld failure surface analysis, both joints failed flexibly.

Zhang et al. [45] showed that by adjusting the filling depth of the pinless tool during the defect-regularizing stage and the rotating direction of the tool with a threaded pin during the repair stage, sufficient metallurgical bonding was achieved, and the kissing bond defect was eliminated due to not only the enlarged stir zone but also the increased pressure and temperature [55].

Babu et al. [46] showed that the joint efficiency of welds made in T6 condition (43 %) was significantly lower than that of welds made in T4 condition (51 %), due to the higher base material strength in the former. The Alclad layers did not present any specific problems in friction stir lap welding [56].

Nian et al. [47] compared friction stir lap welding (FSLW) with a new technology called seal-flow-multi-vortex friction stir lap welding (SM-FSLW). The results showed that, compared to traditional FSLW, SM-FSLW achieved higher welding temperatures, more intense material flow, and a larger area with high flow velocity, thereby creating macro and micro mechanical interlockings and increasing the joint's load capacity. The tensile shear strength of the lap joint under SM-FSLW was 27.8 % higher than that under traditional FSLW [57].

3.3 Micro-hardness variations

Fig. 17 illustrates the variation trends of microhardness values in different regions of the weld based on the variable process parameters.Fig. 17 Trends of microhardness variations in different regions of the weld based on: (a) 4 mm pin diameter, (b) 8 mm pin diameter, and (c) pinless tool.

Fig. 17

Comparing Fig. 17, Fig. 17 (c) reveals that the overall trend of microhardness variations in all three tool pin conditions is almost similar to each other. The microhardness graphs are roughly asymmetric W-shaped, due to the use of dissimilar aluminum plates with different base metal hardness. The microhardness value of aluminum alloy 6061-T6 is about 100.4 H V, while the microhardness of aluminum alloy 5052-T32 is about 75.6. Generally, aluminum alloy 6061-T6 has higher hardness compared to aluminum 5052-T32, which is attributed to its chemical composition and the elements present in it. When using shallow tool penetration depth, the microhardness in the heat-affected zone remains almost constant for both 4 mm and 8 mm pin diameters. However, with increasing tool penetration depth to 1.5 and 2.5 mm, the microhardness in this zone has shown a decreasing trend. The heat-affected zone is farther from the nugget zone and does not undergo mechanical work and plastic deformation by the tool in this area. In fact, this area is only affected by the frictional heat of the welding process, and under conditions of low tool penetration depth, the welding process heat is relatively low and the microstructural changes in this area are insignificant, leading to the constancy of microhardness under these conditions. With increased tool penetration depth and more engagement of the tool with the plates, the frictional heat of the welding process has increased, and in these conditions, more heat is transferred to the heat-affected area, resulting in coarser grain size in this area. Expansion of grain size is accompanied by a decrease in microhardness. Approaching the nugget zone and due to the increase in temperature, further reduction in microhardness is observed. In the thermo-mechanically affected zone, microhardness has reached its minimum values. In this area, deposits are dissolved due to the frictional heat of the process, and a reduction in the density of voids occurs, leading to a sudden decrease in microhardness. When using shallow tool penetration depth and low frictional heat, a lesser reduction in void density and dissolution of deposits occurred, and with increasing tool penetration depth, these conditions intensified. The use of a larger pin diameter leads to increased heat generation during the welding process, which further decreases microhardness in the heat-affected area. In the nugget zone, microhardness values have significantly increased. In this area, materials come in a soft and pasty form due to the frictional heat and are mixed with each other under the rotational movement of the tool. This mixing of materials, along with the mechanical pressure exerted by the tool shoulder, leads to the formation of a dense and compacted region. Due to severe plastic deformation and mechanical work of the tool, microhardness of this compacted area has increased significantly. Increasing the pin diameter has caused more severe plastic deformation and further compaction of materials. Therefore, it is observed that microhardness in the nugget zone increases to a greater extent when using a larger pin diameter.

The lowest microhardness value was obtained in the thermo-mechanically affected zone at the maximum tool penetration depth when using a small pin diameter, while the highest microhardness value was achieved at the top tool penetration depth and in the nugget zone, measuring 60.2 and 142.1 H V, respectively. The lowest microhardness value when using a large pin diameter was 58.1 H V in the thermo-mechanically affected zone and at the top tool penetration depth, while the highest microhardness value was 146.4 H V at the top tool penetration depth and in the nugget zone. As seen from the microhardness profile, the obtained values in the heat-affected zone, thermo-mechanically affected zone, and nugget zone were close to each other for both pin thicknesses and did not show significant changes.

Using a pinless tool, microhardness values in the heat-affected zone were not constant and showed a continuous decreasing trend. Due to increased tool shoulder engagement and frictional heat in this condition, the heat was transferred to the heat-affected zone, leading to grain coarsening and decreased microhardness values. With increasing tool penetration depth, these conditions intensified, and the decreasing trend of microhardness became more severe, with the lowest microhardness values obtained in the heat-affected zone when using a tool penetration depth of 2.5 mm. Increased frictional heat when using a pinless tool led to a decrease in void density and further dissolution of deposits in the thermo-mechanically affected zone, resulting in further reduction in microhardness values in this area. In the nugget zone, microhardness significantly increased, more than in the pin tool condition. It is also observed that microhardness changes in the nugget zone are more uniform with pinless tools. The removal of the pin resulted in more uniform and increased pressure exerted by the tool shoulder on the nugget zone, leading to further compaction and densification of this area, accompanied by increased microhardness values. With increasing tool penetration depth and more engagement of the tool with the plate materials in the nugget zone, the microhardness of the weld nugget area has increased, attributed to the increased mixing and compaction of materials and increased penetration depth of the tool into the deeper layers of the plates. The lowest microhardness value when using a pinless tool was 51.1 H V in the thermo-mechanically affected zone at the top tool penetration depth, while the highest microhardness value was achieved in the nugget zone and at the top tool penetration depth, measuring 162.5 H V. Fig. 18 displays the EDX analysis of the fusion zone in different tool pin conditions and at a 2.5 mm depth.Fig. 18 Presents the EDX analysis of the fusion zone in various tool pin conditions: (a) 8 mm pin diameter, (b) 4 mm pin diameter, and (c) pinless tool. In all analyses, the tool penetration depth is 2.5 mm.

Fig. 18

The predominant element in the fusion zone of all three conditions is aluminum. When using a smaller pin diameter, an increase in the participation of aluminum in the fusion zone is observed. Aluminum, with a weight percentage of 97.17 %, is the most abundant element in this area. Additionally, compared to larger pin diameters, a decrease in the participation of other elements, especially iron, is noticeable. Magnesium constitutes the second most abundant element in the composition, with a weight percentage of 2.29 %. Iron, with a weight percentage of 0.39 %, is present in a small amount in the composition, while a very small amount of silicon, with a weight percentage of 0.08 %, is also identified. With an increase in pin diameter, the weight percentage of aluminum in the fusion zone decreases, and the participation of iron, which provides greater strength and hardness, increases in the chemical composition of this area. Magnesium remains the second most abundant element in the composition, with a weight percentage of 3.3 %. Iron, with a weight percentage of 0.47 %, is identified as the third major element in this area. Silicon, with a weight percentage of 0.14 %, constitutes the fourth element present in this zone. Lastly, a small amount of copper, with a weight percentage of 0.11 %, is identified in this area.

Additionally, placing an aluminum alloy plate of 6061-T6 on top of the stack increased the hardness of the joint area. This is due to the higher strength and hardness of the 6061-T6 aluminum alloy compared to 5052-T32 aluminum and the higher content of Zn and Cr in the 6061-T6 alloy compared to 5052-T32.

Similar results were obtained in the study by Abbas et al. (2016), where hardness profiles of dissimilar aluminum welds showed that the lowest hardness values appeared in the TMAZ and HAZ due to the intermetallic compounds Mg2Si and Al3Mg2 and the presence of AlCu, Al2Cu, and Al9Cu4 precipitates.

Furthermore, Andalib et al. [10] demonstrated that the grain size of the nugget zone of samples prepared under optimized welding conditions decreased by about four times compared to the base metal.

Similar results were also obtained in the study by Sarila et al. [48], where hardness profiles of welds showed a W-shaped appearance, and the TMAZ and HAZ had lower hardness values compared to other areas. Along the joint between the welded sheets, a thin intermetallic layer of Al12Mg17 was identified, contributing to increased hardness.

Additionally, Venukumar et al. [30] obtained the hardness distribution among dissimilar welds from the top and bottom plates and observed that the minimum hardness was created in the HAZ.

Similarly, another researches showed that significant grain refinement with the best grain sizes occurred, and the highest hardness value was obtained in the nugget zone [[49], [50], [51], [52], [53], [54], [55], [56], [57]].

Ekinci and Imak [44] showed that reducing the size of weld microstructure grains leads to an increase in the hardness and tensile strength of the weld. The higher hardness of the fabricated joint was due to the formation of smaller grains in the microstructure. The highest hardness measurements were found in the agitation zones and the lowest measurements were found in the heat-affected zone and the mechanical heat-affected zone. Because they had a finer grain microstructure.

Similar results were obtained in the research of Feizollahi et al. [2]. They showed that the microhardness of the heat-affected area is reduced and the minimum values are obtained in the thermo-mechanically affected area. Also, the microhardness reaches its peak values in the disturbed area.

4 Conclusion

In this study, the joining of aluminum alloy sheets 6061-T6 and 5052 was successfully carried out using the friction stir spot welding process. The effects of variations in pin thickness, tool penetration depth, and sheet arrangement on mechanical and metallurgical properties were investigated. The experimental results are as follows:1. The grain size in the heat-affected zone increased, and with an increase in tool penetration depth, the grain size also increased. Using a larger pin diameter resulted in a more significant grain growth compared to a smaller pin diameter. With the removal of the pin, the largest grain size was formed in this area due to increased frictional heat. Placing the 6061-T6 aluminum sheet at the top of the arrangement led to an increase in grain size in this region.

2. With the increase in tool penetration depth, the thermomechanically affected zone was more distinctly formed, and with the removal of the pin, this zone became slightly thicker.

3. The stirred zone became a dense and compact region. As the tool penetration depth increased, the stirred zone became wider and had a denser microstructure. Increasing the pin diameter led to a denser grain structure in this area. With the removal of the pin, a more compact microstructure was obtained in the stirred zone than in other conditions.

4. The reduction of pin diameter and the increase in tool penetration depth led to an increase in the thickness of the intermetallic layer, which played an important role in the tensile strength of the weld.

5. Placing the 6061-T6 aluminum sheet at the top of the arrangement improved the tensile shear strength. With the increase in tool penetration depth and the reduction of pin thickness, the fracture force of the welds continuously increased. The maximum values of tensile shear force were achieved with the removal of the pin.

6. The failure modes of the welds under all parametric conditions were in the form of nugget pull-out. With the reduction of pin diameter and the increase in tool penetration depth, the area of the pulled-out region significantly increased. All produced joints exhibited ductile failure, indicating high joint strength.

7. The microhardness variations in the 4 and 8 mm pin diameters were similar. The microhardness in the heat-affected zone remained constant at shallow tool penetration and showed a continuous decrease with increasing tool penetration depth. Increasing the pin diameter led to a reduction in microhardness in the heat-affected zone.

8. Due to the dissolution of precipitates and the reduction in dislocation density, the lowest microhardness values were observed in the thermomechanically affected zone. With the increase in tool penetration depth and pin diameter, the microhardness values in this area showed a further decrease.

9. The highest microhardness values were obtained in the stirred zone. Increasing the tool penetration depth led to a significant increase in microhardness in the nugget zone, while the microhardness in this zone relatively improved with an increase in pin diameter. The highest microhardness values were achieved using the pinless tool. When using the pinless tool, the microhardness values were more uniform and exhibited fewer variations compared to other conditions.

10. Placing the 6061-T6 aluminum sheet resulted in higher microhardness values in the nugget zone compared to the 5052-T32 aluminum alloy.

11. The optimal welding process conditions were achieved using a pinless tool, high tool penetration depth, and placing the 6061-T6 aluminum sheet at the top of the arrangement.

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We the undersigned agree with all of the above.

CRediT authorship contribution statement

Vahid Feizollahi: Writing – original draft, Data curation. Yousef Gerami: Methodology, Investigation. Ahmad Saki: Methodology, Investigation, Formal analysis. Behrooz Adelzadeh: Validation, Supervision, Conceptualization. Mahmood Zamani: Supervision, Resources, Formal analysis, Data curation. Mehdi Ghobeiti Hasab: Visualization, Validation, Project administration, Investigation, Data curation. Ali Heidary Moghadam: Writing – review & editing, Project administration, Methodology, Investigation.

Declaration of competing interest

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

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