Microstructural and Mechanical Behaviors of Friction Stir Welded Dissimilar AA6082-AA7075 Joints

In this research, microstructural events and mechanical behaviors in dissimilar friction stir welding (FSW) of aluminium (Al) alloy AA6082-AA7075 joints have been evaluated to apply aerospace, defense, and military sectors. FSW parametric eﬀects have a more signiﬁcant impact on the mechanical performances and microstructure of produced joints. FSW tool rotational speed, welding speed, and tool plunge speed were chosen to make the weld joints. The rotational tool speeds of 1600rpm and 2300rpm, welding speeds of 40mm/min and 60mm/min, and tool plunge speeds of 20 mm/min and 30mm/min were set as the upper and lower limits. A constant axial force of 5kN was maintained throughout the joint fabrication process. A taper pin-proﬁled tool was utilized to produce the butt welded joints. Mechanical properties of microhardness, tensile strength, yield strength, elongation, and bending strength of the joints were analyzed. The response of the stir zone microstructure to processing parameters was evaluated using optical microscopy (OM) and fractographic analysis of a tensile specimen shown by scanning electron microscope (SEM). The weld joints produced at 2300rpm, tool traveling rate of 40mm/min, and tool plunge speed of 30 mm/min showed the greatest tensile strength of the 191MPa hardness of 145Hv at the weld center and also the maximum bending strength of 114.23N/mm 2 was achieved. The lowest bending strength of 25.38N/mm 2 was obtained at 1600rpm with 60 mm/min due to inappropriate mixing of the base metals and poor joint quality. Furthermore, this study revealed that a higher tool plunge speed facilitates the formation of equiaxed grains in the thermomechanically aﬀected zone (TMAZ) on the advancing side (AS). Additionally, the increment in tool rotational speed signiﬁcantly improved the tensile strength, weld joint quality, and joint eﬃciency.


Introduction
Aluminum alloys are lightweight and have more applications in the industrial sector. By conventional fusion welding techniques, aluminum alloys are tough to weld, and the joint quality is also unsuitable due to welding defects like distortion, cracks, and porosity [1]. FSW has recently been selected as a reliable method for retaining the alloy's properties when the joining takes place in the solid state.
is technique is performed for joining different material combinations, such as aluminum, copper, magnesium, brass, and other material types. FSW tool plays a very important role in the material joining processes [2]. e usage of aluminum alloys is rapidly increasing due to their castability, lightweight structures needing a higher strength ratio, and extraordinary corrosion resistance. e various FSW tool pin profiles are used to make weld joints between two materials. It has been reported that the tapered pinprofiled tool produced sound (defect-free) weld joints when compared to other pin profiles [3]. Aluminum alloys find various applications in the wing structure, fuel tanks, railway tanks, vehicle bridges, high-speed ships, aerospace, engine chassis, automotive industries, and military applications due to their lightweight and greater strength and weight ratio [1][2][3][4]. FSW is a solid-state material joining technique developed by e Welding Institute (TWI) in 1991.
To fabricate weld joints between two metals, a nonconsumable rotating tool was used, which eliminated the need for additional filler materials. is process does not release toxic or nontoxic fumes during material joining and develops welds free from solidification effects.
is technique was recently developed and is highly important for the material joining of similar and dissimilar materials [5,6]. e FSW process produces an attractive solid-phase bond in the material joining process without melting and recasting the parent materials [7]. FSW weld joint consists of a heataffected zone (HAZ) and TMAZ. It has produced defect-free weld joints with more excellent mechanical properties and minimum welding defects [8]. e rotational tool speed was the major influential factor in producing sound weld joints.
is method has more significant potential for joining aluminum alloys since it can reduce welding defects like porosity, welding cracks, and distortions, commonly encountered in other fusion welding techniques [7][8][9]. e various effects of the FSW parameters have been discussed, like welding speed, tool revolving speed, axial load, and tool pin profile. However, the effect of the tool plunge speed has not been reported to date. It was identified that the grain size of the particle and hardness value of the nugget zone varies, purely depending on the welding tool rotational speed and tool traveling speed [10]. e FSW welding parameters play an essential function in the quality of the weld joints while fabricating them. e mechanical and microstructural properties were investigated in a pair of AA2024 and AA7075 aluminum alloys. e obtained hardness value of the weld joints is decreased when moved away from their center [11]. Better hardness values were obtained in the nugget zone more than other zones affected by the welding process [12]. e FSW tool features like shoulder diameter and taper pin profile are instrumental in producing adequate weld surface quality and better mechanical behaviors of sound weld joints [13]. e weld joints' properties and features were investigated. A large amount of frictional heat is produced in the obtained weld joints based on the FSW process parameters like revolving tool speed (rpm), tool traveling speed (mm/min), pin plunge depth (mm), and axial force (kN) [14]. e effects of the FSW parameters and tool pin profile on the tensile strength, microstructural analysis, hardness, and fatigue behavior of weld joints were experimentally investigated. e welding speed and tool traveling speed were found to help in deciding the joint strength [15].
Many researchers have successfully investigated and reported the microstructural characteristics and mechanical behaviors of dissimilar aluminum alloys processed by FSW.
However, the literature available on the bending characteristics of dissimilar aluminum butt joints between the AA6xxx and AA7xxx series is limited. erefore, this research presents the mechanical properties, microstructural analysis, and bending characteristics of AA6082-AA7075 dissimilar weldments fabricated with various FSW parameters.

Materials and Methodology
2.1. Materials. AA6082 and AA7075 aluminum alloys were taken as the parent materials in this investigation with 2000 mm × 1500 mm × 5 mm. e base materials were prepared to the required dimensions utilizing a milling machine. e base materials for both AA6082-AA7075 aluminum alloys were resized to the required length (100 mm) and width (50 mm) utilizing an automatic material cutting machine (power hacksaw), and a milling machine was utilized to remove the sharp edges of the base materials. Besides, the maximum amount of manganese present in the parent material controls the grain structure. ese features could be helpful to obtain good results and quality of the weld joints. Tables 1-3 represent the mechanical properties, physical properties, and chemical composition of the base materials.
A detailed investigation of previous literature according to Sevvel et al. [18] reports that tool geometry of FSW is the most significant feature of the material joining process. Tool pin geometry plays an essential role in the material flow and turn and governs the transverse rate at which FSW can be calculated. M35 grade tool steel material is selected to fabricate the friction stir welding tool due to its high strength, excellent wear resistance, and low material cost. e taper cylindrical pin-profiled tool was utilized to make the weld joints. A shoulder diameter of 12 mm, pin length of 4.85 mm, and pin with 7 mm to 4 mm diameter were maintained while fabricating the FSW tool. is tool material provides more resistance to softening, especially while working at high temperatures likes 550-600°C at maximum operating tool speeds. Figure 1(a) and 1(b) show a schematic view and dimensions of the fabricated FSW tool.

FSW Parameters and Experimental Work.
A conventional vertical milling machine FSW setup was used to make the weld joints. e FSW equipment is equipped with a 5 kW motor power pack. e length of the bed is 810 mm and the width 40 mm; this is housed at SA Engineering College, Chennai. e photographic view of the FSW arrangement and joining process is shown in Figure 2(a) and 2(b). e base material AA6082-AA7075 dissimilar aluminum alloys were used to fabricate the weld joints. Before starting the FSW process, the base material surface was thoroughly cleaned of the rust, burs, dirt, and oxide layers. e thickness of both materials was maintained at 5 mm. en, the alloy plates were securely clamped with the help of a manual top clamp set up on the machine bed surface in a butt joint configuration so that it does not disturb the welding process.
Al alloy AA6082 was placed on the AS, and AA7075 alloy was placed on the retreating side (RS). e taper pin-profiled stir tool was fixed on the machine spindle. e material joining process took place along the rolling direction of base materials. e welding process was conducted at different spindle speeds of 1600 rpm and 2300 rpm, tool traveling speeds of 40 mm/min and 60 mm/min, and tool plunge speeds of 20 mm/min and 30 mm/min, maintaining a     Table 4, and the process parameter combinations formed as per Taguchi L8 orthogonal array are presented in Table 5. Initially, the FSW spindle rotation is started with the required speed without touching the base material surfaces. en, the designed FSW stirred pin is inserted to a predetermined plunge depth between the two base materials to be welded at the desired plunge speeds. e tool is revolved continuously to maintain the dwell time of 1 min without moving the stirred pin; meanwhile, the frictional heat produced softens the base materials. After the dwell time, the FSW tool moves along the weld direction to the base materials, producing the weld joints. e tool is retracted after the welding process does it. e photographic view of the joints before and after welding is

Tensile Test.
e tensile test specimens were developed as per the ASTM-E8-04 standard. e tensile specimen dimensions are depicted in Figure 4. e sample was cut to the proper dimensions perpendicular to the weld direction by using wire cut EDM. e prepared tensile specimen was inspected after machining. e universal testing machine (UTM) was utilized to assess the tensile test on the weld samples. e weldments were tested as per the standard. e % of elongation, yield stress, and tensile strength of the welded joints were obtained. e tensile strengths of the weldments are presented in Table 6.

Hardness Test.
A hardness test was conducted across the various FSW zones of the weld joints, utilizing the Vickers hardness testing machine. Hardness values are reported in Table 4. e weld specimens were fabricated and processed by the FSW technique. e size of the grains in the different welding zones plays an important role in evaluating the hardness of the weld joints. e small-sized grain particles give massive strength or the highest hardness values, due to which the grain boundaries avoid slip and dislocations [19]. It is identified that the grain sizes were reduced in the weld zone, which increases the hardness values simultaneously. In the present experimental work, defect-free weldments, due to excellent equiaxed grain particles, were found in the weld   e maximum hardness values are recorded in the nugget zone nearly close to the base material hardness value due to the presence of refined equiaxed grains homogeneously distributed in the nugget zone. Mechanical behaviors of the weld specimens are given in Table 6. e effect of temperature on welded joints was studied, and it was shown that greater welding speeds and feeds resulted in higher temperatures. e weld bead softened and the microhardness decreased as the values of these parameters increased [20].

Bending Test.
e bending strength of the base materials was successfully tested by the three-point method, and the characteristics are presented in Table 7. e bending test samples were developed following the ASTM-E290-08 standard. Figure 5(a) illustrates that a sample size of 100 mm × 12 mm × 5 mm was maintained for all the bending samples. e indenter corner radius is 10 mm. Figures 5(b) and 5(c) show the base alloys' photographic view. Figure 5(d) indicates weld samples used for the bend test. A bending test was successfully conducted to assess the weld samples' joint strength (bond strength).    Figure 3. e FSW process successfully processed the weld samples using a tool steel M35 grade FSW tool with a tapered pin profile. e physical appearance of the weld joints joined with 1600, 2300 rpm tool rotational speeds and 40, 60 mm/min welding speeds seems good without any defects. e pin plunging speed of 20, 30 mm/min was utilized in this process. Due to the FSW tool plunging into the interface of the weld joint, the surface of the weld region contained a slight concavity. As a result, a small number of weld flashes was produced in the weld region of the specimens.

Macro-and Microstructural Analysis.
Following the joint fabricating process, samples were prepared for microand macroanalysis using standard metallographic methods. First, the etching process was performed using 50 mL H 3 PO 4 and 1 gram of NaCl dissolved in 125 mL of ethanol to the weld zone for 60 seconds. Further, the same process was followed for 12 seconds using 4 grams of KMnO 4 and 1 gram of sodium hydroxide (NaOH) dissolved in 120 mL of distilled water [21]. After that, each weld sample was observed with a high-resolution optical microscope. Figure 6(a) illustrates the microstructure of the parent material AA6082 aluminum alloy. It shows the size of the grain particle changing from 7 μm to 12 μm. Figure 6(b) illustrates the microstructure of the base material AA7075 aluminum alloy. It shows the size of the grain particle changing from 6 μm to 12 μm. e frictional heat increased at the weld region due to the increment in the rotational tool speed. Also, the increment in the welding speed produces the highest cooling rate [22]. e weld sample joined by the combination of the maximum tool rotational speed of 2300 rpm and minimum welding speed of 40 mm/min yielded the high tensile strength of 191 MPa, bending strength of 114.23 N/mm 2 , and the highest hardness of 145 Hv, which are nearly 85% of the base metals. In the welding of Ti6Al4V plates, the tensile strength improved significantly. e primary contributing factor was found to be the formation of lamellar structures. During the operation, however, the hardness of the stir zone decreased due to grain coarsening and dynamic recrystallization [23]. e maximum FSW tool speed and lower   [24]. e highest tool speed and lower tool traveling (welding rate) give longer interaction time between the FSW tool pin and the weld samples, which leads to perfect material transportation and string starting from the AS to RS and from the RS to AS [25]. erefore, fine grain with equiaxed particles evenly distributed (Figure 7(a)) in the nugget (weld region) zone led to producing defect-free weld joints (Table 8). Tunnel defects were found just above the bottom of the welded region in the examination of dissimilar welding of Al 5052 and 304 SS alloys. During the microstructural examination, the defects were found to be on the retreating side. Other defects, including voids and cracks, were found regardless of process parameter selection [26]. e weld sample produced by the maximum FSW tool rotation speed of 2300 rpm and higher welding rate of 60 mm/ min exhibited average tensile strength (162 MPa), as shown in Figure 7(b). Due to increasing spindle speed and tool traveling rate, the cooling rate might be higher when compared to the other weld samples. e increasing welding speed and tool rotation speed reduce the interaction time of the weld samples and the tool, leading to inappropriate material mixing in the nugget zone and inadequate material consolidation. e weld sample joined by the minimum tool rotation speed of 1600 rpm and minimum welding speed of 40 mm/min exhibited lower tensile strength (125 MPa). Insufficient frictional heat was developed while using a lower tool rotating speed during the joint fabricating process. e cooling rate is too low for this parameter. e nugget zone reaction between materials purely depends on the lack of heat generation. Figure 7(c) shows low frictional heat produced during the joint making process (welding), leading to the tunnel defect, breaking, and distribution of particles in the nugget region, which provides inadequate material transport and inappropriate material string nugget region in the weld zone. e back-assisted FSW technique was used to examine the mechanical and microstructural characteristics of dissimilar NiTi-Ti6Al4V joints. It was revealed that defect joints can be obtained to increase the plastic fluidity of the welded metals. e mechanical characteristics of the joints were deteriorated by the coarse and brittle intermetallic compounds formed during the process [27]. Table 8 represents the volume defects that have been noticed in the nugget zone (weld region) with a minimum tensile strength of the weld joint. e existence of approximately equal volumes of austenite and ferrite phases in the stir zone was reported in a study of dissimilar FSW of 304 ferritic and austenitic steels, which contributed to the creation of defect-free joints [28]. e weld joints fabricated with a higher tool rotational speed of 2300 rpm and minimum tool traveling speed of 40 mm/min exhibited the highest tensile and hardness values due to equalsized fine grain particles distributed in the weld region. e high tensile and hardness values were obtained using a lower spindle speed and tool traveling speed. Hence, the tensiletested samples were fractured on the RS [29]. e lower hardness value of the weldments was observed in HAZ. A microhardness study was conducted on dissimilar AA6061-T6 and AA5052-H32 welded joints revealed the presence of lower hardness on both sides of HAZ. ough the nugget zones exhibited higher hardness, they also showed the presence of some cracks due to higher welding speeds [30]. An advanced numerical model was developed and the experimental studies were carried out to validate the properties of underwater FSW dissimilar aluminium and steel joints. e maximum temperature was recorded on the steel side due to the higher melting point. e peak temperature increased with the heat input index defined in terms of the ratio of tool rotation speed and welding speed. Heat input index played a major role in the mixing and diffusion of aluminium into steel for the submerged underwater welding condition [31]. e influence of cooling media on the microstructure of dissimilar AA3003 aluminum and A441 AISI steel joints was investigated with CO 2 , air, and water. CO 2 suppressed the grain growth due to recrystallization and reduced the thickness of the intermetallic compound layer at the joint. However, water proved to be the better cooling media as it produced joints with no notable segregations and better mechanical properties [32]. weldments. e bent test sample was prepared following the ASTM-E290 standard to evaluate the material consolidation, ductility of the weldments, and bond strength. A size of 100 × 12 × 5 mm dimensions was maintained to fabricate the bend test samples using EDM. Each weld sample subjected to the bend test is evaluated with a mandrel size of 4 Tat 180°. e experimental observations of the bending samples like bending strength, maximum displacement, and breaking load are listed in Table 9. For all the FSW experimental work, rotational tool speed and welding speed significantly alter the bending strength by influencing the heat generated in the weld zones. In the center of the weld joint, the bending force was applied, and the test sample's back surface was observed. At the end of the bend test, no surface cracks were observed, indicating that the weld joint possessed good ductility. However, a few weld samples were identified with bottom side irregularities resulting in the poor bending strength of the joints. e applied force and maximum displacement of the test samples were recorded precisely during the testing process.
e bending strength of all the FSW AA6082-AA7075 different weld samples was lower than that of the parent materials. A minor crack was noticed at the back surface of the weld specimen joined at 1600 rpm due to poor mixing of materials caused by the lower axial force during the welding process. Another reason for attaining minor cracks was the lack of FSW tool pin penetration on the root site of the weld sample. After the bending test, the defects were identified in the root side for a few samples due to tunnel defects. Even though the weld sample surface appeared friendly, tunnel defects were observed on the RS and the nugget zone of the weld. Figures 8(a)-8(d) show a graphical representation of the bending strength values for defect-free and defective weld samples. e plasticized material below the shoulder is insufficient during the material joining process due to improper frictional heat generation.
is problem can be identified by increasing the welding speed and reducing the rotational tool speed [29]. Figure 9 indicates fracture morphology of FSW weld samples tested as per ASTM-E8-04 for tensile strength. e majority of the tensile test samples were fractured in their lower level of the hardness region. e weld specimens were fabricated at 2300 rpm at the welding speed of 40 mm/min fractured on the base material side. e weld samples fractured on the weld and HAZ zone were fabricated at 1600 rpm and the welding speed of 60 mm/min. e tensile strength of the weld samples is reduced by increasing the FSW tool rotation speed and at minimum welding speed. e high tensile strength of 191 MPa was obtained from a tool speed of 2300 rpm, tool travel rate of 40 mm/min, and tool plunge speed of 30 mm/min. e SEM images of the fractured weld samples depicted in Figure 10 were produced at the rotational tool speed of 2300 rpm. e fractographs of the welded samples revealed a ductile mode of fracture. SEM images of the fractured surfaces presented in Figures 10(a) exhibit dents and clefts. On the serrated surface of the fractured tensile samples, cleavage planes and fine dimples were apparent. e increment in the strength of the joints is attributed to grain refinement. e integrity of the dissimilar 2024 and 7075 aluminium alloys was assessed in terms of residual stress distribution. Residual stress profiles generated by the neutron diffraction method indicated that the transverse and the normal tensile stresses were present on the 7075 sides. However, the 2024 side exhibited compressive stresses [33].

Conclusions
FSW joints were successfully fabricated with the tapered pin tool for various process parameter combinations. In addition, the microhardness, tensile strength, yield strength, bending strength, and microstructural characteristics of AA6082-AA7075 dissimilar weld joints were studied, and the following conclusions were derived; (i) e micro-and macrostructures of the dissimilar FSW joints are analyzed utilizing rotational tool speed, tool traverse speed, tool pin profile, and tool plunge speed. and tool travel speed significantly influence the microstructure and the behaviors of dissimilar AA6082-AA7075 weld joints. However, the tool plunge speed modified the properties due to its interaction with the other two parameters.

Data Availability
e data used to support the findings of this study are included in the article.