An experimental method for choosing the tool pin profile and shoulder size to join dissimilar aluminum alloys AA7075-T651 and AA6061-T6 joints

Many military and light weight aircraft designs call for joints between two different grades of aluminium alloy. This present study looked at the effect of tool shoulder diameter and tool pin profile on the tensile strength properties of dissimilar aluminium alloy AA6061-T6 and AA7075-T651 joints created by friction stir welding. The joints were fabricated utilizing three distinct tool shoulder diameters 15 mm, 20 mm and 25 mm along with two distinct tool pin profiles namely taper and square. The microstructure and microhardness of weld stir zone (WSZ) were examined and linked with the strength parameters of the joints. Adequate frictional heat generation from 20 mm shoulder diameter and pulsating stirring action of square pin revealed the formation of very fine grains in the weld nugget zone (WNZ). Due to enhanced material flow and the production of a defect-free stir zone, the joint created using a tool with a 20 mm shoulder diameter and square pin profile had the maximum hardness of 117 HV and the highest tensile strength of 284 MPa. The ductile mechanism of fracture is revealed by the presence of fine dimples accumulating in the tensile fractured surface of the joint fabricated by 20 mm tool shoulder diameter and square pin.


Introduction
Dissimilar materials can be joined in several industries, including shipbuilding, electronics, aircraft, and the automobile sector [1].Additionally, the above-mentioned applications confront a number of difficulties as a result of the substantial physical and chemical variations between the materials that need to be joined.The heattreatable aluminium alloy AA6061 is moderately strong and has better welding capabilities than high-strength alloys [2].As a result, alloys in this grade are widely used in naval frames, storage tanks, and aeroplanes.Aluminium alloy AA7075-T651 aluminium alloy, which is high-strength and precipitation-hardening, is widely utilised in aircraft major structures [3].On the other hand, achieving a robust fusion weld for dissimilar grades of aluminum alloys poses a significant challenge through traditional welding procedures.This complexity arises from the formation of a dendritic structure in the weld stir zone, leading to a considerable deterioration in the mechanical properties of the resulting weld joint.Friction stir welding (FSW) represents a promising approach since it allows for the solid-state welding of different materials while overcoming the shortcomings of the fusion welding process [4].It is crucial to regulate the essential process parameters in order to enhance the tensile strength of the joint on which the quality of the weld joint is based.The tool pin and shoulder are the two major elements of the FSW tool geometry [5].The regulation of heat generation and material flow is the most significant issue of tool geometry.The peak temperature, energy, and torque are all influenced by the shoulder configuration.In the weld stir zone, the tool shoulder diameter is the most important characteristic for plastic deformation of the work piece.Furthermore, plastically deformed material was stirred by tool pin and it improves the WSZ material transfer [6].Vijayakumar et al [7] made an attempt to optimize the FSW parameters by using Taguchi L27 method.In order to investigate the tensile strength of FSW dissimilar Al6262 and Al5456 joint, this study combined an H-13 tool pin with a number of process parameters, including tool rotating speed, welding speed and tool tilt angle.While keeping a tool rotational speed of 1400 rpm, welding speed of 30 mm min −1 , and tool tilt angle of 2 degrees, the extreme tensile strength is attained.Ramamurthy et al [8] investigated the joining of the dissimilar aluminium alloys AA2014 and AA6063 employing the FSW technique in order to increase yield strength, ultimate tensile strength, and microhardness.The tool pin profile, rotational speed, axial force, and traverse speed were the four input parameters for the experiments, which were planned using the response surface methodology (RSM) based central composite design (CCD).The weld's grain refinement as well as proper material fusion is observable in micrographs, enhancing the weld's mechanical strength and bonding.
Elangovan and Balasubramanian et al [9] explored the correlation between pin profiles and shoulder diameter while welding AA6061 aluminium alloy joints.The square pin shape with an 18 mm shoulder diameter has proven increased tensile characteristics.Zhao et al [10] looked into the effect of pin geometry on FSW joints mechanical qualities.The findings is that the pin profile affects the plastic flow of deformed material and that a tapered tool with a screw thread delivers the best weld quality with good outcomes.Mugada et al [11] explored into the role of the shoulder end function in temperature generation around the tool's shoulder and discovered that it facilitates for further material flow and generates more heat than a traditional flat tool.Kaewkham et al [12] made an attempt to optimize the FSW parameters to enhance the mechanical behaviour of the FSW joints using Response surface methodology.FSW parameters used for fabricating the joints are tool rotational speed, feed rate and stirring tool.Deep rolling pressure and offset variables are also used in addition to FSW parameters.Any type of stirring tool can be employed and the optimum values for the tool rotational speed, feed rate, deep rolling pressure and offset were 979 rev/min, 65 mm/ min , 300 bar and 0.2 mm respectively, exhibited the best mechanical properties.Ravikumar et al [13] explored the influence of FSW parameters on mechanical properties of friction stir welded AA6061-T651 and AA7075-T651 joints.Tool pin profile, tool rotational speed and welding speed were utilized to produce the FSW joints.During tensile testing, all of the produced joints fractured at the heat-affected zone on the AA6061-T651 side, where the micro hardness value is lower.At lower welding and higher rotational speeds, a satisfactory mixing of the two materials that were bonded was achieved.Rao et al [14] examined the triangular and conical pin profiles and discovered that the triangle pin tool produced very fine grains.They discovered that the geometry and non-soldering circumstances of the tool pin had a significant impact on the shape and structure of the welding tool.The influence of tool geometry on 5 mm thick AA5086-O and AA6061-T6 joints was studied by Aval et al [15].He examined three tool geometries and discovered that due to higher heat input, the tool with a concave shoulder and a taper probe with three grooves generated more uniform stir zones than the other tools.Yuvaraj et al [16] made an attempt to analyze the role of tool axial force on mechanical behaviour of friction stir welded AA7075-T651 and AA6063-T6 joints.Four different axial load conditions were employed to fabricate the joints.Fine grain formation and uniform distribution in the joint made with 6kN revealed the maximum tensile strength of 269 MPa.Garg et al [17] studied the impact of FSW tool rotation direction on microstructure evolution, mechanical strength and fracture behaviour of AA6061 and AA7075-T651 joints.Stronger joints with no flaws were made for the joint when the second weld's tool rotation was provided in the anticlockwise orientation.An EBSD study showed that the finer grains at the centre of the SZ compared to the grains below the tool shoulder were indicative of the region's strong heat generation and plastic deformation.
Despite significant advancements in Friction Stir Welding (FSW) across diverse materials, it is crucial to acknowledge that there is still considerable work ahead.The feasibility stage is undergoing refinement, and it's imperative to recognize that the success of a weld is connected to both the design and material of the FSW tool.The effectiveness and quality of the weld can also be influenced by the dimensions and shape of the tool shoulder and tool pin.The current study examines the effects of tool shoulder diameter and pin profile on the materials flow, microstructures, and mechanical characteristics of dissimilar joints using the Al alloys AA7075-T651 and AA6061-T6 as the primary materials.

Materials and method
The base materials for this experiment were rolled plates of various grades of aluminium alloys, specifically AA6061-T6 and AA7075-T651.The 6 mm thick rolled plates were machined to the desired size (150 mm × 100 mm).The plates were cleaned with acetone before the FSW process to eliminate dirt and oil [18].The FSW joints were made using an industrially designed computer numerically controlled FSW machine is shown in figure 1(a).Mechanical clamps were used to secure the plates in the right position, resulting in the butt joint configuration is shown in figure 1(b).A non-consumable tool with square and taper pin profiles made of high-speed steel was used to make the joints is shown in figure 1(c).The FSW parameters employed for fabricating the joints are tool rotation speed of 2000rpm, welding speed of 40 mm min −1 and tool offset of 0.9 mm.The joints were created using a single pass welding process is shown in figures 2(a) and (b).Table 1 shows the experimental plan for the current study.The tensile test was carried out using a computerised universal testing machine [19] built by Fine Spavy Associates and Engineers Pvt.Ltd with a cross head speed of 1 mm min −1 and a load capacity of 1000 kN.Three specimens were tested for each experimental condition, and thus the average of three results is reported.An optical microscope with image analysis software was used to   carry out microstructural evaluations [20].For metallographic investigation, the specimens were cut to appropriate sizes from the joint, which included both weld metal and base metal regions, and polished with various grades of emery sheets.Finally, the microstructure was revealed by etching the specimens using Keller's reagent [21].The optical microscope developed by MEIJI was used for examining the microstructures and macrostructures.The Vickers microhardness tester, made by SHIMADZU, was used to perform the microhardness test.The loading dwell period was 15 seconds , and indenter load was 10 N the during the test [22].The microhardness test was performed in the plane perpendicular to the welding line.Furthermore, the FESEM technique was predominantly used to investigate the tensile specimens fractured surface.It is done by using Field Emission Scanning Electron Microscope made by Carl ZEISS -EVO 18.

Analysis of macrostructure
The non-consumable rotating tool's principal role is to regulate the heat generation, stir the plasticized metal in the WNZ (Weld Nugget Zone) and transport it behind it in order to produce an effective joint.The FSW joints are usually free of solidification-related problems since no melting occurs during the welding process.Owing to inadequate material flow and insufficient material consolidation within the Weld Nugget Zone (WNZ), FSW joints were prone to various defects, including pinholes, tunnels, piping and cracks [23].Material transport occurs on the weld's top surface and in the WNZ due to the rotating tool shoulder and pin profile.The tool shoulder has an impact on the transfer of one-third of the material near the weld's top surface.The amount of material being plasticized and swirled in the WNZ is determined by the pin profile and size.Utilizing the optical microscope produced by MEIJI, the macrostructures and microstructures were analysed.The table 2 shows the macrostructure of joints made with different tool shoulder diameter and pin profile.Tunnel formation in the WNZ is caused by excessive frictional heat generation and material turbulence flow due to the greater tool shoulder diameter of 25 mm and taper pin profile [24].Due to a lack of frictional heat and metal aggregation in the WNZ, specimens manufactured with a tool shoulder diameter of 15 mm had defective tightness and root defects.The joint created using a tool shoulder diameter of 20 mm and square pin profiles exhibits a faultless weld surface and joint due to sufficient heat generation and effective material stirring action.The crucial factors governing the stirring action and material transfer within the Weld Nugget Zone (WNZ) are precisely determined by the ratio of static to dynamic volume in the tool pin profile [25].

Evaluation of microstructure and material flow
The microstructure of an FSW joint can be categorized into three parts: the stir zone, the heat affected zone, and the thermomechanically affected zone.The analysis of flow patterns, grain size and orientation in diverse regions of the weld joint was conducted with precision using an Optical Microscope.Regardless of tool shoulder diameter, onion ring formation was detected in the regions influenced by the square pin [26].Material flow from the warmer top layer to the cooler bottom layer results in the formation of onion ring pattern.There is no indications of onion ring in specimens manufactured with a taper tool pin and tool shoulder diameters of 15 mm and 25 mm.As a result of the insufficient heat input and inadequate material flow, no onion rings could be created.Because of the smaller contact area, there is less interaction between the tool and the plasticized material in the specimen 2. The non-uniform material swept throughout the joint's depth disrupts regular material flow.A notable amount of extruded material was observed at the bottom of the joint and tunnel is formed as a result of inadequate sweeping of plasticized material and reduced frictional heat [27].Along with the pulsating stirring action of the square pin tool, the shoulder diameter of 20 mm created appropriate frictional heat and plastic deformation.In the WNZ, it causes dynamic recrystallization [28] and the creation of onion ring flow with very fine equiaxed grains in specimen 3 as shown in figure 3(a).The plasticized material is simply permitted to flow on the sides of the pin in the case of taper pin profile without flat face, resulting in severe agglomeration of material around the pin and coarse grains formation in the WNZ of specimen 6 as shown in figure 3(b).As shown in figure 4, SEM analysis was used to examine the five different weld zones, including the weld stir zone (WSZ), retreating side heat affected zone (RSHAZ), retreating side thermomechanically affected zone (RSTMAZ), advancing side heat affected zone (ASHAZ), and advancing side thermomechanically affected zone (ASTMAZ).The microstructure study reveals that the interplay between the tool shoulder and pin shape significantly influences various aspects, including the frictional contact area, heat generation, material mixing, as well as grain size and its orientation [29].

Distribution of microhardness
The microhardness profile is demonstrated in figures 5(a) and (b), and it is measured along the line that runs through the middle of the joint's cross section.It demonstrates that the design of the tool shoulder and pin has a significant impact on the weld and heat-affected regions.The characteristic W-shaped hardness plots were recorded in all of the joints [30].The presence of distributed fine or coarse grains in WSZ might cause the microhardness profiles to oscillate.The WSZ has a higher hardness than the TMAZ in all joints.The TMAZ on both the advancing and retreating sides of the joints showed reduced hardness across the relevant areas.
According to the Hall-Petch correlation, finer grains are correlated with higher hardness levels [31].The flat faces of the square pin profile tool generate pulsating stirring in the flowing material.The square pin design generates 80 pulses per second, whereas the taper pin shape does not.As a result of the aforementioned parameters, the maximum hardness of the specimen 3 manufactured with a 20 mm tool shoulder diameter and  square pin shape is 117HV.The inadequate performance of the plasticized material, coupled with the absence of pulsating action, results in a comparatively larger grain size in the FSP zone of specimen 6 produced by the combination of taper pin and 25 mm tool shoulder diameter.Microstructure study revealed that the hardness of the joint is determined by the relationship between heat generation and grain size formation [32].Consequently, grain boundaries emerge as the primary impediment to dislocation slip.Materials characterized by smaller grain sizes exhibit superior hardness or strength, as the movement of dislocations is more effectively constrained [33].

Tensile strength analysis
The transverse tensile properties including tensile strength and joint efficiency of dissimilar aluminium alloy joints have been thoroughly examined.Under each set of conditions, three specimens were subjected to testing and the average of the results obtained from these three specimens is presented.The flow stress of the two aluminium alloys are different.In comparison to AA6061-T6, the aluminium alloy AA7075-T6 has a higher resistance to plastic flow.The tensile strength is influenced by various of microstructural variables, including grain size, dislocation density and the interaction between the base metal and tool pin profiles [34].Electrical Discharge Machining (EDM) was employed to extract tensile specimens from welded plates, adhering to the    demonstrated in the findings.Furthermore, for a given pin tool and the larger tool shoulder diameter, the more heat is generated, resulting in grain growth.Grain development causes hardness to rise and tensile strength to decrease.At tool shoulder sizes of 15 mm and 25 mm, a tunnel at the bottom of the joint is always present, diminishing the joint's tensile strength.When a tensile load is applied to the above-mentioned joints, cracks appear in the tunnel and void region, causing premature failure [36].When the square pin profile tool and 20 mm tool shoulder diameter is used, the movement of plasticized material in the WNZ is homogeneous from top to bottom of the joint.Furthermore, the joint is defect-free and demonstrates the relationship between hardness and tensile strength.

Tensile fractography
Tensile fractography analysis has been systematically carried out to explore both the mode and mechanism of fracture in the specimens that underwent tensile testing.This examination provides valuable insights into the characteristics and patterns of fracture, contributing to a deeper understanding of the material's behavior under tensile loading conditions.Optimal heat generation and strain rate result in good bonding at the stir zone of specimen 3 and 4. The presence of small and deep dimples indicates ductile fracture mode and confirms the specimens enhanced ductility as shown in figure 7(a).Due to the minimal frictional heat created in the 15 mm tool shoulder diameter, these joints are broken at the stir zone of the joint.Layer-by-layer material consolidation occurs at lower heat generation, resulting in inadequate bonding at the stir zone and reduced joint strength [37].
A massive population of microscopic voids, varying in size and shape, cover the tensile fractured surfaces of specimens 1 and 2 as shown in figure 7(b).The depth of micro voids developed using a tapered pin tool is larger than those created with a square pin tool, which can be related to the premature coalescence of micro voids.EDAX analysis were performed on the tensile fractured surfaces to determine the presence of strengthening precipitate and intermetallic compounds [38].The fracture surface is not generated by intermetallic deposits, according to the findings.Due to the uniform distribution of strengthening precipitate MgZn 2 in the WSZ [39], the FSW joint manufactured with a 20 mm tool shoulder diameter and square pin tool demonstrated maximum tensile strength and hardness, as evidenced by the EDAX study is shown in figure 7(c).

Conclusion
The impact of tool shoulder diameter and tool pin profile on the microstructure and tensile strength of the dissimilar friction stir welded aluminium alloys AA6061-T6 and AA7075-T651 was explored in this study, with the following findings: 1.The mechanical qualities of the joints are influenced by the shoulder diameter and pin profile of the welding tool used in the FSW process.
2. The FSW tool pin profile and shoulder play a significant role in heat generation.Moreover, the stirring and movement of the plasticized material are influenced by the design features of the tool pin profile.
3. The FSW joint fabricated using a square pin tool and a 20 mm shoulder diameter has the best mechanical properties of 117 HV hardness and 284 MPa tensile strength when compared to other specimens.
4. The increased pulsing action in the stir zone of the square pin profiled tool resulted in fine equi-axed grain size and uniformly dispersed strengthening precipitates, resulting in increased strength and hardness.
5. When the tool shoulder diameter is 15 mm and the pin profile is taper, less heat is produced, resulting in absence of plastic deformation and lack of material transfer in the WSZ.
6.The specimen 3 made with a square pin tool and a 20 mm shoulder diameter exhibits ductile mode of fracture with strengthening precipitate MgZn 2 , according to FESEM with EDAX analysis.

Figure 1 .
Figure 1.Schematic illustration of (a) FSW machine set up (b) Fixture and clamping arrangement (c) FSW Tool.

Figure 4 .
Figure 4. SEM images of different Zones of Specimen 3.