Impact of the tool shoulder diameter to pin diameter ratio and welding speed on the performance of friction sir-welded AA7075-T651 Al alloy butt joints

This study investigates the friction stir welding (FSW) of aluminum alloy 7075-T651, mainly focusing on managing heat generation during the process. The critical parameters influencing heat amount and the material flow including FSW tool shoulder diameter (SD) and travel speed (TS) were investigated. Two far different SD of 10 mm and 20 mm with constant pin diameter (PD) of 5.70 mm that resulted in PD: SD ratios of 1:1.75 and 1: 3.50, respectively, were employed. Furthermore, three different travel speeds of 25, 50, and 75 mm min−1 at a constant rotation rate of 600 rpm were used in combination with the two PD: SD ratios. The macrographic and radiographic results indicated that the smallest PD: SD ratio has successfully achieved sound friction stir welded (FSWed) joints for the same travel speeds. Results also indicated that a significant amount of material deformed under a high PD: SD ratioat a high TS of 75 mm min−1, while flash increased with reducing PD: SD ratio.Mechanical properties were compared, revealing that hardness in the nugget zone (NZ) decreased with a lower TS of 25 mm min−1. A small PD: SD ratioallowed for more symmetrical heat distribution, supported by the hardness map. The ultimate tensile strength decreased with increasing TS, and the highest ultimate strength, reaching 319 MPa, was observed with a 1:1.75 ratio and TS of 25 mm min−1. X-ray diffraction analysis (XRD) found an increase in peaks with increasing shoulder diameter and the number of peaks increased with decreasing travel speeds.


Introduction
Friction stir welding (FSW) tools have witnessed a significant research efforts and development for industrial applications since inventing the process in 1991 [1][2][3].The time needed to prepare specimens for the welding process, the welding time, and the number of welding passes [4] are all advantages of the FSW over TIG and MIG welding processes.On the other hand, considerable heat input, mechanical preparation, and severe occupational safety measures are necessary for TIG and MIG technologies [5].Shoulder diameter (SD) and welding travel speed (TS), along with other parameters like rotation speed (Rs), tilt angle, and downward force, are also important to control the heat input for the welding material [6][7][8][9][10].A review by Vaidyanathan et al [11] on the impact of the Shoulder Diameter to pin diameter (PD: SD) ratio on the joint strength of aluminum alloys reported that it compares one of the important parameters that govern tensile strength.Research by Saravanan et al [12] used different SD of 18, 21, and 24 mm.Increasing SD increases the heat input in different measuring points from the welding region.The effect of using different SD by Ramanjaneyulu et al [13] beginning from diameter 11.5 mm to 24 mm used for FSW of AA 2014 found that with increasing SD heat input increased.30 joints were fabricated by Rajakumar et al [14] of AA7075-T6 and parameters examined like tool TS, Rs, axial force, and different SD ranging from 9 mm to 21 mm with pin diameters between 3 mm and 7 mm.Shah et al [15] used three SD 18, 20, and 22 mm with a pin diameter of 6.35 mm on AA 7075T651 and found that using 18 mm SD reduced heat distribution and achieved a high tensile strength of 262 MPa.SD of 20 mm is compatible with a pin of 9 mm used on 6.0 mm thickness sheets of AA7075-T651 by Kim et al [16] on different conditions concluded that the optimal conditions are determined on 600 rpm and 0,8 mm sec −1 .A fine-grained structure of about 3 μm grain size of FSW AA 7075 was obtained by heat treatment for 30 min in a furnace on different temperature ranges and then quenched on water by Goloborodko et al [17].High angle misorientation resulted in 90% grain boundaries, and static annealing under the temperature of 723 K made a stable finegrained structure.Comparison made by Dimopoulos et al [18] between different parameters in butt joint FSW of AA 7075 configuration for 2 mm sheet thickness.Two different tool geometries with different pin diameters of 3 mm and 4 mm, tool Rs of 1000-2500 rpm, and various TS of 80-800 mm min −1 were used in the experiment.The results show that TS affected the mechanical properties than Rs.Rezaei et al [19] reported that with increasing the rotational speed the welding properties have a good correlation.The Rs varies from 600 to 1550 rpm and the high mechanical properties were obtained at 825 rpm.At high rotational speed, a reduction of strength and hardness of material were obtained.Another investigation into thin sheets of AA7075 revealed that the thin starting materials caused the peak temperature of the weld zone to be roughly constant for all ranges of welding rotating speeds.Additionally, the widths of the softened areas across the welding regions were nearly the same for all travel speeds [20].
One of the primary factors, along with the Rs and travel speed, that determines the heat amount is the SD [21,22].The softened surface heights were reduced, and the bonded area expanded as the shoulder diameter increased [23] Scaling of tool aspect ratios studies is used to lower process forces for FSW applications.The shoulder ratio's diameter was gradually decreased with the probe to consider frictional heat losses.Simultaneous adjustments were made to the rotational speed and downward force.Conclusions on the European Norm -Aluminum Wrought 6060 T66 material under investigation with a sheet thickness of 2 mm found that spindle torque can be reduced by up to 80% and process forces can be reduced by up to 60% [24].Another analysis for AA2014 of the temperature and plastic deformation by Ramanjaneyulu et al [13] used a different range of SDs from 11.5 mm to 24 mm with a minimum PD: SD ratio of 1:1.91 concluded that with increasing SD, the volume of shoulder driven metal-flow decreases at a given TS and rotational speed.
A 4.75 mm thick sheet of AA6063-T6 was welded using a cylindrical pin-profiled H13 tool to study the impact of the PD: SD ratio.The finding of the 1:2.6 PD: SD ratio gives the maximum tensile strength while 1:2.8 PD: SD gives the minimum tensile strength [25].According to Patel et al [26] AA 7075 welded by using a Rs of 685 rpm, TS of 18 mm min −1 , and SD of 18 mm has the highest tensile strength and the joint efficiency decreased with increasing rotational speed, and TS.Surface flaws resulting from insufficient fluidity in the stirred zone were present at the minimum SD of 11.5 mm.Dewangan et al [27] previous work on the effect of travel speeds of dissimilar FSW AA7075 and AA5083 concluded that at travel speeds of 45 mm min −1 and 20 mm min −1 identical obtained joint strengths of 54.2% and 59.3% respectively.
A novel technology to weld high-strength materials like Titanium and other materials with the same properties is called stationary shoulder friction stir welding (SSFSW) [28].Hammad et al [29] used this technology on AA7075 and checked the joints' quality using different TS and Z-forces.A slight reduction in the hardness of the heat-affected zone (HAZ) was obtained.Elimination of heat generation by the shoulder permits more control of the hardness reduction area.According to li et al [30] on AA7075 by SSFSW, the presence of component particles and the dissolving of precipitates in the HAZ were the main causes of the SSFSW joint's reduced tensile strength.This welding technology enables symmetrical heat distribution throughout the welding cross-section, but it also presents challenges outlined in reference [31], such as the need for specialized tool design and tool wear.While this technology offers the advantage of uniform heat distribution across the thickness of the welded workpiece and a high-quality surface finish, it is subject to limitations as discussed in references [32,33].
These constraints inspire the exploration of leveraging the benefits of employing smaller shoulder diameters to potentially emulate the advantages of utilizing a stationary shoulder, as demonstrated in the study by [34,35] which evaluated three distinct designs of FSW tools.Striving to decrease the shoulder diameter while ensuring its impact on welding properties like tensile strength can facilitate the realization of the benefits associated with efficient heat distribution through FSW, while requiring fewer tool preparation steps.
Significant improvements in tensile strength, joint efficiency, and elongation are observed when changing the ratio of the PD:SD.A review by Vaidyanathan et al [36] on different PD: SD ratios found that a small ratio of PD: SD used was 1:2 on dissimilar welding AA 2014-AA7075.Lowering the PD:SD to 1:1.75 ratio compared with 1:3.5 ratio in FSW can enhance heat regulation and symmetry in heat dispersion throughout the welded surface and cross-section.This adjustment has the potential to yield more consistent material consolidation and improved weld quality.Furthermore, raising the welding travel speed from 25 mm min −1 to 75 mm min −1 in friction stir welding can result in decreased heat input and reduced duration of exposure to high temperatures.This may result in faster processing times and lower distortion.However, excessively high travel speeds can also lead to incomplete material consolidation and lower joint strength.

Experimental procedures 2.1. Starting materials tool design, and FSW parameters
Friction stir welding will be used to apply a butt joint on rolled AA7075-T651 plates with 5 mm thickness, 200 mm length, and 100 mm width.Table 1 lists the chemical composition of the as-received AA7075-T651 alloy and table 2. lists the properties of the alloy according to the supplier (Constellium Valais Company).Two FSW tools having different SD of 10 and 20 mm with a fixed pin diameter of 5.70 mm were machined from H13 hard steel and heat-treated to attain a hardness value of 58 HRC.The shoulders were flat without any geometry and the pin shape was cylindrical with a length of 4.90 figure 1.The two tools were designed to attain different SD toPD ratios of 1:1.75 and 1: 3.50.These tools were used to achieve the welding experiments for AA7075-T651 butt joints at a constant tool Rs of 600 rpm, a constant tilt angle of 2°, and different travel speeds of 25, 50, and 75 mm min −1 .Table 3 summarizes the FSW program for AA7075-T6 butt joints.Based on investigations on the parameters of employing FSW in AA 7075-T651 by [23,29,30,37] it is concluded that changing the TS and SD will affect welding quality.The FSW machine used in the experimental work was the Model: EG-FSWM1, Suez University, Suez, Egypt [38].The accuracy of a friction stir welding machine is tolerances within +/− 0.05 mm.

FSW AA7075-T6 characterization
The jointed samples were first checked using radiographic x-ray for 5 min.Using portable x-ray generator ERESCO MF4 by Germany with a source power of 1500 KW, focal size of 2.5 cm, film distance of 40 cm, density 2-2.5, sensitivity 1.8% and led screen front and back 0.1 mm.After that the samples were cut with electrical discharge wire cutting machine DK7735, China in a direction perpendicular to the FSW weld pass.
The studied samples, which were taken from each perpendicular cross-section of the welded materials, were grounded using conventional grinding papers of 100, 320, 500, 1000, 1200, and 2400.They were then polished up to 0.05 alumina surface finish before being etched with Keller's reagent (2 ml HF, 2 ml HCl, 2 ml HNO 3 , and 100 ml distilled water) to macro and microstructure examination.The microstructure evaluation was achieved using a Soptop optical microscope CX40P Series made in China.Vickers hardness (HV) tester type Huayin made in China was used to conduct the hardness test at a load of 200 gf and a dwell time of 10 s.The hardness test was conducted on the material after three months of welding.The hardness maps were achieved by measuring the hardness on three lines through the cross-sections of the welded joints in distance within the sample 1 mm.An Instron 4210 by USA testing equipment used for tensile testing at a cross-head speed of 0.06 mm/sec.For every welding condition, three samples were tensile tested, and the tensile test specimen was prepared according to ASTM-E8M.X-ray diffraction analysis (XRD) is used to investigate phase transformation.XRD Parco Equipment of brand Panalytical, model Empyren used under voltage of 45 KV and current of 40 mA.The fracture surface of the failed tensile test samples was examined by Scanning electron microscope (SEM) type ZEISS Ultra Plus X-Max 50 X made in Germany.

Total energy and temperature
Research made by [39][40][41][42][43][44] calculates the total energy and power used during FSW.Emam et al [39] refine the model for calculating energy depending on using two mains sources of energy, the first energy coming from friction and the second energy from plastic deformation.The total energy E is illustrated in equation (1) and consists of energy from friction E f , energy from plastic deformation E p and S is the scale factor controlling the heat due to plastic deformation.Equation (2) describes the two parts of E f and E p as illustrated.
Where m is the coefficient of friction, F compressive force, the r o radius of the shoulder, r i radius of the pin, h height of the pin, w pin angular speed, o n travel speed with mm/sec, e s the equivalent effective stress and e e the effective strain.Equation (3) illustrates the effective energy E eff generated in the weld for every unit length J mm −1 .
The temperature generated is illustrated by equation (4) where T max is the maximum temperature generated by the weld and T s is solidus temperature by Kelvin degree.The parameters used in the experiment like Rs, travel speed, SD, pin diameter, and forging force beside the resulted torque of the experiment are illustrated in table 4. The results show that by using SD of 10 mm and increasing TS from 25 mm min −1 to 75 mm min −1 the effective energy reduced from 2110.3 J mm −1 to 1922.4 J mm −1 respectively.On the same side with using SD of 20 mm and increasing TS from 25 mm min −1 to 75 mm min −1 the effective energy reduced from 2629.8 J mm −1 to 2159.5 J mm −1 respectively.Nearby readings were found by Emam et al [39] on AA 7050-T7451 with 700 rpm and Ts 60 mm min −1 which reached 2163 J mm −1 .
The same affection reflects the temperature calculated by equation (4) as shown in figure 2 the temperature reduced with increasing welding velocity and with temperature increased with increasing SD.The calculated temperature with SD of 10 mm reduced from 651.9 K at TS of 25 mm min −1 to 629.9 K at 75 mm min −1 , also  with SD of 20 mm the temperature reaches 712.6 K at 25 mm min −1 and reduced to 657.6 K at 75 mm min −1 .The experimental temperature results were confirmed by Sadoun et al [45] on AA7075 which used three types of pin designs.One of the pins was a cylindrical pin and the temperature reading reached 638 k using the parameter of 500 rpm and TS of 50 mm min −1 .

Top surface appearance of the FSW AA7075-T6 joints
During the friction stir welding (FSW) of AA 7075-T651, three different travel speeds were employed with two distinct SD.The impact on the top surface of the welded specimens varied noticeably with different parameters, as illustrated in figure 3(a) significant amount of flash appeared on the surface of specimens welded with an SD of 20 mm due to increased surface heat compared to using a 10 mm shoulder diameter.The temperature effect was mitigated by utilizing the minimum SD.While the overall surface did not exhibit a pronounced flash with a 20 mm SD, a noticeable flash occurred with a 10 mm SD, as shown in figure 3(f).The rough flash resulted from the inability of the small 10 mm shoulder to contain all the softened material under it.This flash increased with both 10 mm and 20 mm SD as the welding velocity increased from 25 mm min −1 to 75 mm min −1 .Despite the increase in heat input with decreased welding velocity [46].FSW outcomes are highly dependent on travel speed.The slower travel speed impacts the stir zone significantly due to increased heat generation [47].Turbulence in the plasticized metal shapes the weld surface, influencing its mechanical properties.However, the flash decreased with lower velocity, as illustrated in figures 3(a)-(b).The use of radiography for the non-destructive testing of the weld provides a clear image focusing on the effect of the parameters on the total length of the weld line.Attention should be particularly directed towards welding flaws arising when SD is employed at a nonoptimal tool ratio.These defects result from the interaction of two streams of plasticized material: one generated by the tool's working surface (tool pin) and the other by the tool shoulders.Even though the experiment maintains the same parameters of Rs, plunge depth, and pin diameter, a critical parameter SD yields different results.A flaw-free weld is achieved when these material flows completely fuse, while defects in the form of tunnels or wormholes arise from incomplete fusion, especially with varying travel speeds of 25, 50, and 75 mm min −1 , forming a system of distributed voids.A search by Mumvenge et al [48] on AA6061-T6 using different TS and Rs found no defect appeared by radiographic test and confirmed that by using microstructure analysis.The macrostructure cross-section's destructive testing validates the radiographic results, aiming to showcase the defect size and location across the section.The defect size varies between parameters and becomes conspicuous with the use of SD 20 mm, as illustrated in figure 5.
The tunnel defect size increases with higher TS for both SD of 10 mm and 20 mm.Moreover, when comparing the same travel speed with a change in SD, the defect increases with increasing SD.However, Fuse and Badheka [49] found that with increasing shoulder diameter from 20 to 24 mm, the joints of AA6061-T6 become free of defects.The defect appears on the advancing side (AS) and is present in the same zone of the middle cross-section of the specimen.The defect undergoes significant changes with altered parameters and disappears with the use of the same travel speed of 25 mm min −1 but varying the SD from 20 mm to 10 mm.Given that AA 7075-T651 is a hardened material, necessitating more homogeneous heat distribution during welding, reducing the shoulder PD: SD ratio from 1: 3.50 to 1:1.75 results in sound welded joints, employing the same parameters of Rs, pin diameter, and tilt angle.The impact of changing the PD: SD ratio on the macrostructure cross-section is evident in the differences in the HAZ under the shoulder, as shown in figure 5.The HAZ area increases with higher SD and is also influenced by increased travel speed, leading to noticeable changes in material flow.

Hardness distribution
AA 7075-T651 is highly sensitive to the heat amount applied.Given its treated alloy nature, increased heat leads to a reduction in hardness within the welding zone.The nugget zone (NZ) exhibits the highest hardness, attributed to the re-precipitation of hardening particles post-cooling and grain refining.As shown in figure 6 the hardness profiles of the three lines show a 'w' with the lowest hardness values found in the HAZ and thermomechanically affected zone (TMAZ) [50].Figure 7 depicts the hardness maps of AA7075 butt joints created at a constant 600 rpm Rs and three different TS 25 mm min −1 , 50 mm min −1 , and 75 mm min −1 .Ahmed et al [51] used FSW on dissimilar AA5083/AA5754 with tool travel speeds varying from 20 to 80 mm min −1 , and tool Rs varying from 300 to 600 rpm.The profile of the dissimilar AA5083/AA7020 revealed a rise in the nugget's hardness because of the proximity combining of the low-and high-strength alloys.Notably, as the travel speed decreases, the hardness of the NZ rises.Higher travel speeds cause NZ to receive more heat, potentially resulting in precipitates that gather and coarsen the grain, reducing its hardness.Ahmed et al [50] studied the effect of tool pin geometry on AA 1050 with various travel speeds and found that increasing travel speed led to a noticeable rise in hardness.Conversely, the use of a smaller SD of 10 mm ensures symmetry in hardness across the thickness, with the hardness area around the tool pin confined to 10 mm on both sides of the NZ centerline, as shown in figure 7. The uniform hardness in the NZ thickness from top to bottom is attributed to the reduced temperature gradient when utilizing a smaller SD compared to a larger SD.The HV in NZ reaches 140 HV with an SD of 10 mm and TS of 25 mm min −1 and 120 HV with an SD of 20 mm and TS of 25 mm min −1 .The TMAZ at the bottom of the specimens exhibits minimum values of 100 HV on the retreating side (RS).Research by Hammad et al [29] on AA7075-T651 confirmed cross-sectional hardness symmetry when employing stationary shoulder friction stir welding, which allows for a reduction in surface heat input and uniformity of heat distribution.The HAZ and TMAZ demonstrate the minimum hardness between welding zones and vary in size based on the SD.

Tensile properties
The Ultimate Tensile Strength and strain of AA7075-T651 using different TS of 25, 50, and 75 mm min −1 and different PD: SD ratios of 1:1.75 and 1:3.50 are indicated in figure 8(a) highlighting the significant impact of parameter variations on outcomes.Despite radiographic and macrostructure analyses revealing defects, assessing welding effectiveness remained crucial.Discrepancies in results underscore the evident influence of different TS on the ultimate tensile strength that decreases with increasing TS for both PD: SD scenarios as  shown in figure 8(b).Notably, high ultimate tensile results of 319 MPa (using 25 mm min −1 and 10 mm SD) and 314 MPa (using 25 mm min −1 and 20 mm SD) are achieved.While these results are closely aligned, the use of a smaller SD (10 mm) produces higher ultimate tensile strength and a sound welded joint compared to the larger SD (20 mm).Furthermore, employing a high TS of 75 mm min −1 yields lower ultimate tensile strength than at 25 mm min −1 for both SD cases, a finding supported by Sharma et al [52].Abd Elnabi et al [53] results on dissimilar AA5454-AA7075 using the Taguchi method indicated that important factors in determining ultimate tensile strength include travel speed, PD: SD ratio, and plunge depth.
The impact on strain differs from the effect on ultimate tensile strength as appeared on figure 8(b), particularly when using high TS.Varied strain interactions reveal an increase in strain with higher TS using a 20 mm SD, coinciding with decreased ultimate tensile strength.Despite imperfect reflections of defects in welding connections, a notable strain increase occurs in specimens welded with higher TS, reaching 13.1%.This aligns with findings by Xu [48], who observed a 15.9% elongation during failure when welding AA7075 with increased travel speeds.
3.6.Microstructure evolution FSW process can be categorized into four regions, the base metal (BM), theHAZ, the TMAZ, and the NZ.On the AS, the demarcation between TMAZ and NZ is more pronounced than on the RS as shown in figures 9(a), (b).When examining various welding zones and evaluating the impact of different parameters on microstructure, disparities become evident with varying TS and SD.The BM exhibits lath-shaped grains due to the rolling operation, as depicted in figure 9(d).The material undergoes mechanical stirring and temperature changes, with the TMAZ characterized by bent, elongated, and deformed grain shapes.The NZ is divided into two main areas with distinct shapes and grain sizes, as shown in figure 9(c).In the RS direction, grains are drawn toward the center, identifiable by fine grains near the shoulder and elongated coarse grains, as depicted in figure 9(b).Conversely, the AS exhibits a sharp boundary in the opposite area, as shown in figure 9(a).The NZ exhibits a barrel shape, resulting from the effect of the shoulder on the surface at a speed different from that at the bottom of the specimen.The area affected under the shoulder is notably reduced compared to the same area with a larger shoulder diameter of 20 mm as shown in figure 10.This reduction is attributed to the higher peak temperature and shear stress on the AS compared to the RS, significantly improving the material's plastic deformation on the AS and creating a distinct boundary between TMAZ and NZ.Material from the AS flows to the RS, forming the upper surface of the NZ, while material from the RS flows down into the AS, forming the lower NZ.Ji et al [54] utilized a modified 3D model to simulate the FSW of the 6061-T6 aluminum alloy, discovering that the region around the shoulder is the primary heat production area.The upper region of NZ experiences high temperatures, which decrease in the thickness direction.This is because the degree of dynamic recrystallization in the lower NZ differs from that in the upper NZ [55].The grain structure was analyzed by Electron Backscatter Diffraction (EBSD) under the shoulder.The average grain size in the NZ reduced from 1.32 mm at 10 mm SD and TS 25 mm min −1 to 1.28 mm at 20 mm SD TS 25 mm min −1 as shown in figure 11.The grains size reduced on the same area with using different SD and the material flow changed as indicated by macrostructure.Similar grain size reduction was reported by Ahmed et al [56] in their investigation of FSWed AA5083 and AA5754 at different welding speeds Also, research by Li et al [30] reveals that in the NZ, constituent particles are crushed into fine particles and uniformly distributed after welding in stationary shoulder friction stir welding (SSFSW) of AA7075-T651.The constituent particles show little difference in the BM and HAZ, but the NZ has a slightly lower percentage of fraction constituent particles.The NZ consists of equiaxed fine grains with a size of 8 mm [30] while the grain size reaches 60 mm in the HAZ.

XRD analysis
As shown in figure 12, XRD was applied as a method to investigate the presence phases for the AA7075-T6 FSW butt joints at 25, 75 mm min −1 travel speeds with using 10, 20 mm SD.At 2θ°as can be observed, every XRD pattern showed normal aluminum phase peaks at 38.4, 44.7, 65.1, 78.2, 82.4, and 99.0 as the majority phase.The detection of MgZn is indicated by the existence of just teeny peaks.There are two instances of this intermetallic at 20.8, 34.7, 37.9, 43.6, 53.5, 65.4, and 73.3 Furthermore, the potential for finding an Al 7 Cu 2 Fe phase cannot be verified using the XRD method as it seems to be overlapping.Another peak of AlCuMg phase appeared at 37.4 and increased as shown in figure 12(B) at 40.6.research by Kalimba et al [57] found the Al 7 Cu 2 Fe and MgZn 2 phases on aluminum alloy 7075 base material.Research by Elshaghoul et al [58] found the same phases with little movements of the peaks on AA7075 using additive friction deposition.
Peak analysis indicates that with increasing heat amount different phases are shaped, and peak numbers increase.The peaks at 10 mm SD were increased from 11 to 13 peaks using travel speeds of 75 mm min −1 and 25 mm min −1 respectively.With increasing shoulder diameter heat amount increased and this was reflected in increasing phase changes.The peaks with 20 mm SD increased from 19 to 23 with travel speeds of 75 mm min −1 and 25 mm min −1 respectively.For example, the tinny peaks are in figure 12(B), for specimens using parameters of 20 mm Sd and 25 mm min −1 different phases shaped because of heat increase.These phases are AlMg at 2θ °37.6.

Fracture analysis
The tension test fracture location in specimens welded with different SD and travel speeds consistently occurs in the direction of the AS, as illustrated in figure 13.Cracks appear on the edge of the AS welding face for SD 10 mm specimens.In SD 20 mm specimens with 25 and 50 mm min −1 , the fracture occurs in the AS, propagating toward the NZ.The high strength and low flexibility of aluminium alloy 7075 contribute to fractures in the AS during FSW, resulting from stress concentration and limited material deformation.
AA7075-T651 is subsequently subjected to heat treatment, the dislocations within the crystal lattice rearrange and reduce.This reduction in dislocation can lead to a decrease in hardness as shown in figure 7.However, despite the reduction in hardness, the tensile strength of the aluminum alloy 7075-T651 may increase as shown in figure 8 after plastic deformation and subsequent heat treatment [59].This increase in tensile strength can be attributed to the optimization of the material's microstructure through the combination of plastic deformation and heat treatment processes.Fine precipitates caused during subsequent heat treatment contribute to strengthening the material, increasing its tensile strength even though the hardness may have decreased as a result of complex interplay between strain hardening, microstructural changes, and precipitation strengthening mechanisms in the material.The microscopic examination reveals remnant flow and welding defects, evident in figure 14.Residual material appears under the shoulder and toward the RS as shown in figure 14(c), with defects obvious in the AS.Incomplete deformation in the welding area, particularly inTMAZ,  contributes to material failure under tensile test conditions.Cracks in the TMAZ result from residual stresses formed during welding, potentially exacerbated by brittle intermetallic compounds.
The microstructure of aluminum alloy 7075 consists of small grains of the aluminum-rich alpha phase and larger grains of the copper-rich eta phase.The size and distribution of these phases may change based on the material's prior processing.Heat input can lead to the dissolution and re-precipitation of the eta phase, affecting grain size and distribution.These changes in the microstructure can impact the material's fracture behavior by influencing the creation and propagation of cracks.Lattice rotation occurs in the material, leading to a decrease in Schmidt factor (SF) values, which indicates difficulty in continuing deformation.It is noted that stored energy is mainly concentrated near the interface between TMAZ and NZ, making this region more susceptible to fracture [60].For example, areas with too-large grains or a high concentration of the eta phase may cause stress concentrations, encouraging the development and growth of cracks, as depicted in figure 15(f).In the case of aluminum alloy 7075, increasing the travel speed (TS) from 25 mm min −1 to 75 mm min −1 during friction stir welding (FSW) resulted in a coarser grain structure and lower strength for the weld.This is due to the reduced time for thermomechanical processing and microstructure refinement when transit rates are faster.Dimples frequently occur during the FSW process, as seen in figures 15(c)-(f), due to the tool's rotation and movement.Improving material flow and minimizing defects such as voids and cracks can enhance welding quality, although excessive dimple formation should be avoided to prevent a loss of mechanical qualities.
The investigation into fractures in the aluminum alloy 7075 during FSWed has revealed that an increase in travel speed (TS) can result in a more brittle fracture mode.In friction stir welding of aluminum alloy 7075, SEM analysis indicates a fine equiaxed grain structure in the fracture shape, with no signs of melting or porosity.Fracture surfaces may exhibit ridges and valleys, providing insights into the fracture process and crack propagation direction [61].The deformation of the fracture surface, illustrated in figure 15, indicates a mixed fracture mode, combining both brittle and ductile characteristics.The fracture surface of the FSW sample of AA7075 by Bayazid et al [62] has a significant number of fine dimples, which show the fine grain structure in the welding zone.This fracture appearance points to micro voids coalescence as the primary fracture mechanism, involving the formation, development, and coalescence of micro voids.The White River areas suggest ductile fracture causes, while black points indicate the initiation of brittle cracks.

Conclusions
In the FSW of AA7075-T651, two welding parameters were compared: different shoulder diameter-to-pin diameter (PD: SD) ratios (1:1.75 and 1:3.50) and various travel speeds (25 mm min −1 , 50 mm min −1 , 75 mm min −1 ), revealing notable changes in results: • The use of the smallest PD: SD ratio (1:1.75) at TS of 25 mm min −1 results in a sound joint with the highest ultimate strength of 319 MPa, compared to a defective joint with PD: SD of 1: 3.50 at 25 mm min −1 .
• Symmetry in heat distribution across the section and around the welding zone is achieved with PD: SD of 1:1.75 and confirmed by the hardness map.
• The average grain size in the nugget zone reduced from 1.32 mm at 10 mm SD and TS 25 mm min −1 to 1.28 mm at 20 mm SD TS 25 mm min −1 .
• The deformation area under the shoulder reduced with using low travels speeds 25 mm min −1 compared to high travel speeds of 75 mm min −1 .With increasing PD: SD of 1: 3.50 material flow around the outer surface shoulder reduced.
• Decreasing the travel speed from 75 mm min −1 to 25 mm min −1 while employing PD: SD of 1:1.75 eliminates defects and enhances the ultimate tensile strength.
• The ductility increased with increasing heat input and TS from 25 mm min −1 to 75 mm min −1 with usage PD: SD of 1: 3.50 ratio.
• A sharp boundary on the advancing side with PD: SD of 1:1.75, compared to deformation and recrystallized grains on the advancing side with PD: SD 1: 3.50, leads to cracks in the thermo-mechanically heat affected zone of the advancing side.

Figure 2 .
Figure 2. Effect of using different travel speeds with different shoulder diameters on temperature.

3. 3 .
Radiograph and macrostructure cross-section of the FSW AA7075-T6 Figure4indicates non-destructive tests by x-ray which were used to evaluate the welding process after using different parameters of PD: SD 1:1.75 and 1:3.50 and different TS of 25, 50, and 75 mm min −1 .Sound welds were achieved using PD: SD 1:1.75 with TS of 25 mm min −1 as shown in figure4(b).The defects production mechanisms and material flow processes within the friction welding zone are evident, as depicted in figures 4(a)-(f).

Figure 4 .
Figure 4. Radiographic non-destructive test of friction stir welding AA 7075-T651 with a, c, e SD of 20 mm, b, d, f SD of 10 mm and TS of 25, 50, 75 mm min −1 respectively.

Figure 7 .
Figure 7. Vickers hardness map of Friction stir welded AA7075-T651 for different SD and different travel speeds.

Figure 9 .
Figure 9. Microstructure of different zones of specimen with 25 mm min −1 and SD 20 mm (a) Advancing side, (b) retreating side, (c) NZ under shoulder, (d) Base material.

Table 2 .
Lists the datasheet for the AA7075-T651 alloy.

Table 4 .
Energy and Temperature were obtained using different travel speeds and different shoulder diameters.