Effect of Post-FSW FSP Pass on Tensile Mechanical Characteristics and Hardness of Welded Samples of AA6061 Aluminum Alloy

Effect of Post-FSW FSP Pass on Tensile Mechanical Characteristics and Hardness of Welded Samples of AA6061 Aluminum Alloy | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of Post-FSW FSP Pass on Tensile Mechanical Characteristics and Hardness of Welded Samples of AA6061 Aluminum Alloy Mahdi Kazemi, Mohsen Hamidi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4377862/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 6 You are reading this latest preprint version Abstract Friction stir welding (FSW) is a solid-state joining process in which a rotating tool is inserted into the interface of two independent parts and the joint is established with friction and intensive plastic deformation. Most important factors contributing to the performance of FSW include welding tool geometry, joint geometry, tool rotation speed, tool compressive load bearing capacity, plunge depth, and tool traverse speed. In this research, effect of post-FSW friction stir processing (FSP) on mechanical characteristics and hardness of stir-welded samples of AA6061 aluminum alloy was investigated. To this end, thirteen series of experimental tests were performed to come up with a comprehensive analysis of different situations that may occur following the FSP pass and investigate the effects the FSP parameters on the weld quality. Results showed that the post-FSW FSP pass can significantly improve the ultimate weld strength and percent elongation in all samples. However, the highest strength improvement (10%) was achieved when the traverse direction in the FSP pass was opposite to that of the FSW pass while the rotation was set to be in the same direction as that of the welding pass. Moreover, most of the samples exhibited increased ultimate strength and percent elongation with increasing the tool traverse speed. Finally, performing the post-FSW FSP pass enhanced the weld hardness, especially when the tool traverse motion in the FSP pass was in the opposite direction to that in the welding pass while the tool rotation direction was the same in the FSW and FSP passes. Friction stir welding (FSW) Friction stir processing (FSP) Mechanical characteristics Aluminum alloy Welding pass Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction Friction stir welding (FSW) is a relatively new technique for obtaining a joint of high mechanical and metallurgic qualities without local melting [ 1 ]. This is a solid-state joining process that works based on the compression and large plastic deformations only. The presence of appropriate plastic flow during the FSW sets the scene for improving mechanical characteristics of the welded specimens and achieving optimal joint quality. Numerous different parameters and factors affect the final quality of a joint, making the FSW a relatively simple process with particular complexities. In general, the pool of factors affecting the FSW can be categorized under three classes, namely process parameters, tool geometry, and joint geometry [ 2 – 4 ]. Each of the three classes of the factors can be subclassified to many subcategories, which complicates the interaction of different factors affecting the FSW process [ 5 ]. The tool and joint geometries are probably the most significant factors that determine the volume and shape of the plastic flow of the material in the FSW process. Accordingly, any change to the tool geometry results in significant changes in the material flow, and this change in the pattern and volume of the flow then affects the ultimate weld quality [ 6 ]. A summary of the literature on this topic is given in the following. Shi et al. [ 10 ] performed a series of FSW tests and found that the presence of any defect in the joint would affect the static and dynamic characteristics of the joint, making them different from the characteristics of the raw material and perfect joints. Alami et al. [ 11 ] published a research where they analyzed thermal coupling and material flow in the FSW of dissimilar metals (copper and aluminum). They showed that, in the welding of dissimilar metals, the most important parameter that determines the dominant metal (the one that comprises the majority of the weld volume) is the tool deviation, so that deviation of the tool toward either side of the workpiece makes the metal on the corresponding side the dominant metal. It was further figured out that these two metals (copper and aluminum) experience different maximum temperatures, with the difference being a function of the tool deviation. In their study, Kima et al. [ 12 ] concluded that many of the defects in the FSW are resulted from overheating of the weld zone. These defects usually appear on the weld surface. An important series of such defects are those that create surface bumps or pilling of the weld surface. Elanguan et al. [ 13 ] investigated the effect of pin type on the FSW. Evaluating five types of pins, they showed that the geometrical shape of the pin imposes significant impacts on the weld zone developed by the FSW of aluminum alloys. Ultimately, they argued that the best weld quality is achieved with square pins regardless of the pin diameter. Morti et al. [ 14 ] proposed concave tool shoulder for the FSW, where the concavity was obtained at a small angle in the range of 6–10°. The forward motion of the tool coupled with the complex of forces applied to the new material in the shoulder concavity pushed the material into the pin flow. Proper performance of this shoulder design requires a tool contact angle of 2 to 4° (measured from the normal line to the workpiece surface). Honarbakhsh Rauf et al. [ 15 ] found that an increase in the relative velocity of the tool for FSW tends to increase the weld hardness, yield strength, and ultimate strength. In the present research, effects of friction-stir processing (FSP) pass on the FSW of AA6061 aluminum alloy were investigated. Following the FSW, it is common to see multiple defects on the weld surface or the base. Various techniques have been proposed for addressing such defects, such as heat treatment. The FSP is among the proposed methods for treating the weld surface and removing the weld defects while enhancing the mechanical quality of the weld. The present research considers the characteristics of the AA6061 aluminum joints established by this welding method. Given the large number of the factors affecting this process, evaluation of the effect of every single parameter would be too costly and time-intensive. Accordingly, we herein consider only the most fundamental parameters, including the rotational and traverse speeds and the traverse direction, on the mechanical performance of the joint [ 7 – 9 ]. Once the required welding material and equipment were prepared, tests were performed on 6061 sheets at a tool rotation speed of 1180 rpm and traverse speeds of 4, 60, and 80 mm/min. Noteworthily, the proper speed range was obtained by following a trial-and-error procedure, with the best speeds picked based on the literature to avoid the appearance of evident weld defects. Following with the research, mechanical tensile and microhardness tests and microstructural analyses were devised to investigate the effects of the traverse speed and direction on the mechanical characteristics of the obtained welds. 2. Materials and experimental procedure This section explains the materials, test apparatus, and procedures for obtaining the FSW of the AA6061 aluminum alloy and evaluating the effect of the FSP pass on the weld. 2.1. Materials and test apparatus This sub-section introduces the used materials and experimental apparatus for doing the tests in detail. In the present research, we used welding tools with modern geometries, which are known to impose significant effects on the material flow. To build the tools, a shaft of hot-work steel with a diameter of 20 mm and a length of 25 cm was prepared and subjected to a 3-stage work hardening process to enhance its hardness [ 16 ]. Conventionally, such a tool has one pin in the middle. Figures 1 and 2 demonstrate the mentioned tools together with their schematic designs. Table 1 indicate elemental composition of the used hot-work steel. Table 1 Chemical composition of the H13 hot-work tool steel. Chemical Composition (wt %) Cr Mo Si V C Ni Cu Mn P S 4.75–5.5 1.1–1.75 0.8–1.2 0.8–1.2 0.32–0.45 0.3 0.25 0.2–0.5 0.03 0.03 In this work, FSW was performed on the AA6061 aluminum alloy. Table 2 presents elemental and chemical compositions of the welded alloy while Table 3 lists mechanical properties of the alloy. We herein considered aluminum sheets of 5 mm in thickness. For all FSW samples, workpieces of 120 × 50 × 5 mm were used. An industrial-level guillotine cutting machine was used to cut the workpieces appropriately. Accordingly, the workpieces had smooth and polished edges and this ensured the absence of any gap between the workpieces during the welding process. Table 2 Chemical and elemental compositions of the AA6061 aluminum alloy. AA6061 Chemical Composition (Wt%) Al Si Fe Mn Mg Cu Ti Cr Zi Balance 0.127 0.31 0.531 4.769 0.06 0.013 0.058 0.123 Table 3 Mechanical properties of the AA6061 aluminum alloy. Yield Stress (Mpa) Ultimate Tensile stress (MPa) Elongation (%) 290 440 12 2.2. Preparation of weld samples This section presents explanations on the sample preparation and number of samples for evaluating the effects of the FSP pass on the FSW performance. To this end, as demonstrated in Fig. 3 , the milling machine and the fixture were initialized for the tests. A total of 13 pairs of samples were prepared and subjected to FSW. Then, 12 pairs of samples were further subjected to FSP pass according to the classification given in Table 4 . Finally, the results were compared to those of the sample pair that underwent FSW only to see FSP-induced differences and improvements in mechanical characteristics ( e.g. , tensile properties and hardness). As is detailed in Table 4 , the FSP pass, which is used to improve tensile properties and hardness of the welded samples by FSW, was practiced at three different traverse speeds. The traverse direction was set to be once in the same direction as the FSW pass and once in the opposite direction. In the FSP pass, the rotational speed was the same for all tests but the rotation direction was set to be once in the same direction as the FSW pass and once in the opposite direction. In this way, four tests were performed at any particular traverse speed of the FSP pass (same direction – same direction, same direction – opposite direction, opposite direction – same direction, and opposite direction – opposite direction, as compared to the FSW pass). Figure 4 presents a schematic of different cases for the post-FSW FSP. Table 4 Classification of performed experiments. FSW Pass FSP Pass Test number Linear speed (mm/min) Rotational speed (rpm) Linear speed (mm/min) Rotational speed (rpm) Direction of linear movement of the tool Direction of rotational movement of the tool 1 60 1180 40 1180 Same direction Same direction 2 Same direction Opposite direction 3 Opposite direction Same direction 4 Opposite direction Opposite direction 5 60 Same direction Same direction 6 Same direction Opposite direction 7 Opposite direction Same direction 8 Opposite direction Opposite direction 9 80 Same direction Same direction 10 Same direction Opposite direction 11 Opposite direction Same direction 12 Opposite direction Opposite direction 2.3. Experimental procedure Figure 5 shows prepared samples for investigating their mechanical characteristics. Tensile and microhardness tests were used in this research [ 17 ]. Tensile tests were performed, according to the corresponding standard code, on a SANTAM STM-250 tensile testing machine at the Material Strength Laboratory of the Malayer University, as shown in Fig. 6 . Knowing that the process was not mature in the starting and ending 2-cm zones, where visible defects were commonly seen, these zones were removed, using a CNC milling machine, from the weld to obtain tensile test samples. For the tensile testing, the samples were cut in normal direction to the weld line. Sample dimensions were set to follow the ASTM-E8M, as shown in Fig. 7. Noteworthily, three samples for tensile testing were prepared out of each weld sample, with the average values reported as the final output. All tensile tests were conducted in a displacement-controlled fashion at a rate of 2 mm/min. In the present research, hardness of the vertical section of the welded samples was evaluated by the Vickers microhardness test. To do the hardness test and investigate the distribution of the Vickers microhardness across different zones of the weld, each weld was cut at a middle section and the section was prepared for the microhardness test by applying sandpapers of different grits (starting with 220-grit and then proceeding to 320, 500, 800, and 1200-grit) (Fig. 8 ). Microhardness tests were conducted on a microhardness tester (Buehler Co., USA) in the Malayer University (Fig. 9 ). As shown in Fig. 9 , each of the prepared samples was subjected to a 50-g load for 30 s in ambient temperature. In order to record the microhardness distribution, each sample was tested at 15 points along the normal-to-weld line at a depth of 1 mm into workpiece surface. 3. Results and discussion 3.1. Tensile test Table 5 reports the obtained values of ultimate tensile strength (UTS) and percent elongation for the welded samples of AA6061 alloy in different conditions in terms of FSP pass traverse speed. Table 5 Percent elongation and UTS of the welded samples. Samples UTS (MPa) Elongation (%) FSW (Base) 195 8.2 S1 214 8.1 S2 228 10.1 S3 241 10.2 S4 220 9.6 S5 215 10.4 S6 246 11.9 S7 268 12.3 S8 233 11.6 S9 221 12.4 S10 253 13.3 S11 269 14.8 S12 229 13.2 In order to better investigate the weld parameters, results of the tensile tests are graphically demonstrated in Fig. 10 . This diagram shows the effects of the FSP pass parameters on the UTS for different test cases in terms of the tool rotation and traverse directions at different traverse speeds ( i.e. , 40, 60, and 80 mm/min). Considering Fig. 10 , it can be figured out that, regardless of the traverse speed, the samples tend to exhibit the highest UTS when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass (see the green line). In contrast, the lowest UTS was achieved when both the traverse and rotational motions in the FSP pass were parallel to those in the FSW pass (see the blue line). As a result, it can be concluded that the optimum result ( i.e. , corresponding to the highest UTS in the samples) is obtained when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass. In most of the samples, the UTS increased with increasing the tool speed. That is, regardless of the tool motion direction, the highest sample strength was achieved at a tool traverse speed of 80 mm/min. Figure 11 shows the samples following the tensile tests. Considering Fig. 12 , the increasing trend seen on all four diagrams shows that any increase in the tool speed tends to add to the percent elongation of the samples. At any given speed, however, the highest percent elongation was observed when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass (see the green line). In contrast, the smallest percent elongation was seen when both the traverse and rotational motions in the FSP pass were in the same directions as those in the FSW pass (see the blue line). 3.2. Hardness test Vickers micro hardness tests were performed to record the microhardness of the welded samples. Each sample was tested at 15 points along the normal-to-weld line at a depth of 1 mm into workpiece surface. According to Fig. 13 , which shows all four test models at a FSW traverse speed of 40 mm/min, it is evident that the S3 sample, for which the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass, exhibits the highest hardness compared to other samples. Looking at the results for S2 sample on the same figure, it is further evident that opposite rotation direction coupled with parallel traverse of the FSP pass, as compared to the FSW pass, tend to enhance the post-welding sample hardness, although this cannot produce any hardness higher than that of the control ( i.e. , S1 sample). Another important point that can be inferred from Fig. 13 is that keeping the same directions of rotation and traverse in the FSP pass as those in the FSW pass imposes no significant effect on the post-welding hardness of the sample. As is evident for S1 sample, the lowest hardness corresponds to the case where both the traverse and rotational motions in the FSP pass were similar to those in the FSW pass. In contrast, reversing both rotation and traverse directions in the FSP pass was seen to boost the weld hardness, although this improvement is not as large as those seen with S2 and S3 samples. Figure 14 presents the results of hardness tests on S5 to S8 samples. In this case, the only difference to the tests with S1 to S4 samples is the change in the tool traverse speed in the FSP pass ( i.e. , 60 mm/min). Generally speaking, average hardness of the welded samples was higher than those of the earlier 4 experiments. Further, when the rotational motion of the tool in the FSP pass was the same as that in the FSW pass while the traverse motion was in the opposite direction to the FSW ( e.g. , S7 sample), the weld hardness increased. In contrast ( i.e. , S6 sample), that is when the rotational motion of the tool in the FSP pass was opposite to that in the FSW pass while the traverse motion was the same as that in the FSW, the weld hardness reduced, although the samples in this test produced still higher hardness levels than S5 sample (for which rotation and traverse speeds in the FSP pass were parallel to those in the FSW). Finally, the samples exhibited the lowest weld hardness when the rotation and traverse speeds in the FSP pass were opposite to those in the FSW pass. Based on the performed experiments and as shown in Fig. 15 , the average weld hardness was higher when the FSP pass traverse speed was increased to 80 mm/min, as compared to the cases where the traverse speed was set to either 40 or 60 mm/min. The highest weld hardness was seen in the S11 sample for which the rotation speed in the FSP pass was parallel to that in the FSW pass while the traverse speed was opposite to that in the FSW pass. The reverse case ( i.e. , S10 sample, for which the rotation speed in the FSP pass was opposite to that in the FSW pass while the traverse speed was parallel to that in the FSW pass) corresponded to the highest hardness. Finally, comparing the results for the S9 and S12 samples, it is clear that higher weld hardness levels are expected when the rotation and transverse motions in the FSP pass are opposite, rather than parallel, to those in the FSW pass. 4. Conclusion In this research, effect of FSP pass on mechanical behavior of AA6061 aluminum alloy was investigated. For this purpose, considering the repeatability, a total of 13 experimental tests were conducted to obtain a comprehensive understanding of different FSP pass conditions and their impacts on the quality of the final weld. The most important conclusions drawn out of this study are listed in the following: Based on the pattern of microhardness of the welded samples of AA6061 alloy, it was figured out that, upon the welding process, cross sectional distribution of the hardness creates a W-like pattern. In all samples, hardness of the SZ, TMAZ, and HAZ were significantly lower than the base metal, which is a result of thermal softening occurred in these zones. Following the HAZ, the TMAZ and SZ exhibited the lowest hardness levels compared to the base metal. Hardness of the welded samples in the weld zone is directly proportional to the traverse speed of the tool in the FSP pass. For all of the performed tests, when the FSP pass traverse direction is opposite to that in the welding pass, one may expect a sample of higher hardness in the weld zone. The weld hardness is maximal when the rotation speed in the FSP pass is opposite to that in the FSW pass while the traverse speed is parallel to that in the FSW pass, although the resultant hardness is still lower than the case where only the rotation direction in the FSP pass is opposite to that in the FSW pass. At any given speed, the highest UTS is achieved when the rotation speed in the FSP pass is parallel to that in the FSW pass while the traverse speed is parallel to that in the FSW pass. Regardless of the tool speed, the lowest strength is achieved when both traverse and rotational motions in the FSP pass are parallel to those in the FSW pass. Any increase in the tool speed tends to increase the percent elongation for all samples. The largest percent elongation corresponds to the case where the rotation speed in the FSP pass is parallel to that in the FSW pass while the traverse speed is opposite to that in the FSW pass, while the smallest percent elongation occurs when both traverse and rotations speeds in the FSP pass are parallel to those in the FSW pass. In all samples, UTS increases with increasing the traverse speed. Accordingly, optimal traverse speed for FSW of the studied aluminum alloy is 80 mm/min. In the study for optimization of the welding process of AA6061 aluminum alloy, it was figured out that adjusting the direction of the FSP pass in relation to the FSW pass can increase the UTS by 12–23%, which is pretty significant considering the industrial applications of this welding method. In the FSW process of AA6061 aluminum alloy, it was figured out that S1 sample was the only sample that exhibited similar mechanical properties to those of the base metal, while other weld samples showed significantly higher percent elongations and UTS levels than the base metal because of the post-FSW FSP. In the FSW process of AA6061 aluminum alloy, it was figured out that the weld surfaces of the S1 and S4 samples exhibited accumulations of surface and body defects. In the mentioned two samples, due to the uncontrolled growth of plastic flow, the weld core was almost detached from the weld surface to create relatively deep cavities into the weld line zone, through which materials could leave the workpiece from the top, thereby deteriorating the mechanical quality of the joint significantly. Considering the results obtained for the S10 and S11 samples of welded AA6061 aluminum alloy, it was found that these samples undergo large volumes of plastic flow and heat generation in the course of the growth process. If not controlled, this increasing growth can negatively impact the mechanical properties of the weld samples. Due to the symmetry of the generated plastic material flow, the results became better and the highest growths in the UTS and percent elongations were observed. Because, the flow symmetry tends to limit the defects and enhance the mechanical quality and characteristics. Declarations Availability of data and materials: The datasets used and/or analysed during the study are available from the corresponding author on reasonable request. Contributions: Mahdi Kazemi: idea formulation, conceptualization, formulated strategies for mathematical modeling, methodology refinement, formal analysis, validation, writing—review and editing. Mohsen Hamidi: effective literature review, experiments, investigation, methodology, writing—original draft. Both authors read and approved the final manuscript. Funding and/or Conflicts of interests/Competing interests: This study was not funded. Ethical approval: This article does not contain any studies with human participants performed by any of the authors. References Greitmann, I. I. M. J. (2013). Welding through the Ages American Welding Society . Beckett, W. (1996). Welding (pp. 704–717). Mosby St. Louis, MO. Madhusudhan Reddy, G., & Rajasekhar, A. (2013). 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Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Major revision 25 Jul, 2024 Reviewers agreed at journal 23 May, 2024 Reviewers invited by journal 23 May, 2024 Editor invited by journal 10 May, 2024 Editor assigned by journal 09 May, 2024 First submitted to journal 07 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4377862","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":305998196,"identity":"bc4f54cd-74bb-45e9-9db1-69971a112d7b","order_by":0,"name":"Mahdi Kazemi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYBACCTBpAKTYG0AMC1K08ByAMojTAmYloPJxAsn24w8f3SiwYNCd+fzqhh8FEgz87d0JeLVI8yQkG+cA3WN2O6fsZg+QIXHm7Aa8WuQYEo5JQ7Wk3eABMgwkcglo4X/Y/hus5eaZtJt/iNEiLZHMxgzWcoP92G2ibJGc8YwZ5DAeszM5bLdlgAyCfpE4n/7wc86fOjmz48ef3Xzzx0aOv70XvxYY4AEiAyiDeMD+gBTVo2AUjIJRMIIAAJzmQPmVPiQuAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-2683-5968","institution":"University of Malayer","correspondingAuthor":true,"prefix":"","firstName":"Mahdi","middleName":"","lastName":"Kazemi","suffix":""},{"id":305998197,"identity":"8e6c29fd-0464-458e-9bd4-6d1a7a182eb6","order_by":1,"name":"Mohsen Hamidi","email":"","orcid":"","institution":"Malayer University","correspondingAuthor":false,"prefix":"","firstName":"Mohsen","middleName":"","lastName":"Hamidi","suffix":""}],"badges":[],"createdAt":"2024-05-06 15:20:38","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4377862/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4377862/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57783997,"identity":"ea2f39a0-6ff1-4439-9bc6-9b537af46ce8","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":437462,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Used tools and (B) their schematic designs and dimensions (in mm).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/6199046ffa7685e4abd32e66.png"},{"id":57784008,"identity":"d24be6f3-d1ba-4878-958b-5f912e3f5766","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":377879,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 3. A view of the milling machine and fixture.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/3ec5c07cce45eec379a486df.png"},{"id":57784012,"identity":"af9419a5-e882-41c8-932c-c9ee93f86f0b","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":38473,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 4. A schematic of the four cases for testing the post-FSW FSP.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/974eff7ea86577682770842b.png"},{"id":57784014,"identity":"27a7fa44-852a-4a3b-b7ee-3667fd72d2df","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":388846,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 5. (A) Produced weld samples according to Table 4 and (B) the control sample with no FSP pass.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/bb84b9892f6540cb6fdbc9a0.png"},{"id":57784657,"identity":"65ea7e96-389e-4854-b91a-24161817c136","added_by":"auto","created_at":"2024-06-05 15:54:05","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":612907,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 6. The SANTAM STM-250 tensile testing machine (Malayer University).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/b3cd1807476940a3bba2a30c.png"},{"id":57784009,"identity":"f1145c4c-042f-45f5-b64a-011e42edf38f","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":492667,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 7. (A) Dimensions of the tensile test sample according to the ASTM-E8M standard code, and (B) tensile test samples.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/377f1b5844dd5634fe5cfd06.png"},{"id":57784013,"identity":"543a792f-b04d-48e9-a574-3918b44fc7ba","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":381796,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 8. Prepared samples for the microhardness test.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/916a5a547c9c6f0144b152dd.png"},{"id":57784658,"identity":"25605191-1e2e-4dd3-ae75-62b8a56073a3","added_by":"auto","created_at":"2024-06-05 15:54:05","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":823527,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 9. Used apparatus for the microhardness test (Malayer University).\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/b357496df03b8d2f32ee019d.png"},{"id":57784002,"identity":"43bce526-8e03-4db9-b1d0-7aaacdbef7ae","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":164370,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 10. Effects of the FSP pass parameters on the UTS.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/3bdcd9dbe7db72d8d667ca74.png"},{"id":57784005,"identity":"67063b6e-43be-4489-92fe-53928dd33b15","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":337395,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 11. View of the samples following the tensile tests.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/6b3d80439c49ab1bd20dbe9d.png"},{"id":57784015,"identity":"1033785d-2d66-4603-a505-abc1d8805604","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":731704,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 12. Effects of the FSP pass parameters on the percent elongation of the samples.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/ba86eb6ad2f31679eb22b1c8.png"},{"id":57784004,"identity":"64c60cb8-76d8-4d71-a608-e83647802177","added_by":"auto","created_at":"2024-06-05 15:46:05","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":80497,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 13. Diagram of hardness versus offset from the weld center for the samples welded at 40 mm/min.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/cb72a069fdca48cec65174a0.png"},{"id":57784010,"identity":"e93b6b72-12af-4792-a9c5-6047ad6fa9b4","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":110024,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 14. Diagram of hardness versus offset from the weld center for the samples welded at 60 mm/min.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/ca73e3936795dcb575842e69.png"},{"id":57784011,"identity":"815a65d0-1a10-43fd-a4c1-3f6daa3ca462","added_by":"auto","created_at":"2024-06-05 15:46:06","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":82976,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 15. Diagram of hardness versus offset from the weld center for the samples welded at 80 mm/min.\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/8ec018233a0ecc333b0ed63f.png"},{"id":57785366,"identity":"77b14860-d1e8-479f-b4ee-7a3a36ab07ec","added_by":"auto","created_at":"2024-06-05 16:02:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6285058,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4377862/v1/7874e3d3-4d76-4aa1-95ae-b4a494601415.pdf"}],"financialInterests":"","formattedTitle":"Effect of Post-FSW FSP Pass on Tensile Mechanical Characteristics and Hardness of Welded Samples of AA6061 Aluminum Alloy","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFriction stir welding (FSW) is a relatively new technique for obtaining a joint of high mechanical and metallurgic qualities without local melting [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. This is a solid-state joining process that works based on the compression and large plastic deformations only. The presence of appropriate plastic flow during the FSW sets the scene for improving mechanical characteristics of the welded specimens and achieving optimal joint quality. Numerous different parameters and factors affect the final quality of a joint, making the FSW a relatively simple process with particular complexities. In general, the pool of factors affecting the FSW can be categorized under three classes, namely process parameters, tool geometry, and joint geometry [\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Each of the three classes of the factors can be subclassified to many subcategories, which complicates the interaction of different factors affecting the FSW process [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The tool and joint geometries are probably the most significant factors that determine the volume and shape of the plastic flow of the material in the FSW process. Accordingly, any change to the tool geometry results in significant changes in the material flow, and this change in the pattern and volume of the flow then affects the ultimate weld quality [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA summary of the literature on this topic is given in the following.\u003c/p\u003e \u003cp\u003eShi et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] performed a series of FSW tests and found that the presence of any defect in the joint would affect the static and dynamic characteristics of the joint, making them different from the characteristics of the raw material and perfect joints. Alami et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] published a research where they analyzed thermal coupling and material flow in the FSW of dissimilar metals (copper and aluminum). They showed that, in the welding of dissimilar metals, the most important parameter that determines the dominant metal (the one that comprises the majority of the weld volume) is the tool deviation, so that deviation of the tool toward either side of the workpiece makes the metal on the corresponding side the dominant metal. It was further figured out that these two metals (copper and aluminum) experience different maximum temperatures, with the difference being a function of the tool deviation. In their study, Kima et al. [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] concluded that many of the defects in the FSW are resulted from overheating of the weld zone. These defects usually appear on the weld surface. An important series of such defects are those that create surface bumps or pilling of the weld surface. Elanguan et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] investigated the effect of pin type on the FSW. Evaluating five types of pins, they showed that the geometrical shape of the pin imposes significant impacts on the weld zone developed by the FSW of aluminum alloys. Ultimately, they argued that the best weld quality is achieved with square pins regardless of the pin diameter. Morti et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] proposed concave tool shoulder for the FSW, where the concavity was obtained at a small angle in the range of 6\u0026ndash;10\u0026deg;. The forward motion of the tool coupled with the complex of forces applied to the new material in the shoulder concavity pushed the material into the pin flow. Proper performance of this shoulder design requires a tool contact angle of 2 to 4\u0026deg; (measured from the normal line to the workpiece surface). Honarbakhsh Rauf et \u003cem\u003eal.\u003c/em\u003e [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] found that an increase in the relative velocity of the tool for FSW tends to increase the weld hardness, yield strength, and ultimate strength.\u003c/p\u003e \u003cp\u003eIn the present research, effects of friction-stir processing (FSP) pass on the FSW of AA6061 aluminum alloy were investigated. Following the FSW, it is common to see multiple defects on the weld surface or the base. Various techniques have been proposed for addressing such defects, such as heat treatment. The FSP is among the proposed methods for treating the weld surface and removing the weld defects while enhancing the mechanical quality of the weld. The present research considers the characteristics of the AA6061 aluminum joints established by this welding method. Given the large number of the factors affecting this process, evaluation of the effect of every single parameter would be too costly and time-intensive. Accordingly, we herein consider only the most fundamental parameters, including the rotational and traverse speeds and the traverse direction, on the mechanical performance of the joint [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Once the required welding material and equipment were prepared, tests were performed on 6061 sheets at a tool rotation speed of 1180 rpm and traverse speeds of 4, 60, and 80 mm/min. Noteworthily, the proper speed range was obtained by following a trial-and-error procedure, with the best speeds picked based on the literature to avoid the appearance of evident weld defects. Following with the research, mechanical tensile and microhardness tests and microstructural analyses were devised to investigate the effects of the traverse speed and direction on the mechanical characteristics of the obtained welds.\u003c/p\u003e"},{"header":"2. Materials and experimental procedure","content":"\u003cp\u003eThis section explains the materials, test apparatus, and procedures for obtaining the FSW of the AA6061 aluminum alloy and evaluating the effect of the FSP pass on the weld.\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1. Materials and test apparatus\u003c/h2\u003e\n\u003cp\u003eThis sub-section introduces the used materials and experimental apparatus for doing the tests in detail. In the present research, we used welding tools with modern geometries, which are known to impose significant effects on the material flow. To build the tools, a shaft of hot-work steel with a diameter of 20 mm and a length of 25 cm was prepared and subjected to a 3-stage work hardening process to enhance its hardness [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]. Conventionally, such a tool has one pin in the middle. Figures\u0026nbsp;1 and 2 demonstrate the mentioned tools together with their schematic designs.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cp\u003eTable\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e indicate elemental composition of the used hot-work steel.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eChemical composition of the H13 hot-work tool steel.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth colspan=\"10\" align=\"left\"\u003e\n\u003cp\u003eChemical Composition (wt %)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMo\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSi\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eV\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eC\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eNi\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCu\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMn\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eP\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eS\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4.75\u0026ndash;5.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.1\u0026ndash;1.75\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.8\u0026ndash;1.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.8\u0026ndash;1.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.32\u0026ndash;0.45\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.25\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.2\u0026ndash;0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.03\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.03\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eIn this work, FSW was performed on the AA6061 aluminum alloy. Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e presents elemental and chemical compositions of the welded alloy while Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e lists mechanical properties of the alloy. We herein considered aluminum sheets of 5 mm in thickness. For all FSW samples, workpieces of 120 \u0026times; 50 \u0026times; 5 mm were used. An industrial-level guillotine cutting machine was used to cut the workpieces appropriately. Accordingly, the workpieces had smooth and polished edges and this ensured the absence of any gap between the workpieces during the welding process.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eChemical and elemental compositions of the AA6061 aluminum alloy.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth colspan=\"9\" align=\"left\"\u003e\n\u003cp\u003eAA6061 Chemical Composition (Wt%)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAl\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSi\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eFe\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMn\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eMg\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCu\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTi\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eZi\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eBalance\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.127\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.531\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4.769\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.06\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.013\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.058\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.123\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab3\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eMechanical properties of the AA6061 aluminum alloy.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eYield Stress (Mpa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eUltimate Tensile stress (MPa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eElongation (%)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e290\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e440\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2. Preparation of weld samples\u003c/h2\u003e\n\u003cp\u003eThis section presents explanations on the sample preparation and number of samples for evaluating the effects of the FSP pass on the FSW performance. To this end, as demonstrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, the milling machine and the fixture were initialized for the tests. A total of 13 pairs of samples were prepared and subjected to FSW. Then, 12 pairs of samples were further subjected to FSP pass according to the classification given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Finally, the results were compared to those of the sample pair that underwent FSW only to see FSP-induced differences and improvements in mechanical characteristics (\u003cem\u003ee.g.\u003c/em\u003e, tensile properties and hardness). As is detailed in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, the FSP pass, which is used to improve tensile properties and hardness of the welded samples by FSW, was practiced at three different traverse speeds. The traverse direction was set to be once in the same direction as the FSW pass and once in the opposite direction. In the FSP pass, the rotational speed was the same for all tests but the rotation direction was set to be once in the same direction as the FSW pass and once in the opposite direction. In this way, four tests were performed at any particular traverse speed of the FSP pass (same direction \u0026ndash; same direction, same direction \u0026ndash; opposite direction, opposite direction \u0026ndash; same direction, and opposite direction \u0026ndash; opposite direction, as compared to the FSW pass). Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e presents a schematic of different cases for the post-FSW FSP.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab4\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eClassification of performed experiments.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth colspan=\"2\" align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth colspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eFSW Pass\u003c/p\u003e\n\u003c/th\u003e\n\u003cth colspan=\"4\" align=\"left\"\u003e\n\u003cp\u003eFSP Pass\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTest number\u003c/p\u003e\n\u003c/th\u003e\n\u003cth colspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eLinear speed\u003c/p\u003e\n\u003cp\u003e(mm/min)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eRotational speed\u003c/p\u003e\n\u003cp\u003e(rpm)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eLinear speed\u003c/p\u003e\n\u003cp\u003e(mm/min)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eRotational speed\u003c/p\u003e\n\u003cp\u003e(rpm)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eDirection of linear movement of the tool\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eDirection of rotational movement of the tool\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd colspan=\"2\" rowspan=\"12\" align=\"left\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"12\" align=\"left\"\u003e\n\u003cp\u003e1180\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"4\" align=\"left\"\u003e\n\u003cp\u003e40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"12\" align=\"left\"\u003e\n\u003cp\u003e1180\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"4\" align=\"left\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd rowspan=\"4\" align=\"left\"\u003e\n\u003cp\u003e80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSame direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eOpposite direction\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e2.3. Experimental procedure\u003c/h2\u003e\n\u003cp\u003eFigure 5 shows prepared samples for investigating their mechanical characteristics. Tensile and microhardness tests were used in this research [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003eTensile tests were performed, according to the corresponding standard code, on a SANTAM STM-250 tensile testing machine at the Material Strength Laboratory of the Malayer University, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. Knowing that the process was not mature in the starting and ending 2-cm zones, where visible defects were commonly seen, these zones were removed, using a CNC milling machine, from the weld to obtain tensile test samples.\u0026nbsp;\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eFor the tensile testing, the samples were cut in normal direction to the weld line. Sample dimensions were set to follow the ASTM-E8M, as shown in Fig.\u0026nbsp;7. Noteworthily, three samples for tensile testing were prepared out of each weld sample, with the average values reported as the final output. All tensile tests were conducted in a displacement-controlled fashion at a rate of 2 mm/min.\u003c/p\u003e\n\u003cp\u003eIn the present research, hardness of the vertical section of the welded samples was evaluated by the Vickers microhardness test. To do the hardness test and investigate the distribution of the Vickers microhardness across different zones of the weld, each weld was cut at a middle section and the section was prepared for the microhardness test by applying sandpapers of different grits (starting with 220-grit and then proceeding to 320, 500, 800, and 1200-grit) (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eMicrohardness tests were conducted on a microhardness tester (Buehler Co., USA) in the Malayer University (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e). As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, each of the prepared samples was subjected to a 50-g load for 30 s in ambient temperature. In order to record the microhardness distribution, each sample was tested at 15 points along the normal-to-weld line at a depth of 1 mm into workpiece surface.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1. Tensile test\u003c/h2\u003e\n\u003cp\u003eTable\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e reports the obtained values of ultimate tensile strength (UTS) and percent elongation for the welded samples of AA6061 alloy in different conditions in terms of FSP pass traverse speed.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab5\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003ePercent elongation and UTS of the welded samples.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSamples\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eUTS (MPa)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eElongation (%)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eFSW (Base)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e195\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS1\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e214\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS2\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e228\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e10.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS3\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e241\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e10.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS4\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e220\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e9.6\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS5\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e215\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e10.4\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS6\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e246\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e11.9\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS7\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e268\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS8\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e233\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e11.6\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS9\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e221\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12.4\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS10\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e253\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e13.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS11\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e269\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e14.8\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eS12\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e229\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e13.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eIn order to better investigate the weld parameters, results of the tensile tests are graphically demonstrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. This diagram shows the effects of the FSP pass parameters on the UTS for different test cases in terms of the tool rotation and traverse directions at different traverse speeds (\u003cem\u003ei.e.\u003c/em\u003e, 40, 60, and 80 mm/min).\u003c/p\u003e\n\u003cp\u003eConsidering Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, it can be figured out that, regardless of the traverse speed, the samples tend to exhibit the highest UTS when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass (see the green line). In contrast, the lowest UTS was achieved when both the traverse and rotational motions in the FSP pass were parallel to those in the FSW pass (see the blue line). As a result, it can be concluded that the optimum result (\u003cem\u003ei.e.\u003c/em\u003e, corresponding to the highest UTS in the samples) is obtained when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass. In most of the samples, the UTS increased with increasing the tool speed. That is, regardless of the tool motion direction, the highest sample strength was achieved at a tool traverse speed of 80 mm/min. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e shows the samples following the tensile tests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eConsidering Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e, the increasing trend seen on all four diagrams shows that any increase in the tool speed tends to add to the percent elongation of the samples. At any given speed, however, the highest percent elongation was observed when the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass (see the green line). In contrast, the smallest percent elongation was seen when both the traverse and rotational motions in the FSP pass were in the same directions as those in the FSW pass (see the blue line).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2. Hardness test\u003c/h2\u003e\n\u003cp\u003eVickers micro hardness tests were performed to record the microhardness of the welded samples. Each sample was tested at 15 points along the normal-to-weld line at a depth of 1 mm into workpiece surface. According to Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e, which shows all four test models at a FSW traverse speed of 40 mm/min, it is evident that the S3 sample, for which the FSP pass traverses in opposite direction to the welding direction while its rotation direction is parallel to that of the FSW pass, exhibits the highest hardness compared to other samples. Looking at the results for S2 sample on the same figure, it is further evident that opposite rotation direction coupled with parallel traverse of the FSP pass, as compared to the FSW pass, tend to enhance the post-welding sample hardness, although this cannot produce any hardness higher than that of the control (\u003cem\u003ei.e.\u003c/em\u003e, S1 sample).\u003c/p\u003e\n\u003cp\u003eAnother important point that can be inferred from Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e is that keeping the same directions of rotation and traverse in the FSP pass as those in the FSW pass imposes no significant effect on the post-welding hardness of the sample. As is evident for S1 sample, the lowest hardness corresponds to the case where both the traverse and rotational motions in the FSP pass were similar to those in the FSW pass. In contrast, reversing both rotation and traverse directions in the FSP pass was seen to boost the weld hardness, although this improvement is not as large as those seen with S2 and S3 samples. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e14\u003c/span\u003e presents the results of hardness tests on S5 to S8 samples.\u003c/p\u003e\n\u003cp\u003eIn this case, the only difference to the tests with S1 to S4 samples is the change in the tool traverse speed in the FSP pass (\u003cem\u003ei.e.\u003c/em\u003e, 60 mm/min). Generally speaking, average hardness of the welded samples was higher than those of the earlier 4 experiments. Further, when the rotational motion of the tool in the FSP pass was the same as that in the FSW pass while the traverse motion was in the opposite direction to the FSW (\u003cem\u003ee.g.\u003c/em\u003e, S7 sample), the weld hardness increased. In contrast (\u003cem\u003ei.e.\u003c/em\u003e, S6 sample), that is when the rotational motion of the tool in the FSP pass was opposite to that in the FSW pass while the traverse motion was the same as that in the FSW, the weld hardness reduced, although the samples in this test produced still higher hardness levels than S5 sample (for which rotation and traverse speeds in the FSP pass were parallel to those in the FSW). Finally, the samples exhibited the lowest weld hardness when the rotation and traverse speeds in the FSP pass were opposite to those in the FSW pass.\u003c/p\u003e\n\u003cp\u003eBased on the performed experiments and as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e, the average weld hardness was higher when the FSP pass traverse speed was increased to 80 mm/min, as compared to the cases where the traverse speed was set to either 40 or 60 mm/min. The highest weld hardness was seen in the S11 sample for which the rotation speed in the FSP pass was parallel to that in the FSW pass while the traverse speed was opposite to that in the FSW pass. The reverse case (\u003cem\u003ei.e.\u003c/em\u003e, S10 sample, for which the rotation speed in the FSP pass was opposite to that in the FSW pass while the traverse speed was parallel to that in the FSW pass) corresponded to the highest hardness. Finally, comparing the results for the S9 and S12 samples, it is clear that higher weld hardness levels are expected when the rotation and transverse motions in the FSP pass are opposite, rather than parallel, to those in the FSW pass.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this research, effect of FSP pass on mechanical behavior of AA6061 aluminum alloy was investigated. For this purpose, considering the repeatability, a total of 13 experimental tests were conducted to obtain a comprehensive understanding of different FSP pass conditions and their impacts on the quality of the final weld.\u003c/p\u003e \u003cp\u003eThe most important conclusions drawn out of this study are listed in the following:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eBased on the pattern of microhardness of the welded samples of AA6061 alloy, it was figured out that, upon the welding process, cross sectional distribution of the hardness creates a W-like pattern. In all samples, hardness of the SZ, TMAZ, and HAZ were significantly lower than the base metal, which is a result of thermal softening occurred in these zones. Following the HAZ, the TMAZ and SZ exhibited the lowest hardness levels compared to the base metal.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eHardness of the welded samples in the weld zone is directly proportional to the traverse speed of the tool in the FSP pass.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eFor all of the performed tests, when the FSP pass traverse direction is opposite to that in the welding pass, one may expect a sample of higher hardness in the weld zone.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe weld hardness is maximal when the rotation speed in the FSP pass is opposite to that in the FSW pass while the traverse speed is parallel to that in the FSW pass, although the resultant hardness is still lower than the case where only the rotation direction in the FSP pass is opposite to that in the FSW pass.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAt any given speed, the highest UTS is achieved when the rotation speed in the FSP pass is parallel to that in the FSW pass while the traverse speed is parallel to that in the FSW pass.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eRegardless of the tool speed, the lowest strength is achieved when both traverse and rotational motions in the FSP pass are parallel to those in the FSW pass.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAny increase in the tool speed tends to increase the percent elongation for all samples.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe largest percent elongation corresponds to the case where the rotation speed in the FSP pass is parallel to that in the FSW pass while the traverse speed is opposite to that in the FSW pass, while the smallest percent elongation occurs when both traverse and rotations speeds in the FSP pass are parallel to those in the FSW pass.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn all samples, UTS increases with increasing the traverse speed. Accordingly, optimal traverse speed for FSW of the studied aluminum alloy is 80 mm/min.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn the study for optimization of the welding process of AA6061 aluminum alloy, it was figured out that adjusting the direction of the FSP pass in relation to the FSW pass can increase the UTS by 12\u0026ndash;23%, which is pretty significant considering the industrial applications of this welding method.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn the FSW process of AA6061 aluminum alloy, it was figured out that S1 sample was the only sample that exhibited similar mechanical properties to those of the base metal, while other weld samples showed significantly higher percent elongations and UTS levels than the base metal because of the post-FSW FSP.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn the FSW process of AA6061 aluminum alloy, it was figured out that the weld surfaces of the S1 and S4 samples exhibited accumulations of surface and body defects. In the mentioned two samples, due to the uncontrolled growth of plastic flow, the weld core was almost detached from the weld surface to create relatively deep cavities into the weld line zone, through which materials could leave the workpiece from the top, thereby deteriorating the mechanical quality of the joint significantly.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eConsidering the results obtained for the S10 and S11 samples of welded AA6061 aluminum alloy, it was found that these samples undergo large volumes of plastic flow and heat generation in the course of the growth process. If not controlled, this increasing growth can negatively impact the mechanical properties of the weld samples. Due to the symmetry of the generated plastic material flow, the results became better and the highest growths in the UTS and percent elongations were observed. Because, the flow symmetry tends to limit the defects and enhance the mechanical quality and characteristics.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAvailability of data and materials:\u003c/strong\u003e The datasets used and/or analysed\u0026nbsp;\u0026nbsp; during the study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eMahdi Kazemi:\u003c/em\u003e idea formulation, conceptualization, formulated strategies for mathematical modeling, methodology refinement, formal analysis, validation, writing\u0026mdash;review and editing.\u0026nbsp;\u003cem\u003eMohsen Hamidi:\u003c/em\u003e effective literature review, experiments, investigation, methodology, writing\u0026mdash;original draft. Both authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding and/or Conflicts of interests/Competing interests:\u003c/strong\u003e This study was not funded.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval:\u003c/strong\u003e This article does not contain any studies with human participants\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp; performed by any of the authors.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGreitmann, I. I. M. J. (2013). \u003cem\u003eWelding through the Ages American Welding Society\u003c/em\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBeckett, W. (1996). \u003cem\u003eWelding\u003c/em\u003e (pp. 704\u0026ndash;717). Mosby St. Louis, MO.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMadhusudhan Reddy, G., \u0026amp; Rajasekhar, A. (2013). Microstructure and Mechanical Properties of 16Cr-2Ni Stainless Steel Fusion and Solid State Welds-Influence of Post Weld Treatments, in Adva nced. \u003cem\u003eMaterials Research\u003c/em\u003e, pp. 289\u0026ndash;304.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKreye, H. (1977). Melting phenomena in solid state welding processes. \u003cem\u003eWelding Journal\u003c/em\u003e, \u003cem\u003e56\u003c/em\u003e, 154\u0026ndash;158.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAkca, E., \u0026amp; G\u0026uuml;rsel, A. (2016). Solid state welding and application in aeronautical industry, Periodicals of Engineering and Natural Sciences (PEN), vol. 4.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMaalekian, M. (2007). Friction welding\u0026ndash;critical assessment of literature, Science and technology of welding and joining, vol. 12, pp. 738\u0026ndash;759.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZeng, D., Guo, Y., Liu, H., \u0026amp; Jin, L. (2012). Analysis of the Behavior of Q460 Forging Welding Neck Flange Joints under Axial Compressive Load [J], Electric Power Construction, 4.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDaniels, H. (1965). Ultrasonic welding, Ultrasonics, vol. 3, pp. 190\u0026ndash;196.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGibson, B. T., Lammlein, D., Prater, T., Longhurst, W., Cox, C., Ballun, M. (2014). Friction stir welding: Process, automation, and control, Journal of Manufacturing Processes, vol. 16, pp. 56\u0026ndash;73, Materials Science and Engineering, vol. 2014, 2014.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShi, L., Wu, C., \u0026amp; Liu, H. (2015). The effect of the welding parameters and tool size on the thermal process and tool torque in reverse dual-rotation friction stir welding. \u003cem\u003eInternational Journal of Machine Tools and Manufacture\u003c/em\u003e, \u003cem\u003e91\u003c/em\u003e, 1\u0026ndash;11.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAalami-Aleagha, M., Hadi, B., \u0026amp; Shahbazi, M. A. (2016). 3-dimensional numerical analysis of friction stir welding of copper and aluminum. \u003cem\u003eJournal of Mechanical Science and Technology\u003c/em\u003e, \u003cem\u003e30\u003c/em\u003e, 3767\u0026ndash;3776.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKim, Y., Fujii, H., Tsumura, T., Komazaki, T., \u0026amp; Nakata, K. (2008). Three defect AA6061 aluminium alloy, Materials \u0026amp; design, vol. 29, pp. 362\u0026ndash;373.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eElangovan, K., \u0026amp; Balasubramanian, V. (2008). 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(1997). 399\u0026thinsp;\u0026ndash;\u0026thinsp;90: Standard test method for plane-strain fracture toughness of metallic materials, Annual book of ASTM standards, vol. 3, pp. 506\u0026ndash;536.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"international-journal-of-mechanical-and-materials-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ijme","sideBox":"Learn more about [International Journal of Mechanical and Materials Engineering](http://ijmme.springeropen.com)","snPcode":"40712","submissionUrl":"https://www.editorialmanager.com/ijme/default2.aspx","title":"International Journal of Mechanical and Materials Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Friction stir welding (FSW), Friction stir processing (FSP), Mechanical characteristics, Aluminum alloy, Welding pass","lastPublishedDoi":"10.21203/rs.3.rs-4377862/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4377862/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFriction stir welding (FSW) is a solid-state joining process in which a rotating tool is inserted into the interface of two independent parts and the joint is established with friction and intensive plastic deformation. Most important factors contributing to the performance of FSW include welding tool geometry, joint geometry, tool rotation speed, tool compressive load bearing capacity, plunge depth, and tool traverse speed. In this research, effect of post-FSW friction stir processing (FSP) on mechanical characteristics and hardness of stir-welded samples of AA6061 aluminum alloy was investigated. To this end, thirteen series of experimental tests were performed to come up with a comprehensive analysis of different situations that may occur following the FSP pass and investigate the effects the FSP parameters on the weld quality. Results showed that the post-FSW FSP pass can significantly improve the ultimate weld strength and percent elongation in all samples. However, the highest strength improvement (10%) was achieved when the traverse direction in the FSP pass was opposite to that of the FSW pass while the rotation was set to be in the same direction as that of the welding pass. Moreover, most of the samples exhibited increased ultimate strength and percent elongation with increasing the tool traverse speed. Finally, performing the post-FSW FSP pass enhanced the weld hardness, especially when the tool traverse motion in the FSP pass was in the opposite direction to that in the welding pass while the tool rotation direction was the same in the FSW and FSP passes.\u003c/p\u003e","manuscriptTitle":"Effect of Post-FSW FSP Pass on Tensile Mechanical Characteristics and Hardness of Welded Samples of AA6061 Aluminum Alloy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-05 15:45:58","doi":"10.21203/rs.3.rs-4377862/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2024-07-26T02:44:03+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-05-23T14:55:02+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-23T14:33:03+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"International Journal of Mechanical and Materials Engineering","date":"2024-05-10T05:56:33+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-09T08:04:21+00:00","index":"","fulltext":""},{"type":"submitted","content":"International Journal of Mechanical and Materials Engineering","date":"2024-05-07T08:09:50+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"international-journal-of-mechanical-and-materials-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ijme","sideBox":"Learn more about [International Journal of Mechanical and Materials Engineering](http://ijmme.springeropen.com)","snPcode":"40712","submissionUrl":"https://www.editorialmanager.com/ijme/default2.aspx","title":"International Journal of Mechanical and Materials Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"52302df6-1646-4edb-a1bf-100faaa59506","owner":[],"postedDate":"June 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2024-07-26T06:44:48+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-05 15:45:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4377862","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4377862","identity":"rs-4377862","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}