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جوشکاری اصطکاکی تلاطمی آلومینیوم تغییرشکل شدید یافته با استفاده از پودرهای هیبریدی
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جوشکاری اصطکاکی تلاطمی آلومینیوم تغییرشکل شدید یافته با استفاده از پودرهای هیبریدی

موسوی، عزت الله Moosavi, Ezzatollah

Friction Stir Welding of Severely Deformed Aluminum Using Hybrid Powders

Moosavi, Ezzatollah | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54894 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba; Kazeminezhad, Mohsen
  7. Abstract:
  8. Severe plastic deformation (SPD) is an ideal approach to fabricate light-weight alloys, such as aluminum alloys, with ultrafine-grained (UFG) structure up to nano-scale. Thus, it is known as a process that boosts the strength of materials. This method is also adopted to enhance the specific strength of materials, which is essential in the transportation and aerospace industries. Because of limitations like load capacity, and as a result, low-size productions, it is not possible to use them in industries. A practical solution is to join the productions and fabricate them on bigger scales. Welding of SPDed materials by employing solid-state welding techniques is appropriate for joining UFG materials. One of these methods is friction stir welding (FSW). The reason is that not only can it produce fine-grain structure in stir zone (SZ), but also low heat input in the process, compared to traditional welding methods, has fewer aggravating impacts on other weld regions. Thus, grain growth, which can potentially occur in all regions due to high stored energy generated from the SPD process, can be partially retarded due to reasonable welding process temperature. However, as the process temperature is unavoidably increased to join the materials, recrystallization and further grain growth are inevitable. This situation leads to poor mechanical properties in weld regions compared to the SPDed base metal. In order to maintain the superb microstructure of SPDed materials and produce high-efficiency joints after performing FSW, a hybrid powder can be employed. This mixture of powders results in strengthening mechanisms and decreasing the friction coefficient between the welding tool and the material. In this study, Al1050 sheets were processed through two-pass constrained groove pressing (CGP), and then they were welded together through single-pass and multi-pass FSW, employing α-Al2O3 nanoparticles and micrometric graphite powder. Other variables were the rotation speed to traverse speed ratio (ω/v) and powder volume fraction. The effects of these variables on the evolution of microstructure and powder distribution in the fabricated hybrid metal matrix composite (HMMC) were studied by employing optical microscopy and Electron Dispersive Spectroscopy (EDS)-equipped field emission scanning electron microscopy (FESEM). In addition, mechanical behavior of these samples, such as Vickers microhardness, tensile strength, and fracture surface after tensile test, were examined. Graphite powder as a solid lubricant was found to play a dominant role in enhancing mechanical properties. In fact, increasing graphite volume fraction provided superior thermo-mechanical stability of severely plastic deformed specimens upon FSW by decreasing welding temperature; for example, the peak temperature of first-pass FSW decreased from 489 ℃ (without any powder) to 210 ℃ by using 25% α-Al2O3 +75% graphite hybrid powder at ω/v=70 r/mm. Nevertheless, it can also deteriorate mechanical properties due to aggravating material flow. Moreover, both powders as hybrid powder contributed to strengthening mechanisms by ensuring grain boundary Zener-pinning and particulate stimulated nucleation (PSN) mechanism. It was also observed that sound weld could not be produced on condition that one-pass FSW was performed. Scattered microhardness values along the weld regions, and low ultimate tensile strength (UTS) and elongation (up to 87 MPa and 3.5% in joints welded by using 50% α-Al2O3 +50% graphite at ω/v=70 r/mm, respectively) showed low joint efficiency after conducting one-pass FSW. However, it was deduced that finer powder distribution significantly enhances powder effectiveness in weld microstructure and improves load-bearing capacity, which was accomplished by increasing the ω/v ratio and FSW number of passes. In fact, higher process efficiency was achieved when three-pass FSW was carried out due to better material flow, cavity elimination, uniform powder distribution, besides minor heat-affected zone (HAZ) softening. For instance, by performing three-pass FSW at ω/v=70 r/mm, using 50% α-Al2O3 +50% graphite, not only more uniform microhardness up to a maximum of 65 HV was achieved, the highest UTS of 101 MPa (81% efficiency) and elongation of 12.5% was obtained, where the base metal hardness and its UTS were 40 HV and 125 MPa, respectively.
  9. Keywords:
  10. Severe Plastic Deformation ; Constrained Groove Pressing (CGP) ; Friction Stir Welding ; Mechanical Properties ; Thermo-Mechanical Analysis ; Metal Matrix Composite (MMC) ; Thermo-Mechanical Stability ; Solid Lubricant ; Hybrid Metal Matrix Composite

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