OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 5 No. 4 2023 plastic deformation, resulting in the formation of a stir zone with very fine recrystallized grains [3], called a dynamic recrystallization zone [4–6]. Friction stir welding is also characterized by low energy consumption [7]. Melting and recrystallization do not occur, so the materials are joining in a solid state. FSW is of particular importance for joining of aluminum and magnesium alloys since it can significantly reduce defects such as solidification cracking, porosity and distortions, which are the commonly observed during fusion welded. These process capabilities of FSW have made it very practical for joining some alloys. The interaction of the tool bottom surface and the pin surface with material being processed results in the generation of sufficient frictional heat to soften the material without melting [8–9]. Thus, the surface of the tool pin plays a critical role in the generation of frictional heat, material flow and plastic deformation. The tool offset is also critical factor that determines the heat generated when the tool tip rubs against the material [10], therefore it determines the thermal-physical properties in the weld zone [11–12]. The quality of the joints depends on the correct choice of FSW process parameter [13, 14]. As practice shows, some investigations are focused on the friction stir welding of hot and cold-worked aluminum alloys [15] using some specially designed tools [16] with various pin shapes such as hexagon, pentagon and square [17]. Some recent studies have shown that FSW is capable of joining both similar and dissimilar aluminum alloys [18–21]. Butt joints in friction stir welding are very common, compared to aluminum alloy lap joints, which have been studied by only a few researchers [22, 23]. Davidson et al. [24] investigated the tensile strength characteristic of the friction stir welded joints of AA8011 aluminum under different process parameter and concluded that the joint fabricated at traverse speed of 45 mm/min and tool rotational speed of 1,400 rpm and axial thrust of 2.15 kN have better tensile strength compared to the other joints. K. Palani et al. [25] fabricated dissimilar FSW joint and focused on the impact of process variables and tool design on joint quality. Consequently, this work attempted to determine the effect of three different tip profiles (square, pentagonal, and hexagonal) and the combination of tool rotational speed and traverse speed on the tensile properties of FSW joints of dissimilar aluminum alloys AA6061-T6 and AA8011. By implementing FSW, Elangovan and Balasubramanian [26–28] examined the performance of five various geometries of pin namely “threaded cylindrical”, “tapered cylindrical”, “triangular”, “square”, “straight cylindrical” on AA2219 aluminum alloy. Although previous research have shown that the tool pin profiles [29—31] or pin shapes [32] and welding speed influence strength enhancement, a connection between low welding speed and pin profile has not yet been established. Therefore, in the present research, AA8011 aluminum alloy was friction stir welded at low welding speed with the different pin profiles, and effect on joint quality is evaluated in terms of hardness and tensile properties. Investigation Technique An experiment was conducted to obtain defect-free FSW joints of AA8011 on a vertical milling machine; the plates to be welded were mounted on the base plate with clamps, as shown in fig. 1. Fig. 2 represents the plastic flow of the material during the welding process. The chemical composition of AA8011 is shown in table 1. The process parameters considered include tool pin profiles as shown in fig. 3, with all the tool characteristics listed in table 2 and few combinations of tool rotational speed and traverse speed. These parameters are known to have an important impact on the strength and other mechanical properties of the joint. A rolled plate 6 mm thick was cut into the required size using a hand hacksaw. Two plates were arranged in square butt joint arrangement to create the FSW joint. The plates were clamped over a base plate, and the welding was performed in the direction of rolling. Welding was carried out using two different nonconsumable tools made from D2 steel. The chemical composition of D2 steel is shown in table 3. The choice of tool material depends on the material being welded. For soft materials, tool steel or stainless steel can be used, while refractory materials are necessary for hard materials. The evaluation of the mechanical properties of the joints was conducted through tensile tests on standard specimens. The specimens were cut transversely to the welding direction, and the tensile tests were performed on a universal testing machine with a constant strain rate, specifically a crosshead speed of 1 mm/min.
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