On the problem of tool destruction when obtaining fixed joints of thick-walled aluminum alloy blanks by friction welding with mixing

OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. No. 3 2021 Introduction Friction stir welding (FSW) is a process in which the joining of workpieces of different materials (such as aluminum and titanium alloys, bronze, steel, etc.) occurs with no liquid phase formation [1–3]. The welding process is carried out by introducing a specially designed rotating tool under a loading force into the butt of two rigidly fixed workpieces. It moves along with the butt line and forms a welded joint. The main advantage of the FSW method is that the tool heats the workpiece material to a temperature of about 0.6–0.8 of the melting point. As a result, the material is plasticized, fragmented, and gripped and due to adhesive interaction, is captured by the tool and transferred layer by layer, forming a weld [4–6]. Depending on the alloy to be welded, welding tools are made with high-speed steels [7, 8], heat-resistant nickel-based alloys [9], and other materials. It ensures both the tools’ ability to withstand the thermal conditions and optimal adhesion-diffusion interaction to form the stir zone. Since the process occurs in the solid phase, it has found an application in the aircraft and spacecraft industries, particularly for the welding of high- strength aluminum alloys [10, 11]. But, in most cases, the manufacture of aerospace components requires welding of thick-walled workpieces with subsequent mechanical processing to ensure the strong and most rigid construction. In this regard, a problem arises associated with obtaining high-quality welded joints with a thickness of 30.0 mm and more: the temperature effect of the tool in the welding zone is uneven, which requires fine control of FSW parameters, as well as the selection of the optimal shape of the welding tool [12]. The work [13] shows that during the welding of thick workpieces, the grain size in the stir zone changes throughout the height of the weld. It leads to a decrease in microhardness from top to bottom of the stir zone. In addition, in [14], attempts were made to optimize the heating of the welded material of workpieces up to 25.0 mm thick by preheating using a specially designed substrate and a laser heating device. However, the additional heating also affects the selection of welding parameters. As a result of poorly selected parameters, there are defects in the welded joint such as voids, tunnel-type flaws, and butt lines (Lazy S) [15–20], besides there is a possibility of welding tool destruction. Therefore, in the present work, a study of 35.0 mm thick welded joints made of aluminum alloy AMg5 by friction stir welding is carried out in order to reveal the regularities of the formation of defects in the mixing zone and destruction of the welding tool, depending on the parameters of the FSW process. Methods The samples for the research were obtained on the special equipment for friction stir welding at CJSC “Cheboksary Enterprise Sespel” Cheboksary, Russia. Welding tools made of high-speed steel with shoulders with a diameter of 50 mm and a movable pin up to 35 mm long were used for the form welded joints. The tool pin had a conical shape with helical grooves, and three surfaces flattened at an angle of 120º. The samples were welded in four modes, presented in Table 1. The scheme of friction welding with mixing with the destruction of the tool and the scheme of cutting samples for research are shown in Figure 1. The fored welds were subjected electrical discharge machining to obtain research samples in the longitudinal and cross-section of the weld using the DK7750 machine. The samples for metallographic studies were sanded on abrasive paper, polished using diamond paste and subjected to chemical etching in Keller reagent to reveal the microstructure of the material. Samples of the material with the tool stuck in the mixing zone were additionally etched in an aqueous solution of nitric acid (HNO 3 ). Metallographic studies were carried out on an optical microscope Altami MET 1C. Results and discussion As a result, by the method of friction welding with mixing, samples of welded joints were obtained using four modes that differ in the loading force and welding speeds (travel speed and the speed of the tool rotation). Figure 2 shows panoramic images of the sample mixing zone in cross-section. When welding