OBRABOTKAMETALLOV technology Vol. 24 No. 4 2022 nological capabilities for manufacturing products for transport and aerospace applications, which explains the research relevance of in this direction. However, the widely used methods of fusion welding, such as laser, electron-beam welding, welding in shielding gases lead to formation of porosity, cracking, decrease in corrosive resistance of a welded joint material [1–3]. Nowadays intensive research is being carried out on friction stir welding (FSW) of titanium alloys, which reduces the factors that form defects in the welded joint and are associated with heating the welded material to the melting temperature [4]. The key problems arising in the FSW process are an optimization of welding parameters for obtaining defect-free joints with minimal degradation of properties, as well as a selection of a tool material that provides wear minimization and high durability of a welding tool [5, 6]. Optimization of titanium alloys welding process parameters is focused on investigation of influence of each of parameters, mainly, welding speed and tool rotation frequency on strength properties of resulting welded joints [7–9]. Processes of formation of welded joint structure as a result of thermomechanical effect of welding are also intensively investigated [10–11]. It has been established that as a result of thermomechanical effect of welding there are synchronized processes of recovery and dynamic recrystallization, leading to redistribution of α- and β-phases, which contributes significantly to strength properties of a resulting permanent joint [12–14]. At the same time, much less attention has been paid to an influence of an axial force, which determines a degree of deformation of material in conditions of torsion under pressure. Alloys based on molybdenum, tungsten, tantalum, niobium, cobalt, various types of carbides are used as tool materials for welding titanium alloys [15]. Tools based on tungsten-rhenium alloys are widely used because such alloys are characterized by high working temperature [16, 17]. Tools made of tungstenlanthanum and cobalt alloys show good durability when welding titanium alloys [18, 19]. Tungsten carbide tools are widely used [20, 21]. However, despite its advantages, the manufacture of such tools is quite expensive and technologically complicated. In addition, a contamination of a welded material by tool wear particles is possible, which has a negative effect on properties of welded joints. All these require a search for new tool materials for welding of titanium alloys. The heat-resistant nickel-based alloy Ti-5Al-3Mo-1V appears to be promising for this purpose, which has proven itself in welding high-plastic (Grade 2, Ti-1.5Al-1.0Mn) and medium-strength (Ti-6Al-4V) titanium alloys [22, 23]. Considering the above-mentioned, the aim of the present work is to study the effect of tool axial force in friction stir welding process using a tool made of heat-resistant alloy ZhS6U on the strength properties of high-strength titanium alloy Ti-4Al-3Mo-1V. Materials and methods Friction stir welding was performed on the special experimental equipment at the Institute of Physics of Strength and Materials Science, Siberian Branch of the Russian Academy of Sciences. Rolled Ti-4Al-3Mo1V titanium alloy with a thickness of 2.5 mm and chemical composition indicated in Table 1 was used as workpieces. When welding, a tool made of a nickel-based heat-resistant alloy ZhS6U was used; its chemical composition is indicated in Table 2. Ta b l e 1 Chemical composition of Ti-5Al-3Mo-1V alloy, wt. % Fe Si N Ti Мо V Al O Rest ≤ 0.25 ≤ 0.15 ≤ 0.05 86.85–92.8 2.5–3.8 0.9–1.9 3.5–6.3 ≤ 0.15 0.3–0.4
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