Features of structure formation processes in AA2024 alloy joints formed by the friction stir welding with bobbin tool

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Introduction Conventional friction stir welding (CFSW) was developed to produce permanent joints of aluminum- based alloys with limited weldability. Among the FSW types, there is the bobbin tool welding (BFSW), which provides synchronous impact on the front and back surfaces of the material to be welded. It enables the production of welded joints of various configurations without the use of a substrate and reduces the rigidity of the welding equipment due to the absence of the axial penetration force typical for the conventional FSW [1, 2]. The BFSW is also characterized by reduced heat losses and temperature gradient across the thickness of the welded material [3, 4]. In comparison with CFSW, it provides homogenization of the welded joint structure, reduction of internal stresses and the property gradient in its main characteristic zones. In work [5] it is shown that during the CFSW and BFSW of AA6061 alloy, properties of produced joints are comparable with each other, but the BFSWprocess has a more limited range of admissible parameters. When comparing the CFSW and BFSW of AA6005 alloy [6], it was found that with the same process parameters, the BFSW process allows producing stronger joints. The authors of the works believed that these effects were caused by the above-mentioned features of temperature effect during the BFSW, which determines the kinetics of structure formation processes in the welded joint. These features make the BFSW process effective for welding heat-treated aluminum alloys. In the work [7], during the BFSW of AA2198 alloy, the authors produced welded joints with an ultimate tensile strength of 82% compared to the initial alloy. The welding of joints made of AA7085 alloy [8] also resulted in tensile strength of more than 80% from the initial alloy. In this regard, the most interesting is the BFSW process of AA2024 alloy, the properties of which are determined by its structural-phase state. In its turn, it is closely connected with the thermomechanical influence [9, 10], therefore the final characteristics of AA2024 welded joints are highly dependent on welding process parameters [11, 12]. When considering the BFSW process, the parameters determining the properties of welded joints are the speed of welding and tool rotation, the depth of tool penetration, as well as the shape of the shoulder and pin [13–17]. According to the research data, the temperature and strain rate of the welded material caused by those parameters have some value ranges, in which strong defect-free joints are formed. At the same time, much less attention is paid to the mechanisms of structure formation in the BFSW process. To date, there are several works that simulate the mechanism of layer-by-layer transfer of the welded material during the BFSW [18, 19]. However, an extended understanding of the basic mechanisms of structure formation during the welding process is required to solve the problem of forming defect-free and strong welded joints by the BFSW process. Consequently, the purpose of this work is to study the mechanisms of structure formation in the welded joint of AA2024 alloy at different welding speeds during the friction stir welding with a bobbin tool. Methods The research was carried out together with CJSC “Cheboksary Enterprise “Sespel”, using the origi- nal equipment and tools of in-house design for the friction stir welding. Welded joints were produced by the BFSW (Fig. 1, a , b ) of AA2024 rolled sheets with the size of 100×250 mm and a thickness of 20 mm. Parameters of the tool are shown in Fig. 1, c . The chemical composition of AA2024 alloy is given in Table 1. The welding was carried out as follows: the welding tool rotating with the frequency ω = 250 rpm was introduced into the butt of the welded sheets from the end surface. At the introduction of the tool, its travel speed V was equal to 5 mm/min. Further, after the tool movement at the distance of 15 mm, the travel speed was increasing in each 5 mm up to values 7.5, 10, 15, 20, 25, 30, 35, 40 mm/min. The welding process was performed at the tool travel speed of 40 mm/min, while the tool rotation speed remained constant. The research was carried out on specimens made using a DK7750 wire-EDM machine by cutting the welded joints according to the diagram in Fig. 2. The structure of the welded joint at its initial/outgoing sections was investigated (IN/OUT in Fig. 2). Longitudinal sections in the central region of the joint (CTR