Jiang D. Influence of technological parameters of refill friction stir spot welding on the mechanical properties of aluminum-based material joints.

The dissertation is devoted to the study of material deformation behavior and plastic flow mechanisms during welding, as well as the influence of process parameters on the microstructure and mechanical properties of welded joints. The aim of the dissertation is to establish the regularities of the influence of technological parameters (rotation speed, welding speed, tool diameter) on the quality of welded joints produced by refill friction stir spot welding of aluminum alloys, in order to optimize the process and ensure high mechanical properties. Object of research: welded joints of 3 mm thick 6061-T6 aluminum alloy produced by refill friction stir spot welding. Subject of research: the influence of stirring tool diameter (6–10 mm), rotation speed (1000–1600 rpm), and welding speed (20–50 mm/min) on the microstructure, mechanical properties, and formation mechanisms of 6061-T6 aluminum alloy joints. Scientific results obtained for the first time: 1. Five types of stirring tools with diameters of 6–10 mm were systematically studied, and optimal technological parameters for defect-free welding of 3 mm thick 6061-T6 aluminum alloy were established. 2. The formation patterns of the characteristic «U» - shaped weld cross-section, slightly larger than the tool dimensions, were identified, and three main zones were distinguished: Pin Affected Zone (PAZ), Sleeve Affected Zone (SAZ), and Thermo-Mechanically Affected Zone (TMAZ). 3. The critical role of the «Friction Stir Riveting Bonding Zone» in ensuring joint performance was determined, and correlations between grain size and zone location were established. 4. A direct relationship between tool diameter and tensile-shear force was established: 9.27, 13.74, 14.79, 16.83, and 18.16 kN for tool diameters of 6–10 mm, respectively, confirming the tool diameter as a key factor in joint strength. 5. A numerical model based on Deform V11.0 was developed, accurately describing the RFSSW process and enabling prediction of temperature fields, stresses, and metal flow in the welding zone. 6. Two mechanisms of joint formation were identified depending on tool diameter: “shear friction–in-situ rotation” for smaller diameters and “bilateral shear–central axial flow” for larger diameters. Research objectives: To develop and manufacture mixing tools of various diameters for friction spot welding of thick aluminum plates. 1. To determine the influence of technological parameters on the formation of microstructure and zonal distribution in welded joints. 2. Investigate the mechanical properties of joints and determine the optimal welding modes. 3. Develop a mathematical model of the friction stir spot welding process to predict the quality of joints. 4. Establish the mechanisms of joint formation at different tool diameters. Practical significance of the results obtained: 1. Criteria for selecting the diameter of the tool depending on the strength requirements of the joint have been established: smaller diameters lead to destruction at the interface, larger ones – to partial destruction of the mixing zone. 2. A methodology for predicting the mechanical properties of joints based on mathematical modelling of the process has been developed. 3. The results have been implemented in the educational process at NTU ‘KhPI’ and can be used in the automotive, railway and aerospace industries. 4. Develop and manufacture mixing tools of various diameters for friction spot welding with mixing of thick aluminium plates. 5. Technological recommendations have been developed for selecting the optimal parameters of friction spot welding with mixing for industrial application in the joining of thick aluminium plates.