Experimental and numerical investigations on friction stir welding of aa7068 t6 aluminium alloy

Abstract

This thesis explores the capability of Friction Stir Welding (FSW) process to join the high newlinespecific strength precipitation hardenable alloy AA 7068-T6. The FSW process development newlinewas initiated by building mathematical models that relate process parameters and responses. newlineExperiments using center composite face-centered design of response surface methodology newline(RSM) were carried out for this investigation. The tool rotational speed, welding speed, and newlineaxial force were identified as the factors that control the responses, such as UTS and elongation. newlineThe developed mathematical models were validated using the analysis of variance (ANOVA), newlineand the actual and adjusted values of the regression coefficients. Multi response optimization newlinehas been carried out using the desirability function approach.The microstructure of the welded newlinejoints produced at high, medium, and low heat input conditions has been studied to correlate newlinemicrostructure development and process parameters. newlineTool geometry is another critical parameter that controls the FSW joint quality. A 3D newlineCFD numerical model was developed to study the influence of tool pin profile on temperature newlineand flow fields of FSW joints. Among the five models selected for the study (viz., threaded, newlinetriangular, square, hexagonal and nonagon) threaded pin profile was found to improve material newlinemixing. The predicted peak temperature by threaded pin profile was observed to be within the newlinemelting range of the alloy AA 7068-T6. The strains developed were sufficient to improve the newlinemicro structure. Considering these aspects, threaded pin profile was selected as the optimized newlineone. Experiments were carried out using threaded pin profile to validate the predictions of the newlinenumerical simulations. The peak temperature and the weld shape were compared with experimental newlinemeasurements. The simulation results over predicted both quantities. The joint was newlineanalysed further to study the microstructure, precipitate formation and hardness using optical newlinemicroscope, EDAX and microhardness tester respectively. Defect free joint was obtained. Grain newlinegrowth was observed in TMAZ. Hardness was found to be lower than the base metal through newlinei newlinethe entire weld zone. newlineBesides optimizing the pin profiles, the developed numerical model provides a better understanding newlineof the material flow and temperature development in the FSW of AA 7068-T6. The newlineunderstanding of the flow and temperature field will assist in the future development and fast newlinecommercialisation of FSW process. The new joining method will enable the alloy AA 7068-T6 newlineto replace heavier materials like steel in automobile and aerospace industries. Moreover, the introduction newlineof lighter aluminium alloy in automobiles and air crafts will improve fuel efficiency newlineand reduce emissions. newline