Study of the Mechanical Performance of Similar and Dissimilar Titanium Alloy Joints Formed by Diffusion Bonding and Friction Stir Welding
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Titanium alloys have turned strategically important in aerospace applications due to their high strength to weight ratio, corrosion resistance, and good strength sustainability at high temperatures. The joining of lightweight materials such as titanium alloys in aerospace industries offer two main benefits of near net shape manufacturing and of reduction in buy to fly ratio. Among the various joining methods, Diffusion Bonding and Friction Stir Welding (FSW) can be combined with superplastic forming, therefore, have been widely accepted technique to join titanium alloys to produce complex sheet structures with reduced weight and fabrication costs compared to mechanically fastened structures. Both the processes are mainly optimized for titanium alloy, Ti-6Al-4V, to produce defect free sheets of it. The other titanium alloys such as Ti-6242 and Ti-54MFG offer advantages of high temperature sustainability and better machinability over Ti-6Al-4V. The joining of titanium alloys with different properties can improve the utilization of titanium alloys and increase the functional properties of joints. However, a very limited research has been conducted on joining techniques with dissimilar titanium alloys. Therefore, the current study examines the processes with dissimilar titanium alloys to broaden the application of the process with titanium alloys. The capability of diffusion bonding process for similar and dissimilar titanium alloys was studied with five titanium alloys, Ti-64SG, Ti-64FG, Ti-6242SG, Ti-54M, and Beta-21S, by maintaining two governing parameters pressure and time constant while the temperature was varied during the formation of joints. The friction stir welding of similar and dissimilar titanium joints was also studied with five titanium alloys, Ti-64SG,Ti-64FG,Ti-6242SG,Ti-6242FG, and Ti-54MFG, by varying two parameters namely, spindle speed and the feed rate, that control the quality of the process. The bond integrity of both the processes was evaluated with metallographic examination and mechanical testing. The surface and subsurface characteristics were examined to evaluate the surface characteristics and bonding strength. The mechanical strength of diffusion-bonded joints was investigated by conducting flexure testing whereas of the friction stir welded joints were examined by performing tensile tests. The probabilistic theoretical model of estimation of bonding time of dissimilar alloys was developed by computing the surface topography at the joint interface. A combined approach of numerical and experimental methodology was used to investigate the mechanical response of different titanium joints. The numerical model was developed to simulate flexural behavior of dissimilar diffusion bonded titanium joints from the material curves of its parent alloys. Similarly tensile behavior of friction stir welded similar titanium alloys for different process conditions was developed to identify the locations of maximum stress and strain in the joints. The numerical results were compared to experimentally determined behavior characteristics of the joints to gage the validity of the modeling approach. It was found that there was good agreement in the numerical and experimental results of the tensile behavior in titanium FSW joints. The diffusion bonding temperatures lower than the β transus temperature of titanium alloys were identified to obtain sound bonds in similar and dissimilar titanium alloys. The quality of bonding was improved as the bonding temperature was increased. In case of friction stir welding, the optimum values of spindle speed and feed rate for the joints formed by similar titanium alloys were 275 RPM/100 mm min-1, whereas it was 325RPM/125 mm min-1 for the joints formed by dissimilar titanium alloys.
- Mechanical engineering