Superplastic Forming and Diffusion Bonding

Friction Stir welding, Superplastic Forming and Diffusion Bonding

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. In addition to this, Titanium exhibits superplastic behavior and the capability of diffusion bonding with the ability to readily absorb the thin surface oxides and dissolve minor amounts of adsorbed gases that inhibits the formation of required metallic bonding. The joining of lightweight materials such as titanium alloys in aerospace industries offer two main benefits: Near net shape and reduction in buy to fly ratio.  Among the various joining methods, Diffusion Bonding and Friction Stir Welding (FSW) has been widely accepted technique to join titanium alloys. Both the processes when combined with superplastic forming produce near net shape of parts and complex sheet structures with reduced weight and fabrication costs compared to mechanically fastened structures. Till date lot of work has been done with similar type of titanium alloys, specifically on Ti-6Al-4V to produce defect free sheets of it. A very limited research has been conducted on joining techniques with dissimilar titanium alloys. Therefore, the objective of this research is to understand the feasibility of joining dissimilar titanium alloys using diffusion bonding and friction stir welding processes that has a potential to improve the utilization and functional properties of titanium.

Current Work

1. Modeling & Simulation of Titanium Diffusion Bonding and Friction Stir Welding Processes
In order to study the capability of diffusion bonding for titanium alloys, two governing parameters pressure and time were held constant while the temperature was varied during the process (50 to 70% of their melting point). The study of diffusion bonding of titanium alloys included six alloys: Ti-6Al-4V (Ti-64), Ti-6Al-4V Fine Grain (Ti-64FG), Ti-6Al-2Sn-4Zr-2Mo (Ti-6242), Ti-6Al-2Sn-4Zr-2Mo Fine Grain (Ti-6242FG), Ti-5Al-4V-0.6Mo-0.4Fe (Ti-54M), and Ti-15Mo-3Nb-3Al (β-21S). The viability of friction stir welding of titanium joints was studied by changing two parameters that control the quality of the process, the spindle speed and the feed rate for five titanium alloys: Ti-64, Ti-64FG, Ti-6242, Ti-6242FG, and Ti-54MFG.  The spindle speed was varied from 225 rpm to 325 rpm whereas the feed rate was changed from 100 mm/min to 150 mm/min. The bond integrity was evaluated with metallographic examination and mechanical testing. The surface and subsurface characteristics were examined to evaluate the surface characteristics and bonding strength. The surface texture was measured using Mahr-Surface Profilometer and the hardness measurement was performed on LECO Micro-Hardness Tester. The mechanical strength of diffusion-bonded joints was investigated by conducting flexure testing whereas of the friction stir welded joints was examined by performing tensile tests on Instron Machine. The main goals of this study are mechanical characterization of the joints and to develop numerical models using FEM.

Research Publications And Conferences

1. Kulkarni and M. Ramulu, “Experimental and Numerical Analysis of Mechanical Behavior in Friction Stir Welded Different Titanium Alloys,” Proceedings of the ASME 2014 International Mechanical Engineering Congress & Exposition IMECE2014, (November 14-20, 2014), Montreal, Canada.
2. Kulkarni, M. Ramulu, and D.G. Sanders “Experimental Study on the Quality of Dissimilar Titanium Joints through Diffusion Bonding,” Poster presentation at International Titanium Association Conference-TITANIUM USA 2014, (September 21-24, 2014), Chicago, Illinois.
3. Kulkarni, H. Bae, R. Mamidala , “A three-dimensional single and multiple shot simulation of shot penning for Steel, Aluminium and Titanium alloys,” to be published in Proceedings of the 12th International Conference on Shot Peening, (September 15-18, 2014), Goslar, Germany.
4. Kulkarni, M. Ramulu, D. Sanders and L. Hefti, “Experimental Investigation of Flexural Behavior of Diffusion Bonded Dissimilar joints of Titanium Alloys,” Presented at the 25^th Advanced Aerospace Materials and Processes (AeroMat) Conference and Exposition, (June 16-19, 2014), Orlando, Florida.
5. Ramulu, N. Kulkarni, K. Gangwar, T.Morton and V. Aditya, “Experimental Study to Develop Joining Methods for New or Modified High Temperature Titanium Alloys: Metallurgical Characterizations, Microstructural Integrity and Mechanical Performance,” Poster presentation at the JCATI Symposium at WSU, (April 21, 2014), Pullman, WA.
6. Kulkarni and M. Ramulu, “Flexural Behavior of Diffusion Bonded Titanium Joint,” Presented at the International Conference on Computational & Experimental Engineering and Sciences (ICCES) Conference, (May 24-28 2013), Seattle.
7. Kulkarni, W.J. Emblom, T. Kozman, J. Lee, and K.J. Weinmann, “The Development of a Strain Model for an Oval Aluminum Stamp Formed Pan,” Transactions of the North American Manufacturing Research Institution of the Society of Manufacturing Engineers (NAMRI of SME), May 2010.

2. Characterization of surface, subsurface, and Mechanical properties of Friction Stir Welded Dissimilar Titanium Alloys

Friction stir welds for alloys, Ti-6Al-4V. Ti54, Ti6242; standard grain (SG), and fine grain (FG), were produced with rpm 225, 275, and 375 and travel speeds 4, 5, and 6 in/min, for the study.

For microstructural study, each specimen was sectioned, mounted, and polished in accordance with ASTM E3-01. Microstructure analysis was conducted in the etched condition with 2% HF etchant per ASTM E407- 07. The microstructure for each specimen was evaluated and documented using a Nikon Eclipse LV150 microscope/camera and NIS image analysis software system. Calibrated images were captured for documentation and analysis with denoted conditions and locations. Optical micrographic analysis has shown the refinement of the grains in the weld nugget region due to severe plastic deformation imparted by the non-consuming tool. Microstructural analysis has revealed the formation equiaxed primary alpha with a transformed beta matrix. HAZ/TMAZ microstructure composition reflects a transition increasingly equiaxed and refinement of Alpha grains in an increasing amount of transform beta matrix approaching the weld nugget. – Weld Nugget microstructure composition is prior beta grains (basket-weave morphology) defined by intergranular Alpha plates, with prior Beta grains increasing in size as approaching the weld center.

The microhardness profiles for this study were conducted in an “as polished” condition on each mounted specimen. All microhardness indents have a spacing of 254 microns. All the microhardness indent traverses were conducted with a LECO AMH43, Automatic Hardness Testing System, using a Vickers indenter with a 500 gram load. All microhardness testing for this study was completed in accordance with ASTM E384-06. Hardness analysis has shown the increase in the values of hardness in the stir zone as compare with the base metal.

Despite the poor conductivity of titanium it has been found that there is a very small TMAZ adjacent to the SZ in titanium welds. Electron Back Scattered Diffraction (EBSD) technique will be employed to understand the nature of TMAZ in titanium alloys. To correlate the microstructure with the mechanical properties the hardness values will be measured on the transverse section of weld in the middle region of weld nugget with spacing about 60 micron between two indents.

With the non-consuming tool slightly tilted, and offsetted between two materials, material transports from retreating side to advancing side in the friction stir welding process. The presence of different materials is vividly visible in the macrographs of dissimilar alloys FSW butt welds. For better understanding the materials transport will be studied by the aid of scanning electron microscope (SEM) equipped with Energy-dispersive X-ray spectroscopy (EDX).

The ability to produce near net shape components, with tailored characteristics to meet durability and damage tolerance requirement, is one of the profound features of friction stir welding (FSW). However, the existence of crack, or notch can significantly affect the crack growth rate, and fatigue life. Location of the crack, whether at the center, interface or, across the weld has predominant role on the fatigue life of the specimen. In the current study high cycle fatigue strength, and crack propagation in Ti-6Al-4V FSW butt welds will be studied using Lineal Elastic Fracture Mechanics (LEFM) approach for the specimens with cracks at the aforementioned locations. Load ratio (R-Value) of 0.01 has been applied during the investigation. Higher fatigue life has been observed for specimens with crack at the center of the weld in contrary with the specimens for cracks at the interface. Furthermore, the inhomogeneity of the weld nugget is discernible by the fatigue life for specimens with cracks located at advancing, or, on retreating interface. Crack growth rate, and crack interaction for specimens with multiple cracks have also been studied and characterized.

 

Figure 1: Macrograph of friction stir butt weld of similar titanium alloys.

Figure 2: Optical micrograph of the friction stir weld butt weld. WN = Weld Nugget. A= Advancing, R = Retreating, C = Center, B = Bottom, T = Top

Figure 3: Hardness profile. a) Similar Alloys. b) Dissimilar alloys

 

Figure 4: SEM images of region of the friction stir butt weld of similar alloys. (Notation same as Figure 2)

 Figure 5: Macrograph of friction stir butt weld of dissimilar titanium alloys.

Publications

• Ramulu, N. Kulkarni, K. Gangwar, Todd Morton and V. Aditya,” Experimental Study to Develop Joining Methods for New or Modified High Temperature Titanium Alloys: Metallurgical Characterizations, Microstructural Integrity and Mechanical Performance” Poster presentation at the JCATI Conference at WSU in Pullman, April 21, 2014