superplastic forming

After developing welding and forming commercially pure titanium and titanium alloy Ti-6Al-4V, we turned our attention to superpastic forming of Ti-6Al-4V.We had been manufacturing gas bottles of titanium alloy for missile application. We wanted to examine whether we can adopt superplastic forming for manufacture of gas bottles.

Superplasticity is the property of certain metals and alloys to be stretched many times their intial length without fracture.Elongations of the order of 800% to 1000% can be expected if certain conditions such as temperature, strain rate, microstructure of the alloy can be controlled.Ti-6Al-4V shows excellent superplastic forming properties in the temperature range of 750degC to 1000degC.Maximum elongation is observe around 930 deg C. The Microstructure must be fine grained and the deforming temperature must be higher than half th melting temperature.In addition to the grain size, grain size distribution. grain aspect ratio and percentage of alpha -beta in the microsructure are also important. Without getting in to more details on theory of superplastic forming let us look into how we did free blowing of a sphere and some other shapes.

We selected a 3mm thin Ti-6Al-4V sheet for free blowing. We attempted free blowing a sphere. Circular blanks were trepanned by holding the plate in a vacuum chuck. The two blanks were held in a jig and the edges were welded by TIG welding. In of the blanks a thick walled tube of commercially pure titanium was welded. The welded blank was coated with a protective coating which reduced oxidation of the blanks during heating, An electric furnace which was earlier calibrated with the temperature uniformity of plus or minus 5degC was used for experiments. A thermocouple was attached to the nozzle and the temperature indicated by digital temperature recorder was accepted as the temperature of blank. The furnace was purged with argon and low volume flow was maintained throughout blowing,When the temperature of the blank reached 930 degC Argon was admitted in to the sandwich and the pressure was increased in increments of few Psi.After the assuring that the free blowing was completed, the pressure was released slowly and the furnace switched off. After the sphere was cooled to room temperature the surface was examined and the thickness was measured by ultrasonic thickness gauge.The thickness at the pole of the sphere was between 2.1mm to 2,2 and at the pole was 1,7mm to 1.8mm.

Although we demonstrated the feasibility of forming a sphere by free blowing we realised that the process is slow and hence we abandoned further trails. We continued with hot forming, machining,welding and testing. But if the number of components required was not many free blowing is a viable option.