Super Plastic Forming Process
Manufacturing of complex lightweight automotive structure that meet cost and product goals is a competitive challenge facing industry. Super plastic forming process (SPF) is appreciated tool for the fabrication of complex parts used in the aircraft and automobile industries. Super plastic forming process of sheet metal has been used to make very complex shapes and integrated structures that are frequently lighter and stronger than the assemblies they replace. Super plasticity in metals is well-defined by very high tensile elongations, ranging from two hundred to several thousand percent. Superplasticity is the capability of certain materials to undergo great elongation at the proper temperature and strain rate.
The process is usually conducted at high temperature and under organized strain rate, can give a ten-fold increase in elongation related to conventional room temperature processes. Components are formed by applying gas pressure between one or more sheets and a die surface, producing the sheets to stretch and fill the die cavity. The development of pressures must be closely controlled throughout the process since the alloys of interest only shows Super plastic performance for certain temperature dependent range of strain rates. particular alloys of titanium, stainless steel, and aluminium are commercially available with the fine-grained microstructure and strain rate sensitivity of flow stress that are essential for Super plastic deformation.
SPF can yield parts that are impossible to form using conventional methods. During the SPF process, the material is heated to the SPF temperature inside a sealed die. Inert gas pressure is then applied, at a controlled rate forcing the material to take the shape of the die pattern. The flow stress of the material at the time of deformation increases rapidly with increasing strain rate. Super plastic alloys can be stretched at higher temperatures by several times of their original length without breaking.
Some of the materials developed for super plastic forming are
1. Bismuth-tin (200% elongation)
3. Titanium (Ti-6AI-V)
5. Aluminum-lithium alloys
This technique consists in hot forming up to 1000 °Celsius super plastic alloys by using an inert gas pressured up to 50 bars. Combined with diffusion bonding, this method permits honeycomb structures made of several sheets in a single operation.
The blank is loaded in the form of die. The hot press heats the die and the blank to the material super-plastic temperature.
Once the temperature is reached, it is exactly controlled, while the gas pressure slowly expands the blank. The gas keeps expanding the part to fit the die. The material at the super-plastic temperature can permit up to 500% elongation.
At the end of the forming cycle, the part perfectly conforms to the die, even in its smallest details.
The method is increasingly being applied in the aerospace industry as a way of producing very complex geometries.