(B.E Mechanical, A.M.I.Prod.E, A.I.E)

Dept. of Metallurgical Engg. & Matl. Science

Indian Institute of Technology – Bombay

I.I.T – Bombay

Introduction

Manufacturing of complex lightweight automotive structures that meet cost and product goals is a competitive challenge facing industry. Superplastic forming (SPF) is a valuable tool for the fabrication of complex parts used in the aircraft and automobile industries. Superplastic forming (SPF) of sheet metal has been used to produce very complex shapes and integrated structures that are often lighter and stronger than the assemblies they replace. Superplasticity in metals is defined by very high tensile elongations, ranging from two hundred to several thousand percent. Superplasticity is the ability of certain materials to undergo extreme elongation at the proper temperature and strain rate.

                   The process typically conducted at high temperature and under controlled strain rate, can give a ten-fold increase in elongation compared to conventional room temperature processes. Components are formed by applying gas pressure between one or more sheets and a die surface, causing the sheets to stretch and fill the die cavity. The evolution of pressures must be closely controlled during the process since the alloys of interest only exhibits Superplastic behaviour for certain temperature dependent range of strain rates. Specific alloys of titanium, stainless steel, and aluminum are commercially available with the fine-grained microstructure and strain rate sensitivity of flow stress that are necessary for Superplastic deformation.

The Process

SPF can produce parts that are impossible to form using conventional techniques. During the SPF process, the material is heated to the SPF temperature within 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 during deformation increases rapidly with increasing strain rate. Superplastic alloys can be stretched at higher temperatures by several times of their initial length without breaking.

Fig. 1 shows the SPF process.

Some of the materials developed for super plastic forming are:

1. Bismuth-tin (200% elongation)
2. Zinc-aluminum
3. Titanium (Ti-6Al-V)
4. Aluminum (2004, 2419, 7475)
5. Aluminum-lithium alloys (2090, 2091, 8090

Advantages of SPF Process

The main advantages of this process are:

1. It is a one step process.
2. The process can be used to form complex components in shapes that are very near the final dimension.
3. Higher material elongations.
4. Elimination of unnecessary joints and rivets.
5. Reduction of subsequent machining.
6. Minimizes the amount of scrap produced.

Applications

• In automotive body panels.
• In forming of aircraft frames and skins.
• Diaphragm forming of plastics.
• Complex shape parts – window frames, seat structures.

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Created: 11/21/2002
Last Rev: