Explosive Forming

Explosive forming is a process in which forces produced by an explosion are used to shape a workpiece. The explosive energy replaces the male part of the tool that is used in regular forming processes.

^{A rocket nozzle produced by Souriau PA&E’s Bonded Metal Division}

Explosive forming – process description

A die is manufactured according to the desired shape of the product. Dies can be made from a number of materials including concrete, ductile iron, fibreglass and kirksite or epoxy and concrete.

Forming takes place inside a water tank or “explosion tank” where the die is placed. The workpiece is situated directly above the die and explosive material placed above that. The explosive material is typically Trinitrotoluene (TNT), a mixture of acetylene and oxygen or Nitroglycerine.

Quite simply, when the explosive material is detonated, the resulting shock wave through the transfer medium (the water) forces the workpiece material into the die, thus assuming its shape

Applications

Some of the applications of explosive forming include:

• Sheet metal panels
• Tubing
• Housings
• Jet engine parts
• Missile nose cones
• Ducts

Materials for explosive forming

• Both ferrous and nonferrous metals including steel, aluminium, magnesium, and their alloys.
• Some metal matrix composites like aluminium matrix, copper matrix and lead matrix composites.

Process Variations

• Explosives can be placed at a fixed distance from the workpiece. After detonation, the explosive forces travel through the intervening medium to reach the workpiece. This method is called the standoff method.
• Alternatively, the explosive can be placed directly on the workpiece. Upon detonation, explosive forces hit the workpiece directly. This is called the contact method.

Design Considerations

• Section thickness at any point should be in the range of 1 mm to 10mm.
• Tolerance of 0.25mm to 1mm should be provided.
• Surface roughness of 1.575mm to 12.3mm can be achieved.

Economic Considerations

• Slow production rate of 0.5 to 6 pieces per hour.
• High labor skills required.
• Low equipment costs, moderate tooling costs.
• Economical for batch production.

Quality Considerations

• Explosives contaminate the transfer media, which need to be cleaned each time for better quality and process control.
• It is difficult to control the mechanical properties and dimensions of the part.

Advantages

• Complex shapes can be produced.
• The smoothness of the contour can be controlled.
• Cheap alternative to Superplastic Forming.

Disadvantages

• Only viable for low production volumes.
• Handling explosives requires great care and safety precautions.

In the video below a maniac demonstrates a homemade version of explosive hydroforming in his back garden.