Shaped metal panels are all around us. The bodies of our cars
are made almost completely from metal panels that have been forced into complex
curves. Inside our consumer goods there are steel and aluminium panels that have
been pressed into brackets and formed into intricate shapes. Aircraft, toys,
kitchenware – the list goes on.
And almost all of these metal components started out as flat
sheet, before being stamped into shape by huge presses. Typically, these
hydraulic tools crush the sheet between male and female dies, using enormous
pressure applied relatively slowly to push the metal into its new shape.
But instead of doing it this way, what about using explosive
forces to force the metal into just one half of the die? Not only do you reduce
the number of (very expensive) dies, but if the movement can be made to happen
fast enough, the metal can even alter the way in which it behaves, acting as if
it’s got a lot more flexibility.
The technology is called high energy metal forming, and despite
the fact that in various forms it’s been around for decades, it is currently one
of the hottest areas being explored in a range of industries.
There are three ways in which sheet metal can be forced into a
die using high energy techniques.
1. Electrohydraulic Forming
In this approach, the sheet metal is clamped into place over
the female die, with the die and metal submerged under water or oil. Two
electrodes are placed to one side of the work-piece and the current from a large
capacitor bank is then discharged through them. A spark jumps the gap, creating
huge shockwaves in the water or oil, and these shockwaves smash the metal into
the shape of the die.
A similar approach can also be taken in the shaping of tubes,
with the shockwaves initiated within the tube and an external die used.
This approach was first used in the 1940s, and was developed
further in the 1950s and 1960s, primarily in the aerospace field.
2. Explosive Forming
Even more dramatic is explosive forming, which uses the same
basic approach as electrohydraulic forming - but replaces the arc discharge with
explosives. The explosion creates a shockwave which causes the metal to take the
shape of the die.
3. Electromagnetic Forming
Electromagnetic forming is the only high-energy technique to
gain widespread acceptance. In this approach, electrical charge from a bank of
capacitors is passed through a coil. This develops a strong magnetic field,
which in turn develops eddy currents in the nearby workpiece. These eddy
currents develop their own magnetic field, which causes a mutual repulsion
between the workpiece and the coils. The coils are placed so that the metal is
repelled at great speed into the die, so shaping it.
This approach can be used to form both shaped cylinders (the
coils are placed within the cylinder) and also more conventional pressings that
start as flat sheet.
Electromagnetic forming is being used by Boeing to swage
end-fittings onto torque tubes in its 777 aircraft. One loudspeaker manufacturer
is also using electromagnetic forming to shape speaker cones from 0.002-inch
thick titanium sheet.
The velocities with which this type of metal forming occurs are
very high - up to 200 metres per second (720 km/h) can be used.
One major advantage of high energy metal forming is that
because the shaping of the material occurs so quickly, the material can alter in
its metallurgical characteristics, becoming much more ductile. This change in
characteristics is known as ‘hyperplasticity.’ Aluminium, for example, has poor
formability at slow velocities, tending to tear at sharp corners and bends.
However, high-energy forming can overcome these problems.
One approach is to use a hybrid of conventional forming methods
to gain the general shape, with high energy forming used to complete the
product. This has the advantage of reducing the capital cost of the high-energy
equipment – using electromagnetic forming to produce large panels requires big,
expensive capacitor banks.
Other benefits of electromagnetic forming are:
Reduced number of operations
Improved strain distribution
Less reliance on lubricants
A vivid demonstration of the reduction in wrinkling that can
occur as forces increase can be seen here. A male die in the shape of a
truncated cone was used, with sheets of aluminium thrown over it by
electromagnetic means. Different capacitor discharge energies were used,
resulting in different impact velocities. As launch energy increased, wrinkling
was startlingly reduced.
Another demonstration of what can be achieved with the
technique is shown here. Aluminium rings were compressed onto a mandrel only
half the original diameter of the ring. With sufficient energy, the ring could
be massively reduced in size without wrinkling.
So Why Isn’t Everyone Doing It?
If high energy metal forming is good, why isn’t it far more
widespread in industrial applications? Some researchers in the field, in a paper
entitled Opportunities in High-Velocity Forming of Sheet Metal, pose the same question. They answer it in this
Suppose that a classically educated, but sheltered, engineer
is asked to devise a procedure to drive nails into wood. If he or she is unaware
of the concept of the hammer, the engineer is likely to develop something that
looks like a modern press. The device might be built so that it precisely aligns
a nail normal to the piece of wood and has an actuator that moves at a
controlled displacement rate (possibly with high force) and drives the nail
slowly into the board.
Other engineers might applaud this approach as it offers
much control and precision. By way of added improvements, the engineering
community would work on issues such as the stability and buckling of the nail as
well as the challenge of making a truly portable nail driver. Over time, others
would improve on this approach. Standards would be developed and the viability
of many companies might become dependent on its continuation.
Now imagine another engineer suggesting that this common but
somewhat elegant process could be replaced by simply banging on the head of the
nail to drive it into the wood. While this has many advantages in terms of
simplicity, cost, stability of the nail and portability, it might encounter some
resistance as it appears some control over the process is lost and there might
be a substantial learning curve in developing good hammers and the skills needed
to wield them properly.
Fortunately the hammer was developed long before
conventional engineering practices!
In some sense this analogy parallels the state of sheet
metal forming technology today. Forming typically is accomplished with the
motion of massive matched tools with precise control of static forces and slow
displacement rates. In effect, we now are suggesting that much might be
accomplished by hurling chunks of metal into dies.