Continuous drive and stored energy are the main two variants by which energy is provided to make a friction weld.
Continuous drive
Taking basic rotary friction welding as an example, the rotating part is continuously driven by an energy source such as an electric motor running at fixed speed and/or pre-set varying speeds. The rate of energy input into the weld is more or less constant, and energy can be supplied for as long as necessary.
Stored energy
Unlike continuous drive, only a finite amount of energy is available to make the weld. This energy is stored in the form of hydraulic oil under pressure or in a rotating flywheel, for example. Small, portable friction stud welding machines typically utilise hydraulic stored energy systems, as do some linear friction welding machines.
When a flywheel is used, it is run up to a set speed at which point its input power is cut and the weld cycle is initiated. Energy stored in the flywheel is dissipated into making the weld while the flywheel is decelerated to zero. In this case the weld is made under falling speed conditions, resulting in a progressive reduction in the rate of energy input.
The flywheel stored energy system is generally known as inertia welding and this is widely used in aero-engine manufacture for assembling very large components. The process is very common in the USA, although continuous drive is preferred in Europe.
Some friction welding machines use a combination of particular features of both continuous drive and stored energy. Known as hybrid friction welding, this is used to good effect on materials which require very large pressures to make them deform during the forge cycle (e.g. nickel-based super alloys).
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