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How do I produce a vacuum for EB welding?

 

Frequently Asked Questions

Several joining processes use vacuum as the joining environment or in the equipment itself, e.g. electron beam welding, CO2 laser welding, vacuum brazing etc.

A vacuum is produced by using a vacuum pump to remove air from a closed volume; the more air that is removed, the better or higher the vacuum, or the lower the pressure. The vacuum level is measured by vacuum gauges.

Vacuum pumps fall into two groups: -

  • Capture pumps. In these, gas molecules are 'captured' by the pump and retained either permanently or temporarily to be released later; examples include sorption and sublimation pumps.
  • Transfer pumps. In these, gas molecules are transferred from a low pressure region to a higher pressure region by one or more stages of compression; examples include rotary vane and piston pumps, turbo-molecular pumps and diffusion pumps.

Each type of pump will only operate over a limited range as shown in the figure. Those pumps which will exhaust gas to atmosphere are known as 'primary' pumps (P); those which will not and hence need a primary pump between them and atmosphere are known as 'secondary' (S) pumps. 

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Rotary pumps, sometimes known as 'backing' pumps, consist of one or more rotating vanes or pistons which draw air into the pump, compress it and then exhaust it to atmosphere. For good ultimate vacuum (the lowest pressure which the pump will achieve), low wear and quiet running, these pumps work with an internal oil bath; when much air is passing through the pump, some of this oil is carried over into the exhaust as an oil mist which must be dealt with. More recent designs work without oil but their cost is greater and their ultimate vacuum is degraded.

Mechanical booster pumps (also known as Roots blowers) use two meshing 'figure of eight' rotors to draw air into the pump, compress it and then exhaust it to a primary pump; these pumps are capable of very high pumping speeds in the 10 to 10-2 mbar range.

Sorption pumps consist of an enclosed container filled with a porous material having the property of absorbing large quantities of gas when cooled to liquid nitrogen temperatures. These pumps produce a very clean, oil-free vacuum but when saturated must be regenerated by warming to room temperature.

Turbo-molecular pumps contain several alternating sets of rotors and stators with inclined blades which impart momentum to the air molecules in the direction from the inlet to the outlet port; rotational speeds are high, up to 60,000 rpm leading to a relatively high cost.

Diffusion pumps use several jets of high speed oil vapour produced by a boiler to move air molecules from the inlet towards the outlet port. Although much cheaper than turbo-molecular pumps, their warm up time, power consumption and possible oil back-streaming make them less attractive in some applications.

Sublimation pumps contain an active cathode material, usually titanium which, when bombarded with high velocity ions, chemically combines with the active gases while the remaining gases diffuse into it. These types of pumps are used only for clean well-sealed systems at high vacuum. Further information is available from any vacuum equipment manufacturer.

For further information see Joining Technologies or please contact us.

For more information please email:


contactus@twi.co.uk