Vacuum Welding Tips

These tips are intended to help the end user in the basic techniques as it pertains to welding for vacuum applications. Building vacuum chambers and fixturing is still a highly specialized area of vacuum technology.

It is recommended for end users to either purchase these items from a qualified vacuum vendor or use a vacuum technology consultant to assist in the selection of materials and all other aspects of construction.

LDS offers a low cost consulting service supported by AutoCad versions 12-14. Email us for more information.

The usual method of joining steel parts is by welding, gas or arc techniques being the most common. In welding parts for vacuum service particular care must be exercised in order to avoid porosity in the joint.

A technique which minimizes oxide formation, porosity, and other structural flaws is that of inert gas shielded arc welding. This method involves doing the welding in an inert atmosphere, often argon. The equipment in which such welding is carried out consists of an enclosure for the inert gas (usually steel), means for removing the air (usually a mechanical vacuum pump), means for filling the enclosure with inert gas to atmospheric pressure, insulators for introducing electrical leads, and means for manipulating the welding rods in the inert atmosphere.

In some cases both mechanical and diffusion pumps are used in order to pump down to a lower pressure. Often viewing and lighting ports are included. The usual method for manipulating the welding rods and the work is by rubber gloves attached to openings in the enclosure. Clearly, these openings must be covered by plates during pump-down.

Some precautions to be followed in making welds are listed below. These are intended to cover arc or gas welding in air, but many of the points are applicable to inert gas shielded arc welding.

1. Whenever possible, use single pass welds. When multiple pass welds are necessary, remove all slag from the preceding pass by chipping and brushing with a wire brush. Do not peen or hammer the preceding pass excessively.

2. Use the longest practical welding rod so as to permit the longest possible uninterrupted weld. With an interrupted weld, make the overlap as generous as practicable to eliminate voids.

3. Avoid an undue amount of machining, grinding, hammering, or peening since cracks or pinholes may thus be temporarily closed and open up later under stress or corrosion. Where machining is necessary, (e.g., with mating flanged surfaces), extra precautions should be taken to make good welds.

4. Whenever possible, welds should be made from the inside of the vessel only. With double welds, gas is trapped and there is a possibility of virtual leaks.

5. Welds of a structural nature should never cross sealing joint welds continuously. Here again gas may be trapped and a virtual leak develop.

6. Structural welds outside the vessel should be discontinuous or just tack welds to allow easy flow of gas from any channels or pockets.

The possibility of virtual leaks, i.e., trapped pockets of gas exposed to the vacuum through cracks, pinholes, or a porous weld, is an important consideration in welding for vacuum service.

In all examples shown the good welds avoid the formation of gas pockets with the possibility of virtual leaks. Particularly serious are the cases where the channel between welds has a connecting leak to both the inside and outside of the vessel. Here the high flow impedance of the leak, coupled with the storage capacity of the pocket or channel, makes it almost impossible to locate the leak.

Where double welds are absolutely necessary for structural reasons, a definite channel between the welds should be provided in the joint. Small pipe taps, located so that the channel can be pressurized, should be provided. The openness of the channel is determined by a flow test from one tap to the next; then one is sealed off and the channel pressurized. The soap bubble technique, or other leak detection methods are then used on the welds inside and out.

Spot welding with an electric current is often used to join metals to be used in vacuum systems. Usually spot welders are designed to operate at low voltage (often about 6 v) and high current (up to several thousand amperes). The required voltage and current are obtained from the secondary of a transformer, the primary of which is connected to the line voltage. Some means such as a rheostat, is provided to regulate the welding current. Many designs of various power capacities are available on the market. Units designed to handle very thin wires (a thousandth of an inch or less) are very useful.

In carrying out the welding, the electrodes are brought in contact with the work, and a definite pressure is applied, usually by means of a foot pedal. The pressure and duration of the current are important. Inadequate pressure results in burning and "spitting" at the joint, while too much pressure decreases the joint resistance and consequently the heating action. Metals which weld together best are those of similar melting temperature and heat conduction.

In many cases, special welding techniques are used. Heliarc welding is common, in which the work is covered by helium to prevent the formation of oxides. Atomic hydrogen welding is also common. In this method the welding is carried out in an atmosphere of hydrogen which is at a high temperature. At such high temperatures as these hydrogen is very effective in reducing oxides or in preventing them from forming, thus eliminating porosity. To obtain particularly pure metals or alloys, the material is melted in vacuum by an electron beam.

Welding by this method prevents the formation of oxides. The method involves heating the metals to be joined by an electron beam in vacuum.

LDS Vacuum Products, Inc.
975 Sunshine Lane, Altamonte Springs, Florida 32714
Phone (407) 862-4643 Fax (407) 862-8723

Email: hivacuum@aol.com