Gas-shielded arc welding includes two processes. Gas-Tungsten-Arc uses a non-consumable tungsten electrode while the Gas-Metal-Arc process employs a consumable electrode that is deposited as weld metal. In both cases the necessary arc shielding is provided by a continuous stream of chemically inert gas such as helium, argon, or helium- argon mixtures. The gas flows axially along the tungsten or consumable electrodes which are mounted in special welding torches. Of the two processes, Gas-Tungsten-Arc, without filler metal, is recommended.

Gas-tungsten-arc is also known as the tungsten-inert gas process but more often referred to as TIG Welding. Although TIG welding is commonly used with filler metals, it is not recommended for welding applications which involve joining metal components to Ceramic-to-Metal sealed assemblies. Welding with the aid of filler metals drastically increases the heat affected zone which, in turn, leads to higher thermal stress at ceramic to metal seal joints. If the thermal gradient is severe enough, seal failure will follow.

Joint Design
Eliminating the use of filler metals greatly reduces the heat required in making fusion welds and minimizes thermally induced stress. To further reduce heat and its effects on the parts being joined, careful consideration must be given to weld joint design.

Joining Ceramic-to-Metal is a specialized technology requiring the use of low-expansion metals which closely match the ceramic’s coefficient of thermal expansion. The match is close but not exact. This means that all Ceramic to Metal seals will experience some differential expansion when subjected to thermal gradients. This differential expansion is affected by part size and severity of thermal gradient. If heat from the welding process is severe enough to generate a thermal gradient in excess of 25°C per minute at a ceramic to metal joint, the stress produced by the differential expansion between ceramic and metal, may be sufficient to cause seal failure.

To minimize stress at Ceramic-to-Metal seals, it is necessary to reduce heat in the welding process. To achieve this reduction, the use of weld prep grooves is recommended. A weld prep groove is a groove which is machined outboard and coaxial to the weld bore. The purpose of this groove is to create a weld lip of shape and size which are equal to the weld lip on the component being joined. Symmetrical and coaxial weld lips of .030" thickness provide an ideal weld joint design which minimizes fusion heat and its influence on nearby Ceramic-to-Metal seals.

Materials
The great majority of Ceramic-to-Metal seal components listed in this Catalog are fitted with 304 Stainless Steel weld adapters. A small number of products are supplied with weld adapters fabricated from other metals and alloys including OFE Copper, Nickel-200, 70-30 Copper- Nickel and Nickel-Iron. These materials can be joined to Stainless Steel with one or more of the previously mentioned fusion welding methods. The most commonly encountered material is Kovar® a Nickel-Iron alloy. When joining Kovar® to Stainless Steel, be sure to use weld prep grooves. Begin by tack welding in three places, continuing the third tack around the entire weld circumference. Do not stop to repair holes, gaps, or other weld imperfections. Instead, repair them with another complete pass.

More than two passes are not recommended when welding Kovar® to Stainless Steel. The alloy formed by fusing Kovar® and Stainless Steel no longer has the characteristics of either metal. Further welding, in the area where this alloy has formed, will create an unstable mixture which will crack as the weld cools.

Special Notes
In cases where the use of weld prep grooves is not practical because of space restraints, Copper heat-sinks are an alternative solution. If ever short or thin weld adapters are encountered where weld prep grooves and heat sinks do not provide adequate heat control, a process other than TIG welding may be required. The most common solution to this problem is the use of precise, localized welding techniques such as electron-beam or laser-beam welding. Note that because the two methods use finely focused energy beams - .005" diameter laser beams - the fit between parts being joined is crucial.

For additional information and assistance, contact the ISI Sales and Service office or your local Sales Representative.

Back To Top