Welding of contact assemblies has both technological and economicimportance. Because of the short heating times during welding the carriermaterials retain their hardness except for a very small heat affected area. Of themethods described below, resistance welding is the most widely utilizedprocess.

Because of miniaturization of electromechanical components laser welding hasgained some application more recently. Friction welding is mainly used forbonding (see Chapter 9). Other welding methods such as ball (spheres) weldingand ultrasonic welding are today used in only limited volume and therefore notcovered in detail here.Special methods such as electron beam welding and cast-on attachment ofcontact materials to carrier components are mainly used for contact assembliesfor medium and high voltage switchgear.

Resistance welding is the process of electrically joining work pieces by creatingthe required welding energy through current flow directly through thecomponents without additional intermediate materials. For contact applicationsthe most frequently used method is that of projection welding. Differentlyshaped weld projections are used on one of the two components to be joined(usually the contact). They reduce the area in which the two touch creating ahigh electrical resistance and high current density which heats the constrictionarea to the melting point of the projections. Simultaneously exerted pressurefrom the electrodes further spreads out the liquefied metal over the weld jointsarea. The welding current and electrode force are controlling parameters for theresulting weld joint quality. The electrodes themselves are carefully designedand selected for material composition to best suit the weld requirements.

The waveform of the weld current has a significant influence on the weld quality.Besides 50 or 60 Hz AC current with phase angle control, also DC (6-phasefrom 3-phase rectified AC) and medium frequency (MF) weld generators areused for contact welding. In the latter the regular AC supply voltage is firstrectified and then supplied back through a controlled DC/AC inverter as pulsedDC fed to a weld transformer. Medium frequency welding equipment usuallyworks at frequencies between 1kHz to 10kHz. The critical parameters ofcurrent, voltage, and weld energy are electronically monitored and allow throughclosed loop controls to monitor and adjust the weld quality continuously. Thevery short welding times needed with these MF welding machines result in verylimited thermal stresses on the base material and also allow the reliable joiningof otherwise difficult material combinations.

Contact tips or formed contact parts produced by processes as described inchapter 3.1.2 are mainly attached by tip welding to their respective contactsupports. In this process smaller contact parts such as Ag/C or Ag/W tips withgood weldable backings are welded directly to the carrier parts. To improve thewelding process and quality the bottom side of these tips may have serrations(Ag/C) or shaped projections (Ag/W). These welding aids can also be formed onthe carrier parts. Larger contact tips usually have an additional brazing alloylayer bonded to the bottom weld surface.

Tip welding is also used for the attachment of weld buttons (see chapter 3.1.3).The welding is performed mostly semi or fully automated with the buttonsoriented a specific way and fed into a welding station by suitably designedfeeding mechanisms.

This contact attachment process is also one of the liquid phase weldingmethods. Solid phase lasers are predominantly used for welding and brazing.The exact guiding and focusing of the laser beam from the source to the jointlocation is most important to ensure the most efficient energy absorption in thejoint where the light energy is converted to heat. Advantages of the method arethe touch-less energy transport which avoids any possible contamination ofcontact surfaces, the very well defined weld effected zone, the exactpositioning of the weld spot and the precise control of weld energy.

Laser welding is mostly applied for rather small contact parts to thin carriermaterials. To avoid any defects in the contact portion, the welding is usuallyperformed through the carrier material. Using a higher power laser and beamsplitting allows high production speeds with weld joints created at multiplespots at the same time.

In high voltage switchgear the contact parts are exposed to high mechanicaland thermal stresses. This requires mechanically strong and 100%metallurgically bonded joints between the contacts and their carrier supportswhich cannot be achieved by the traditional attachment methods. The twoprocesses of electron beam welding and the cast-on with copper can howeverused to solve this problem.

The electron beam welding is a joining process which has shown its suitabilityfor high voltage contact assemblies. A sharply focused electron beam hassufficient energy to penetrate the mostly thicker parts and generate a locallydefined molten area so that the carrier component is only softened in a narrowzone (1 – 4 mm). This allows the attachment of Cu/W contacts to hardand thermally stable copper alloys as for example CuCrZr for spring hardcontact tulips ''(Fig. 3.12)''.

The cast-on of liquid copper to pre-fabricated W/Cu contact parts is performedin special casting molds. This results in a seamless joint between the W/Cu andthe copper carrier. The hardness of the copper is then increased by asecondary forming or deep-drawing operation.

Functional quality criteria such as bonded area percentage or shear force areusually agreed upon between the supplier and user and defined in deliveryspecifications.

Functional quality criteria such as bonded area percentage or shear force areusually agreed upon between the supplier and user and defined in deliveryspecifications.