Resistance butt welding

by Steve Westgate

Description

Resistance butt welding is the simplest form of a group of resistance welding processes that involve the joining of two or more metal parts together in a localised area by the application of heat and pressure. The heat is generated within the material being joined by resistance to the passage of a high current through the metal parts, which are held under a pre-set pressure.

The process is used predominantly to make butt joints in wires and rods up to about 16mm diameter, including small diameter chain. The faces of the pieces to be joined may be flat and parallel or profiled in the case of larger sections. This reduces the initial contact area and further concentrates the heating at the interface. The components are clamped in opposing copper dies, with a small amount of stick-out, and abutted under pressure. Current is passed between the dies causing resistance heating of the weld area. The heat generated during welding depends on the current, the duration of the current, and the resistance. As the resistance is highest at the joint interface, heating is most intense in this area. When the material softens, it deforms under the applied load, giving a solid phase forge weld. No melting occurs. The current is terminated once a pre-set upset length has occurred, or the duration of the current is pre-set. The joint is then allowed to cool slightly under pressure, before the clamps are opened to release the welded component. The weld upset may be left in place or removed, by shearing while still hot or by grinding, depending on the requirements.

Present status

Equipment is well established for joining steel wire and rod up to about 16mm diameter, and narrow strip. Automated dc welding equipment is available for joining wider strip, up to about 300mm wide for automobile road wheel rims, at rates up to about 12 per minute.

Important issues

Depending on the application area, issues include control of weld quality, production speed, and control of upset shape.

• Weld quality is normally maintained by good process control coupled with consistent end-preparation of materials. Programmed force and current are used for the most demanding applications, such as wheel rims
  
   
• In some applications, such as wire frames, a small, smooth upset is preferred, to avoid the need to remove it. Careful choice of welding parameters allows a balance between adequate strength and minimum upset

Hardenable steels can be welded and given a subsequent in-machine, local heat treatment to restore weld area toughness. Alloy and stainless steels require more care to ensure that tenacious surface oxides such as chromium oxide are sufficiently dispersed from the interface during welding.

Benefits

Resistance butt welding is a high speed, clean process and is preferred to flash welding for many small components.

Risks

There are some limitations on component size and geometry: very thin or large sections are unsuitable. The main hazards are (i) the risk of crushing fingers or hands and (ii) burns or eye damage from splash metal.