Moving contact arc welding

by Wayne Thomas

Description and explanation

In moving contact arc welding (MCAW), current is supplied to a shaped metal consumable electrode through a sliding or rolling contact tool. The lying consumable (known as a Ridgeback^TM consumable) is flux-covered and features an exposed metal ridge that protrudes above the flux. Electrical contact between the tool and the consumable is applied at a relatively short distance from the arc, eliminating the resistive heating path of the electrode.

The MCAW technique is under development and offers an easy-to-use alternative cladding, welding and repair method. The process is suitable for either manual or mechanised operation. It can be applied with restricted access and is amenable to remote operation.

Fig 1. Basic principle of MCAW using Ridgeback^TM consumable

Present status of MCAW

The process is novel in that it differs from the traditional Firecracker (Elin-Hafergut) and other lying consumable welding techniques because the contact point is kept as close as possible to the burning arc. This minimises resistive heating between the contact point and the arc, enabling the Ridgeback^TM consumable to carry greater currents, or, almost infinite lengths to be used in one operation. Restricted access and remote operation capability are two of the major advantages unique to the MCAW process.

Although the feasibility has been demonstrated, the MCAW process has not yet been used commercially. Underwater welding under dry conditions in a local habitat is a possible application. The practicability of restricted access conditions has been demonstrated using a local habitat chamber. In addition, repair welding in hazardous surroundings (e.g. in the nuclear industry) and welding or repair inside steel tubes and pipes are likely to provide ideal application for the process.

Important issues

Ease of manufacture of profiled consumables by hot rolling or extrusion, etc and application of the flux covering on consumables in a more cost-effective way are important development issues in successful commercialisation of the process. A further research requirement will be to develop the process to become positional, i.e. for vertical and overhead applications.

A potential variant of the MCAW technique is sub-arc in which a plain bar stock or powder filled consumable is used. The sub-arc variant would not need a specially shaped rod, and the consumables need not be flux covered. The technique would make use of a hopper feed, granulated flux and insulated flux saddles. The sub-arc variant would eliminate the need for any consumable feed system.

Fig 2. Assessing the feasibiltiy of moving contact arc welding (MCAW)

Benefits

Potentially, the MCAW process has many beneficial features compared with existing processes which utilise consumables.

The process gives greater productivity as the number of start/stops is reduced, and longer and/or larger consumables can be used in one welding operation. Stop/starts can be sites for welding defects such as cold laps; reduction in stop/starts reduces this associated risk. The use of larger consumables can reduce/eliminate the need for multi-pass welding of thick plate.

MCAW uses low-cost, portable equipment and is readily automated. It is simple to operate and is essentially a low-skill welding/cladding technique with less associated operator fatigue. It is ideal for applying hardfacing and corrosion overlays and can be designed to operate in restricted access.

Risks

Typically, the risks associated with traditional arc welding equipment will also apply to MCAW.

Future prospects

It is expected that MCAW will be used for niche applications were existing techniques are unable to operate.