Resistance projection welding

by Steve Westgate

Description

Projection welding is a development of resistance spot welding. In spot welding, the size and position of the welds are determined by the size of the electrode tip and the contact point on the workpieces, whereas in projection welding the size and position of the weld or welds are determined by the design of the component to be welded. The force and current are concentrated in a small contact area which occurs naturally, as in cross wire welding or is deliberately introduced by machining or forming. An embossed dimple is used for sheet joining and a 'V' projection or angle can be machined in a solid component to achieve an initial line contact with the component to which it is to be welded, see Fig.1.

Fig.1. Example of projection welding configurations: a) embossed projection; b) stud to plate; c) annular projection

In sheet joining using embossed projection welds, a melted weld zone is produced, as in spot welding. However, when a solid formed or machined projection is used, a solid phase forge weld is produced without melting. The plastic deformation of the heated parts in contact produces a strong bond across the weld interface.

Present status

The process is well established and is applicable mainly to low carbon or microalloyed steels. The process is widely used on sheet metal assemblies in automotive and white goods industries for both sheet joining and attaching nuts and studs. Wire mesh welding is also a large industry.

Important issues

Projection welding tends to be more application dependent than spot welding and is less easy to standardise. However, similar issues can arise:

• Weldability of coated steels and attachment of fasteners - control of weld quality and electrode life
• Weld quality is controlled mainly by good process control together with periodic testing of samples. While a number of monitors have been developed, there is still a desire to produce a low cost, reliable and robust in-process weld quality monitor, in order to reduce or eliminate periodic destructive tests. The added difficulty with projection welding is that multiple welds are made
• Recent equipment developments have included use of capacitor discharge power supplies to make large diameter welds for applications such as gear assemblies

Benefits

The advantages of projection welding include its versatility, the speed and ability to automate, the ability to make a number of welds simultaneously and minimisation of marking on one side of joints in sheet materials. Capacitor discharge supplies used with machined annular projections can compete with power beam welding, as the weld is completed in a single shot within milliseconds.

Risks

There are some limitations on material weldability but attention to correct setting up and good process control can solve most production problems. The main safety factors are trapping hazards and splash metal. Little fume is produced but may need attention when welding coated steels or when oils or organic materials are present.