Upset-welding,

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Upset-welding (UW) is a form of resistance welding process using both heat and pressure to perform quickly a weld, even with relatively large joint areas.

Heat is produced from the resistance to the passage of electric current at the contact interface of those surfaces.

Can Upset-welding help you?
Yes, if you need a reliable, quick process for repetitive end-to-end joints of any shape, even in hard to weld materials.

If you are familiar with Band Saws you know how saw band loops are welded: you may not know that the process is called Upset-welding. Some thermocouple wire welders are also based on the same principle.

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Pressure is applied before heating is started and is maintained throughout the heating period. Upset-welding can be used only if the parts to be welded have the same cross-sectional area. The abutting surfaces must be very carefully prepared to provide for proper heating.

Many different shapes like wire, bar, strip and tubing of various materials can be joined end to end by Upset-welding. Coalescence is produced simultaneously over the entire area of the abutting surfaces or progressively along a joint.

An example of a progressive process is that used in the manufacture of pipes or tubes in a continuous production line whereby a coiled strip is roll formed by having it pass through a set of pairs of forming rolls. The seam is then closed by Upset-welding after heating the edges with the current provided by wheel electrodes and pressing the edges together with other conforming rolls.

In a line like this one, Upset-welding is used also, with a different set-up, to weld together the end of a coil to the beginning of the next one, to assure uninterrupted production.

Significant deformation of the joint results from the applied pressure on the heated, softened material.

Control of the weld is maintained by selection of optimized current, force, and time. The entire area to be joined is welded at one time.

Resistance welding is usually considered a fusion welding process as in spot welding where the weld nugget presents, in its section, the aspect of microstructure solidified from the melt.

However, in the form of Upset-welding, the process produces no melting at all. It is therefore similar to Forge Welding, another solid-state process. An article on this subject was published in the Issue for July 2006 of Practical Welding Letter. To read the article click on PWL#035.

The deformation at the joint provides secure contact between clean surfaces with the formation of strong metallurgical bonds. Any occasional melting at the joint is promptly squeezed out of the weld by the applied pressure.

This solid state resistance welding process combines the best of both resistance welding and solid-state welding. Therefore Upset-welding can be applied to parts much larger than those conventionally considered for fabrication by resistance welding.

The resistance Upset-welding process is suitable for many joining applications, like fabrication of small spherical and cylindrical vessels, because it consists in a reliable, short welding cycle, and employs equipment that is relatively simple, easy to operate, maintain and control.

Furthermore, the solid-state welds produced have improved mechanical and metallurgical properties compared to those obtained by fusion processes.

Extensive tests, both nondestructive and destructive, have shown excellent performance.

Advantages of Upset-welding:

• Fast process
• Ease of parameters control (only current, time and force)
• High quality, absence of typical fusion defects
• Metallurgical properties comparable to those of hot worked material.
• Simple, sturdy and reliable equipment operated by unskilled workers
• Tolerance for minor alloy deviations
• Large selection of materials, including difficult to weld ones.

Limitations:

• Equipment generally suitable to one type of applications only
• Peak current drawn from power line.

This limitation may be reduced by using homopolar generators that store energy in a flywheel and convert quickly back the energy to a current peak without unduly loading the power line.

See the following reference:

ANSI/AWS C1.1M/C1.1:2000
Recommended Practices for Resistance Welding
American Welding Society, 01-Jan-2000
105 pages
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For certain selected applications Upset-welding is the most convenient solution providing rapidly and consistently reliable joints made with simple equipment by unskilled workforce.