Is a copper chrome (Class 2) backup bar suitable to reduce weld indentation?

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Yes, a flat backup bar could be used to reduce indentation on the one side. It may be more prudent to use a large flat faced electrode on the side you would like to reduce indentations. A flat faced “C” nose electrode of the same body diameter as the electrode on the other side of the weld is commonly used. A larger diameter could also be used, for example a #5 flat face opposite a #4 pointed “A” nose. Of course if the tooling arrangement is easier to use a backup copper chrome (Class 2) bar that can accomplish the same goal.

Flat electrode or bar, the goal is to spread the heat on the good/no mark side of the part out over a large area. Using a smaller face on the other side will concentrate the heat on that half of the part. The nugget should form mostly on the non-show side of the part. Permit the weld nugget to grow into the good side only enough to make an acceptable weld. We are describing what is called a cosmetic weld.

 Cosmetic Weld Nugget No electrodes

Offset weld nugget

The indentation can be caused by excessive force or contraction of the molten nugget when it solidifies. The large weld face reduces the force issue and moves the nugget toward the other side of the part to reduce contraction on the good side. We are talking about a law of physics. Material contracts when it solidifies and could still show on the good side. Frequently this means that you are walking a tight rope between visible indentation and an acceptable weld.

Backup bars are common in series welding.

Reference articles: WHAT IS A COSMETIC WELD?
                              WHY DO I HAVE EXCESSIVE WELD INDENTATION?
                              WHAT IS A SERIES WELD?

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