Backing

A material (base metal, weld metal, or granular material) placed at the root of a weld joint for the purpose of supporting molten weld metal.

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Backing | Definition of Backing
Backing | Definition of Backing at Dictionary.com
Backing Synonyms, Backing Antonyms | Thesaurus.com
Backing | Definition of Backing by Lexico
BACKING | meaning in the Cambridge English Dictionary