Backing material used to retain molten metal or to assure complete fusion. Successful welding of various materials often depends on the type of backup bar or plate that is used. The high heat conductivity of a copper backup bar or plate, for example, will prevent it from sticking to the weld metal, while its chill-mold effect will assure a clean, smooth weld metal surface.

Electrolytic copper has proven to be the most satisfactory material for backing up a weld.

Copper backup bars are usually made by cutting pieces from copper plate or sheet. Electrolytic copper in cold rolled bars and plates is available in a variety of sizes for these applications. While these copper pieces give more satisfactory results than other backing materials, they must be made carefully to provide accurate dimensions and good surfaces.

Figure B-4 illustrates the use of a copper backup bar to obtain a full penetration weld in heavy plate.