Aluminum Workshop: Backing bar weld for large-diameter pipe

Q: I need to fabricate many pieces of aluminum pipe from plate. The pipe will be 16 in. in diameter and 8 ft. long with 0.25-in. wall thickness. I want to weld the seam in one pass from the outside, but I don’t want to use any backing material and I don’t want to do anything to the root of the weld on the inside of the pipe. What should the joint geometry be?

A: What you are proposing is essentially an open-root weld. Unfortunately, I must tell you that this is extremely difficult to do on aluminum.

Backer Bar Welding

A far better approach is to use some sort of temporary backing material. This type of application commonly is welded in a weld seamer or other hard-automation fixture using copper or stainless steel backing. If you do this, be sure the backing bar has a groove in it to allow you to get a good back bead. You can also use a ceramic backing tape without a fixture. These tapes usually have the groove already incorporated into the ceramic.

You should be able to completely penetrate a ¼-in. wall using a square butt preparation. Use a 1/16-in.-dia. filler wire of the proper alloy for the plate being used. For pipe with a thicker wall, you will need to make a V preparation, a 1/8-in. land, and a 60-degree included groove angle. Do not use a root gap, You may need to deposit multiple passes depending on the wall thickness.

What about the back bead? Will it be acceptable as-welded? It depends. In many applications it will be, but it will probably contain several irregularities. While these are not defects per se, they may be deemed as unacceptable indications if the weld seam is radiographed.

Performing a GTAW wash pass is a common practice to smooth the back bead of the weld. A GTAW arc fuses the back bead and smoothes out irregularities. If the pipe diameter is large, you can perform the wash pass inside the pipe. You can use an automated track system or a simple wheeled carriage to hold the GTAW torch inside smaller pipe diameters.