Flange Joints

When very thin plates, foils or diaphragms need to be welded, it is often unfeasible to use either a butt joint  or even a lap joint, on the grounds that even a minimal amount of power would burn through the pieces. In this  case, a flange joint is employed. To create a flange joint, the edges to be welded together are first bent upright for so that when the parts are places together, the faces of the flange are touching and the joint is upright and properly aligned. The laser or electron beam (Low energy density devices are rarely used for this  operation) is then run over the joint, melting a relatively large portion of it, making the weld.

Although this joint type works very well in some applications, there are strict limitations regarding its use. The first  of these is that the joint must be under minimal stress.  If excessive tensile force is applied, the bent portion  will deform, placing a tremendous twisting force on the weld. This will, either suddenly or over time, result  in the weld failing. Another limitation is that the method only works well with relatively thin plates, as it is not possibly to add enough energy to the edges of thick plates to melt the entire top edge. Finally, the parts must be approximately the same thickness, or one will melt before the other and not form a proper weld bead.

The variations of the flange joint are differentiated by the way that the parts are bent before welding, as well as the thickness of the plates. The weld itself is approximately the same for all of them. To form the most common flange joint, both parts are flanged before they are welded. This results in the two parts lying in the same plane. In another variation only one of the parts is flanged, resulting in the parts being perpendicular to each other.

A third type of weld, known as an edge joint, is used when the parts to be joined are thicker than about .100 inches. A common example is found in assemblies that recess a cover inside a container so that the edge of the container extends beyond the recessed cover. Care should be taken in fabrication and tolerance control to assure that the cover and container are in close contact with minimum edge extension. This ensures that minimum weld melts are produced and avoids excessive weld melt roll over that often results in the necessity of removing the excess in a subsequent finishing operation. This can be a costly operation. Corners of these assemblies are frequently poorly matched due to radii tolerances resulting in lack of fusion and melt back in these areas. This condition is especially critical if parts are to be machine welded.