Butt Joint

One of the most common types of weld joints in manufacturing is the butt joint. This joint is formed when the two pieces to be welded are simply placed face to face and the welding head run over it. In the case of GTAW  and PAW this joint can only be used on very thin pieces without extensive preparation and the use of filler  wire. Both the laser and the electron beam, on the other hand, can butt weld very thick pieces, up to 30 cm for the electron beam. This is accomplished by using the keyhole method, in which the beam is used to bore a path through the piece for itself, allowing it to distribute energy evenly across the joint, regardless of its depth. This makes the electron beam, and to a lesser extent the laser, the ideal welding system for many jobs.

The butt joint has many advantages over other types of  joint. The first of these is that it results in a uniform  surface, which allows them to be used in places where fit or appearance is extremely important. A second advantage is strength. Due to the fact that the area of the weld is nearly the same as that of the pieces being welded, the tensile strength can be comparable to that of the base metal. The third advantage of the butt joint is simplicity to set up and weld. Unlike some of the other joints, which require complicated geometry, such as flanges, to work, the butt joint only requires a smooth interface.

There are also disadvantages to the butt joint.  The first of these is that they are especially sensitive to weld  defects. Because all of the forces on the joint are absorbed by the weld, defects such as porosity, inclusions, cracks, etc, can cause easily cause the joint to fail catastrophically.  A second disadvantage of the butt weld is that it is usually not self-aligning. Whereas some other joint types will hold together before welding, the butt joint will not. In many cases this greatly increases the complexity of the fixturing necessary to hold the pieces to be welded in place before and during the welding processes. A third disadvantage of the butt weld is that it is nearly impossible to butt weld very thin materials, due to the fact that aligning the faces properly is very difficult.

There are three major types of butt weld. The simplest of these involves simply butting two smooth faces together and welding down the joint. This is quick and easy and requires little in the way of joint preparation. However, it is susceptible to all of the disadvantages mentioned above.

The first variation on the standard butt weld involves matching notches in the two pieces. This is relatively easy to machine and has two major advantages over the standard butt weld. First, the corresponding cuts provide some self-alignment of the joint, reducing fixturing and potentially increasingaccuracy. Second, the lip that is created prevents drop through, where surface tension can no longer support the molten weld and it falls through the bottom of the joint. There are also disadvantages to this method, primarily that it reduces the weld area an therefore strength, and that it can increase the residual stress due to the less even heat application.

The second variation of the butt weld involves adding a flange to the bottom of the parts, rather than machining it off as is the case with the first alteration. This provides similar advantages to those of the first alteration, but without decreasing the area of the weld and with less potential for residual stress. It can, however, be harder and more expensive to make this lip than the simple cuts required for the first alteration.