A lap joint is formed by overlapping two plates and welding them either in the joint where they meet, as is done in GTAW and PAW, or through the top plate and into the bottom plate. This is usually done using either an electron beam or laser welder. Lap joints can be used to weld pieces of dissimilar thicknesses and materials. Lap joints also greatly reduce the number of critical parameters in the weld. Unlike a butt weld, which performs a similar function, the lap joint does not require that the cut faces be perfectly flat and parallel. Rather, in a lap joint, the only critical surfaces are the faces of the parts where they overlap, and the tolerances on this overlap are fairly high.
Lap joints have several advantages over many other types of joints. The first of these, as mentioned, is that the technique reduces the number of critical weld parameters. This makes the joint easer to prepare and increases the likely hood of a successful weld. A second advantage of the lap joint over the butt joint is that it easily accommodates materials of different thicknesses, although the thinner piece should be welded on top. A third advantage is that the technique can be used to weld thin materials such as foils and diaphragms.
One disadvantage of lap joints is that they tend to have a much lower tensile strength than butt joints, as the total effective area of the weld is lower. Another related effect is that the welds tend to be much less stiff than their counterparts, as the thin weld tends to act as a pivot point. This effect can be greatly reduced by welding both of the joints between the two plates. A third disadvantage of lap joints is that they provide no self-alignment. This can require the use of more complicated and expensive welding fixture to ensure that the parts are properly aligned before welding. Finally, there are situations in which creating the necessary overlap may be undesirable for aesthetic or mechanical reasons.
There are two major variations of lap joints. The first of these involves placing a weld bead along one or both joints between the two parts. This process is usually used in low energy density welding processes such as GTAW and PAW, although it can be used with the laser or electron beam. This process tends to be stronger than the alternative, but requires that the seam be precisely followed. This makes it a popular joint for welding by hand.
The second variation of the lap joint is used almost exclusively with high energy density systems, such as the laser or electron beam. In this process, the plates are placed on top of one another and the beam is aimed at the top plate. Soon the beam melts through the top plate and part of the bottom plate. This forms the weld bead. One benefit of this process is that the beam path is far less critical, as it only has to follow the overlapping region, rather than precisely follow a joint. Another benefit is that although the welds tend to be weaker on an individual bases than those made with the former lap joint process, as many welds as necessary can be made in the overlapping region. This can potential bring the strength of the weld into a region comparable to the base metal.
When using either welding method, the plate can be bent before welding so that the two plates lie in the same plane when the weld is complete. This may be necessary when one side of the joint must be relatively smooth, either for mechanical or cosmetic reasons. This process can also provide some measure of self-alignment to the weld, reducing the complexity of the fixturing.