You can’t make an omelet without breaking eggs, and in the same vein, you can’t make a weld without melting metal. The problem with melting metal though is that it needs heat. That causes expansion and changes the material properties. Stitch welding is a way of putting less heat into the workpiece, but it does carry a cost.
Thermal effects of welding
Expansion may be the biggest challenge the welder faces. Components being welded grow as they are fused together. Then as they cool they contract, but because they are now joined the fabrication distorts.
Welding also creates a heat affected zone (HAZ) where material properties are changed. In the case of aluminum for example, the heat of welding removes all temper and takes the material more or less back to it’s initial properties. When welding steel there’s a risk of creating a less ductile region more prone to cracking.
Welding thin sheet is another situation where excess heat is a problem. In this case the risk is of burning through and creating a hole in the material.
This consists of a short weld followed by a gap. For example, along a joint the pattern might be 2” of weld, followed by 2” of gap, repeating for the whole length being joined. This can be done for both filet (corner) and butt (flat) welds. It’s sometimes referred to as intermittent welding, and less often as skip welding.
The main benefit of stitch welding like this is that it puts half as much heat into the workpiece. That reduces distortion and makes for a smaller HAZ. It also means less filler metal is used, which saves money, and the weld is usually completed in less time than if it was continuous. Some authorities also argue that stitch welding prevents a crack from propagating along the join.
The alternative to stitching is a continuous or seam weld. Here the welder strikes an arc at one end of the joint and maintains the weld pool as he traverses the full length. Seam welding can be done for filets of flat joints and is often seen where pipe or tube is welded.
Seam welding will cause more distortion, simply because more heat goes into the fabrication. For the same reason, the HAZ will be larger. It also consumes more filler and will usually take longer than stitching. However, a seam welded joint is inherently stronger than one stitch welded simply because more of the metal is fused. This also results in a more rigid fabrication. Possibly the biggest argument for seam welding though is when fabricating tanks. Stitch weld these and they’ll leak!
A design decision
Standard practice is to put welding instructions on the fabrication drawing. This tells the welder, among other things, whether to use stitch or seam welding. The designer bases this on the strength required in the joint, plus other aspects of the design such as the need to hold fluids. Stitch welding is the default choice in most applications, because it minimizes distortion and saves a little money. Every metal fabrication job is different though, and that’s why the best designers have welding experience or consult with welders about the best route.
It’s application specific
Welding produces the strongest, longest-lasting joints in a metal fabrication, but it can’t be done without putting heat into the metal pieces being joined. That heat causes distortion and alters material properties in the HAZ. Fabrication designers understand these issues and specify joints accordingly. If stitch welding is feasible, that’s usually the way to go, but there are times when seam welding yields a better result.