One of the simplest welding joints is the butt joint. It is not the strongest, but it is one of the most useful especially for automotive body work. This type joint is used whenever you butt 2 pieces together and then weld between where the two meet. Butt welding thin sheet metal can be complicated because thinner metal has a tendency to burn through on the edges. This doesn't mean it's impossible, just that there are techniques that can be used to minimize these issues.
Because the edge of a piece of metal absorbs heat faster than a solid surface, you need to modify your technique with the electrode. Whether you are using wire feed, stick or TIG welding, you need to move the electrode quickly and dance around the weld area, to avoid burn through. With the stick welding technique, this can be done using a stitch welder, which moves the electrode in and out like a sewing machine needle when you pull the trigger.
When done properly, a butt joint should show bead on both sides of the metal. One way to help insure this is to clamp the 2 pieces with a uniform gap between them. The Eastwood Butt Weld Clamp and Backer Set holds sheet metal slightly apart for better weld penetration and also helps hold the work tight to prevent warpage from the heat. These clamps also help to prevent crawl, which occurs when the metal tries to move away from the heat of the work area.
Even with clamps, the first step in a butt joint is to tack weld along the entire length of the joint. Start with a weld every few inches, at a uniform distance, then go back and fill in with more tack welds between the first set. Before moving on to the final bead you should have welds about an inch apart along the entire joint. Even with this technique, there will be some distortion that needs to be hammered out afterwards, but this will help minimize it.
Some welders prefer to use a weaving/zig zag or circular technique with thinner metals. This leaves you with a wider bead than you need, but it helps to spread out the heat of the weld to minimize burn through and warping. Before doing butt welds on something important, practice different angles with the electrode, rate of travel of your welding and length of your arc until you are comfortable with the thickness you need to weld, and establish your technique to avoid burning through it.
The picture above shows 4 different welds in cross section that you are likely to see when making TIG welded butt joints. Figure B shows correct technique, while examples A, C and D have various issues.
A: A common mistake beginners make is to pile too much bead on the top side of the joint, in an attempt to keep from burning through. This can be because the weld was not hot enough or more likely because the electrode was not close enough to the surface for proper penetration. For a butt joint to be acceptable, the bead should envelope both edges on both sides of the work, so no trace of the original edge can be seen.
B: This is an example of the correct penetration of a butt joint. There is bead showing on both sides of the work, with the lower bead being slightly smaller than the upper weld.
C: When you have too much penetration, the weld will begin to show undercutting and take this shape. You can see how the bead has begun to sag through the joint and not fill the top side fully. Undercutting is where the thickness of the weld is actually less than the work being welded, which means a weak joint.
D: This shows even more penetration and undercutting; the top bead has taken an almost concave shape. This is an even worse example than the one directly above.
In addition to the common problems, there are also a few different ways you can prep the metal before it is welded. The following only really applies when welding metal 1/8" or thicker because any thinner and you will almost always burn through.
The first and most common is known as a square butt weld. This is done when two flat pieces are up against each other. this is used for thinner metals and TIG welding. If welding metal thicker than 3/16" this method should not be used because the weld will not penetrate far enough into the metal and will not be as strong.
The next type of but joint is known as a bevel or double V joint. This type of weld is a must when welding metal 1/4" - 3/4". When the edges of each piece are ground down it creates a valley or grove for the weld to sit in, this gives more surface area for the weld to bond the two panels.
The third type of butt joint is known as a double bevel or double 'V" and is the strongest type of butt joint. This type of joint is used in areas where weld strength is critical. This type of weld is usually used when welding metal thicker than 3/4" but can also be used on thinner metals if more strength is needed.
It is true that the strength of welds when doing butt joints to body panels is not as critical as when doing structural repairs. However, properly welded butt joints will make the repair look better with less grinding and body filler. A well done joint will also last much longer, while a poorly done repair may crack and ruin the paint and body work after just a few miles of driving. Because of this, it’s important to practice and get the underlying repairs correctly done before moving on to the next phase. Learn the proper butt joint technique, and you will use it on countless welding jobs.