Troubleshooting weld defects
- August 3, 2014
Welding errors can result in costly downtime and rework
When a weld defect appears, it's important for welding operators to have the knowledge they need to rectify the situation as quickly as possible. Faster troubleshooting leads to greater opportunities to add value to the welding operation through increased productivity and quality improvements.
Following are some common weld defects along with suggestions for quick troubleshooting.
Lack of fusion and cold lap
Cold lap and lack of fusion are terms often used interchangeably but they are slightly different and can happen independently of one another or in conjunction.
Lack of fusion is the result of the weld metal failing to fuse completely to the base metal or to the preceding weld bead in multi-pass applications. An incorrect welding gun angle and incorrect travel speeds are among the leading causes of this weld discontinuity. Correct the problem by maintaining a 0° to 15° gun angle during welding and keeping the arc on the leading edge of the weld pool. To maintain the correct arc position, increasing travel speeds is sometimes necessary.
Insufficient heat can also cause lack of fusion and can be remedied by increasing voltage settings or wire feed speeds.
Like lack of fusion, incorrect travel speeds can cause cold lap, leading to the weld overfilling and essentially overlapping on the toes of the weld. When the weld pool absorbs most of the energy of the arc, as opposed to the base material, cold lap is more likely to happen. Increasing travel speeds can help the weld fill in more smoothly and prevent this problem.
This is the result of the weld metal penetrating completely through the base material. Burn through is especially common when welding thin materials, typically less than 1/8 in. or around 12 gauge. Excessive heat is the main cause of burn through. Lower the voltage or wire feed speed to help rectify this weld discontinuity. Also consider increasing travel speeds, particularly when welding on thin aluminum, which is especially prone to heat buildup.
Excessive spatter can appear when using a variety of different filler metals, including solid wires and flux-cored wires, particularly the self-shielded variety, as well as stick electrodes. This happens when the weld puddle expels molten metal and scatters it along the weld bead, resulting in molten metal that fuses to the base. Typically, spatter needs to be removed mechanically, by scrapping or grinding.
Insufficient shielding gas and/or dirty base materials, as well as too high of voltage, too high of travel speeds and too long of wire or stickout can all cause excessive spatter. Ensuring proper shielding gas flow, cleaning base materials thoroughly, lowering the given weld parameter settings and using a shorter stickout can help.
When a gas mixer does not feed a consistent gas blend or has been adjusted incorrectly, it could change the arc characteristics drastically, resulting in increased spatter.
For example, if an application requires a conventional 75 per cent argon/25 per cent CO2 shielding gas blend and this weld discontinuity develops, it is possible that the mixer is delivering too much CO2, which tends to generate higher levels of spatter. Check the mixer and adjust accordingly.
For self-shielded flux-cored wires, be certain to weld with straight polarity (electrode negative) and use a drag technique when welding to minimize the opportunity for spatter buildup.
While using flux-cored or metal-cored wires, too low of voltage could produce an excessive amount of spatter.
The wrong size contact tip, a worn contact tip or the wrong contact-tip-to-nozzle recess can also cause excessive spatter and should be addressed accordingly.
Concave and convex weld beads
Regardless of the welding process, the goal is to create a smooth, flat weld bead. Welds that are too concave or too convex can compromise the integrity of the finished product.
Concave weld beads are particularly prevalent when welding in vertical down applications and are simply the result of working against gravity. It is difficult to keep the weld pool in the joint in this position, so the weld tends to be thinner at the throat. A good remedy is to adjust the welding parameters to a lower setting so the weld pool is less fluid and more able to fill in the joint. If a concave weld bead appears in the flat or horizontal position, it is typically the result of too high of voltage, too slow of wire feed speed or too fast of travel speed. Reduce these factors accordingly.
Convex weld beads present themselves as high, ropey-looking welds, and generally happen in flat and horizontal welding. They are the result of too cold of weld parameters for a given material thickness. Increase the voltage accordingly. Using the wrong shielding gas for a given wire can also cause a convex weld bead. For example, if an application calls for a mixed argon shielding gas, but the welding operator uses 100 per cent argon, the weld pool will not be fluid enough, preventing the weld from penetrating the joint; it will build up on top of it instead. Always follow the recommended welding procedure. Be sure to use the correct polarity for the given wire, as well, to prevent this discontinuity.
Crater cracks are small cracks that develop at the end of the weld. They can happen during any welding process and are sometimes called shrink cracks. This occurs when the weld crater has not been fully filled when completing a pass; the weld pool solidifies and the center of the crater pulls from the center of the weld bead. This is especially common in welding aluminum.
To prevent crater cracks, pause at the end of the weld to ensure enough filler metal has filled the weld pool or "back step" to fill it, moving back from the end of the weld, then forward. Some welding operators choose to solve crater cracks by pausing and welding a "curlicue" at the end of a pass. A "double trigger" technique also works. Release the trigger (in a MIG welding application), pull it again to start the weld and then release the trigger again to fill the crater.
There are also crater functions on some wire feeders that can remedy this problem. To activate this function, the welding operator releases the trigger and the system automatically fills the crater.
In some applications, particularly those using submerged arc welding, run-off tabs are a common way to prevent crater cracks. These are metal tabs that the welding operator tacks to the end of the joint and welds over, thereby eliminating any opportunity for a crater to form.
While not an exhaustive list of the weld defects that can develop, those discussed here are common across many types of materials and welding processes. To minimize costs and labour for rework, as well as downtime spent addressing weld issues, take a systematic approach for troubleshooting each one, should they appear. Look at any variables that have changed during the course of welding during a shift or from shift to shift (e.g. welding parameters or technique) and consider their potential impact on the problem. Then consider these tips as potential remedies.
As with any portion of the welding process, should a problem persist, consult with a trusted welding distributor, or filler metal or welding equipment manufacture for assistance.SMT
Dave Almy is a welding engineer with Miller Electric Mfg. Co., Appleton, WI