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Like any aspect of metal fabrication, TIG welding can cause difficulties for even the most experienced welders and certainly for novice operators. Still, these problems don't have to slow you down. Arming yourself with some good troubleshooting skills can help you quickly get back to work — and save valuable time and money.
Weld discontinuities (those imperfections that adversely affect the integrity or strength of a weld) are common problems that can occur during TIG welding. One such discontinuity is porosity, the formation of gas pockets in the weld metal. In many cases, porosity is visible with the naked eye in the form of small holes (Figure 1). In the instance of critical welds, X-Ray or other similar testing can identify it.
Porosity in a TIG weld occurs from one or more of the following factors: improper shielding gas coverage, the wrong filler metal or shielding gas, too much heat and/or a dirty base material. To remedy the problem, be certain all torch fittings are tightened and visually inspect hoses for cracks that could be causing shielding gas leaks. If you have trouble finding loose connections, apply soapy water to the hose and fittings until you find the leak, then tighten the fittings accordingly. Replace damaged hoses as needed.
Also remember a simple rule of thumb for shielding gas flow rate: use a range of 10 to 20 cubic feet per hour (CFH) to ensure proper coverage. Rates higher than this can lead to weld puddle turbulence, while lower rates will not protect the weld puddle from the atmosphere — both circumstances that can lead to porosity.
Another way to avoid porosity is to choose the right filler rod and shielding gas. Specifically, choose a filler rod with the proper designation for your base material. When in doubt, contact a filler metal manufacturer or welding distributor to confirm the best option. Use Argon shielding gas when TIG welding. Do not use CO2.
Stay within the 80 to 120 amperage range for 1/8 in to 3/16 in thick material, especially when welding on carbon steel. Too much heat on this material can cause porosity.
Finally, be certain that you have properly cleaned your base material. Aluminum especially is prone to porosity when dirt, oil, grease or moisture is present. Be certain to wipe aluminum with a clean cloth before welding and, if the surface appears dull gray, brush it with a stainless steel brush designated for that purpose to remove surface oxides. If you are TIG welding carbon steel, be certain to grind off any mill scale beforehand. Note that a wire brushing will not remove mill scale.
Two other common weld discontinuities are undercutting and lack of fusion. Undercutting occurs when a groove forms along the edges of the weld, and is not filled with weld metal (Figure 2). It is primarily caused by uneven distribution of heat on the joint being welded, which is in turn caused by an improper torch angle. If, for example, when TIG welding a T-joint you have 60 percent of the heat directed toward the top piece and only 40 percent toward the bottom piece, undercutting is likely to occur. Subsequently, the joint will be weak.
To correct undercutting, follow these steps: initiate the arc, establish a weld puddle and visually check to see that the weld puddle is equally spread across both sides of the joint you are TIG welding. Adjust your torch angle accordingly to ensure even heat distribution. Also, watch to see that the filler metal flows evenly into the weld puddle, as this is an indication that the heat is being distributed equally to both parts of the joint.
If the weld fails to fuse to the base material or to the preceding weld bead when you are TIG welding multiple passes, lack of fusion (also called incomplete fusion) results, as does a weak joint (Figure 3). Using too little amperage for a given thickness of material is a primary cause of lack of fusion. A good rule to follow is this: use 10 amps for every thousandth of an inch of material you are TIG welding.
Improper joint preparation can also cause lack of fusion, and there are two remedies. You can create a space in the joint or bevel the pieces forming the joint. For example, if you are TIG welding two pieces of ¼ in material, space the pieces approximately 1/8 in apart and fill in the gap with weld metal. Doing so allows for full joint penetration. Beveling the joint to a V-shape can also help prevent the problem because it minimizes the amount of filler metal needed to fill the given space.
If you are TIG welding thick aluminum (around 1/2 in), adding helium to your Argon shielding gas will create a hotter weld puddle, increase weld penetration and minimize the chances of lack of fusion.
Another common weld discontinuity, called excessive penetration, results when the weld metal melts or 'falls' through the weld joint (Figure 4). It is almost always the result of too slow of travel speed or excessive heat. The slower you weld on a joint, the more the heat builds up on the base material. When that heat builds up enough, it can eventually cause the base metal to melt through the joint.
To prevent excessive penetration, maintain a travel speed that creates a weld puddle approximately twice the diameter of the tungsten you are using. This may take some practice, but you can look for a few clues to help you along the way. First, if you see that the weld puddle is narrow, high and crowned it is an indication you are traveling too fast and/or the heat is too low. Conversely, a wide weld puddle (two and a half to three times the diameter of the tungsten) indicates you are TIG welding too slowly. Adjust your travel speed accordingly in both instances.
The most typical culprit of excessive penetration is too high of amperage for the given thickness of material. Remember your rules of amperage again: use 10 amps for every one thousandth of an inch of metal.
Remember, a bit of mindfulness and practice can help minimize the chances of any of these weld discontinuities from occurring at all. Still, everyone makes mistakes and having the knowledge to correct them can get you back to work sooner than later!
Jack Fulcer is the product and marketing manager for Weldcraft, 2741 N. Roemer Road, Appleton, WI 54911, 920-882-6800, Fax: 920-882-6840, www.weldcraft.com, customerservice@weldcraft.com.
author: By Jack Fulcer, Weldcraft