November 11, 2008
Understanding common gas tungsten arc weld (GTAW) joints, knowing when to use them, and mastering the proper method for each can help you achieve better welds. This article focuses on butt, corner, and T joints and discusses considerations such as material type and thickness.
The three most common types of gas tungsten arc weld (GTAW) joints— butt, corner, and T (fillet)—each serves a particular function in product engineering and design that other joints cannot fill. Likewise, each type calls for specific considerations to be successfully gas tungsten arc welded, including an understanding of why to use them and the best welding methods for each.
You can configure all three joint types with mild steel, stainless steel, and aluminum, although each type of material demands its own special precautions. Before welding on any joint of any material, be sure to clean and prepare the material properly.
Also, for each joint type, position the GTAW torch at a 70-degree angle to the seam of the joint, with the filler metal at a 20-degree angle to the joint (Figure 1). Regardless of the joint type, use a high-frequency start for DC work on mild steel and stainless steel, and use continuous high frequency for AC aluminum applications.
Commonly used for pressure vessels, piping, tanks, and other applications that require a smooth weld face, butt joints extend the length or width of a part by connecting the edges of two pieces of material in the same plane (Figure 2).
Butt joints on thick material require a bevel or a gap between the edges of the material, but you can make a full-penetration weld on tightly fitted joints when the material is 1/8 in. thick or less.
To achieve even penetration on both pieces of material (if they are the same thickness), angle the torch at 90 degrees to the plane of the workpiece (Figure 3). For materials of different thicknesses, angle the torch slightly toward the thicker piece.
Too much heat can cause distortion and residual stress in carbon and stainless steel welds, so butt joints on thin material should be tack welded or skip welded—weld an inch, move down several inches and weld another inch, until you reach the end of the joint. Then go back and repeat welding between the existing welds.
Because of its heat dissipation properties, aluminum does not distort as much as stainless steel, so the tacks can be placed farther apart at approximately 3 in.
T joints (Figure 4) consist of two pieces of material connecting at right angles to form a T shape. These joints require a fillet weld and are common in many fabrication and construction applications, such as structural steel, tubing, and equipment fabrication. A T Joint in a tubing application requires a curved fillet weld as the connecting tube contours to the curve of the cross-member of the T.
Although T joints can yield very strong welds, you must place the weld on the same side of the joint where force against the weld will be applied (Figure 5). Pressure from the opposite side of the joint can create weakness and cause the weld to break. Weld both sides of the joint to achieve maximum weld strength, or when pressure is applied from both sides.
Because the vertical piece of material with its edge at the joint melts faster than the flat piece, the torch angle in a T joint needs to focus more of the heat on the flat piece to avoid undercutting on the vertical piece. The torch should be at a 35- to 40-degree angle from the vertical piece, and the filler metal should be at a 20-to 30-degree angle from the flat piece (Figure 6).
With T joints, the base material often prevents the torch cup and tungsten from getting close enough to the weld joint. If this occurs, extend the tungsten from the edge of the cup to get the tungsten to roughly its own diameter away from the center of the weld joint.
A flat bead profile provides optimal strength and weld efficiency in T joints. Use a filler metal that is one size larger than normal to fill the space between the two members and achieve a flat profile. Avoid convex and concave weldments. Convex welds are overwelded, which costs more, whereas concave welds can produce weaker weld joints.
A corner joint consists of two pieces of material joined at their edges to form an L shape. When the edge of one piece lies flush against the edge of the other piece, it is called a closed corner joint. When two edges meet at their corners and there is an opening where the thicknesses of the members are exposed, that is an open corner joint (Figure 7).
You usually can fusion weld a closed corner joint without adding filler metal. Simply tack the joint at its edges and melt the two pieces together from one end to the other.
An open corner weld always requires filler metal. Common in furniture and other cosmetic applications, open corner welds demand very precise fixturing because of the shallow joint depth and the ease with which the edge of the material melts.
You should make a convex bead profile in open corner joints so that the throat of the weld is at least the thickness of the base material. Also, maintain a fast travel speed on thin material to avoid melt-through to the inside corner of the material.
The torch in an open corner weld should bisect the angle formed by the two pieces of material, so that it applies an equal amount of heat to each piece. If the two pieces are different thicknesses, it may be necessary to angle the torch slightly toward the thicker piece.
Stainless steel open corner joints are particularly susceptible to problems, such as warping. You might need to place tack welds every 2 in. or so, depending on the material thickness, to maintain a consistent joint. In many cases, you also might need to clamp the material in place.
Corner, butt, and T Joints share many commonalities, but also have many differences that require consideration. Understanding their unique applications and the proper techniques for each will provide a solid foundation for successful GTAW.