Best practices, differences between aluminum and steel pipe welding
October 11, 2005
Steel and aluminum have distinctly different properties, and this is important if you're manufacturing aluminum piping systems. Learn the differences in welding steel and aluminum pipe and how they apply to the best practices you can use to weld aluminum pipe effectively.
|Aluminum piping is test-welded in a laboratory.|
A large number of fabricators manufacture carbon and stainless steel piping systems, and in general, the welding procedures and techniques for manufacturing them are well-known.
But few fabricators manufacture aluminum piping systems, and the manufacturing techniques aren't as widely known. In fact, the material differences between steel and aluminum make the fabrication practices for them decidedly different.
Steel pipe joints commonly are welded using an open-root joint geometry (see Figure 1). You can use gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) in short-circuit mode, or shielded metal arc welding (SMAW) to weld the root pass. You then can fill the joint using SMAW or GTAW, although you also can use GMAW in spray transfer mode if the pipe joint can be welded in the flat rotated (1GR) position. Carbon steel doesn't require backpurge inside the pipe, but stainless steel piping does require an argon backpurge.
Open-root Weld Preparation Commonly Used for Piping Joints in Steels
Consumable Insert Joint Design Used in Welding Stainless Steel and Other Piping
Often you can weld stainless steel and other piping systems by using a consumable insert, often called an EB insert. You weld the root pass by fusing the consumable insert, which melts and flows to form a root pass of good geometry.
Again, stainless steel and nickel piping require an internal backpurge. The only other disadvantage is that this technique requires a machined weld preparation.
Unfortunately, neither backpurging nor consumable inserts works well for producing X-ray-quality welds in aluminum piping. In fact, the technique of "pulling" a consumable insert doesn't work at all on aluminum. The reason is fairly simple to understand.
If you look atFigure 2, you can see that the weld joint area has a large surface where aluminum oxide can form. However, the only areas where the arc will strip the aluminum oxide from the surfaces to be welded are those within the tungsten electrode's line of sight, so to speak. So in many areas, the arc can't strip the oxide off the surface of the aluminum. Because of this, the consumable insert won't melt and flow out easily, and it can't be pulled.
You can weld aluminum pipe joints using an open-root technique, but it's more difficult to do than it is with steel. Aluminum's high thermal conductivity means that the weld pool is larger than it is in steel. The weld pool also is more fluid, so it's harder to control the molten pool. Open-root welding almost always is performed using GTAW. It's difficult to perform a root pass using GMAW, especially if the pipe isn't rotated.
Joint Geometry for Welds Made Using a Temporary or Permanent Backing
In some applications, you can weld aluminum pipe using either a permanent or temporary backing ring (see Figure 3). If the backing is permanent, it should be made from the same type of aluminum as the pipe. Temporary backings can be made of materials such as copper, stainless steel, or segmented ceramic tiles. You usually don't need to machine backing ring joints to prepare them for welding, which is an advantage.
A good way to make X-ray-quality pipe welds in aluminum is to use an extended-land joint geometry (see Figure 4). In this type of joint, machining is necessary for weld preparation, which is a disadvantage. However, aluminum is relatively soft, so the weld preparation can be machined using a wood router with a router bit.
Extended-land Pipe Weld Preparation
Then fit the two pieces of pipe together with no root gap and tack-weld them using GTAW and the appropriate filler wire (4043 or 5356 filler metal can be used to weld most pipe; this usually is extruded 6XXX alloy, so check the contract documents to determine the correct filler metal).
The welding technique is fairly simple. Use AC welding with a pure or zirconiated tungsten electrode. Use pure argon as a shielding gas. No internal backpurge is necessary, although it's used sometimes in critical
The most important thing to remember in welding the root pass is that most piping alloys are extruded 6XXX alloys, and these alloys are fairly crack-sensitive. If you try to weld the root by fusing the extended land without adding filler, the root pass will crack. Be sure to add filler to the weld as it is being made. If the weld cracks as it cools, you need to add more filler metal when welding.
You can weld the root pass this way either with the pipe fixed in position or rotated during welding. Once you complete the root pass, you can fill the remainder of the weld preparation using either GTAW or GMAW.
It might take a little time to get used to the idea of machining a weld preparation using a wood router. It probably will take some practice to develop the technique of welding a root pass using the extended-land technique. But once you do, you will have a new tool that can help you make good-quality aluminum pipe welds.
Frank G. Armao is group leader, nonferrous applications, at The Lincoln Electric Co., 22801 St. Clair Ave., Cleveland, OH 44117, 216-481-8100, fax 216-486-1751, Frank_Armao@lincolnelectric.com, www.lincolnelectric.com.