APRIL 2015 / WELDING JOURNAL 43 thermal conductivity, it requires using larger wire diameters and power sources than those used for welding steel sections of the same thickness (see lead photo). In shop terms, fabricators should, “get on an aluminum part, form the pool, make the weld, then get off it as quickly as possible to prevent warping.” The authors, as part of their technical service roles, have advised hundreds of fabricators needing assistance in aluminum filler metal selection and processes. This article presents some of the interesting problems and how they were solved. Colorful Oven Cleaner Tips One caller needed help repairing a hydraulic box attached to a collapsible, temporary bridge that he believed was made from aluminum. After welding, he said the welds promptly cracked. We suspected the cracking was not caused by using the wrong filler metal, but that he had incorrectly identified the base material. Let’s note right away that to positively identify the base metal, it must be sent to a metallurgical lab for analysis with a mass spectrometer. With that caveat, here’s a quick method for detecting certain alloying elements. Spray the metal with Easy-Off® oven cleaner in the yellow can (not the blue can). The sodium hydroxide (lye) in this cleaner etches the aluminum. Because the various grades of aluminum use different alloying elements, they anodize and corrode at different rates to produce the different colors — Fig. 1. The following colors most likely indicate the following base materials: • Gold indicates the presence of copper: 2xxx series • Black (or gray) indicates a silicon alloy: 4xxx series • Whitish indicates the presence of magnesium: 5xxx series • Bluish indicates magnesiumsilicon alloys: 6xxx series. When this caller sprayed the box with oven cleaner the metal did not display any of the above colors. We concluded that the box he thought was made from aluminum was probably made from magnesium, because it provided the low weight and highstrength properties required for the application. We recommended he work with a lab to conduct a proper metallurgical analysis before proceeding. The oven cleaner technique can also help with repairs, such as on semitrailer landing gear, which are typically welded with a 5xxx series filler alloy. Since these landing gear often break, the repair technician should first determine whether the gear has been repaired, and whether the correct filler material was used. To make the test, first degrease the weld area with a solvent such as Zero Tri® using a stainless steel brush dedicated to cleaning aluminum. Then spray the weld area with the oven cleaner. If part of the weld area becomes frosty white (5xxx) and another area becomes black (4xxx), it indicates the weld was incorrectly repaired using mixed alloys. Before repairing this joint, it is necessary to remove all of the mismatched materials either with a grinding wheel designed for aluminum or a rotary tool fitted with a carbide bit. What Filler Works with 6061-T6, etc.? As with selecting filler for any grade of aluminum, the choice depends on the application requirements, welding procedures, and such variables as ease of welding, joint strength, ductility, corrosion resistance, sustained service temperature, color matching, and postweld heat treatment. Following are six practical examples to illustrate some factors affecting filler material selection: 1. For a general application, such as welding a flange to a steam tube, 4043 would work. This grade can sustain service temperatures above 150°F and has good welder appeal. 2. However, note that 4047 also supports high service temperatures. Further, because it has higher silicon levels, it provides better leak-tight capabilities, helps reduce cracking, and supports faster welding speeds. 3. To weld an engine bracket on a radiator, the magnesium in 5554 would provide higher strength and since the Mg level is below 3%, it is not affected by high heat. 4. For forming after welding, 5356 will have better ductility in forming operations. 5. To color match a hand railing that will be clear coat anodized after welding, 5356 would come closest to the bright silver appearance. 6. For a safety-critical weldment that will undergo postweld heat treatment and returns the entire structure to the T6 temper, use, for example, 4643 or one of the other heattreatable filler alloys. Rather than simply selecting a filler material from a catalog, the above examples should encourage fabricators to discuss their application with the filler material supplier or other reliable resource. To demonstrate why this is important, the authors often hear fabricators requesting a 6xxx series filler to weld 6061-T6. Their rationale is that they “know” the filler needs to “match” the base material. The following section expands on this topic. Why Do Corner Welds Crack? While outside corners on some grades of aluminum can be GTA welded autogenously (without filler), 6xxx Fig. 1 — The image on the left shows the black/gray color of anodized 4XXX series, and the image on right shows the whitish color of anodized 5XXX series.
Welding Journal | April 2015
To see the actual publication please follow the link above