by Rob Krause
Good preparation for better weld performance
Appropriately preparing aluminum before welding can be done for one of two reasons. The first is to incorporate a good pre-weld cleaning procedure into standard production practices to prevent potential weld quality issues like porosity and lack of fusion. The second is to use the pre-weld cleaning practices to troubleshoot issues if they occur.
Plan for a successful weld through good preparation.
Step 1: Degrease
In an ideal situation, first clean the aluminum with an industrial degreaser that does not contain chlorinated solvents. One of the more popular cleaners is ZeroTri, which comes in spray can and bulk containers. Since many shops prefer not to have aerosol cans littering their shop, they use refillable spray bottles instead. Non-chlorinated brake cleaners can be a good choice, and acetone also works. For the person at home, they could even use a mild solution of a good grease cutting dish soap and hot water. If using this option, be sure to rinse the material well using hot water so it evaporates quickly.
As for applying the degreaser, the best choice is a basic industrial roll of white paper towels, which should then be disposed of according to shop procedures (the wipes must be managed in closed, non-leaking, labeled containers, among other government rules). While shop rags are popular, they often serve multiple purposes and could accidentally introduce contamination onto the part. Sometimes new shop rags contain lanolin, and laundered shop rags can contain detergent residue.
A few other finer points about degreasing and cleaning in general. Always degrease before tacking, otherwise the solvent (and contamination) could become trapped between the faying surfaces of the weld joint. Also, if you need to bevel the edge of the material, the preferred method is to use a high speed carbide bit and a process that doesn’t require lubricant. If the process requires lubricant, follow the degreasing steps described above.
Step 2: Remove Aluminum Oxide
When exposed to air, aluminum instantly begins forming a layer of aluminum oxide. Where aluminum melts at about 1,200°F (the exact temperature varies depending on the alloy), aluminum oxide melts at about 3,600°F.
On clean aluminum, the natural forming oxide layer is self-limiting in thickness; it’s measured in angstroms, a unit primarily used to measure the wave length of light. In short, the layer is really thin. In the perfect scenario, you would want to mechanically remove (or break up) the oxide layer prior to welding, just make sure the material has been cleaned first. However, the plasma created by the arc’s interaction with the shielding gas, coupled with the reverse flow of electrons from the base material to the electrode, will typically break up the layer enough to obtain good fusion between the base and filler materials. This is true whether you’re AC TIG welding or MIG welding, which requires DC reverse polarity.
On material with a hydrated oxide layer (aluminum with white stains on it), you must mechanically remove the oxide. Because the oxide melts at a temperature three times that of the base material, it causes problems with establishing (or maintaining) the weld puddle. Basically, by the time the arc cuts through the oxide layer, you’ll have lost control of the weld puddle (sometimes you can’t even establish an arc on really dirty material). Further, even if you can weld, the arc will break down the hydrated oxides and can create significant porosity in the weld.
In either case, if you’re going to remove the oxide, use a tool that results in shavings rather than dust. A couple examples are aggressive hand files (8 to 10 teeth per inch) and hand planers. Whatever the tool, just keep in mind that you are trying to “scrape” a thin layer from the material surface. Most grinding discs for aluminum will simply
smear the surface. For the same reason, stainless wire-wheels that
are grinder mounted should be
used only with light pressure. As an added bonus, shavings can be easily swept up and recycled.
As for size of the cleaning area, you need to clean only an area the size of the weld face (unless procedures specify otherwise, of course). And finally, do not use sandpaper or abrasive kitchen cleaning pads for removing oxides. Their binder chemicals could actually contaminate the surface or embed residue into the plate.
The question often asked is: “what if we only have time for one step, what would you recommend?” As long as you are working with properly stored material (e.g., no hydrated oxides), clean and degrease the material. That said, good preparation always promotes better weld performance. SMT
Rob Krause is technical services manager, AlcoTec Wire Corp. AlcoTec is an ESAB brand.