We fabricate railings (hand rails) that are primarily steel. However, sometimes the jobs call for aluminum railing. The railing can consist of just about any structural shape from schedule 40 pipe to rectangular tubing to bar and angle. The alloy is usually 6061 but occasionally we use 6063. Because this is structural railing the welds have to be strong enough to meet the specs. Nearly all of the aluminum rail is sent out for anodizing. Our problem is that after anodizing some of the parts show a dark area or a multi-colored halo around the welds. The discoloration ranges from gray to brown to black. Sometimes some of it can be wiped off, but it still doesn’t look quite right. We suspect this condition is either a welding issue or an anodizing problem or perhaps a little of both. Our anodizer says it’s a welding problem. What do you think? T.F.
Simply put, your anodizer is correct. This is not an anodizing problem, it is a welding problem. Virtually all aluminum railing fabricators have had to deal with it at one time or another. It’s largely a matter of perfecting a welding technique that works, but that’s not quite all there is to it. You didn’t mention the weld filler (welding wire) itself, so I presume that you have dealt with that issue and solved it. Nevertheless, I’ll address it briefly for the benefit of the other readers.
I think it’s safe to say that the most common filler wire used for welding structural aluminum is 4043. This works only if the parts are not to be anodized. Alloy 4043 is great welding wire and what makes it great (easy to weld and strong) is the high level of silicon in that alloy. Since silicon doesn’t anodize and is not homogenous with aluminum, the result when it is used to weld parts that are to be anodized is a black, powdery weld that is tough to deal with aesthetically.
The most common alloy filler wire used when the welds are both “architectural” and structural is probably 5356. I think that for 6061 alloy, a 5154 alloy wire works well, also. About the only difference in these two wires is that 5154 has slightly more chromium and a little less magnesium than 5356.
The secret is to produce the weld strength required but without getting the welded railing members too hot during welding. At some given temperature, the chemistry of the extruded members changes and the alloying elements, namely silicon, magnesium and chrome, come out of solid solution and precipitate on, or near, the surface of the extrusion. This is what causes the various color halo effects. You cannot see it until after the railings are anodized. As you say, sometimes it can be wiped off after anodizing, but it rarely looks unblemished.
The best way to solve this problem is to set up testing of different welding techniques using several different samples that are welded with various “heats” and speeds to finally arrive at the correct formula. The correct formula will achieve adequate structural strength to meet the specs without getting the metal hot enough to change the temper. Keep track of exactly what is done to each sample and carefully mark the samples so they can be identified after being anodized in the same batch. It’s exacting work, but obviously well worth the time and effort.
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