Why do weld areas on some parts look different than the rest of the part?How can such anomalies be minimized?

Welds can discolor for a couple of different reasons. First, the metallurgy of the welding wire is different than that of the alloy being welded. Since the finish produced in the anodizing process is somewhat dependent on the metallurgy, the metallurgical difference will show up as a shade difference. Secondly, during the welding process a significant amount of heat is built up around the weld. This heat buildup actually changes the temper of the aluminum immediately surrounding the weld bead. Since a temper change is really a metallurgical change, again this shows up after anodizing as a color difference. These areas are commonly called halos or ghosts.

There are a few things that can be done to minimize the color differences. First, excellent results may be produced by using welding wire alloy 5356. This alloy reportedly produces the best color or shade match when used to weld 6xxx series alloys. The second problem, concerning halos or ghosts, is a little more difficult to solve. Try using as little heat as possible to accomplish the job. This can be something of an art and is dependent on the individual doing the work. Another possibility is to put the aluminum that is being welded in contact with a chill block that will draw the heat away from the working area. One other note: Some people grind the weld bead smooth and mechanically finish the weld area in an attempt to avoid the color difference. This practice, however, will not help hide the discoloration.