The filler designation doesn’t tell you anything about the strength of the filler or of the weld. Aluminum fillers are different from carbon steel fillers. Most carbon steels have a fairly similar chemistry, so you pick the filler that will give you a strength level in the weld that is as strong as the carbon steel. However, the chemistries of aluminum alloys often are very different from each other, and in most cases you can’t match the strength level of the parent material in the weld. Aluminum filler alloys are designed to have chemistries that will have the lowest crack sensitivity when used with a specific aluminum material. Consult a filler metal selection chart, such as the one contained in AWS D1.2, “Structural Welding Code—Aluminum,” to find out what the best filler is for the specific alloy you are welding. The applicable fabrication code will then tell you what the minimum required strength is for the base/filler combination you are using. What determines the tensile strength of the filler wire and the weld? The biggest factor in determining weld strength is the chemistry of the base and filler metals. Of course, other factors come into play as well. What determines the strength of the wire? Again, chemistry is a big factor, but the process of how the wire is drawn to size is a bigger factor. It starts as an annealed rod about 3/8 in. dia. and is drawn through a series of dies to reduce its diameter. Each drawing step cold-works the wire and makes it stronger. After a few dies, the wire is so strong and brittle that it will snap if it is drawn anymore. So it is annealed and then drawn again. This process is repeated until the wire reaches the desired diameter. At that point, the major factor determining the strength of the wire is how much cold-working it has received since the last annealing. That’s why we don’t report the strength of filler wires—because you can get almost any value you want, within limits, by adjusting the drawing and annealing schedule.