There are a number of reasons your aluminum alloy wheels may crack. One of the most common is “hot cracking,” which is the result of high-temperatures during the welding process. Also called hot fissuring, solidification cracking, hot shortness and liquation cracking, it’s ugly and annoying, but it can be avoided. When arc welding is employed, cracking can be eliminated. Aluminum welding gets you started on the wrong foot from the beginning—even if you avoid hot cracking, hydrogen cracking (aka cold cracking) might show up.
Cold cracking can happen when welding low alloy steels and carbon steels. Depending on the type of aluminum you’re using, you might become the victim of cold or hot cracking and, either way, you don’t want to be on the receiving end of this conundrum. When dealing with hot cracking, you need to consider three major issues: The alloy chemistry, appropriateness of the filler alloy, and choosing the best joint decision.
Picking the Right Alloy Additions
To sidestep hot cracking, consider four potential alloy additions: Silicon, copper, magnesium and magnesium silicide. By including trace amounts of one of these elements, which is common, cracking is more likely to occur. The “chemistry” of the alloy, which is dependent upon which additions are used and how much, can be a major factor in cracking. Silicon is the most crack-resistant, followed by a magnesium/silicone hybrid, but all fillers can be crack-resistant if used in the right amount.
To get the best results and resist cracking, it’s best to completely avoid very crack-sensitive materials that are also considered non-weldable. Picking the right filler or addition, while considered the crack sensitivity ranges, is step two. You should pinpoint a filler with a solidification point that’s similar to (or below) that of the base material. Edge preparation choice, root gap to allow for the filler addition, and the creation of a “weld metal chemistry” that’s suitable is also paramount.
Preparing for a Crack-Free Future
Choosing the right filler is just part of the process; you also need to choose one that has additions of grain refiners (i.e. zirconium or titanium). This will best prevent cracks, and complements the makeup of the materials. The actual welding process itself can also be a literal breaking point—utilize the highest welding speed because the faster it’s done, the faster the cooling rate. Basically, the least amount of time you spend in the “hot cracking temperature spectrum,” the better.
When welding, tap into your skill set and use assembling techniques that cut down on restraint, minimize stress and product the cleanest welds possible. A pressure application on the joint during welding can also help to prevent cracks from forming. Generally speaking, sometimes aluminum base alloys are difficult to weld at best—especially for those who aren’t exceptionally well-versed in the properties of the materials. Sometimes arc welding just isn’t possible, and that’s when mechanically joining parts via bolting or riveting is the best approach. No matter which avenue is taken, remember that cracking isn’t “just something you have to live with.” It can be addressed, but it takes knowing the materials and some serious skills to avoid it.