Yes, it’s true that sometimes cracks grow while being repaired. This is because welding locally heats and expands the metal, which causes very high stresses around the repair area. You can reduce these stresses by preheating the metal to be welded. If you do, remember not to preheat any of the heat-treatable alloys above 250 degrees F. It also helps to drill holes at the end of the crack. The round hole distributes the stresses more evenly and should keep the crack from growing. Is aluminum different? Yes, in one very important way. Remember that aluminum forms an oxide layer almost immediately upon exposure to air. The crack’s surface also has an oxide layer. If you try to repair the crack merely by welding over it, the oxide layer will keep the crack from mending, and it will still be there after welding. Therefore, to repair a crack in aluminum, you must first excavate the cracked area with a grinding wheel or burr and completely remove the crack. Only then can you repair weld it. Is it better to use GTAW or GMAW? Actually, either one can be used. However, remember that the major advantage of GMAW over GTAW is its relatively high speed. While this is a big advantage in production welding, it can actually be a disadvantage in doing fine work like repairing cracks. In general, you have more control over the puddle with GTAW. Additionally, because the heat source and the wire feed are independent with GTAW, you can use your torch to provide extra heat where needed without adding wire. I recommend using GTAW for repair whenever possible.