Alloy 356 and its variants (A356, B356, C356) is an aluminum-silicon-magnesium (AISiMg) alloy containing about 7 percent silicon and 0.35 percent magnesium. It is heat-treatable in the same manner as 6XXX wrought alloys and is most often supplied in the T6 temper. In fact, A356 is the most commonly used aluminum casting alloy. So how do you weld it? Welding A356-T6 is very similar to welding 6061-T6. For both alloys, the heat from welding over ages the heat-affected zone (HAZ) and reduces mechanical properties, regardless of what filler alloy you use. The correct filler metal choice depends on what, if any, heat treatment will be given to the casting after welding. Two situations are common. The first, and most common, is the 356 casting is part of an existing component or is being assembled into a larger weldment. In this case, use either 4043, which is aluminum with 5 percent silicon added, or 4047, which is aluminum with 12 percent silicon added. Yes, the HAZ will be weakened, but this is the most typical manufacturing sequence. The reduced strength is taken into account in the design. In the second situation, the weld strength must match that of the casting. Such cases include the welding of components that will be totally heat-treated again after welding and when welding is used to repair small defects in castings, which are then heat-treated to the T6 condition. For such applications, it is important that the strength of the repaired area match that of the rest of the casting so that the casting does not contain strong spots or weak spots after heat treatment. In this case, use alloy 4008 filler because its chemistry closely matches that of 356. Alternatively, several filler metal manufacturers make a matching filler for 356. Once repaired with any of these fillers and heat-treated, the repaired area will blend smoothly into the casting.