5.8 Fusion welding
In fusion welding, the parts to be joined are brought together, melted and fused to each other. In some processes the interface is filled with a molten substance, supplied by a filler rod that is similar in composition to the materials being joined.
During fusion welding the areas that are being joined comprise an intimate mixture of parent material, and filler rod (if one is used), within the welded zone. In all methods of fusion welding, heat must be supplied to the joint in order to melt the material. Inevitably, temperature profiles are created and the resulting differential expansions and contractions can cause distortion, and in extreme cases, the formation of cracks, in the assembly. As welds are, in fact, small castings, welds contain both the microstructure and porosity endemic in cast material.
Soldering and brazing can minimise some of these problems, as the parent material is not melted, so temperature profiles are not as great. But in brazing and soldering there is a discrete join between the materials as opposed to an intimate mixture of material in welding; welding is by far the strongest of the processes.