Solidification shrinkage

When the weld cools, the metal contracts, that's called shrinkage. The weld wants to contract as it solidifies, but the surrounding material holds against it. This creates stresses that can lead to distortion or cracks. You don't need to excessively cool or artificially cool the weld, that often makes it worse. It's important to weld the weld cleanly with appropriate heat input and not to clamp the part too tightly. This way the weld can settle a bit without cracking. For critical parts, it helps to choose the welding sequence and the order of the welds so that the stresses cancel each other out.