Constitutional Supercooling Zone
A region ahead of the solidification front where the actual temperature lies below the liquid-solid equilibrium temperature, caused by local concentration gradients. It destabilizes planar fronts.
Constitutional supercooling occurs when a concentration gradient builds up ahead of the solidification front due to alloying elements. This causes the actual temperature to fall below the equilibrium temperature, making solidification unstable and leading to an uneven structure. In welding, you notice this as hardness variations and stresses in the weld zone, and in the worst case, cracks. This often happens when you cool too quickly or when heat dissipation is uneven. Keep the cooling controlled and uniform, then the structure remains homogeneous. If you run the parameters cleanly and don't cool too hastily, you'll hardly have any problems in practice.