Solidification Front Instability
Instability of the solidification front leads to the breakdown of a smooth phase boundary into cellular or dendritic structures. This is caused by thermal and chemical gradients in the undercooled melt.
Physical-Chemical Mechanism
Solidification front instability occurs when the undercooled melt contains thermal and chemical gradients. These gradients cause uneven stresses at the phase boundary, leading to instability. The front begins to break down into cellular or dendritic structures.
Metallurgical Relevance & Welding Risks
The instability can cause uneven stresses and cracks in the weld. It creates porosity and a finer grain structure, which impairs the material's toughness. In extreme cases, it can lead to hot cracks or incomplete fusion in the workpiece.
Workshop Practice & Instructions
To minimize solidification front instability, it is important to ensure uniform cooling of the weld pool. This can be achieved by optimizing the cooling rate and using cooling water with a suitable pH. For heavy metals like steel or nickel, controlled cooling is especially important to avoid porosity.