Solidification Segregation
Solidification segregation is the process where certain metals or alloys in a liquid state crystallize faster into solid crystals than others. This leads to uneven crystal sizes and can cause porosity and loss of toughness.
Physical-Chemical Mechanism
Solidification segregation occurs when, in a liquid weld pool, certain elements or alloys crystallize faster into solid crystals than others. This is often due to different solubilities of the elements in the liquid and solid states. For example, some elements may be strongly arranged on certain crystal faces in the liquid, increasing the attraction for new nuclei and accelerating the crystal growth rate.
Metallurgical Relevance & Welding Risks
Solidification segregation can lead to uneven crystal sizes and irregular structures, which in turn causes porosity and loss of toughness in the weld. These pores can significantly impair the mechanical strength of the weld and lead to cracks or hot cracks. Additionally, uneven crystal growth rates can create stresses in the structure, increasing the risk of fracture.
Workshop Practice & Action Items
To minimize solidification segregation, it is important to control the melting process so that all elements crystallize simultaneously and uniformly. This can be achieved by choosing the right weld alloy, correct temperature control, and applying suitable deoxidizers. An example is the use of silicon as a deoxidizer, which binds oxygen and simultaneously regulates the crystal growth rate. It's also important to ensure the melting process is slow enough to allow homogeneous crystal formation, but not so slow that porosity occurs.