Homogeneous nucleation
Homogeneous nucleation describes the spontaneous formation of crystal nuclei directly in a pure melt without any foreign surface. For this, the melt must undercool strongly, often 100 to 300 Kelvin below the melting point. In welding practice, this case rarely occurs because the weld pool always provides enough foreign nuclei for the usual heterogeneous nucleation (on foreign surfaces).
Physical-Chemical Mechanism of Action
In a homogeneous melt, the atoms move randomly. During cooling, small atomic groups randomly form that dictate the crystal structure. These groups usually disintegrate immediately because their surface costs energy. Only when the melt is strongly undercooled do some of these nuclei grow further. The necessary undercooling is often 100 to 300 Kelvin below the melting point. Homogeneous nucleation thus requires a high energy barrier and only occurs in very pure, particle-free melts.
Metallurgical Relevance & Welding Risks
In the weld pool, there are always foreign surfaces: the joint flanks, oxide skins, welding flux residues, alloying elements. On these surfaces, crystal nuclei form much more easily because the interface energy decreases. Therefore, solidification almost always occurs heterogeneously. Homogeneous nucleation only occurs in extremely clean melts, e.g., in special welding under vacuum. Risk: If homogeneous nucleation does occur, the pool solidifies abruptly after strong undercooling. Gases that were previously dissolved can no longer escape and form pores. Additionally, high residual stresses arise due to the sudden volume change.
Workshop Practice & Action Instructions
The practitioner cannot directly influence homogeneous nucleation, but he can make it unlikely. For this, he ensures many foreign nuclei in the weld pool: alloy inoculants like titanium or aluminum, use welding consumables with finely distributed oxides or carbides. These particles act as nuclei and start crystallization at low undercooling. This results in a fine-grained, tough structure without porosity. Do not keep the weld pool liquid longer than necessary and avoid excessively high purity of the materials if there is no special application behind it.