Dendritic growth
Crystallization of the melt forming branched, tree-like structures during solidification. This branching network determines the distribution of pores and segregation.
When the melt cools, the crystals grow from the cold base material into the weld. They grow like fir trees with side branches, called dendrites. Between these branches, melt remains and becomes enriched with impurities. If this gets coarse, you have weak material at these spots that can crack during shrinkage – that's hot cracking. So you want a fine structure. You achieve that by not putting too much heat into the weld and not preheating the material unnecessarily. The filler metal also plays a role: with certain alloying elements, you get more nuclei and thus finer dendrites. Make sure sulfur and phosphorus in the filler are low, they only make things worse. And once the weld has solidified, you have to watch the cooling to avoid hardness problems.