Dendritic Solidification

When the melt cools, the crystals don't grow evenly but branch out like trees – that's called dendrites. That's normal, but if the structure gets too coarse, the weld's toughness suffers and cracks can occur. This often happens with rapid cooling or when heat can't dissipate properly. So you want even cooling to keep the structure fine and clean. Preheat the weld or let it cool slowly, then the crystals won't have a chance to branch out so coarsely.