Dendritic Arm Spacing
The dendrite arm spacing is the distance between the growth lines (dendrites) in a metal during the solidification process. A small spacing results in a fine, strongly connected structure, while a large spacing results in a coarse, less connected structure.
Physical-Chemical Mechanism of Action
During solidification of metals, dendrites grow from crystal nuclei. The distance between these dendrites, the dendritic arm spacing, is determined by the cooling rate. With rapid cooling, the crystal structure grows quickly and forms tightly connected structures with a small spacing between dendrites. With slow cooling, the structure grows slower and forms wide, less connected structures with a larger spacing.
Metallurgical Relevance & Risks in Welding
An excessively large dendritic arm spacing can lead to a fine grain, which impairs mechanical properties such as strength and toughness. This increases the risk of cracks and fractures. An excessively small spacing can lead to a strongly connected, glass-hard structure, which also leads to hot cracks and low formability.
Workshop Practice & Instructions
To optimally control the dendritic arm spacing, it is important to regulate the cooling rate of the weld pool. This can be achieved by choosing the appropriate welding process (e.g., TIG welding for thin steels) and adjusting the delay times after welding. A goal is to achieve a dendritic arm spacing of about 10 to 50 micrometers, depending on the specific metal and the desired mechanical properties.