[Henry Utley]

Metallurgical question: What is actually happening to the metal as it is fusing? I know that at least with iron, when it cools quickly from a liquid state its grains naturally form in a brittle state and that filler metal additives offset this, but besides that, I know nothing.

[Poland308]

Lots of metals act the same way when fused. Your description is accurate. Oxidation from incomplete gas shielding can also become an issue, that’s why to do a proper fusion weld on thin wall SS tube you need an internal purge.

[Franz©]

DISCLAIMER: Don't blame me if you get a headache reading this.

http://ispatguru.com/carbon-steels-and- ... e-diagram/

Actually, the first paragraph might pretty well answer your question.

[A36]

Metallurgy is a complex subject. A book every welder should own is "Metals and How to Weld Them" by Lincoln Electric. Silly name but a great book covering welding/metallurgy. Not an engineering textbook, but technical enough to get a good understanding of metals and their properties.