How can oxide layers make a welded joint brittle?

When metals are heated during welding, they often react with the surrounding atmosphere, causing them to oxidize rapidly. In the context of metallurgy, these oxide layers can compromise the integrity of a weld in two primary ways:

• Prevention of Bonding: The formation of an oxide layer on the surface of the metal can act as a barrier, preventing the proper coalescence or bonding of the base materials.1
• Entrapment: If molten-metal droplets become coated with oxide, these particles can become trapped within the weld pool as it solidifies. The presence of these entrapped oxides creates structural discontinuities that make the resulting joint brittle.1

To mitigate these issues, welders often employ fluxes to form a protective blanket over the molten material or use inert atmospheres—such as argon—to displace oxygen and prevent the metal from reacting with the air during the welding process.1