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How does flux protect metal during fusion welding?

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point. — welding Shielded metal arc welding.

In fusion welding, flux plays a critical role in protecting molten metal from the detrimental effects of the atmosphere, such as rapid oxidation, which can cause brittleness and prevent proper bonding.1

The flux functions by facilitating a controlled reaction and forming a protective blanket over the molten material. This blanket prevents the metal from coming into contact with oxygen in the air. Additionally, the flux helps remove impurities, which are then sequestered into a substance known as slag. Because slag is less dense than the molten metal, it floats on the surface, providing a physical barrier that keeps the weld pool clean and shielded from atmospheric contamination during the cooling process.123

Depending on the specific welding application, fluxes can be active, helping to facilitate the process, or inactive, serving solely to protect the surfaces during joining.1