What can cause porosity in MIG welds?

Porosity in MIG welds occurs primarily due to the presence of gas pockets trapped within the weld bead, which can compromise the integrity and strength of the weld. Contaminated base materials, such as those with oil, rust, or moisture, can introduce gases that form bubbles during the welding process. When the molten metal solidifies, these bubbles become trapped, resulting in porosity. Additionally, improper shielding gas coverage can allow atmospheric contaminants such as nitrogen or oxygen to react with the molten weld pool, leading to the formation of porosity.

In contrast, conditions like proper gas flow and clean materials are crucial for preventing porosity; they promote good shielding and a clean welding surface. Overheating the weld area may cause poor fusion or burn-through but does not inherently lead to porosity unless there are other contributing factors. Similarly, while a fast travel speed can affect bead shape and penetration, it is not a direct cause of porosity unless coupled with other issues such as inadequate shielding. Thus, contaminated materials and improper gas coverage are key contributors to this defect in MIG welding.