What type of discontinuities are categorized as porosity in welding?

Porosity in welding is defined as cavity-type discontinuities that occur when gases are trapped in the weld pool during the solidification process. These gases can be a result of various factors, including moisture, contaminants, or even atmospheric exposure during the welding process. The gas bubbles that form can become entrapped as the molten metal solidifies, leading to small voids or cavities within the weld bead or the heat-affected zone.

This type of discontinuity is particularly problematic because it can compromise the mechanical properties and integrity of the weld, making it less strong and susceptible to failure under stress. Identifying and understanding porosity is crucial for weld quality assurance, as it indicates issues related to cleanliness, shielding gas coverage, or welding technique.

The other options describe different types of weld discontinuities. The joining of two different metals pertains to compatibility issues rather than porosity. Excess weld material refers to added weld metal dimensions and underfilled weld joints concern insufficient weld material. However, neither directly relates to the gaseous entrapment that characterizes porosity. Understanding porosity helps welders focus on the quality and performance of their welds, reducing defects in the final product.