Which type of discontinuity is specifically caused by a welding operation?

The correct answer is lamellar discontinuity, which refers to a specific type of defect that can occur as a result of welding operations. It typically involves the presence of layers or flakes within the material, often caused by the effects of stress during the cooling process after welding.

Welding introduces significant thermal gradients that can lead to uneven contraction as the material cools. This uneven cooling can create internal stresses, which may manifest as layers of different microstructures or inclusions that are not bonded together properly. Lamellar discontinuities can compromise the integrity of the weld and the surrounding material, making it crucial to identify them during non-destructive testing processes.

Porosity, while also a potential welding defect, is not specifically caused by the welding operation itself but rather by trapped gas within the molten weld pool. Cracking can occur due to thermal stresses as well, but it generally refers to the formation of cracks that can happen under various conditions, not limited to welding. Fusion is related to the melting and joining of the base materials, and while it is integral to the welding process, it does not constitute a specific type of discontinuity.