Which type of defect arises from lack of fusion between the weld and base metal?

When considering the type of defect that arises from a lack of fusion between the weld and the base metal, the correct answer focuses on incomplete fusion. This defect occurs specifically when the weld metal does not properly bond with the base materials, leading to a weak joint that may not have the required strength and integrity.

Incomplete fusion can result from inadequate heat input during the welding process or improper technique, which prevents the molten weld metal from fully penetrating and bonding with the base metal. This defect can cause significant issues in the structural performance of the weld, as it may lead to reduced load-bearing capabilities and increased risk of failure under stress.

In contrast, porosity refers to the presence of gas pockets within the weld, which can weaken it but is not directly related to fusion with the base metal. Undercutting involves the erosion of the base metal along the edges of the weld, leading to a groove that can weaken the joint, but it doesn't concern the bonding itself. Crater cracks typically occur at the termination point of a weld when the molten metal solidifies unevenly, and while this can compromise weld quality, it is not primarily about fusion with the base material.