Final answer:
Welding failures often occur in the heat-affected zone due to microstructural changes from rapid thermal cycles, which result in altered mechanical properties and thermal stresses.
Step-by-step explanation:
Most welding failures occur in the heat-affected zone (HAZ) due to the microstructural changes that occur as a result of the rapid heating and cooling cycles during welding. These thermal cycles can cause a number of metallurgical changes such as grain growth, phase transformation, and hardening or softening of the material, which can lead to decreased mechanical properties such as toughness and ductility within the HAZ. In addition, when materials with differing coefficients of thermal expansion are joined, the HAZ can experience significant thermal stresses. These stresses may result in the formation of cracks, especially if the metal's inherent toughness is not sufficient to absorb the stresses produced during contraction and expansion.