Meaning
Microscopic voids distributed within the weld bead or the heat-affected zone of a metal joint arise from trapped gases or shrinkage during solidification. High levels of micro-porosity reduce the effective load-bearing cross-section of the weld, making it more vulnerable to mechanical failure. This defect is a common concern in the laser welding of aluminum battery cans where the volatile alloying elements can vaporize and become trapped.
It is defined by void sizes that are typically below one hundred micrometers in diameter.
Weld Weakness
Tensile and fatigue strengths of the joint are compromised when these tiny gas pockets acting as stress concentration points are densely clustered together. Under cyclic mechanical loads, such as those caused by vehicle vibration or thermal expansion, cracks can initiate at these voids and propagate through the joint. This propagation eventually leads to a complete tear of the weld and the loss of the hermetic seal.
Designers must limit the allowable density of these defects to ensure long-term reliability.
Process Origin
Unstable weld pools and high levels of surface contamination are the primary drivers for the formation of these voids. During laser welding, if the vapor keyhole collapses prematurely, it traps shielding gas and metal vapor within the rapidly solidifying weld metal. Moisture or organic residues on the metal surfaces also vaporize and contribute to the gas volume.
Inspection Strategy
Radiographic and computerized tomography methods are required to detect these subsurface defects.
