Meaning
Internal cavities form within a solidifying metal when dissolved gasses are rejected from the liquid phase. In aluminum castings used for battery housings, gas porosity typically results from the presence of hydrogen which becomes less soluble as the temperature drops. These voids appear as small spherical holes distributed throughout the section thickness.
High humidity or contaminated scrap metal often increases the severity of the condition during the melting process.
Formation Logic
Hydrogen enters the melt through the dissociation of water vapor on the liquid metal surface. Because the solubility of hydrogen in solid aluminum is lower than in the liquid state, the gas must escape or form bubbles during solidification. If the cooling rate is too fast for the gas to reach the surface, gas porosity becomes trapped within the grain structure.
Technical Inspection
Radiographic testing and ultrasonic evaluation provide the primary means of identifying these internal defects without destroying the part.
Structural Impact
Reduced cross-sectional area directly lowers the fatigue life and pressure tightness of the component. Voids act as stress concentrators. In cooling plates or pressurized enclosures, these internal defects create leak paths that compromise the thermal management system of a high-voltage battery pack while increasing the risk of structural failure under load.
Parts with excessive void fractions are rejected.
