Casting Porosity — Root Cause Analysis & 8D Correcti...

Symptoms Visible voids on machined surface; leak test failure Reduced strength; X-ray/CT shows internal void clusters Affected Components Al castings (gravity, LP, HPDC); iron (sand, shell) Engine blocks, heads, transmission housings; pump/valve bodies Detection Methods X-Ray/DR: Porosity >2% section thickness CT (3D): Volume fraction, location, morphology Leak Test: Pressure decay or He spectrometer DPI: Surface-breaking after machining RPT/Density Index: Hydrogen content in aluminum Possible Causes Gas porosity (H₂): dissolved hydrogen → bubbles on solidification Shrinkage: insufficient feeding, wrong gating/riser design Entrained air: turbulent flow, gate velocity >0.5 m/s Core gas: binder decomposition from sand cores Die hot spots: inadequate cooling line placement Excess lubricant: vaporized die spray → trapped gas 5-Why Example Problem: Al transmission housing leak test fail. X-ray shows 2–5mm porosity at flange Why porosity? → Shrinkage near thick boss — insufficient feeding Why insufficient? → Riser froze before section solidified Why premature? → Riser modulus 0.8 vs 1.2 cm required Why undersized? → Simulation used generic material properties Why generic? → No measured solidification data for this alloy TRC: Shrinkage from premature riser freezing MRC: Casting simulation not validated with alloy-specific thermal properties Fishbone (6M) Man: Short degassing for production rate Machine: Furnace TC reading 15°C low...