Casting Porosity — Root Cause Analysis & 8D Corrective

What Is Casting Porosity?

Casting porosity is internal void formation in a casting, produced either by dissolved gas (gas porosity) or by shrinkage during solidification (shrinkage porosity). Machined surfaces reveal the voids as round pores or irregular sponge-like cavities, and pressure-tight castings such as engine blocks, pump bodies and valve housings fail leak tests because of them. Porosity is a process problem: melt quality, gating and risering design, and solidification control. It is investigated in D4 with process data, simulation and metallurgical evidence.

Symptoms

• Visible holes on machined surfaces: round for gas porosity, irregular for shrinkage
• Leak test failures on pressure-tight castings (blocks, pump bodies, valve bodies)
• Tensile test bars breaking at pore locations, reducing mechanical strength
• X-ray or CT showing internal void clusters
• Porosity exposed at different depths after machining, causing surface finish defects

Affected Components

• Aluminium castings (gravity, low-pressure and high-pressure die casting)
• Iron castings (sand casting and shell moulding)
• Automotive: engine blocks, cylinder heads, transmission housings, brake calipers
• Industrial: pump bodies, compressor housings, hydraulic valve bodies
• Aerospace: structural brackets and landing-gear components

Detection Methods

• X-ray / DR: 2D projection detects internal porosity above about 2% of section thickness
• CT scan (3D): quantifies pore volume fraction, location and morphology
• Leak testing: pressure-decay or helium mass spectrometry detects through-wall porosity
• Dye penetrant inspection (DPI): finds surface-breaking porosity after machining
• Metallographic sectioning: measures pore size, shape and distribution under a microscope
• Reduced pressure test (RPT) / density index: assesses hydrogen content in the melt

Possible Causes

• Hydrogen gas porosity: dissolved hydrogen in the aluminium precipitates as H2 bubbles during solidification; sources include moisture in charge, tools and flux, humid air and insufficient degassing.
• Shrinkage porosity: insufficient feeding during solidification from poor riser design, undersized risers or risers too far from the hot spot.
• Entrained gas and oxide film: turbulent filling traps air and oxide films; aluminium gravity casting ingate velocity above 0.5 m/s is a common trigger.
• Sand core gas: binder decomposition releases gas that enters the metal when venting is poor.
• Die hot spots: local overheating in high-pressure die casting creates localised shrinkage, aggravated by inadequate cooling channels.
• Excess release agent or coating: volatiles from too much die release agent are entrained into the metal.

5-Why Example

Problem: Aluminium transmission housing failed leak testing; X-ray showed 2-5 mm porosity clusters near the mounting flange.

1. Why was there porosity? → Shrinkage in the thick boss — insufficient feeding during solidification
2. Why was feeding insufficient? → The riser froze before the thick section
3. Why did the riser freeze early? → The riser modulus (volume / surface area) was 0.8 cm versus the 1.2 cm required
4. Why was the riser small? → The casting simulation used generic material properties instead of measured alloy solidification data
5. Why were generic properties used? → The foundry had no measured solid fraction versus temperature data for that alloy grade

TRC: The riser froze early, causing shrinkage porosity. MRC: Casting simulation was not verified with measured alloy thermophysical data.

Fishbone (6M)

• Man: Operator shortened the degassing time to chase throughput
• Machine: Holding furnace thermocouple read 15 °C low, so the metal was colder than indicated
• Method: Pour time was 12 s against an 8 s specification, so the metal cooled before filling
• Material: Return scrap ratio rose from 30% to 50%, increasing oxide inclusion risk
• Measurement: Density index measured once per shift and X-ray sampling at 1 in 50
• Environment: 85% humidity in the foundry in summer, increasing hydrogen pickup

D3 Containment

• 100% X-ray or CT inspection of stock and work in progress
• Extend degassing from 8 to 12 minutes and reduce return scrap to 30%
• Raise pouring temperature by 20 °C to compensate for cooling during filling
• Quarantine all castings from the affected production period

D5 Permanent Fix

• Redesign the gating and risering: larger risers, chills at thick walls, and simulation verified with measured alloy data
• Install a rotary degasser with automatic hydrogen measurement (ALSCAN or equivalent)
• Closed-loop pouring temperature control at ±5 °C with automatic furnace power adjustment
• Add dehumidifiers in the melting and pouring areas to hold humidity below 60% RH

D7 Prevention

• Add measured alloy solidification parameters (thermal analysis) to the simulation database
• Make casting simulation review mandatory before a new die is launched
• Real-time density index monitoring with automatic alarms
• 100% CT inspection for safety-critical castings

Key Takeaways

1. Always separate gas porosity from shrinkage porosity first — the fixes are different.
2. Riser modulus and feeding distance explain most shrinkage defects.
3. Simulation is only as good as the alloy data behind it; measure the real grade.
4. Melt hydrogen control depends on degassing time, scrap ratio and humidity.
5. The management root cause is usually missing process validation.