Casting Defect Prediction
Theory: each defect has its own indicator
Overview
When casting simulation says 'defect here', what has actually been computed?
Not the defect itself but indicators of conditions that favour it. Casting defects fall into those formed during pouring (misruns, entrained oxide films and air), those formed during solidification (shrinkage cavities, porosity, hot tears) and distortion after cooling. Filling analysis computes metal velocity and temperature; solidification analysis computes where metal freezes last and whether liquid can keep feeding it. Indicators such as surface velocity, isolated liquid surrounded by solid, and combinations of thermal gradient and cooling rate like the Niyama criterion are then displayed. They are judged by empirical thresholds, so calibrating against trials of the same alloy and process is assumed.
Three indicators
v is the velocity gained in a fall of height h; Ny is the Niyama criterion (G thermal gradient, Ṫ cooling rate); V_feed is the liquid volume needed to compensate solidification shrinkage (β shrinkage ratio, m casting mass, ρ density).
The science of 'pour gently'
The rule of thumb to pour quietly is old, but explaining it through oxide-film entrainment and giving a concrete velocity guide of around 0.5 m/s came from the 1990s work of John Campbell and colleagues in the UK. The oxide skin on aluminium alloys is extremely thin, but when folded in it remains as a double film (bifilm) that seeds pores and cracks. The idea reshaped running-system design and made surface velocity a key indicator in simulation.
Worked examples
Ny units are √(K·s)/mm. The threshold of 1 is illustrative, not universal. The 0.5 m/s guide is condition-specific and cannot guarantee absence of oxide entrainment. Feeding volumes simply multiply the assumed reference volume by the stated volumetric shrinkage.
Example 1: fall height and metal velocity
| Fall height | Velocity | vs 0.5 m/s guide for Al |
|---|---|---|
| 12.7 mm | 0.50 m/s | 1.0 |
| 50 mm | 0.99 m/s | 2.0 |
| 100 mm | 1.40 m/s | 2.8 |
| 300 mm | 2.43 m/s | 4.9 |
A drop of only 13 mm reaches the guide velocity. Designs that drop metal tens to hundreds of mm from the sprue to the mould floor almost certainly disturb the surface and entrain oxide films.
Example 2: Niyama criterion at three locations
| Location | Gradient G | Cooling rate | Ny | Verdict (guide 1) |
|---|---|---|---|---|
| A thin wall | 3.0 K/mm | 4.0 K/s | 1.50 | Sound |
| B section centre | 0.6 K/mm | 0.5 K/s | 0.85 | Watch |
| C hot spot | 0.2 K/mm | 0.3 K/s | 0.37 | Porosity risk |
G and cooling rates are illustrative. At thick-section centres and hot spots the gradient is small and feeding paths close early, so Ny is low.
Example 3: feed metal for solidification shrinkage
| Casting | Volume | Shrinkage | Feed needed |
|---|---|---|---|
| Aluminium alloy 1 kg | 370 cm³ | 6% | 22 cm³ |
| Steel 10 kg | 1,282 cm³ | 3% | 38 cm³ |
Shrinkage values are typical. If this much liquid reaches every region in freezing order from the riser, no shrinkage cavity forms. The riser must be well larger than this and freeze after the casting.
Niyama was above 1 everywhere in simulation, yet trials showed porosity. What should I suspect?
Suspect the threshold and the thermal conditions. The Niyama threshold was originally set from steel experiments; for long-freezing-range alloys such as aluminium, values far below 1 may apply, and the number also changes with units (K/mm vs K/cm). So 'above 1 is safe' may not hold for your alloy and definition. Second, a constant mould-casting heat transfer coefficient misses the air gap that opens with shrinkage and slows heat loss, tending to overestimate the gradient. Also, high hydrogen in the melt forms gas pores even with little shrinkage, invisible to Niyama. Overlaying porosity positions from sectioned trials on the Ny map to recalibrate the threshold for that alloy is the shortest path.
Practical workflow
- Prepare geometry including runners and risers, and temperature-dependent alloy and mould properties.
- Check surface velocity, misruns and entrainment risk with filling analysis.
- Check isolated liquid, Niyama values and last-to-freeze regions with solidification analysis.
- Revise runners, risers and chills and repeat until indicators improve.
- Compare with X-ray and sectioned trials to calibrate thresholds and heat transfer.
“Right location, wrong size”
Predicted shrinkage often appears in the right place but more than twice the size seen in trials. Location depends mainly on geometry and freezing order and is relatively easy to get right; size depends on shrinkage volume, feeding paths, mould expansion, gas content and more. In practice, use 'where' for design decisions and treat 'how much' as a trend calibrated against trials.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Borrowing thresholds from another alloy | Missed or false defects | Calibrate with trials |
| Constant interface heat transfer | Gradient overestimated | Model gap formation |
| Solidification without filling | Wrong initial temperatures | Start from filling |
| Running system with free falls | Oxide film entrainment | Fill quietly from below |
| Ignoring dissolved gas | Gas pores missed | Control melt hydrogen |
I'd like to learn related topics.
Related pages include casting filling analysis, casting solidification analysis, investment casting simulation and casting residual stress analysis.
Frequently asked questions
Which defects can casting simulation predict?
Typically misruns (freezing before filling completes), entrained oxide films and air, shrinkage cavities and porosity (insufficient feeding of solidification shrinkage), gas porosity, hot tears and distortion from residual stress. Filling analysis gives flow and temperature, solidification analysis gives freezing order and thermal gradients, and defect-specific criteria highlight risky regions.
Why does metal velocity matter?
A fast-moving metal surface folds its oxide skin inward, entraining films that become crack origins. For aluminium alloys a common guide is to keep velocity below about 0.5 m/s. Free fall reaches 0.5 m/s after only 12.7 mm and 1.4 m/s after 100 mm, so avoid designs that drop metal from the sprue and fill quietly from below instead.
What is the Niyama criterion?
The thermal gradient G near the end of solidification divided by the square root of the cooling rate. Lower values mean feeding struggles to reach the region and fine shrinkage porosity is likely. For steel a threshold near 1 (K·s)^0.5/mm is often used; in the example a hot spot shows 0.37 and needs better feeding. Thresholds vary with alloy and unit conventions, so set them against trials.
References and scope of verification
Numbers are educational reference calculations under the stated assumptions. Sources document theory and setup, not proof that these numerical values were reproduced in each package. Engineering decisions require validation for the actual conditions.
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