Common Defects in Aluminium Alloy Die Castings and Their Solutions

Published: August 29, 2026

In the purchasing and acceptance of aluminium alloy die castings, defect judgement is often the point where buyer and supplier most easily disagree. The same depression on a surface may be an acceptable flow mark, or it may be a cold shut that has to be scrapped; likewise, for a hole inside the part, porosity and shrinkage porosity have completely different causes and corrective directions. This article reviews the five most common types of defect in aluminium alloy die castings, their causes and the corresponding corrective approaches, for purchasing and quality staff to refer to in practical communication.

1. Porosity: the Most Common Internal Defect

1. Manifestation: after sectioning or X-ray inspection, round holes with smooth walls are visible, mostly distributed in thick-walled areas of the casting, at the last-filled positions of the cavity or near the overflow wells.
2. Main causes: gas in the shot chamber and in the cavity is not discharged in time during high-speed filling and is entrained in the molten metal; or the release agent is used in excessive amounts or the coating is not fully volatilised, generating gas locally.
3. Corrective direction: optimise the runner, overflow and venting system to improve venting efficiency; use vacuum die casting or local vacuum extraction; control release agent concentration and spray volume; reduce the slow shot speed appropriately to limit gas entrapment.

2. Shrinkage Cavities and Shrinkage Porosity

1. Manifestation: the hole walls are rough and the shapes irregular, and they usually appear at hot spots — that is, at abrupt wall-thickness changes, at the roots of reinforcing ribs or in areas with a large machining allowance. The difference from porosity is that shrinkage cavity walls are rough and are mostly accompanied by dendritic structure.
2. Main causes: when the aluminium alloy shrinks during solidification it does not receive enough feeding, so a hole forms at the position that solidifies last.
3. Corrective direction: adjust the casting structure to avoid abrupt wall-thickness changes, using uniform wall thickness or a thinned reinforcing rib design; add cooling or apply local pressurisation at hot spots; increase intensification pressure and extend the dwell time; reduce the pouring temperature appropriately.

3. Cold Shuts and Flow Marks

1. Manifestation: a cold shut shows as the place where two streams of molten metal meet without fully fusing, forming a linear mark that is penetrating or semi-penetrating; flow marks are stripes that appear on the surface and follow the direction of metal flow.
2. Main causes: the molten metal temperature is too low, the die temperature is insufficient, the filling speed is too slow or the runner cross-section is too small, so the front of the molten metal solidifies prematurely.
3. Corrective direction: raise the die preheat temperature and stabilise the production cycle; raise the pouring temperature and shot speed appropriately; enlarge the ingate cross-sectional area; improve the balance of the die temperature field.

4. Galling and Die Sticking

1. Manifestation: strip-like scratches appear on the casting surface along the ejection direction, with skin tearing in severe cases; die sticking, by contrast, is aluminium alloy adhering to the cores or cavity walls.
2. Main causes: insufficient cavity surface finish or the presence of reverse draft; too small a draft angle; insufficient lubrication from the release agent; low iron content in the alloy, giving it a strong affinity with the die steel.
3. Corrective direction: polish the cavity and ensure the part is ejected along the ejection direction; increase the draft angle; adjust the release agent formulation and spray positions; keep the iron content of the alloy within a reasonable range.

5. Short Filling and Poor Forming

1. Manifestation: metal is missing locally and the contour is incomplete, which is especially common in thin-walled sections and in areas far from the ingate.
2. Main causes: insufficient ingate cross-sectional area, insufficient shot energy, a biscuit that is too thin, poor die venting or a locally low cavity temperature.
3. Corrective direction: check the match between casting weight and shot curve, and switch to a larger-tonnage die casting machine if necessary; enlarge the ingate and the venting grooves; raise the local die temperature.

6. Move Defect Management Forward to the Prototyping Stage

1. Carry out mould flow analysis during the die development stage and identify gas entrapment and hot spot positions in advance; this is far more economical than correcting problems after mass production begins.
2. Agree clear acceptance criteria at the prototyping stage: which defects are permitted, to what extent they are permitted, and by what means they are inspected (visual, dye penetrant testing, X-ray, air-tightness test).
3. Fix the process parameters and keep records during mass production, putting shot speed, intensification pressure, die temperature and biscuit thickness into routine checks, so as to avoid batch fluctuations caused by parameter drift.
4. When a defect appears, first locate the change point: whether it is the material batch, the die condition or a change in process parameters, and then adjust accordingly.

HUACHUAN Die Casting is located in Chengdu and focuses on aluminium alloy die casting, with an annual capacity of 3,000 t and monthly output of up to 500,000 pcs. We provide one-stop service from die development and die casting production to CNC machining and surface finishing. For a process evaluation or quotation calculation based on specific drawings, add us on WeChat at +86 199 8304 0872, or call +86 28-8712 5500.