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Casting Defects in Low-Pressure

Die-Cast Aluminum AlloyWheelsB. Zhang, S.L. Cockcroft, D.M. Maijer, J.D. Zhu, and A.B. Phillion

1. During solidifi cation, control of cooling rates is important for productquality. In the bottom die, cooling is augmented by forcing air through internalchannels at various times during the casting cycle. In the top die, cooling iscontrolled by air jets aimed at various sections of the exterior face. On theside dies, cooling may be retarded by the addition of insulation to the cold

face at certain locations or augmented by air cooling, depending on thecasting conditions.

2. During solidifi cation, control of cooling rates is important for productquality. In the bottom die, cooling is augmented by forcing air through internalchannels at various times during the casting cycle. In the top die, cooling iscontrolled by air jets aimed at various sections of the exterior face. On theside dies, cooling may be retarded by the addition of insulation to the coldface at certain locations or augmented by air cooling, depending on thecasting conditions.

3. Of these three quality-control tests, control for cosmetic appearance is

generally the most diffi cult. However, wheels failing this criterion can often berepaired instead of being scrapped and remelted.In contrast, rim-leak defectscannot be repaired; wheels failing this criterion must be scrapped andremelted at considerable expense since this defect can only be testedfollowing fi nal machining. An evaluation of the mechanical criteria usuallyoccurs in the preproduction stage, during development of the wheel and dieassembly.

4. Macro- or shrinkage porosity forms in locations where there is insuffi cientfeeding of liquid metal to offset the volumetric shrinkage associated with thesolid-to-liquid transformation.

5. This porosity formed because directional solidifi cation was lost, resultingin liquid being encapsulated by solid.

6. One area requiring much development, of a more fundamental nature, isthe infl uence of hydrogen content and alloy modifi cation on macroporosity.For example, it is well known within the foundry industry that excessively lowhydrogen contents can lead to increased occurrences of macroporosity.Likewise, it is known that alloy modifi cation can infl uence mass transportwithin the interdendritic liquid, which will alsoalter the tendency formacroporosity to form

7. As the name implies, microporosity (more often termed hydrogen-based

porosity) relates to porosity that is relatively small in scale (

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8. owing to its dependence on the interaction of a number of phenomenaoccurring during the manufacturing process including the pickup of hydrogenfrom the ambient air by liquid aluminum, the decrease in hydrogen solubilityduring solidifi cation, behavior of oxide bifi lms, volumetric shrinkage duringsolidifi cation, and the development of the solidifi cation structure.

9. The result is a gradual pickup of hydrogen with increased exposure toambient air until the solubility limit is reached, ~0.69 cc H2/100 g Al. Incontrast, the solubility limit of solid aluminum is quite small, only ~0.036 ccH2/100 g Al.

10. In addition to cosmetics, hydrogen based porosity can affect themechanical properties of a wheelespecially fatigue performance.

11. (the white structures in the photomicrographs are the primary aluminumstructures that form first and the light grey structures are the Al-Si eutectic

constituents that form during the last stage of solidifi cation).

12. The poor fatigue performance of aluminum castings is the result of both ashort fatigue crack initiation period and small crack propagation period whichis exacerbated by the presence of pores, oxide inclusions, and oxide fi lms