9322 Casting Quality

Foundries only hold those tolerances that are specified by the purchaser. This is unlike the case for wrought products, for which mills will meet standard mill tolerances as a minimum. The dimensions of castings can be difficult to control because it is sometimes difficult to predict the shrinkage during solidification and the warping that may be produced by nonuniform cooling. The quality of cast material may also vary widely, and any inspection methods must be specified by the purchaser. The most commonly used inspection techniques are radiography and penetrant methods. Radiography is performed by X-raying the part to show discontinuities such as gas holes, shrinkage, and foreign material. These discontinuities are then rated by comparing them to reference radiographs shown in ASTM E155. The ratings are then compared to inspection criteria agreed to beforehand by the customer and the foundry. The inspection criteria for quality and frequency of inspection can be selected and then specified from the Aluminum Association's casting quality standard AA-CS-M5-85, which provides seven quality levels and four frequency levels from which to choose. The penetrant inspection method is only useful for detecting surface defects. Two techniques are available. The fluorescent penetrant procedure is to apply penetrating oil to the part, remove the oil, apply developer to absorbed oil bleeding out of surface discontinuities, and then inspect the casting under ultraviolet light. The dye penetrant method uses a color penetrant, enabling inspection in normal light. Frequency levels are given in AA-CS-M5-85 for penetrant testing also.

A test bar cast with each heat is also useful. It can be tested and the results compared directly to minimum mechanical properties listed for the alloy in Table 9.16.

9.3.3 Aluminum Powder

There are many uses for aluminum powder particles, which can be as small as a few microns thick. Larger particles are used in the chemical and metal production industries; one of the first uses of aluminum was as particles to remove oxygen from molten steel during its production. Finer particles are used as an explosive in fireworks and flares and as a solid fuel for rockets. Each launch of the space shuttle uses 350,000 16 (160,000 kg) of aluminum powder. Powder is also flattened into flakes in a rotating mill and used as a constituent for paints to provide a metallic finish. Finally, aluminum powder may be pressed into parts, referred to as powder metallurgy, competing with conventionally cast aluminum parts.

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