Understanding Quality in Aluminum Castings
By establishing a scientific basis for evaluating casting quality, casting engineers can determine the relative quality of a part and whether further improvements are possible. GEOFFREY SIGWORTH, FOSECO, CLEVELAND, OHIO
to offer a precise definition. Fur- ther, it is common for metalcasters producing the same part with the same tooling to achieve different mechanical properties.What causes this difference? Dissolved gas, iron and oxide
T
inclusions in aluminum all have a deleterious effect on casting quality. Iron limits are specified for commer- cial alloys, but how do we measure gas and oxides? One approach to
he concept of metal qual- ity has been pervasive yet elusive in the casting industry. Everyone uses the term, but few are able
this problem is the use of a quality index. Another approach is to use a test casting to measure the metal’s quality index. The test casting may be a production casting with a good historical pedigree or a standardized casting produced using a standard, as specified by a governing body. In order to assess the relative quality of
a production or test casting, one may use a quality index developed by the industry.
Theoretical Basis for Quality Index
Te most common index for defining the quality of net-shaped castings was first proposed by French metalcasters. Te researchers studied
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June 2011 MODERN CASTING | 31
the effects of casting conditions, metal composition and aging time on the mechanical properties of 356-type aluminum alloys. As they analyzed the aging process, the metalcasters noticed that for a given “quality” of casting—as determined by freezing rate, poros- ity and iron content—the T6 aging process produced tensile properties
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