AFS NEWS
ASTM Approves New Permanent Mold Test Bar from
AFS Research Project ASTM International has accepted a new aluminum
permanent mold separately-cast test bar developed through an AFS Research Project as an alternative methodology applicable to the ASTM B108 standard. The new mold configuration, called the “Case Mold,” has been accepted as an alternative approved design. The AFS research project originated within its Aluminum Division in 2009, with the initial objective of developing and confirming best practices for casting A356 alloy with highest, or optimum, mechanical prop- erties. Separately-cast permanent mold test bars were chosen as the means to evaluate the mechanical proper- ties. However, the standard test bar mold, the “Stahl mold,” which underpins the ASTM B108 standard, suf- fers from microporosity in normal application and does not always yield best mechanical properties. Based on a successful experience using a step-mold for magnesium, researchers at Case Western Reserve University, Cleve- land, Ohio, undertook a computer simulation study to investigate whether a knife-ingate into the gage section of the “Stahl Mold” design could theoretically reduce microporosity. Comparative results with the standard ASTM B108 mold confirmed the micro-porosity could be reduced from a simulated 3% to as little as 1%. Based on the computer simulation results, research- ers designed the Case Mold with a thin knife-ingate into the best bar gage section. Separately-cast test bars using the Stahl and Case Molds were poured in both A356 and 319 alloys at Case Western Reserve and at two commercial metalcasting facilities. The test bars were treated to the T6 condition. The 319 alloy was also treated to the T7 condition. In all instances, the test bars consistently showed a measurable increase in mechanical properties with the Case Mold versus the Stahl Mold.
One of the key advantages of the new Case Mold test
bar is the “sweet spot” for optimal mechanical proper- ties, achieved at a mold temperature 600-625F, which is readily and easily achieved in metalcasting floor prac- tice. Embedded heaters and thermocouple connections in the new Case Mold design also facilitate foundry- floor capability for pouring repeatable test bars. Separately-cast test bar properties are not necessar-
ily fully representative of the actual properties attained in an actual casting. The solidification conditions and microstructural characteristics must be similar in both the test bar and the casting. Metalcasting facilities now can manufacture the Case Mold design as depicted in the new ASTM standard. It is also available commercially. For more information, contact AFS Senior Technical Director Steve Robison at
str@afsinc.org.
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