Improvement in Mechanical Properties of A356 Tensile Test Bars Cast in a Permanent Mold by Application of a Knife Ingate
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ALUMINUM alloy castings are widely used in many applications, often as a substitute for steel to reduce weight. Since steel has higher mechanical properties than aluminum alloys, improving the mechanical properties becomes an important task. A large volume of research has been performed to optimize the chemical composition,[1–7] liquid metal processing,[8–13] chemical modification,[14–17] and heat treatment,[18–21] and metal alloys with increased properties have been successfully developed. However, the mechanical properties of aluminum alloy are very sensitive to casting conditions.[22] A high-integrity cast microstructure is critical to the desired mechanical properties. One important requisite to produce high mechanical properties is a properly designed mold. The standard ‘‘Stahl’’ permanent test-bar mold is used by many aluminum foundries to evaluate the quality of the molten aluminum alloy and the effectiveness of metal treatments such as degassing, modification, and grain refining, as well as heat treatment response. The mechanical properties of aluminum alloys cast in a permanent mold are affected by many factors,[23] such as grain size, inclusions, and porosity. In order to obtain the best properties, several permanent mold designs have been developed. Among those designs, the most widely used in foundries are the ‘‘Step’’ Mold and the ‘‘Stahl’’ Mold. YAOU WANG, Postdoctoral Researcher, is with The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] DAVID SCHWAM, Professor, DAVID V. NEFF, Retired, CHAIJUNG CHEN, Doctoral Candidate, and XUEJUN ZHU, Research Associate, are with Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted February 28, 2011. Article published online November 17, 2011 1048—VOLUME 43A, MARCH 2012
The Step Mold was developed by the Aluminum Association 25 years ago.[24] This mold produces sections of different thicknesses, forming a steplike shape. The variation in thickness allows one to examine the mechanical properties of castings solidified under different cooling rates in one pour. However, since the mold does not produce a test bar-shaped casting, significant machining labor is required to prepare test bars. In order to save the labor and make it easy to test the mechanical properties, the test-bar permanent mold is frequently used. The first test-bar mold design can be traced back to 1948, when Ebert et al.[25] designed a mold producing two 12.7-mm (0.5-in.) test bars. In 1972, ISO[26] adopted the design of Grandier-Vazeille and Jacob,[27] which produces two 11.2-mm test bars. The ISO mold was found to be very sensitive to casting conditions. Whaler,[28] Stahl Specialty Company, examined the ISO mold and made a new design by replacing the 11.2-mm test bars with 12.7-mm (0.5-in.) test bars. The design of Whaler was then incorporated into ASTM B108 in 1980. This mold has been widely adopted by foundries over the years. The mold is often named as the ASTM-B108 mold or Stahl Mold. In the Stahl Mold design, the sprue is narrow and tapered so as
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