A Diffusing Runner for Gravity Casting
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LUMINUM alloy, as the liquid aluminum contacts the surrounding atmosphere containing the oxygen, has a tendency to form an insoluble oxide film on the surface. As Runyoro and Campbell[1] observed the emerging liquid aluminum in various speeds through the gates, they found that conditions for surface turbulence in the casting start at 0.5 mÆs1. With velocity higher than this critical ingate velocity, ‘‘fountaining’’ with a mushroom shape was observed, which generates severe rates of oxides entrainment. When Grube and Kura[2] investigated water flow through the transparent mold of their running design, the velocity of the liquid leaving the sprue was approximately 2.1 mÆs1, whereas the velocity at the exit end of the gate was 0.457 mÆs1. From photographs recorded at 128 frames per second, they found that a quiet manner of flow entered the riser (or gate) and that the severe surface turbulence was eliminated. Later, no surface defect was found when they cast aluminum alloy (with 7% Mg) into the same running system. Supposedly, this aluminum alloy experienced a similar condition to the water. Thus, the velocity of the liquid metal entering the gate was around 0.5 mÆs1 or less, which was certainly beneficial for the quality of the casting. Green and Campbell[3] found that the Weibull twoparameter distributions best describe the scatter of tensile test data and allows discrimination between ‘‘good’’ and ‘‘bad’’ casting. The Weibull distribution is convenient as a tool for casting quality assessment since it enables a single figure quantification (modulus k) to be FU-YUAN HSU, Assistant Professor, and HUEY-JIUAN LIN, Professor, are with the Department of Materials Science and Engineering, National United University, MiaoLi 36003, Taiwan, R.O.C. Contact e-mail: [email protected] This article is based on a presentation given at the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online August 12, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
used to describe the scatter in strength and, thus, is an excellent parameter for the quantification of reliability. They found that high-reliability aluminum alloy (i.e., Al-7Si-Mg) castings were obtained by bottom gating a sand mold at an initial gate velocity less than or equal to 0.5 m.s1. The analysis also showed that the bottom gating at a velocity greater than the critical ingate velocity is equally as likely to produce bad castings as a top gating. The skewed distributions of tensile strengths seem to be associated with entrained oxide films produced by surface turbulence. These oxide-film defects introduced during filling dominate the reliability of any set of aluminum castings. Green and Campbell[4] later found that the tensile strength distribution is controlled by the runner system design. Use of a runner system that promoted the formation of large amoun
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