New Test Bar for Permanent Mold Casting Developed An AFS research project updated the standard permanent mold test bar design for better representation of clean metal’s highest mechanical properties. david neff and geoffrey sigworth est bars are used in alumi-num foundry practice for both sand and permanent mold casting applications. They may be incorporated into the casting gat-ing system, or more commonly, metal is poured into a separate mold. The purpose of the test bar is to give an indication of the mechanical prop-T erties achievable with a particular melt composition. However, test bar mechanical properties only correlate with the actual properties in a casting when the solidification conditions and defect population are similar in the test bar mold and a given section of the casting. The permanent mold test bar design Fig 1. Mechanical properties of 356-T6 alloy test bars are displayed as a function of iron content and aging time. Castings were aged 2, 6 and 18 hours at 310F (155C). widely used in the U.S. was developed nearly 50 years ago by Ken Whaler at Stahl Specialty Company. He cut molds of various designs from plexi-glass blanks. By pouring colored water into the transparent molds, he found a design that produced a smooth, qui-escent fill of liquid metal in the mold cavity. Stahl Specialty also produced and sold foundry equipment, including their test bar mold, so it became known as the “Stahl” mold. The mold design was subsequently incorporated into ASTM standard B108. Whaler studied the effect of aging time and iron content on tensile prop-erties in A356-T6 alloy castings, using the B108 mold. His results are shown in Figure 1. This figure clearly shows how a standard mold can be used to produce extremely useful results. The original Stahl mold had to run fairly hot, or shrinkage porosity would occur in the gauge section of the bars. (The mold casting tempera-tures recommended were 800-860F or 427-460C). A hot mold was also required to fully fill the cavity. This was not so convenient in practice—15 to 20 shots were needed to preheat the mold—so foundries sometimes made modifications. The sprue was 36 | MODERN CASTING March 2021
Modern Casting March 2021: Page 36
