Fig 2. The dimensions of a fatigue test bar are shown.


mold and Case-H mold were preheated to 400F (204C) for mold coating application, then poured at 625F (329C) mold temperature in all foundries; the step mold was preheated and poured at 400F (204C) mold temperature. Te step mold produces sections of different thickness, form- ing a step-like shape. Te variation in thickness allows one to examine the mechanical properties of castings solidified under different cooling rates in one pour. However, because the mold does not produce a test bar shaped casting, significant machin- ing labor is required to prepare test bars. Tensile test bars were machined from the 2-inch section and fatigue samples were machined from the 1-inch section (Fig.1d). Te A356 test bars poured at CWRU and Foundry A


were heat treated with T6 condition: solution treatment at 1,000F for 12 hours, quench, artificial aging at 320F (160C) for 6 hours. Te 319 test bars poured at Foundry A were heat treated with T6 and T7 conditions. T6: solution treatment at 950F (510C) for 8 hours, quench, artificial aging at 320F for 6 hours. Both commercial operations applied T7 to the 319 alloy test bars: solution treatment at 925F (496C) for 8 hours, quench, artificial aging at 462F (239C) for 4.75 hours. Te gauge section diameter of Stahl mold and Case-H mold is 0.5 inch. Te test bars in the as-cast, T6 and T7 condition were pulled to fracture with a tensile test machine at room temperature at a strain rate of 10-3s-1. In addition to the UTS and elongation, the Quality Index was used to evaluate the overall mechanical properties of the A356 test bars. Te Quality Index is defined by the equation:


QI = UTS+150log10 (elongation) Te UTS and elongation values were determined from an


average of six bars for the Stahl mold and the Case-H mold and at least three bars for the step mold and the sand mold. In the latter stages of this work, the yield stress also was determined. Fatigue test bars were cut and machined from the step


mold’s 1-inch section as shown in Figure 2. Testing was conducted at C-T-C at 125 Mpa in fully reversed sinusoidal loading at 60 Hz.


Fig 3. The porosity of Stahl/Case-H mold (a) and step mold 2-inch section test bars (b) is shown.


50 | MODERN CASTING November 2014


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