Experimental Methodology
The tensile test bars were fabricated and tested as indicated by the ASTM B557 protocol. Geometry is shown in Fig- ure 1. To ensure the exact placement of the tensile test bar blanks with respect to the chill, a template was used to draw the exact location for the blanks before sectioning.
Two different chill designs (Chill #1 and #2) for a prototype casting were used to provide a refined microstructure from which tensile test samples could be extracted. The details of the two chills are indicated in Table 1 and Figure 2. Specific design details of the engineered prototype casting will not be discussed further in this paper as they are proprietary, how- ever the resulting microstructure achieved will be discussed as it is relevant to the discussion of the tensile test results. In addition, the heat treatment used was a T7 for all castings. The specific parameters are 480°C (896°F) for 5.5 hours, followed by a forced air quench producing a 0.80°C/min cooling rate
(confirmed with K-type thermocouple trace in-situ where tensile test samples are extracted) followed by an age treat- ment at 240°C (464°F) for 5.5 hours. The temperature con- trol in both solution and age was ± 2°C from target. Table 1 specifically lists the measured λ2
Figure 2) and the calculated solidification time that reflects the chilling force achieved.
(from the λ2
Three iterations of Chill #1 will be investigated where the effect of elevated dissolved hydrogen in the aluminum bath, and the position of the tensile test bar in the cast structure, with respect to the chill, are investigated. Three more itera- tions using Chill #2 will be used to establish the effect of sample preparation and testing laboratory proficiency of generated tensile test results. Two tensile test bars were ex- tracted for each casting and 30 tensile test bars are needed to generate reasonable Weibull statistical analysis, thus 15 castings were made for each iteration, or 90 castings in total for this study.
Table 1. Specific Conditions of the Chill Types and Targeted Microstructure array seen in
Table 2a. Chemistry as Determined from Optical Emission Spectrometer (OES)
Table 2b. Dissolved Melt Hydrogen Concentrations as Determined by the In-situ Alspek™ Probe & Reduced Pressure Test. The Target Vacuum Used in the RPT Apparatus was -27 ± 2 in Hg.
International Journal of Metalcasting/Winter 10
33
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