A 40,000 lb. capacity furnace with an electromagnetic pump was used to cast the different iterations of engi- neered prototype castings. A nitrogen blanket exists over the bath surface to deter excessive oxide formation. The chemistries used for the different iterations are listed in Table 2a. The melt temperature used was 730 ± 5 °C and levels of dissolved hydrogen were kept below 2.70 grams/ cc via the density of the Reduced Pressure Test (RPT) samples (this corresponds to 0.11 H2
cc/100 grams Al, as
measured by the Alspek™ hydrogen analyzer, see Table 2b). The authors do not contend that the relationship be- tween the RPT density and hydrogen concentrations as measured by the Alspek™ hydrogen analyzer is direct, as oxide concentration could also lower RPT density. However known practices, namely the already mentioned nitrogen blanket, were used in melt handling which con- sistently provides very low oxide concentrations in the melt. The quiescent liquid metal filling provided by the electromagnetic pump, coupled with the very low hydro- gen levels, gave a cast structure which did not have spuri- ous pores (e.g. from unfurled bi-films) resulting in exces- sive scatter in tensile testing results. This was confirmed by the lack of G-rated indications as indicated by ASTM E155 radiography standard (for coarser structure) and the lack of furled oxide films on the tensile fracture surfaces (finer structure). After the casting was made, a T7 treat- ment was used.
Two metallurgical labs (Lab #1 and Lab #2) were involved with the tensile testing for this work, and both used a UTS (SFM–60) tensile machine at a strain rate of 8 x 10-4
(mm/
mm)/second to test the tensile samples. A completed test (at the point of fracture) took approximately 10 minutes. A strain gauge extensometer was attached to the tensile test samples for the measurement of elongation. After the test was conducted the Yield Stress (0.2% offset), Tensile Stress (UTS), and Plastic Elongation were measured as de- fined in the ASTM B 557 specification. Both testing labs (#1 and #2) are accredited to ISO/IEC 17025 by A2LA for tensile testing.
Once the tensile test sample has fractured, one end con- taining the fracture surface was mounted and polished for metallographic analysis using an Image Analyzer. At a distance of 3 mm below the fracture surface, the Porosity (%), Largest Pore Measured (µm), λ2
(µm), AFS Si Modi-
fication rating, and finally the Vickers microhardness were measured from dendrite cells. The details of the field size, magnifications used and the frequency of a particular mea- surement as done for each test sample is listed in Table 3. Tests covered under the labs’ A2LA accreditation scope are indicated by an asterisk.
Weibull Statistical Method
Although this type of analysis has been developed with the investigation of mechanical properties of ceramics in mind, it has been successfully applied in previous analysis of the tensile test data for aluminum based cast components.14-16
The approach used in Weibull analysis was proposed by Weibull.17
Its
usefulness lies in the fact that it accounts for the cast structural defects during the analysis of the tensile properties.
Since Yield Stress (YS) and Yield Strain (εYS ) are rather
not follow the Gaussian distribution, but rather a skewed distribution. As a result Weibull statistics lends itself as a useful alternative when analyzing large quantities of UTS and
El.TOT
insensitive to casting defects, they tend to follow a nor- mal distribution. In that case, the statistical analysis tools are the mean, the median and the mode, in combination with single/double/or triple standard deviations (σ/2σ/3σ). However, the Ultimate Tensile Stress (UTS), and the To- tal Elongation (
El.TOT
) are casting defect sensitive, and do tensile data. been treated using this approach.
The analysis begins with rearrangement of the data in as- cending order. A frequency plot can then be constructed where the occurrence of a specific data point is quantified within narrow range sets. In this work both the Ultimate Tensile Strength (UTS), and total Elongation (
El.TOT
) have
Table 3. Details of the Metallographic Analysis Preformed 3 mm Below the Fracture Surface of the Tensile Test Sample. The Array Size was 9.496 mm2
34
International Journal of Metalcasting/Winter 10
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