pressure testing was conducted during selected experiments using a stainless steel cup with a Dynarad reduced pressure test unit, where a vacuum of -100 kPa was applied. Follow- ing these procedures, density was calculated using the Ar- chimedes technique.
Scanning electron microscopy (SEM) was conducted using either a Leica S440 SEM microscope, or alternately, an FEI Quanta 400FEG environmental SEM. For select cases, Im- age Pro Plus analysis software was used with color segmen- tation to approximate the area of the defect on the fracture surface.
results and Discussion statistical Analysis
Initial experiments were conducted to determine if mean- ingful results related to casting quality could be derived for HPDC components. As a baseline measure of performance, in previous work on HPDC’s12,13
it has been demonstrated that
some differences exist in the tensile properties derived from as-cast samples produced with a velocity at the gate of 26 m/s, compared to those produced with higher velocities such as 82 m/s. The difference in properties was later shown to become more significant in heat treated conditions,14
where, as would
be expected, a higher quality as-cast part will result in a higher quality heat treated one. Data for an A380 composition (Alloy 1 from Table 1) were generated using 25 individual tensile tests for each of the two melt velocities.
Values of the mean (µ), one standard deviation (σ), and µ-3σ for the tensile data of each of the two conditions are shown in Table 2. As a particularly simple technique for ex- amining the comparative quality of the two sets of castings, this technique shows an excellent representation, since only 0.135% of actual data should fall below µ-3σ. (Note that µ ±1σ represents 68.2% of the data, µ ±2σ represents 95.4% of the data, and µ ±3σ represents 99.7% of the data). As shown in Table 2, values of µ -3σ for the 82 m/s samples were sub- stantially better than for the 26 m/s samples. In this regard the values of µ -3σ for the 82 m/s condition are actually reasonably close to the mean values for the 26 m/s condi- tion. Here it is important to note that these tensile property values are only valid for the specific geometry tested, and importantly, are not necessarily comparable with castings of other geometries or gating systems.
Representative samples having close to the mean values of elongation and tensile strength were chosen for further study. Figure 1 shows fracture surfaces of an as-cast sample pro- duced with a melt velocity at the gate of 26 m/s. The fracture surface is shown as a secondary SEM image in Fig. 1(a) and as an equivalent Backscattered Electron (BSE) image in Fig. 1(b). Figure 1(c) and (d) are higher magnification images of the central region of the fracture surface, also in BSE mode. Casual examination of Fig. 1(a) suggests that no large pores or defects were present on the fracture surface. However, the BSE image in Fig. 1(b) reveals a defect that occupies a very large proportion of the fracture surface (the defect was found to have an area of approximately 6.1 mm2
using Image Pro Table 1. Alloys Examined
Table 2. Data for Alloy 1 Samples Produced at 26m/s or 82 m/s
40
International Journal of Metalcasting/Summer 2011
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