In Fig. 11, the equivalent fraction of defects (generated by solving Eqn 14 for a strain hardening exponent of n = 0.259) may be examined against the spread of data pres- ent. For the samples produced at 26 m/s, the proportional spread of equivalent defects present on the fracture sur- face ranges from around 0.32 to 0.44. For the samples produced at 82 m/s, the equivalent values are 0.22 to 0.34. It is important to note that these numbers generated us- ing Eqn 14 appear to be very large since it is most com- mon to have less than 2% average porosity by volume in a HPDC. However, as noted by Cáceres and Selling,7
the
volume fraction of defects present in a casting is a poor representation of the tensile fracture behaviour. In addi- tion, as pointed out by Sigworth,2
the defects present on
the fracture surface may be of the order of three to twenty times the volume fraction of porosity present in the alloy. Examination of Figs. 1, 2, 6, 7, 8 agree with this state- ment and suggest that a very large proportion of the frac- ture surface may be part of a composite defect structure. In addition to porosity, oxides and Fe bearing particles, other microstructural features such as Cu bearing inter- metallics, coarse silicon plates, heterogeneous grain sizes arising from pre-solidified grains24
and weaker than aver-
age aluminium grains, are also likely to be contributing to the defect equivalence, where complex flow localization is considered. In addition, the skin of an A380 HPDC may have tensile properties higher than the average,* meaning the weaker, interior material, occupies a relatively large proportion of the cross sectional area and therefore will also play an important role in strain localization leading to failure. (*Note: Preliminary experiments on a limited number of samples [three] of Alloy 1 produced at 26 m/s, where the centre was carefully machined out of the cast- ing leaving only the skin layer, suggest the strength prop- erties are approximately 20% higher than the average, for the test bars used in the current work.) In the current samples, the “skin” layer is 400-500µm thick, and there- fore accounts for approximately 1/3 of the cross sectional area. Therefore, although the numbers generated by solv- ing Eqn 14 for the equivalent defect contents are high, they do not appear to be unrealistic in light of the actual defect clusters observed on the fracture surfaces.
Effect of melt Degassing
To further test these proce- dures and to examine one potential way to improve casting quality, an addition- al set of experiments were conducted utilizing the prin- ciples outlined above. Alloy 4 from Table 1 was manu- factured at a melt veloc- ity of 82 m/s from recycled runners, biscuits, and reject samples of A380 used for
making Alloys 1-3. It was expected that the use of recycled material containing larger proportions of oxides and other defects would simply produce a greater spread in the re- sults and would therefore produce a good comparison. Two batches of samples were prepared. In the first batch of sam- ples, the recycled material was cast with no special prepara- tion. Once molten and the temperature stable, the dross was skimmed from the surface prior to casting. A reduced pres- sure test sample taken just before casting commenced (but after skimming) is shown in Fig. 12(a) and it will be seen that, as expected, the material contained substantial amounts of hydrogen. It is also reasonable to assume, oxides are pres- ent in suspension. Following preparation of the first set of samples, the melt was treated with the rotary degassing unit operating at 300 RPM with high purity argon flowing at 7 litres/min. for a total of 20 minutes, and a reduced pressure test specimen was again taken before casting. The results are shown in Fig. 12(b), and the absence of pores confirms that the melt had been successfully degassed. In addition, it would be reasonable to assume, a percentage of the ox- ides present in the melt were also eliminated.1,25
Gallo25 has
shown that by melt degassing alone without the use of flux, around 60% of oxide films are removed from the melt, along with up to 80% of the spinel-like oxide inclusions that are present. One interesting observation in this regard appears in Fig. 12(c), which shows the results of a reduced pres- sure test specimen taken after holding the molten metal for 16 hours after the test of Fig. 12(b). The sample presented in Fig. 12(c) has almost the same measured density as Fig. 12(a) due to the re-hydrogenation of the molten aluminium, but the appearance of the porosity present is somewhat dif- ferent to that observed in the lower section of Fig. 12(a) and may have resulted from oxides in the melt. Irrespective of the reasons for this curious difference, both hydrogen and oxides are well known to adversely influence casting quality and it would be quite reasonable to suggest samples made from the material shown in Fig. 12(b) should be of superior quality to those made from the material shown in Fig. 12(a).
Twenty-five tensile tests were conducted with material cast both before and after the rotary degassing, therefore representative of the metal obtained from the reduced pressure test samples shown in Figs. 12(a & b). Values of the mean (µ), one standard deviation (σ), and µ -3σ
Table 6. Individual and Combined Strain Hardening Exponents, n and Strength Coefficients, K, for Alloys 1 to 3
International Journal of Metalcasting/Summer 2011
49
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