ture surface that were visible to the naked eye were routinely segregated for further examination. Three samples from the untreated batch of material were placed aside, whereas only one showed this type of defect in the treated material. After testing was complete, and the data analyzed, it became clear that these four samples also represented the lowest four data points in the combined set of test results, falling within the range of q = 0.05 to q = 0.1 in Fig. 14.
Figure 16 shows secondary electron images of the frac- ture surfaces of the four segregated samples in (a, c, e and g). Corresponding backscattered electron images are provided in (b, d, f, and h). For reference, the specimens with the fracture surface shown in Fig. 16 (g & h) demon- strated the lowest properties (159 MPa 0.2% proof stress, 283 MPa UTS and 2.1% Ef
), with all three of the other
tween the fracture surfaces. Figure 16 (a & b) represented the one sample showing a visible oxide defect where the molten metal was treated by degassing. In (c & d), close examination revealed there were two oxide defects pres- ent on the fracture surface (arrowed). A small amount of the foam-type dispersed shrinkage porosity mentioned earlier was also able to be detected from the BSE im- age. Alternately, in (e & f), only a small oxide defect was present but a larger amount of the foam-type shrinkage porosity was evident as the darker region in the centre of Figure 16(f). In (g & h), it is important to note that, not only is the amount of the foamy shrinkage defect present relatively significant, the size of the oxide defect on the fracture surface was also larger than in all other exam- ples. This sample corresponded to the lowest mechanical properties derived from the combined set of tests.
samples showing almost identical properties to each other (157-165 MPa 0.2% proof stress, 304-306 MPa UTS and 2.7-2.8% Ef
What is most significant is that the four lowest values of tensile properties within the combined data sets all dis- played the presence of one or more oxide films present on the fracture surface. This would suggest that the presence or absence of these films is critical to the properties and relative quality of HPDC components. In this instance, it may be concluded that rotary degassing clearly had very little effect on average tensile properties, but did have an effect on the spread of results. Quality was improved in the treated material since the Weibull modulus was in- creased from 25 to 44.7. This increase corresponded to the proportion of samples displaying large, visible oxide defects on the fracture surface being reduced by melt degassing, and as a result, the quality of the final cast product was improved.
summary and conclusions
Here it is important to comment on the different measures of what constitutes casting “quality,” and the various ways in which it can be evaluated. What is the best method to
52 ). Interesting comparisons may be made be-
use is the subject of much debate,11 but it is important to
note each technique has widely varying complexities. The techniques used in this current work all provide excellent information regarding casting quality and all can clearly identify what constitutes a “better” casting. The methods developed by Cáceres9,10
may be considered to be robust in
that they can represent different casting geometries from different sources, for a single alloy specification, and allow them to be compared against a unified measure of casting quality. An even more powerful tool for analysis of as-cast high pressure diecastings is to use these accumulated re- sults in combination with a derivation of the Weibull mod- ulus of both tensile strength and elongation.
Whichever technique that is used, most crucial is the ability to establish the baseline performance with high reliability. Once the baseline condition and importantly, the spread of data, are known, changes in quality and the reasons for these changes are relatively simple to detect. Secondly, samples generated during the evolution of the baseline data, provide a wealth of reference information. In the current work, it has been demonstrated that casting quality may be improved by increasing the melt velocity at the gate, which has the effect of decreasing the size of defect clusters appearing on the fracture surfaces. Cast- ing quality is also influenced by alloy composition, but this effect would appear to be complex, and further in- vestigation regarding the actual basis of the differences is warranted. Rotary degassing to remove hydrogen and ox- ides from the melt does not influence quality in the same way as melt velocity at the gate, but it does decrease the spread of results generated. In this instance, casting qual- ity would appear to be improved through the removal of a proportion of oxides from the melt, which has an effect on the value of Weibull modulus. As may be appreciated, it is impossible to completely eliminate oxides from the HPDC process as many will originate simply due to the extreme turbulence that exists during casting. However, eliminating large oxides already present in the melt which arise from the use of recycled metal would appear clearly to be advantageous.
Finally, it is important to note that defects with substantially different size, shape and distribution may have similar or equivalent effects on the tensile failure. In HPDC defects are usually present in complex clusters where many differ- ent features have an inter-relationship, and, as a result, such defect clusters may behave as singular defects with a sub- stantially different size.
Acknowledgements
The authors would like to thank Andy Yob and Gary Sav- age for assistance with casting, reduced pressure testing and rotary degassing. The authors would also like to thank Drs. Carlos Cáceres, Geoffrey Sigworth and Prof. Ian Polmear for their valuable comments on this manuscript.
International Journal of Metalcasting/Summer 2011
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