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residues were metallurgical spinel, reacted refractory and alumina needles. The latter is an indicator of increased use of recycled material. Spinel forms in the presence of magnesium and reacted refractory stems from refractory coatings, furnace linings or scrap additions. The overall inclusion content, especially the increase of alumina nee- dles provides evidence that the turnings were successfully introduced into the melt. The results of the PoDFA tests indicate a significant increase in oxide films contamination with higher scrap metal additions which is not revealed by the bifilm index. However, it has been indicated that the PoDFA technique has some limitations16 like thin bifilms may easily pass through the filter.3, 17


and defects The


nature of oxides was characterized as “fine.” This can be explained by the research of Liu and Samuel.18


They


found that due to density differences most of the larger inclusions will settle in the bottom of the crucible when the melt stands for a longer period of time. In turn, de- gassing and upgassing are very effective in removing most of the inclusions, namely chunk size oxide particles and thick films.18-19


For the work described here, this means that


due to the same treatment performed, every melt has lost its majority of larger oxide inclusions by settling, floating and skimming before the pour, leaving behind thin oxide films. Due to their nearly neutral buoyancy, the floating and settling is especially slow for these thin films. In a gen- tly convecting melt this can take hours or days.1


Therefore,


the expected increase in the bifilm index is not apparent. On the other hand, it is important to note that the bifilm index is the measure of total oxide length. Although ox- ides were successfully introduced into the melts, the simi- lar bifilm index measurements could only indicate the total oxide content which is the overall quality of that particular melt. It may be important to include the number of pores in the RPT samples as an additional indi- cation of melt quality which may result in a more straight-forward explanation of the change in the mechanical properties and the distribution of the pores in the step mould samples. Currently Dispinar20


has a publi-


cation in progress about this subject. It is important to note that hydrogen helps the inflation of bifilms. Thus, one has to no- tice that the combination of high oxide and relatively low hydrogen concentration in the melt may lead to an underestimation of bifilm values.


Porosity characterization


Castings are ideally designed so that the last metal which solidifies is the liquid metal inside the feeder. That means the hydrogen enriched liquid ahead of the solidification front is the last to solidify in the used step die. The combination of inappropriate feeding and low hydrogen


48


concentration has led to large shrinkage porosity near the top of the castings, especially in the junction of the feeder and 30 mm (3 cm) thick section, which is a hot spot. This resulted in elongated, interconnected and interdendritic pores near the surface and within the bulk of the material. Shrinkage and gas porosity was found in all cast parts. Contaminated melts with oxides leads predominantly to shrinkage porosity whereas the cleanest melt exhibited higher fractions of gas porosity.


A possible reason for this is that large oxide films block the feeder path in the mushy zone during interdendritic feed- ing. Oxide films may have been pushed upwards towards the feeder of the casting due to directional solidification,3 density differences and convection which results in feeder blockage and formation of surface linked porosity. More- over, it is reported that oxidized melts have a reduced ability to flow. According to Campbell21


it is also possible that large


bifilms, created during filling of the mould, block the feeder neck. Campbell proposes that, once bifilms settle inside the feeder, the reducing pressure in the casting pulls the lowest bifilm apart from the unbonded upper half. This mechanism leads to under-feeder porosity.


Figure 12 explains the formation of surface sinks and in- ternally nucleated porosity under various conditions. Since, in comparison, Melt 1 is clean, the castings are expected to have more surface sink problems. However, this was not the case because Melt 1 has only few entrained oxide films. Hence, the liquid metal was not hindered from flowing to feed the shrinkage. In the presence of high hydrogen concen- tration and oxide films, porosity can completely compensate the volumetric loss due to shrinkage and vice versa.


Figure 12. Influence of hydrogen concentration and oxides on the shrinkage behaviour; (a-b) clean melt that contains neither oxides nor hydrogen; (c-d) melt contains oxides but no hydrogen; melt contains oxides and hydrogen (e) high and (f) low hydrogen content.


International Journal of Metalcasting/Spring 2012


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