Letter to the Editor
- PDF / 342,078 Bytes
- 3 Pages / 593.972 x 792 pts Page_size
- 22 Downloads / 277 Views
authors carry out an investigation to ascertain whether there are oxide bifilms (cracks introduced into the liquid alloy by turbulence during pouring) in the Ni-based superalloy IN100. They are the first investigators to use such powerful techniques as high-resolution electron microscopy and scanning auger microscopy for the study of bifilms. The authors report many fascinating new observations, but curiously concluding that the evidence for bifilms is either absent or questionable. It seems to this writer that the authors have misinterpreted their interesting data, and that, in general, their study is excellent support for the bifilm concept. We shall consider their papers in order, avoiding the repeat of discussions common to both papers.
PAPER I The authors select to compare top and bottom gating since these differing filling conditions have previously been reported to give different results, as would be expected, with more bifilms being created by the poor top filling geometry (sometimes supplemented by turbulators). However, of course, if both top and bottom gating geometries are poor, as is usually the case with investment casting, and if the original poured melt already contains a high proportion of bifilms, which is also probable, it is likely that any differences between top and bottom filling will merely disappear as ‘noise’ when studying a liquid metal which already contains a dense population of defects. (it may be significant that
JOHN CAMPBELL, Emeritus Professor of Casting Technology, is with the Department of Metallurgy and Materials, University of Birmingham, Birmingham, UK. Contact e-mail: [email protected] Manuscript submitted March 23, 2016. Article published online June 15, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
the melts were 70 pct virgin and 30 pct revert material?) In the experience of this writer, this is a common circumstance, so that the observed similar properties delivered by both geometries are predictable. It is well known that the alloy provided by the alloy producers to the foundry is poured via a system seemingly designed to generate the greatest density of bifilm defects. The current melting and casting process in the foundry will only add to this already massive population. It follows that all the mechanical property results of this study were poor, and differences, where observed at all, were practically negligible. The slight reduction in properties of air cast material vs material cast in a vacuum or argon environment is understandable; the invariability of the yield strength is commonly observed as a result of the loss of area by the defects being to some extent compensated by the increased work strengthening of the region surrounding the defect. The fact that the mechanical properties of vacuum and inert gas environments were so little improved compared to air cast material, which is surely seriously degraded, confirms the poor properties of the traditional and conventionally vacuum-cast Ni-based superalloy. There is a powerful implication here that enormous improvements are
Data Loading...
