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A growing body of evidence shows that Ni base alloy castings, especially of superalloys, harbor cracks because of the poor casting techniques that are used currently to shape these materials.[1–3] These defects arise naturally during the turbulent pouring of metals (Figure 1). The importance of this subject was confirmed by reports on three in-service failures of Ni-based superalloy turbine blade castings.[4–6] Naeem et al.[6] list a review of previous blade failure reports. Although it is acknowledged that engine manufacturers go to great lengths to ensure that a ‘‘blade off’’ event does not cause the engine to fail, it seems unhelpful to continue to put huge efforts into metallurgical research on alloy development while ignoring the major casting defects necessarily introduced by current manufacturing techniques. The defects in metallic liquids and final castings are principally bifilms. They seem to give rise to a wide spectrum of phenomena including porosity,[1] hot tearing,[1] cold cracking,[1] fatigue initiation[7–10] corrosion,[1] and stray grain initiation.[11] This list is formidable. Bifilms are a serious issue that so far has not received the attention that it deserves. Bifilms are created easily and quickly during casting. The surface of the melt oxidizes rapidly (whether in air or in so-called ‘‘vacuum’’ that has to be viewed as merely ‘‘dilute air’’) so that when folded in or when experiencing collisions between droplets, the surface oxide contacts dry-face-to-dry-face when impinging JOHN CAMPBELL, Emeritus Professor, is with the Department of Metallurgy and Materials, University of Birmingham Edgbaston, B15 2TT, U.K. Contact e-mail: [email protected] MURAT TIRYAKIOG˘LU, Director and Professor, is with the School of Engineering, University of North Florida, Jacksonville, FL 32224. This article was presented at the 4th Shape Casting Symposium in Honor of Prof. John T. Berry. Manuscript submitted January 6, 2012. Article published online April 10, 2012. 902—VOLUME 43B, AUGUST 2012

against other masses of liquid. The resulting unbondable interface, which is a double film called a ‘‘bifilm,’’ is then entrained in the bulk liquid as a crack. Our existing pouring systems are mostly turbulent and fill the liquid metal with cracks. The defects remain in suspension sufficiently long to become frozen into the casting. A significant advance in recent years in the design of filling systems for castings has been the concept of the critical velocity, which has also led on to the concept of the critical fall distance.[1] This velocity for all engineering liquid metals is close to 0.5 m/s, which, if exceeded, means that the melt has sufficient energy to jump and splash, and so is in danger of enfolding its surface to create bifilms. This velocity is exceeded when the metal falls under gravity after a distance of only about 10 mm. This trivial distance effectively forbids gravity pouring of metals if bifilm-free material is desired. If castings with reduced bifilms are desired and if gravity pouring is still required to be used, the