Characterization of the Melt Quality and Impurity Content of an Lm25 Alloy
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INUM has a high affinity for oxygen and as a consequence readily forms an oxide. If a molten aluminum surface reacts with atmospheric oxygen, an oxide film is created immediately. This film can take different morphologies, depending on the alloying elements of the melt and the contact time with the oxygen.[1] A film created on the melt surface cannot be dissolved in the liquid solution; it remains solid. The formation of bifilms has been extensively described by Campbell.[1] Essentially, surface turbulences are the only possibility to entrain folded films (bifilms) into the melt,[1] which subsequently lead to casting defects, e.g., cracks or pores.[2,3] All oxides are created by the described entrainment mechanism. Furthermore, the following two forms of inclusions can be distinguished[4]: new oxides (bifilms) and hard inclusions (old oxides, nitrides, carbides, etc.). New oxides are created during filling and casting, and their shape is two-dimensional. In contrast, hard inclusions are created before or during the melting. Their shape can be described as three-dimensional. It has been proposed that trapped atmosphere in the form of N2 and O2 can react with the bifilm, so that the reaction KATHARINA HABERL, Scientific Researcher, and PETER SCHUMACHER, Professor and Head of Department, are with the Chair of Casting Research, Metallurgy Department, University of Leoben, 8700 Leoben, Austria. Contact e-mail: katharina.haberl@ unileoben.ac.at. PETER SCHUMACHER, Managing Director, and GEORG GEIER, Head of Tomography Group, are with the Austrian Foundry Research Institute, 8700, Leoben, Austria. BERNHARD STAUDER, Head of Process Development, is with NEMAK Linz GmbH, 4030 Linz, Austria. This article is based on a presentation given in the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online August 25, 2009. 812—VOLUME 40B, DECEMBER 2009
products fill the bifilm and a hard inclusion is formed in the end.[5,6] In addition, new bifilms can fold and ravel during bulk turbulences into a compact mass.[7] For the nucleation of hydrogen pores, bifilms (new oxides) are particularly relevant. Moreover, the influence of other hard inclusions in the melt has to be considered for mechanical properties. Porosity has a negative influence on the casting part. It decreases the mechanical properties (fracture elongation, tensile strength, fatigue life) and hinders potential applications for the castings. Two extreme morphologies of porosity are gas-driven and shrinkage-driven porosity.[8] The formation of each one cannot be separated from the other as they are both caused by bifilms; hence, porosity is a mixture of both these features,[1,9] as shown in Figure 1. However, in the reduced pressure test (RPT) method, samples are well fed during solidification. Thus, the influence of shrinkage on porosity formation can be viewed as negligible
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