Sedimentation analysis of inclusions in aluminium and magnesium
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IT is well established that aluminium and magnesium melts contain small amounts of inclusions ~,2 with high densities. The common types are oxides, carbides and borides in aluminium and oxides, nitrides, chlorides and iron-rich particles in magnesium. Standard optical microscopy techniques for analysis of inclusions in ingots are tedious and normally give only qualitative results. Thus we decided to try and develop a new method based on centrifugation of melts. This is a general technique for separation of phases with different densities. The main advantage of the method is that the particles in the liquid are collected in a small volume so that the subsequent analysis of particles is simplified. Centrifuging of molten aluminium has been used so far for investigations of inclusions in grain refined materials 3,4 and for evaluation of melt cleanliness: According to Mollard et al, 5 borides and carbides settle at the bottom of the crucible, while oxides move toward the top. Thus, we decided to build a centrifuge suitable for melts and test if sedimentation could be used to improve the methods for measurement of content and size distribution of inclusions in aluminium and magnesium. The work was concentrated upon three issues: 1) Development of a centrifuging technique suitable for magnesium and aluminium melts. 2) Analysis of oxides, carbides and borides in aluminium ingots. A general survey of inclusions in magnesium and aluminium, using among other techniques sedimentation analysis, has been published previously) ,2 3) Dissolution of A13Zr in aluminium and (Fe,Mn)3Si in magnesium at high temperature. This part was carried out since we found that a portion of these CHR. J. SIMENSEN, Project Manager, formerlywith Central Institute for Industrial Research, Oslo, Norway, is now with Ardal & Sunndal Verk a.s. PO Box 51,6601 SunndalsOra,Norway. Manuscript submitted December8, 1980. METALLURGICALTRANSACTIONSB
phases dissolved during heating of the melts. Comparative analyses were carried out on noncentrifuged samples using emission spectrography, gas chromatograhy, neutron activation analysis and chemical analysis. During the investigation it became evident that centrifuging could also be used for determination of impurity concentration of atoms in liquid solution. One example is given in this paper, namely that of titanium, boron, and vanadium in an aluminium ingot. SEDIMENTATION THEORY Consider an aluminium melt being centrifuged with rotational frequency f until it freezes. During this period, the particles move towards the bottom or the top of the crucible with a velocity v given by Stoke's law (Fig. 1): 6'7 V ~--.
21I 2 d2 f2(pp - pu)czr 9 ,/
[ll
where rl --- viscosity coefficient of the liquid, d = spherical diameter of the particles (defined as the diameter of a sphere with the same volume as the measured inclusion), r = distance between the rotation axis and the particle, Ou, 0e = density of the melt and the particle, respectively, and t~ = V/Vsphere = is the relative speed of the particle to that of a sphere
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