Structural Homogeneity of Direct-Chill Cast Ingots of Aluminum Alloy EN AW-5083
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ve was to determine the structural homogeneity of ingots of aluminum alloy EN AW5083, designated according to EN 573-3,[1] which were cast by the vertical direct-chill (DC) casting process on a reconstructed and revamped casting line,[2,3] by measuring the size and the mean number of grains per unit area. The investigation was planned to follow the statistical design of the Latin square.[4] Sampling was done from slices of about 3 cm in depth, cut transversally from the front and rear sections of the cast ingots. NATALIJA DOLIC´, Research Assistant, ANTE MARKOTIC´, Professor, and FARUK UNKIC´, Associate Professor, are with the Faculty of Metallurgy, University of Zagreb, 44 103 Sisak, Croatia. Contact e-mail: [email protected] Manuscript submitted April 25, 2006. Article published online May 12, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
Investigation of structural homogeneity was carried out in six ingots cast by the semicontinuous vertical DC process. The ingots 1430 · 520 · 5100 mm in size were manufactured from six different charges of alloy EN AW-5083 (designations 3112, 3113, 3114, 3116, 3117, and 3120). Before casting, the melt was refined with an argon and chlorine mixture in an Alpur unit. For grain refinement, the Al-Ti5-B1 master alloy was used in an average amount of 1.9 kg/t melt; small bars of the alloy were added to the casting furnace, and a wire was introduced in a launder positioned in front of the Alpur unit. Figure 1 is a schematic representation of the Latin square-based sampling design, which belongs to a group of orthogonal models that find large application when no interactions between variability sources–process factors are expected,[4] where i is the slice height/ingot depth and j is the slice width/ingot width. The specimens cut from the slice carry the following designation: charge number–specimen number, letter F or R. The letter F refers to specimens taken from the ingot front section and R refers to specimens taken from its rear. To identify the grain boundaries, the specimens were subjected to electrolytic etching (anodization) with Barker’s reagent.[5,6] Viewed under polarized light, with the addition of a sensitive tint filter, the grains were well discernible and appeared as different shades of the appropriate colors.[7,8] The mean grain size was determined using a semiautomatic method for mean lineal intercept length measurement (the intercept procedure).[7,8,9] Table I shows the chemical compositions of the examined charges determined by optical emission spectrometry. Specimens were taken during casting, from ingots of about 0.5 m in length. Analysis of the microstructures demonstrated considerable differences in grain size depending on slice height/ ingot depth, although equiaxed grain structures were noted at all three positions. The results of grain size determination by semiautomatic measurement of mean lineal intercept length (the intercept procedure) are given in Table II. From the computed mean lineal intercept length, l (lm), the mean number of grains per unit area, A (No
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