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Metal Casting Technologies : March 2011
38 www.metals.rala.com.au TECHNICAL FEATURE achieving stability and for hot topping. Table 1 shows the chemical composition of the ESR ingot. The sectioned ingot showed absence of any internal blowholes and porosities. Macrostructure confirmed the presence of equiaxed grains of about 1 mm uniformly distributed in the longitudinal section. Figure 4 shows the equiaxed macrostructure. There was an absence of columnar grains which is typically present in steels. It was due to the low temperature gradient in the liquid melt owing to higher thermal conductivity of the aluminium. The results of mechanical testing for the ESR ingot are given in Table 2. The average impact strength is 68.218 KJ/m2 whereas the ultimate tensile strength is 117.5 MPa. The total elongation is 10.775% and the hardness value is 64.95. Table 3 shows the properties of cold rolled ESR ingot. After rolling UTS increases from 117.5 MPa to 238.5 MPa. There is an increase in hardness after cold rolling. The impact energy is reduced after rolling. Cold rolling decreases the impact toughness of alloys and makes the material brittle. Figure 5 shows the SEM image of tensile fractograph. The image shows the presence of cup shaped depressions called dimples throughout and are the result of microvoid coalescence. The fracture mode is dimple rupture. Figure 4. Equiaxed macrostructure ESR Ingot Charpy Impact Energy Impact Strength KJ/m2 UTS, MPa Elongation % Hardness Transverse section (Ingot Bottom) 5.48 J 68.5 110 9 63.8 5.8 J 72.5 112 10.3 64.6 Longitudinal section (Ingot Centre) 5.38 J 67.25 117 10.8 65.1 5.17 J 64.625 131 13 66.3 Average 5.4575 J 68.218 KJ/m2 117.5 MPa 10.775% 64.95 Table 2. Properties of aluminium ESR ingot Rolled Impact Energy J Impact Strength KJ/m2 UTS, MPa Elong-ation % Hard- ness CTE 10-6 0C-1 Rolled Sample 1 4.12 51.5 242 MPa 7.9 69.1 22.7 Rolled Sample 2 4.8 60 235 MPa 6.5 73.2 23.2 Average 4.46 J 55.75 KJ/m2 238.5 MPa 7.2 % 71.15 22.95 Table 3 Properties of Rolled ESR Elements wt% Si 0.3 Fe 0.7 Cu 0.15 Mg 0.25 Zn 0.13 Al Bal. Table 1. Chemical composition of Al ingot