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mechanical Properties


In order to study the impact of oxide additions and the ac- companied change in porosity upon mechanical properties, tensile tests were carried out. Figure 10 shows the ultimate tensile strength (UTS) (a) as well as the elongation versus section thickness (b) in comparison to a clean reference melt. Here, the reference melt had a hydrogen content of 0.20 ml/100g and the volume porosity as assessed by density measurements was 0.5 ± 0.1 %. PoDFA residues revealed an average number of 20 oxide films per kg liquid metal. UTS for the Reference Melt and Melt 1 decrease from 195, 170 MPa to 153, 148 MPa respectively, with increasing section thickness. Likewise, properties of the heavier contaminated melts deteriorate, but in the 20 and 30 mm (2 and 3 cm) sec- tion the UTS leaps to 167 MPa again. The same behaviour was found for the elongation at fracture. The Reference Melt and Melt 1 differ in a way that elongation decreases steadily with increasing section thickness from 7.5, 3.5 % to 2.7, and 2.3 % in each case. Again, Melts 2 and 3 exhibit better properties (3.2 and 3.7 %) in the 30 mm (3 cm) section. By


comparing the melts with scrap additions, the overall pic- ture reveals a continuous loss of mechanical properties, both in UTS and elongation. However, castings made of heavily contaminated melts show slightly better results than those for castings made of an almost clean melt.


The fracture surfaces of tensile test specimen were inves- tigated by scanning electron microscopy (SEM). Figure 11 represents typical defects leading to early fracture of speci- men from the 15 mm (1.5 cm) thick section of casting pro- duced with Melts 2 and 3.


discussion reproducibility and melt cleanliness


It could be shown that all castings were conducted with a high degree of reproducibility. The melt and pouring tem- peratures were kept constant at 750C (1382F) and 720C (1328F), respectively and the die was opened as soon as it reached 470C (878F). The inclusions found in PoDFA


(a)


(b) Figure 10. Mechanical properties vs. section thickness of step castings, (a) UTS and (b) elongation.


Figure 11. SEM micrographs from fracture surface of tensile specimen, (a) iron rich intermetallics and (b) interdendritic pores. (a)


(b) International Journal of Metalcasting/Spring 2012 47


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