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machined from Samples A and C. The same procedure was followed for each casting and step thickness. Round tensile specimens were made from steps with section thickness of 10, 15, 20 and 30 mm (1, 1.5, 2 and 3 cm). Rectangular flat tensile specimens were produced from steps with section thickness of 5 mm (0.5 cm). The tensile tests were run on an MTS 810 tensile testing machine where the crosshead movement was applied using a hydraulic system. The load was 100 kN and the crosshead speed 5 mm/min. An exten- someter was used to precisely measure the elongation and the initial set up was 25 mm (2.5 cm). Data were recorded at a frequency of 10 Hz.


The Archimedes’ principle was applied to measure the den- sity of the castings in order to determine their porosity. The reference density of the pore free alloy was considered to be 2.678 g/cm.3,5


The porosity distribution was charac-


terized by image analysis. Sectioned samples were prepared by metallographic methods. Nine optical micrographs of each of the selected samples were taken at a magnification of 25x. The commercial software package, Image Access EasyLab 6, was used to analyze the images. The data were evalu- ated in terms of pore size distributions, equivalent diameter, pore number density and shape factor. The latter two parameters were calculated using Equations 2 and 3.


Equation 2 Where; εP is the pore fraction which is dimension-


less and d is the average equivalent pore diameter in [m].


Equation 3 Where; A is the pore area in [µm2 ] and P is the pore


perimeter in [µm]. The shape factor is a measure of the roundness of a pore. Thereby, a value of “1” corresponds to an ideal round pore and “0” stands for a straight line.


results reproducibility and melt cleanliness


Important process parameters such as hydrogen content, melt temperature, pouring temperature, die and die opening temperature were carefully con- trolled during all experiments. Figure 3 shows the readings from the ALSPEK-H analyzer. Due to the initially low hydrogen content (Figure 3) the melt was upgassed with Ar-10% H2


mixture to reach


0.17 ml/100g. Step castings were poured when the hydrogen content in each melt was stabilized


International Journal of Metalcasting/Spring 2012


Figure 3. Hydrogen concentration and melt temperature during a casting trial are shown.


43


Figure 2. Sectioning scheme for porosity characterization and mechanical testing.


around 0.17 ml/100g. The dashed line in Figure 3 represents the melt temperature which fluctuated within a narrow tem- perature range (approximately 740–755C [1364–1391F]).


Figure 4 presents a summary of the recordings from the ther- mocouples during ten consecutive casting experiments. The thermocouple TC4 displays the temperature for die opening 470C (878F), while TC9 displays the pouring temperature 720C (1328F). In general, all recordings except TC9 indicat- ed temperatures above 320C (608F). This value was chosen as the lowest acceptable die temperature and was maintained by circulating heat exchange oil in the die channels.


Results from the PoDFA analysis are presented in Figure 5(a). Alloy melt numbers 1, 2 and 3 correspond to ad- ditions of 10, 25 and 50 % turning chips, respectively.


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