rear charge well. A continuous protective N2
Degassing to reduce hydrogen concentrations in the melt was carried out by bubbling Ar2
gas via a perforated lance in the gas atmosphere
each measurement), as shown in Figure 3. The same mea- surements were performed for the RPT sample densities of 2.66, 2.71 and 2.72g/cc. The density of 2.70g/cc was below the reported hydrogen concentration threshold in the melt needed to produce gas porosity.21-23
It was found that for the Al-7Si-4Cu alloy a 2.70g/ cc RPT density reading corresponded to the mean hydrogen concentration of 0.123ccH2
Before the initial chemical composition (Al-7Si-1Cu alloy) was altered (according to Table 1) the Aluminum Thermal Analysis Platform (AlTAP) was used for thermal character-
existed over the bath surface to ensure that further hydrogen absorption and oxidation did not occur. Casting would not commence until the Reduced Pressure Test (RPT) sample density achieved 2.70g/cc. The RPT sample’s density was correlated with the Alspek® in-situ melt hydrogen analyzer data.15
/100g Al (repeated 3 times for
ization of three test samples. The mass of the AlTAP test samples was 250 ± 12gms. Each sample was poured into a steel cup made of thin rolled 304 steel. The calculated solidi- fication rate between the non-equilibrium liquidus and soli- dus was 0.15°C/s that was designed to simulate the casting component bulkhead solidification conditions. Low expan- sion fused silica foam (Rescor 310-M) was used on the top and bottom of the solidifying test sample to promote Newto- nian radial heat transfer.
Once the test sample’s cooling curve was established, a first derivative (dT/dt) was determined using the simplified least squares method. The smoothing of the cooling curve data was performed using the Savitzky et al.,24
methodol-
ogy. Abrupt changes in the dT/dt plots provide characteris- tic temperature points, caused by latent heat released from a specific phase growth. The fraction solid (fs
in Figure 4. The base line curve represents the tested material that hypothetically does not go through semi-sol-
) curves were
determined via a baseline method reported by Kierkus et al.,23
International Journal of Metalcasting/Fall 10
35
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