How the quality of liquid aluminium can be ruled out?
Metal cleanliness influences nearly all parameters of casted pieces and the casting process. Thus, since many years there are worldwide efforts to improve test procedures and methods to determine the quality of metal melts. Today diverse methods are known to get information about cleanliness in a very cheap and quick way. The aluminum test units of FMA allow to measure the hydrogen content, determine the density, and check non-metallic oxides and inclusions.
Sievert’s law says that the concentration of a dissolved gas in a liquid depends on the partial pressure of the gas in the liquid and a constant for the temperature of the liquid and the solubility of the gas.
For alloy correction you type in the alloying components in weight %. The CPU calculates the constants A and B. The temperature is measured with a thermo couple. The pressure is measured by the first bubble method or with the CHAPEL® portable probe.
CH = Concentration of hydrogen dissolved in metal
pH2 = Partial pressure of hydrogen
T = temperature
A, B = Sievert’s constant depending on alloy composition
There are 2 samples taken from the metal ( each with about 80 grams ) . The density of both samples is measured and the density index is calculated. The density index is an indicator for metal purity.
DI = Density Index
Da = Density of sample solidified under atmosphere
DV = Density of sample solidified under vacuum ( 80 mbar )
NON-METALLIC OXIDES AND INCLUSIONS
Judging the non-metallic inclusions. The Drosstest method is based on the fact that the impurities in an evacuated sample (< 10mbar) come to the surface together with the gas bubbles. After solidification the sample can be compared with other samples.
For the Straube-Pfeiffer-Test, the solidified sample is cut apart in the centre and compared with other samples. The amount, size and distribution of holes give information about impurities. A vacuum of 30 – 50 mbar is used for this test.