A novel method for the determination of the hydrogen solubility in aluminum and aluminum alloy melts
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INTRODUCTION
IN the casting practice, the control of the hydrogen content of the melt is a basic requirement. For the analytical determination of the hydrogen concentration, samples must be taken from the melt. This procedure is time-consuming and sensitive to uncontrollable errors.[1,2] In aluminum melts, the amount of dissolved hydrogen is very small. Complicated equipment must be used, and corrections for systematical errors have to be made. The direct-reading control methods (Telegas,[3] CHAPEL,[4] and Alcan[5]) display only the equilibrium hydrogen pressure that is related to the hydrogen solubility by Sieverts’ law: c eq H (pH2, T) 5 KS(T) =pH2
[1]
where ceq H is the solubility of hydrogen in the melt, pH2 the equilibrium hydrogen pressure above the melt, and KS the Sieverts constant. The value of KS depends on temperature and alloy composition. Particularly, the composition dependence of KS may be significant. Even small amounts of alloying elements can alter the hydrogen solubility remarkably. The direct-reading methods need no sampling of the melt and provide reliable data on the activity, =pH2 , of the dissolved hydrogen in the melt. However, to convert the measured equilibrium pressure data into solubility, the value of the Sieverts constant must be known. The KS data are taken from measurements of the equilibrium hydrogen concentration at known hydrogen pressures at different temperatures. ´´ ´ . SZ OKEFALVI-NAGY, A Research Scientist, and E. FROMM, Senior Scientist, are with the Max-Planck-Institut fu¨r Metallforschung, D-70174 Stuttgart, Germany. L. STOJANOVA, formerly Visiting Scientist, MaxPlanck-Institut fu¨r Metallforschung, is Senior Scientist, Institute for Metal Science, Bulgarian Academy of Sciences, 1574 Sofia, Bulgaria. Manuscript submitted February 4, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
Hydrogen solubility data determined by various authors with absolute methods deviate remarkably from each other even for pure aluminum melts.[6,7,8] This inconsistency results from systematic errors of the methods used. After accurate redetermination of the solubility in the temperature range between 943 and 1123 K and the critical assessment of other data available, Talbot and Anyalebechi[9] concluded that the most reliable determination of the solubility was provided by the original method of Sieverts,[10] where the volume of hydrogen absorbed by a metal sample of known mass is measured directly. This method, however, is extremely time-consuming and requires meticulous corrections for experimental errors. Therefore, it is not well suited for fast determination of the Sieverts constant of alloys in a wide range of temperature and composition as required for the direct-reading control methods in the casting practice. Systematic measurements are rare and restricted to narrow ranges of alloy composition. For example, for aluminum alloys, little experimental data are available.[6,11] Therefore, extrapolations from these few data are often combined with empirical estimations to evaluate the hyd
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