INVESTIGATION OF THE COMPLETENESS OF SPECIMEN DEGASSING IN AN ANALYSIS OF THE HYDROGEN CONTENT OF ALUMINUM ALLOYS

Analyzing the hydrogen content of aluminum alloys is a mandatory part of the quality control of ingots of such alloys. Analytical methods based on vacuum-heating and vacuum-melting and involving recording of the absolute amount of hydrogen released by a specimen were developed as far back as the 1930s. The time required to perform an analysis by these methods ranges from 45 min to 2 h. Modern technology makes it possible to use the more productive method of melting in a flow of a carrier gas. The leading instrument manufacturers (LECO, CIIIA JUWE, Germany) make hydrogen analyzers that operate on the basis of this method and employ induction heating. The main distinction of this approach is the short analysis time ‐ 2‐5 min for aluminum alloys. Given the global competition that exists, the time factor has become decisive in choosing a method to analyze hydrogen content at the factory. The prevailing opinion of specialists in the given field is that the classical methods are obsolete and should not be used in factory laboratories. It is generally believed that the time required to fully extract hydrogen from a metal specimen by melting it in a flow of a carrier gas can be reduced significantly by using induction heating to rapidly heat the crucible holding the specimen to the melting point of the metal and then having hydrogen rapidly displaced from the specimen by the crystallization front formed after the induction heating is discontinued. To prevent sublimation from taking place, the induction heating is ended the moment the specimen has completely melted. Thus, the crystallization of the alloy begins immediately after melting has ended. At the same time, there are empirical data that are in conflict with the above opinions. One study of the diffusion of hydrogen during the crystallization of aluminum [1] did not reveal any changes in the flow of hydrogen extracted from the specimen at different crystallization rates. The explanation for this is that the crystallization front is not continuous but instead consists of single crystals that are growing in different directions and are separated from one another by a liquid phase.