Controlling hydrogen gas in aluminium melt

Reduced Pressure Test is used to measure  hydrogen gas in aluminium melt. Hydrogen is the only gas soluble in aluminium and may at solidification precipitate as porosity in the cast component. Therefore the level of hydrogen in aluminium should be controlled.

Purpose

The method gives the possibility of measuring gas content in an aluminium melt in order to ensure a melt with as low hydrogen levels as possible.

Hydrogen may precipitate as porosity in aluminium castings, which can affect mechanical properties as well as pressure tightness. Hydrogen results in gas porosity but can also contribute to larger problems in porosity arising at the solidification shrinkage of the material. By measuring hydrogen content in the melt, the foundry may learn what affect the content and be given an indication on how the quality of the casting may be affected. There are research theories that increased oxide content in melt gives more porosity, which means that the method could also be used to evaluate purity in general in the melt. This can then be a tool for evaluating the process in terms of routines for handling the melt, as there is a great risk that the oxide content increases during these processes.

Method

Two small samples are taken from the melt, where of one solidifies under vacuum. The vacuum makes the hydrogen expand, which will give a difference in volume related to the sample solidified in normal atmosphere. The samples can be examined by microscopy, but also be used as samples in a following measuring of specific density which is the most common way to evaluate the results. By using an equipment based on the principle of Archimedes a density index may be calculated, which gives the difference in density between the two samples.