The control of dissolved hydrogen in liquid aluminium alloys is a critical requirement for the aluminium industry. Aluminium producers and foundries worldwide make significant investments in capital equipment, consumables and human resources in order to ensure that the dissolved hydrogen content is within acceptable limits.
If the hydrogen level is too high, during solidification, there is the potential for the formation of cavities / pores. Conversely, a moderate hydrogen level may be desirable under certain conditions, in order to combat shrinkage porosity. As such, there is an optimum hydrogen level which ideally should be established and controlled during casting. Although dissolved hydrogen is critical, pore formation is also influenced by several other factors, the most significant being melt cleanliness i.e. the level of inclusions such as metal oxides.
Hydrogen measurement techniques such as the ‘reduced pressure test’ provide an indication of the combined effect of both the hydrogen content and the metal cleanliness. However, in order to be able to analyse the hydrogen content specifically, it is necessary to measure it directly. Furthermore, in order to control the hydrogen content, it is necessary to measure it accurately in real-time.
“The most accurate, reliable and cost-effective solution”
Above, an impervious section of hydrogen ion conducting ceramic (green) is coated with metal electrodes on both faces. The concentration of electrons is higher on the right side than on the left due to the greater hydrogen concentration. This results in the right side becoming negatively charged with respect to the left.
If the voltage across the ceramic is known and the hydrogen concentration on one side is known (the “reference” hydrogen concentration), then the hydrogen concentration on the other side can be calculated. This type of arrangement is called a “hydrogen concentration cell”.
In order to be able to use a hydrogen concentration cell to measure hydrogen, the reference hydrogen concentration must be known. A patented “solid-state reference” (solid material which generates a known hydrogen concentration) is sealed within the sensor to effect this reference. The electrode exposed to the solid state reference is thus the “Reference Electrode” and the electrode exposed to the measured gas is the “Measuring Electrode”.
The schematic below shows the arrangement of the probe during measurement of hydrogen concentration in molten aluminium. At the end of the probe, there is a porous diffusion barrier. Hydrogen dissolved in the melt comes out of solution, and the hydrogen gas forms within the measurement chamber as shown. The measuring electrode of a hydrogen concentration cell is exposed to this gas. The voltage from the hydrogen concentration cell is measured, and as the reference hydrogen concentration is known, the concentration of hydrogen gas in the measurement chamber is calculated.
To work out the hydrogen concentration in the melt, the solubility of hydrogen must be known. The solubility of hydrogen in pure aluminium is temperature dependent. Therefore the melt temperature must also be measured to calculate the dissolved hydrogen level.
Hydrogen solubility also depends on the alloying components in the melt. In this respect, an adjustment is made to solubility factors within the analyser to compensate accordingly.
The Hycal device range has been specificaly designed to address the varied demands of distinct production environments, spanning from advanced aerospace rolling slab manufacturers to artisanal foundries.
|Portable||No external connections required whatsoever. Battery powered. Internal gas cylinder.|
|Durable||Withstands arduous working environment (up to 70 deg C).|
|Remote Control||Included PC software allows every function to be viewed and controlled from a remote location, such as control-room or office.|
|SCADA||Easy integration into an existing SCADA system (MODBUS TCP protocol).|
|User Interface||A sophisticated user interface has been developed specifically to meet the re- quirements of the production environment at the operator level. Security is controlled by login based access privileges (Operator / Manager / Engineer).|
|OBD||Sophisticated On-Board Diagnostics constantly monitor the probe’s impedance, flow rate and internal pressure etc and outputs the appropriate alarm message with suggested remedies, should an issue arise - such as the the prompt to change probes when maximum use has been detected.|
|Control||Can be used to control hydrogen during a ‘degassing’ or ‘gassing-up’ procedure. Rather than operate a treatment for a fixed amount of time, with Hycal it is possible to define a setpoint in terms of hydrogen content.|
In close co-operation with end-users and partners, EMC has developed a sophisticated optional user interface which guides the operator through a number of ‘steps’ from the selection of the alloy and lot number through to the final result which outputs and logs an averaged hydrogen value plus other statistical data such as the span (max. minus min. value), analysis time and temperature range during the analysis etc.
The data within the Hycal is stored in a protected format such that only validated data can be imported into the PC software thus providing data archiving security.
The software is very simple to use. All the data from multiple analysers and probes can be searched via melt code and lot number, probe number etc can be filtered thereafter. Either a graphical (shown below) or spreadsheet report can be generated.