VAC-Test & Density Index

QA systems for determining and analysing the pore potential in aluminium alloys

Using the IDECO VAC-Test system it is safe and simple to determine the potential pore behaviour of a melt, including corresponding quality-relevant documentation. This is achieved through the production and analysis of negative pressure density samples. The Density Index automatically determined on the density terminal provides precise and reproducible information about gas content, oxides and inclusions.

Hydrogen is the only gas that is soluble to any appreciable extent in liquid aluminium. In everyday foundry work, the hydrogen contents in an aluminium melt are between 0.05 and 0.5 cm³/100 g Al. However, quality-influencing effects can be expected from hydrogen contents in the melt of just 0.1 cm³/100 g Al. In addition to the hydrogen content, pore formation is also influenced by impurities due to oxide inclusions and the casting conditions (cooling rate). Oxide inclusions can be seen as seeds for the formation of pores. The VAC-Test system facilitates determination of the entire actual pore potential of the spectrum of possible porosity formations. 

The required density index is largely determined by the casting process (die casting, low-pressure casting, gravity die casting, sand casting), the part to be cast and the internal processes.

During sample preparation, a sample of approx. 80 gr. is allowed to solidify in the crucible at a negative pressure of approx. 80 mbar in the VAC-Test system. With the onset of solidification, the dissolved hydrogen is largely molecularly precipitated, whereby it adheres to non-metallic impurities (e.g. oxides). Solidification of the sample in the second crucible takes place under atmospheric pressure. Due to the atmospheric pressure acting on the melt, a significantly smaller pore volume precipitates in this sample if a contaminated melt is present. The solidification time of the vacuum sample is defined as 4 minutes to ensure complete solidification of the samples. 

Once the samples have cooled down, the densities are determined in the Density Index terminal according to Archimedes' principle and set in relation to each other. 

The respective sample is first weighed, then suspended in a water bath (located on a scale) and the volume is determined as the amount of water displaced through this process. The density of this sample can then be determined from this finding in combination with the weight.

The two density values are then compared (comparative method) to determine the "density index".

If IDECO software is used to document the measurements, the samples are identified before the measurement, the measurement result is saved and is subsequently available for further analyses.

The density index determined from the respective densities is a reliable indicator for the expected solidification character of the melt.

The following figure shows four pairs of samples with different density index values.

Findings clearly show that in addition to hydrogen (free, bonded, dissolved), an unknown quantity of oxides and impurities can always be present, which are responsible for the formation of micro cavities, for example, and therefore have a significant influence on the potential pore behaviour of a melt.

The following figure compares the typical solidification defects caused by micro cavities or gas porosity. The samples contain different levels of hydrogen despite an almost identical density index.

Knowledge regarding the current amount of free hydrogen therefore only corresponds to part of the possible porosity. The proportion is unknown because only a conditionally stable ratio naturally exists between free and bonded hydrogen. Measurement of the pure hydrogen content takes place using a Hycal or Hydrogen-Analyser.

The differentiated knowledge of the hydrogen/oxide ratio is becoming increasingly important; in particular since mechanical melt cleaning systems (Melt Clean plants) have been in use. Using density index systems, it is possible to assess the cleaning effect and therefore guarantee the melt quality! In this way it is possible to reduce waste and production costs and improve the CO2 balance.

The accuracy of the achievable measured values depends on the following influencing factors and must be kept constant: 

  •  Ambient temperature of the crucible 
  •  Surface coating of the crucible 
  •  Constant sample volume
  •  Casting temperature 
  •  Constant distance (pouring height) between casting ladle and crucible (kept to a minimum) 

Today, the use of these systems is indispensable for ensuring the melt quality. In striving to improve your market share, make good use of our technological expertise and our high quality standards to safeguard and document your quality system.

These systems also require regular maintenance and calibration; processes that are naturally documented with a certificate and seal. This is essential in order to comply with the requirements of your standard quality assurance certificates (e.g. quality audit, ISO9001, etc.).

IDECO offers you a range of different devices to provide the optimum solution for your operation. The type of device is dependent on your melting and casting process, as well as your customer requirements.