Determining Density Index

Purpose

The Reduced Pressure Test (RPT) is used to evaluate the tendency of an aluminium melt to form porosity during solidification. The method is based on the expansion of dissolved hydrogen in the melt when the pressure is reduced, resulting in increased pore formation. However, the extent of porosity is affected not only by the hydrogen content but also by the presence of nucleation sites, such as oxide films and other inclusions in the melt. 

Since porosity can impair the mechanical properties, pressure tightness, and machinability of cast components, the method can be used as a tool for assessing the risk of porosity-related quality issues. 

Method

Two samples are taken from the aluminium melt. One sample is allowed to solidify at atmospheric pressure, while the other solidifies under reduced pressure. The reduced pressure causes any dissolved hydrogen to expand and form pores to a greater extent than under normal atmospheric conditions. At the same time, pore formation is facilitated at existing nucleation sites, such as oxide films and other inclusions. 

The samples can be evaluated metallographically to study the extent and characteristics of the porosity. However, the most common method is to determine the density of the samples using equipment based on Archimedes' principle. The density difference between the samples is then used to calculate a Density Index (DI). A high DI value indicates an increased tendency for pore formation in the melt, which may be due to a high hydrogen content, a high concentration of nucleation sites, or a combination of both factors. 

Deliveries

The results are summarized in an investigation report.