Critical assessment of reduced pressure test. Part 2: Quantification

Abstract

The presence of bifilms in aluminium alloys has not so far been widely accepted, because there is no single metal quality test that is capable of identifying and classifying them, despite them appearing to be among the most serious defects that can exist. The reduced pressure test (RPT) is widely known and already used throughout the industry. However, the purpose of the present study was to evaluate its potential for the assessment of metal quality in terms of hydrogen content and bifilm content, and assess whether it has potential to be developed into a quantitative technique. To investigate these effects, two alloys from the previous work were used: LM4 (Al–5Si–3Cu) and LM24 (Al–8Si–3Cu–Fe). Various quality index concepts were explored in an effort to quantify the results of the reduced pressure test. The 'Bifilm Index', the total length of bifilms on the sectioned surface of the RPT sample solidified at 100 mbar, appears to have promise as a fundamental parameter to quantify quality. Good products appear to have a typical total bifilm length up to 1 mm, whereas possibly tolerable products are up to 30 mm. Poorly handled alloy can contain bifilms of total length approaching 300 mm. In terms of area of bifilms in a standard weight of melt, for a RPT sample weighing about 90 g, these values correspond approximately to 1.0, 1000 and 100 000 mm2 per 100 g.