The comparison of methods of non-ferrous alloys solidification testing by means of thermal analysis and acoustic emission

This paper deals with the applicability of acoustic emission (AE) testing methods for real-time evaluation of the phenomena which can generate AE signals during the solidification of alloys, and which can have a significant influence on casting quality. First, some papers on AE during phase transition in allo:ys are described. Then the authors' own research on Pb-Sb (Pb-8 wt.% Sb) and Al-Si(Al-11.6wt.% Si) alloys by means of AE is described with reference to simultaneously conducted thermal and derivative analyses. Based on the correlation between the results of these analyses, AE signal measures were sought which could be used for the identification of alloy changes from liquid into solid state and kinetics of the solidification front. At the beginning, the alloys were made in sand moulds and then in a 76 mm diameter ball shaped cast iron mould with a metal supply system. After initial testing, the sand moulds were abandoned, because high temperature caused the cracking of quartz grains, which was an additional AE source causing the useful signal to be noisy. Temperature, impluses and AE envelope were recorded continuously during the solidification of metals. Numerical computation provided the derivative of the self-cooling curve and the RMS values of AE signals for both the centre of the casting and a point located 15 mm (or 20 mm) from the surface. It has been found that in different phases of the solidification of Pb-Sb and AI-Si alloys, the AE phenomena differ in their intensity. In the AI-Si alloy the amplitudes of the AE signal are almost twice as, high as those in the Pb-Sb alloy. In almost identical characteristics of temperature changes in subsequent solidification tests, the AE signals were not always repeatable.