Defects related to incipient melting in Al–Si–Cu–Mg alloys

Abstract

The present study was undertaken to investigate the effect of Mg on the occurrence of incipient melting in experimental and industrial 319 alloys, using porosity measurements. Castings were prepared from experimental and industrial alloy melts containing Mg levels of 0–0.6wt%. Tensile test bars were cast using an ASTM B-108 type permanent mold. The bars were solution heat treated in the range of 490–540°C for the single step solution treatment, and at 505°C followed by 520°C or 530°C for the two-stage solution heat treatment. Porosity (arising due to incipient melting) was also measured to monitor the occurrence of the incipient melting. Optical microscopy technique was used for microstructural analysis, and quantification, of porosity. The results showed that both Mg concentration and solution temperature play an important role in the occurrence of incipient melting. The porosity measurements were found to be in accordance with the tensile properties and confirmed the results obtained in terms of the incipient melting observed for each alloy/solution treatment condition. Compared to the experimental alloys, the industrial alloy shows a greater resistance to incipient melting, which may be accounted for in terms of the reaction between Cu and trace elements such as Fe and Ni present in the alloy, leading to an increase in the incipient melting temperature.