Abstract
This research has evaluated the effect of surface turbulence introduced during the filling of moulds on the reliability of Al-7Si-Mg alloy (2L99)investment castings. Four different running systems were designed to provide different amounts of surface turbulence: top filled, uncontrolled bottom filled, and controlled bottom filled with and without a filter. Computer modelling has been used to simulate the filling of the different designs and the results have been compared with the actual flow behaviour established using real time X-ray radiography. Castings have been produced using the different running systems, tested in four point bending, and the results analysed using the Weibull statistical technique. It has been shown that top filling produces significant turbulence, which results in the least reliable castings (Weibull modulus of 25.5). There was no significant difference between this and the Weibull modulus of 27 found for uncontrolled bottom filled systems. This indicates that a poorly controlled bottom filled system is no better than a top filled system. Both were inferior to the controlled bottom filled systems without and with filters, which had Weibull moduli of 38 and 54 respectively. SEM examination and oxygen analysis of representative fracture surfaces has provided supporting evidence for the important role that oxide films played in reducing properties.
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