Influence of vacuum counter-gravity filling on reliability of 2L99 aluminium investment castings

The reliability of 2L99 (Al-7Si-0 center dot 4Mg) alloy investment castings produced using gravity and vacuum counter-gravity mould filling processes has been compared. Large numbers of bars have been cast and tested in four point bending and the scatter in their strengths characterised using the Weibull statistical technique to give the Weibull modulus l. Both top and bottom gated designs of gravity poured moulds produced unreliable castings (lambda=517 for both). This contrasted with previous work which showed much higher moduli for bottom gated moulds (49 and 54); this discrepancy has been attributed to poor control over mould filling and evidence for this was wide variation in filling times in the present work. The standard CLA vacuum counter-gravity filling process significantly improved the reliability (lambda=29), but this was marginally reduced (lambda=27) when a filter was introduced into the sprue. Possible reasons for this are discussed. A modification to the mould design further increased the reliability (lambda=44) but this was again decreased (lambda=34) by the addition of a filter. The most reliable castings were produced by the recently developed centrifugal counter-gravity (C3) process, which provided a Weibull modulus of 50.