An analysis of shrinkage defects in aluminium and aluminium-copper castings

Abstract

The influence of casting parameters and alloy composition on the formation of shrinkage defects has been studied using a T-piece test casting. Macro shrinkage defects were found in pure aluminium and aluminium 0.4wt% copper castings, but not in aluminium 1, 2, and 4.5 wt.% copper alloys. The thermal data which were in general, reproducible to +/-5% showed that a negative temperature gradient prevailed in the pure and aluminium 0.4wt% aluminium alloy right up to the end of freezing. In the aluminium 1, 2 and 4.5 wt.% copper alloys the initial negative gradient changes to become positive after approximately one half of the total solidification time has elapsed. The gradient was only of the order of 2°C/mm. Variations in pouring temperature of about 50°C and filling times of a factor of three changed the solidification times by 10% only. Such variations did not change the relative freezing times at different locations in the uniform section castings. Solidification times for the 15 mm horizontal section of the castings (a/b = 0.6) were similar to those of the 25 mm sections (a/b = 1) showing that even in this simple case the modulus concept is invalid. The form of the cavities in the feeder heads is a function of the metal cast, the gas content and the grain structure. The results formed the basis of validation tests for a computer shrinkage model.**