The influence of internal chills on the structure and properties of aluminium alloy castings

Many of the drawbacks associated with the production of conventionally cast ingots and castings, such as the presence of pipes, centre-line segregation and columnar grains, can be attributed to the manner and extent of cooling inside the mould cavity and the problems of heat transfer from the centre to the outside of the casting. These result in lowering of the average mechanical properties and the yield of the casting. In the past, the use of external chills to reduce the above defects has had limited effects. This is because the influence of external chills becomes marginal beyond a certain distance. One of the potential methods of overcoming such problems is to employ heat sinks in the form of internal chills. Despite reported work by Russian and Japanese investigators on the use of internal chills, in the form of powder or strip, in iron and steel castings, no detailed information on the use of such chills in aluminium or other non-ferrous alloys is available in the literature.

This paper presents details and findings of an investigation carried out with Al-4.5% Cu (LM11) alloy using chills of cylindrical form of the same composition. The influence of such internal chills, placed centrally and non-centrally was assessed in terms of changes in solidification time, temperature gradient, percentage melting, microstructure, density, and the ultimate tensile strength of the castings.

The investigation has shown that solidification time decreases linearly with the percentage volume of the chills. Progressive structural refinement, corresponding to this reduction in solidification time, has also been observed. Distribution of the chills is seen to play an important role. One centrally-placed chill is essential in the refinement of the structure of the central region of an ingot, as well as to reduce the size of the central open pipe. The use of microchills in the form of turnings and powder has also been found to refine the structure considerably. The density and ultimate tensile strength of castings has been found to increase up to the optimum volume of chill, i.e. 2.5% at 75–115 K and 0.63% at 35 K superheat, and then decrease.