Assessment of melt cleanliness and analysis of inclusions in Al-Si alloys using the Prefil pressure filtration technique

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The aluminium casting alloys A356.2 and A390.1 are two of the most popular Al-Si alloys used in automotive and die casting applications. To obtain premium quality castings, their alloy melts must possess optimum melt cleanliness, i.e., the inclusions present therein, particularly those harmful to the alloy properties, must be minimised. In recent years, melt cleanliness of aluminium alloys has been assessed by means of various filtration techniques. In the present work, the measurement of inclusions was carried out using the recently introduced pressure filtration ‘Prefil’ technique which has the advantage of providing in situ results in the form of flow rate charts. These minimise the need for time-consuming metallographic examinations for inclusion analysis demanded by other pressure filtration techniques such as PoDFA. Experiments carried out using the ‘Prefil’ technique indicate that for obtaining appropriate results, the pressure chamber and filtration crucible of the ‘Prefil’ appratus should be heated to ~300-350°C, to reduce heat loss during transfer of the liquid metal from the melt crucible to the filtration crucible. Also, the filtration temperature should be high enough to avoid possible sedimentation of inclusions in the ladle during the transfer. It was found that the main inclusions in A356.2 alloy are Al4C3, dispersed oxides, MgO and MgAl2O4, while those in A390.1 alloy are bone ash and alkaline compounds. Generation of oxide films (associated with the use of mechanical stirring to dissolve grain refiner/modifier additions to the melt) can be avoided by employing degassing instead.