Classification and Survey of Defects and Imperfections in Al-alloy castings

Defects and imperfections are intrinsically generated by casting processes, due to several reasons. The final properties and in-service behaviour of castings are always related to microstructural features and to defects and imperfections: both microstructure and defects/imperfections are the results of process stages, alloys properties and dies & tools design. As an example, it should be considered that in HPDC filling stage extreme conditions are established: complexity of components leads to complex dies, and the high production rates required (up to 120 shots/h) lead to very high filling velocities for the molten alloy (up to 40 m/s) with strong generation of turbulence in the flow. Solidification takes place in few seconds, and the die is first in contact with a molten alloy at more than 700 °C and, after 30-40 seconds, with a sprayed lubricant at room temperature. For these reasons HPDC (as well as other Aluminium alloys casting processes, such as permanent mould casting process) can be considered a “defect generating process”. Not only an average 5-10% scrap is typically produced, but the type, size and severity of defects and imperfections are varying. From these considerations, it seems that the potential of high-pressure die-casting and permanent mould casting processes will be completely exploited only when the quality level will be perfectly optimised. In this regard, the analysis of defects and imperfections allows the foundry to monitor the products quality respect to a quality standard. Further, the analysis of defects and imperfections provide to the foundry useful correlations between their type/distribution and their origin, so that it could be possible to define process modification for improving the quality. One of the current targets of the StaCast project is the compilation and dissemination of common tools to allow foundries to define a proper, comparable, quality standard. The first tool to be proposed is a common basis of language, i.e. a terminology and classification of defects and imperfections in order to help die-casting foundries to face with increased confidence and ability the defect-related issues, to assure quality and reliability of their products.