Optimization of the Strontium Modification Process in Gravity Permanent Mould Tilt Cast AlSi6Cu4 Cylinder Heads

Sr addition rates between 80 and 400 ppm were tested in industrial-scale casting campaigns. An adequate level of modification is achieved across the entire section of the cylinder head at a Sr addition rate of 160-200 ppm, and further increase in Sr levels fails to offer additional benefits. Porosity increases at Sr addition levels exceeding 200 ppm. The fraction of porosity is as high as 2.5% at a Sr addition rate of 400 ppm. Mechanical properties do not change over these Sr addition rates. It can be argued that the adverse and favourable effects of Sr addition more or less balance each other. However, the underlying issue of the industrial-scale experimental work is that the mechanical properties targeted for this particular AlSi6Cu4 cylinder head casting have been confidently and consistently achieved with the gravity permanent mould tilt casting process, without a separate heat treatment