The significance of microstructure heterogeneities on the VHCF life of cast aluminum alloys

• Ultrasonic fatigue tests and crack propagation tests combined with high resolution analysis. • Industrial cast cylinder head (primary AlSi7Mg0.3) and engine block (secondary AlSi8Cu3) • Laboratory castings with defined variations in SDAS and eutectic Si, combined with HIP. • Lower SDAS improves fatigue limit but reduces near threshold crack propagation resistance. • Hydrogen pores can reduce crack propagation rate by crack deflection and crack tip blunting. The very high cycle fatigue (VHCF) of cast aluminum is strongly associated with casting defects and microstructure features, e.g. porosity, (secondary) dendrite arm spacing (SDAS) and intermetallics. Complex components have an inhomogeneous microstructure due to locally varying solidification conditions. Due to complex geometries, directional solidification and sufficient feeding are often not possible, giving rise to solidification defects and shrinkage pores. These microstructural aspects influence the service life, as demonstrated by fatigue tests on automotive engine blocks and cylinder heads. However, understanding the damage process and correlating it with the microstructure is difficult due to the large number of influencing factors and their interdependencies. To this end, laboratory melts with controlled microstructure evolution were used, allowing a variation of SDAS, eutectic Si and pores under otherwise identical boundary conditions. Mechanical resonance and ultrasonic fatigue testing in combination with high-resolution microstructure analysis show a reduction in fatigue life with increasing SDAS. Main reasons are large α-aluminum cells, which provide low resistance to crack propagation. The impact of pores is depending on their size, shape and location. Large fissured pores in near-surface regions act as crack initiators, whereas small spherical pores can retard crack propagation due to crack deflection and crack tip blunting.