Fatigue crack growth mechanisms in Al–Si–Mg alloys

Abstract

Owing to the increasing use of cyclically loaded cast aluminium components in automotive and aerospace applications, the fatigue and fatigue crack growth characteristics of aluminium castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys’ behaviour when subjected to dynamic loading is still lacking. The present research investigates the mechanisms active at the microstructure level during dynamic loading and failure of Al–Si–Mg alloys. Five model alloys were cast to isolate the contribution of constituent phases on fatigue resistance. The major constituent phases, α-Al dendrites, Al/Si eutectic phase, and Mg–Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth D δK_th and crack propagation regimes (regions II and III) for alloys of different Si composition/morphology, grain size, (secondary dendrite arm spacing) SDAS and heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data has been successfully developed based on a complex elastic plastic fracture mechanics (EPFM/J-integral) approach. Residual stress–microstructure interactions, commonly overlooked by researchers in the field, are also comprehensively defined and accounted for both experimentally and mathematically. Revisions of ASTM E647 are in progress.