Elevated temperature low cycle fatigue of a gravity casting Al-Si-Cu alloy used for engine cylinder heads

摘要

The strain controlled low cycle fatigue behavior (LCF) of a gravity casting (GC) Al-9Si-3Cu (333) alloy for engine cylinder heads was investigated at 423 and 523 K. Detailed studies were performed on cyclic hardening/softening behavior, hysteresis loops, fatigue lifetime, and fracture modes, as well as dislocation structures during cyclic straining. At any given total strain amplitude (from 0.20% to 0.275%), the material exhibited cyclic hardening at 423 K and cyclic softening at 523 K, which also corresponded well to the observed phenomena in hysteresis loops. Meanwhile, under the same total strain amplitude, the hysteresis loop at the half of lifetime at 523 K was wider than that at 423 K. The cyclic hardening characteristics observed at 423 K at the selective total strain amplitudes (0.20% and 0.275%) were mainly attributed to the tangled dislocation structures, along with the interaction of dislocation with second phase particles. At 523 K, the dislocation climb through Al_2Cu precipitates was considered a key to cyclic softening at any total strain amplitude. At a low total strain amplitude of 0.20% at 523 K, the slight cyclic softening behavior was bound up with not only dislocation climb but also subgrain boundaries and dislocation loops. Finally, the fatigue life of the studied alloy at 423 K was higher than that at 523 K under the same total strain amplitude. The drop in fatigue life of this material at 523 K correlated closely with many factors, such as plasticity, presence of PFZs together with coarse AlMg_4Zn_(11) precipitates, as well as oxygen diffusion on the crack surfaces.