Thermo-mechanical fatigue property of a surface-hardened AC4CH cast aluminum alloy

The thermo-mechanical fatigue life and microstructural change under a compression-tension thermo-mechanical cyclic loading of a surface-hardened Al-Si-Mg alloy casting are investigated. A surface cold working technique using steel balls is utilized for the surface hardening. Out-of-phase type thermo-mechanical fatigue tests have been performed with the temperature range of 323-523K and the applied mechanical strain range of 0.75-2.0%. The surface-hardened material exhibits better thermo-mechanical fatigue property especially in a low cycle regime, together with higher generated stress, suggesting that it is even superior under stress-controlled loading. The differences in the stress-strain hysteresis loops and hardness variation near specimen surface between the materials with and without surface hardening have not vanished until final fracture. The observed difference in the thermo-mechanical property is attributed to some microstructural differences. The effects of damaging at the silicon particles to this difference are also investigated using the high-resolution microtomography technique. It has been clarified by microstructural observations that recovery and recrystallization are suppressed in the surface-hardened material. It can be inferred that such thermal stability is attributed to the multi-axial deformation by a combination of the surface hardening and the thermo-mechanical loading.