Effects of solidification structure and aging condition on cyclic stress-strain response in Al-7% Si-0.4% Mg cast alloys

Stress-strain response under cyclic loading at fixed plastic strain amplitude condition was examined for age-hardenable Al-7% Si-0.4% Mg (A356) cast alloys. The specimens examined include the cast alloys with ordinary dendrite structure and sem-liquid die-cast alloys with fine effective grain structure. Al-0.5% Mg-0.4% Si (6063) alloy was also tested for comparison. Special attention was paid to the effect of solidification structure and aging condition on cyclic hardening behavior, Cyclic hardening behavior was sensitive to the solidification structure. The refined grain size, DAS and unmodified acicular eutectic Si particles increased stress levels of cyclic hardening curves. Drastic change in cyclic hardening behavior was obtained by changing aging condition. After the initial rapid hardening, the stress amplitude kept increasing steadily until fracture in as-quenched and under-aged materials containing shearable GP zone. In contrast, for over-aged (and peak-aged materials), which were hardened by non-shearable beta'-precipitates, the initial hardening occurred more rapidly and the stress amplitude reached the saturation stress in a quite early stage of the fatigue life. The attained stress level was almost constant until fracture. Essentially the same aging condition dependence was obtained for the 6063 alloy with no dendrite structure and eutectic Si particles. This indicated that nature of strengthening precipitates controlled the cycle hardening behavior of the present cast alloys. (C) 2002 Elsevier Science B.V. All rights reserved.