Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Al-Si foundry alloys are widely used in aerospace and automobile industries, such as the engine blocks and cylinder heads. These engine components are often exposed to cyclic mechanical stress at high operation temperature (250-350 °C) and such dynamic loading frequently results in the occurrence of fatigue failure. Hence, a fully understanding of fatigue behaviors at elevated temperature of Al-Si foundry alloys is the critical concern for safety designs. In the present study, two typical Al-Si foundry alloys, namely 356-T7 and 319-T7, were investigated to discover their low cycle fatigue (LCF) behavior at 300 °C. The LCF tests were performed in the strain-controlled compression-tension mode with the strain ratio of Rε = -1 at various strain amplitudes of 0.1-0.6%. The cyclic strain-stress response, hysteresis loops and the Masing behavior were analyzed and compared between two alloys. Fatigue parameters were calculated from experimental data for fatigue life estimation and fatigue fracture surfaces were also analyzed at various strain amplitudes. Results showed that both alloys present nearly ideal Masing behavior with symmetric hysteresis loops. The cyclic softening occurs in both alloys but the softening rate varies with alloys, which is lower in 356-T7 than 319-T7. Moreover, 356-T7 behaved more ductile while 319-T7 exhibited higher strength during cyclic deformation. 356-T7 achieved higher low-cycle fatigue life than 319-T7. The different LCF behavior of these two alloys may be attributed to the presence of various precipitates due to the variation of Mg and Cu contents.