The Study Of Fatigue Properties Of Al-Si Piston Alloy And Its Composite

This work tested the low cycle fatigue properties and fatigue crack growth rates of near-eutectic Al-Si piston alloy and Al2O3f/Al-Si composite at room temperature,200℃and350℃.By observing their fracture morphologies, the fatigue crack initiation and propagation characteristics were acquired and the fatigue fracture mechanisms were analyzed.Microstructures of the Al-Si piston alloy and Al2O3f/Al-Si composite were examined. It was found that the heavily alloyed near-eutectic Al-Si piston alloy consisted of complicated phase composition and microstructure, such as a-Al, primary Si, eutectic Si, various intermetallics (ε-AI3Ni, y-Al7CuiNi, T-Al9FeNi, a-AlFeMnSi and Q-Al5Cu2Mg8Si6) and dispersed precipitates (θ’-Al2Cu and Q’-Al5Cu2Mg8Si6). By reinforcing with alumina fibers, the Al2O3f/Al-Si composite showed more complex microstructure and the uniform distribution of fibers showed well adhesion with matrix alloy.Low cycle fatigue (LCF) properties of the Al-Si piston alloy and its composite were tested. The results showed that the near-eutectic Al-Si piston alloy exhibited unique twice cyclic hardening behaviors at room temperature. The first stage of cyclic hardening happened at the beginning20cycles and the dislocation evolution results under TEM showed that dislocations were beginning to form during this stage and dislocation/precipitate interactions led to the cyclic hardening. After200cycles, the second cyclic hardening stage occurred and continued to the end of fatigue testing. This stage of cyclic hardening was caused by dislocation/dislocation and dislocation/intermetallic interactions. With the rise of temperature (200℃and350℃), the Al-Si piston alloy showed normal cyclic softening behavior. Meanwhile, the Al2O3f/Al-Si composite all showed cyclic softening behavior at room temperature,200℃and350℃.Fatigue crack growth rates (FCGR) of the Al-Si piston alloy and its composite were tested. It was found that the fatigue crack growth rates of the two materials gradually increased as the temperature rose from RT to350℃. Under a same temperature condition, the composite’s fatigue crack growth rate was lower than the matrix alloy. This illustrated that the fibers could slow down the fatigue crack propagation effectively.Fatigue crack initiation sites of the Al-Si piston alloy and its composite were observed and summarized. Large primary Si particles and eutectic Si clusters near the surface of sample usually acted as fatigue crack initiation sites in the Al-Si piston alloy. For the Al2O3/Al-Si composite, bulky fibers or agglomerated fibers were the major reasons for fatigue crack initiation.Fatigue crack propagation mechenisms of the Al-Si piston alloy and its composite were analyzed and discussed. At room temperature and200℃, the primary Si particles and intermetallics often debonded from the Al matrix during fatigue crack growth and fatigue cracks would pass through the eutectic Si structure directly. The alloy showed typical dimple fracture behavior at350℃. Moreover, the fatigue cracks in the Al2O3f/Al-Si composite mainly propagated along the fiber/matrix interfaces at room temperature and200℃, and the reinforcing alumina fibers showed an obvious tendency to cracking with the temperature increased to350℃.