Control Of Si Morphology In Eutectic Al-Si Alloy And Its Effect On Fracture Toughness And Fatigue Crack Growth Of The Alloy

Al-Si alloys have been used extensively Si morphology, which is the main second-phase particles in the alloys, has very important influence on the mechanical properties of the alloys. In this paper, the methods to control the Si morphology of eutectic Al-Si alloy and the effect of the Si morphology on the tensile properties, the fracture toughness and the fatigue crack growth behavior of the alloy were investigated.The effects of Na-salts modification on the Si phase morphology were studied. The results show that 1% Na-salts are enough to make the acicular Si phases in the as- cast microstructures change to fibrous morphology. The reason is that the Na absorbed on the surface of Si phases change the growth of the Si phases.The effects of the mould temperatures on the Si morphology were studied. It is found that with the same modification the samples produced using the lower temperature moulds have smaller fibrous Si phases which are prone to spheroidize in the solution treatment and the Si phases with the small size and high aspect ratio are obtained. The reduction of mould temperatures leads to the increasing of the undercooling and the nucleation of Si phases during the solidification, which restricts the growth of Si phases.The changes of the Si phase morphology during the solution treatment were investigated. In unmodified alloys, with the increasing of the solution hours the large acicular Si phases passivate and fracture gradually, but it takes a long time to complete the spheroidization. In modified alloys, the spheroidized structure is obtained after 1h solution treatment because of the fibrous Si morphology and prolonged solution treatment leads to coarsening of the particles. Consequently, the appropriate solution time is lh in this experiment.The tensile properties and the fracture toughness of Al-Si alloys that contain different sizes of Si phases were investigated. It is found that the acicular Si phases do harm to the elongation, the YS (Yield strength) and the UTS(Ultimate Tensile Strength), while the alloys that contain smaller Si particles show a better plasticity and strength. With the increasing of Si size, the fracture toughness of the Al-Si alloy declines gradually. In conclusion, it is necessary to make Si phases small and to be spheroidizd in order to gain the alloys that show fine tensile properties and fracture toughness.The influence of Si size on the fatigue crack initiation life and the fatigue crack propagation behavior was carefully studied. We find that the increasing in the Si size reduces the fatigue crack initiation life. The alloys that contain smaller Si phases have lower fatigue crack propagation rate. The smaller Si phases are less likely to fracture, so the fatigue cracks tend to go round Si phases when they confront them. Besides of the increasing of the effective crack, deflected cracks also lead to the increasing of the fracture surface roughness and the crack closure. In this way, the rate of fatigue crack propagation declines.