High Temperature Fatigue Behavior Of The Al-Si-Cu Aluminum Piston Alloy

Piston aluminum alloys are widely used in engines, for their excellent high-temperature strength, elevated temperature resistance, low coefficient of linear expansion, good size stability, wear resistance, heat resistance, corrosion resistance and so on. Due to working in high temperature and high pressure environment in long period, the fatigue failure of the piston often occurs. In order to study its fatigue failure mechanism, this article chooses three kinds of piston aluminum(single-cast material before and after heat treatment and component ontology material after heat treatment) as the object, and the tensile and fatigue tests were carried out at room temperature, 350℃ and 425℃. And then microstructure, tensile and fatigue properties at different temperatures and fracture surfaces were investigated. Finally the experimental data was analyzed to find out the relationship between tensile and fatigue properties and its corresponding fracture mechanism.Experimental results show: the phases of the component ontology material is more coarser, and phase structure of heat treatment single-cast material is more refined than before heat treatment one. After the tensile test, the component ontology material appears Mg2 Si phase at 350℃ and 425℃, and the single-cast material appears MgSiCuNi-Aluminide phase. The quantitative analysis results of the microstructure of different piston aluminum alloy show that the tensile strength of the material are improved with the decrease of the number of the primary silicon. After fatigue tests, cracking on a large block of primary silicon and the bulk aluminum-rich phase is obvious on component ontology materials, but the single-cast material does not find this phenomenon.Tensile properties: no matter component ontology material or single-cast material, the tensile strength decreases and the elongation increases with the increase of temperature; whether at room temperature or at high temperatures, tensile strength and elongation of single-cast material are higher than component ontology material; the tensile strength of the heat-treated single-cast material is higher than it before heat-treated, but the plastic is not as good as before heat-treated one. In the yield stage, the primary silicon of component ontology material appears cracking. As the crack continues to expand and the small crack converges, the crack reaches a certain size and fracture occurs; there are many hole defects in the single-cast material. In the yield stage, the crack initiates at the casting defects, and then continues to rapidly expand when encounters the holes. the specimen will fracture when cracks reach a certain size.Fatigue properties: with the increase of temperature, the fatigue strength of component ontology material decreases, and the decrease of tensile strength with temperature is more pronounced than the decrease of fatigue strength; at room temperature, the fatigue strength of single-cast material is higher than that of component ontology material; heat treatment process does not make the fatigue strength improved, and the fatigue strength can not be improved only by improving the tensile strength. The crack initiations of the component ontology materials and the single-cast material are casting defects in the near surface. Fatigue crack of component ontology material always tends to the crack growth on the primary silicon and the aluminum-rich phase; single-cast material is not in these positions, while tends to a large number of hole defects. Cracks continue expanding rapidly when encountering the holes. Coupled with the convergence of small cracks, the specimen will fracture when cracks reach a certain size. Tensile strength and fatigue ratio showed a linear relationship, and tensile strength and fatigue strength showed a quadratic relationship.The research on the microstructure, tensile properties, fatigue properties and fracture mechanism of the piston aluminum alloy was studied, which provides reference for improving the mechanical properties of the materials and the safe use of the components.