| Light weight, high efficiency, energy-saving and environmental protection are inevitable trend of the development of automobile manufacturing. The key is high-performance material.The excellent performance aluminum alloy is the best and the most realistic choice. At present, there is a big gap of research and application of aluminum in automobiles between our country and developed countries in Europe and America. So there is a lot of room for improvement. Based on this, in this paper, the excellent comprehensive properties of A356 aluminum alloy which mainly used in automobile engine core components such as cylinder and cylinder head was studied. By taking appropriate measures to optimize the process, improve its mechanical properties and meet the bear performance requirements of high power density of aluminum alloy cylinder, cylinder head. Engine internal components will generate considerable thermal stress and mechanical stress in the process of engine operation. Especially in the start,stop or acceleration, deceleration process, rapid heating or cooling effects adding together will generate various transient thermal and mechanical stresses, resulting in thermal fatigue phenomena in localized area. At the same time accompanied with the severe friction and wear which seriously affect the service life of components.Aluminum alloys are required higher wear resistance and thermal fatigue resistance under these circumstances. Therefore, in this paper, the sliding friction and wear performance test and thermal fatigue performance test of aluminum alloys were studied.Not only can fully excavate the potential of the material, but also expand the application of aluminum alloys in automotive as well as to provide a solid theoretical foundation and practical reference value for improving aluminum ratio of China’s automobile manufacturing, accelerating the development lightweight of China’s automobile. In this paper, the main study contents and results are as follows:1. Since mechanical properties of cast A356 aluminum alloy depends mainly onα-Al phase and eutectic Si phase. Property of α-Al phase is similar to pure aluminum.Therefore, the refinement of pure aluminum was studied firstly by new style Al-5Ti-1B-1RE master alloy grain refiner synthesized by melting method to better study on refining and modification of A356 aluminum alloy. Thermodynamics and kinetics were used to analysize feasibility of synthesis process of Al-5Ti-1B-1RE master alloy grain refiner. The synthesis temperature, dynamic synthesis, structural heredity effect test and other process parameters on refinement effect of the pure aluminum werestudied, and its refinement mechanism was analyzed. Through performing structural heredity effect test to the refiner and dynamic refinement process, the distribution of the second phase particles of Al-5Ti-1B-1RE mater alloy and the refinement effect of pure aluminum can be further improved. So that the coarse columnar grains of pure aluminum are transformed into fine equiaxed grains. Mechanical properties such as the tensile strength σb and elongation δ of pure aluminum after refinement can respectively reach to 77.82 MPa, 54.36%. It increased by 46.83% and 132.51% respectively compared with that of pure aluminum before refinement.2.To achieve a better refining and modification effect, self-made Al-5Ti-1B-1RE master alloy grain refiner and Al-10 Sr master alloy modification were used to perform conventional composite refining and modification to A356 aluminum alloy. The optimum adding amount is 0.80%, 0.30%, respectively. By conventional composite refining and modification treatment,coarse dendrites of α-Al phase in A356 aluminum alloy are transformed into fine and dense equiaxed grains. And the thick plate-like or long needle sheet-like of eutectic silicon are transformed into disperse and tiny fibrous.Also, secondary dendrite arm spacing decreases significantly. And lamellar eutectic silicon is almost completely disappeared and obtained complete modification effect. To further achieve refining and modification effect, tests are adopted to dynamic refining and modification treatment, structural heredity effect test process, the melt mixing process and the different alloy elements such as Cu, Mn, Ti, were purposefully added to A356 aluminum alloy and carried on dynamic composite refining and modification treatment to obtain the new aluminum alloy. With the help of orthogonal test, the optimized comprehensive mechanical properties of new aluminum alloy formula was as follows: Cu, Mn, Ti adding amount is 1.80 wt %, 0.30 wt % and 0.25 wt %, respectively.At the same time, DSC differential thermal analysis of new aluminum alloy was carried on. Mechanical properties of the new aluminum alloy after T6 heat treatment wereσb=352.45 MPa, δ=5.75% and brinell hardness was 114.30 HB, can meet the automobile cylinder, cylinder head and other aluminum alloy castings.3. The friction and wear behavior and wear mechanism of the cast A356 aluminum alloy, refining and modification A356 aluminum alloy, new aluminum and new aluminum alloy by T6 heat treatment were studied. The study shows that with the increase of the load and wear time, the wear rate increases, and the quality of the new type of aluminum alloy treated by T6 is the smallest, and the wear resistance is the highest. Followed by the new aluminum alloy, again is the refining and modification ofA356 aluminum alloy, and wear rate of the quality of as-cast A356 aluminum alloy is the most. The wear resistance of aluminum alloy depends on not only the content of oxides in the friction layer, but more important depends on their compactness,distribution uniformity and bonding strength with matrix.When the load is low load,oxidation growth rate of tribo-layer is less than rate of wear, leading to tribo-layer oxide-free or contain trace amounts of oxides, almost did nothing in reducing the friction. The wear mechanism is abrasive wear, adhesive wear and plastic extrusion wear co-existed. When the load is medium load, tribo-layer had more uniform distribution, compactness and high bonding strength with the matrix, which leads to good anti-friction effect. The wear mechanism is abrasive wear, adhesive wear and oxidative wear light cooperated. When the load is high load, despite there existed more oxides than the previous two, the tribo-layer is no longer compactness, and easily broken, easily removed and the protective effect is decreased. With the load increased significantly, the wear mechanism is composite wear of severe oxidative adhesive wear and delamination wear.4. Thermal fatigue behavior and crack propagation mechanism of four different processes of aluminum alloys were studied. The investigation shows that the growth mode of thermal fatigue crack is mainly the inoculation, initiation, propagation and failure. At the same temperature amplitude, the crack initiation life of thermal fatigue specimens from short to long are as-cast A356 aluminum alloy, refining and modification of A356 aluminum alloy, the new alloy and the new alloy by T6 heat treatment. Under different temperature range, the larger temperature amplitude is, the shorter crack initiation life of thermal fatigue specimen of the same aluminum alloy is.Also, it is found that the orientation of second phase particles can importantly influence on the thermal fatigue crack propagation path. When the crack tips propagation path and the second phase particles in long axis direction have an angle less than 45°, the crack propagation will be along the edge of the second phase particles to continue forward growth. When the crack tips propagation path and second phase particles in short axis direction have an angle less than 45°, the crack will pass through the second phase particles and to continue forward growth. Under thermal stress, thermal fatigue crack relies on the crack tip sharpening→passivation→sharpening alternating intermittently forward growth→ceasing→growth, until the specimen failure. |
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