Microstructure And Properties Of A356 Aluminum Alloy Reinforced By In-Situ（TiB₂、ZrB₂） Particles

Under the background of"Carbon peaking and carbon neutrality",A356aluminum alloy,with its excellent casting performance,low density but high specific strength,can be formed into precise and complex workpiece at the same time to meet the green energy-saving policy,so it has a very wide range of applications in aerospace,automotive fields.With the continuous development of science and technology,the comprehensive mechanical properties of the workpiece are increasingly improved,the traditional A356 aluminum alloy used for key parts often can not achieve high strength and excellent fatigue resistance.In this paper,particle reinforced A356 aluminum alloy was prepared by in-situ reaction.The effects of different in-situ preparation methods on the microstructure of composites were studied,and the static mechanical properties at room temperature and 150℃as well as the high cycle fatigue properties at room temperature were investigated,which provided relevant data support and technical guidance for in-situ particle reinforced Al matrix composites.The microstructure of TiB₂/Al and（TiB₂+ZrB₂）/Al prepared by different methods showed that:The particle size of TiB₂/Al and（TiB₂+ZrB₂）/Al composites prepared by fluoride method at 850℃for 20min are 893nm and 1.13μm,and the amount of by-product is less.The results show that this is the optimal process,but there is particle agglomeration.The agglomeration of TiB₂/Al and（TiB₂+ZrB₂）/Al was improved by alloy method and extrusion treatment.The particle sizes were 798nm and 578nm respectively.The particles were closely bonded to the matrix in a semi-coherent relationship.The microstructure and tensile properties of TiB₂/A356 and（TiB₂+ZrB₂）/A356composites prepared by different methods show that the composites obtained by fluorine salt method and alloy method have finer effect than A356 alloy matrix,mainly because the reinforcement particles generated by in-situ reaction are distributed at grain boundaries and can achieve finer effect.The distance between secondary dendrite arms of（TiB₂+ZrB₂）/A356 prepared by alloy method is the smallest.The corresponding tensile properties（T6 state）have the same rule.The room temperature Rm of matrix alloy A356 is 262MPa,and the corresponding elongation is 3.2%.The room-temperature Rm of 5wt%TiB₂/A356 and 5wt%（TiB₂+ZrB₂）/A356 obtained by fluoride and alloy methods are 298MPa,330MPa,318MPa and 341MPa,respectively,and the corresponding elongation is 4.4%,5.2%,4.7%and 5.8%,respectively.The high temperature Rm of matrix alloy A356 is 207MPa,and its corresponding elongation is9.2%.The tensile strength at 150℃of 5wt%TiB₂/A356,5wt%（TiB₂+ZrB₂）/A356obtained by fluoride method and alloy method are 239MPa,254MPa,249MPa and267MPa respectively.The corresponding elongation is 3.8%,6.0%,3.2%and 4.9%.The corresponding tensile fracture shows mixed ductile brittle fracture characteristics.Tension-compression high-cycle fatigue experiments of（TiB₂+ZrB₂）/A356composites at room temperature show that:By fitting the S-N curve,the functional relation between stress amplitude and cycle life is obtained,S=76.29+3389.3e^{-0.72lg Nf},and the ultimate fatigue strength of the material is determined to be 105MPa,which is16.7%higher than the fatigue strength of 90MPa of the matrix A356 alloy.According to the fatigue fracture analysis,the crack growth rate of high loading stress is faster than that of low loading stress.The crack of high loading stress specimen causes the brittle phase to crack preferentially,while the crack of low loading stress specimen cracks preferentially at the interface.The in-situ particles can slow down the crack propagation rate by restraining the dislocation motion and thinning the microstructure,thus prolonging the fatigue life of the composites.