Microstructure And Properties Of A356 Matrix Composites Synthesized Under The Pulsed Magnetic Field

The in-situ (Al₂O₃+Al₃Zr)p/A356 and (ZrB₂+Al₃Zr)p/A356 are successfully prepared from A356-Zr(CO₃)₂, A356-(K₂ZrF₆+KBF₄) systems via melt in-situ reaction under pulsed magnetic field. The Optical Microscope (OM), Scanning Electron Microscope (SEM), EDS, X-ray Diffractomer (XRD) and electronic universal testing machine are used to determine the solidification structures of the as-prepared composites, the morphologies and distributions of the in situ reinforcement particles in the aluminum matrix and the mechanism of pulsed magnetic field is investigated. The mechanical properties of the composites are investigated and their nature and mechanisms are analyzed.The results show that the best proportion of strengthening and toughening elements are 0.02% Sr, 0.2% A1-5Ti-1B and 0.3% RE in the A356 matrix alloy, the reinforcements synthesized from A356-Zr(CO₃)₂and A356-(K₂ZrF₆+KBF₄) systems by melt in-situ reaction under pulsed magnetic field are Al₂O₊Al₃Zr and Al₃Zr+ZrB₂, respectively. With the assistance of the pulsed magnetic field, the process of the melt in situ reaction is accelerated, and the nucleation ratio of the in situ particles increased, the particles fined, and the microstructure of the composites optimized the over-all properties of the composites improved. Furthermore, the process of reaction is stable, and there is no need of mechanical stirring, therefore it plays a role in industrial production and application of composite. The mechanism of the in situ reaction by the pulsed magnetic field is due to the micro turbulent action force initiated by the pulsed magnetic field, the different stress status among the particles and the macro vibration and the stirring force of the melt.The SEM results of the composites fabricated from A356-Zr(CO₃)₂ reaction system under pulsed magnetic field indicates that the in situ reinforcement particles Al₂O₃ and Al₃Zr are well distributed in the aluminum matrix, the size is fine, between 0.5 and 1μm, and some are nano-sized, the interfacial banding between the in-situ Al₃Zr particle and matrix is good as well. The morphologies of Al₃Zr are particulate-shape or spheroid-shape; the morphologies of Al₂O₃ are mainly particulate-shape. When strengthening and toughening element Sr, Al-5Ti-1B and RE are added, the structure of matrix and reinforcements are refined at the same time, the volume fraction of the particles is increased and the distributions of the particles are uniform, the size is between 0.1 and 0.3μm, as well as, the eutectic Si is modified. In the composite prepared from A356-(K₂ZrF₆+KBF₄) system, the particles are fine and blunt, and the diffusion is uniformly.The mechanical properties test results show that the tensile intensity and yield intensity of (Al₃Zr+Al₂O₃)p/A356 composite and (Al₃Zr+ZrB₂)p/A356 composite are enhanced with the incensement of the pulsed magnetic field intensity. For (Al₃Zr+Al₂O₃)p/A356 composite, when the magnetic intensity is 0.07T,σ_b andσ_s of the composite are up to 395.12MPa and 338.25MPa, increased by 14.05%and 20.94% respectively which coMPared with the composite with no magnetic field .For (Al₃Zr+ZrB₂)p/A356 composite,σ_b andσ_s increased by 16.14% and 19.07% respectively, are up to 390.55MPa and 333.87MPa. With the time of the pulsed magnetic field increasing,σ_b andσ_s are increasing and then decreasing. Especially when the time is up to 20min, theσ_b andσ_s of (Al₃Zr+Al₂O₃)p/A356 composite are 393.82MPa and 331.96MPa, respectively, and increased by 13.68% and 18.69% which coMPared with composites with no magnetic field; theσ_b andσ_s of (Al₃Zr+ZrB₂)p/A356 composite are increased by 19.25% and 20.13% respectively, reached up to 401.03MPa and 336.83MPa .The tensile strength of composite increased by 11.01% than composite under zero-magnetic field ,and is up to 384.56MPa with the addition of Sr, Al-5Ti-1B and RE; on the basis of above ,when the pulsed magnetic field is added ,the tensile strength of the composite increased 18.92%,reached up to 411.97MPa.The strength mechanisms of the as-prepared aluminum matrix composites are Orowan strength, dispersion strength, fine-grained strength, solid -solution strength and dislocation.