Research On The Preparation And Mechanical Property Of In-situ（TiB₂+ZrB₂）/AlSi9Cu3 Composites

The AlSi9Cu3 matrix composites reinforced by in-situ TiB₂ and ZrB₂ particles are fabricated from Al-K2TiF6-K2 Zr F6-KBF4 system via direct melt reaction（DMR） technology. The effects of the technical parameters, mechanical stirring, heat treatment on the microstructure of（TiB₂+ZrB₂）/AlSi9Cu3 composites are investigated by X-Ray diffraction（XRD）, optical microscope（OM）, scanning electronic microscopy（SEM）, energy dispersive spectrometer（EDS）and transmission electron microscope（TEM）. The size, morphology and distribution of TiB₂ particles, ZrB₂ particles and CuAl2 phase are analyzed as well. Furthermore, mechanical properties of the（TiB₂+Zr B₂）/AlSi9Cu3 composites are investigated and those of the strengthening and toughening mechanism are also analyzed by tensile testing and impact testing.The results of the analysis of cooling rate show that the dendrite arm spacing（DAS） of copper moulding is better than that of iron moulding in the AlSi9Cu3 alloy. The DAS of the AlSi9Cu3 alloy is about 20⁴0μm by iron mold casting, while the DAS size by copper casting is about 10μm. Therefore, the pouring type by copper moulding is used on the preparation of（TiB₂+ZrB₂）/AlSi9Cu3 composites.The results of the optimal preparation technical parameter show that the best reaction time and mass fraction reactants are 30 min and 20 wt.% respectively at the temperature of 860℃. It is revealed that the morphology of TiB₂ and ZrB₂ particles are presented in rectangular and nearly hexagon with average size of about 40 nm. Moreover, the particles are agglomerated as many of“Reinforced Group” which distribute in the crystal boundary of the matrix at the size of about10²0μm. Besides, the morphology of silicon phase changes from needle-like to short-rod, which forms a net structure at the crystal boundary. Grain refinement effect is obvious with the size of about 20⁶0μm.The results of mechanical stirring on the microstructure of（TiB₂+Zr B₂）/AlSi9Cu3 composites reveals that the distribution of the local TiB₂ and ZrB₂ particles has been improved when stirring speed and optimal stirring time is 700 rmp and 10 min respectively,which makesparticles distribute uniformly with the size of about 20⁶0nm and the nano reinforced groups have been broken.The results of mechanical properties show the tensile strength and elongation of（TiB₂+ZrB₂）/AlSi9Cu3 composites with the best reaction time（30 min） and mass fraction of reactants（20wt.%） by the cast condition reach 265 MPa and 14.8% respectively, which are 1.2and 2.11 times greater than that of AlSi9Cu3 alloy. The impact performance of AlSi9Cu3 composites is better than matrix alloy and the shock absorption function is 3J. When mechanical stirring time is 10 min, tensile strength and elongation reach 277 MPa and 15.6%, respectively.The strengthening mechanisms of（TiB₂+ZrB₂）/AlSi9Cu3 composites are dislocation strengthening, grain size refinement and interface strengthening. The tensile fracture is presented as cleavage fracture and the impact fracture show that the material has a certain impact toughness.The results of heat treatment on the microstructure and mechanical properties indicate Si phase tend to be granular with the size of about 2μm by solution+overaging treatment.Besides the bond-shape CuAl2 phase dissolved in the matrix which is help to achieve the formation of stable phase and stabilization material size.The tensile strength and elongation reaches 305 MPa and 15.8% respectively,which is higher than others. The shock absorption function is 6J.