Study On The Microstructure And Mechanical Properties Of TiB₂ And ZrB₂ Reinforced Al-Cu Matrix Composites Under Vibration And Pressure Fiel

With the increasing demands of energy conservation and emission reduction and the constantly development of green casting trend,the demand for high strength,toughness and lightweight safety structures in transportation,aerospace and other fields is increasing.Nano-phase reinforced aluminum matrix composites has become one of the preferred materials for manufacturing structural parts in transportation,aerospace and other fields due to its advantages of high specific strength,high specific stiffness,high elastic modulus,high wear resistance and low density.However,nano-sized particles are easy to aggregate and become crack initiation points and propagation paths,which leads to the failure of both strength and toughness.This limits the use of nano-particle reinforced aluminum matrix composites.The particle distribution can be significantly improved by using vibration and pressure field,thus improving the strength and toughness of the composites.In this study,based on Al-Ti-Zr-B reaction system,TiB₂ and ZrB₂ particles were in-situ generated by mixed salt reaction,ultrasonic and mechanical vibration were applied to the composite melt above the liquidus,and extrusion casting pressure molding was used to prepare（TiB₂+ZrB₂）/Al-Cu composites.The application of external field can effectively improve the agglomeration of nanoparticles,improve the uniformity of particle distribution,and thus significantly improve the mechanical properties,wear resistance and creep resistance of the composites.Specific research contents are as follows:（1）Nano-sized TiB₂ and ZrB₂ ceramic particles were successfully prepared by adding fluorine salt into Al-Cu alloy molten liquid and mechanical stirring by in-situ formation of mixed salt reaction.The application of vibration and pressure field improves the mechanical properties of composites.After ultrasonic and mechanical vibration,the ultimate tensile strength and the yield strength of UMSC are 370 MPa and 320MPa,respectively,and the elongation is 22%.These parameters increase by 37%,128%and 152%of gravity casting（GC）composites,respectively.Compared with squeeze casting（SC）composites,the results are 31%,92%and 100%higher,respectively.Compared with ultrasonic vibration+squeeze casting composites（USC）,the results are 10%,48%and 47%higher,respectively.The reason why UMSC composites obtain high strength and toughness mechanical properties is that the cavitation effect caused by ultrasonic vibration and mechanical vibration breaks the particles which are aggregated and clustered in the melt,and makes the broken particles dispersed evenly in the melt.In addition,the turbulence effects of mechanical vibration can promote a relative movement between liquid and solid phases,and the impact of the liquid flow causes the dendrites to break and form the crystal cores.Finally,under the action of pressure field,the solidification rate and nucleation rate of Al matrix composites are accelerated,and the grains and nanoclusters are further refined.（2）The application of vibration and pressure field improves the wear resistance of composite materials.Friction and wear experiments at room temperature show that the wear mechanism of gravity casting（GC）and squeeze casting（SC）composites is mainly characterized by severe adhesive wear,associated fatigue wear and slight abrasive wear under 5N load.The adhesion wear of ultrasonic vibration+squeeze casting（USC）composite decreases,and the abrasive wear is obvious.The wear mechanism of UMSC composites is typical abrasive wear.All the UMSC composites exhibit severe adhesive wear under 10N loading.Compared with GC composites,the wear resistance of composites with vibration field and pressure field is significantly improved at room temperature,and UMSC has the best wear resistance at room temperature,mainly because the vibration field and pressure field can significantly improve the agglomeration phenomenon of nano-TiB₂ and ZrB₂ particles,making their relative distribution more uniform,thus reducing the stress concentration at particle agglomeration and reducing the possibility of cracking.At the same time,the evenly distributed particles can effectively improve the hardness of the whole material and resist the plastic deformation of the material,thus improving the wear resistance.（3）The friction and wear experiments at high temperature show that the wear resistance of（TiB₂+ZrB₂）/Al-Cu composite is obviously higher than that of Al-Cu alloy.The wear properties of GC,SC,USC and UMSC are 6%,15%,20%and 65%higher than those of Al-Cu alloy,respectively.In this case,the pinning effect of TiB₂ and ZrB₂ particles at the grain boundary effectively resists the plastic flow of the material,and the application of physical field improves the size and distribution of reinforced particles,which strengthens the effect.（4）The change rule of steady creep rate of Al-Cu matrix composites is:UMSC<USC<SC<GC.The creep resistance of UMSC samples is 3.5～19 times that of GC samples,2.3～6 times that of SC samples and 1.5～1.8 times that of USC samples,respectively.The stress index n_a and the activation energy Q_a indicate that the creep deformation rate of matrix alloys and composites is controlled by dislocation climbing.