Study On Rheological Behavior And Strengthening-toughening Mechanism Of XTiB₂/Al-17Si-4Cu Composite

The hypereutectic Al-Si alloy with low density,high wear resistance and high thermal stability was widely used in automotive,aerospace,military equipment and other fields.However,due to the coarse and irregular organization characteristics of the primary Si phase,which can reduce the mechanical properties of the alloy,the conventional hypereutectic Al-Si alloy is unable to meet the application environment of higher strength.In order to improve the strong toughness of hypereutectic Al-Si alloys,the strong toughness of hypereutectic Al-Si alloys is generally improved by non-equilibrium solidification conditions,particle enhancement,elemental modulation and heat treatment.In this study,xTiB₂/Al-17Si-4Cu composites were prepared by the original autogenous TiB₂ particles and semi-solid rheological extrusion casting process by choosing A390 alloy as the research object.The flow behavior of xTiB₂/Al-17Si semisolid slurry was analyzed by the apparent viscosity test,numerical simulation of semisolid metal forming and metallographic observation.Meanwhile,the material composition and forming process were determined for the next study.The selected composites were subjected to T6 heat treatment,and the effects of TiB₂ particle addition and T6 heat treatment on the composites were analyzed by characterization means of XRD,OM,SEM,EPMA,TEM,nanoindentation experiments,and room temperature tensile experiments.The basic physical parameters of xTiB₂/Al-11Si composites were obtained by OM and melt apparent viscosity experiments.TiB₂ particles promoted the nucleation of the incipient Si phase,thus affecting its size and the number of nucleation.At the same time,TiB2 particles regulate the apparent viscosity of the composites and enhance the synergistic solid-liquid two-phase flow.Based on the measured physical parameters,a solid-liquid two-phase model was established and numerically simulated.The results show that increasing the TiB₂ particle content significantly reduces the solid phase segregation rate of the composites.The observation results of the microstructure of xTiB₂/Al-17Si composite triode casting show that when the content of TiB₂ is 1wt.%,the solid-liquid synergistic fluidity is poor and the solid phase distribution is uneven.With the increase of TiB₂ content,the solid-liquid two-phase synergistic fluidity is improved and the microstructure uniformity is increased.When the TiB₂ content is6wt.%,the microstructure of the lower pipe of the composite casting is the most uniform.Based on the simulation results and experimental results,it is decided to take samples from the lower pipe of the semi-solid rheological extrusion 6TiB₂/Al-17Si-4Cu three-stage pipe casting for further research.The effects of TiB₂ particles and heat treatment process on the microstructure of Al-17Si-4Cu composites have been analysed.After the addition of TiB₂ particles,the size of the primary Si phase decreases to 23.2μm.TiB₂ particles can refine the primary Si phase by promoting the nucleation of the primary Si phase and embedding into the surface of the primary Si phase to limit its growth.The ageing time gradient T6 heat treatment of 6TiB₂/Al-17Si-4Cu composites was carried out.After solution treatment at 520°C for 9 h,followed by ageing treatment at 170°C,the average equivalent diameter of the primary Si phase in the composite decreases from 27.4μm to 17.5μm.The interface between TiB₂ particles andα-Al substrate and primary Si phase in6TiB₂/Al-17Si-4Cu composites has been analysed by TEM.There is a semi-coherent interface betweenα-Al and TiB₂,and a semi-coherent interface between primary Si and TiB₂.It is shown that TiB₂ particles can be used as a nucleation substrate forα-Al phase and primary Si phase.The interface between the precipitated phase in the composite and theα-Al substrate has also been analysed.There is a semi-coherent interface between theθ′phase andα-Al,and a coherent interface between theθ′phase and theθphase.The precipitation behaviour of the Al₂Cu phase was analysed using STEM images.With aging,the volume fraction and average particle size of the precipitated phase increase,and the main precipitated phase gradually transitions from theθ′′phase to theθphase.The copper-rich phase redissolves into the matrix during the solution treatment,and the Cu element gradually precipitates uniformly in the form of GPI-θ’′-θ’-θduring the ageing treatment.6TiB₂/Al-17Si-4Cu composites were strengthened by TiB₂-primary Si diphasic structure.Compared with the primary Si phase,the hardness and Young’s modulus of the TiB₂-primary Si diphasic phase increased by 12.19%and 29.41%,respectively.At the same time,the hardness and Young’s modulus of the composite matrix were increased by 13.48%and 28.08%,respectively,after the addition of TiB₂ particles.The tensile properties of the 6TiB₂/Al-17Si-4Cu composites were further analysed.The ultimate tensile strength and maximum elongation of the composites were improved after the addition of TiB₂ particles.After T6 heat treatment,the short ageing time reduces the tensile strength and elongation of the material.When the ageing time was increased to 8 h,the tensile strength and elongation of the material increased to263.3 MPa and 0.51%,respectively.When the ageing time reached 12 h,the strength of the composite was improved and the toughness was reduced.The tensile strength and elongation were 281 MPa and 0.49%,respectively.The main strengthening mechanisms of the composites are strain strengthening,CTE strengthening,fine grain strengthening and Orowan strengthening.Quantitative analysis of A₃ sample with the best comprehensive mechanical properties showed that the contribution of different strengthening mechanisms to the strength improvement of composites was as follows:CTE strengthening>Orowan strengthening>loading strengthening>fine grain strengthening.