Research On The Effects Of Both In-situ Nano-ZrB₂ Particles And Sr On The Microstructure And Properties Of AlSi9Cu3 Alloy

In recent years,with the increasing importance of energy shortage and environmental issues,the weight reduction of vehicles and other vehicles has become imperative.t,high-strength characteristics,Aluminum alloys gradually replace traditional steel materials and are increasingly used in the automotive industry due to its lightweight and high strength.Al-Si-Cu series aluminum alloys are widely used in the international market for key components such as automotive shock absorbers,brackets,and oil pump housings because of its high strength,good formability,and excellent casting properties.However,the development and application of Al-Si-Cu serious alloy in our country started relatively late.Also,the traditional Al-Si-Cu serious alloy is too low in strength,plasticity and the fatigue performance to meet the higher performance requirements of the rapid development of the manufacturing industry.Therefore,it is of great significance to further improve the mechanical properties of Al-Si-Cu serious alloy,and especially the fatigue performance.Based on the introduction of in-situ reinforced particles combined with element Sr can significantly improve the properties of AlSi9Cu3 alloys.Thus,the AlSi9Cu3-K₂ZrF₆-KBF₄system was chosen to fabricate the in-situ ZrB₂ particles in order to reinforce the AlSi9Cu3matrix alloy via direct melt reaction?DMR?method.Furthermore,element Sr was chosen to optimize the microstructural structure in order to improve the strength and fatigue properties of the alloy.OM,XRD,SEM,EDS,TEM,tensile testing and fatigue testing machine were used to test and analyse the microstructure,mechanical properties and fatigue properties at room temperature of the AlSi9Cu3 matrix composites with different ZrB₂ mass fraction,the AlSi9Cu3 alloy with different Sr mass fraction,and the ZrB₂/AlSi9Cu3 composites with different Sr content.The structure analysis of ZrB₂/AlSi9Cu3 composites demonstrated that the morphology of ZrB₂ particles presented in rectangular and nearly hexagonal shape with an average size of30⁶0 nm.Due to the effect of heterogeneous nucleation of nano-particles,the grain was refined.While with the increase of the content of particles,the agglomeration phenomenon appeared which was confirmed to be harmful to the properties of the composites.The introduction of Sr can transform the coarse plate-like eutectic Si into fine fiber or granular morphology.In addition,it can also refine the secondary dendrite arm spacing?SDAS?of the matrix alloy.Then the strength propeties and fatigue resistance performance of the alloy can be significantly improved.However,excessive Sr will lead to an increase in the tendency of the alloy melt to inhale,which is unfavorable to the properties of the materials.In this paper,the optimum addition amount of Sr is 0.06 wt.%.The mechanical properties of ZrB₂/AlSi9Cu3 composites showed that with the increase of the content of ZrB₂ nano-particles,the strength of the composites first increased and then decreased.Especially when the mass fraction of ZrB₂ particles was set as 3 wt.%,the ultimate tensile strength?UTS?reached 283.7 MPa,which was 23.1%higher than the matrix alloy.When0.06 wt.%Sr was added to the 3 wt.%ZrB₂/AlSi9Cu3 composites,the tensile strength and elongation of Sr-modified composites reached 330.5 MPa and 15.7%.The two parameters increased by 43.4%and 121.1%compared with the AlSi9Cu3 alloy,respectively.The fatigue test at room temperature of ZrB₂/AlSi9Cu3 composites indicated that the fatigue life of the composites was obviously higher than that of the matrix alloys at the same stress level.With the increase of cyclic stress level,the fatigue life of materials decreased and the curves tended to ease.The anti-fatigue ability of the composites after Sr modification was obviously improved,especially in low stress and high cycle fatigue areas.When the fatigue life was 10⁶,the fatigue strength of the composite modified with Sr reached 104.98 MPa,which was 27.3%higher than the matrix alloy.The fatigue fracture morphology analysis showed that the fatigue source of the material was mainly coarse eutectic Si and casting defects such as porosity.The residual salts might become the fatigue source and excessive addition of Sr would increase the melt hydrogen absorption tendency,both of which were harmful to the fatigue performance of the materials.Therefore,the amount of nano-particles and Sr should be strictly controlled.In this topic,the best content of ZrB₂ and Sr was 3 wt.%and 0.06 wt.%,respectively.