Optimization Of Al-Nb-B Grain Refiner For Hypoeutectic Al-Si Casting Alloys And Prediction Model Of Grain Size

Hypoeutectic Al-Si casting alloys has been widely applied used in complicated casting components with relative low strength due to the advantages of low density,high specific strength,and low tendency in shrinkage.The mechanical properties of Al-Si alloys mainly depend on their microstructure and defect level.Generally,a refined microstructure,which can be commonly achieved with the addition of grain refiners,is crucial for improving mechanical properties.However,the widely used Al-5Ti-1B grain refiner is ineffective in refining the hypoeutectic Al-Si casting alloys due to Si poisoning.Recently,the developed Al-Nb-B grain refiner with good resistance to silicon poisoning and insensitivity to cooling rate has been applied in automobile power system and chassis parts.However,the poor grain refinement performance（400-500μm）,anti-fading performance（<60 min）,and low particle efficiency（<0.01%）need to be overcome.Therefore,an efficient grain refiner for refining hypoeutectic Al-Si casting alloy should be developed to refine as-cast microstructure,enhance the mechanical properties,and further expand the application fieldAl-Nb-B system is studied in this work.Firstly,the nucleation potency of NbAl3and Nb B2 is calculated by lattice mismatch.Secondly,thermodynamic calculation is used to evaluate the chemical range containing NbAl3 and Nb B2.Finally,the chemical composition of Al-Nb-B system is designed based on the phase fraction of NbAl3 and number density of Nb B2.The grain refiner with high Nb/B ratio refines hypoeutectic Al-Si casting alloy to less than 220μm.The anti-silicon poisoning property and heterogeneous nucleation behavior of NbAl3 are studied by analyzing NbAl3/α-Al interface.Meanwhile,the influence of particle agglomerations on nucleation potency and sedimentation behavior are discussed.It can be decreased by the extrusion process to improve particle efficiency and anti-fading performance.An as-cast prediction model of grain size describing the solidification process of Al alloy is proposed.The evolution of the solidification structure of Al-Si alloys after various inoculations is quantitatively described.The results are summarized as follows.（1）With inoculated by 0.1 wt.%Nb,the Al-2.57Nb-0.27B grain refiner with Nb/B ratio～10:1 provides the number density(7.9×10¹⁴ m^-3)of Nb B2 to Al melt,and the phase fraction of NbAl3 is 0.13 wt.%,accordingly refines theα-Al grain size to less than 220μm.The orientation relationship between NbAl3 andα-Al is:（001）NbAl3//（010）α-Al,[010]NbAl3//[001]α-Al.Meanwhile,the mismatch between them is 4.5%,which indicates NbAl3 acts as a heterogeneous nucleation site forα-Al and increase the nucleation rate ofα-Al.The interfacial energies of NbAl3/α-Al with/without Si doped is less than that of solid/liquidα-Al（0.158 J/m²）,indicating that NbAl3 has excellent resistance to silicon poisoning and its nucleation potency is not influenced by Si doped.（2）The formation of particle agglomeration and nonuniform distribution is caused by van der Waals force.The smaller sizes of particles,the more serious agglomeration.Particle agglomeration introduced from grain refiner to base material decreases numbers of nucleation sites and increases sedimentation behavior.The recrystallization behavior of matrix during hot extrusion process improves agglomeration distribution.Compared with as-cast Al-2.57Nb-0.27B grain refiner,agglomeration size of NbAl3and Nb B2 of as-extruded is reduced by 1/2 and 2/3,respectively,and number density of nucleation particles is increased by 1-2 orders of magnitude.The anti-fading performance and particle efficiency of grain refiner are increased by more than 1 time.（3）An as-cast grain size prediction model based on particle agglomeration and sedimentation behavior describing solidification process of Al alloy is proposed.The actual existence type and size distribution of refined phases are discussed.The influence of factors on low particle efficiency of Al-Nb-B grain refiner is found.The influence of casting parameters on refined phases and the nucleation kinetics ofα-Al grains are studied.The grain refinement performance and the particle efficiency with prolonged holding time is analyzed.The model as a guide is used to design the microstructure and process parameters for developing long-holding and high-efficiency Al-Nb-B grain refiner.