Preparation Technology Of Al-5Nb-B Grain And Refinement Of Al-Si Alloys

Al-Si casting alloys have good comprehensive mechanical properties and are widely used in industrial production.The grain refinement method of the Al-Si alloy can significantly reduce the grain size,thereby improving its mechanical properties.However,a"Si poisoning phenomenon"often occurs in Al-Si alloys with high Si content when added the currently used commercial Al-5Ti-B grain refiner,thereby reducing the refining efficiency.In this work,based on the current research on Al-Nb-B grain refiner,Al-5Nb-B grain refiner was prepared and the refining effect of binary Al-Si alloy was studied.Taking the optimal preparation process of Al-5Nb-B grain refiner as the research goal,based on the thermodynamic calculation and kinetic analysis of Al-Nb-B system,the refining effects of Al-Nb-B grain refiner were studied.First,Al-Nb-B grain refiner was prepared by reacting Nb powder and Al-6Nb alloy with KBF₄ powder in Al solution,and the resulting Nb-based intermetallic compounds Al₃Nb and Nb B₂ size,distribution,were studied,respectively,Then,two kinds of refiners were added to hypoeutectic(Al-4Si,Al-7Si)alloys and near-eutectic Al-12Si alloys,and the addition of different content(1.5wt.%,3.0wt.%,4.5wt.%)of the refiners on the microstructure of the alloy,the room temperature tensile properties of the as-cast alloys were tested.The fracture mechanism of Al-Si alloys with different contents of refiners were compared and analyzed.Finally,the effect of Al-Nb-B grain refiners on the microstructure and mechanical properties of binary Al-Si alloys is summarized.The main conclusions are as follows:Two kinds of Al-5Nb-B grain refiners prepared by two different processes,that is,Nb powder+KBF₄ powder and Al-6Nb alloy+KBF₄ powder,were used in Al solution.Including intermetallic compound Al₃Nb and Nb B₂ particles,the average size of Al₃Nb and Nb B₂ in the best quality refiner prepared by the first process is 19.52?m and 2?m.The particle size of Al₃Nb in the refiner prepared by the second process is larger,mainly distributed between 15-30?m,and the particle size distribution of Nb B₂ is between 1-4?m,and the particle size distribution of the two compounds is uneven.The size of the intermetallic compounds Al₃Nb and Nb B₂ particles generated in the former refiner is smaller,and the particle distribution is more uniform.The number of Al₃Nb and Nb B₂ particles used as heterogeneous nucleation particles of Al-Si alloy?-Al grains is more,so the refining effect will be better.The Al-5Nb-B grain refiners prepared by the two processes were added to the binary Al-Si alloy.The results shows that the secondary dendrite spacing of the Al-Si alloy decreased with the increase of the amount of refiner.The prepared refiner used two processes has refining effect on Al-Si casting alloy,but with the increase of the amount of refiner.The secondary branches of Al-Si alloy refined by the refiner prepared by Nb powder+KBF₄ powder The smaller the intergranular spacing,the higher the proportion of fine petal-shaped grains,resulting in a more uniform distribution of eutectic silicon,the refiner prepared by adding 4.5wt.%can reduce the secondary dendrite spacing of Al-4Si alloy by 57.63%;the tensile strength is also better,the elongation after fracture is increased by 90.7%,and the fracture morphology reflects that the size of the dimples is smaller and the distribution is more uniform,the flatness of the fracture is improved.