Study On The Microstructure And Mechanical Properties Of ZrB₂/AA6111 Aluminum Matrix Composites By Friction Stir Processing And Heat Treatment

The interfaces between particles and matrix of in-situ particle-reinforced aluminum matrix composites are clean and the bonding strength is high,but there are problems of uneven as-cast microstructure and coarse grains,which need to be improved to adapt to subsequent processing and application.This study has chosen Al-K₂Zr F₆-KBF₄ in-situ reaction system to prepare Zr B₂/AA6111 aluminum matrix composite.And then the composite was modified by friction stir processing（FSP）with different parameters.The evolution mechanism of matrix structure and the change rules of particle size,shape and distribution were explored.Aiming at the macroscopic inhomogeneity of the stirring zone,the reverse secondary processing after FSP is used to improve the microstructure and study the evolution.For the composite after FSP with the best parameter,the strength and plasticity are further improved by different T6heat treatment processes.Finally,a set of FSP and T6 heat treatment composite modification process suitable for Zr B₂/AA6111 aluminum matrix composites was obtained.The contribution of each strengthening mechanism in the whole process was quantitatively clarified.The as-cast microstructure and properties of Zr B₂/AA6111 aluminum matrix composite were analyzed.The results show that the as-cast microstructure of the composite is coarse,and the average grain size is 63.60μm.The particle shape is round,the interface is well bonded,and the average particle size is 2.0μm.The particles agglomerated seriously,mainly in the size of 20-50μm.Due to the inhomogeneity of the as-cast structure,the strength of the composite is low,and the tensile strength is 149.51 MPa.The plasticity is also very poor,and the elongation is 17.9%.The fracture form is brittle fracture.The microstructures and properties of Zr B₂/AA6111 aluminum matrix composites after FSP were analyzed.The results show that the grains were significantly refined.The particles were fully broken to be round and fine,and uniformly dispersed under stirring.The tensile strength and elongation of the composites increase significantly.The effect comes to the first-class when the rotating speed is 1200 r/min and the travel speed is 60 mm/min.The grains in the nugget zone have been refined to 2.59μm,the average particle size is 0.35μm,and the average hardness is 88.8 HV.The tensile strength reaches 193.40 MPa,the elongation is29.3%,and the product of strength and elongation is 5.67 GPa%,which increased by 29.4%,63.7%and 111.6%respectively compared with the base metal（BM）.The microstructures and properties of Zr B₂/AA6111 aluminum matrix composites after double-sided FSP were analyzed.The results show that only when the rotating speed is 1000r/min and the travel speed is 40 and 60 mm/min respectively,the stirring zone is well formed and there are no defects.With the increase of rotating speed and travel speed,channel defects throughout the travel direction appear in the processing area,and the tensile strength and elongation also decrease.The average particle size of the nugget zone is refined from 5.56μm to 4.65μm,and the average aspect ratio is reduced from 1.92 to 1.49.The highest tensile strength is 200.42 MPa.The elongation is 24.5%,which is 24.4%higher than 19.7%of the single-sided FSP composite,with the rotating speed of 1000 r/min and the travel speed of 40mm/min.Under the appropriate parameters,double-sided FSP can achieve better results.The microstructures and properties of FSPed composites after T6 heat treatment were analyzed.The results show that the particles are stable without growth and dissolution after T6 heat treatment.Mg₂Si precipitate in the matrix and at the particle-matrix interface,which greatly improve the properties.When the solution temperature is 540°C,the grain uniformity of the T6 state nugget zone is the best,which is standard equiaxed,and the average size is3.21μm.The average hardness is 123.3 HV.The tensile strength is 316.75 MPa,which is111.9%higher than that of BM.The elongation is 30.1%,which is 68.3%higher than that of BM.The product of strength and elongation is 9.54 GPa%,which is 255.97%higher than that of BM.The theoretical values of yield strength of BM,FSPed-and T6-composites are86.18 MPa,128.82 MPa and 291.37 MPa,respectively,which are more than 96%consistent with the experimental values of 89.66 MPa,129.60 MPa and 298.18 MPa.