Friction Stir Processing Of In Situ Al₃Zr Particle Reinforced Aluminum Matrix Composites And Its Microstructure And Properties

In-situ particle-reinforced aluminum matrix composites have the advantages of high specific strength,low density,and low thermal expansion coefficient.Compared with ex-situ aluminum matrix composites,the reinforcing particles in the in-situ self-generated aluminum matrix composites nucleate and grow in the aluminum matrix by chemical reaction,and the bonding interface between the reinforcing particles and the matrix is clean.However,casting defects such as particle segregation,coarse grains,and pores are inevitable in as-cast in-situ Al-matrix composites,making it difficult to significantly improve the mechanical properties of in-situ Al-matrix composites.Conventional secondary processing techniques（such as Rolling,extrusion,etc.）play an obvious role in the elimination of casting defects and grain refinement,but the improvement effect on problems such as particle segregation is not obvious.Stirring friction processing,as a solid-state processing technology with large plastic deformation,can be used for secondary processing of aluminum matrix composites,which can effectively refine and homogenize the particles and refine the matrix grains,thus achieving the purpose of enhancing plasticity and improving material properties.In this thesis,the in-situ Al₃Zr/AA6082 particle reinforced composites were prepared by chemical reaction of molten AA6082 aluminum alloy and K₂Zr F₆.First,the microstructure,particle morphology,size,distribution and mechanical properties were studied.The material is subjected to secondary processing to study the influence law between different processing techniques and the microstructure,particle morphology and properties of aluminum matrix composites,and to explore its strengthening mechanism and corrosion mechanism.The preparation and characterization results of in situ Al₃Zr/AA6082 particle reinforced composites show that Al₃Zr particles are relatively uniformly distributed in the aluminum matrix composites,and the particle size is about 4-5μm.The reinforcing particles nucleate and grow in the matrix through chemical reaction,the surface of the particles is free of contamination,and the metallurgical bonding between the particles and the matrix is good,and the bonding interface is clean.The as-cast aluminum matrix composites have relatively coarse grains,with a grain size of 70-80μm.The grain orientation distribution inside the material is relatively random.The tensile strength of the aluminum matrix composite material is 152.3MPa,and the fracture form is ductile fracture.The composite materials were processed by friction stir machining at different speeds.The test results showed that:under the action of friction stir machining at different speeds,the grains of aluminum matrix composites changed from coarse and irregular to fine equiaxed grains.The grain size is 3-4μm under the post-processing condition of/min,which is about1/16 of the grain size of the as-cast composite.The reinforcing particles are uniformly dispersed and obviously refined by the vigorous stirring of the stirring needle,and the particle size is 0.8-1μm under the processing conditions of the rotating speed of 1200 r/min.The tensile strength of Al₃Zr/AA6082 aluminum matrix composites processed at different speeds is up to 224.14MPa,which is 45%higher than that of the base metal.After friction stir processing,the sensitivity of pitting corrosion of aluminum matrix composites is significantly reduced,and the corrosion rate in corrosive environment is significantly reduced.Multi-pass friction stir processing is used to process the composite material twice.The test results show that with the increase of processing passes,the degree of grain refinement in the stirring zone is further improved.When the number of overlapping passes is 2,the grain refinement effect in the stirring zone is the best,and the degree of uniformity of the structure is the best.The refinement degree of the reinforced particles（Al₃Zr）is further improved with the increase of the number of passes,and the uniformity of dispersion is also improved.When the overlapping passes are 2 times,the particle size is 0.1-0.5μm.The improvement of the grain refinement degree in the stirring zone and the improvement of the refinement and dispersion degree of the reinforcing particles makes the tensile strength of the composite material increase,reaching 263MPa,which is 70.8%higher than that of the as-cast composite material.After multi-pass friction stir processing,the dynamic recrystallization of the composite material is fully carried out,the electrochemical corrosion performance is significantly improved compared with that under the single-pass processing condition,and the self-corrosion current density of the composite material is significantly reduced.