Effect Of Friction Stir Processing On Microstructure And Properties Of In-situ TiB₂Reinforced Al-alloy Composites

TiB₂reinforced Al-alloy composites fabricated by in situ method exhibitsoutstanding mechanical properties due to its excellent interfacial bonding andsmall particles size. Nevertheless, the particle clustering of micro-scale andcasting defects can still degrade the properties composites. In order toimprove mechanical properties of composites, it is necessary to explore away to eliminate casting defects and uniform the microstructure of thecomposites. Friction Stir Processing （FSP） can arouse intense material flow,thus it becomes an effective way to modify microstructure of alloys.In this research, in situ TiB₂reinforced Al-alloy composites wereprocessed by Friction Stir Processing. Then, Optical microscope, SEM andTEM were used to investigate particles distribution, grain, hardness andstrength of the composites. The influence of FSP on microstructuralevolution and mechanical properties of composites were characterizedaccordingly. The strengthening mechanisms were analyzed.The results of microstructure show that, FSP can evidently optimize themicrostructure of Al-alloy composites. In the stir zone, large clustering ofparticles has been broken up, leading to a more homogeneous particlesdistribution. The casting defects produced in preparation process are alsoeliminated during FSP. The average grain size changes from45μm to lessthan2μm. The plastic deformation occurs in advancing side and crown zone,inducing the dynamic recrystallization. Hence, it helps refine the grain.However, grains in the retreating side have different sizes. Even though the particles of retreating side distribute more homogeneous compared to that ofbase materials, small clustering of sub-micron scale exists. This phenomenonis due to the non-uniform material flow during FSP, which can not provideenough deformation energy for recrystallization to all grains. So part of thegrains still keep the state of deformation. This non-uniform material flow alsoresults to small scale clustering in retreating side.Testing of mechanical properties proves that, FSP improves the generalmechanical properties of Al-alloy composites. The distribution ofmicro-hardness shows that the micro hardness of stir zone is higher than thatof base material, while micro-hardness of advancing side is higher than thatof retreating side. According to the results of tensile test, the yield strength,tensile strength and elongation of the composites are all improved after FSP.The4-pass FSP has more profound effect than1-pass FSP. After4-pass FSP,the tensile strength of the composite is1.3times of base material and theelongation is8times of base material. The improvement of the mechanicalproperties is attributed to strengthening mechanisms, including grainrefinement strengthening and dislocation strengthening. FSP can refine thegrain and uniform the distribution of particles, which hinder the movement ofdislocation, and accelerate the multiplication of dislocation.