Study On Microstructure And Properties Of Nano-SiC Reinforced Aluminum Matrix Composites Prepared Via Friction Stir Processing

With the advantages of excellent corrosion resistance,low expansion,high specific strength and low density,particulate-reinforced aluminum matrix composites（AMCs）are in growing demand in marine,automotive,electronics and aerospace applications.Although high-strength AMCs can be developed with micro-size particles as the reinforcements,the ductility is generally decreased considerably,exhibiting a trade-off relation between strength and ductility,which restricts its application and development.To address this challenge,some researchers have proposed utilizing nano-particles as reinforcements for preparing AMCs to enhance the strength of AMCs while retarding their ductility degradation.Compared with traditional processing techniques,friction stir processing（FSP）can manufacture fine-grained materials with uniform and dense structure at temperatures below the melting point of metals,enabling it to have unique advantages in preparing ceramic particle-reinforced AMCs.In this study,nano-SiC was used as the reinforcement to investigate the microstructure evolution and its effect on the properties during the preparation of aluminum matrix composites through FSP.The research results aim to broaden the approach for strengthening and toughening AMCs and provide theoretical support for related research while promoting the development and application of FSP.The main research contents and conclusions are as follows:（1）Al-SiC specimen was prepared via FSP.Effects of FSP and nano-SiC on the recrystallization mechanism and texture evolution in the stir zone（SZ）were evaluated.The results showed that a fine-grained structure of about 3.5μm was formed in the SZ of Al-SiC specimen due to the effects of continuous dynamic recrystallization,particle-stimulated nucleation（PSN）and Zener-pinning.Compared with FSPed Al,the percentage of high angle grain boundaries in the SZ of Al-SiC specimen increased from 62.0%to 76.8%,and the level of dynamic recrystallization enhanced,forming 85.0%recrystallized grains,which was mainly related to the nano-SiC promoting dislocation multiplication in the SZ during the FSP and accelerating the recovery kinetics process with the effect of high temperature.The intensity of the texture in the SZ is significantly weaker than that of the base metal（BM）,with the main texture component of the{001}<100>recrystallized cubic texture.There are also the weak{001}<110>,{110}<111>texture components,as well as"γFiber texture"composed of{111}<110>E and{111}<112>F textures.（2）Exploring the effect of FSP on the recrystallization mechanism and texture variation in the thermo-mechanical affected zone（TMAZ）of Al-SiC specimen.It was found that since the deformation intensity and temperature are much lower than that of the SZ,a high percentage of low angle grain boundaries in TMAZ,where dynamic recovery and partial dynamic recrystallization predominate.In comparison with the retreating side thermo-mechanical affected zone（RS-TMAZ）,the advancing side thermo-mechanical affected zone（AS-TMAZ）exhibits a lower level of recrystallization and mainly contains{001}<110>shear texture.The texture components of the RS-TMAZ mainly include{123}<634>“S texture”,{012}<100>and{112}<110>shear texture,but the overall texture intensity is weaker than that of the AS-TMAZ.（3）The results of tensile and wear tests in the SZ of Al-SiC specimen showed that the tensile strength and yield strength of Al-SiC specimen have increased from 176±3 MPa and66±2 MPa of the BM to 220±4 MPa and 116±2 MPa,respectively,while maintaining a high elongation and increasing strain hardening sensitivity.The fracture mechanism of the Al-SiC specimen is ductile fracture.Orowan strengthening derived from nano-SiC is the dominant responsible for the mechanical performance enhancement of Al-SiC specimen,as analyzed through the theoretical model calculations.The specific wear rate of the Al-SiC specimen decreased from 44.4 mm³/N·m of the BM to 33.3 mm³/N·m.The improved wear resistance of Al-SiC specimen is mainly related to its increased hardness and the reduction of the direct contact area between the aluminum substrate and the friction pair during the wear process by nano-SiC.（4）The electrochemical behavior was analyzed by detecting the open circuit potential versus time curve,potentiodynamic polarization curve and electrochemical impedance spectra of different regions of Al-SiC samples combined with the microstructure variation.It was found that the electrochemical corrosion resistance was from weak to strong in the order of BM,AS-TMAZ,RS-TMAZ,SZ.The open circuit potential in the SZ rose at the fastest rate,and the potentiodynamic polarization curve fitting yielded the minimum self-corrosion current density of 0.39μA/cm²,while the low-frequency region of the electrochemical impedance spectra showed no shrinkage of the capacitive arc and the maximum impedance value,indicating the best electrochemical corrosion resistance.The increase in the number of grain boundaries leads to an increase in crystal defects and amplifies electron scattering effects,while the high-energy state of grain boundaries can provide the driving force for the rapid formation of thicker passivation films,resulting in increased charge transfer resistance,which is the main reason for the enhancement of electrochemical corrosion resistance.Additionally,the nano-SiC minimizes damage to the integrity of the passive films,and the close-packed plane preferred orientation also contributes to improving the electrochemical corrosion resistance.（5）Al-SiC-[C]specimen was prepared using FSP.The phase composition of the Al-SiC-[C]specimen was detected by X-ray diffraction,Raman spectroscopy and transmission electron microscope,while the mechanism of mechanical and tribological performance variation was analyzed.The results suggest that the combined effect of the resistance caused by the nano-SiC and the severe plastic deformation makes the graphene exfoliate from the nano-graphite,resulting in the increase of the 2D-band to G-band intensity ratio in the Raman spectroscopy of the Al-SiC-[C]specimen from 0.71 of the nano-graphite to 0.93.The increased defects position at the edge of the nano-graphite and the high temperature contributed to the in-situ formation of Al₄C₃phase.The tensile strength and yield strength of the Al-SiC-[C]specimen are 227±2 MPa and 130±3 MPa,respectively,and the fracture indicates ductile fracture characteristics.Due to the self-lubricating effect of nano-graphite,as compared to the Al-SiC specimen,the average friction coefficient of the Al-SiC-[C]specimen decreased from 0.65 to 0.61,and the specific wear rate decreased by 33.3%,while the wear mode was mainly delamination and abrasion wear.