| The nanoparticle reinforced aluminum matrix composites were fabricated by using ultrasonic vibration and combination of ultrasonic vibration and wetting reaction,respectively.The effects of nanoparticle content on microstructure and mechanical properties of as cast composites were investigated.Mechanisms of particle dispersion,grain refinement and strengthening were analyzed.Thermomechanical analysis was carried out for the wetting reaction,and the influences of ultrasonic time and ultrasonic temperature on microstructure of composites were studied.Compared to traditional techniques,nanoparticles were effectively dispersed in the matrix by these methods.Microstructural studies showed that the addition of nanoparticles caused the grain refinement and that the resultant microstructures of composites depended on the dominant size and degree of nanoparticle clustering.The results of thermodynamic calculation revealed that the wetting reaction could occur spontaneously.Effective particle dispersion could be attributed to ultrasonic dispersion and fatigue rupture induced by it.Meanwhile,the addition of wetting agent promoted the wettability of nanoparticle by the melts,accordingly resulting in improvement of particle dispersion.The grain refinement could be mainly attributed to particle pushing mechanisms and heterogeneous nucleation behavior.Compared to the matrix alloy,the tensile strength,compressive strength and hardness of the as-cast composite with 1.5 wt.% nanoparticles and 1.0 wt.% wetting agents fabricated at 800 °C were significantly increased.TEM analysis showed that high density dislocations and clear interfaces between particles and matrix were present in the grains,suggesting that the superior improvement of mechanical properties can be attributed to dislocation-type strengthening of matrix alloy and load transfer from matrix to hard nanoparticles.In addition,the fracture surfaces of the as cast tensile and compression specimens were studied.Interactions between the solidification front and the remaining suspended particles take place during solidification of particles reinforced composites.The particles are either engulfed by the solidification front or pushed by the solid-liquid interface.The relation of distribution and clustering degree of nanoparticles in 7075 alloy with the microstructure of alloy was investigated by conducting casting experiment.A hydrodynamic model for calculating the critical interface velocity required to push nano-particles by the interface was proposed.In addition,the influence of the nano-particles present ahead of solidification front on interface shape was discussed.Microstructural studies showed that the grain size of composites depended on the distribution of nano-particles and nano-particles clustering in the matrix.Based on the model,it was found that for particles below approximately 36% of the critical radius,r < 0.36r* = 0.9973 μm,the critical interface velocities were four orders of magnitude lower than those obtained by micron particles,suggesting that smaller particles are easily engulfed by the interface rather than pushed.The experimental results were validated by the prediction of the model.Result showed that the constitutional super-cooling of local melts and the accumulation of solute behind the particle caused formation of a cellular interface.For semisolid forging,it is necessary to have a fine globular microstructure with appropriate liquid fraction in a semisolid range.The semisolid composite slurry was fabricated by using ultrasonic vibration and particle induction for a short process of semi solid forging.By adopting the method,nanoparticles were effectively dispersed in the matrix,and a semisolid microstructure of fine,uniform and globular grains was obtained.The effects of cooling rate and ultrasonic vibration(USV)with different power levels in various temperature ranges on semisolid microstructure of the composites were investigated.Also,the effect of second phases on the microstructure was analyzed.Microstructural studies revealed that good semisolid slurry with average grain size of 73 μm,shape factor of 0.84 and solid fraction of 0.715 could be obtained.The chemical homogeneity resulting from USV and addition of wetting agents led to formation of the nano-sized Al7Cu2 Fe and Mg Al2O4 particles.According to transmission electron microscopy(TEM)analysis,it was found that there are the well-defined crystallographic orientation relationships between the hard particles and α-Al,suggesting enhanced heterogeneous nucleation in 7075 alloy and accordingly premature precipitation of primary α-Al grains.Mechanisms involved in the development of microstructure are discussed.Semi-solid isothermal compression experiments were carried out on the composites,by using thermal simulated test machine(Gleeble-3500).The effects of deformation temperature,strain rate and presence of nanoparticles on mechanical behavior of isothermal compression of composites were studied.the fracture surfaces of samples were analyzed and,its deformation mechanism was discussed.The constitutive model for semisolid forging of nanoparticle reinforced metal matrix composites was established.According to the results of the compression experiments,the coefficients of the constitutive model were solved by multi-element linear regression.It was found that the peak and steady stresses were decreased with the increase of deformation temperature and reduction of strain rate.The reduction of deformation resistance could be mainly attributed to the lubrication action of liquid phases between solid grains.Also,it was found that the stresses were increased with the increase of nanoparticles content and reduction of particle size,and that the enhancement effect of fine,uniform distribution nanoparticles was obvious.Fracture analysis indicated that high temperature or high strain rate could contribute to a continuous distributing of liquid phases along grain boundary,resulting in the formation of liquid film on the surface of solid grains and accordingly the improvement of lubrication action.During deformation process,the sample undergone three stages,i.e.,strengthen,soften and steady state,and the deformation mechanism is liquid flow,flow of liquid incorporating solid grains,and plastic deformation of solid grains,respectively.It was found that the experiment results were reasonably consistent with the computed result obtained by the model.According to the constitutive model,numerical simulation for semi-solid forging was carried out.The effects of volume of semi-solid billet,deformation temperature,and strain rate on the distribution of effective strain and stress were analyzed.Meanwhile,a short process of semi-solid forging for universal joint part was successfully implemented.Also,the effects of deformation temperature,travel rate of bottom die and dwell time on mold filling behavior of part were discussed.According to the results of numerical simulation,it was found that under higher deformation rate and higher deformation temperature,the uniform distribution of effective strain and stress could be obtained in the billets,and the maximum deformation force was lower,and yet the defined amount of billet was in favour of improving the ability of complete mold filling of formed parts.The semi-solid forging results indicated that higher deformation temperature and higher travel rate of bottom die helped in improving the mold filling ability of semi-solid slurry,and that higher dwell time was advantageous to fill the cavity and to compact the forge pieces.The nanoparticle content and particle size affected the mechanical properties of parts.The results showed that the strength of semi-solid forged part was increased with the increase of nanoparticle content and reduction of particle size,and that the plasticity of materials was also somewhat increased.The improvement of mechanical properties could be attributed to grain refinement,homogeneity of microstructure,Orowan strengthening of nanoparticles,and pinning effect of nanoparticles along the grain boundary.In addition,the fracture surfaces of semi-solid forged parts after heat treatment were analyzed. |
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