The Research Of MWCNTs/AZ31 Magnesium Matrix Composites And Equal Channel Angular Pressing

Because magnesium matrix composites have a series of advantages, such as lightweight, high specific strength, good dimensional stability and so on,they have wide application prospects in the fields of automobile, aeronautic instruments and 3C products.However, due to their relatively low plasticity and poor corrosion resistance, the wide application of magnesium matrix composites has been seriously hindered.Multi-Walled Carbon Nonotubes(MWCNTs)/AZ31 alloy composites were fabricated by introducing MWCNTs into AZ31 Mg alloy melts.Composites were prepared for severe plastic deformation by means of equal channel angular pressing (ECAP) deformation process after hot extrusion. The corrosion behavior of composites which were under different conditions, such as as-cast and after ECAP different passes, were studied by immersion and electrochemical corrosion experiments in 3.5wt%NaCl corrosion medium. The optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscope (AFM) were used to investigate their microstructures, tensile fracture appearance analysis, surface topograpHic analysis, etched surface morpHology analysis;tested the mechanical properties at room temperature and corrosion resistance of composite materials.The texture evolution of composites was investigated by X-ray diffractometer (XRD) during ECAP.Starting with improvement of the dispersion of MWCNTs in an AZ31 alloy, investigated the technological characteristics of joining perform of MWCNTs during melting; the effect rules of mechanical properties at room temperature, microstructure and corrosion resistance of the cast composites when enhancing the mass fraction of MWCNTs addition were investigated. The effect rules on grain refinement, mechanical properties at room temperature, texture evolution and corrosion resistance of composites produced by ECAP were investigated.The results showed that the preparation method which joined perform of MWCNTs during melting for MWCNTs/AZ31 alloy matrix composites can effectively add MWCNTs into AZ31 alloy matrix with good dispersion; most MWCNTs were distributed in divorced eutectic pHase(β-Al12Mg17) and quadratic pHase (β-Al12Mg17) of grain boundaries;the pHenomenon not only refined the grains of the composites, but also overlap and strengthen the grains and grain boundaries. With the adding of MWCNTs, the comprehensive mechanical properties of composites have been significantly increased:when the addition of MWCNTs was 1.0wt%, the tensile strength, elongation and micro hardness achieved simultaneously the maximum, obtaining high-strength at the same time better elongation, but the mechanical properties of composites would be fell down if have more addition of MWCNTs, because MWCNTs have strong surface effect and easily cause agglomeration.The ECAP process can significantly refine the grains of the MWCNTs/AZ31 alloy composites. The MWCNTs were broken under high shearing stress, and the particle distribution in the matrix alloy was homogeneous. During the process of higher temperature ECAP, MWCNTs could pin grain boundaries and hinder grains growth; with the increase of ECAP pass numbers, the proportion of ultra-fine grain increased. After 4 pressing passes, the grains were effectively refined to a mean size of 2μm. The elongation of composites were improved but the strength decreased with the grains'refinement after ECAP, which was an inverse Hall-Petch relationship; the dispersion of MWCNTs in AZ31 alloy, grain size and texture together have an important role on the mechanical properties at room temperature of MWCNTs/AZ31 alloy composites after ECAP.The addition of MWCNTs in AZ31 alloy can be obviously improved the corrosion resistance of composite materials. After immersion in 3.5wt%NaCl corrosion medium for 24h, the average corrosion rate changed from 3.2068 mg/(m2·s) to 1.1069mg/(m2·s)when the addition of MWCNTs was 1.5wt%. The corrosion current density (icorr) which was measured by electric polarization curve decreased from 5.279μA/cm2 to 2.994μA/cm2 at the addition of MWCNTs was 1.5wt%.The CNTs dispersed in the composite materials prevented the surface oxide layer of composite material from formation of cracks and reinforced the oxide layer; meanwhile, strong interface between MWCNTs and AZ31 alloy prevented the surface oxide layer separating from AZ31 alloy matrix.Therefore, delayed the further oxidation of the composite materials and improved their corrosion resistance.After ECAP deformation, the corrosion resistance of the MWCNTs/AZ31 alloy composites has been significantly enhanced than the cast composite material.Besides grain refinement, with the increase of ECAP pass number, the improvement of dispersion of MWCNTs in AZ31 alloy was the main reason. After ECAP 4 passes, the average corrosion rate of AZ31-1.0wt%MWCNTs composites immersed in 3.5wt%NaCl corrosion medium for 24h changed from 0.6035 mg/(m2·s) to 0.2963mg/(m2·s) and the corrosion current density(icorr) decreased from 3.363μA/cm2 to 2.269μA/cm2.