Preparation And Mechanical Properties Of Carbon Nanotubes Reinforced Aluminum Matrix Composites

Carbon nanotubes?CNTs?have received extensive attentions due to their superior mechanical properties and physical properties,and are the excellent reinforcement for aluminum-based composites.Choosing a suitable process to improve the dispersion of carbon nanotubes in a metal matrix is the key to the preparation of carbon nanotube reinforced aluminum matrix composites.In this paper,the method of preparing carbon nanotube reinforced aluminum-silicon alloy is explored by the combination of traditional casting and high-energy ultrasonic.Meanwhile,the carbon nanotubes was coated to increase the wettability of the carbon nanotubes and the aluminum matrix.The hot extrusion deformation of 0.9 wt.%CNTs/A356 aluminum matrix composites was carried out later.The effects of different extrusion temperatures and extrusion ratios on the microstructure of the composites were systematically studied,and the wear properties of the composites prepared by different processes were compared.The main contents can be summarized below:?1?The carbon nanotubes are added to the alloy melt in the form of an intermediate alloy.By comparing the high energy ultrasonic treatment and the conventional stirring dispersion method,it is known that the high energy ultrasound is better than the conventional mechanical stirring process for dispersing the carbon nanotubes in the melt.?2?Exploring the preparation of CNTs/A356 aluminum matrix composites with different ultrasonic power and different amounts of carbon nanotubes.The experimental results show that the optimal process for preparing CNTs/A356aluminum matrix composites by ultrasonic assisted stirring casting method is 2.1Kw and 0.9 wt.%.?3?The addition of carbon nanotubes can reduce the grain size of the composite to some extent.The yield strength of the composite under the optimum process parameters is 202 MPa and the tensile strength is 261 MPa,which is 21.9%and 38.3%higher than that of the matrix under the same conditions.?4?The carbon nanotubes can be coated with a layer of titanium oxide particles by the method of heating in a hydrothermal reactor.The titanium oxide distributed on the surface of the carbon nanotubes may be in the form of fine particles or needles.Treatment of the carboxyl functional groups introduced on the surface of the carbon nanotubes with acid is the key to the success of the coating process.The coating of titanium oxide can effectively improve the wettability of carbon nanotubes and the melt.?5?Hot extrusion can eliminate the carbon nanotube segregation of as-cast CNTs/A356 composites and improve the distribution of carbon nanotubes.The higher the extrusion temperature,the larger the extrusion ratio,and the more obvious the effect of carbon nanotube distribution improvement.The silicon phase structure in the matrix can be broken by hot extrusion,and the dispersibility of the silicon phase is optimized along the extrusion direction.However,the higher the extrusion temperature,the larger the silicon phase size in the tissue.Under the process parameters of extrusion ratio of 20 and temperature of 430°C,the tensile strength and yield strength of 0.9 wt.%CNTs/A356 composites were 305 MPa and 252 MPa,and the elongation of composites was 13.2%.The strengthening mechanism of carbon nanotubes in aluminum matrix composites mainly includes various strengthening mechanisms such as load transfer mechanism and fine grain strengthening mechanism.It can be seen from the calculation that the carbon nanotube load transfer mechanism is the main strengthening mechanism in the composite.?6?It can be seen from the comparison of the wear tests that the addition of carbon nanotubes can effectively improve the wear resistance of the composite and reduce the friction coefficient of the material.At 40N load,samples of CNTs/A356nanocomposites have better friction stability in the wear test than the matrix.?7?The friction coefficient and wear rate of the nanocomposite were reduced to0.359 and 13×10-3 mm3/m due to the addition of 1.2 wt.%of carbon nanotubes,which was 26%and 39%lower than that of the base alloy.Under different loads,the friction coefficient of composites decreases with the increase of applied load and the wear rate increases with the increase of applied load.?8?TiO₂@CNTs has a greater influence on the wear resistance of composites than raw CNTs.When the content of TiO₂@CNTs in the composite is 0.3 wt.%,the wear rate of the sample is about 19.45 x 10^-3 mm³/m.With the further increase of the content of carbon nanotubes,the wear rate of the material continued to decrease.When the content was 0.6 wt.%,0.9 wt.%and 1.2 wt.%,the wear rate was 17.24×10^-3mm³/m,14.2 x 10^-3 mm³/m and 11.4 x 10^-3 mm³/m,respectively.The friction coefficient and the variance value also showed a generally consistent trend,which decreased as the content of carbon nanotubes increased.?9?The hot extrusion process has obvious effects in improving the wear resistance of the composite.0.9 wt.%TiO₂@CNT/A356 composite,the wear of the composite is 8.35×10^-3 mm³/m after hot extrusion process.The amount of wear is reduced by nearly 41.9%compared to composites that are not hot extruded.It shows that the hot extrusion process can greatly improve the wear resistance of the composite.?10?The content of carbon nanotubes is the main factor affecting the wear behavior of composites.As a result of the EDS,it is known that the carbon nanotubes are peeled off from the matrix during the abrasion test,and the carbon is pulverized or worn to form a carbon film used as a solid lubricant,thereby reducing the wear rate and coefficient friction.Meanwhile,the exposed carbon nanotubes attached to the matrix reduce the area of direct contact between the aluminum matrix and the friction pair,thereby reducing the composite wear rate and coefficient of friction.