Fabrication And Performance Of Short Carbon Fiber Reinforced Magnesium-based Composites

Magnesium metal has broad application prospects in aerospace, transportation and electronics industries because it has low density, good damping capacity, good thermal conductivity and electromagnetic shielding characteristics. However, it is not been directly used as structural materials due to its poor mechanical properties and corrosion resistance. The application of magnesium metal is usually take the following two ways: one is alloying and another is magnesium matrix composites. The introduction of different reinforcements into the magnesium alloys can significantly improve the mechanics performance, wear-resisting performance, damping performance and high temperature resistant performance of magnesium matrix composite.Carbon fiber is a very ideal reinforcement to make the magnesium based composite on account of its excellent performance such as high specific strength and modulus, thermostability, fatigue resistance, low inflation and self-lubricating etc. But it is well known that the wettability between it and magnesium is poor. So it is often required to carbon fiber's surface treatment. Electroless nickel plating is one of surface treatment measures. On the one hand the good wettability between nickel and magnesium could improve the mechanical properties of carbon fiber reinforced magnesium based composite; On the other hand the introduced metal nickel in the interface can significantly improve the damping capacity of magnesium matrix composite.In this thesis the short carbon fibers reinforced magnesium matrix composites were fabricated by powder metallurgy and hot extrusion technology. Then its preparation technology, organization structure, mechanical properties and damping properties were studied. The deposition mechanism of nickel,and its influence on the mechanical properties and damping performance of the magnesium-based composite was studied. Finally the hot extrusion process of composite was simulated by finite element method. The influences of extrusion process parameters to the composite material's microscopic structure were studied. The following main achievements were obtained in this paper:1?The L16 (4⁵) orthogonal experiments were designed based on the preliminary experiments and it was got the best plating technology conditions: T = 65?, t = 3min, pH = 8.0, complexity agent content 25g/L. Using this technology conditions, the uniform nickel coating about 0.5?m thickness was obtained.2?It was found the whole electroless plating process could be divided into four obvious stages: induction period - accelerating period - slow period - stabilization period. The orders of reaction of [OH -] and [C₆H₅O₇^-] were 0.265 and -0.233 respectively. The activation energy of electroless plating was 62.82 KJ/mol. Along with the heat treatment temperature elevation, the following results were appeared. The graphitization appeared because the interior diffusion of nickel coating from the surface carbon fibers. The grains were obvious grown up in the coating and its crystallization improved nearly doubled. Its phase composition changed from the initial preferred orientation along (111) faces of metal Ni and a small amount of P to the disorder. Ni3P and Ni.3?Using the magnesium as matrix, the magnesium-based composites which the short carbon fiber distribution was distributed uniformly and directionally aligned were fabricated by powder metallurgical and hot extrusion. The average length of short carbon fibers was about 30?m which was shorter than its initial because the shearing action of irregular magnesium granules to the short carbon fiber during the pressure billets process. TEM test results showed that the thickness of good interface layer was 500nm and the interfacial product of Mg₂Ni had a certain catalytic graphitization role.4?Comprehensive theoretical calculation and test results,it was found that the enhancement mechanism of magnesium matrix composite reinforced by coated short carbon fibers was mainly "load transfer effect"; At ambient temperature, the composites possess the more excellent damping performance above the critical amplitude with the increase of volume fraction and there existed other damping mechanisms in addition to dislocation damping mechanism in it. Whether it be Pure Mg, 5.5 vol % uncoated cf/Mg or 5.5 vol % Ni - coated cf/Mg, only one damping peak on the damping-temperature curves and the peak temperature shift to high temperature with the increased frequency which shows the characteristics of heat activating relaxation process. The thermal activation energies of above three different materials were 1.287eV, 1.129eV and 1.725eV, respectively. There existed mainly the dislocation damping mechanism in the first two kinds of materials. However, the dislocation density near the interface reduced due to the introduction of nickel coating which improved interfacial wettability in the 5.5vol%Ni-coated cf/Mg composite. The dissipated energy by dislocation motion was decreased at low temperature and the same applied stress. The maximum damping values appeared in 200 260?due to the increased dissipated energy through the interface and grain boundary sliding with the rise of temperature.5?The AZ91D magnesium-based composites reinforced by short carbon fibers were successfully prepared and the length of carbon fibers was 30 40?m in them. The results of TEM observation shown that the bonding of the interface between metal substrate and carbon fibers is good. The mechanical properties of 5.0vol%Ni-coated cf/AZ91D composite was optimal compared with pure AZ91D and 5.0vol%uncoated cf/AZ91D composite. The damping peak temperature shifted to higher temperature with the increase the frequency and also displayed the characteristics of heat activating relaxation process. Its thermal activation energy was 3.448 vetches damping capacity of 5.0vol%Ni-coated cf/AZ91D composite decreased with the rising hot extrusion temperature and increased extrusion ratio at the temperatures higher than 220?.This was responsible for the grain size internal the composite.6?The simulation results of hot extrusion process carried out by DEFORM -3D finite element software shown that the decrease of carbon fiber's length was due to the magnesium particle's shearing action during the pressing process. In hot extrusion process the stress field, strain field and temperature field were distributed symmetrically and the obvious stress and strain concentration was appeared in the sizing area. The metallographic structure near this regional in the composite shown that the metal particles were gradually been stretched and the obvious dynamic recrystallization was appeared .But the grain size become bigger due to the release of the internal stress and strain when the composite outflow the sizing region.