Investigation On Mg₂B₂O₅ Whisker Reinforced Magnesium Matrix Composites Fabricated By Powder Metallurgy

Magnesium borate whisker reinforced Mg and AZ91 matrix composite were fabricated by powder metallurgy. The processing parameters were optimized according to the experiment results. Nickel-plating process was taken on the whisker surface and its effects on the bonding of the interface in the composites were discussed. The influences of the addition of the whiskers on the microhardness, Brinell hardness and abrasive and wear properties of the composites were systematically investigated. In addition, a finite element (FE) model was used to analyze the distribution of stress in the composites applied the tensile stress.The mixed powders were ball-milled in a planetary ball grinder with different size steel balls under the ball to powder ratio 8:1, rotation speed 800rpm for 1h. The cold isostatic pressing is beneficial to the compressive deformation of composite. The relationships between pressure and green density for various powders were discussed in the range of 100 to 400MPa, which agree with linear relationship. Sintering experiments were carried out at 400℃, 450℃, 500℃and 550℃, the results indicate that it is reasonable to sinter composites in the temperature range from 450 to 500℃.Formations of nickel layers with the thickness of 0.1-1μm were observed after electroless nickel plating on the whisker surface. The bonding of the interface between the whisker and matrix is improved. And a transition layer between the reinforcement and the matrix was observed in SEM images.Hardness testing indicates that, the alloying of the matrix improves the macro hardness of the composite. The nickel-plating improves the mechanical properties and reduces the oxidation at elevated temperature and the tendency to break due to the thermal stress existed at the interface between the reinforcements and the matrix. Abrasive and wear experiment show that the major wear mechanism of Mg and AZ91 is adhesion mechanism. While Mg-Mg₂B₂O₅w, Mg-Mg₂B₂O₅w(Ni) and AZ91-Mg₂B₂O₅w present grain-abrasion mechanism. And AZ91-Mg₂B₂O₅w(Ni) shows a complex mixed-wear mechanism.The FE analysis shows that the whiskers effectively bear the applied loads. But, the concentration of the stress may occur at the tips of the whiskers. The nickel-plated layer on the surface of the whisker relaxes the stress concentration and reduces the possibility of interfacial failure.