Study On Fabrication Of Al Composites With Al-Ti-B₄C-CuO Powder Mixtures By Quick Spontaneous Infiltration Process And Concerned Reaction And Infiltration Mechanism

In the past20years, MMCs have progressed from primarily a laboratory enterprise with only narrow commercial significance to a diverse and robust class of materials with numerous important applications across a number of commercial markets. Interest in MMCs for use in the aerospace and automotive industries, and other structural applications, has increased over the past20years as a result of the availability of relatively inexpensive reinforcements and the development of various processing routes which result in reproducible microstructure and properties. The attractive physical and mechanical properties that can be obtained with MMCs, such as high specific modulus, strength, and thermal stability, have been documented extensively. Recently, combustion reaction processes for metal matrix composties have emerged as novel processing techniques. Combustion reaction involving a chemical reaction results in the formation of a very fine and thermodynamically stable reinforcing ceramic phsse within a metal matrix. In particular, the reinforcement surfaces are likely to be free from comtamination and a strong bonding between matrix ans reinforcement can be achieved.In this thesis, Al-Ti-B4C-CuO mixed powders were compacted into a cylindrical shape to make a pellet and then put it into molten Al melt. A combustion reaction occurs in the pellet, and thus enhances the temperature of the pellet. As a result, the molten Al melt infiltrated into the pellet spontaneosusly, and a pellet with high volume fraction particles reinforced Al composites is obtained. Without any pretreatment and protection, the pellet sustains its shape thoroughly. Based on the enhanced properties of high volume fraction particles reinforced Al composities, if a pellet with a shape of some component can be made, a component with good properties can be obtained after the process.In order to get the ideal composition for fabricating the composites, how the compact sustained its shape during the quick spontaneous infiltration process, and effect of CuO addition on the combustion reaction were studied in this thesis. A consistent RVF of the solid particles in the compact is essential for sustaining the compact shape during the combustion reaction. The solid (Al+Ti) particles in the powder compact tend to dissolve into the aluminum melt before the combustion reaction and this may result in crumbling of the compact. Highly exothermic CuO reduction is employed to facilitate the complete combustion reaction of Al-Ti-B4C in an Aluminum melt. The compact fabricated with CuO retains its original shape whereas the compact decomposed partically in the absence of CuO. The compact shape was greatly affected by the initial composition of the powder mixture.In addition, the effect of the Ti/B4C mole ratio on the fabrication behavior of Al composites is investigated using Al-Ti-B4C powder mixtures as reactants. The quick spontaneous infiltration (QSI) process combined with the combustion reaction and DTA analysis were used. According to the thermodynamic predictions, which are verified in the experimental results, TiB2is formed in all the samples whereas TiC is only formed in reactants with a Ti/B4C mole ratio of more than two. The C atoms from the reacted B4C do not move into TiC but instead they move into Al3BC or AI4C3when the Ti/B4C mole ratio is less than two. In addition, the reaction mechanism with a Ti/B4C mole ratio of0.75is investigated extensively.To verify the rapidity of the process, the infiltration kinetics was calculated based on the Washburn equation in which melt flows into a porous skeleton. However, there was a big deviation from the calculated results with the experimental results. Considering the cross-sections of the samples at different processing times, a new infiltration model consisting of macro-infiltration and micro-infiltration was suggested. The calculated kinetics results in light of the proposed model agree well with the experimental results. The effect of the addition of Mg to aluminum melts on the infiltration was also examined. Unlike the pressure infiltration, Mg addition was not beneficial to improving the infiltration capability for the spontaneous infiltration process. The reason is discussed based on the different role of the capillary pressure in pressure infiltration and spontaneous infiltration.