Research On Synthesis And Grain-Refining Efficiency Of Al-Ti-C Based Master Alloys

After decades of research efforts, several effective routes have been developed to enhance the strength of metallic materials. Correspondingly, the ductility or the plasticity dramatically deceases, thus resulting in the imbalance between the strength and the ductility (plasticity), which is a great obstacle for the advancement of material discipline and also a technical bottleneck for the practical applications. How to improve the strength and ductility (plasticity) simultaneously is a significant scientific issue widely concerned by domestic and international researchers in metallic material field.For metallic materials, purifying, grain-refining and homogenizing are three techniques to improve traditional materials and develop new alloy systems. Grain refinement of aluminum, magnesium and their alloys is an important subject in the modern metal processing industry. Meanwhile, the grain refinement, a mechanism for grain-boundary strengthening, can elevate the mechanical properties and plastic deformation capabilities and is an effective way to modify the quality of aluminum and magnesium section bars. The addition of grain-refiners can restrict the formation of columnar and twin-columnar grains and promote the equiaxed grains, which is widely utilized in the industrial processes of light alloys. Al-Ti-C master alloy shows excellent grain-refining efficiency on aluminum and its alloy and superior technical predominance over Al-Ti-B master alloy. Its appearance is a great breakthrough in terms of the grain-refining technique for aluminum and its alloy, and great attentions have been paid by countries around the world. Therefore, researching on this subject is of great significance from theoretical and practical perspectives.Self-propagating high-temperature synthesis (SHS)-melting technique has become a new technology for the fabrication of master alloys in that it has the advantages of simple experimental setups and processes, low energy consumption, low cost in production and rapid reaction procedures. The SHS-melting technique shows its superiority during the synthesis of Al-Ti-C-X master alloy since it effectively solve the problems such as low content and inhomogeneous distribution of reaction products including TiC, TiAl3 and AlX compound. Great progress has been made in terms of the fabrication of various composite, structural ceramics and functionally gradient mateials by the SHS technology. However, very limited information is available regarding the reaction processes, microstructural transformation and evolution during the fabrication of Al-Ti-C-X grain-refining master alloy using SHS-melting technique. Supported by Foundations of Shanxi Province, research work is investigated in the following aspects.In this paper, Al-Ti-C grain-refining master alloys have been prepared by combining SHS (self-propagating high-temperature synthesis) technology and conventional casting technology. Various powders were mixed homogeneously and pressed to form the preforms, which were added into molten aluminum to ignite the SHS reaction, resulting in the formation of Al-Ti-C master alloys.The thermodynamics and kinetics of the reaction between [Ti] and the graphite in the perform was analyzed, Constituent phases and compositions in the master alloys were identified by optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy.Experimental results show that the main factors influencing the reaction velocity in Al-Ti-C system are the system temperature, aluminum content, original size of graphite particles, and the thickness of reaction layer surrounding the graphite particles. At high-temperature region, higher system temperature, lower content of excess aluminum powder, smaller the particle size of graphite, and thinner the reaction layer surrounding the graphite particles, are positive for the reaction between [Ti] and the graphite of Al-Ti-C system.Based on Al-Ti-C ternary grain-refining master alloy and the SHS-melting technique, Al-Ti-C-X (X=Y, La, P) multiple master alloys have been successively prepared through adding proper amount of Y, La and P, respectively. The Al-Ti-C-X multiple master alloys were introduced into aluminum, magnesium alloys so as to obtain materials with fine grain size and excellent comprehensive mechanical properties. The synthesis, phase constitutes and hereditary effect of Al-Ti-C-X multiple master alloys were investigated; the evolution and the origin of the microstructure and mechanical properties of aluminum, magnesium alloys after the addition of multiple master alloys were also studied.It is well-known that the wettability between carbon and liquid aluminum is crucial for the synthesis of TiC particles. Experimental results show that pre-immersion treatment of graphite particles and adding special activators, are indispensable to improve the wetting of the graphite with the aluminum melt. Meanwhile, the addition of Y, La or P could further promote the wettability and subsequent synthesis of TiC. In addition, the Al-X phases in the Al-Ti-C-X multiple master alloys show grain refining effect on various aluminum, magnesium alloys.Experimental results indicate that the phase constitutes of Al-Ti-C-Y master alloy differs as the Y content varies. When the Y content is 0.5%, the master alloy consists of needle-like TiAl3, TiC andα-Al matrix; while increasing the Y content to 1.0%, the master alloy comprises rod-like and blocky TiAl3, TiC, Al3Y andα-Al matrix; further increasing to 2.0%, the master alloy is mainly composed of large lath-shaped TiAl3, net-like Al3Y, TiC andα-Al matrix. The addition of La into Al-Ti-C alloy results in Al-Ti-C-La master alloy that comprises TiAl3, TiC, AlTiLa andα-Al matrix, while the addition of P into Al-Ti-C alloy results in Al-Ti-C-P master alloy that comprises TiAl3, TiC, AlP andα-Al matrix.In order to refine and strengthen aluminum, magnesium alloys, the Al-Ti-C-X multiple master alloy is introduced. Since the Al3Y,AlP,TiAl3 or TiC particle can either act as effective nucleating site or restrict the diffusion of solute atomic Al and Mg, the grain size of the matrix is reduced and morphology, distribution of divorced eutectic phase are modified, which is positive to the improvement of mechanical properties of aluminum, magnesium alloys.In this paper, based on the research in terms of the synthesis of AlX, TiC phase in Al-Ti-C-X multiple mater alloy, the development of aluminum, magnesium alloys reinforced by AlX, TiC phase, and microstructural evolution of the newborn refining phases, hereditary effect and grain-refining behavior of the refining phases during the solidification and crystallization process of aluminum, magnesium alloys have been explored.Experimental results show that the newborn refining phases, with dispersive distribution at the inner part of the primary phase, can refine the grain, strengthen the matrix and grain boundaries, lower the diffusion rate of solute element, hinder dislocation movement in the matrix, prevent the grain boundary sliding; reduce the size of the phase at the grain boundary, render its morphology to be spherical; finally, aluminum, magnesium alloys with fine grain size and excellent mechanical properties are achieved.