Microstructure Evolution During Partial Remelting Of (Al-Si)-Al-Ti Bulk Alloy Prepared By Cold Pressing Of Mixed Powders

Particles reinforced A1 matrix composites have been widely used in the aerospace, advanced weapon, automobile and electronic industries due to their superior comprehensive mechanical properties. In attempt to further enhance the ultimate tensile properties as well as its ductility at the same time, a kind of Al3Ti-Ti shell-core structures particle reinforced A1 matrix composite was proposed in this paper. A novel processing technology for the fabrication of Al3Ti-Ti shell-core structured particle reinforced A1 matrix composite, powder thixoforming, was developed by combining the advantages of powder metallurgy and thixoforming. The microstructural evolution of the A1-Si-Ti mixed powders prepared by cold pressing during partial remelting was mainly investigated in this paper. Simultaneously, the dropping experiment was utilized to study the reaction kinetics between the Ti powders and A1 matrix so as to lay a foundation for the later research of thixoforging.The experimental results indicate that a semisolid micro structure with the fine and spheroidal primary α-A1 particles uniformly suspended in liquid phase as well as a dynamic equilibrium between the solid-liquid phases with a liquid fraction of 45% can be obtained when the bulk composite was heated at 595 ℃ for 30 min. The microstructural evolution process can be divided into four stage:the migration of Si phase from the center to the edge of the Al-Si powders and the alloying of pure Al powder, namely, the transformation of the original powders into the primary particles; the formation of a liquid phase and the increase in its amount with the extension of the heating time resulting in the formation of a continuous liquid layer; the rapid coarsening of the primary particles and the increase in the liquid phase amount; the final coarsening of the primary particles. With the rise of reheating temperature, the size of the primary particles increased to 65 μm along with the increase in the liquid fraction. An ideal liquid fraction of 45% with the primary particle size of 55μm can be achieved when reheated at 595 ℃.Chemical reaction between the Ti powders and A1 element in the matrix alloy occurs in the process of reheating at 595 ℃. First, core-shell structured, reinforced particles composing both an intermetallic shell and a soft Ti metal core formed in situ.The compact shell subsequently ruptured and peeled off when its thickness increased to a agiven value for a given size of Ti powder particles. Repeating the process of compact layer formation and subsequent peeling, the whole Ti particle was consumed completely. The results indicated that the compact τ2-Ti shell-core structured reinforcements can be obtained after being heated at 595 ℃ for 30 min. A nanoindenter was employed to evaluate the microhardness of the reaction products and the results reveal that the microhardness of the shell and core attained the values are 6.97 GPa and 1.22 GPa, respectively. As the reheating temperature rised, the thickness of the reaction layer slightly increased and consequently ruptured due primarily to the increase in the reaction rate.The XRD results indicate that the phase compositions of the reaction product were τ2 ((AlxSi1-x)2Ti) and a little Al3Ti phase when the bulk alloy was heated at 595 ℃ for 30 min. However, its composition would change into (A1,Si)3Ti phase when the time was prolonged to 120 min. That is, the reaction product changed from τ2 to (A1,Si)3Ti phase with the extension of reheating time. Besides, the reaction product was composed of τ1 phase (Al5Si12Ti7)when reheated at a low temperature of 590 ℃ for 30 min. The τ2 phase and small amounts of Al3Ti phase were formed as temperature rised. Further elevating the temperature reduced the τ2 phase, but increased the Al3Ti phase. Summarily, the variation of the shell core phase was from τ1 to τ2 and finally to (Al,Si)3Ti.The results from the dropping simulation experiment demonstrate that the reaction product layer growed in a linear kinetic way and growed towards two directions, characterized by activation energy of 374 kJ/mol.