Formation And Evolution Of Defect During The Vertical Centrifugal Casting Of Titanium Alloy

The complex components of investment cast titanium alloys fabricated in the vertical centrifugal field have their unique superiority. The investigation of the forming and the evolution of defects for titanium alloys in vertical centrifugal field has important theoretical meaningness and practical worthiness to fabrication of high quality castings, optimize the casting technology, reduce the experiment cost and shorten the production cycle. In this thesis, titanium alloy melt feeding and infiltration flow was deeply analyzed by the combination of theoretical analysis, similar physical simulation theory and experimental research. The forming law of shrinkage defects during the solidification process was discussed. Besides, the forming and migration law of the gas bubble and inclusions in vertical centrifugal field were found according to the solidification thermodynamics and kinetics theory. Finally, the evolution of microstructures and mechanical properties in the castings were systematically researched. The methods against the defects of titanium alloy in vertical centrifugal field were presented.Aiming at the limitation of traditional linear Darcy law in the calculation of melt feeding infiltration flow, general Darcy law for the feeding infiltration flow which is fit for all Reynolds numbers was established according to infiltration hydrodynamics theory. The condition in the physical simulation of titanium alloy melt feeding infiltration flow was given according to the similar physical simulation theory and general Darcy law. Titanium alloy melt feeding infiltration flow law was studied by this condition. The physical simulation results show that the infiltration volume in feeding process increases with increasing feeding time and rotative velocity or decreasing fluid viscosity and eyelet number of porous medium.The shrinkage cavity and volume criterion during the solidification process in vertical centrifugal field were established considering to the frontal general Darcy law, feeding infiltration flow law and solidification shrinkage theory. At the same time, the shrinkage cavity defects of centrifugal casting were researched by physical simulation. In order to validate the shrinkage cavity analysis and physical simulation, titanium components were casted practically. Physical simulation of shrinkage cavity correlates well with experimental observation.The forming and migration law of gas bubble and inclusion in vertical centrifugal field were researched. The gas solubility distributing law and condition of gas bubble nucleation was studied. By means of quantitative analysis method, gas solubility, gas bubble nucleation condition, nucleation work, critical nucleation radius, and nucleation rate in centrifugal casting liquid metal filling and flow process were thoroughly studied. The results show that with increasing the centrifugal radius and angular velocity, gas solubility as a gradient vector in centrifugal field, gas bubble nucleation work, and critical nucleation radius increase. Physical simulation results of gas bubble and inclusion movement law in vertical centrifugal field correlate well with experimental research.The fitted relational expression was obtaincd after the deep and through analysis of microstructure and mechanical properties for titanium alloy melt solidification in vertical centrifugal field. The results show that the grain size and lamellar space were refined while the mechanical properties were enhanced with the increasing of centrifugal radius and rotating time. Rotation speed increases, grain size was reduced, then improves the mechanical properties but the segregation worsened. The higher the cast modulus, the larger the grain size and lamellar space, but the microhardness was reduced. The microstructure and mechanical properties grads distribution law in vertical centrifugal field was discussed. The correlation of the technological parameters, microstructure and mechanical properties in vertical centrifugal field was newly established.