Experiment And Simulation Of Microstructural Evolution And Gas Pore Formation During Solidification

Microstructural evolution,pore formation and their interaction during solidification have a significant impact on the final mechanical performance of metals.In this thesis,the dendritic/eutectic growth and gas pore formation are studied using experiment and numerical modeling methods.In situ observation experiments of directional solidification are carried out using the transparent materials of cyclohexane and SCN-2 wt.% ACE.Various phenomena are observed,which include the stable and competitive growth of primary dendrites,interaction between dendrites and bubbles.The effects of temperature gradient and pulling velocity on the primary dendrite arm spacing are investigated.It is found that under a same temperature gradient,the primary dendrite arm spacing decreases with increasing pulling velocity;for a same pulling velocity,the primary dendrite arm spacing becomes narrower with increasing temperature gradients.The interaction between bubbles and dendrites at different positions and sizes during the directional solidification of a SCN-2 wt.% ACE transparent alloy is observed.After dendrites engulfing bubbles,the primary dendrite arm spacing is found to approach the stable average value by adjusting the number of primary dendrite arms.Besides,the bubbles with larger size between the dendrites have a greater influence on the dendritic morphology.On the other hand,the dendrites have a significant influence on the final shapes of the bubbles with smaller size.In addition,the phenomena such as bubble motion and co-growth of bubbles and solid phase are observed.The cellular automaton-finite difference method(CA-FDM)is employed to simulate the dendritic competitive growth modes of a SCN-2 wt.% ACE transparent alloy during directional solidification.The effects of temperature gradient and cooling rate on the growth morphology of divergent dendrites are investigated.It is found that the tertiary dendritic arms are more likely to become the primary trunks with the increase of temperature gradient and cooling rate.The simulation results are in good agreement with the results obtained from the in situ observation experiments of a transparent alloy.The CA-FDM model coupled with the lattice Boltzmann method(LBM)is applied to simulate the growth of columnar dendrites and bubble evolution in a SCN-2 wt.% ACE alloy during directional solidification.The results show that when the bubble is located in the front of dendrite tip or in the dendrite channel,the influence of the bubble on the morphology of dendrites increases with the increase of bubble size,while dendrites always affect the bubble morphology.The above CA-FDM-LBM model is extended to the multi-phase system,including liquid,dendrites,eutectics,and gas bubbles,for simulating the equiaxed dendrite/eutectic growth and gas pore formation of Al-Si alloys.The effects of initial Si and hydrogen concentrations on the evolution of microstructure and gs pores are investigated.It is found that the percentage of porosity and the average gas pore radius in the end of solidification increase with decreasing the initial Si concentration and increasing the initial hydrogen concentration.