Numerical Modeling And Experimental Study On Dendritic Solidification And Gas Pore Formation For Binary And Ternary Alloys

The phenomena of metal solidification takes place in various means of materials processing,such as welding,additive manufacturing,and casting,etc..Thus,the control of dendrite growth and pore formation during solidification is vital to optimize the mechanical properties of materials.In this thesis,numerical modeling and in-situ observation experiments are performed to investigate the interaction between dendrite growth and gas pore during solidification for binary and ternary alloys.The present study could reveal the mechanisms of dendrite growth and gas pore formation,which are significant for predicting microstructure morphology and eliminating defects in academia and engineering.The directional solidification experiments with succinonitrile-2.2 wt%camphor(SCN-2.2 wt%DC),succinonitrile-1.65 wt%acetone(SCN-1.65 wt%ACE)binary and succinonitrile-2.2 wt%camphor-0.5 wt%neopentyl glycol(SCN-2.2 wt%DC-0.5 wt%NPG)ternary transparent alloys are conducted by using a Bridgman-type solidification setup that can observe experimental process in situ under different solidification conditions.For the binary alloy solidification,it can be found that the steady-state primary dendrite spacing decreases with increasing the pulling velocity and temperature gradient.The growth direction of the newly formed dendrite after engulfing the bubbles is not affected by the bubble size,while the bubble can change the local primary dendrite spacing.Dendrite melting,fragmentation and columnar-to-equiaxed transition(CET)phenomena are also observed during the solidification of a SCN-1.65 wt%ACE binary alloys.A cellular automaton-lattice Boltzmann(CA-LB)model is established and applied to investigate the effect of convection on the dendritic morphology for binary alloy.It is found that symmetrical solute enrichment zones appear in the liquid under the effects of thermocaplliary convections,which influences the dendritic growth after overlapping the solute zones at the dendritic tips.The interactions between a bubble and the solidification interface,under the conditions of external ultrasonic field,are simulated by using a coupled LB model.The results show that the solidification interface melts due to the increase of temperature near the bubble,and the melting distance increases with the ultrasonic intensity.The simulated morphologies of the bubble and the solidification interface are consistent with the experimental result.As compared to the SCN-2.2 wt%DC binary alloy,in-situ observation experiments of a SCN-2.2 wt%DC-0.5 wt%NPG ternary alloy reveal that the angle between the primary dendrite trunk and the secondary dendrite arms increases,while the steady-state primary dendrite spacing decreases.A cellular automaton-finite difference-lattice Boltzmann(CA-FD-LB)model coupled with the thermodynamic data of the ternary alloy calculated by Calphad is applied to simulate the dendrite growth and pore formation for ternary alloy during the solidification in the welding process.The temperature field is calculated by using an analytical model.The results show that the nucleation and growth of the gas pores occur in the interdendritic space,and the gas pores are squeezed into irregular shape by the growing dendrites.With the reduction of heat input,the dendrite arms become finer.However,the porosity is affected by the simulated region.Overall,this thesis not only reproduces the interaction between solidification dendrites and gas pores during material processing,but also quantitatively describes the solidification process of binary/ternary alloys in academic research,which is potential for engineering application and has academic significance.