Phase Field Investigation Of Solidification Behavior In The Welding Pool Of Stellite 6 Alloy

Stellite 6 alloy is a kind of cobalt based alloy,with CO,Cr as the main constituent elements,which have good wear resistance and hightemperature properties.Stellite 6 alloy is often used as a cladding material for various surface strengthening applications.The as-welded microstructure of Stellite 6 alloy usually consists of dendrite-like primary solid solution phase and eutectic phase distributed between dendrites,in which the primary phase plays a major role on the macro scale.It is very important to study the solidification process of Stellite 6 alloy,especially the solidification process of primary phase,for exploring the formation mechanism of microstructure and guiding the optimization of welding process.In this thesis,Stellite 6 alloy system is treated as pseudo binary system by using pseudo binary cut of Co-Cr-C-W quaternary phase diagram.Through quantitative phase field model of binary alloy solidification,the growth process and its influencing factors under welding conditions are simulated and analyzed.The main research contents and relevant conclusions are as follows:1.In this thesis,the influence of capillary effects and constitutional undercooling on the morphology and size of Stellite 6 alloy are analyzed.The amplification behavior of perturbation on the interface is in good agreement with Mullins-Sekerka theory.Under the welding condition,the influence of pulling speed on the primary dendrite spacing is much greater than that of temperature gradient.2.In the process of free dendrite growth when the tip undercooling is small,the tip radius tends to decrease as the tip undercooling increases while the tip velocity tends to increase as the tip undercooling increases,the contribution of local curvature to the total undercooling is small.3.In this thesis,a phase field method for welding that directly couples the prior temperature field is taken,simulations have been done to compare with the results from the phenomenological model based on the pool geometry.The grains starting from the bottom of the molten pool undergo planar-cell-competitive stages,and finally reach a quasi-steady state.