Numerical Simulation Research About The Effect Of Flow On The Solidification Structures Of Alloys

The external fields like ultrasonic treatment and electromagnetic shocks,are often introduced in solidifying alloys to refine the microstructures by forced flow.And sometimes the natural convection has obvious effect on solidification.Thus,studying the effect of flow on solidification of alloys has profound academic significance and important value for engineering application.In this dissertation,which is based on the background of applying external fields in the solidification of alloys,the effect of forced flow and natural convection on solidification structures of alloys were studied by modeling and simulaiton.The coupled model of flow-phase-fie ld for simulating dendrite growth under flow fie ld based on Lattice Boltzma nn Method and Phase Field Method was establis hed.3-D programs for s imulating dendr ite growth under flow based on paralle l comp uting and Adaptive Mesh Refine me nt were developed.The solid ificatio n can be simulated eve n whe n fraction so lid approaches 100%,the shortcomings of SIMPLE algor ithm,such as low effic ie ncy and stability ha ve been overcome by adopting t his algor ithm,without compromis ing any accuracy,it could improve the computatio na l effic ienc y by 2 – 3 orders of magnitude.The difficulty o f perfor ming massive 3-D simulation of dendrite growth under convection due to the limitation of computing capacity has been successfully overcome.Based on this,the effect of forced flow on the dendrite growth of Al-4%Cu was studied by simulation,it was found that only when the incoming velocity of forced flow is over 0.001m/s,the dendritic morphology can be obviously affected.In a certain range,higher incoming velocity resulted in faster growth of dendrites.The effect of shear flow on the growth of columnar dendrites is weaker than that of forced flow parallel to the primary arms.Forced flow cannot eliminate columnar crystal competitive growth,it can only affect the speed of dendrite growth.Based on the simulation o f microstr uctures at high fractio n solids,the permeability of mus hy zo ne was calculated,making it possible to prediect the permeability of equi-axed and columnar crysta l zones in the whole range of fraction so lid whether with or witho ut forced flow.New models o n describ ing the permeability o f mus hy zone under forced flow ha ve been proposed.The permeability columnar dendrites decreases exponentia lly with the increas ing of incoming flow speed,for equi-axed dendrites,the permeability becomes obviously anistropic under forced flow.The effect o f natura l convection caused by the dens ity d ifference between solute and me lt on solid ification microstructures of alloys was studied.For Ga-25%In,many stacks of funne l-like floating were formed at the dendrite tips during so lid ificatio n due to the low dens ity o f solute I n.For Al-4%Cu,the sinking solute is prone to accumulate in the interdendr itic spaces,resulting in the formation of microsegregation.It was studied by experime nts and simulatio n that,the inte ns ity of ultrasonic streaming is close ly related to the melt itse lf,the streaming becomes weaker as the density and viscosity increase.Ultrasonic treatment is he lp ful for reduce the temperature gradient in the me lt.The effect of treatment decreases with the increase of distance fro m the ultrasound entra nce.In this d issertatio n,the Reynold number and veloc ity magnitude in the me lt treated by 300 W ultrasound are in the range of 0~20000 and 0~0.8m/s respectively.