Simulation And Experimental Study On Electron Beam Remelting Of APS-W Coatings

The study of Plasma facing materials(PFMs)becomes increasingly important during the process of controlled nuclear fusion researches.W is the most promising plasma-oriented material due to the high melting point,high thermal conductivity,low vapor pressure,extremely low sputtering rate,and enthalpy retention.While the brittle behavior of W makes processing difficult,it is considered to deposit a W coating on the heat sink material to use on the first wall.Atmospheric plasma spraying(APS)is commonly used to fabricate the W coatings since the advantages like high deposition rate,flexible deposit pattern,low cost,and easy to situ repair.Nevertheless,disadvantages including exist high porosity and oxygen content,which are greatly affected the quality of the coatings.Besides,the huge physical difference between W and 316 L steel attributed to poor bonding strength.Electron beam remelting is considered in this essay to reduce porosity and oxygen content,enhance the thermodynamical properties.Besides,prefabricated W-316 L interlayers were melted to explore the possibility of making interlayers by the electron beam.ANSYS was used to simulate the temperature distributions and thermal stresses of ideal W coatings and W-316 L interlayers under electron beam treatments for learning the best treating time and treating heat flux during the experiment and the effect of thermal stress.Furthermore,a porous model was introduced since both W coatings and W-316 L interlayers were porous media.The effect of porosity on the temperature field of the coatings under thermal load is studied using COMSOL Multiphysics.It is concluded that the surface temperature of the coating increased with the porosity,and the higher the porosity,the greater effect on temperature,Electron beam treatments were used to modify the surface microstructure of plasma sprayed W coatings in two most representative remelting parameters.Then the properties of the coatings such as porosity,oxygen content,microhardness,wear,and corrosion resistance are characterized and studied.The experimental results reveal that a compact remelted layer with a columnar structure is generated on the top surface of the W coating after remelting.As a result,the oxygen content has been significantly reduced by 90%,and the lowest porosity slump to about 0.156%.The microhardness of the remelted W coating by certain remelting parameters has nearly doubled to 475.79 HV and the thermal diffusivity was four times as large as it before.Compared with the as-sprayed W coating,wear and corrosion resistance of remelted W coatings have been considerably improved,which were shown as less mass loss in wear experiments and lower corrosion current.The possibility of using electron beam melting to make the W-316 L interlayers was explored.The prefabricate interlayer was preset on the 316 L substrate by QPAC40 binder using mixed W and 316 L powders.The coating oxygen content,about 0.93%,and the porosity were low fabricated by electron beam melting and the inner was compact.However,unmelted W particles showed an apparent agglomeration tendency,which still needs more exploration in the future and gives a new approach to fabricate the interlayers.