Numerical and Experimental Study of Porosity Evolution during Plasma Spray Deposition of W

Abstract

The porosity that is commonly associated with discrete droplet processes, such as plasma spraying and spray deposition, effectively degrades the quality of the sprayed material. In the present paper, numerical and experimental studies on porosity evolution in plasma spray deposition of W are undertaken to provide insight into the formation and evolution of porosity. In the numerical study, deformation, interaction and solidification of molten droplets impinging onto a flat and non-flat substrate during plasma spraying are investigated. The full Navier-Stokes equations coupled with the Volume Of Fluid (VOF) function are solved to determine the exact movement and interaction of droplets. A 2-domain method is employed for the treatment of the thermal field and solidification problem within the flattening droplet to track the moving solid/liquid interface. A two-phase flow continuum model is employed for the simulation of the flow problem with a growing solid layer during droplet impingement. On the basis of the VOF function and the two-phase flow continuum model the micro-porosity is quantitatively calculated. In the experimental study, a W deposit of 2 - 3 mm in thickness is prepared using low pressure plasma spraying (LPPS). The microstructure of the deposit is characterized in detail, paying particular attention to the presence of porosity. The mechanisms that govern the formation of porosity during LPPS are proposed in light of numerical and experimental results. On the basis of the mechanisms, some fundamental trends and effects of important processing parameters on micro-porosity may be reasonably explained and optimal processing conditions for reducing microporosity may be determined.