Homogeneity of metal matrix composites deposited by plasma transferred arc welding

Tungsten carbide-based metal matrix composite coatings are deposited by PTAW (Plasma Transferred Arc Welding) on production critical components in oil sands mining. Homogeneous distribution of the reinforcement particles is desirable for optimal wear resistance in order to reduce unplanned maintenance shutdowns. The homogeneity of the coating can be improved by controlling the heat transfer, solidification rate of the process and the volume fraction of carbide.;Infrared thermography was used to measure the temperature of the surface of the deposit during the welding process. The emissivity of the materials was required to acquire true temperature readings. The emissivity of the deposit was measured using laser reflectometry and was found to decrease from 0.8 to 0.2 as the temperature increased from 900°C to 1200°C. A correction algorithm was applied to calculate the actual temperature of the surface of the deposit. The corrected temperature did increase as the heat input of the weld increased.;A one dimensional mathematical model of the settling profile and solidification of the coatings was developed. The model considers convective and radiative heat input from the plasma, the build-up of the deposit, solidification of the deposit and the settling of the WC particles within the deposit. The model had very good agreement with the experimental results of the homogeneity of the carbide as a function of depth. This fundamental model was able to accurately predict the particle homogeneity of an MMC deposited by an extremely complicated process. It was shown that the most important variable leading to a homogeneous coating is to operate at the packing saturation limit of the reinforcement. In the case of the MMC explored, a fully homogeneous coating was obtained with 50 vol% WC in a NiCrBSi matrix.;The degree of settling of the particles in the deposit was quantified using image analysis. The volume fraction of carbide was the most significant factor in obtaining a homogeneous coating. Lowering the current made a modest improvement in homogeneity. Changes made in other operational parameters did not effect significant changes in homogeneity.