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Heat and Mass Transfer Aspects of Coaxial Laser Cladding and its Application to Nickel-Tungsten Carbide Alloys Open Access


Other title
Thermocapillary Flows
Bead Geometry
Catchment Efficiency
Type of item
Degree grantor
University of Alberta
Author or creator
Wood, Gentry D
Supervisor and department
Mendez, Patricio
Examining committee member and department
Li, Leijun (Materials Engineering)
Sanders, Sean (Chemical Engineering)
Lang, Carlos (Mechanical Engineering)
Mendez, Patricio (Materials Engineering)
Colaço, Rogério (Materials Science and Engineering)
Department of Chemical and Materials Engineering
Materials Engineering
Date accepted
Graduation date
2017-11:Fall 2017
Doctor of Philosophy
Degree level
Simple engineering expressions capable of predicting the cross sectional geometry of weld beads deposited using laser cladding technologies are presented. The formulae can determine the width and maximum height of a single clad bead directly from fundamental engineering principles. These parameters have practical implications in targeting specific clad thickness and predicting the overlap between beads to create continuous protective surface layers. This work has been developed to address the problems associated with implementing state of the art numerical simulations that are often too difficult, costly, and time consuming for practitioners to use and empirical expressions that cannot be applied outside a rigid set of parameters or for a particular material system. The approach in this work decouples the heat and mass transfer aspects of the cladding process considering first the heat transfer in the substrate to estimate the molten pool boundaries and secondly, the interactions of the powder cloud with the molten pool to predict the mass transfer and resulting clad build up. For the thermal analysis, scaling principles and asymptotic considerations are applied to Rosenthal's point heat source. Expressions are presented for the maximum width of any isotherm directly, which, applied in the context of the melting temperature, output the maximum width of the molten pool. This characteristic value of the molten pool is the width of cross section of the solidified clad. Point heat source estimates are shown to be consistently within 70% for a wide range of laser powers, powder feed rates, and travel speeds in coaxial laser cladding of nickel-tungsten carbide alloys (Ni-WC). To improve the prediction, a numerical solution was developed to Eagar's dimensionless representation of isotherm geometry for a Gaussian heat source. For the same set of experiments, the numerical approach predicts the cross section within +/-10% of actual measurements for clad width and height. The role of convection in the heat transfer of the molten clad pool is evaluated using an existing framework for welding systems. This analysis is applied to the Ni-WC composite system, which indicates that conduction is more significant than convection under typical process conditions for this high solid fraction weld overlay. This result supports the use of a conduction based model to predict isotherm geometry in the proximity of the heat source and melt zone. Considering the mass transfer of the process, the bead profile is shown to be accurately represented by a parabola for the circular geometry of the laser beam and experimental conditions in this work. A new model for catchment efficiency (mass transfer efficiency) is proposed relating the area ratio of the projected powder cloud and molten pool to this efficiency. An expression for the height of the bead is proposed by combining the curvature of the bead surface, the catchment efficiency, and an overall mass balance of the cross section. Predictions for catchment efficiency for the Ni-WC experiments in this work were shown to be within +/-10% for all but the low laser power tests. For these same tests, estimates for the calculated height were shown to consistently over predict the bead height by 20%. The final result is a series of simple equations for width and maximum height of a single clad bead that can be solved easily based upon parameters known prior to cladding. The results of this work are based upon fundamental engineering principles and therefore can be generally applied outside of a particular material system and in some cases are even applicable to other cladding and welding processes.
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
Citation for previous publication
Wood. G. and Mendez, P.F. Role of Thermocapillary Flows in the Laser Cladding of Nickel-Tungsten Carbide Alloys. Welding in the World, 2016. Submitted.Wood, G. and Mendez, P.F. First Order Prediction of Bead Width in Coaxial Laser Cladding, Proc. of Numerical Analysis of Weldability, IIW Commission IX Mathematical Modelling of Weld Phenomena, 2015.Wood, G. and Mendez, P.F. Disaggregated Metal and Carbide Catchment Efficiences in Laser Cladding of Nickel Tungsten Carbide. Welding Journal, Vol. 94(11), pp.343-350, 2015.Wood, G., Islam, S. and Mendez, P.F. Calibrated Expressions for Welding and their Application to Isotherm Width in a Thick Plate. Soldagem and Inspeção, Vol. 19. No. 3, pp.212-220, 2014.

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