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

• Author / Creator

  Wood, Gentry D

• 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.

• Subjects / Keywords

  • Thermocapillary Flows
  • Catchment Efficiency
  • Bead Geometry
  • Overlays

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3V11W060

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. 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.