Microstructural Analysis of Ni-WC Metal Matrix Composites Deposited Using Plasma Transferred Arc Additive Manufacturing

  • Author / Creator
    Rose, Dylan
  • The objective of this thesis is to examine how the composition and solidification rate influences the thermal degradation of WC and the resulting microstructure of WC-Ni metal matrix composites (MMCs) deposited using plasma transferred arc additive manufacturing (PTA-AM). Scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDX) were used to gather semi-quantitative compositional data. The thesis explores the effect of the composition of the metal matrix composite on the microstructure by altering the WC content, WC type, and metal matrix composition. The WC type and Cr content in the matrix played a significant role in the amount of dissolution-diffusion damage to the WC, resulting in the formation of complex carbides. The matrix microstructure consisted of Ni dendrites, a Ni halo around the primary dendrites, and a Ni/Ni3B eutectic.

    Convolutional neural networks (CNN) were used to quantify the distribution of the WC in the PTA-AM samples by determining the carbide volume percent and mean free path. The final model could segment the WC particles with above 90\\% accuracy for the intersection over union (IOU), dice coefficient, and area under the receiver operating characteristic curve across the entire test dataset. Compared to the ground truth, the model calculated statistically similar values for the carbide volume percent and slightly more conservative values for the mean free path. It was found that the mean free path between carbides tended to increase with the height of the sample, which suggests more carbide degradation.
    Electromagnetic levitation was used to estimate a relationship between the secondary dendrite arm spacing (SDAS) and the solidification cooling rate for 70wt% angular monocrystalline WC  in a NiBSi matrix. The relationship between SDAS and cooling rate was then used to determine the cooling rate at different heights throughout a PTA-AM build of the same composition. The composition of the phases present in the microstructure was determined for the different cooling rates that are experienced during the complex thermal cycling of PTA-AM. The phase and composition data are compared to Scheil solidification conditions using Thermo-calc software. The culmination of this data provides a microstructural analysis of 70wt% WC - NiBSi of cooling rates up to 1000°C/s.

  • Subjects / Keywords
  • Graduation date
    Fall 2023
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • 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.