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Phase field analysis of solidification structures and interface composition in WC-Co hard metals

  • : Tatu Pinomaa1, Nana Ofori-Opoku2, Sebastian Gurevich3, Anssi Laukkanen1, Nikolas Provatas3
  • : 1VTT Technical Research Centre of Finland, 2Northwestern University, 3McGill University
  • : PDF Download
  • : 2015

Abstract

This work demonstrates an accurate model for solidification of WC-Co powder in thermal spray deposition, where particles –typically metal or ceramic powder– are heated and propelled towards a substrate, leading to a thick and lamellar coating. The WC tends to dissolve partially while the cobalt is molten in flight, and upon impact, the cobalt solidifies rapidly. This leads to a microstructure with WC particles decreased in size, and the matrix phase –originally pure cobalt– is accompanied with tungsten semicarbide and Co-W-C carbide phases, which embrittles the coating.

A simple diffusion model is used to model dissolution of WC in liquid cobalt in flight, where the time-temperature history of the thermal spray droplet is taken from an earlier process simulation. We use a 2D phase field method to simulate the rapid solidification of WC particles immersed in liquid cobalt. We assume that the dendrites nucleate at the bottom center of the molten droplet. Phase field simulations are used to predict a heterogeneous distribution of dendrites, their sizes, morphologies, alloying element distribution, and competitive growth of different phases. The phase field generated microstructure is then used to simulate the material properties such as strength and toughness in the coating layer using high-fidelity finite-element methods.

Our work presents a model that can be used to generate virtual microstructures of thermal spray coatings, and additionally, the model can be employed to model other powder consolidation and sintering processes, such as selective laser sintering.

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