Micro residual stresses in hard metals

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Using the example of tungsten carbide cobalt hardmetals a finite element model is developed which enables the precise prediction of type II residual stresses in liquid phase sintered, heterogeneous metal matrix composites. Based on a temperature dependent, crystal-(visco)-plastic constitutive description of the cobalt binder, an orthotropic thermo-elastic constitutive formulation of the tungsten carbide and the consideration of the influence of volume changes on the type II residual stress response, caused by precipitation of tungsten carbide the following objectives are pursued: 1. The accurate and precise prediction of the residual stresses and the development of the internal stresses depending on the cooling history and the microstructural features of the hardmetal. 2. The qualitative investigation of the influence of precipitation of tungsten carbide out of the binder during the cooling from sintering temperature. 3. The statistical validation of the simulation results based on the investigation of multiple 3D models based on electron backscatter diffraction data (EBSD) and focused ion beam (FIB) techniques. 4. Verification of the simulated results by residual stress measurements performed on hard metals with varying processing history. 5. Identification of a combination of microstructural features and cooling cycle which leads to a minimal residual stress state. For achieving the above mentioned targets the constitutive laws are implemented into the l finite element software Abaqus by the use of different user defined subroutines. The parameters for the crystal-(visco)-plastic model are derives form temperature dependent mechanical experiments on single crystals made of a representative cobalt binder alloy. Finite element models are automatically derived from EBSD data by a self-developed software which detects the crystal orientation and cobalt-/-carbide boundaries. By performing multiple simulations, which are based on realistic processing parameters, the residual stress distribution within the microstructure is predicted and information for the optimization of the manufacturing process can be derived. The simulation results are verified by measuring the residual stresses in hard metals with varying processing history using X-Ray and neutron diffraction techniques. Besides a more accurate prediction of the residual stress state in tungsten carbide materials, the model will lead to a better understanding of the mechanisms causing residual stresses in these materials.