Simulation of fatigue crack growth in coated cemented carbide milling inserts

Abstract: The aim of this work is to create a nite element model that simulates fatigue crack growth in coated cemented carbide milling inserts. A typical fatigue wear of coated milling tools are comb crack formation on the cutting edges of the tool inserts. Comb cracks initiate from pre-existent cooling cracks in the coating layers. Severe temperature changes occur during the intermittent machining and when using liquid coolant. As a result thermal stresses cause the initial cooling cracks to propagate. With the model this work also aim to model a possible explanation for lateral comb crack formation by introducing a material alteration zone. Chemical attack occurs when wet-milling as the cooling liquid enters the principal comb crack and reacts with the substrate. The presented model is based on WC{Co substrate with TiCN and Al 2O3 coatings. The model allow for a quick and comprehensive understanding of fatigue crack growth in milling tools due to simplied loading conditions, which include thermo-mechanical and mechanical stresses. Thermo-mechanical stresses are calculated corresponding to the temperature change during one milling cycle as well as stress contribution corresponding to residual stresses from the coating application process. The mechanical stresses are approximated and applied as pressures. The fatigue crack growth simulation is governed by Paris law and is performed using Extended Finite Element Method technique in ANSYS Mechanical APDL. The result of the simulations indicates that the residual stress state is more accurate using non-linear instead of linear material models, when comparing to experimentally measured stress states. The calculated fatigue cracks path were found to be similar in shape and length to that of a comb crack in early growth stage. The stress levels in the substrate are found to be compressive except for close to the crack tip where the stresses are tensile. The stress level in the proximity of the crack tip are several magnitudes larger than in the surroundings. The crack paths change in anglewere due to the applied mechanical pressure. The length of the crack is found to be inuenced by the thermo-mechanical stresses. The chemical attack zone model was introduced as a circular zone in the WC{Co layer. The zone had its stiness altered to 80 and 120 % of the initial stiness during fatigue crack growth phase. The proposed model did not provide any indication that lateral comb crack forms due to such a stiness change.