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New tool materials with a structural gradient for milling applications

New hardmetals with a structural gradient for milling applications were investigated experimentally and numerically on several length scales. On the macroscale, milling tests were performed and a numerial model for up-milling was developed. On the mesoscale, the interaction of cracks with the graded surface zones, especially large soft binder islands were studied. On the microscale, the influence of shape and volume of such inclusions were investigated and crack propagation in realistic microstructures was simulated. On the sub-microscale crack propagation and void growth in the ductile binder were modelled with crystal plasticity.

Instrumented milling tests were performed and tool life was studied for three different milling grades, where two contain a gradient structure underneath the coating. One of these grades, gamma Free, has a gamma-phase depleted zone while in grade CoStri cobalt striations are included. Two coatings were considered, a chemical vapour deposition (CVD) and a plasma assistant (CVD) coating. Initial cracks were detected in the CVD coating layer. Clear differences were detected in tool life of the gradient variants gamma Free and CoStri. The type of coating has strong influence on chipping at the edge. It also effect contact length, chip thickness ration, chip curl, edge radius and friction coefficient. Numerical simulations of milling was carried out for the tools to predict tool temperatures and stresses. The largest stresses in up-milling appear just after tool exit due to thermal stresses or just before tool exit due to reversed chip flow. Mesoscopic simulations were set up where the critical region of the tool has been modelled. The results from the calculations are comparable to the experimental results. The crack resistance curves show the important influence of residual stresses due to the coating procedure. In presence of initial cracks a soft gradient zone may retard crack propagation, in large cobalt islands, crack blunting and arrest may occur. The crack path in a microstructure can be modelled using the damage parameter. The critical values can be obtained from sub-microscopic simulations using a crystal plasticity theory.

Reported by

Max-Planck-Institut für Metallforschung
Seestrasse 92
70174 Stuttgart
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