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 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 have been 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.
In machining operations the tool material is exposed to severe conditions with respect to mechanical load,wear and temperature.This imposes conflicting requirements on the tool material such as good high temperature wear resistance(high hot hardness)along with sufficient toughness.It is a well established technology in continuous machining operations like turning to meet these requirements by coating the substrate.
One elegant way of approaching this problem is to create a material with high resistance to crack propagation in the very surface region at the coating and with a harder core some tenths of micrometers below the coating.This technology of manufacturing hard metals with a gradient structure is now an emerging technology for intermittent turning applications.American companies have taken the lead in this field with European hard metal companies attempting to catch up. -The next challenge in this technology is to develop coated hard metals with gradient structures for milling applications.The present project aims at developing and verifying computational design aids for the new generation milling tools based on coated hard metals with gradients structures. With the new computational tools it will be possible to test different combinations of coatings and carbide and binder distributions in the substrate to obtain optimum crack resistance and load carrying capacity.
With these computational aids it will be possible to design new coating and hard metal structures for the complex application of milling.In this process the high level of European research in modelling in these fields will be exploited to assist a new development stage in the European hard metal industry.
Funding SchemeCSC - Cost-sharing contracts
114 28 Stockholm