Evolution of residual stress and damage in coated hard metal milling inserts over the complete tool life

In coated hard metal milling inserts the main damage mechanisms are thermal fatigue induced by interrupted tool-workpiece contact and wear. Dependent on the magnitudes of thermal and mechanical loads in two applied test setups, either wear or thermal fatigue in the form of combcracks is induced. The evolution of residual stress and damage in the used milling inserts was documented over their complete lifetime. In a region of interest on the tool rake face a significant buildup of tensile residual stress was observed via a synchrotron based technique. A special preparation technique enabled position resolved measurements in this area by in-house X-ray diffraction facilities to study the evolution of residual stress over the entire tool lifetime. The onset of cracking was observed to happen in this region of interest by means of focused ion beam milling and scanning electron microscopy. The residual stress levels observed are comparable in used inserts at early stages of application, independent of the different cutting conditions and the applied characterization technique. At the end of tool life wear damage dominated inserts showed tensile residual stress, whereas thermal fatigue as the dominant damage mechanism resulted in compressive residual stresses.