TOOL LIFE ENHANCEMENT OF COATED CARBIDE TOOLS USED FOR MILLING OF H13 TOOL STEEL

Abstract

Dry High speed and wet milling strategies have both been used to machine hardened die and mold H13 tool steel (HRC 45-58). The TiAlCrSiYN-based family of PVD coatings prepared with various architectures (mono-, multi- and multilayer with an TiAlCrN interlayer) were studied to evaluate the coating micro-mechanical properties that affect tool life during dry high-speed milling of H13 tool steel. A systematic design of varying TiAlCrN interlayer thickness within a multilayer coating structure was developed and its influence on coating properties and cutting performance was investigated. A comprehensive characterization of the coatings was performed using a transmission electron microscope (TEM), focused ion beam (FIB), scanning electron microscope (SEM), X-ray powder diffraction (XRD), room-temperature nanoindentation, a nano-impact, ramped load scratch and a repetitive load wear test. The incorporation of an interlayer into the multilayer coating structure was found to increase the crack propagation resistance (CPRs) to 5.8 compared to 1.9 for the multilayer and 1.6 for the monolayer coatings, which resulted in a 60% tool life increase. The wear test at a load of 1.5 N showed that although the 500nm interlayer exhibited the best coating adhesion, a decline in the H3/E2 ratio was observed to worsen the machining performance. An approximate 40% increase in the tool life was achieved with the 300 nm interlayer by obtaining a balance between mechanical and adhesion properties. To investigate the tool performance during the wet milling of hardened tool steels, the (AlCrN-TiAlN) bi-layer PVD coating was post-treated by WPC (Wide Peening Cleaning) at various pressures and times. Fatigue resistance of the coating following the application of post treatment was observed to improve as the micro-mechanical characteristics (such as H3/E2 ratio, yield stress) were increased. A deterioration in the coating’s adhesion with increasing WPC pressure was also observed as measured by wear test applying a load of 1 N. Through experimentation a balance between fatigue resistance and adhesion was found with tool life being improved by 35% at a WPC applied pressure of 0.2 MPa.