White layer formation and tool wear in high speed milling of 57HRc tool steel using coated and uncoated tools

Abstract

Advances in process technology have opened new possibilities for rapid manufacturing. High Speed Machining (HSM) is one of these innovative areas. One demanding application is the HSM of hardened steels for die and mould tooling applications. A significant impediment in the wide-spread use of HSM in hard machining is a lack of understanding and subsequent control of possible micro-structural changes to the surface of machined components. These changes can occur in the form of surface and sub-surface layers induced by grain refinement, rapid heating and quenching and or reactions with the environment. Some surface layers are known for decreasing the material fatigue life due to their brittleness. Generally, these affected surface layers are referred to as the white layer. This paper focuses on formation of white layers during high speed milling of hardened tool steels. The machining was carried out using uncoated and TiAlCrN coated micro-grain carbide end mills. The cutting tools were also analysed for tool wear. The paper explores the correlation of white layer formation to tool wear progression and how this is affected by the PVD coating. Surface hardening, sub-surface tempering, surface finish and compositional changes are also presented. The results show that in milling, tool wear is a significant driver for white layer thickness progression. Moreover, increased oxygen content suggests that oxidation could also play a role in white layer formation.