ICMCTF2008 Session E4/G4: Tribological Studies of Coatings for Green Manufacturing and Dry Machining

The gradually stricter environmental regulations and their enforcement are eliminating much of the flexibility in the use of cutting fluids and lubricants in the metal machining industry. Therefore manufacturers of cutting fluids and lubricants are developing new formulations, for example formulations without the elements Pb, S, or Cl which improves machinability but are detrimental from a health and environmental point of view. It is, however, a long way to go before these new lubricants can be considered completely harmless and acceptable. Furthermore, the cost associated with the use of cutting fluids is estimated to be several billion dollars per year. Consequently, reducing the use of cutting fluids or metal forming lubricants, i.e moving towards so called green manufacturing (environmentally friendly manufacturing), can be a considerable economic motivation for the metal machining industry.

One approach towards dry machining is to improve the properties of the tool material by making them more wear resistant, i.e. more refractory and/or to make tools that generate less friction heat during machining. There has been a continuous development of tool materials over the past century starting with high-speed steels, cast cobalt alloys, cemented tungsten carbide, ceramics, coated carbides and coated HSS, cubic boron nitride and diamond, i.e. towards more and more heat and wear resistant materials. It is very often forgotten though, that these more advanced tool materials also are becoming more and more brittle as compared to traditional tool steel materials. It is also too often forgotten that coated tools can not be subjected to an excessive wear during break in of the tool, if the coating should be able to protect the less wear resistance substrate material in a reliable and predictable way. Hence, it is important to optimise not only the inherent properties of new tool materials but also the brittle substrate and coating composite. This should be done with the goal to obtain excellent resistance against thermal and mechanical wear without jeopardizing the reliability of the tool. A reliable tool is a prerequisite for high productivity and successful utilisation of advanced tool materials, which in turn is a prerequisite for green manufacturing. The question is how to do this? Until today, too little attention has been given to this subject, i.e. optimisation of a given tool, including the tool material (substrate) and the coating, with respect to its ability to carry load (thermal or mechanical) and how such an optimisation affects the reliability and useful life of the tool. The presentation will focus on how to optimise the performance of coated tools with respect to their ability to carry load and increase tool life in a predictable way to avoid excessive wear, premature brittle fracture or fatigue wear of coated (as well as uncoated) tool materials.

The present paper proposes a novel methodology based on analytical and experimental test methods for predicting coated tool efficiency in milling Inconel 718 and Ti6Al4V, using coated cemented carbide inserts. The applied coatings were CrAlN coatings, containing or not Zr or Y dopants. The stress-strain curves and the fatigue critical loads at various temperature levels of these coatings were determined by nanoindentations and impact tests respectively, employing various FEM-supported procedures for results evaluation. In this way coatings’ Woehler diagrams were established for specific film operational temperatures.

The milling investigations were conducted at various cutting speeds and feedrates. An insight of the developed stress and temperature fields in the cutting wedge region, during the milling process at various cutting conditions, was obtained by force measurements and calculations, based on FEM-simulations of the material removal.

These results, along with FEM-based calculations of the cutting wedge thermal and mechanical loads during the material removal process, facilitated the explanation of the coated inserts’ cutting performance at various conditions. The obtained results revealed a sufficient correlation of the coatings’ impact resistance at various temperatures with the cutting performance at corresponding cutting speeds.