In the current study, the potential of dry machining of the titanium alloy Ti-6Al-4V with uncoated tungsten carbide solid endmills was explored. It is demonstrated that tribo-oxidation is the dominant wear mechanism, which can be suppressed by milling in an extreme high vacuum adequate (XHV) environment. The latter was realized by using a silane-doped argon atmosphere. In the XHV environment, titanium adhesion on the tool was substantially less pronounced as compared to reference machining experiments conducted in air. This goes hand in hand with lower cutting forces in the XHV environment and corresponding changes in chip formation. The underlying mechanisms and the ramifications with respect to application of this approach to dry machining of other metals are discussed.

The machining of difficult-to-cut materials such as titanium plays a key role in several industries such as aerospace or medical. Approaches to overcome many difficulties when machining these materials can be an appropriate coating system for cemented carbide cutting tools. However, the atmosphere under which machining takes place, influencing the chemical tool wear, has not been taken into consideration. This work examines the tribochemical wear resistance of TiN, TiAlN and CrAlN coated carbide tools under different atmospheric conditions when cutting Ti6Al-4V. Air, technically pure argon and silane-doped argon is used to determine the influence of different oxygen levels on the wear behaviour of the tools. It has been found that oxidation of tools and tool coatings plays a significant role in tool wear when dry cutting titanium. Best results were generated using CrAlN and uncoated inserts where an increase in tool life up 50 % can be achieved when cutting in oxygen levels corresponding to extreme high vacuum (XHV) adequate atmospheres by using silane-doped argon. The benefits of XHV adequate atmospheres also have an effect on TiAlN-and TiN based coatings, but the chemical interaction of Ti element in the coating with the workpiece material, which presumably reduces wear resistance of cutting tools, cannot be outweighted or equalised by applying oxygen free atmospheres.

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Titanium and titanium alloys have high strength at low density, good corrosion resistance and excellent biocompatibility. Therefore, the use of titanium materials is well established in high performance applications such as aerospace and biomedical. However, titanium and titanium alloys such as Ti-6Al-4V have low thermal conductivity, exhibit unfavorable chip formation with typical segmented chips, and have high chemical affinity to surrounding elements such as oxygen. Tool wear and the properties of the component surface and sub-surface are significantly influenced by the presence of oxygen and resulting chemical interactions. Among other things, chemical reactions such as oxidation occur due to the high temperatures and presence of oxygen. In this work, the chip formation of Ti-6Al-4V at different cutting speeds in discontinuous orthogonal cutting process under different atmospheres is investigated. A conventional air atmosphere, a pure argon atmosphere, and a silane-doped atmosphere were used. The oxygen content of the silandoped argon atmosphere corresponds to an extremely high vacuum (XHV), which is practically oxygen-free. It was found that chip formation is affected by the surrounding atmosphere. At the cutting speed vc = 80 m/min, non-periodic segmentation is present under oxygen-free atmosphere, while segmental chip formation occurs under air. This is accompanied by up to 16.5% lower feed force under inert gas atmosphere, which is due to reduced friction caused by the use of an oxygen-free atmosphere.

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