Titanium alloys are known for being difficult to machine. Within the groups of Ti alloys, the machinability is reduced when going from the α-alloys, to α + β, and finally to near-β or β-alloys. Uncoated cemented carbide is traditionally used for machining these alloys and finding a suitable coating to improve the performance is a challenge due to the high strength of Ti alloys and the high chemical reactivity of Ti with tool and coating materials at the temperatures achieved during cutting. The PVD applied TiAlN (x = 0.4–0.7) is generally recommended for machining Ti alloys and a top layer of NbN has shown promising performance in milling applications. This study systematically explores the wear mechanisms of either uncoated WC-12%Co or its TiAlN-NbN coated version in the milling of Ti alloys ranging from near-α (Ti-6Al-2Sn-4Zr-2Mo), α + β (Ti-6Al-2Sn-4Zr-6Mo), and near-β (Ti-5Al-5Mo-5 V-3Cr) alloys. The wear evolution is explored by studying as-worn tools and their cross-section using SEM-XEDS and EBSD after having reached 10%, 30%, 50%, and 100% of the full tool life at a flank wear criterion = 200 μm. The coating is removed within seconds of engagement and fails by cracking within the bulk and PVD droplet defects can initiate such cracks. Diffusional dissolution of the coating may be active on a minor scale, but the rapid mechanical failure shows the need for a more robust coating formulation. Exposed cemented carbide is worn at varying intensities when machining the Ti alloys. The highest wear rate is achieved in milling α + β Ti-6246, followed by a moderate wear rate in near-β Ti-5553, and with a slower wear rate in milling near-α Ti-6242 which is explained varying intensities in oxidation wear, diffusional dissolution, and mechanical cracking. Diffusional loss of C gives rounder WC grains and remaining W at the interface is removed by the chip flow. Diffusional loss of Co gives reduced grain bonding and loss of the dampening effect that leads to fractures in WC grains. Specifically in milling α + β Ti-6246 and in minor scale in near-α Ti-6242, there is formation of CoWO ceramic present several μm into the tool and its formation is facilitated by the oxidation of binder with resolved W. Fractures within the CoWO explains the high wear rate. Cracks also propagate in binder regions initiated from weakened interface regions due to diffusional loss of C and Co.

中文翻译：

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铣削钛合金时无涂层和涂层硬质合金刀具的磨损机制


众所周知，钛合金难以加工。在钛合金组中，从 α 合金到 α + β，最后到近 β 或 β 合金时，切削加工性会降低。传统上使用无涂层硬质合金加工这些合金，由于钛合金的高强度以及在切削过程中达到的温度下钛与刀具和涂层材料的高化学反应性，寻找合适的涂层来提高性能是一项挑战。通常建议使用 PVD ​​应用的 TiAlN (x = 0.4–0.7) 来加工钛合金，并且顶层 NbN 在铣削应用中显示出良好的性能。本研究系统地探讨了未涂层 WC-12%Co 或其 TiAlN-NbN 涂层版本在铣削 Ti 合金时的磨损机制，范围从近 α (Ti-6Al-2Sn-4Zr-2Mo)、α + β (Ti -6Al-2Sn-4Zr-6Mo) 和近β (Ti-5Al-5Mo-5 V-3Cr) 合金。在后刀面磨损标准 = 200 μm 下达到整个刀具寿命的 10%、30%、50% 和 100% 后，通过使用 SEM-XEDS 和 EBSD 研究磨损后的刀具及其横截面来探索磨损演变。涂层在接合后几秒钟内就会被去除，并因本体内部破裂而失效，而 PVD ​​液滴缺陷可能会引发此类裂纹。涂层的扩散溶解可能在小范围内活跃，但快速的机械故障表明需要更坚固的涂层配方。加工钛合金时，暴露的硬质合金会以不同的强度磨损。铣削 α + β Ti-6246 时磨损率最高，其次是近 β Ti-5553 中等磨损率，铣削近 α Ti-6242 时磨损率较慢，这是由于氧化强度不同造成的磨损、扩散溶解和机械开裂。 C 的扩散损失使 WC 晶粒更圆，界面处剩余的 W 被切屑流去除。 Co 的扩散损失会导致晶粒结合减少和阻尼效应丧失，从而导致 WC 晶粒破裂。特别是在铣削 α + β Ti-6246 和小规模近 α Ti-6242 时，刀具中会形成几微米的 CoWO 陶瓷，并且通过溶解 W 的粘合剂的氧化促进其形成。 CoWO 解释了高磨损率。由于 C 和 Co 的扩散损失，裂纹也在粘结剂区域中从弱化界面区域开始扩展。