• Andersson, Jon M.
• Bushlya, Volodymyr
• Msaoubi, Rachid
• Lindvall, Rebecka
• Bjerke, Axel
• Vikenadler, Elias
• Bermejo, Juan Manuel Bello

Abstract

<p>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 Ti_xAl_1-xN (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 Ti_0.45Al_0.55N-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 VB = 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 CoWO₄ ceramic present several μm into the tool and its formation is facilitated by the oxidation of binder with resolved W. Fractures within the CoWO₄ explains the high wear rate. Cracks also propagate in binder regions initiated from weakened interface regions due to diffusional loss of C and Co.</p>

Topics

• impedance spectroscopy
• grain
• scanning electron microscopy
• grinding
• crack
• milling
• physical vapor deposition
• strength
• carbide
• titanium
• titanium alloy
• electron backscatter diffraction