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On the wear mechanisms of uncoated and coated carbide tools in milling titanium alloys

Lindvall, Rebecka LU ; Bello Bermejo, Juan Manuel LU orcid ; Bjerke, Axel LU orcid ; Andersson, Jon M. ; Vikenadler, Elias ; M'Saoubi, Rachid LU and Bushlya, Volodymyr LU (2024) In International Journal of Refractory Metals and Hard Materials 124.
Abstract

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 TixAl1-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... (More)

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 TixAl1-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 Ti0.45Al0.55N-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.

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author
; ; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Cemented carbide, Milling, Titanium, Tool wear
in
International Journal of Refractory Metals and Hard Materials
volume
124
article number
106806
pages
14 pages
publisher
Elsevier
external identifiers
  • scopus:85199393349
ISSN
0263-4368
DOI
10.1016/j.ijrmhm.2024.106806
language
English
LU publication?
yes
additional info
Publisher Copyright: © 2024 The Authors
id
c1d07a3b-5923-4d79-a7d4-1492918479e9
date added to LUP
2024-08-05 10:34:03
date last changed
2024-08-07 10:58:02
@article{c1d07a3b-5923-4d79-a7d4-1492918479e9,
  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<sub>x</sub>Al<sub>1-x</sub>N (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<sub>0.45</sub>Al<sub>0.55</sub>N-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 CoWO<sub>4</sub> ceramic present several μm into the tool and its formation is facilitated by the oxidation of binder with resolved W. Fractures within the CoWO<sub>4</sub> 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>}},
  author       = {{Lindvall, Rebecka and Bello Bermejo, Juan Manuel and Bjerke, Axel and Andersson, Jon M. and Vikenadler, Elias and M'Saoubi, Rachid and Bushlya, Volodymyr}},
  issn         = {{0263-4368}},
  keywords     = {{Cemented carbide; Milling; Titanium; Tool wear}},
  language     = {{eng}},
  publisher    = {{Elsevier}},
  series       = {{International Journal of Refractory Metals and Hard Materials}},
  title        = {{On the wear mechanisms of uncoated and coated carbide tools in milling titanium alloys}},
  url          = {{http://dx.doi.org/10.1016/j.ijrmhm.2024.106806}},
  doi          = {{10.1016/j.ijrmhm.2024.106806}},
  volume       = {{124}},
  year         = {{2024}},
}