Combcrack Generation in PVD Coated Hardmetal Milling Inserts

Tamara Teppernegg

Research output: ThesisDoctoral Thesis

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Milling is a frequently applied method for material machining in several industries like automotive and mechanical engineering industry. PVD Ti-Al-N coated WC-Co milling inserts are widely used in several machining applications. Milling is characterized by a rotating tool, which moves relative to a fixed workpiece material. This leads to non-constant cutting conditions with alternating mechanical and thermal load acting on the tool during the cutting and idle period. Thermo-mechanical fatigue in form of characteristic combcracks and abrasive wear are the main damage mechanisms in coated hardmetal milling inserts. For further improvements of hardmetal substrates and hard coatings, it is necessary to expand the knowledge of damage evolution during the milling application. Thus, the aim of this thesis is to enhance the understanding of the main damage mechanisms, which are wear and thermal fatigue. Therefore, the location, magnitude and evolution of residual stresses and the corresponding damage state in the substrate as well as in the coating were determined till the end of service life. Special emphasis was laid on the investigation of combcracks, their location, and their extension in an insert. To investigate the evolution of residual stresses in milling inserts over their complete service life, a new special preparation technique was developed. This preparation technique enables the determination of residual stresses in the WC-Co substrate as well as in the coating in a small region of interest by using X-ray diffraction facilities. The finite element simulation is a further important tool to estimate the cutting temperatures and cutting forces during application. The realistic simulation of strains and stresses in coated WC-Co based milling inserts requires detailed knowledge of the thermo-mechanical and thermo-physical properties as function of temperature of the coating as well as of the WC-Co hardmetal. Due to the lack of respective data in literature, the thermo-physical and thermo-mechanical properties of the used hardmetal grade were determined at temperatures up to 800°C (in some cases up to 1000 °C). Within this thesis it is shown that the flow strength of the hardmetal is exceeded during service due to the combination of stresses resulting from thermo-mechanical and mechanical loading during cutting. During the idle period, the material is hindered in contraction in an area, in which plastic deformation was apparent during cutting. After 1000 cuts, for the first time tensile residual stresses in a region of about 1.6 × 0.3 mm² were detected. This region of interest is located about 0.4 mm away from the cutting edge. The position of the area of significant tensile residual stress build-up in the milling insert corresponds well with the location of the most pronounced combcrack as well as the position of the highest simulated thermal and mechanical loads via finite element simulation. All developed testing methods and the found knowledge, shown within this work, help to improve selectively either the substrate or the hard coating of the cutting inserts.
Translated title of the contributionKammriss Entstehung in PVD beschichteten Hartmetall Wendeschneidplatten in der Fräsanwendung
Original languageEnglish
  • Pippan, Reinhard, Assessor B (internal)
  • Ebner, Reinhold, Assessor A (internal)
Publication statusPublished - 2016

Bibliographical note

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  • WC-Co hardmetal
  • Ti-Al-N hard coating
  • Milling
  • Cutting Insert
  • Residual Stresses
  • Thermal Fatigue
  • Wear
  • Synchrotron X-ray diffraction
  • X-ray diffraction
  • Post Treatment
  • High Temperature
  • Elastic Properties
  • Yield strength
  • Fracture Toughness
  • CTOD
  • FE Simulation

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