Condition monitoring of milling tools is crucial to achieve autonomous and resource-efficient production. It is, however, a challenging task due to the dynamic nature of the cutting process and the complex wear mechanisms. The majority of existing approaches is relying on data that is only accessible through additional instrumentation, such as cutting forces. Furthermore, high frequency data is often required. The current paper presents a novel approach for condition monitoring of milling tools using low-frequency machine-tool data. For the industrial-scale milling of titanium alloy, a decrease in signal roughness in servo-motor torque data is identified through exploratory data analysis. As a consequence, we propose a condition monitoring strategy that uses fractal analysis to quantify this effect. A threshold was set for the moving range of the fractal dimension to detect a significant drop and recommend tool exchange. For several milling experiments under identical conditions, tool exchange was recommended by our approach when the cutting tool is subject to significant damage, as documented in microscopy images, demonstrating consistency and robustness of the proposed approach. Additionally, only data stemming from sensors installed in standard machine tools in a relatively low frequency is required, leading to high industrial applicability.

Whether a cryogenic treatment enhances or diminishes the mechanical properties of WC-Co cemented carbides remains a subject of debate. In this study, we systematically evaluated the effects of cryogenic treatment (-196 degrees C, 24 h) and its subsequent time-dependent influence on hardness and Palmqvist toughness in three WC-Co cemented carbides: WC-10Co, WC-10Co-1.5Ru-0.6Cr(3)C(2), and WC-10Co-0.75Cr(3)C(2)/VC-0.3Mo(2)C. A statistically significant 9.6% increase in Palmqvist toughness, accompanied by a similar to 3% rise in hardness, was observed in the WC-10Co-0.75Cr(3)C(2)/VC-0.3Mo(2)C cemented carbide, whereas the other two compositions showed no measurable change. The toughness enhancement in the WC-10Co-0.75Cr(3)C(2)/VC-0.3Mo(2)C cemented carbide was most pronounced immediately after a 24 h cryogenic soak, then gradually diminishes during 1-3 weeks of room-temperature relaxation. Fractographic and crack-propagation analyses reveal shorter crack lengths and a higher frequency of transgranular cracking (similar to 23.5%) in the cryogenically treated condition compared with the relaxed conditions, consistent with a higher resistance to crack advance. Our findings clarify the role of cryogenic treatment in modifying hardness and toughness in WC-Co cemented carbides and offer insights into strategies for extending tool life.

Composed of hard tungsten carbide (WC) particles and a soft cobalt (Co) matrix, WC-Co cemented carbides exhibit significant differences in the material removal rates of these phases during metallographic preparation. This disparity, combined with the susceptibility of the soft Co phase to deformation-induced martensitic phase transformation from its face-centered cubic (CoFCC) to hexagonal close-packed (CoHCP) structure, poses substantial challenges for microstructural characterisation. The resulting ambiguity complicates the identification of the pristine Co phase and raises concerns about the presence of preparation-induced artefacts. In this study, we present a detailed comparative analysis aimed at minimizing ambiguities in the characterisation of the pristine Co phases in a series of WC-Co cemented carbides. We quantitatively report on the controllability of various preparation parameters under multiple conditions, for plane-polished cross-sections used in (e.g.) EBSD analysis and for thin-sections such as used in (e.g.) TEM analysis. We report on the interplay between material removal and the deformation-induced martensitic CoFCC-CoHCP phase transformation during metallographic preparation, identifying "GO" and "NO GO" regimes for the unequivocal identification of the pristine Co phase in WC-Co cemented carbides. The optimal metallographic preparation method for the "GO" regime involves an Ar+ ion polishing energy density of similar to 10 MJ/m(2) and a duration of similar to 80 min. This work establishes a robust workflow for accurately determining the pristine Co phase, providing a pivotal aspect for the characterisation of microstructure-property relationships in WC-Co cemented carbides.

Grain boundaries (GBs) generally exhibit complex structural and compositional features that significantly affect material hardness. Here, we establish a methodology to correlate the local hardness contributions of the GBs with their frequency distribution and their structural and compositional characteristics, using a submicron WC-Co cemented carbide as a model. An exceptional local hardness of (14.68 +/- 0.12) GPa is observed from a 90 degrees WC{0001}/WC{1010} GB, unlike the low contributions from other WC/WC GBs. This is linked to pronounced Cr and Co segregation at this GB, due to Cr affinity at the WC{0001}/Co and WC{1010}/Co phase boundaries and Co infiltration during liquid-phase sintering. Density functional theory results indicate that a large lattice mismatch, strong W-C covalent bonding, and Cr and Co accumulation increase the elastic strain field, resulting in strong atomic distortion near the interface and contributing to exceptional strengthening. Our findings highlight the critical influence of GB complexities on material hardness.

The present work investigates an indexed cutter with round cutting inserts manufactured from hard metal grade M10 with 10 wt% Co binder and WC grain size of 2 mu m used for the application of milling the titanium alloy Ti-6Al-4V. The focus is set on the cyclic thermomechanical loading in combination with tool degradation. For this sake, a combined 2D and 3D finite element modelling approach is introduced in order to determine the thermal and mechanical tool loading locally resolved and to reveal the specifics of a round cutting insert geometry. The finite element model allows establishing the linkage between the material damage observed in uniaxially loaded laboratory specimens with real tool degradation observed in cutting experiments for the calculated critical loading conditions. For the investigated round insert with 10 mm diameter applied in a cutting operation with 55 m min(-1) cutting speed and 0.125 mm feed per tooth, a maximum cutting edge temperature between 600 degrees C and 700 degrees C and stress amplitudes between 1500MPa and 2000 MPa at a stress ratio R-sigma < - 4 are calculated. This loading is close to the load limit for cavity formation and accumulation in M10 which for the temperature of 700 degrees C and a stress ratio R-sigma = - infinity was experimentally found to be at a stress range of 1500 MPa. This reveals that the investigated process conditions are close to the limit of safe tool application which is in accordance with findings from cutting tests. The presented approach provides a novel and highly valuable method for future tool material and cutting process development and research.

Monitoring of the end milling tool wear in real-time is very crucial for the quality control of a surface finish. In the model-based approaches, usually the tool wear is monitored by tracking the force model coefficients during the milling operation. In this work, we explore a monitoring approach based on two metrics derived from the goodness-of-fit of the milling mechanistic model. The cumulative sum control charts are constructed to monitor changes occurring to the tool condition during a milling operation. Compared to previously suggested methods based solely on the values of force model coefficients, our approach facilitates a preciser and less conservative identification of the point of transition from homogeneous wear to onset of breakout formation at the cutting edges. The proposed strategy is an alternative approach for real time monitoring of tool condition based on mechanistic models.

Milling tools are subjected to severe loading conditions causing different wear mechanisms. Among others, the dominating tool wear mechanism depends on the combination of workpiece material and tool material, cutting parameters and the mode of operation. Usually, in industrial milling processes, the mode of operation is a combination of up- and down-milling. The present work is devoted to the question how up-milling and down-milling processes differ with respect to the thermomechanical loading and the tool wear, particularly in the case of milling titanium alloy Ti-6Al-4V. To this end, cutting tests for both modes of operation have been performed. The cutting inserts have been evaluated in certain intervals via optical and electron microscopy. Finite element simulations provide the corresponding thermal and mechanical loading at the cutting edge during service. Experiment and simulation consistently show the detrimental effect of up-milling in comparison to down-milling. However, the wear analysis suggests the same mechanism for both modes of operation but with higher rate of wear in up-milling. The cutting experiments show that wear is driven by the growth of fatigue cracks and thus indicate a mechanical reason for tool wear. From the simulations, it is clear that the specific interaction between thermal and mechanical loading in up-milling is the reason for accelerated tool wear in up-milling compared to down-milling. The higher rate of wear in up-milling is attributed to its specific thermomechanical loading.

The current paper presents a new computational approach to detect wear and damage to milling tools' cutting edges. The proposed approach is independent from exact information on tool-workpiece interaction conditions and only requires that they remain constant for compared milling operations. Additionally, the approach was thoroughly tested on time-series data obtained from an industrial-scale milling process, instrumented by commercially available instrumentation equipment, during which 18 identical parts were milled. The time-series data contains the bending moments in the x and y directions as well as the torque and tension acting on the milling tool. Some measures used are systematic in nature, based on shape, rotation and work needed for milling, whereas others are statistical in nature, describing the change in the distribution of the data. All of the measures proposed in the current work are relative and mutually invariant, meaning they address different information content of the data independently. A comparison of the mentioned measures with the real-world damage evo-lution of the milling tool's cutting edges for multiple produced parts yielded consistent results and suggests a high potential for practical tool damage detection in industrial production.

When machining duplex steel 1.4462 the choice of the hard coating of the cutting tool plays a major role due to the high process temperatures, caused by relatively low thermal conductivity of this material. Hard coatings can have different thermo-physical properties, e. g. thermal conductivity and specific heat capacity. Thus, they influence the process temperatures in the shear zones and have an impact on the chip formation. For a mechanisms-oriented tool development it is necessary to understand these influences. Experimental cutting tests based on a linearorthogonal kinematic are a suitable method for an experimental approach. A challenge with experimental methods is to vary the thermo-physical properties of the coatings without changing the friction effects in the contact areas of the tool and workpiece at the same time. An unwanted change of the friction leads to superimposed effects and difficulties during the interpretation of the influence of the thermo-physical properties. This work discusses the influence of two different wear protection layers on the behavior of an austenitic-ferritic duplex steel during linearorthogonal cutting experiments. In the investigations, one monolayer coating (TiAlN) and one multilayer coating (TiN-TiCN-Al2O3) were examined. The experiments were carried out under different cutting conditions and wear conditions of the tools to generate various friction conditions. In the results analysis, focus is mainly given to the cutting and feed forces as well as the chip thickness values. The experimental results are supplemented with analytical analyses to understand the influence of thermo-physical properties of coatings and interfacial effects caused by friction.

In service, milling tools have to cope with severe levels of thermal and mechanical load. Especially temperature influences the damage behavior of a tool’s cutting edge by influencing material properties and thermally induced stresses. It is therefore of relevance to gain quantitative information on the thermal tool load situation. Information on temperatures in milling tools is not readily available today. Therefore, extensive experimental effort was necessary to determine temperatures in-situ during milling in the axial center of a rotating end mill and in a Ti6Al4V workpiece near the milled surface. The used end mill was a WC-Co hard metal tool protected by a TiAlN coating. Since the damage-relevant cutting edge temperature is not directly accessible by experimental means, a simulation was employed. The transient temperature field in the tool was calculated by an iterative and synergetic use of two-dimensional finite element cutting models, three-dimensional finite element end mill models and two-dimensional workpiece models. The simulation allows for the description of the time-dependent temperature distribution from the chip formation site at the cutting edge to the axial tool center and into the workpiece, where thermocouples were placed in experiments. Validation of the calculated cutting edge temperatures was performed for 5000 individual consecutive cuts via comparison of results for tool core temperature in experiment and simulation. The model yields a very pronounced concentration of the thermal load maximum of T>650 °C near the cutting edges in a very small volume of only 1 ppm of the tool’s volume. In particular, the model’s spatial discretization is able to resolve the gradient of temperature in the hard coating towards the coating/substrate interface, showing temperature shielding effects of the hard coating.

Ti-Al-N based coatings are presently state-of-the art for severe cutting applications. Further improvement of these coatings can be reached by multilayer architectures in combination with different layer materials. Within this work, multilayer arrangements consisting of Ti-Al-N and Cr-Al-N sublayers with different chemical composition and sublayer thicknesses were investigated. All coatings were deposited via cathodic arc evaporation. The individual sublayers of the multilayer coatings exhibited thicknesses of 10, 30, 100 and 300nm, respectively. The morphology and microstructure of the coatings were investigated by scanning and transmission electron microscopy and X-ray diffraction. Mechanical properties were evaluated using nanoindentation and milling tests. In addition, the thermal stability of the coatings was characterized by annealing experiments. An optimum in hardness and cutting performance was obtained for multilayers with 10nm sublayer thickness and high Al contents within the Ti-Al-N layers, whereas the Al content in the Cr-Al-N sublayers showed only a minor effect.

In milling applications thermal and mechanical loadings are affecting the damage behavior of milling inserts. There are several open questions regarding the influence of loading and tool temperature on inelastic deformations and damage mechanisms. The aim of the current work is to simulate industrial milling processes with the finite element method and generate knowledge about the acting damage mechanisms. The validation of the results is made by two recently reported experimental milling setups. The focus of the simulations is set on investigations of the evolution of tensile residual stresses orientated parallel to the cutting edge of a milling insert. These tensile residual stresses foster the formation and growth of so-called comb cracks growing in planes perpendicular to the cutting edge which are detrimental to the performance of the tool. The insert is made of WC-Co hard metal with 8 wt.% Co-binder and an average WC grain size of 1 mu m. It is coated with a 7 mu m thick TiA1N layer acting as a thermal shield. The workpiece material is 42CrMo4, described by a Johnson-Cook constitutive material model. The milling process is modeled with a 2D Arbitrary Lagrangian-Eulerian (ALE) approach. Results of the 2D simulations are used to generate the temperature and contact load imposed on a 3D solid-model of the milling insert that is in turn used to predict the evolution of stress and temperature over 50 milling cycles. The simulations reproduce a shift of residual stresses toward tension in the milling insert at the same location as observed in experiments from which one failed due to comb cracks and one due to wear.

The special properties of titanium and titanium alloys, such as their high strength-to-weight ratio, their corrosion resistance, and their biocompatibility are of high importance for aerospace, medical and other industrial applications. However, components made of these difficult-to-cut materials entail major challenges for machining processes. High thermo-mechanical tool load results in rapid tool wear, and the already significant machining costs are further increased by premature tool exchange to avoid tool breakage, damaging the valuable workpieces. Both, tool wear progress and risk of tool breakage can be reduced by gaining extensive knowledge about the thermo-mechanical tool load over the tool life time. In this paper, the influence of different machining parameters on the end milling tools’ core temperature and the resulting active force affecting the tool are investigated over tool life time. To measure thermal and mechanical tool loads, an adapted sensory tool holder was used. The results show a strong interdependence between tool load, tool core temperature, machining parameters and tool wear.

Difficult-to-cut materials, such as titanium- and nickel base alloys, are of great importance for the aerospace industry. However, they entail major challenges for machining processes, due to the high thermo-mechanical tool load. To meet these challenges, various lubrication and cooling strategies have been developed, but their effect on the load collective has not been sufficiently investigated. In this paper, the influence of different lubrication/cooling strategies on the end milling tools’ core temperature and the resulting active force affecting the tool while milling Ti6Al4V are investigated. Therefore, an adapted sensory tool holder with wireless data transmission was used.

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100°C and compressive yield strength up to 900°C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800°C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800°C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.

Within this work, a state-of-the-art Arbitrary Lagrangian-Eulerian finite element model of a milling operation using coated hard metal cutting inserts is presented. During milling, the cutting depth constantly decreases, thus, to obtain the correct cutting depth, the model considers movement of the tool in a vertical direction. The behavior of the 42CrMo4 workpiece material is described using a standard Johnson-Cook material model. A detailed tool model able to represent both an uncoated and coated tool is created. The tool model is based on an industrial hard coated fine-grained hard metal tool with 8wt.% Co. Three hard coatings are investigated: (i) an arc evaporated TiAlN single layer, (ii) a chemical vapor deposited TiCN/α-Al2O3 bilayer and (iii) a chemical vapor deposited TiAlN/α-Al2O3 bilayer. An uncoated tool model is used as a reference to compare the results. The tool loading during milling is investigated. The calculated variables are cutting forces and the tool-workpiece contact length. The influence of the coatings on temperature, von Mises stress and accumulated equivalent plastic strain is simulated in the coating and the substrate. Measured and literature based thermal and mechanical material parameters are used to describe the material behavior of the coatings and the substrate.

In coated hard metal milling inserts the main damage mechanisms are thermal fatigue induced by interrupted tool–workpiece contact and wear. Dependent on the magnitudes of thermal and mechanical loads in two applied test setups, either wear or thermal fatigue in the form of combcracks is induced. The evolution of residual stress and damage in the used milling inserts was documented over their complete lifetime. In a region of interest on the tool rake face a significant buildup of tensile residual stress was observed via a synchrotron based technique. A special preparation technique enabled position resolved measurements in this area by in-house X-ray diffraction facilities to study the evolution of residual stress over the entire tool lifetime. The onset of cracking was observed to happen in this region of interest by means of focused ion beam milling and scanning electron microscopy. The residual stress levels observed are comparable in used inserts at early stages of application, independent of the different cutting conditions and the applied characterization technique. At the end of tool life wear damage dominated inserts showed tensile residual stress, whereas thermal fatigue as the dominant damage mechanism resulted in compressive residual stresses.