Definierte strukturierte Hartmetallwerkzeuge für abrasive Prozesse

The challenge of this dissertation concerns the surface structuration of the WC-CoNi hardmetal with defined geometry, despite the fact that the embedded WC grains have irregular geometrical properties and distribution. An advanced method should be found and applied to structure the WC-CoNi hardmetal tool surface. The structured surfaces should be favorable and beneficial to reduce friction or to remove material in abrasive machining processes. Based upon the surface topography characterization of existing abrasive tools, e.g., CBN honing stone, geometrical properties of abrasives can be measured and quantified. The obtained geometrical information can contribute to the reproduction of the abrasive tool surface on WC-CoNi hardmetal. Laser surface texturing is an advanced machining method with high precision and it can effectively avoid some common thermal damage. Therefore, this method is implemented to machine WC-CoNi hardmetal surfaces. It is found that the structured WC-CoNi hardmetal tool can effectively remove material and improve surface quality of the counterpart (workpiece). These surface patterned hardmetal tools emerge then as potential alternative to conventional abrasive tools. Meanwhile, other patterns have also been produced on the hardmetals, and they can be used in the tribological system to reduce friction and improve wear resistance. It is a methodological and technical innovation to fabricate abrasive machining tools using laser to produce defined structures on a hardmetal surface, because it not only expands the utilization of hardmetal as an abrasive tool material but also enables the control and design of abrasive tool surface topography with high precision.Die Herausforderung dieser Arbeit besteht in der geometrischen Strukturierung von WC-CoNi-Hartmetall-Oberflächen, wobei die eingebetteten WC-Körner geometrische unbestimmte Eigenschaften sowie zufällige Verteilungen aufweisen. Es sollen neue Methoden gefunden und angewendet werden, um WC-CoNi Hartmetallwerkzeugoberflächen zu strukturieren. Mit Hilfe dieser Strukturen soll die Reibung reduziert oder Material durch abrasive Bearbeitung gezielt abgetragen werden. Durch eine geeignete Charakterisierung der Oberflächentopographie vorhandener Abrasivwerkzeuge können die geometrischen Eigenschaften von abrasiven Körnern ermittelt und quantifiziert werden. Die erworbenen geometrischen Informationen können zur Reproduktion der Oberflächen von Abrasivwerkzeugen auf Hartmetalloberflächen genutzt werden. Die Laseroberflächenstrukturierung ist ein innovatives Bearbeitungsverfahren mit hoher Präzision und Effizienz. Diese Methode kann wirksam die üblichen thermischen Schäden vermeiden. Daher ist dieses Verfahren zur Bearbeitung von WC-CoNi Hartmetall vorteilhaft. Es konnte bestätigt werden, dass die neuartigen strukturierten WC-CoNi Hartmetallwerkzeuge die Materialien der Werkstücke abtragen und die Oberflächenqualität dieser verbessern. Diese oberflächenstrukturierten Hartmetallwerkzeuge können als potentielle Alternative zu konventionellen abrasiven Werkzeugen dienen. Parallel hierzu wurden weitere Strukturmuster auf Hartmetalloberflächen erzeugt. Die Strukturen können in einem tribologischen System angewendet werden, um Reibung zu reduzieren und Verschleißbeständigkeit zu verbessern. Die definiert erzeugten Strukturen können nicht nur bei Hartmetallen als Schleifwerkzeugmaterial eingesetzt werden, sondern ermöglichen auch eine gezielte Einstellung der Werkzeugoberflächentopographie

Laser surface texturing of a WC-CoNi cemented carbide grade: surface topography design for honing application

Abrasive effectiveness of composite-like honing stones is related to the intrinsic surface topography resulting from the cubic boron nitride (CBN) grains protruding out of the metallic matrix. Within this framework, Laser Surface Texturing (LST) is implemented for replicating topographic features of a honing stone in a WC-base cemented carbide grade, commonly employed for making tools. In doing so, regular arrays of hexagonal pyramids (similar to CBN grains) are sculpted by a laser micromachining system. Micrometric precision is attained and surface integrity does not get affected by such surface modification. Finally, potential of laser-patterned cemented carbide tools, as alternative to conventional honing stones, is supported by successful material removal and enhanced surface smoothness of a steel workpiece in the abrasive testing.Peer ReviewedPostprint (author's final draft

Analysis of different surface structures of hard metal guiding stones in the honing process

Honing is a precise abrasive machining process with high standards for the resulting form, dimension, and surface quality. Additionally, honing further improves geometrical tolerances of the machined workpieces, especially when compared to the drilling process. In order to achieve a high adherence it is essential that the honing tool and the workpiece interact accordingly. The following paper will describe the static and dynamic correlations of the process forces of a honing tool equipped with one honing stone and two guiding stones for bores with small diameters (less than 20 mm). When working with bores of such small diameters, a direct measurement of the process forces with an integrated sensor is usually difficult to realize. Therefore, a theoretical model will be used to calculate the process forces within the honing tool. Missing coefficients of friction or tangential force coefficients (TFC) within the system will be determined with the help of an external test bench. Moreover, guiding stones made of hard metal with two different types of surfaces will be investigated and then compared with conventional guiding stones. The following measurement results are based on a MATLAB® simulation calculating the forces of the honing and guiding stones.Peer ReviewedPostprint (published version

Assessment of wear micromechanisms on a laser textured cemented carbide tool during abrasive-like machining by FIB/FESEM

The combined use of focused ion beam (FIB) milling and field-emission scanning electron microscopy inspection (FESEM) is a unique and successful approach for assessment of near-surface phenomena at specific and selected locations. In this study, a FIB/FESEM dual-beam platform was implemented to docment and analyze the wear micromechanisms on a laser-surface textured (LST) hardmetal (HM) tool. In particular, changes in surface and microstructural integrity of the laser-sculptured pyramids (effective cutting microfeatures) were characterized after testing the LST-HM tool against a steel workpiece in a workbench designed to simulate an external honing process. It was demonstrated that: (1) laser-surface texturing does not degrade the intrinsic surface integrity and tool effectiveness of HM pyramids; and (2) there exists a correlation between the wear and loading of shaped pyramids at the local level. Hence, the enhanced performance of the laser-textured tool should consider the pyramid geometry aspects rather than the microstructure assemblage of the HM grade used, at least for attempted abrasive applications

Wear characterization of cemented carbides (WC-CoNi) processed by laser surface texturing under abrasive machining conditions

Cemented carbides are outstanding engineering materials widely used in quite demanding material removal applications. In this study, laser surface texturing is implemented for enhancing, at the surface level, the intrinsic bulk-like tribological performance of these materials. In this regard, hexagonal pyramids patterned on the cutting surface of a tungsten cemented carbide grade (WC-CoNi) have been successfully introduced by means of laser surface texturing. It simulates the surface topography of conventional honing stones for abrasive application. The laser-produced structure has been tested under abrasive machining conditions with full lubrication. Wear of the structure has been characterized and compared, before and after the abrasive machining test, in terms of changes in geometry aspect and surface integrity. It is found that surface roughness of the machined workpiece was improved by the laser-produced structure. Wear characterization shows that laser treatment did not induce any significant damage to the cemented carbide. During the abrasive machining test, the structure exhibited a high wear resistance. Damage features were only discerned at the contacting surface, whereas geometrical shape of pyramids remained unchanged.Peer ReviewedPostprint (author's final draft

Critical Assessment of Two-Dimensional Methods for the Microstructural Characterization of Cemented Carbides

Cemented carbides, or hard metals, are ceramic–metal composites usually consisting of tungsten carbide particles bound by a cobalt-based alloy. They are the backbone materials for the tooling industry, as a direct consequence of the outstanding range of property combinations, depending on their effective microstructural assemblage, i.e., the physical dimensions and relative content of their constitutive phases. Hence, reliable microstructural characterization becomes key for hard metal grade selection and quality control. This work aimed to assess the practical twodimensional characterization methods for the most important one- and two-phase properties of cemented carbides, i.e., the carbide grain size, phase fraction, carbide contiguity, and binder mean free path. Three different methods—point, line, and area analysis—were implemented to characterize four microstructurally distinct grades. The images were acquired by optical and scanning electron microscopy, with the latter through both secondary and backscattered electrons. Results were critically discussed by comparing the obtained values of properties and the different characterization methodology. Inspection technique combinations were finally ranked based on accuracy, accessibility, and operability considerations. The line method was used to analyze all the properties, the area method, for the one-phase properties, and the point method, for only the phase fraction. It was found that the combination of optical microscopy and the line analysis method was suitable for a direct inspection and rapid estimation for carbides above fine grain size. The most precise results were achieved using line analysis of the images obtained by the backscattered electrons of the scanning electron microscope

Ablation investigation of cemented carbides using short-pulse laser beams

As an excellent engineering tool material, cemented carbides are capable to shape and cut metallic materials with high surface finish quality and precision. However, in regard to conventional abrasive methods cemented carbides are difficult-to-machine materials due to their extreme hardness combined with relatively low toughness. In contrast, laser beam machining is an advanced non-contacting cutting method which is therefore suitable for shaping hard materials. In particular, the application of short-pulse laser beams enables the cutting of hard materials meeting high precision requirements. Moreover, it can effectively reduce defects induced by mechanical contacts and thermal reactions. In this paper, a general study of the ablation mechanism of cemented carbides using short-pulse laser is conducted. Special attention is paid to the correlation between the material ablation and machining parameters within the nanosecond regime: pulse number and pulse energy. In doing so, two cemented carbide grades with similar composition but different grain size have been chosen as investigated materials. An experimental set-up equipped with a nanosecond laser and an auto-stage is implemented to produce dimples on the cemented carbide surfaces with variable pulse number and pulse energy. The experimental design and characterization of geometrical features of produced dimples are presented and discussed. The work is complemented with a thorough surface integrity assessment of the shaped materials. It is found that ablation increases proportionally with the pulse number and applied energy. Regarding microstructural effects, ablation is discerned to be more pronounced in the coarse-grained grade as compared to the medium-sized one.Postprint (author's final draft

Surface topography quantification of super hard abrasive tools by laser scanning microscopy

Non-conventional super hard abrasive tools are made of composite materials containing super hard grains, e.g., diamond or cubic boron nitride (CBN) grains, bound by a metallic constitutive phase. These tools are usually produced by means of sintering, and are widely applied in the abrasive machining processes of modern manufacturing, especially in precision machining. The abrasive grains, which induce the material removal processes, are embedded in the metallic binder. They emerge as a consequence of self-dressing, resulting in a self-sharping effect. Therefore, the cutting surface of the tool displays an irregular topography. Quantification of surface topography scenario may supply valuable information to evaluate and understand its correlation to wear mechanisms. In this study, an experimental protocol consisting of five steps: specimen preparation, surface scanning, image assembly, image digital processing and surface quantification, was proposed and validated by characterizing two CBN honing tools used for precision machining: B151/L2/2010/50 (B151) and B91/128/x44/35 (B91) CBN honing stones. It involved the use of laser scanning microscopy and digital imaging processing for assessing significant dimensional, geometrical, and positional properties of CBN grains at the surface of super hard abrasive tools. It was shown that surface topography quantification is an effective method to evaluate and obtain the defined parameters. However, smaller grains may require images with higher resolution; thus, scanning must be refined. Finally, a critical comparative analysis of the experimental results attained for the studied tools pointed out honing stone B91 as more appropriated than B151 one for achieving a higher machining quality of the workpiece.Peer ReviewedPostprint (author's final draft

Surface patterning of cemented carbides by means of nanosecond laser

A nanosecond laser combined with a two-axis reflection control unit is used to shape polygonal pyramids with defined geometry on a specific cemented carbide grade. In total, 12 different surface patterns have been fabricated, including four pyramid shapes, i.e., triangle, square, hexagon and octagon, and three lateral side angles, i.e., 30°, 45° and 60°. Characterization of the geometrical features shows satisfactory agreement between produced patterns and aimed ones. The precision is improved when the number of polygon sides and/or both side and slope angles increase. Side effects, such as re-deposition, cracks and pores, were discerned through scanning electron microscopy inspection. They become less obvious when the polygon side length or side angle increases, as the material melting becomes less important. Based on the observations, a borderline curve can be plotted for describing the production capability of such surface patterns on cemented carbides using the laser technologyPostprint (author's final draft