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Aluminium matrix composites (AMCs) represent an important group of high-performance materials. Due to their specific strength and a high thermal conductivity, these composites have been considered for the large-scale production of brake discs. However, preconditioning the friction surfaces is necessary to avoid severe wear of both the brake discs and the brake linings. This can be achieved through controlled friction against commercially available brake-lining materials and the formation of transfer or reactive layers (tribosurfaces). Homogeneous tribosurfaces allow for nearly wear-free brake systems under moderate brake conditions. In this work, preconditioning was carried out with a pin-on-disc tester, aiming for the fast creation of homogeneously formed and stable tribosurfaces. The influence of surface microedges perpendicular to the direction of friction on the machined AMC surfaces on the build-up speed and homogeneity of the tribosurfaces was investigated. The microedges were generated using ultrasonic-vibration-superimposed face turning. Thereby, the vibration direction corresponded to the direction of the passive force. For research purposes, the distance of the microedges was changed by varying the cutting speed and feed. The experiments were carried out using AMC disc specimens with a reinforcement content of a 35% volume proportion of silicon carbide particles. Machining was realised with CVD-diamond-tipped indexable inserts. The evaluation of the generated surfaces before and after preconditioning was achieved using 3D laser scanning microscopy and scanning electron microscopy. It was demonstrated that ultrasonic-vibration-superimposed face turning effectively generated microedges on the AMC surfaces. The results show that larger distances between the microedges enhanced the formation of stable tribosurfaces. Thus, the tribosystem's steady state was reached quickly. Therefore, the benefits of AMC-friction-surface microstructuring on the generation of tribosurfaces under laboratory conditions were proven. These findings contribute to the development of high-performance AMC brake systems. [ABSTRACT FROM AUTHOR]

Machining processes are characterized by thermal loads generated during the shearing and friction work of the cutting process. For the in-process temperature measurement by a tool-workpiece thermocouple a consideration of the change in temperature of the cold junction at the specimen rear side is vital for the accuracy of the measurement. Thus, a macroscale thermodynamic simulation model implemented in COMSOL Multiphysics is used to model the heat distribution within the specimen during turning of the aluminum alloy EN AW-2017. The thermal input conditions are derived from experimental tests. For the assumption of the relative measurement error the simulated cold junction temperature is compared to the in-process temperature measured by a tool-workpiece thermocouple. The model shows that the machining time is the most important factor for heating of the cold junction. On the one hand, for low cutting speeds the measurement error in relation to the measured temperature is significantly high which discloses the need for a temperature compensation in this parameter range. On the other hand, for high cutting speeds and thus short machining times, the cold junction is not subjected to a significant increase in temperature. Consequently, the model validates the applicability of the temperature measurement based on a tool-workpiece thermocouple for dry machining of aluminum alloys with high cutting speeds. [ABSTRACT FROM AUTHOR]

The processing of high-entropy alloys (HEAs) via laser metal deposition (LMD) is well known. However, it is still difficult to avoid chemical intermixing of the elements between the coating and the substrate. Therefore, the produced coatings do not have the same chemical composition as the HEA feedstock material. Single-layer CrFeCoNi and AlCrFeCoNi HEA coatings were deposited using high-speed laser metal deposition (HS-LMD). Elemental mapping confirmed a good agreement with the chemical composition of the powder feedstock material, and revealed that chemical intermixing was confined to the immediate substrate interface. The coatings are characterized by a homogeneous structure with good substrate bonding. The machining of these coatings via turning is possible. Subsequent diamond smoothing results in a strong decrease in the surface roughness. This study presents a complete manufacturing chain for the production of high-quality HS-LMD HEA coatings. [ABSTRACT FROM AUTHOR]

Finishing of sprayed coatings is required for applications in tribological systems. The mostly hard and wear-resistant coatings cause increased tool wear in machining. Functional coating materials with hardening capacity during machining represent an alternative approach that reduces tool wear and ensures sufficient wear resistance. The research objective is to analyze the hardening potentials of coatings while machining. For the investigations specimens of the steel X120Mn12 are manufactured by spark-plasma sintering and thermal spraying, respectively. Finishing is performed by face turning using tools characterized by different rake angles and by diamond smoothing under varying smoothing force. The geometrical properties and the hardness of the surface are determined. After tribological tests the specimens wear depth is measured. The tool rake angle and the smoothing force affect the surface hardness and the depth of the hardened layer. Additionally, specimens manufacturing process influences the resulting surface properties and the wear resistance. Purposive hardening of coatings by finishing is analyzed for the first time. The results enhance the knowledge about machining of coatings and contribute to an enlargement of their field of application. [ABSTRACT FROM AUTHOR]

The generation of heat during chip removal can affect the residual-stress state and thus the performance of machined parts. For this reason, the aim of this study is to determine and modify the process temperatures in turning of the aluminum alloy EN AW-2017. Therefore, the influence of dry machining, cold-air cooling (CAC), flood lubrication (FL), and minimum quantity lubrication (MQL) is investigated by measuring the temperature with a tool–workpiece thermocouple. In regard to the correlation between the cutting power and the temperature, the cutting speed is varied in the range of 50 m/min to 550 m/min while the depth of cut and the feed are kept constant. The results show that the contact area between the tool and the workpiece has to be considered for the evaluation of the measured temperature as it represents an average value over the tool–workpiece interface area. Since there is only a slight decrease of the process forces with increasing cutting speed, the modification of the surface properties has to be mainly influenced by the change of temperatures. Thus, process cooling entails a reduction of tensile residual stresses as a consequence of a decrease of the cutting temperature. Furthermore, the surface roughness values (Ra, Rz) are smaller when applying cooling lubricant. In contrast for low cutting speeds, the intensified built-up edge (BUE) formation leads to compressive residual stresses but deteriorates the surface quality. Hence, the combination of an effective cooling strategy and the temperature measurement allows for the generation and monitoring of advantageous surface properties during machining. [ABSTRACT FROM AUTHOR]

Aluminium matrix composites (AMCs) consist of a comparatively soft aluminium alloy and hard ceramic particles. Due to elevated properties, these materials are used for high-performance lightweight components. Their application in tribologically loaded brake systems requires a defined friction behaviour, which can be gained by appropriate geometrical surface properties as a prerequisite for a specific tribolayer formation. The aim of this research is to enhance the generation of nearly non-wearing tribolayers on AMC specimens reinforced with 35% volume proportion of silicon carbide particles. For that purpose, the influence of the corner radius and the feed on the surface properties is analysed in face turning using CVD diamond tipped indexable inserts. On the machined AMC specimens, tribolayers are generated in a conditioned pin-on-disc arrangement applying pins made of automotive brake lining material. The focus lies on the tribolayer build-up phase as the stage in which the protective tribolayer begins to form. Surface analyses show that a high kinematic roughness resulting from facing leads to a more intensive build-up of the tribolayers, which is further promoted by voids in the AMC surface. This study provides a first overview of the facing conditions that can be applied to support the subsequent formation of the tribological layer. [ABSTRACT FROM AUTHOR]

Austenitic high-manganese steels (HMnS) offer very high wear resistance under dynamic loading due to their high work hardening capacity. However, resistance to static abrasive loading is limited. Various approaches to increasing abrasion resistance are known from traditionally manufactured metallurgical components. These confirm the high potential for surface protection applications. In this work, the powder of the Hadfield HMnS X120Mn12 is prepared and processed by high-velocity oxy-fuel (HVOF) spraying and spark-plasma sintering (SPS). A good correlation was observed between the results of the HVOF and SPS specimen. Different surface conditions of the coatings and the sintered specimens were prepared by machining. Compared to the polished state, turning and diamond smoothing can increase the surface hardness from 220 HV to over 700 HV significantly. Regardless of the surface finish condition, similar good wear resistance can be demonstrated due to strong work hardening under sliding and reciprocating wear loading. In contrast, the finish machining process clearly influences abrasion resistance in the scratch test with the best results for the diamond smoothed condition. Especially against the background of current trends toward alternative coating systems, the presented results offer a promising approach for the development of HMnS in the field of coating technology. [ABSTRACT FROM AUTHOR]

Substitution of grinding by hard turning in order to achieve twist-free surfaces promises significant economic and environmental advantages. The modifications in feed kinematics needed to manufacture such twist-free surfaces by a special start-stop turning process are very demanding in terms of dynamic performance of the machine tool. An approach consists in the integration of an additional highly dynamic drive system. The dynamic properties of an air bearing drive system regarding the demands for start-stop turning are analysed in detail. Furthermore, the ability for manufacturing of twist-free surfaces by start-stop turning is proven. [ABSTRACT FROM AUTHOR]

Tool wear is an important criterion for economic machining. It does not only determine the tool life, but also affects the surface layer properties of metallic components and thus their performance. In addition to the machining parameters, there are numerous other influencing factors like batch variations, workpiece inhomogeneities, process vibrations, or the formation of built-up edges that can impact the wear progression. Consequently, it is difficult to predict the tool wear analytically. Therefore, the determination of the tool condition during machining is of great importance. The tool-workpiece thermocouple enables the in-process measurement of a thermoelectric voltage and current during machining generated by the Seebeck effect at the interface between two electrically conductive materials. Thus, the thermoelectric signals are sensitive for the changing contact interface between the tool and the specimen. To allow for an in-process characterization of the tool wear, finding an empirical relationship between the flank wear land width of cemented carbide indexable inserts and the thermoelectric signals in turning is aspired. To vary the flank wear land width in a reproducible manner, tools with a flank face chamfer resulting in a clearance angle of 0° and five different flank face land widths (80 µm – 280 µm) are used. These geometrical properties are measured with a 3D laser scanning microscope. Afterwards, cylindrical specimens of the aluminum alloy EN AW-2017 are machined by turning applying the modified tools and cutting speeds in the range of 300 m/min to 550 m/min. The depth of cut (0.4 mm) and the feed (0.04 mm) are kept constant. Besides the tool-workpiece thermocouple, the components of the resultant force are measured by a dynamometer. The geometrical properties of the machined surfaces are characterized using a stylus measurement instrument. The experimental investigations show an increase of the temperature and the components of the resultant forces with increasing flank wear land width, which impacts the geometrical properties of the surface. An empirical regression model derived from the in-process measurement results depicts the relationship between the thermoelectric signals, the force components, and the changing area of the tool which is in contact with the specimen. Consequently, the combined in-process measurement of the process forces and thermoelectric signals allows for an in-process determination of the tool wear progression and therefore enables the prevention of a strong impairment of the surface-layer properties by a timely tool change or a controlled adjustment of the machining parameters. [ABSTRACT FROM AUTHOR]

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Lightweight materials can contribute significantly to an enhancement of energy and resource efficiency. In this context, the properties of the surface layer (e.g. residual stresses) influence the performance of machined parts considerably. However, the machining process often entails a change of the surface properties. Consequently, a predefined modification of the surface properties in final machining enables a highly efficient manufacturing of reliable lightweight components. For ensuring the required properties, the machining process has to be monitored. In the experimental investigations, stepped cylindrical specimens consisting of the aluminium alloy EN AW-2017 are machined by turning. The cutting speed is varied in the range from 50 m/min to 550 m/min. The depth of cut (0.2 mm) and the feed (0.05 mm) are kept constant. The influence of the chamfer angle of the flank face (0°, 5°, 10°, 15°) is analysed using cemented carbide indexable inserts. For process monitoring, the components of the resultant force are recorded by a dynamometer. Additionally, the voltage and the current between the tool and the workpiece resulting from the Seebeck effect are detected, enabling the measurement of the temperature near the shear zone. The geometrical properties of the machined surfaces are characterised using a tactile measurement instrument and a 3D laser scanning microscope. To determine the residual stresses, X-ray diffraction analyses of the machined surface are performed. The experimental results show that an increase of the cutting speed leads to higher Seebeck voltages between the tool and the workpiece due to elevated temperatures in the shear zone. Additionally, the surface roughness depth Rz and the built-up edge formation are reduced. Enlarging the chamfer angle results in increasing surface roughness values and decreasing absolute values of the tensile residual stresses in the tangential direction at the surface. The process monitoring allows for the implementation of real-time control adjustment of the surface layer properties during machining. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) represent a comparably novel class of materials composed of approximately equimolar fractions of at least four chemical elements. Applied by thermal spraying, the material can be used as a wear protection layer. In order to achieve predefined functional surface properties, finish machining of the applied layers is typically necessary. In this context, the scientific field of HEA machining is hardly opened up requiring intensified research activities. Accordingly, cutting experiments based on turning are addressed. The specimens are represented by cylindrical sections of the aluminium wrought alloy type EN AW-5754 thermally spray coated with a HEA of the composition AlCoCrFeNiTi. For the experimental investigations, indexable inserts with a standardised geometry CCGW 09T304 are used. Especially, the influence of the cutting speed and the feed on the resulting surface properties is focused, while the depth of cut is kept unchanged throughout the cutting tests. The geometrical properties of the machined surface of the sprayed coating are characterised by roughness parameters, valley void volume and a qualitative assessment using scanning electron microscopy (SEM). Residual stresses in the surface layer are determined by X-ray diffraction analysis using sin2 ψ method. The results show a decrease of Rz, Rvk , and Vvv with an increase of the cutting speed. This is primarily attributed to a decreased proportion of pulled-out coating material. Additionally, compressive residual stresses are determined in the axial direction. The absolute values of these stresses decrease with an increasing cutting speed. The presented research expands the field of HEA machining with geometrically defined cutting edges. Further research activities should address a deeper understanding of the mechanisms in machining of HEAs. Moreover, the findings should be transferred to cutting processes with rotating tools such as milling and drilling. [ABSTRACT FROM AUTHOR]

An approach for a reduction of weight and fuel consumption of passenger cars consists in a replacement of grey cast iron by aluminium alloys as base material of combustion engine blocks. Regarding the insufficient chemical resistance of these alloys to the fuels with its additives, material compounds with thermally sprayed coatings are used as cylinder linings. For machining and in service a high connection strength between the substrate and the coating is demanded. This can be gained by a predefined microstructuring of the substrate surface to attain a local form fit with the coating. But, the microstructured surface also influences the local morphology of the layer. Another possibility to improve the connection strength is the application of a primer between the substrate and the coating. For basic investigations, spiral dovetail microstructures with different pitches (200 µm – 500 µm) are machined by face turning using tools with CVD diamond tippings. The substrates consist of the aluminium alloy EN AW-5754 and their microstructured surfaces are coated with a nickel-aluminium layer. The coating with a thickness of about 200 µm is produced by high-velocity arc spraying. The material is usually used as a primer between the substrate and an iron-based coating. The substrate microstructures and the morphology of the coatings are analysed by SEM and 3D laser scanning microscopy. To determine the adhesive tensile strength of the coating tensile adhesion strength tests are performed. The results show that a higher number of structure elements per length increase the tensile adhesion strength, the hardness and the oxide proportion of the coating. Furthermore, tensile adhesion strength tests reveal failure mechanisms of the compound of substrate and coating depending on the substrate microstructure. [ABSTRACT FROM AUTHOR]

Abstract: Aluminium matrix composites (AMCs) consist of a comparatively soft aluminium alloy and hard ceramic particles. Due to their elevated properties, these materials are used for high-performance lightweight constructional components. Applications in tribologically loaded systems often require a defined friction behaviour, which can be gained by an appropriate microstructuring of the surface. Experimental investigations in turning of AA2124 with 25 % volume proportion of SiC particles have shown that an ultrasonic vibration assistance enables the generation of a microstructured surface in finish cutting without any additive process. Furthermore, an ultrasonic vibration assistance in the radial direction or in the cutting direction respectively leads to stronger compressive residual stresses in the surface layer, which increases the fatigue strength of the components. [Copyright &y& Elsevier]

Abstract: Currently grinding is commonly used as the finishing operation to manufacture seal mating surfaces and bearing surfaces, especially in the automotive industry. It would lead to more resource-efficient production if the cost- and energy-intensive grinding process could be replaced by machining with geometrically defined cutting edges, such as hard turning [1,2]. However, turning operations usually cause a twist structure on the surface, which can convey lubricants like a pump. Several methods exist to overcome this problem, for example, tangential turning, rotation turning and turn broaching, etc. Due to the high costs of tools and special machines required by these methods, the industrial application is still limited. This paper describes a more efficient approach by applying a modified feed kinematic. When using this approach, hard turning produces twist-free surfaces. The results of the latest twist test methods have confirmed that the surfaces are free of twist, hence free of conveying effect of lubricant and that they are suitable for application in manufacturing of seal mating surfaces and bearing surfaces. Furthermore, this method requires only minimal investment in any turning machine. [Copyright &y& Elsevier]

Magnetic properties are known to be crucial in the application of electrical steel and they are therefore covered by manifold studies. Other ferromagnetic materials are out of scope in this respect, even if the importance of magnetism of conventional steel is evident. Additionally, there is a contradiction regarding the major influence on magnetic properties. Machining, transport, and storage are possible influencing variables. In the experimental investigations, specimens consisting of the bearing steel 100Cr6 are machined by turning and partly by subsequent diamond smoothing. While machining using several cutting speeds and smoothing forces, the thermoelectrical voltage, current, and the components of the resultant force are recorded. The results show how the near-surface plastic deformations evolve throughout the machining process. Additionally, it was found that the magnetic properties and other properties of the surface layer are influenced in different ways depending on turning and diamond smoothing parameters. Correlations between in situ and ex situ measured values are shown. This study aims to solve the aforementioned question by quantification of machining impacts of cutting speed in turning and force in diamond smoothing and its dependence on transport and storage. [ABSTRACT FROM AUTHOR]

In the presented work, cylindrical specimens of aluminium EN AW-5754 are pre-machined by turning to provide fine-structured surfaces before the subsequent coating with Fe17Cr2Ni0.2C by Atmospheric Plasma Spraying (APS). The pre-machining is done with specific MCD-tipped tools that generate helical dovetail structure elements in μm-range and offer high adhesive tensile strength of the applied coating. In this way, finish-machining of the iron based coating is possible. It is carried out at varied tool feed (0.05 mm – 0.15 mm) by turning with CBN-tipped indexable inserts. Depth of cut (0.05 mm) and cutting speed (250 m/min) are kept constant. Specimens are then machined by diamond smoothing with different radial forces (50 N – 200 N), constant tool feed (0.05 mm), and constant machining speed (100 m/min). Properties and geometric features of the machined surfaces are characterised by tactile measurement, 3D laser scanning microscopy, SEM, and tribological analyses. The analyses include the determination of opened pores and pulled-out coating material, that are brought together in the combined surface parameter of the valley void volume. Machining by diamond smoothing after turning generally leads to decreased surface roughness. In addition, the smoothing forces lead to increased absolute values of surface-near compressive residual stresses. The results of this work contribute to the substitution of common honing processes as finish machining steps of thermal spray coatings, especially focussing on the non-homogeneous character of these coatings. In this work, for the specific application and when comparing similar surface finish qualities, it is proven that at diamond smoothing results in less tool wear than finish-machining by turning. As a main result, a material and energy efficient finish-machining process for thermal spray coatings is positively evaluated. • Iron based thermal spray coatings were successfully machined by diamond smoothing. • Diamond smoothing after preliminary turning leads to similar surface properties over a wide range of parameters. • Smoothing forces, valley void volume, coefficient of friction and wear rate were correlated. • Diamond smoothing induces strong compressive residual stress in the feed direction. • The absolute values of these stresses can be increased with raising smoothing force. [ABSTRACT FROM AUTHOR]