Tribological Conditions Research Articles

Load-dependent wear in sheet metal forming

Wear in sheet metal forming processes is influenced by numerous variables. Their impact has so far been investigated in model tests neglecting forming-specific tribological boundary conditions such as continuous infeed of new sheet metal material and lubricant as well as high local contact stresses. This paper proposes a new testing method which overcomes these deficits and allows for a high resolution of wear formation in time. Thus, the wear behavior of different tribological systems under different load conditions is compared. Both the reproducibility and transferability of the results are discussed. Furthermore, a new characteristic value is derived in order to characterize the tribological load of different tribological systems so as to provide the possibility of wear prediction.

Identification of friction coefficients when drilling titanium TiAl6V4

This paper investigates the friction behaviour around a carbide drill when drilling titanium with an oil mist lubrication. It questions whether friction is constant along the cutting edge and along the margins. The application of specially designed tribometer reveals that tribological conditions on margins are fully different that the ones on rake and flank faces. Friction coefficients are quantified for both tribological conditions (open and closed tribosystem). The influence of contact pressure and of an AlCrN coating are also investigated. It is shown that friction varies a lot between both tribosystems, and that a closed tribosystem (i.e. margins) leads to much higher friction, adhesion and wear rate than an open tribosystem (i.e. flank faces).

Modular system to measure and control the force distribution in deep drawing processes to ensure part quality and process reliability

Force distribution is one of the most important variables in deep drawing. Together with the tribological conditions, it determines the quality of the formed component. This paper presents a novel mechatronic system for measuring and controlling the normal force distribution in deep drawing. The concept is based on an arrangement of a force measuring platform between the upper die and the press ram. Furthermore, its modular design with multiple sensors allows it to be applied to any tool shape. Using the location of the resulting total force enables novel approaches in the process evaluation. Experimental investigations with the measuring system in a controlled process show the potential of new methods to increase process reliability.

Investigation on friction behaviour of deep drawing radii using volatile media as lubricant substitutes

Abstract Understanding and controlling tribological systems and thus the friction conditions during deep drawing processes represent two important challenges for stable and economical forming processes. In order to optimise the friction conditions in sheet metal forming processes, mineral oils with additives and oil-water based emulsions are usually used. When using such conventional lubricants two major disadvantages may emerge on the one hand, environmental aspects due to the partly environmentally hazardous additives have to be considered and on the other hand, cleaning steps after forming processes to enable further production steps such as welding, gluing and painting are not avoidable. To solve these problems, the Institute for Metal Forming Technology (University of Stuttgart) is currently investigating the use of volatile media as lubricant substitutes. Thereby, media are used which volatilise into the gaseous state without any residue after forming. In the present work, friction conditions at tool radii are particularly considered, since highest surface pressures between workpiece and tool usually occur at these areas. In this context, the work deals with experimental investigations on the tribological friction conditions at tool radii while using volatile lubricants, such as CO2 and N2. To keep the multitude of possible influences on the tribological system as low as possible, the investigations were carried out on a deflected strip-drawing testing rig. The aim of the investigations was the characterization of the tribological system and the friction conditions at tool radii under varying boundary conditions such as surface pressure and injection angle of the lubricant.

Friction in sheet metal forming: influence of surface roughness and strain rate on sheet metal forming simulation results

The quality of sheet metal formed parts is strongly dependent on the tribology and friction conditions that are acting in the actual forming process. These friction conditions are then dependent on ...

Investigation on blasted tool surfaces as a measure for material flow control in sheet-bulk metal forming

Highly integrated and closely tolerated functional components can be produced by sheet-bulk metal forming which is the application of bulk forming operations on sheet metal. These processes are characterized by a successive and/or simultaneous occurrence of different load conditions which reduce the geometrical accuracy of the parts. One challenge within sheet-bulk metal forming is the identification of methods to control the material flow to improve the product quality. A suitable approach is the local modification of the tribological conditions. Within this study, requirements regarding the needed adaption of the tribological system for a specific process were defined by numerical investigations. The results reveal that a local increase of the friction leads to an improved geometrical accuracy. Based on these results, abrasive blasting as a method to modify the tool surface and thus influencing the tribological behaviour was investigated. For the determination of the tribological mechanism of blasted tool surfaces, the influence of different blasting media as well as blasting pressures on the surface integrity and the friction were determined. Additionally, the functional stability of a modification was investigated. Finally, the correlations between surface properties and friction conditions were used to derive the mechanisms of blasted tool surfaces.

Investigations of blistering in the sealing zone of radial lip seals dependent on the operating factors and the power loss

The causation of damage to radial lip seals (RLS) is largely unexplained, especially with regard to blistering. Initial tests indicate a dependency of blistering on the circumferential speed and the sump temperature, whereat solely a high sump temperature does not result in blisters. It is assumed that a specific combination of operating factors leads to a tribological state in the sealing zone, which favours damage. Though, the key drivers leading to blisters are still unknown especially as influencing parameters interact. The power loss in the sealing zone is a measure for the thermal energy generated in the sealing zone. The RLS Tribometer offers the possibility to control the power loss via different operating parameters such as sump temperature, speed and line load. Consequently, it is possible to investigate the influence of one operating parameter independent from the power loss. This aims at identifying the tribological conditions leading to blistering. In this work, initial tests controlling the power loss are presented and associated with their influence on the tribological state of the sealing zone.

Experimental investigation of tool-sided surface modifications for dry deep drawing processes at the tool radii area

Abstract In conventional deep drawing processes lubricants are applied to reduce friction and wear. Demands for increased sustainability, environmental protection and resource efficiency motivate the realization of lubricant-free forming processes. The direct tool-workpiece contact during dry deep drawing leads to changed tribological conditions. Especially in the areas of increased normal and shear stresses, such as the die radius, the dry contact causes an increased risk of wear and damage to the sheet surface. A well-known approach to face the challenges of dry deep drawing of steel sheets and aluminum alloys is the application of amorphous carbon based coatings on the tool surfaces. Within this study the tribological behavior of conventional and modified tool surfaces at the critical radii area is investigated with a strip bending rotation test. In this test, the friction coefficient is determined based on the relative movement between a rotating test cylinder and a sheet-metal strip bent in a die. For the tribological tests the sheet materials AA6014, AA5182 and zinc coated DC04 are applied. The modification of the tool sided surface was achieved by applying ta-C and a-C:H coatings to the test cylinder. In addition to tests with coated surfaces, tests with uncoated tools are carried out for reference. By analyzing the resulting friction coefficients for the different sheet materials, the suitability of ta-C and a-C:H coatings for dry deep drawing processes is evaluated comparatively. In order to investigate cause-effect relations as well as wear mechanisms, the tool and workpiece surfaces are characterized before and after testing.

Complex Behavior of Nano-Scale Tribo-Ceramic Films in Adaptive PVD Coatings under Extreme Tribological Conditions.

Experimental investigations of nano-scale spatio-temporal effects that occur on the friction surface under extreme tribological stimuli, in combination with thermodynamic modeling of the self-organization process, are presented in this paper. The study was performed on adaptive PVD (physical vapor deposited) coatings represented by the TiAlCrSiYN/TiAlCrN nano-multilayer PVD coating. A detailed analysis of the worn surface was conducted using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) methods. It was demonstrated that the coating studied exhibits a very fast adaptive response to the extreme external stimuli through the formation of an increased amount of protective surface tribo-films at the very beginning of the running-in stage of wear. Analysis performed on the friction surface indicates that all of the tribo-film formation processes occur in the nanoscopic scale. The tribo-films form as thermal barrier tribo-ceramics with a complex composition and very low thermal conductivity under high operating temperatures, thus demonstrating reduced friction which results in low cutting forces and wear values. This process presents an opportunity for the surface layer to attain a strong non-equilibrium state. This leads to the stabilization of the exchanging interactions between the tool and environment at a low wear level. This effect is the consequence of the synergistic behavior of complex matter represented by the dynamically formed nano-scale tribo-film layer.

Influence of contact configuration and lubricating conditions on the microtriboactivity of the zirconia-Ti6Al4V pair used in dental applications

Loosening and fracture of implanted dental crowns is a consequence of relative micromovements between the zirconia abutment and the titanium alloy of the implant, in a biochemical aggressive environment. Thus, it is important to establish the in vitro tribological testing conditions that better mimics such environment. The present work aims to evaluate the effect of ball-on-plate tests configuration on the tribological behavior of ZrO2/Ti6Al4V pair in dry and lubricated conditions, using different lubricants: water, artificial saliva solution and human saliva. Ceramic balls sliding on metallic plates (TiPlate) and metallic balls sliding on ceramic plates (TiBall) were tested and the coefficient of friction (CoF) and wear response was monitored trough nanotribological tests. Open circuit potential was also measured during the tests carried out in saline solution (artificial saliva) to access the tribochemical response. The wear mechanisms were evaluated by scanning electron microscopy and atomic force microscopy analysis. Relevant differences were found between the two configurations, with and without the presence of human saliva: TiPlate presented always a higher CoF than TiBall, which may have resulted from differences in the degradation and regeneration processes of the titanium passive film during sliding. TiBall demonstrated to be the best choice to reproduce the in vivo conditions, since the metallic surface contacts permanently with zirconia, impairing the titanium repassivation. Regarding the effect of the lubricants, it was observed that human saliva had a protective action of the surfaces, leading to the lowest CoF among the lubricants used (0.19 ± 0.05 for TiBall and 0.35 ± 0.08 for TiPlate) and neglectable wear.

Analysis of Formability of face-centered-cubic Structured Aluminum Alloy And Hexagonal Closed Packed Commercially Pure Titanium

The continuous demand for light weight and cost reductions, in the aerospace industry lead to development of different metal forming techniques with higher formability. In this study the deforming behaviour of face-centered-cubic Structured Aluminum Alloy and Hexagonal Closed Packed Commercially Pure Titanium under different lubrication condition is analyzed. Due to different types of bonding among atoms in face-centered-cubic Structure and Hexagonal Closed Packed structure exists, this leads to differences in capability of forming to a required shape. As the light weight components are desired for aircrafts, the maximum formability is desirable to achieve a uniform thickness of formed component without reduction of thickness. Generally, formability depends upon forming conditions, crystal structure as well as on coefficient of hardening at room temperature. The parameters which influence the formability are analyzed and optimized. It is evaluated that high temperature deforming improves formability for both materials. It is also evaluated that the tribological conditions varied due presence of lubricant, Hence influence the forming behaviour and formability.

Experimental Analysis of Formability of Commercially Pure Titanium under PTFE Lubricated Conditions

It is essential to know the correct thickness to which a product can be developed with adequate strength and ductility. In this study the Influence of sheet thickness on formability of material Commercially Pure Titanium under Polytetra Floro Ethylene (PTFE) lubrication condition is experimentally investigated according to BIS-IS 10175 standard using Erichsen Cupping Test. Generally formability of a material depends on intrinsic material properties and prevailing process parameters. The process parameters such as lubricating conditions which alter tribological conditions during forming are taken into considerations. The process parameters are analyzed by selecting different thickness specimens of commercial pure titanium alloy. It is evaluated that the formability of the material commercially pure titanium alloy varies on thickness variation. It is also evaluated that formability of materials selected in the study differs under different lubricating conditions.

Adaption of tool surface for sheet-bulk metal forming by means of pressurized air wet abrasive jet machining

Surface structures are used for several applications in industry to enhance the characteristics of surfaces and therefore processes. One example is the Sheet-bulk metal forming process which combines the advantages of bulk and sheet metal forming. Due to the complex material flow and load, the tool surfaces do need adapted tribological properties. High feed milled surface structures have been used in the past to control the material flow of the sheets. But due the high stresses appearing, the surfaces show a running-in behavior which altereds the tribological conditions. Within this study, a pressurized air wet abrasive jet machining (PAWAJM) process is used for adapting the high feed milled surface in order to manipulate the tribological conditions. Therefore, the surfaces characteristics, residual stresses as well as the tribological effects were investigated. It is shown that the PAWAJM process can be used to modify the surface characteristics and thus the friction of the surface. In addition, it can be seen that residual compressive stresses are applied during the PAWAJM.

3-D reconstruction by extended depth-of-field in tribological analysis: Fractal approach of sliding surface in Polyamide66 with glass fiber reinforcement

The objective of this work was to inspect the fractal behavior of sliding surfaces in Polyamide66 with 30% glass fiber reinforcement. Tribological tests were performed under different conditions and the fractal values were correlated to tribological parameters and evolution of morphological aspects along the surface. Elevation maps of the sliding surfaces were built from images obtained from a conventional optical microscope. Extended depth-of-field method allowed the ordering of stacks of images to quantitative investigation of behavior of surfaces. A robust method to establish the threshold between micro-scale and macro-scales for multi-fractal behavior was proposed, allowing to compare mono-fractal behavior with the ‘‘structural’’ (micro scale) and ‘‘textural’’ (macro-scale) mixed fractal components. It was found that fractal dimension data were depended on tribological conditions and positions along sliding surfaces.

Experimental studies using minimum quantity cooling (MQC) with molybdenum disulfide and graphite-based microfluids in grinding of Inconel\xae alloy 718

In the paper, the results of experimental studies related with determination of the influence of the supply of a grinding fluid (GF) doped with powdered graphite and MoS2 into the machining zone with the minimum quantity cooling (MQC) method on the course and results of the reciprocating internal cylindrical grinding of rings made from Inconel® alloy 718 have been presented. As a grinding fluid, water aerosols were used. The aerosols delivered the following into the grinding zone: water slurry MoS2 with a concentration of 30 g/dm3, water slurry of graphite with a concentration of 30 g/dm3, 5% water solution of Syntilo RHS oil and pure demineralized water. The obtained results of carried out experiments showed that the most favorable conditions of grinding wheel operation were obtained when MQC-based delivering an aerosol of water slurry made from demineralized water doped with MoS2 and graphite with a minimum flow rate and when delivering an aerosol of 5% water slurry of Syntilo RHS oil. It was proved that doping GF with powdered MoS2 and graphite, with delivery in the form of an aerosol with a minimum flow rate, has a substantial influence on the intensity of clogging grinding wheel active surface (GWAS). Additionally, it has been demonstrated that the solid grease MoS2 and graphite particles reached the area of contact of the GWAS and the machined surface effectively, actively influencing its tribological conditions of the grinding process.

Limiting conditions for application of PVD (TiAlN) and CVD (TiCN/Al2O3/TiN) coated cemented carbide grades in the turning of hardened steels

The development of new coated cemented carbide tool grades, based on grain refining and the unidirectional orientation of crystals in the coating layer, allows this material to withstand the severe tribological conditions imposed in hard turning. Previously, this was only possible using PCBN or oxide ceramic tool materials. The aim of this research was to determine the limiting conditions for two coated cemented carbide grades (with MT CVD and PVD coatings) aimed at allowing them to withstand at least a machining time of 15 min in the turning of hardened steels, considering the cutting parameters usually adopted for PCBN tools. Turning experiments were performed on AISI 4340, AISI 52100 and AISI D2 hardened steels, and the worn cutting edges were analyzed using focus variation microscopy (FVM) applying the three-dimensional wear parameters. The use of the PVD-coated cemented carbide grade was feasible in the case of AISI 4340 steel with a level of hardness up to 55 HRC, allowing a machining time longer than 30 min. For AISI 52100 and D2 steels, feasibility was only observed with hardness levels up to 50 and 45 HRC, respectively. Interestingly, in these cases, longer machining times were achieved using the MT CVD-coated cemented carbide grade. Wear and consequent coating layer deterioration were found to be the limiting factors for tool life, and the three-dimensional wear parameters allowed these to be identified. The results indicated the appropriate limiting conditions for the application of coated cemented carbide grades in hard turning as a function of the steel microstructure and level of hardness.

Combined Analytical and Experimental Evaluation of Frictional Performance of Lubricated Untextured and Partially Textured Sliders

The study of textured surface performance is one of the highly researched topics in recent times. This is mainly due to the advantages that such surfaces can potentially provide in practice, in mitigating adverse tribological conditions, such as friction and wear. However, considering the complexities found in practice, a methodological analysis and evaluation procedure is essential in order to gain an understanding of the benefits from utilising such features in a given contact. The current study provides a combined analytical and experimental approach towards an enhanced understanding of the behaviour of textured surfaces relative to their untextured counterparts. The developed analytical models are invaluable in providing an insight into the relationship between the many parameters involved in defining even simple surface texture feature geometry and the expected outcomes in practice, when corroborated with experimental results. The current study reports on such an endeavour. With the studied texture configuration, the results have shown the possibility of reducing friction by as much as 25%.

A combined numerical and experimental approach for determining the contact temperature in an industrial ironing operation

Tribological conditions in forming operations depend on several parameters such as tool-workpiece interface pressure, surface expansion, sliding length, sliding speed, tool and workpiece materials and the roughness of the parts. Among indirect parameters, the most influential one is the tool-workpiece interface temperature, which directly influences the lubricant performance. Prior to testing new tribo-systems to determine their limits of lubrication, it is therefore important to find the interface temperature. However, measurement of the interface temperature in metal forming is difficult. The present work investigates the determination of the interface temperature in an industrial ironing operation, where severe process parameters lead to lubricant film breakdown and galling after several strokes. The methodology combines finite element simulations and experimental measurements. The overall procedure is based on a steady-state thermal analysis to determine the temperature distribution within the tool and a transient thermo-mechanical analysis of the ironing process when steady-state conditions are achieved. Results show that the proposed methodology applied to a single stroke can effectively and accurately predict the interface temperature in the test tool, thus avoiding the otherwise required thermo-mechanical FEM analyses of hundreds of strokes to reach steady-state. Furthermore, the influence of parameters, such as the predicted steady-state tool temperature, the friction coefficient and the heat transfer coefficient on the contact temperature, is analysed. It is concluded that the frictional heating is the primary cause for the peak temperature. By calibration of the friction coefficient and the heat transfer coefficient to ensure matching of the numerical results and the experimental measurements, a maximum tool-workpiece interface temperature of 158 °C was determined during the forward stroke and 150 °C during the backward stroke.

Synchronized Cu-Cantilever Structure for Kinetic Friction Characterization

A synchronized Cu-cantilever structure, used for characterizing the kinetic friction, is proposed by integrating frictional motion with synchronized amplification for the first time in this paper. A theoretical expression is further derived to predict the entrainment range of synchronization under periodically variable perturbation, rather than the invariable one. A two-cantilever resonator system, mainly consisting of a long cantilever and a short one coupled by an overhang, is designed in this paper. A friction block is fixed on the free end of the long cantilever to contact with target surfaces. The kinetic friction force representing a periodically varying perturbation is applied to the long cantilever via the friction block. The resonance frequency shift of the long cantilever, found to be related to the kinetic friction force, is magnified multiple times by that of the short cantilever. This not only implies the feasibility of applying the synchronized amplification to periodically variable perturbation but also improves the sensitivity of detecting such kind of perturbations. If both the frictional pair and tribological conditions are fixed, the abovementioned periodically variable perturbation, kinetic friction force in this paper, tribologically correlates to the surface feature, which is an invariable parameter like surface roughness and surface hardness. This paper validates that the proposed method based on the synchronized amplification is capable of characterizing extremely small differences in invariable parameters which cannot be detected by conventional imaging methods, especially those out of identification limits (like no change in color or surface feature) of conventional imaging methods.

Friction in Sheet Metal Forming Simulations: Modelling of New Sheet Metal Coatings and Lubricants

The quality of sheet metal formed parts is strongly dependent on the tribology and friction conditions that are acting in the actual forming process. These friction conditions are then dependent on the tribology system, i.e. the applied sheet material, coating and tooling material, the lubrication and process conditions. Although friction is of key importance, it is currently not considered in detail in sheet metal forming simulations. The current industrial standard is to use a constant (Coulomb) coefficient of friction, which limits the overall simulation accuracy. Since a few years back there is an ongoing collaboration on friction modelling between Volvo Cars, Tata Steel, TriboForm Engineering, AutoForm Engineering and the University of Twente. In previous papers by the authors, results from lab scale studies and studies of a door-inner part in Volvo Cars production have been presented. This paper focuses on the tribology conditions during early tryout of dies for new car models with an emphasis on the effect of the usage of new steel material coatings and lubricants on forming results. The motivation for the study is that the majority of the forming simulations at Volvo Cars are performed to secure the die tryout, i.e. solve as many problems as possible in forming simulations before the final design of the die and milling of the casting. In the current study, three closure parts for the new Volvo V60 model have been analysed with both Coulomb and TriboForm friction models. The simulation results from the different friction models are compared using thickness measurements of real parts, and 3D geometry scanning data of the parts. Results show the improved prediction accuracy of forming simulations when using the TriboForm friction model, demonstrating the ability to account for the effect of new sheet metal coatings and lubricants in sheet metal forming simulations.