Papers by Keyword: Friction

Abstract: The damage state in a formed component has a significant influence on the performance of the component in service. Controlling damage evolution during forming through specific modifications of the process parameters will therefore allow an improvement of this performance. The evolution of the stress-strain state during the forming process is the primary influencing factor of the resulting damage state. The stress-strain state is influenced by the friction between tools and workpiece. To investigate the cause-effect relationship between friction and damage evolution in the deep drawing process, Finite Element simulations of the deep drawing of rotationally symmetric cups were performed. Punch velocity and blank holder force were varied. Damage was predicted using a Lemaitre damage model. The damage states predicted using a Coulomb friction law and a model incorporating a dependence on contact normal stress and relative velocity were compared. The parameter-dependent friction model predicted a change in the damage distribution after forming when varying the process parameters, which was not found using the Coulomb friction law.

Abstract: For the realization of efficient production processes, an understanding of the appropriate application of metal working fluids (MWF) is necessary. In addition to knowledge about the process-related aspect of chip transport and the macroscopic cooling effect, the characteristics and properties of the lubrication film thickness and the cooling conditions in the area of the secondary shear zone on the chip surface, i.e. in the direct vicinity of the material separation, represent a fundamental scientific issue within production technology. In particular, these areas generate a high proportion of heat during machining, so that the local friction phenomena have a significant influence on the resulting edge zone of the produced component and the thermomechanical load on the tool. Currently, there are no numerical models and methods for mapping and predicting the lubrication film thickness that can be used in the sense of a targeted design of the cooling lubricant supply. The aim is to transfer the methods from the field of tribology of machine elements, which have already led to significant knowledge gains in this discipline, to machining and couple them to approaches already established in machining. To this end, experiments on tribometers have been performed as a first step. For example, an oscillating pin-on-plate tribometer was used. In this setup, a steel plate is doing oscillating motion against a fixed ball (diameter of 6 mm) under a defined load. The frictional force is recorded during the test. A MWF in a heated tank is used for the lubricant. Additional investigations on the film thickness were performed on an optical EHL (elasto-hydrodynamic lubrication) tribometer. In this setup, a ball rolls on a glass-disc and the resulting film thickness is measured by interferometry.For comparison, the influence of the MWF on the chip formation process in metal cutting was investigated on a special test rig (machine tool). This test rig allows high speed imaging and force measurements of an orthogonal cutting process while using MWFs. The first results show a reduced contact length between chip and tool as well as lower process forces for processes with MWFs compared to dry cutting processes. In future investigations, this test rig will be applied for the identification of the local friction coefficient between chip and tool. The data gained from the cutting test are compared with the output of the tribological test rigs.

Abstract: This study combines the laser surface texturing technology and heat treatment process to fabricate a dual surface engineered Ti6Al4V consisting of micro-groove crosshatch pattern texture covered with hard TiO₂ oxide coating to reduced friction and improve the wear resistance at the bio-lubricated interface. Crosshatch texture with 25 μm width, 5 μm depth at 25% area density were fabricated using nanosecond Nd:YAG laser over Ti6Al4V surface and then heat treated at 600 °C for 48 hours. XRD result showed rutile TiO₂ phase formation along with the presence of anatase TiO₂, Al₂O₃ and Ti₃O minor phases whereas, the surface hardness was increased to 1538±41 HV. Bio-tribology experiments were carried out for 45 and 90 o oriented micro-groove crosshatch textures, with and without heat treatment, under partially replicating hip implant articulation. Results demonstrated 60% friction reduction corresponding to the 45 ^o oriented crosshatch texture with heat treatment. Further, the worn-out surface morphology showed reduced wear damage and good wear debris entrapment inside the micro-grooves.

Abstract: To improve the sensitivity of the steady combined forward and backward extrusion test proposed in previous work, an optimization job based on the finite element simulations was carried out. A raw material of 0.45% carbon steel was tested under different stain rates from 0.001s^-1 to 1s^-1 and different temperatures from 30°C to 400°C, and the material flow stresses were modelled by Hensel-Spittel equation. The deformation degree of the forward extrusion was set as 50%. The key parameters including the deformation degree of the backward extrusion, the ratio between the radius of the punch nose and the radius of the punch, the taper angle of the punch, the die angle, the sizing lands of the punch and the die were optimized. The sensitivity of the optimal design is improved about 20% compared with previous design when the friction factor is assumed as 0.03~0.15. The new group of calibration curves presents more scatter than the old group. The sensitivity improvement is also validated by the experimental works.

Abstract: The present work proposes to emphasize the effects of friction and wear formulations for wear prediction for turning operations. It is shown that friction models play a major role on local variables such as pressure, sliding speed and temperature (σ_n, V_sl, T) and thus on the simulated tool wear. This work highlights that both formulations and parameters of these equations should be carefully considered to achieve an actual predictive capability.

Abstract: The paper presents a developed a technology for producing a new abrasive material, which is a refractory ceramic matrix made of aluminum oxide with embedded diamond particles. The tests have shown its advantage over traditional diamond-bearing materials when processing high-hardness ceramics. To synthesize a new material with a predetermined set of operational parameters, it is necessary to build a wear model of a friction pair composed of diamond-bearing ceramic material and ceramics. After analyzing the results of other authors’ studies and assessing the morphology of the contacting surfaces, it is assumed that the contact of microroughnesses on the friction surfaces of abrasive diamond-bearing ceramic tools is linearly elastic. The model is built within the framework of the basic wear equation. The composite material surface was modeled by a set of same radius spherical segments; diamond grains were distributed in the material with a given bulk density. The paper uses the concept of an equivalent surface with the power law distributed of microroughnesses tops. A theoretical relationship is obtained between the micro-and macrocharacteristics of the wear process of a composite ceramic friction pair. The authors established theoretically and confirmed experimentally that the grinding productivity increases with an increase in sliding speed, applied load and diamond grain size. The diamond concentration has no significant effect on the workpiece wear. The established dependencies can help to achieve high productivity of diamond ceramic tools.

Abstract: Increasing of the hardness and wear resistance of the working surfaces of parts made of aluminum and its alloys is achieved by modifying them by the method of micro-arc oxidation in alkalescent electrolytes. This technology consists in the formation of a highly hard and wear-resistant ceramic layer by means of the spark discharges and implies an enormous investment of energy and time. The finished product surface roughness in numerous cases fails to meet the requirements, and demands additional machining. High hardness of the hardened layer, at the same time causes an increased wear of the processing tool. A method in which the technological process is carried out in two stages allows improving energy efficiency of this technology. At the beginning, when using an electrolyte with a high content of liquid glass, a primary oxide layer, consisting predominantly of silicon oxide, is formed. At the next stage of micro arc oxidation, the content of the liquid glass in the electrolyte decreases, which allows the formation of predominantly high rigid aluminum oxide in ceramic layer. This technology can significantly reduce the time of formation of a coating and reduces energy consumption. To reduce the processing tool wear it is proposed to keep the surface of the modified by oxidation workpiece in hydrofluoric acid for 5 - 30 minutes. This operation allows to reduce the hardness of the upper layer, consisting mainly of silicates, without affecting the lower layer, formed mainly of aluminum oxide. As a result, it is possible to receive the required surface micro geometry with less tools wear, less time and energy. There was achieved a reduction of time for getting the finished product by 1.6 - 2.3 times, electricity by 2 - 2.7 times, tool wear by 8.7 - 12 times.

Abstract: The given study is dedicated to optimization of dispersion-strengthened material based on copper powder of the Cu-Al-C-O system used for valve guides of internal combustion engines. The developed material containing 3 % wt aluminum, 0.6 % wt carbon and not more than 0.02 % wt oxygen has the recrystallization temperature of 1000^оС and exhibits higher tribotechnical properties comparing to the prototype. In particular, the wear out intensity of the developed material is 2.5 times lower. The material production technology is based on the method of reactionary mechanical alloying in the attritor and powder and granular metallurgy technologies.

Abstract: The technology of friction-electric modification of the surfaces of machine parts with W₂C and WC tungsten carbides is considered. A method of modifying the surface layer by implantation of materials based on tungsten carbides is investigated in order to increase the wear resistance of parts forming tribo-conjugations in heavily loaded units of multi-purpose tracked and wheeled vehicles.

Abstract: The article considers the issues of tribological properties of a titanium alloy when sliding without lubrication on zirconium ceramics. The coefficients of friction and wear resistance of friction pairs are investigated in relation to conditions in which it is impossible to use liquid lubricants. The statement about the possibility of normal operation of a pair of titanium - zirconium ceramics at a temperature of 150 ° C and more is substantiated. It is shown that the working capacity of a friction pair can be ensured by alphanizing the friction surface. Taking into account that titanium alloys are widely used in aircraft engine technology, special attention is paid to the coefficient of friction, because a high value can lead to failure of the friction unit. On the basis of the study, the application perspectiveness of zirconium ceramic materials for increasing the reliability and service life of friction units operating without lubrication at elevated temperatures in contact with a titanium alloy have been established.ds.