Papers by Keyword: Friction

Abstract: Using additive layer manufacturing techniques, such as Fused Deposition Modelling (FDM), it is possible to produce complex geometry relatively quickly and cheaply. For this reason these processes offer intriguing possibilities for tooling in metal forming processes. A common material utilised in the FDM process is Acrylonitrile Butadiene Styrene (ABS). A series of compression and tensile tests were carried out on FDM ABS test specimens built in a range of orientations. The tensile tests were carried out until fracture and the specimen cross-sections analysed to investigate the cause of failure. In uniaxial tension the vertical build direction was found to be the weakest, failing in a brittle fashion. The FDM material elastic modulus and Poisson’s ratio were found to be isotropic in nature within experimental scatter. The ‘yield’ strength in compression was found to be higher than that observed for equivalent tensile orientations. Following a series of strip pull friction tests using commercially pure titanium as the blank material, it was found that without the utilisation of an interfacial lubricant a favourable frictional performance was achievable on ABS tool surfaces. Due to tool wear however, the frictional performance of these tool pieces deteriorated with increasing sliding distance.

Abstract: Increasing technological requirements, as well as the demand for an efficient production demands high performance materials and enhanced manufacturing processes. The development of a new manufacturing process, sheet-bulk metal forming (SBMF), is one approach to produce lightweight forming parts with an increased number of functional properties while, at the same time, combining the advantages of sheet and bulk metal forming. For SBMF processes, the specific adjustment of the friction between tool and workpiece for a specifically designed material flow, which is called tailored friction, is of great importance. The reduction of friction is essential in order to ensure a homogeneous forming zone. However, a higher friction can be used to control the material flow to increase the local thickness of the work piece for additional functional integration. This paper shows the development of surface structures for SBMF tools by means of high-feed milling. Process parameters like the tilt angle or the feed are varied to influence the surface parameters of the structures, which results in different tribological properties of the forming tool. The structured surfaces are subsequently coated with a wear resistant CrAlN coating, processed by a magnetron-sputtering process (PVD) to enhance the lifetime and performance of the forming tool. Finally, a ring compressing test is used to investigate the tribological behavior of the coated structures.

Abstract: Friction has a considerable influence in metal forming both in economic and technical terms. This is especially true for sheet-bulk metal forming (SBMF). The contact pressure that occurs here can be low making Coulomb’s friction law advisable, but also very high so that Tresca’s friction law is preferable. By means of an elasto-plastic half-space model rough surfaces have been investigated, which are deformed in such contact states. The elasto-plastic half-space model has been verified and calibrated experimentally. The result is the development of a constitutive friction law, which can reproduce the frictional interactions for both low and high contact pressures. In addition, the law gives conclusion regarding plastic smoothening of rough surfaces. The law is implemented in the framework of the Finite-Element-Method. However, compared to usual friction relations the tribological interplay presented here comes with the disadvantage of rising numerical effort. In order to minimise this drawback, a model adaptive finite-element-simulation is performed additionally. In this approach, contact regions are identified, where a conventional friction law is applicable, where the newly developed constitutive friction law should be used, or where frictional effects are negligible. The corresponding goal-oriented indicators are derived based on the “dual-weighted-residual” (DWR) method taking into account both the model and the discretisation error. This leads to an efficient simulation that applies the necessary friction law in dependence of contact complexity.

Abstract: The objective of this paper is the investigation of the implemented pressure dependent friction model in the FE simulation program AutoForm. Since the values for the required parameters of this approach are not defined in the actual FE simulation program, this paper contributes to a first definition of these parameters. Therefore, experimental investigations with the cup drawing tests are carried out. With the support of the experimental results regarding the maximum stamping force, the model is calibrated and subsequently validated by the comparison of the sheet thicknesses of the experimental and numerical investigations. The results of this validation reveal a good accordance of simulation and reality in nearly all areas. However there are also areas in which the prediction accuracy decreases in comparison to the basic simulation. Therefore, additional investigations have to be carried out which concentrate on the modification of this pressure dependent approach towards a better universal prediction accuracy of the simulation.

Abstract: Objective: This study aimed to evaluate the effect of surface texture mold by stainless steel (SS) mesh on Invisalign material friction coefficient. Materials and methods: Ten kinds of SS mesh were used to mold surface texture on Invisalign material. Experiments were conducted using a zirconia ball that slid against Invisalign material plates with different surface textures. In the experiments, the coefficients of friction under normal loads of 100, 200, and 300 g under dry and artificial saliva conditions were recorded. Results: The coefficient of friction was relative to the pore size and wire diameter of SS mesh. Conclusions: The Invisalign material coefficient can be effectively managed by the surface texture mold by SS mesh.

Abstract: Concrete pavements and airfields in the curing process of protection against desiccation, particularly, the application of the wax-polymer emulsion which forms a solid film, which persists for a long time in the initial period of operation of roads and airfields. The presence of the film changes the nature of the friction on the surface that could adversely affect traffic and aircraft. This paper investigates the friction of rubber on the surface of the treated concrete with an aqueous emulsion of paraffin series Emcoril. It was established that the friction on the paraffin film, unlike friction on the surface of the concrete does not respect the law of linear Coulomb. The degree of deviation from linearity, the higher the greater the thickness of the film. The frictional force on the film increases with the slip velocity. At high speed, friction in the film is greater than the frictional force on the concrete, and at low speed is much lower than the previous one

Abstract: Molybdenum (Mo) film is deposited by Ion Beam Assisted Deposition (IBAD) on the 316L steel substrate surface in this experiment. The micro structure and nanohardness of the film are tested by Scanning Electronic Microscope (SEM) and nanoindentation tester. The tribological behaviors of Mo film under ZDDP and MoDTC lubrication are evaluated by SRV test machine. The tribo-film formed on the worn surfaces is investigated by X-ray photoelectron spectroscopy (XPS) to find out the tribological mechanisms between the Mo film and the two additional additives. The result shows that Mo film not only has friction reduction property but also has wear assistant property under ZDDP and MoDTC lubrication. As element Mo has advantage on promoting the deposition of these two lubricating additives.

Abstract: In this paper, we present a global tribological approach of friction dry contact polymer with SGF on steel, in terms of consequences on metallic surface condition (comparative wear coefficients of polished steel surface) based on extensive experimental determinations between the value and the evolution of the friction coefficient, wear of steel surfaces and contact temperature, in the case of linear dry contact, for thermoplastic material reinforced with short glass fibers (SGF) and various steel surfaces. The aim was to highlight the evolution of the wear process depending on the friction coefficient, as well as the dependence of the loading wear and the sliding speed. Wear depth and volume were calculated based on the “wear imprint” method. As a result, it was possible to graphically illustrate the evolution of the friction coefficient, of the contact temperature, and the change of the wear process, emphasizing the abrasive, adhesive and corrosive wear. The evolution of the plastic material transfer function of the contact temperature, namely of the power lost by friction was highlighted. It has been demonstrated that in the case of a 30% SGF content it can reach and even exceed contact temperatures very close to the yield limit of the plastic material. The influence of the normal load and sliding speed was evaluated in detail. The influence of SGF content, normal load, relative sliding speed and contact temperature over the metal surface wear and over the nature of wear mechanism was recorded. The reaction to wear of different steel surfaces in linear dry friction contact on polymers with SGF (polyamide + 20% SGF, polyamide + 30% SGF and polycarbonate with 20% SGF), observing the friction influence over the metallic surfaces wear. The paper includes also its analysis over the steel’s wear from different points of view: the reinforcement content influence and tribological parameters (load, contact pressure, sliding speed, contact temperature, etc.). Thus, authors' findings related to the fact that the abrasive component of the friction force is more significant than the adhesive component are presented, which generally is specific to the polymers’ friction. Authors’ detections also state that, in the case of the polyamide with 30% glass fibres, the steel surface linear wear rate order are of 10^-4 mm/h, respectively the order of volumetric wear rate is of 10^-6 cm³/h. The resulting comparative volumetric wear coefficients are of the order (10^-11 to 10^-12) cm³/cm and respectively linear wear coefficients of 10^-9 mm/cm.

Abstract: This work presents the results of treatments by CO₂ laser of automotive rings covered with carbon black slurry, aiming to improve the friction rate and the ring lifetime. The results of this treatment were characterized by scanning electron microscopy, Vickers hardness, ball on disk test and X-ray energy dispersive spectroscopy. The scanning electron microscopy showed that the laser treated area extent reaches near to 120 μm depth. The measured hardness was about 950 HV_0,05 and the friction coefficient lower than 0.2. The energy dispersive spectroscopy along the treated cross section showing carbon diffusion into the metallic substrate.

Abstract: Hot stamping parts made of boron steel have been widely used in automotive industry. In forming process of a hot-stamped vehicle bumper, non-uniform contact and friction between the boron steel and tools caused the bumper groove sidewalls excessive thinning or crack. Increasing stamping speed could improve the non-uniformity of blank temperature field and reduce the temperature difference. Influence of the stamping speed and the friction on the forming quality were studied by numerical simulation. The results showed that increasing the stamping speed within a certain range or using reasonable lubrication could reduce the thickness reduction of the sidewall. A bumper without cracking was obtained with a high stamping speed of 300 mm/s and a certain lubrication method.