Advanced Materials Research Vols. 966-967

Abstract: Finding the correct friction coefficient for the simulation of bulk metal forming processes is crucial. The practical approach nowadays for this objective is to conduct a friction sensitive process-test and the corresponding numerical simulation in order to reveal the friction coefficient. The Double Cup Extrusion Test (DCET) is one of the widely used friction tests for bulk metal forming. Although, there is a large body of literature on DCETs, there are still important aspects which have not been addressed yet. Motivated by this fact, this study emphasizes and demonstrates the importance of thermo-mechanical modelling to evaluate the DCET for the characterization of friction coefficients even for cold forging processes. To this end, thermo-mechanical material characterization covering necessary temperature and strain rate spectrum is conducted and used in the thermo-mechanically coupled finite element analysis (FEA) of the DCET. These findings are compared with the results of single flow curve based purely mechanical FEA in terms of cup height ratios as well as force-displacement curves for two different press speeds.

Abstract: An ever increasing mobility and a shortage of resources lead to restrictive politically driven limits for fuel consumption as well as an increasing demand of customers for efficient vehicles. Though electrification of cars proceeds, combustion engines will play an important role for conventional and hybrid concepts within next decades. Thus, for a contribution to increasing energy efficiency of vehicles it is vital to trace sources of friction losses and to identify possibilities for friction reduction in combustion engines. Therefore, the follower as a main contributor to friction losses in valve trains was chosen as a demonstrator for friction reduction effects by microstructured components. However, the realization of theoretically advantageous microstructures with filigree geometries is challenging for manufacturing technologies. The present study focuses on the elaboration of a technological basis for a repeatable production of components with microstructured surfaces by a combined cup backward extrusion micro coining process, coping with the demands of large-lot production. For realization of a high accuracy the influence of friction on geometry of microstructured components was investigated. In addition, running-in of components is decisive for final geometry and tribological behavior of microstructured surfaces and hence considered as well.

Abstract: The recent trend towards miniaturization of products and technology has boosted a strong demand for such metallic micro-parts with micro features and high tolerances. Conventional forming technologies, such as extrusion and drawing, have encountered new challenges at the micro-scale level due to the ‘size effects’ that tends to be predominant at this scale level. Friction is one of the predominant factors exercising strong effects in micro-forming. Previous studies varied grain size of the test pieces in order to examine size effects in micro-extrusion. In addition, the effects on the extrusion load, forming shape, as well as hardness of different grain sizes, die coatings and lubricants were compared. DLC coating has been proven effective as a die coating. Increasing grain size was effective with lubricants having high viscosity. In this study, the effect of different die angles and lubricants is compared and examined.

Abstract: The use of different process media such as cutting fluids, coolants, honing oil and washing media in typical machining operations exceeds 5,000 m³ per year. These media support critical functions such as lubrication, corrosion protection, cleaning and cooling, and have an enormous effect on the manufacturing performance. The tribological properties of these media are improved by using additive molecules, which are physically or chemically adsorbed on the surface of tools and workpieces. The additive performance is especially important in water lubricated tribosystems, where the environment is highly corrosive. The role of corrosion inhibitors typically applied is to neutralize the pH of contaminants in the fluid. Ethanolamines and ethylamines are known as ligands which can form chelate bonds with metals via their amino, hydroxyl and deprotonated hydroxyl-groups. In tribology they are widely spread, as corrosion inhibitors and detergents especially for water based lubricants. This study inquires the tribological performance of amine-based solutions in two types of tribotesters which apply different contact conditions. The dissimilar behaviour under rolling and sliding contact is explained in terms of the structure of the adsorbed compounds. Understanding the performance of the first chemisorbed layers of additives on the workpiece provides essential information for optimizing lubrication in aqueous solutions.

Abstract: Most of the several billion liters of metalworking fluid (MWF) used worldwide and annually are water-based and thus prone to a microbial contamination. The microbial growth leads to a deterioration and therefore to a loss of quality and technical performance. In most cases, biocides, which pose a potential risk to health and environment, are used to reduce the microbial load. To avoid these limitations, the paradigm shift of using microorganisms in a positive way in a manufacturing process as a lubricant is investigated in this paper. Some microorganisms are able to synthesize equivalent MWF components like e.g. fatty acids or sulfur compounds. Due to this fact, it is possible to create a regenerative system on a microbiological basis for the substitution of mineral oil containing MWF components. To demonstrate the lubrication potential of bacteria, preliminary investigations were conducted on a Brugger-tribotester. Against this background, the approach presented here intends to investigate the lubrication properties of special microorganisms and the influence of the microbial cell counts on the lubrication behavior respectively. The results of the tribological tests show that the microbial-suspensions exhibit Brugger-values as high as highly concentrated conventional MWF and indicate the potential to replace these respective components.

Abstract: This work focused on the wear performance of the clad layers which were formed with cladding titanium nitrides (TiN) powder on the JIS SKD11 tool steel by the gas tungsten arc welding (GTAW) method. A rotating type tribometer was used to evaluate the wear behavior of the clad specimens under different sliding conditions. Furthermore, a nanoindenter was used to measure the hardness and elastic modulus of the reinforcements. According to the wear test results, the wear performance of the specimens cladded with TiN powder was better than that of the JIS SKD11 tool steel specimens. During dry sliding wear test, the clad layers exhibited a strong wear resistance because they contained the hard TiN reinforcements. Therefore, the wear performance of the clad layers was substantially better than that of the SKD11 specimens under all the test conditions in this study. In addition, produced oxide films might influence the wear behavior of different specimens during the wear testing, and oxidation wear would even dominate the wear behavior of the clad layers under some conditions.

Abstract: The present paper deals with a sequential fluid structure coupling approach in order to solution the roughness prediction. The cold rolling model involves the strip with its asperities, the lubricant and the working roll. The strip asperities are modeled in 2D (trapezoidal shape) forming valleys and plateaus. Fluid flow rate between each valley full of lubricant is solved using local Couette's equation. Thus, the volume of lubricant trapped and its pressure are updated on the cold rolling model. During computations, the asperity is deformed from the entry to the exit to obtain its final shape. Global parameters such as front, back tensions, speeds are taken into account but also rheological (fluid, solid) and tribological behaviours.

Abstract: The austenitic stainless steel (SS) of AISI 304L is widely used in industrial applications because of its superior anti-corrosion resistance. However, the material suffers from a lower hardness, thus reducing wear resistance. In this study, AISI 304L was clad with tungsten boride (WB) ceramic powder using the gas tungsten arc welding (GTAW) process to increase surface hardness and improve wear resistance. The microstructure of the cladding layer was investigated using an X-ray diffractometer (XRD), an electron probe microanalyzer (EPMA), and a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The hardness distribution of the cladding layer was measured using a micro-Vickers hardness tester. Wear tests were conducted with a pin-on-disc tribometer at the ambient condition, while simultaneously monitoring friction coefficient variation. Surface frictional temperature was recorded with K-type thermocouples during wear tests. The worn morphology of the tested specimens was observed by SEM to identify wear characteristics. The results show that WB cladding successfully increased the hardness and the wear resistance of AISI 304L. Keywords: GTAW, WB, wear resistance, microstructure

Abstract: The recently developed machine hammer peening process is used at the die shop of the Mercedes-Benz plant in Sindelfingen in order to replace manual surface finish of deep drawing dies. The goal of the process is surface roughness reduction after milling to ensure the tribological properties, which are necessary for the sheet metal forming process. Using machine hammer peening it is also possible to create defined surface structures that may be employed to influence local friction conditions and therewith overcome current limitations of the forming process. To take advantage of the surface structuring capabilities it is necessary to understand how to create defined surface structures using machine hammer peening and how the created structures affect friction and material flow behavior. In this work an approach is presented to describe the interaction of milling and machine hammer peening parameters on the created topography by wave theory. Especially the influence of tool path parameters of milling and consecutive machine hammer peening is investigated. The results, which are calculated using wave theory, are verified by FEM simulations and real experiments. In addition, suitable process parameters for machine hammer peening are derived from the obtained results, as they are used at the Mercedes-Benz die shop today.

Abstract: Deep rolling process is a mechanical surface treatment that provides several advantages, such as low friction on the interface between the tool and workpiece in the process, controlled profile of induced compressive residual stress to enhance the HCF and LCF strength, enhancement of the stability of the near-surface structure at high temperature, and improvement of surface finish after the process. This paper investigates the deep rolling process under lubricated condition for a complex deep rolling path. A three-dimensional finite element model incorporating the strain hardening and strain rate effects on the material responses is developed to sequentially simulate the continuous multi-axis roller motion in the process. This model can capture the horizontal and normal forces acting on the roller so that a time-varying apparent coefficient of friction can be obtained. In addition, due to the complex roller path, the model also predicts a complex residual stress distribution in the near-surface material.