Defect and Diffusion Forum Vol. 414

Abstract: The classical tribological systems which are used in cold forming are still based on zinc phosphate as a separation layer and soap as a lubricant. Due to the globalization more and more processes are required following a globalized environmental protection and chemical registration concept. Therefore, the search for resource-efficient and environmentally friendly pretreatment concepts in cold forming are essential to fulfill these demands. One possible solution can be found in the phosphate free cold forming processes. Globally seen, there are different development routes to gain phosphate free tribological systems. On one hand, lubricants have been developed which are working without a conversion layer but leading to less forming performance which is often unacceptable out of profitability. On the other hand, there is the possibility to use a zinc phosphate free conversion coating layer to realize a phosphate free tribological system, without leaving the common system consisting out of a separation layer with a lubricant on top through remaining the same performance known from common systems. Beside this, it is discussed how a further development concept of this system leads to easier and shorter processes introducing a reactive lubricant concept. With the help of the reactive lubricant it is possible to apply a separation layer and a lubricant in one treatment step and obtain a phosphate free separation layer with a self-assembled lubricant on top. This system enables to realize a phosphate-free cold-forming process without changing the application of the lubricant and the forming process.

Abstract: The frictional properties of diamond-like carbon (DLC) films under boundary lubrication with a synthetic base oil were investigated. The specimens tested were a-C:H, Cr-a-C:H, Ti-a-C:H, W-a-C:H, WC-a-C:H, Mo-a-C:H, MoS2-a-C:H, and Sn-a-C:H films. The tribological properties of DLC/steel contacts were evaluated using a reciprocating-type cylinder-on-disk tribotester. The DLC films exhibited different frictional properties depending on the type of doping elements and compounds used. Notably, the Sn-a-C film gave the lowest friction. The friction tests and static contact angle analyses revealed that the average friction coefficient decreased with a decrease in the static contact angle of the worn DLC film surface, demonstrating that the DLC films with relatively high surface wettabilities exhibited relatively low friction properties. Therefore, surface wettability plays an important role in determining the frictional properties of DLC films under boundary lubrication with synthetic base oil..

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: 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: In recent years, hybrid manufacturing combining additive and subtractive processes is gaining increasingly importance in the industry. One of the issues related to this association of processes concerns the use of cutting fluids, important to optimize the machining part, but that can strongly affect the additive part by generating pores in the laser metal deposition. The present work deals with the performance of a new ecological cutting fluid that dries just as water, eliminating the need for a cleaning step between the machining and the laser metal deposition. This lubricant is an emulsion mainly composed of water and alkylphosphonic acids known to allow creating a low-friction tribofilm on metals. This study is carried out by comparing the machining performance of this new cutting fluid with two more classical lubricants, a straight oil and a soluble oil. It was found that machining forces and surface roughness were not very affected by the change of the lubrication mode, while the tool wear showed a significant difference between the dry and the lubricated cases. Considering that the performance of all the cutting fluids was very close, it was concluded that the new lubricant has a great potential for machining applications, since it is ecologically more friendly, non-harmful to the operator and does not need a degreasing step.

Abstract: Functional surfaces have been widely used for control of physical and/or chemical properties of substances on the surface. In the manufacturing industries, some of micro fabrication approaches such as laser processing have been applied to form textured surfaces, which control the surface functions with topographies. This study presents the surface texturing in cutting with micro-scale structured end mills. Micro-scale nicks are fabricated on the cutting edges of PCD (Poly-Crystalline Diamond) end mills in laser finishing. The cutting operation is conducted to form the chips on each nick with the cutter axis inclination in the feed direction. An analytical model is applied to control the surface structure for the spindle speed, the feed rate, the nick geometry and the inclination angle of the cutter axis. Then, the surface structures were fabricated in the actual cutting process. The machining operation in this study is available in fabrication of the micro-scale structures at high production rates and the structures shape can be controlled in the surface simulation.

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: Mechanical loading and unloading of silicon is a characteristic feature of grinding and diamond turning processes. Such rapid loading and unloading induces damage and phase transformations. While, indentation tests are often used to study such normal loading and unloading via characteristic events in the force-depth plot, such tests involve only normal loading and lack tangential loading. A better alternative is scratch test, both constant and varying depth ones, involving normal and tangential loading on the scratching tool; this better simulates conditions of machining, or grinding. In this research, the mechanical load/unload behavior response of silicon is studied under scratching conditions by comparing increasing and decreasing depth scratch behaviour. In-situ force responses show that after ductile-brittle transition occurs, higher forces, at a given scratch depth, are required to deform the material during increasing depth scratching for a given depth than in decreasing depth scratch. Large surface and sub-surface damages with the presence of radial, median, and lateral cracks are seen to make the material weaker, ahead of the advancing tool, in decreasing depth scratch. Raman intensity ratio of amorphous silicon (a-Si) to nanocrystalline silicon (nc-Si) shows that high amorphization of silicon occurs during increasing depth scratching than decreasing depth. Using such force-depth plots an attempt is made to compare the normal loads while indenting and scratching. This study can help optimize the processing of silicon by grinding and diamond turning.

Abstract: In the automotive industry, the development of electrically powered vehicles has become a major forward-looking topic. For improving the range and thus the efficiency of electric cars, lightweight construction has gained even more importance. In this regard, hot stamping has been established as a suitable and resource efficient process to manufacture high-strength and lightweight body-in-white components. This method combines hot forming and quenching of boron-manganese steel 22MnB5 in a single process step. As a result, complex structures with thin sheet thicknesses and high ultimate tensile strength up to 1500 MPa are generated. However, the use of lubricants is not possible at elevated temperatures, which subsequently leads to high thermo-mechanical tool stresses. As a side effect, high friction and severe wear occur during the forming process, which affect the resulting part quality and maximum tool life. Therefore, the aim of this study is to improve the tribological performance of hot stamping tools by using a laser implantation process. This technique is based on manufacturing highly wear resistant, separated and elevated structures in micrometer range by embedding hard ceramic particles into the tool material via pulsed laser radiation. As a result, highly stressed areas on the tool surface can be modified locally, which in turn influence the tribological and thermal behavior during the forming process. In this regard, laser implanted and conventionally tool surfaces were investigated under hot stamping conditions. A modified pin-on-disk test was used to analyze the friction coefficient and occuring wear mechanisms. Furthermore, quenching tests as well as hardness measurements were carried out to gain in-depth knowledge about the cooling behavior of the modified tool surfaces and its impact to the resulting mechanical part properties.