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hp tooling 2020 #4

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The journal of hp tooling is an english, global publication on all aspects of high precision tools, accessories and their applications.

processes Oxygen-free

processes Oxygen-free production: New manufacturing approach in grinding written by Prof. Dr.-Ing. Berend Denkena, Dr.-Ing. Alexander Krödel and M. Sc. Nils Hansen, authors from the Institute of Production Engineering and Machine Tools IFW Production engineering is constantly driven by increasing demands regarding economically efficient manufacturing processes as well as increasing requirements on the precision of machined workpieces. With respect to precision machining, surface quality plays a key role in evaluating machined surfaces. Because of its ability to enable high surface qualities, grinding is the most commonly used finishing process [1, 2]. However, grinding of materials that are difficult to machine, like titanium (Ti), can lead to rapid tool wear and therefore high process costs [3]. Since titanium is of great importance to the industry as a material for applications in lightweight construction (e. g. aerospace), there is a big market for machined titanium parts. Conse quently, there is a considerable interest in optimizing existing production processes. In a new approach, an oxygen-free atmosphere will be used within manufacturing processes, which could potentially lead to longer tool life and higher workpiece qualities. Due to the presence of oxygen in usual machining processes, metal workpiece surfaces tend to oxidize at elevated temperatures, which can lead to an increased hardening and embrittlement in the subsurface microstructure. Especially when it comes to machining materials like titanium that possess a high chemical affinity to oxygen, oxidation effects are distinct [4]. This results in higher tool wear and also poor surface qualities of the workpiece [3]. Based on this premise an entirely new approach will be investigated within the “Collaborative Research Centre: Oxygen-free production” over the next years to improve manufacturing processes like grinding. For this purpose, an oxygenfree atmosphere within the grinding process will be used to eliminate oxidation effects and therefore increase the life of grinding tools. Another aspect of the research project is the sintering of metal bonded grinding tools under an oxygen-free atmosphere. Oxygen within the sintering process also leads to oxidation of powder particles, which potentially has a negative influence on the strength of the abrasive layer as well as the retention of diamond abrasive grains. This article will present results that were gained in first experiments. A new approach to the process chain of grinding tools To investigate the influence of the oxygen-free manufacturing of grinding tools and the behavior of those tools in grinding processes the gas mixture of argon/silane (Ar/SiH 4 ) is used in sintering and grinding experiments. Oxygen can be displaced by a heavier and chemically inert gas like argon (Ar). Since argon is not able to displace oxygen entirely low contents of monosilane (SiH 4 ) can be doped in argon to further reduce the residual content of oxygen. The reaction of silane with oxygen results in the formation of water and a silicon dioxide powder [5]. At ambient pressure, this reaction generates an atmosphere with very low oxygen partial pressures (≤ 10 -23 bar). With respect to the oxygen content, the resulting atmosphere is adequate to an extremely high vacuum (XHV). Compared to this approach generating a conventional technical ultrahigh vacuum (UHV) is much more expensive and complex and can only enable oxygen partial pressures down to 10 -15 bar. For sintering and grinding experiments, a mixture of 98.5 Vol.-% argon and 1.5 Vol.-% silane is used. The associated process chain consists of four steps whereas the sintering step is divided into “grinding tools” and “specimens” (figure 1). Grinding wheels are sintered and conditioned to carry out grinding experiments to evaluate their behavior (e. g. tool wear). The results can then be correlated to the sintering parameters in order to individually adapt the grinding tool manufacturing process to the grinding application [6]. Specimens on the other hand are sintered at similar conditions and are used for scientific characterizations of abrasive layers. This enables to investigate a large variety of abrasive layer compositions and different process parameters. In addition, compared to the sintering of entire grinding wheels specimens can be produced at much lower production costs [6 - 10]. 32 no. 4, November 2020

processes figure 1, process chain of grinding tools and specimens Potential of oxygen-free atmosphere in sintering of abrasive layers All sintering experiments are carried out on a sintering press DSP 510 from Dr. Fritsch. The specimens are sintered in a graphite mold with graphite dies (figure 2, top left) at given constant process parameters in a resistance heating process. The sintering process is performed under air with ambient pressure and under a silane doped inert gas atmosphere (Ar/SiH 4 ) with low oxygen partial pressure. The bond material consists of 100 % titanium that tends to form carbides when using diamond as abrasive grain. In order to eliminate additional chemical reactions of the Ti-powder with the grain during sintering, no abrasive has been used in these first tests. In doing so the results of the investigation only base on the bond material and can therefore be evaluated separately. After this, a three-point flexural test can be conducted to determine the force F Z at which the specimens break. This enables to calculate the critical bond stress σ by taking the diameter d, height h and support length l of the specimens into account (figure 2, center left); [6]. The calculation assumes an area moment of inertia of I y = (d * h 3 )/12. The critical bond stress is a characteristic value that describes the ability of the specimen to withstand mechanical loads. It can also be used to compare specimens that are sintered under different conditions. Previous studies have shown that a higher bond stress results in a decreased wear of the grinding tools [7]. The ambient atmosphere has a significant influence on the resulting bond stresses of sintered Ti-specimens (figure 2, center). The bond stress of specimens sintered under Ar/SiH 4 (335 N/mm 2 ) is over five times the amount of the specimen bond stress that results at sintering under air (65 N/mm 2 ). The main reason for this result is the influence of oxygen within the process. Since oxygen is not able to diffuse out of the titanium powder during sintering because of ambient pressure conditions the reaction of titanium particles with oxygen is distinct. The forming of titanium oxides intensifies with increasing temperatures during sintering, which leads to a less homogeneous microstructure [11]. In the case of the Ar/SiH 4 atmosphere, a significantly lower content of oxygen is present that enables much higher critical bond stresses. Due to the reduction of oxidation effects titanium can now form a more homogeneous microstructure that is able to withstand higher mechanical loads. A visual analysis of the specimen cross-sections after the fractural test shows a noticeable difference between the surfaces. Specimens that are sintered under air have blue and brown colored areas (figure 2, right). The distinct coloration is a result of the oxidation of titanium during sintering and can consist of various titanium oxides (e. g. TiO 2 , TiO) [12]. The color gradient indicates different oxide types as well as different Ti-oxide layer thicknesses [13]. These visual observations confirm the forming of titanium oxides that lead to a decreasing material strength. When sintering under Ar/SiH 4 atmosphere no oxide layers can be visually found on the cross-section surface of the specimens. This can be explained by the low oxygen partial pressure that prevents the oxidation of titanium, which results in higher bond stresses. Later investigations will also include the use of diamond and cBN grains to explore the influence of the atmosphere on the interface between bond and abrasive grains. The temperature-dependent oxidation behavior of titanium could potentially prevent the forming of carbides and thus the chemical retention of the grain [4, 11]. By using an Ar/ SiH 4 atmosphere oxidation effects will be reduced, which could lead to higher grain retention forces and would therefore increase the critical bond stresses. no. 4, November 2020 33

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