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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 figure 2,

processes figure 2, left: mechanical characterization of specimens; right: cross-section analysis of specimens Oxygen-free atmosphere as enabler to reduce tool wear Further experiments are performed to determine the influence of the oxygen-free environment on the result of the grinding process. For this investigation, a bronze-bonded grinding wheel with diamond grain is used to machine titanium (Ti6Al4V) under air and Ar/SiH 4 atmosphere. For the latter, the face grinding process is conducted within a gastight housing to ensure low oxygen partial pressures (figure 3, center). For grinding under air, the housing is removed to enable normal grinding conditions. An EDXanalysis is performed afterward which enables to determine the elemental composition of the material by measuring the characteristic X-rays. The results of the EDXanalysis of the machined workpiece surfaces show that the atmosphere has an impact on the oxygen content of the workpiece material. The atomic percentage of oxygen decreases by 60 % comparing the results of grinding under air and Ar/SiH 4 (figure 3, right). The main reason for this outcome is the low content of residual oxygen within the figure 3, left: experimental setup for grinding under air and Ar/SiH 4 atmosphere; right: results of EDX-analysis 34 no. 4, November 2020

processes process under Ar/SiH 4 , which leads to lower gas diffusion into titanium and consequently less pronounced oxidation effects in the subsurface microstructure. The residual oxygen content of 6.8 atom.-% can partially be explained by the period of time between the grinding process and the EDX-analysis in which a thin oxide layer has formed due to the natural reaction of titanium with oxygen. This result is confirmed by additional tests in which a recently ground Ti-surface (oxygen content < 1 atom.-%) and an unmachined Ti-surface (oxygen content ≈ 10 atom.-%) were examined. The time between the grinding process and the analysis was approximately 15 minutes. These explanations can also be applied to the visual observation of the grinding process and the result of the surface quality of the Ti-workpiece, which is described below. This can mainly be attributed to oxidation effects during grinding. The resulting surface colors are comparable to those on the sintering specimens and can also be attributed to different oxide types as well as different oxide layer thicknesses [13]. Although the effect of Ti-oxidation at elevated temperatures also thermally damages the surface, it cannot be entirely equated with grinding burning since the latter does not include damage caused by chemical reactions like oxidation [14, 15]. The formation of oxidation on the surface can be compared to the anodizing process anodic spark deposition. The material is heated locally in the contact area and stays molten for a short period of time. This leads to a strong reaction of titanium with oxygen [16]. In contrast to the anodizing process in which a chemical reaction is wanted to create a wear-resistant passive layer, oxidations on the workpiece during grinding should be avoided. The reason for this is that oxide layers cause a hardening of the original surface, which in turn leads to an increase of grinding forces and tool wear [3, 4]. Conclusion and outlook The results of the experiments show that oxygen has a significant impact on the processes sintering and grinding. On the one hand, oxygen causes a weakening of the abrasive layers due to oxidations that results in inhomogeneous structures. On the other hand, oxygen within the grinding process leads to surface oxidation and thus to possibly higher tool wear compared to grinding under Ar/SiH 4 atmosphere. figure 4, microscope images of ground Ti-surfaces Due to the experimental setup, no cooling lubricant can be used within the gastight housing. This leads to higher friction and temperatures in the contact area between tool and workpiece. While strong sparking can be observed when grinding in the presence of air, this effect is much less pronounced under Ar/SiH 4 atmosphere. This confirms the significantly lower content of oxygen in Ar/SiH 4 atmosphere since the exothermal reaction of removed titanium chips with oxygen is less distinctive. However, due to minor leakages in the current setup small quantities of oxygen are still present within the process. The visual results of titanium workpieces show that the machined surfaces are less affected by chemical reactions when ground under Ar/SiH 4 atmosphere (figure 4). Based on the presented results further scientific investigations will be conducted to gain a deeper understanding of the processes and mechanisms. This includes sintering experiments under different atmospheres and various abrasive layer material compositions as well as different grinding tests to determine the influence on grinding tools and workpieces. To investigate, for example, the material removal mechanisms during grinding a quick stop device will be used. It enables the interruption of cut so that the contact zone on the workpiece surface can be seen as a snapshot of the current chip formation mechanisms. Furthermore, an improved gastight housing will be developed to allow the feed of cooling lubricant into the contact zone between tool and workpiece. A measurement module will also be applied to measure the exact amount of residual oxygen content within the housing and especially near the contact zone. Acknowledgements The authors would like to thank the German Research Foundation (DFG) for their organizational and financial support within the “Collaborative Research Centre 1368: Oxygen-free production” and the subproject “Cooling concepts for Grinding” in particular. no. 4, November 2020 35

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