Machining is a key technology in modern industries. In sectors such as aerospace, medical technology, energy technology, and automotive manufacturing, it often constitutes the final, quality-determining manufacturing step. In addition to producing the desired component geometry, machining significantly influences the properties of the surface and the edge zone. Parameters such as surface roughness, residual stresses, and microstructure play a key role in determining the wear behavior, fatigue strength, and thus the service life of high-performance components.
Titanium: a high-performance material, but difficult to machine
Titanium alloys such as Ti-6Al-4V are used in many modern high-performance applications due to their property profile. Thanks to their high specific strength, corrosion resistance, and biocompatibility, they are ideal for lightweight structures in aircraft engines, medical implants, and power generation. As their use increases, so does the need for efficient, reliable machining processes.
However, the technological advantages of titanium alloys in practical applications contrast sharply with the significant challenges posed by their machining. Titanium is among the most difficult materials to machine. This is due to a combination of several material properties. For one thing, titanium has low thermal conductivity. Unlike when machining steel, the heat generated during the machining of titanium is dissipated only to a limited extent into the workpiece and the chip. The thermal load on the tool is therefore significantly higher compared to machining steel. Furthermore, titanium exhibits high chemical reactivity, which becomes particularly pronounced at elevated temperatures.
Challenge: oxidation as a limiting mechanism
When machining titanium under conventional conditions – that is, in the presence of air – the oxygen in the ambient atmosphere presents several disadvantages. On the one hand, the high temperatures in the contact zone, combined with the oxygen, lead to oxidation-induced wear, which significantly reduces tool life. On the other hand, the titanium itself reacts with the oxygen. A passivating layer of titanium oxides, such as TiO2, forms on the surface of the workpiece and on the chips. However, oxidation degrades the quality of the titanium chips, as it makes their material recycling more difficult. This limits the resource efficiency of the manufacturing process.
These are precisely the issues that the scientists at the IFW are addressing. They are investigating the influence of the atmosphere on local contact and friction conditions during turning, as well as changes in chip formation, surface development, and wear mechanisms under oxygen-free conditions.
Experiment: turning tests in an oxygen-free process atmosphere
To investigate this, a specialized experimental setup was developed at the IFW. This consists of a sealed process chamber that was integrated into a Gildemeister CTV 400 vertical lathe. A two-stage purging process is used to remove ambient air – and thus oxygen – from the experimental setup. In the first step, the process chamber is filled with the inert gas argon. Subsequently, small amounts of the reactive gas monosilane (SiH4) are added, which reacts with the remaining oxygen to form silicon dioxide and hydrogen. The resulting process atmosphere has an extremely low oxygen partial pressure of less than 10-23 mbar, placing it at the level of an extremely high vacuum (XHV). Due to the low oxygen content, oxidation processes can be completely ruled out during the rotational tests.
The integration of a high-speed camera and a force measurement device into the test rig also enables in-situ investigations. This allows both chip formation and surface development to be analyzed and correlated with the acting process forces. By comparing different process atmospheres, the influence of an oxygen-free process atmosphere can thus be determined.
Findings: improved tool life, altered edge zone
The investigations to date show that reducing the oxygen content – and thus suppressing oxidation – has a significant impact on the machining of titanium. A clear effect is evident in the wear of carbide tools. Under oxygen-free conditions, there is a significant reduction in oxidation-induced wear. This leads to an increase in tool life of up to 170% compared to the machining process in ambient air.
The resulting chips also exhibit distinct changes. When machining titanium alloys, highly segmented shear chips typically form, which induce cyclic, thermomechanical alternating loads on the tool. Oxygen-free machining, on the other hand, results in significantly more homogeneous chip formation. Furthermore, the chemical compositions of the chips produced in air and in an oxygen-free atmosphere differ. Analyses show that chips produced in an oxygen-free process atmosphere have a significantly lower oxygen content than chips produced in ambient air. This finding is particularly relevant from a resource management perspective, as titanium with a lower oxygen content has a higher potential for material recycling.
Furthermore, due to the changing thermomechanical stress conditions, an influence of the atmosphere on the surface and edge zone properties of the titanium workpieces was demonstrated. Compared to a turning process in ambient air, an oxygen-free process atmosphere at low cutting speeds helps to reduce surface roughness and increase compressive residual stresses within the edge zone. This suggests that the process atmosphere during machining also influences the fatigue strength of the final product. To quantify the exact effects of oxygen-free machining on the service life of cyclically loaded components, experimental investigations using cyclic bending tests are currently underway.
Long-term perspective: controlled oxidation processes
The findings obtained at the IFW provide the scientific basis for a new generation of high-performance machining processes based on controlled atmospheres. In the long term, the plan is to integrate the process atmosphere – and thus the oxygen concentration – as an additional control parameter into industrial machining strategies. The goal is thus to control oxidation processes in order to reduce tool wear, increase the recyclability of titanium, and enhance the performance of the components.
The research conducted by SFB 1368 is aimed at making the economic and ecological potential of oxygen-free production processes accessible to broad sectors of industry. Looking ahead, the researchers at the IFW have therefore set themselves the goal of gaining a cross-material and cross-process understanding of oxygen-free machining. For this reason, investigations are being conducted into the machining of additional materials, such as steel, as well as under various cutting conditions. This research project is embedded in interdisciplinary collaborations with other institutes participating in SFB 1368 at Leibniz University Hannover, the Laser Zentrum Hannover e.V. (LZH), Clausthal University of Technology, and the University of Paderborn.
