Towards rational manufacturing of titanium-based alloys

05-10-2016

he transportation sector moves towards greater energy efficiency in all areas in order to meet the latest performance and environmental targets. Titanium alloys exhibit higher specific strength than other structural materials as well as excellent corrosion and creep resistance up to about 500 °C. These properties represent many performance advantages for the transportation industry. The ESRF allows researchers to monitor the kinetical evolution of the microstructural phases of titanium-based components during thermal and thermomechanical treatments, which can help scientists rationalize their processing.

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Company

Vienna University of Technology (TU WIEN), German Aerospace Center (DLR)

Challenge

The transportation sector moves towards greater energy efficiency in all areas (i.e. from component production over to fuel consumption) in order to meet the latest performance and environmental targets. The success and progress fulfilling these goals is highly dependent on the availability of new engineering materials and manufacturing methods that enable significant weight savings associated with simultaneous improvements in component performance. Titanium alloys exhibit higher specific strength than other structural materials as well as excellent corrosion and creep resistance up to about 500 °C. These properties represent many performance advantages for the transportation industry. Despite these benefits and the relatively large resource reserves (titanium is the fourth most abundant metal in the earth’s crust), titanium alloys still come at high production costs that limit their industrial usage.

Titanium-based components are mainly produced via the classical process of ingot metallurgy (cast and wrought) which provides alloys with high strength levels. At this stage of manufacturing, thermal and thermo-mechanical treatments determine the microstructural characteristics, i.e. the mechanical properties of titanium alloys. On the other hand, selective laser melting (SLM) is a very promising powder-bed based “3D printing” technique that manufactures near net-shape metallic components with higher resource-efficiency than conventional fabrication methods. Consequently, considerable cost savings can be achieved. One of the main key strengths of SLM is that extremely complex geometries inaccessible using conventional manufacturing techniques can be manufactured. In this way, structures of minimal weight and optimized functional performance can be produced. However, the very fast cooling rates reached during solidification of molten metal pools during SLM produce brittle titanium based components with poor mechanical performance.

Monitoring the kinetical evolution of the microstructural phases of titanium based components during thermal and thermomechanical treatments would help scientists rationalize their processing either via ingot metallurgy or advanced SLM while improving their mechanical performance (e.g. strength and fatigue resistance).

Sample

The a+b Ti-6Al-4V and Ti-6Al-6V-2Sn as well as the metastable b Ti-10Al-2Fe-3Al, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-5Al-5Mo-5V-3Cr-1Zr titanium alloys presenting different initial microstructures.

Solution

The researchers carried out in situ high energy synchrotron X-ray diffraction experiments at the ID15B beamline. Image sequences of complete Debye–Scherrer rings from the bulk of the studied alloys were recorded in transmission mode while heating and cooling the sample within the same temperature ranges used in the industry. This allowed univocal determination of the phase transformations kinetics (i.e. the evolution of the volume fractions and lattice parameters of phases) which confine the microstructural changes of the alloys, i.e. their mechanical properties. Moreover, three-dimensional (3-D) imaging was performed at the ID22NI beamline by high-energy magnified synchrotron tomography using Kirkpatrick–Baez focusing optics, to analyse morphological features of the microstructure (e.g. non-uniform distribution of phases, formation of complex structures, or contiguity between them) and understand their relationship with the mechanical properties of the studied alloys.

Benefits

The studies provide an advance in the current knowledge of the phase transformation kinetics of titanium alloys mostly discussed in the basis of stable conditions (e.g. isothermal aging and ex-situ experiments). This will help to develop new theoretical models for microstructure prediction leading to improvements in functional alloy design, lead-time and cost savings via knowledge-based thermal treatment optimization. Particularly, the results obtained will contribute to overcome the present manufacturing restrictions of SLM manufacturing.

Journal of Materials Science 50, 1412-1426.