New application of titanium for bipolar plate material of proton exchange membrane fuel cells

From the viewpoint of climate change、proton exchange membrane fuel cells (PEMFCs) have been expected as a clean energy source and many automotive companies have advanced the development and commercialization of PEMFC vehicles.
Recently titanium has been used as a material for bipolar plates which are the crucial component of PEMFCs. Bipolar plates have many functions such as corrosion resistance preventing metal ion elution leading to the degradation of PEMFCs, press formability for fuel gas flow channels and high surface conductivity to collect electric current. In addition, lightweight and high mechanical strength are required for automotive applications. The most attractive material to satisfying these functions is titanium. However, improvement of surface conductivity by means of coatings is an important issue, since a passive film on titanium have low conductivity. At the same time, the reduction of bipolar plate cost is also an important issue. From the standpoint of manufacturing cost, a pre-coating process (coating before forming) is beneficial in which the time-consuming handling is eliminated to carry bipolar plates one by one to a coating process after forming. The pre-coating process is applicable to titanium due to its excellent corrosion resistance unlike to stainless steel which may elute ferrous ions from cracks of coating caused by forming. Indeed, Kobe Steel developed pre-coating type titanium coated with carbon nanoparticle composite named as ‘NC titanium’. NC titanium has been equipped in the fuel cell vehicle ‘MIRAI’ launched in 2020 from Toyota Motor Corporation.
As described, although titanium is more expensive than stainless steel, it has merits such as excellent corrosion resistance, lightweight and reduction of the manufacturing cost. The use expansion of titanium is expected as a bipolar plate material of PEMFCs for not only passenger vehicles but also commercial vehicles, railways, ships, and aircrafts in the future.
 

Recent development of Ti-Mo and Ti-Cr based superelastic alloys

Ni-free beta Ti-based shape memory alloys (SMAs) are fascinating for biomedical devices to replace Ti-Ni.  However, the shape recovery strain and force are limited in common Ti-Nb based  SMAs. Thus, in the presentation, recent development and achievements of Ti-Mo and Ti-Cr based shape memory and superelastic alloys are presented.   

 

 


Hydrogen in Titanium and Stress Corrosion Cracking

Hydrogen in titanium has become an exciting research area as it has recently become possible to measure hydrogen accurately at microstructural length scales, first using FIB-SIMS and more recently atom probe tomography. This enables longstanding issues such as stress corrosion cracking, an irritant in jet engine operation, to be re-examined. Cold chain FIB preparation and isotopic labelling are increasingly enabling confidence to be developed that the results are not affected by pickup during preparation or H atoms in the atom probe itself. It also allows issues like the role of Fe in CP Ti to be re-examined. Increasingly, these results are leading to new insights that enhance our understanding of the role of hydrogen in titanium alloy degradation. This many become increasingly important with the advent of the hydrogen economy, for instance in electrolysers and also in hydrogen-powered air transport.

Modelling, measurement, and management of residual stress during manufacturing of titanium components

Residual stress, a tensor quantity, are locked-in stresses within a component without external loading, generated as a result of complex non-linear thermal-mechanical processing during manufacturing. Most manufacturing processes introduce residual stress that has a direct bearing on manufacturing (e.g., undesirable distortion) and on the resilience of products in service and their design life. Historically, residual stresses have primarily been incorporated into structure critical component design through a significant safety factor because they are challenging to characterise and control, and there is little design guidance in codes and standards. Consequently, components have thicker sections than needed, increasing the resource use and entry cost of the product as well as the cost of ownership through extra weight. The grand challenge is to bring residual stress into the 4th industrial revolution to engineer its effect during material manufacture to minimise waste and production time and maximise product performance in-service.

Based on current state-of-the-art technology, for forging high value components, this is done by developing a baseline model simulating the metal forging and heat treatment processes, considering microstructural changes using physically based and constitutive materials models. A baseline model is also developed for the subsequent machining operation (metal removal rate, heat generated, etc.) using simplified approaches for metal cutting through material removal operations and integrating cutting forces, machining induced effects, and clamping configurations. These baseline models are then integrated into a more holistic approach to create a quasi-“digital-twin” that also estimates microstructure changes, generation and evolution of residual stress, and distortion throughout the whole processes. This presentation provides the latest development in this vein and sheds light on the prediction, measurements, and control of residual stress during manufacturing processes of high value components made from titanium.

Understanding the transformation behaviour of metastable beta alloys
 
A number of metastable beta titanium alloys can undergo a reversible transformation between the parent beta phase and the orthorhombic αʺ martensite, giving rise to superelastic and shape memory behaviours.  The low elastic moduli, high specific strengths and large recoverable strains exhibited by these alloys make them particularly attractive for applications across a wide range of technology areas, including those within the aerospace, defence and biomedical sectors.  However, to date, industrial uptake of these materials has been limited as tailoring their properties to specific conditions and controlling their transformation across multiple cycles has proven challenging. 

The inability to overcome this issue is indicative of an incomplete appreciation of the factors that influence the behaviour of these materials.  This view is reinforced when considering the literature where significant variations in reported behaviour exist between different studies even for alloys with nominally identical compositions.  For example, differences of over 100˚C in the martensite start temperature have been reported for several alloys and in some cases the transformation was not observed even when cooling to cryogenic temperatures.  However, key details, such as measured interstitial contents or processing routes, are not always reported, which hinders mechanistic understanding.

To address this, a large body of research has been undertaken, assessing a range of alloys all fabricated, processed and analysed in the same manner. Through a combination of high energy in situ diffraction and complementary ex situ characterisation effects such as the variation in martensite start temperature, the critical stress for transformation and evolution of cyclic behaviour have been studied.  Here, an overview of this work will be presented and used to provide an enhanced understanding of the key factors that influence the transformation, in particular the role of internal stresses and the influence of the omega phase.

 

 

Texture development during the hot forming and annealing of dual-phase Ti alloys
 
Texture control is of great importance during the processing of dual phase Ti alloys, since texture affects their properties and performance in service. Although we have a good empirical understanding of the texture development during processing, our fundamental understanding of the mechanisms responsible is lacking. We know that deformation, phase transformation and recrystallization are all involved, but there is no consensus on their relative importance and how it changes with changes in processing conditions. This underatanding is particularly important in the forging of Ti alloys, where processing conditions vary within a part and during the process. This talk will highlight results from recent studies carried out in the EPSRC programme grant LightForm and the TIFUN consortium, that aimed to unravel the contribution of these different mechanisms to the texture development in these important alloys. This work included lab scale hot working simulations, in-situ synchrotron and EBSD studies, as well as computational experiments using crystal plasticity and phase-field modelling. These results provide a new insights into the complex process of texture evolution in these important alloys, and provide a basis for making improved predictions of texture evolution during processing. 

 

Fabricating titanium and titanium alloy parts and structural members cost and energy effectively by a novel PM technology

Starting with sponge titanium and master and/or elemental alloy particles or powders and without going through melting and solidification, titanium and titanium alloy parts and structural members can be fabricated by a novel powder metallurgy technology involving powder fabrication and thermomechanical powder consolidation and forming. The PM process steps include powder fabrication by hydrogenation of sponge titanium granules and mechanical milling, powder blending and compacting, rapid heating and short time holding of the powder compact, and then thermomechanical processing and forming of the heated powder compact. A large amount of research has been done and published by the speaker’s group and other research groups with important findings which show the titanium and titanium alloys in the parts and structural members exhibit novel microstructures and excellent mechanical properties encompassing high strength and excellent ductility. This presentation is to report and discuss the scientific principles of the process, the process condition-microstructure-mechanical property correlations of the titanium and titanium alloys and the potential applications of this PM technology.

 

Production Cost of Current Titanium Metallurgical Process and Possibility of New Alternative Process
 
The application of titanium is, to a great degree, restricted by the costly extraction method, e.g. the Kroll process, although titanium is the fourth richest element among all structural metals in the earth’s crust and has many desirable engineering and functional properties. In this paper, the metallurgical process of titanium was analyzed, and compared with the processes of iron and aluminum. The details of the production cost of the current metallurgical process were analyzed. New titanium metallurgical processes were reviewed, and the possibility of reducing the production cost was discussed.  The energy consumption and operation cost of titanium metallurgical process will be remarkably reduced through the combination of the carbon thermoreduction and molten salt electrolysis, using ilmenite (FeTiO3) as the raw material. 

 

Aerospace Materials

Undeniably, unprecedented challenges due to COVID-19 slowed research and development efforts across all sectors of aerospace from March 2020 to December 2021; however, the recent economic recovery and high demand for titanium supply has come with renewed focus on research and development programs for aerospace. This presentation will provide an industry perspective on the opportunities and challenges for the next generation of titanium products targeted to meet higher performance demands and address identified needs. A review of the current state of the art and future opportunities will be discussed across the industry, including airframes and aeroengines.
 

 

 

 

Additive manufacturing of titanium aerospace and space components
 
Additive manufacturing of metals is currently paving its way into industrial applications at high pace. While in medical applications there is already a widespread use of AM for customized solutions, the strongest innovation boost in AM is coming from aviation industry, followed by the energy sector, automotive industry, space and toolmaking industry. The focus of this lecture is on aerospace and space applications that have recently attracted major attention, some of them already being in series production.

Using powder bed-based and nozzle-based (wire and powder) AM processes a large variety of customized solutions is feasible, ranging from micrometer-size parts with filigree features to the meter scale of large-size components. With regard to the processing requirements either high accuracy or high productivity can be achieved, whereas a combination is difficult. Among others, examples of industrialized solutions of micro-AM structures for aeroengine use will be presented as well as a demonstrator component for space applications with a total diameter of about 3 meters.

The presentation will highlight recent developments in AM related to different processes, titanium alloys and part sizes/geometries. Unlike any other manufacturing technology, AM of high quality parts requires an in-depth understanding of the close relationship between the AM process, the material and the resulting component properties. As a matter of fact, customized hardware, online diagnostics and control systems are required for robust processing of AM parts.  Moreover, the effects of defects on part quality shall be studied in detail. Some of the results presented are derived from a 80 Mio. Euro research project on AM, initiated and coordinated by the presenting author.

 

 

Titanium Aluminides – Status of the Production of Ingots, Semi-Finished Products and Powders

Low Pressure Turbine blades made of TiAl alloys are successfully used in aircraft engines for regular commercial service for more than one decade now. TiAl materials production technologies at GfE were developed and adjusted to both small sized semi-finished products and over the time to larger volumes.
The technology developed for TiAl alloys is based on Vacuum Arc Remelting (VAR) and subsequent homogeneization in a VAR Skull Melter followed by centrifugal casting in permanent moulds. This technology leads to small sized products which are able to meet the exceptional high requirements concerning the homogeneity of the material. With increasing production volumes the need for a recycling route of the valuable material came up. This was solved and implemented via an Induction Skull Melter (ISM) followed by the approved centrifugal casting technology of the VAR production line. Industrial TiAl recycling was a key for decreasing production costs.
The technology and the melting equipment is also basis for the production of feedstocks for the EIGA atomization unit at GfE. The gas atomizer is not limited to TiAl alloys but can also produce spherical powders of different material classes as Ni based alloys, Ti alloys, refractory metal based alloys.
The presentation gives an overview on the metallurgical technologies for the production of TiAl ingots, semi-finished products and powders. AMG Titanium Alloys and Coatings operates a well balanced set of VAR furnaces, VAR Skull Melters, Induction Skull Melters and EIGA Gas Atomizers for the production of Titanium Aluminides in order to strengthen its position as the technology and market leader in this materials segment.

 

New strategies for strain-hardening improvements in titanium alloys

Regarding the mechanical properties of titanium alloys, one of the longstanding challenges still concerns their plastic properties and in particular their lack of intrinsic stain-hardening generally leading to a penalizing loss of ductility. Research efforts have recently been devoted to improvement of the strength/ductility trade-off through work hardening. Thus, new “strain-transformable” Ti alloys (TRIP/TWIP alloys) have been developed and are now reaching a reasonable level of maturity for further development. However, in spite of these promising developments, the recurrent problem remains the insufficient yield strength level (between 500 Mpa and 700 Mpa, on average) of this family of alloys.

As an alternative, new strategies, generically based on the control of microstructural heterogeneities in the alloys, also allow today to considerably increase the strain-hardening effects (and consequently the strength/ductility combination), while keeping high values of yield strength.

Among these approaches, we can mention the RIP effect (Reorientation Induced Plasticity) which allows, in so-called "dual-phase" alloys (a+a', for example), a combination of mechanical properties potentially superior to TRIP/TWIP alloys, from the point of view of yield strength. In a+b type alloys, a+a’dual-phase microstructures are easily achievable by quenching and allow to reach both very high yield strength (up to 1000 Mpa) accompanied by an important kinematic strain-hardening generated by a very strong mechanical contrast between the two phases.

In this presentation, the underlying mechanisms associated with the RIP effect are discussed in relation to the mechanical properties. We will try to shed additional light on the crystallography and mechanical behavior of hexagonal α' martensite to rationalize the configuration adopted by the martensite during deformation and to highlight the crucial role of interfaces in explaining its fine-scale mechanical behavior.

Finally, we will address the aspects related to the alloy design strategies that are a prerequisite for further developments of this family of alloys.

 

 

Titanium: a promising metal full of challenges
 
Titanium has been already identified as a very promising metal for its outstanding specific mechanical characteristics, meeting expectations of aerospace industry. However, it also requires very attentive manufacturing process to ensure lifetime performances.
Geostrategic, environmental and economical contexts have reinforced the need of maximizing the recycling of scraps. This paper presents the recent developments of Aubert & Duval to face these challenges.