“Less is more”! 3D printing of new high-performance Ti-O-Fe alloys

On May 31, 2023, the team of Ma Qian Distinguished Professor Ma of the Additive Manufacturing Center of RMIT University, Professor Simon Ringer and Professor Liao Xiaozhou of the University of Sydney, together with Assistant Professor Chen Zibin of the Hong Kong Polytechnic University and Hexagon Manufacturing Intelligence of Melbourne, jointly published a title entitled “Strong and ductile” in the journal Nature Titanium-oxygen-iron alloys by additive manufacturing”

In this study, a new class of high-performance titanium-oxygen-iron (Ti-O-Fe) alloys was successfully prepared by fitting the titanium alloy design and 3D printing process design. The original intention of alloy design fully considers the idea of “less is more” (that is, low alloying) and circular economy, that is, considering the use of iron and oxygen beyond the grade of sponge titanium, residual titanium powder from high oxygen content in the printing cycle or titanium powder with high oxygen content in other ways, and processing “waste” with high oxygen content as raw materials to prepare such new titanium alloys. These new high-performance Ti-O-Fe alloys are expected to have a wide range of applications, including aerospace, biomedical, chemical engineering, space and energy technology. In addition, the study’s findings in fundamental innovation are expected to drive similar research and development of other titanium, zirconium and related alloys.

The corresponding authors are Professor Ma Qian (RMIT) and Professor Simon Ringer (University of Sydney), the first author is Dr. Song Tingting (RMIT), and Assistant Professor Chen Zibin (formerly University of Sydney, now The Hong Kong Polytechnic University).

Left: Dr. Song Tingting; Right: Ma Qian’s distinguished professor.

Research background

Titanium alloy is an advanced lightweight and high-strength metal structural material used in many key fields. α-β duplex titanium alloy is the backbone material of the titanium industry, accounting for more than half of the titanium alloy application market (α-phase titanium and β-phase titanium are both a way of titanium as a metal crystal, each corresponding to a specific atomic arrangement). Since 1954, the production of this class of titanium alloys has been achieved mainly by adding aluminum (Al) and vanadium (V) to titanium. Among them, aluminum is used to stabilize and strengthen α-phase titanium, while vanadium is used to stabilize and strengthen β-phase titanium.

Oxygen (O) and iron (Fe) are two abundant, inexpensive elements that can strongly stabilize and strengthen α-phase titanium and β-phase titanium, respectively. Oxygen stabilizes α titanium about 10 times as much as aluminum, while iron-stabilized β titanium is about 4 times as capable as vanadium. However, oxygen is widely known as the “nemesis of titanium” because it significantly increases the brittleness of titanium alloys beyond a low threshold. Although iron is the strongest β-phase titanium stabilized element, when more than 2% of iron is added to the titanium alloy as the main β-phase titanium stabilization element, under the usual solidification conditions, it will often form lumpy β spots that are difficult to eliminate, thereby seriously affecting the uniformity of the tissue, and then causing many adverse effects on the performance of the titanium alloy. The above two factors seriously restrict the development and preparation of high-performance α-β duplex Ti-O-Fe alloys by traditional manufacturing processes.

From the perspective of raw materials, since the establishment of the titanium industry in the United States in 1948, the production of sponge titanium metal (the basic raw material of titanium, titanium powder and other titanium structural parts) has basically used the energy-intensive Kroll process. About 5-10% of the sponge titanium contains excessive iron or oxygen, which belongs to low-grade or grade out-of-sponge titanium products, and is generally not used to produce high-performance titanium alloys. If these low-grade or out-of-grade out-of-sponge titanium can be converted into high-performance titanium alloys, it will have important economic value and emission reduction effects.

In addition, oxygen and titanium have a very strong binding ability. In the process of 3D printing cycle, with the increase of the number of cycles, the oxygen content of the last remaining titanium powder will gradually increase, which may exceed the standard. Moreover, in the production process of non-spherical titanium powder, a part of titanium powder inevitably contains a higher oxygen content. At present, the effective reuse of these high-oxygen titanium powders has been a difficult problem. This study also provides a new way to solve this difficult problem.

▌Research work

The research team successfully combined the alloy design concept with the 3D printing process design to achieve the preparation of a series of Ti-O-Fe alloys with high strength, good plasticity and easy printing. The team used Simufact software’s metal 3D printing DED (directed energy deposition) module, and after several experimental verifications, through detailed simulation, obtained the temperature and cooling rate information of each point in the laser powder directional energy deposition titanium alloy 3D printing process. Based on this, combined with the characteristics of the phase transition process of this type of alloy and the evolution law of its microstructure with the thermal cycling process, a wide printing window suitable for this type of alloy was determined, and the alloy design ideas of the research team were verified inside and outside the printing window. The series of Ti-O-Fe alloys prepared within the print window fully meet the design objectives (see Figure 1).

The introduction of oxygen and iron directly affects the morphology and size of the original phase titanium crystals (called grains) after solidification. After solidification of Ti-0.14O-3.23Fe alloy with low oxygen content, a relatively coarse primitive β phase grain mixed with short columnar and nearly spherical (called equiaxial) is formed, but with the increase of O content, the original phase titanium grain tends to become uniform equiaxed grain. Another feature is that in the printing window, the printed room temperature structure of the designed Ti-O-Fe alloy is a α-β duplex ultra-fine slat. Among them, the volume fraction of the α phase is about 70% (the phase accounts for about 30%), and the thickness of its slats is generally less than 400 nm. The printed series of Ti-O-Fe alloys have a uniform structure, and no β defects are found. Ti-O-3Fe alloys with 0.3-0.5% oxygen content have similar ductility compared to Ti-6Al-4V alloys, but have higher strength (see Figure 2). These alloys have fine, primitive isoaxial grains and an ultrafine duplex α-β slats structure inside.

To further explain the excellent tensile properties exhibited by these Ti-O-Fe alloys in their printed state, the research team used three-dimensional atomic probes (see Figure 3) and first-principles calculations (see Figure 4) to investigate the distribution of oxygen and iron atoms in the α phase and β phase ultrafine duplex slats. The study found that the oxygen content was close to zero in the β phase and the iron content was close to zero in the α phase; Oxygen in the α phase and iron in the β phase showed nanoscale gradient distribution characteristics. The distribution of oxygen in the α-phase ultrafine slats divides the α-phase slats into low-oxygen and high-oxygen regions. The interior of the α phase slats is a low-oxygen zone with good plasticity or ductility, while the high-oxygen part is adjacent to the α/phase boundary and has high strength. This distribution of oxygen atoms or the low-high oxygen combination in the α phase ultra-fine slats helps mitigate the risk of oxygen embrittlement. The observation of the internal dislocations of the α phase β after tensile deformation confirms the above hypothesis.

Figure 1: Microstructure of Ti-O-Fe alloy in laser powder deposition titanium alloy printing window (green zone in c) and laser powder deposition printing state. The scale of d – g is one hundred microns, and the scale of h – k is one micron.

Figure 2: Tensile mechanical properties of Ti-O-Fe alloy in laser powder deposition 3D printing state at room temperature (alloy composition changes, 3D printing process does not change).

Figure 3: Distribution of Fe atoms and O atoms in a laser powder deposition 3D printed Ti-O-Fe alloy.

Figure 4: DFT simulation of Fe and O atom distribution in BCC (β) and HCP (α) phases of α β-O-Fe alloy.

▌Significance and impact

This work shows that by fully fitting the alloy design and 3D printing process design (based on detailed process simulation), a new high-performance alloy that is completely different from the traditional design thinking can be developed. For example, the use of promising alloying elements that have been “snubbed” by traditional manufacturing processes can be explored, similar to the alloy design shown in this work for Ti-O-Fe using oxygen and iron.

Secondly, this new alloy design is more conducive to the idea of low alloying or primitization (saving resources, less is more). For example, the total amount of alloying elements in the Ti-O-Fe alloy designed by this work does not exceed 3.5 wt% (0.3-0.5%O + 3%Fe), while the current mainstream Ti-6Al-4V requires 10% of the total alloying elements (6%Al + 4%V). Among them, vanadium is a high-priced and toxic metal (clinically vanadium poisoning is mostly acute poisoning). Low alloying or primitization can go a long way towards achieving sustainable development.

Specific to the 3D printing Ti-O-Fe alloy developed by the institute, as mentioned earlier, it is expected that the iron and oxygen produced in the production process of sponge titanium can be effectively used as a powder raw material to achieve the printing of these alloys. In addition, spherical or non-spherical high-oxygen titanium powder and high-oxygen waste generated during titanium processing can also be effectively used as raw materials for the production of such Ti-O-Fe alloys. This has a positive effect on the realization of a sustainable circular economy model with the goal of reducing consumption, reducing emissions and reducing costs.

Still taking titanium alloys as an example, nitrogen (N) has a better ability to stabilize and strengthen α phase titanium crystals than oxygen, and nitrogen is also easy to make titanium brittle, so the nitrogen content in titanium alloys is strictly controlled (< 0.05%). With the ideas presented in this institute, it should be possible to develop a high-performance Ti-N-Fe alloy based on 3D printing.

The production of zirconium sponge is the same as that of titanium sponge. The above concepts applicable to titanium alloys are also expected to be applied to zirconium alloys in principle.

Finally, the brittleness problem caused by oxygen occurs not only in titanium alloys, but also in other metals and alloys, such as niobium and molybdenum, and the aforementioned zirconium. How to solve or reduce the brittleness caused by such gap elements is a big challenge in physical metallurgy. The 3D printing Ti-O-Fe alloy designed by this work has certain enlightenment, that is, the introduction of a second component phase that can “laugh” oxygen or other gap elements through alloy design, and the distribution of the gap elements predicted by first-principles calculation, and the implementation of tailor-made 3D printing process may provide an effective solution for solving or alleviating the brittleness problem caused by oxygen or similar gap elements. (Source: Science Network)

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