Titanium is already widely used in aerospace applications, but advances in 3D printing technology mean that it’s now feasible to produce titanium components in geometries and timeframes not previously possible.
In this article, I will discuss the benefits of using titanium in aerospace engineering, and how 3D printing is already changing how companies like General Electric are manufacturing components.
Current titanium usage
Classified as a transitional metal, titanium possesses characteristics in its natural state that make it useful for certain aerospace applications, such as strength, corrosion resistance, heat resistance, machinability, toughness, creep resistance and favourable fatigue properties. It possesses roughly the same strength as steel while being much lighter, and can absorb large amounts of energy per unit volume. It can be machined, and while it requires tougher tooling than aluminium (for example), its increased strength means that it can withstand deformation under sustained load and elevated temperatures. It is resistant to both very high and cryogenic temperatures, and is ideal for environments that experience extreme temperature fluctuations.
Examples of aerospace applications that currently use titanium include: landing gears, where its high strength-to-weight ratio and fatigue resistance are unmatched; jet engines, where extreme mechanical stresses and temperature fluctuations require a variety of titanium alloys; and rockets, missiles and space applications, where the weight limitations, vibrations during launch and high g-force loads make titanium an ideal material for certain components.
Digital manufacturing is the term being used for the technology that employs 3D printing for metallurgical consolidation. Wire-based 3D printing focuses on creating near-net shapes involved in close die forgings. For titanium-based 3D printing, though, powder is generally preferred as the feedstock material, especially for smaller parts and more complex geometries.
In general, 3D printed parts are economically attractive for relatively small production runs, while larger runs favour traditional manufacturing methods such as close die forging or investment casting. The traditional manufacturing methods involve a considerable initial investment in dies or moulds, which are difficult to amortize when only a small number of units are required.
3D printing advancements now enable virtually any geometry to be produced, many of which would be impossible using other manufacturing processes. Consequently, new design possibilities are available, such as to further optimise aircraft component designs and significantly reduce weight. In most cases, the design engineer no longer needs to design while keeping tooling / machine limitations in mind.
As 3D printing technology continues to develop, larger production runs will soon be possible, and add further weight to the argument for using 3D printing ahead of traditional manufacturing techniques.
3D printing using titanium requires a two-step process: powder acquisition, and metallurgical consolidation by additive manufacturing.
Two basic approaches exist to obtain titanium powder, known as the ‘blended elemental’ approach and the ‘pre-alloying’ approach. The ‘blended elemental’ method involves the blending of (virtually) pure titanium with alloying elements, while ‘pre-alloying’ uses solid scrap, billets or machined turnings as feedstock.
The powder that needs to be produced must be fine and semi-spherical, to allow a higher packing density and low porosity, which results in stronger components and better fatigue properties. The powder particles must be compositionally homogenous and relatively uniform in size. As a general rule, aerospace applications will use relatively small particles, between 50 and 150 microns. Smaller particles afford smoother finishes, which is especially important to prevent crack growth. However, smaller particles are more difficult to manage and have much lower deposition rates.
Some aerospace-relevant powder acquisition methods are PREP (Plasma Rotating Electrode Process), TGA / VIM (Titanium Gas Atomization / Vacuum Induction Melting), and PA (Plasma Atomization).
Also known as ‘additive manufacturing’, 3D printing is suitable for synthesizing three-dimensional objects with a wide range of materials. There are various technologies currently in development, but each currently share the following common characteristics:
- Parts are constructed in a layered fashion.
- Material is added to produce the final product, instead of subtracting material from castings or billets through milling or machining (hence the term ‘additive manufacturing’).
- An energy or heat source is required to fuse the particles together (such as a laser or electron beam).
In the aerospace industry, the primary additive manufacturing processes being developed are laser-based or electron beam-based, and include techniques known as ‘Selective Laser Sintering’ (SLS), ‘Selective Laser Melting’ (SLM), ‘Direct Metal Laser Sintering’ (DMLS), and ‘Electron Beam Melting’ (EBM).
In terms of using titanium in 3D printing, EBM appears to offer the greatest benefits. In contrast to laser-based methods, with EBM the part is constructed under vacuum. This makes it particularly suitable for use with titanium, since the metal has a high affinity for oxygen, which may lead to unfavourable characteristics of the end product. Moreover, titanium powder is highly combustible, and the inert environment helps to significantly reduce the risk of explosion / fire.
The EBM process typically uses pre-alloyed material, and involves a higher energy density than with SLS, resulting in a higher build rate. High quality, dense parts without porosity can be produced.
Industry example: GE fuel injection nozzle
In aerospace, safety is of paramount importance. Consequently, when implementing new technologies, a conservative approach is needed.
When applied to the technologies discussed here, this means that we tend to go from non-load carrying elements to elements or structures that are statically-loaded, and from there to dynamically-loaded structures that may be fatigue critical.
General Electric has developed a 3D printed fuel injection nozzle for application in its LEAP engine. Nineteen of the nozzles are used in the combustion system of the engine.
Benefits of the 3D printed parts include:
- About 25% weight reduction compared to its predecessor part while still being stronger.
- A much simpler design, reducing the number of parts from 18 to 1.
- New design features, allowing more advanced cooling pathways and other improvements.
- An estimated 5x improvement in durability.
When using 3D printing with titanium instead of traditional manufacturing techniques, there are a number of things to consider.
In terms of product quality and testing, data regarding the mechanical properties of 3D printed titanium parts are not readily available, although not completely non-existent. In general, 3D manufactured parts cannot currently match the mechanical performance of conventionally manufactured counterparts, but this will change over time as development continues.
Titanium powder is flammable and sensitive to electrostatic charges, and can also be harmful if inhaled. As a result, precautionary measures must be in place when handling the material, including safe storage techniques, using anti-static handling measures, maintaining a safe perimeter around equipment to protect from heat- and spark-generating processes, and special fire extinguishing media.
Titanium is an expensive material in its own right; roughly speaking, it is around six times as expensive as steel. It is not traded on the London Metal Exchange, and there is a tendency for considerable price fluctuations, which results from the fact that over 40% of titanium is used in aerospace and is closely linked to the value of the aerospace industry as a whole.
Titanium powder is still very expensive, currently valued at around 600 USD per kg. Curiously, most titanium powder is produced within the US, while most 3D printing machines are produced in Europe.
Clearly, additive manufacturing technologies are still maturing, and as the technology develops, 3D printers will become more economical. In time, both quality – particularly process repeatability – and cost will improve to firmly challenge conventional manufacturing methods.
3D printing with titanium can yield a very high raw material to product conversion rate, and means that highly complex geometries, which may not otherwise be possible using traditional manufacturing techniques, can be produced relatively easily, without needing bespoke tooling. There is greater design flexibility with potential for weight savings, and the possibility for reducing complexity in the manufacture process by reducing the number of parts and associated joints.
Quality, or consistency thereof, is still an issue, and mechanical properties of the end products are still insufficiently known or characterised. The production rate is currently slow and the processes for certification and qualification are still in development. The costs of the material and manufacturing are still very high. However, the technology is improving rapidly, the available data is increasing quickly, and costs will continue to decrease and permit wider adoption.