How to print a Jet Engine
3D printing, or the printing of materials layer-by-layer to form a 3D shape, has emerged as a leading technology in the manufacturing industry and is destined to revolutionise the way that jet engines are designed and manufactured...
Building an aircraft and its engines is a conservative business. Components have to be manufactured with extreme precision and are usually made from custom-made materials, including specialist alloys. In the case of the components experiencing the hottest temperatures, a class of materials known as superalloys is used. Superalloys are highly-engineered materials able to withstand the severe conditions inside the jet engine, where the passing gas stream routinely reaches 1500°C and the physical loads are equivalent to hanging a London bus off of each of the jet engine's turbine blades. As well as being super strong and resistant to high temperatures, jet engine materials also have to be chemically durable (as highlighted by Cathie Rae and Bill Clegg), and they need to be deliverable at an acceptable manufacturing and operating cost.
In this article, materials scientists Robert Krakow and Robbie Bennett explain how 3D printing, or additive manufacturing as it is also known, is revolutionising the way engineers approach designing and making components, and how this may be used to reduce the costs of jet engine construction, operation and, ultimately, flights.
From Small Print to Capital Letters
Since the 1980s engineers have used additive manufacturing primarily for prototyping, a method in which one-off parts are built based on new designs, or new materials, in order just to validate them. The advantage here is the short time it takes to produce a first component without the need for creating moulds, dies or extra tools. This means that the turnaround of the design process is much quicker and engineers can try out several designs first before they commit to a single one. 3D printing also fosters the digitisation of manufacturing processes.
The first industrial applications of 3D printing were repair processes. Rather like repairing a stone-chip in a windscreen, the advent of additive manufacturing techniques meant that new material could be added back to fix worn components and extend their time in service. This is now state of the art and used on a daily basis.
The 3D-printing of entire components de novo, on the other hand, offers a number of even larger advantages, although the process itself is more challenging. The first advantage is related to the amount of input material. Because parts are made close to their final shapes, less material is needed for the overall process. This means that material costs - and material waste - goes down. And when engineers are dealing with rare alloy ingredients like rhenium that are only one third as abundant as gold, minimising the materials you use can literally save a fortune. The second main advantage is the versatility of the technique, which allows a wide variety of shapes and forms to be manufactured. Often, this means that engineers can simplify a component by producing a single structure at once, rather than first making several separate parts that then need to be assembled by welding or bolting them together.
One disadvantage of current additive manufacturing techniques is that they are slow. Building components layer-by-layer takes considerably longer than casting and forging. Similarly, the usual economies of scale do not apply to currently available 3D printers as they can produce only one item with essentially the same material. This makes 3D printing - in its current form - unsuitable for mass production. Also, ensuring that components are manufactured in a reproducible way, which is key for component safety and impacts life cycle management, is also a challenge. A critical question from a materials science viewpoint, though, is whether the parts made by additive manufacturing are of the same metallurgical qualities and capabilities as their traditionally forged or cast counterparts. Nevertheless, many companies have been quick to spot the opportunities offered by this new technology and capital investment in 3D printing in Europe and the USA continues to rise year-on-year.
Pen, or Pencil?
Just like the choice of writing tool, there are a wide variety of 3D printing techniques. Perhaps the most popular of these involves selectively melting metal powder particles using either a laser or an electron beam. After an initial layer of powder is spread onto the build platform, a cross-section of the part is melted with surgical precision by the laser or e-beam, fusing the new material in place. A fresh layer of powder is then applied and the process is repeated so that a component can be built up.
In some instances, a complex geometry is required only for a small region of a larger component, so the bulk of that part can be produced by cheaper methods. An alternative is to deposit the powder where it will be melted using direct energy deposition (DED), using either a laser to produce a net shape (LENS) or an electron beam (EBAM). These DED techniques are used extensively in metal additive manufacturing, largely for repair of existing components. Components produced in this manner are proven to reduce material waste, fuel emissions and manufacturing time, making them ideal for use in aerospace applications.
Recently, Rolls-Royce successfully tested the largest 3d-printed aerospace component yet made to power an aircraft. The tractor tyre-sized component (shown above and, in cut-away, below) is 1.5m in diameter and is 0.5m thick. It was 3d-printed from titanium using electron beam melting technology and forms the front bearing housing for the XWB-97 engine, although the 3d-printed parts have not been included in the production version of the engine yet. Use of 3d-printing techniques, Rolls-Royce say, can help to cut development and production times by 30%. Meanwhile, GE, who also operate in this space, unveiled plans in mid 2017 for what the company says will be the world's largest laser-powered 3d-printer, which will use precision lasers to fuse metal powders to form parts up to a cubic metre in volume. According to GE Additive vice president Mohammad Ehteshami, "It will print aviation parts suitable for making jet engine structural components and parts for single-aisle aircraft." Highlighting the wider manufacturing appeal of the process, he also pointed out that "It will also be applicable for manufacturers in the automotive, power, and oil and gas industries.”
Overall, the future promises to be very exciting for additive manufacturing as engineers begin to discover the full potential the technology has to offer. For instance, since the material is added as a component is being built, the material properties, microstructure, texture and composition can theoretically be tweaked and altered across the component to endow specific areas with special qualities. Such changes would be wasteful, costly or deleterious to performance if they were applied throughout a component, but, in the right places, the effects could be dramatic. So, while we don't know yet exactly what 3d-printing will enable us to achieve, what we do know is that it will be transformative...