Every year, aircraft turbofan technology propels billions of people into the sky. To allow engines to remain safely secured to aircraft wings for several years, the material properties required in turbofan engines are, naturally, exacting. Engine parts must withstand extreme conditions (stress, corrosion and heat) but still guarantee predictable behavior even in extraordinary situations.
Fuel efficiency can be improved by getting rid of excessive weight and improving thermodynamics. Unsurprisingly, this is a key driver of the aerospace industry’s pursuit of lightweight parts (via advanced composite structures) in aircraft and engine construction. Fuel efficiency leads to great savings and, nowadays, there is growing interest in additive layer manufacturing (ALM) – especially metal ALM for advanced applications.
It’s like night and day when metal additive manufacturing (DMLS or SLM and EBM) are compared to the one-off CNC machining of systems assembled from numerous complex parts, made of expensive metals (super-alloys or titanium and aluminum alloys). Machining requires specific tooling, fixtures and the purchase of large amounts of material that will just end up as chips.
There are several benefits to the high geometrical freedom of ALM:
- lightweight design that saves expensive metal and reduces exploitation costs
- design consolidation
- performance improvement or integration of new functions.
When applied appropriately to the right component and material, metal additive manufacturing becomes more viable than traditional manufacturing, even though it is usually more costly.
Although still in its infancy, metal ALM is sometimes perceived as a relevant alternative for high value/low volume engine components, typically made of high grade metals.
Inconel 718 and Titanium 64: two highly important metals for turbofan engines design
The exceptional combinations of the material properties of Titanium 64 and Inconel 718 naturally make them the workhorses of titanium and super-alloys industries. Over the years, they have become crucial for the aircraft industry, both materials now accounting up to 60-80% of the weight of an aircraft engine. These high-grade metals are employed within the main three sections of any turbofan engine: the compressor, the combustor, and the turbine.
Before the air flow enters the combustor, it is pressurized in the compressor, where the temperature is moderately low. The blades and other elements of the compressor are predominantly made of Titanium 64.
The combustor components, in which the air is mixed with fuel, ignited, and burnt, are typically made of a nickel super-alloy such as an Inconel (625, 718, etc…) for its heat and corrosion resistance.
The third section, the turbine, is where the strides in materials and processing are best rewarded. When higher combustion temperatures can be tolerated, the Carnot efficiency increases. A higher efficiency means more mechanical work extracted by turbine blades from the hot and high-pressure gases exiting the combustion chamber.
Titanium 64, a premium choice for light structural components undergoing fatigue or corrosion
Titanium 64 (also known as Ti64, TA6V, Ti6Al4V or Grade 5) has been used for 50 years in the advanced aerospace applications. It weighs roughly half as much as steel parts, but is far stronger.
Thanks to the extremely high strength-to-weight ratio and resistances to fatigue, corrosion, creep, and fracture (at temperatures up to 380°C), Titanium 64 can make up a third of the dry weight of a jet engine. It is used for load-bearing and even fatigue/fracture-critical components (front bearing with airfoil-shaped vanes, compressor blades, some turbine components, airframes, fasteners, and discs).
Inconel 718, a keystone of hot structural aerospace applications
Inconel 718 is considered to be refractory and is able to be permanently used above 600°C. This super-alloy has an exceptionally high yield, tensile and creep-rupture properties at moderately high temperature, along with a good resistance to fatigue and corrosion.
As an iron-nickel superalloy, “718” was designed with a high amount of iron to overcome the general low weldability of superalloys and particularly their susceptibility to cracks in the heat-affected zone of welds. More iron also makes the alloy more affordable, which is another reason for its wide adoption.
Since Inconel 718 can withstand severe mechanical stresses and strains in extreme environments, it serves for discs, blades and casings of the high pressure section of the compressor and discs as well as some blades of the turbine section.
However, when creep resistance is crucial, applications of Inconel 718 are restricted below 650°C.
The flipside of excellent mechanical properties is tough forming and material removal
Traditional methods present some limitations when applied to super-alloys or other high-strength metal alloys. Additional processing steps are often required to ease manufacturing. The material is preliminary annealed to induce ductility and better deformability, leaving only the finishing steps in a hardened condition. Heat treatments are often specified to recover or induce optimal property level after processing.
Hazardous and expensive machining: high forming loads, tool wear and material wastes
The machining of materials with low thermal conductivity and high strength is difficult, even more when complex designs are desired. The cost rise can keep back machining.
The deformation mechanism during machining is complex and causes basic interdependent problems such as high stress on cutting tool, high temperature and high wear rates, chipping, notching and sometimes cutting edge breakage.
When an important volume of Titanium 64 must be machined away, it becomes expensive and time-consuming (low cutting speed and feed rate, large quantities of cutting fluid, sharp tools and a rigid setup).
Inconel 718, for convenience, may be machined annealed, in a softer state and at temperatures exceeding 540˚C to reduce the high forming loads. The work hardening that occurs during the first machining pass tends to plastically deform during subsequent passes, either the workpiece or the tool. Therefore, a slow but aggressive cut is used with a hard tool to minimize the number of passes required.
Superalloys, as heat resistant metals are obviously more difficult to work at high temperatures
Metals can be processed at higher temperature, in a softer state. Plastic deformation at temperatures sufficiently high for recovery and recrystallization to counteract work-hardening is called “hot working”. While not surprising because they were invented to resist deformation at elevated temperatures, the high mechanical and heat-resistant Inconel 718 and other superalloys limit the aptitude for hot work.
Moreover, the continuous improvements in service capabilities specified by aerospace applications tend to result in new superalloys with alloying additions and material properties that decrease the workability and narrow the hot-working temperature range.
Inconel 718 also has a limited aptitude for investment casting
To bypass the difficult solid-state forming, hard and heat resistant materials can be cast. Investment casting of titanium alloys is increasingly recognized as an affordable solution capable of meeting the stringent requirements for structural airframe components.
Compared to forging, investment casting allows the efficient use of expensive materials and produce complex geometries cheaply.
However, it can be disadvantageous for Inconel 718 due to some susceptibility of metallurgical defects (porosity, segregation and very coarse grains).
When casting metal alloys, the element with the highest melting point will start to solidify first. As the casting cools from the surface towards the center, concentration of the alloying elements in the grains will vary throughout the part and the material properties will not be uniform. Local segregation of Niobium produces isolated regions, which, after heat treatment, have a very high hardness. In case of machining for finishing, such regions favor an increased tool wear and occasionally, provoke a catastrophic tool failure. Because load bearing applications often require heat treatment, “hot isostatic pressing” or “HIP” is an additional step required after casting to reduce porosity and alloying elements segregation. It doubtlessly increases the cost of finished components.
ALM is a logical evolution of forging and investment casting
Difficult manufacturing is an opportunity for ALM. Parts produced by investment casting typically have inferior properties than those from wrought material. However, casting has brought the ability to produce very complex shapes easily, which is what ALM now offers with extended freedom.
By discarding investment casting for metal ALM, better mechanical properties can be realized without sacrificing the ability to design complex and freeform shapes. Metal ALM processes, powder-bed or powder-blown, elaborate the material and the part in a parallel process, without any pre-processing.
Printed Inconel 718 parts exhibit directional solidification and a fine microstructure. While anisotropy is not desired, mechanical properties have been shown to equate or exceed those of cast and forging after heat treatment.
In metal ALM, the energy beam melts a very small amount of metal at a time. Despite the rapid solidification, some alloying elements’ segregation occurs but on a much smaller scale than casting. Chemical composition is more uniform throughout the part and material properties exceed those of casting and approach forged counterparts.
…and is way more material efficient than machining
Despite that, all metals in powder form are more expensive than their bulk counterpart, ALM produces few material waste. Coupled to the light-weight design freedom, ALM reduces the amount of needed material and the buy-to-fly ratio of components. For expensive materials such as titanium and super-alloys, material savings push ALM to be an economically favorable technique.
Welding is a critical step for aircraft parts and fewer welds means “reliability”!
Metal elements are routinely joined by several fusion welding techniques. To join high-grade metals, one must emphasize on the exclusion of detrimental interstitial elements and contaminants from the joint region and avoid prejudicial phase changes in order to maintain the joint ductility. In fatigue-critical aerospace applications, welds are generally stress-relieved. Therefore, it is not trivial to joining highly functional parts made of high-grade metals while preserving material properties.
For aerospace there is a real advantage in additive manufacturing in the ability to consolidate design. Reducing the number of subcomponents and the need for joining, brazing, welding and assembling not only frees more capital and time for quality assurance but also offers mechanical systems a gain in reliability.
ALM empowers heat treatment of Titanium 64
Inconel 718 and Titanium 64 can be heat treated for different purposes, such as ”relieving” residual stresses developed during fabrication or “annealing” the material and produce an optimum combination of ductility, machinability or stability. Likewise, heat-treatments often serve to “age” the material in order to increase strength or optimize special properties such as fracture toughness, fatigue or creep strengths.
With Titanium 64 parts, the microstructural configuration doesn’t only depend of the deformation rate during processing or temperature/duration of solution heat treatment, but also of the section thickness. It becomes a concern if age hardening is foreseen because the maximum realizable strength depends on the quench’s effectiveness.
Ideally, alloys should have a good quenchability, to reach in fairly thick sections, the minimum cooling rates that will give rise to a fine microstructure. Titanium 64 doesn’t! It is effectively hardened only in sections thinner than 25 mm. Center regions in thicker quenched sections won’t achieve the desired microstructure and will fail to reach optimum property levels after aging.
ALM offers aerospace engineers unrivalled freedom to deal with complexity and unlocking their creativity to leverage the high strength-to-weight ratio of Titanium 64. They can push light-weighting to the limits with free-form, topological optimization or complex and intricate geometries while allowing parts to comply better with heat treatment requirements.
Not all turbine blades can be additively manufactured
ALM will go far, but there are limitations. Creep-resistant turbine blades are typically fabricated by complex investment casting procedures that allow the introduction of elaborate cooling schemes to control the grain structure. Blades may contain grains randomly-oriented (equiaxed), directed and aligned with the effort (columnar) or they may be cast as single crystals, completely eliminating grains boundaries (sites for damage accumulation).
The blades in the early stages of the turbine (higher temperatures) are usually single crystals super-alloys, whereas the blades in the later stages of the turbine contain equiaxed grains.
For cooling, turbine blades contain complex ventilation ducts that have to be drilled or casted. These methods are now reaching their limits and turbine blades could be cooled better with superior duct geometries. Reducing the amount of cooling air that turbines need would improve efficiency.
However, turbine blades must withstand extreme conditions. At high rotational speeds, the tips of the blades move so fast that they endure centrifugal forces comparable to the weight of 20 cars. Metal ALM parts are still not creep-resistant enough to withstand such conditions.
Additive processes are versatile, and thus, hard to qualify
The real challenge is to certify the powder, the process and the part so that every single component performs as required in the engine.
ALM is a truly relevant technique to shape high-grade metals into complex components but it is not suitable for all aircraft engines applications. The technique will most likely supplement traditional ones and provide an economical solution in some niche aerospace applications, for products with unusual shapes that must be produced in small batches or with shorter lead times.