DED offers significant advantages over conventional repair processes in aerospace applications.
Engineering is all about balancing trade-offs: strength versus weight, cost versus quality, and repairing versus remanufacturing. In the aerospace industry, deciding whether to repair or replace a part is not only a question of cost, but also one of safety.
It’s not enough for repairing a vital component to be less expensive than manufacturing a new one—the repaired component must also demonstrate performance that’s up to the rigorous standards of one of the most highly regulated industries there is.
For this reason, aerospace engineers may be understandably wary of employing new technologies, such as additive manufacturing (AM) in repair applications, but there is one 3D printing technology that shows particular promise when it comes to repairing aerospace components.
What is directed energy deposition (DED)?
Directed energy deposition (DED) is one of the seven process categories of additive manufacturing according to ISO/ASTM 52900:2021. In contrast to powder bed fusion (PBF), in which a laser or electron beam selectively fuses regions within a powder bed, DED deposits molten material from a powder or wire feedstock in conjunction with the application of the laser or electron beam used to melt it.
While DED has the advantage of being able to produce fully dense parts with highly controllable microstructural features, especially compared with powder bed fusion, DED also has relatively poor resolution and surface finish compared to PBF. As a result, DED is often combined with machining processes to achieve tighter tolerances and better surface finishes. As with additive manufacturing processes more generally, DED faces trade-offs between faster build times through higher deposition rates and lower resolution.
How does DED enable aerospace repairs?
By depositing new material directly onto the surface of a damaged part, DED enables engineers to repair components that would be prohibitively costly to replace. Many aerospace components – for example, those found in engines and made from expensive, high-strength alloys such as Ti6Al4V or Inconel – fit this description.
The repair process using DED can be broken down into ten steps:
- 3D scanning the damaged part
- Comparing the part’s nominal and scanned geometry
- Evaluating and preparing the damaged surface
- Material characterization and process parameter optimization
- Tool path definition via CAM software
- Repair via DED
- Machining
- 3D scanning the repaired part
- Comparing the part’s nominal and scanned geometry
- Repeat steps 6-9 as needed
Why should you use DED to repair aerospace components?
While there are similar but simpler methods for repairing damaged components, such as tungsten inert gas (TIG) welding, these introduce considerably more heat in the repaired component, leading to higher residual stresses and distortions. Other methods, such as plasma transferred arc welding (PTAW) and electron beam welding (EBW), introduce less heat but also require complex and expensive equipment to implement.
What makes DED well suited for repairs is the combination of lower heat (hence less distortion) and higher precision, combined with mechanical performance of repaired components that is comparable to bulk material in terms of yield strength and ultimate tensile strength, though elongation still tends to be lower.
What are some examples of using DED for repair in aerospace?
Jet engine manufacturer Rolls-Royce uses CMSX-4, a single-crystal nickel superalloy as the base material for its turbine blades. The company has reportedly explored using IN718-RAM3 (another nickel superalloy with a proprietary composite additive) with 3% reinforcement material with directed energy deposition to repair the blades rather than replacing them.
In 2014, engineers at Purdue University used an Optomec LENS 750 to repair a damaged turbine blade made from 316L stainless steel. According to the researchers, the accuracy of the repaired blade was within 0.03mm of nominal geometry and tensile tests comparing undamaged and repaired samples showed 793 MPa and 815 MPa, respectively.
More recently, researchers at Tokyo University of Science have developed a numerical method for DED simulation that automatically generates metal powder deposition elements and predicts processing conditions, temperature distribution, deformation state and residual stress distribution. Their findings showed that residual stresses in deposited layers of repair parts were significantly lower than those resulting from more conventional repair processes.
