FEATURE: Additive manufacturing goes large – Overcoming turbulence in AM application

Researchers using additive manufacturing for aerospace applications face several key challenges, which the Open Architecture Additive Manufacturing project is aiming to overcome. David Steer from Airbus explains how.

Read part one, "Additive manufacturing goes large – Aerospace prints bigger and better parts".

Residual stress/ distortion

In order to manage residual stresses, which build up during material deposition, Airbus has developed a combination of subtle geometrical adaptations, build strategy, use of inter-pass cold working, and rigid tooling to maintain geometry during stress-relieving heat treatment. Airbus is confident of achieving near-net-shape and low residual stress even for demanding component geometries. 

Lack of fusion/ complex intersection quality

Early applications included solids of revolution and relatively simple walls with few challenging discontinuities in deposition. As more complex geometries have been attacked, it has been necessary to establish deposition techniques to guarantee good fusion at intersections while preserving proximity to final form without over-building and deposition rate. Development by the partners that have now come together in OAAM has established effective techniques to achieve this.

Material morphology/ properties

Many AM processes are today focused on improving the utilisation of Ti6Al4V alloy which is the titanium ‘workhorse’ of aerospace. It is frequently the case that a grain structure with persistent vertical boundaries (prior beta columnar grains) results from AM processes. Once established, these are impossible to remove by heat treatment and risk anisotropy and preferential cracking directions. Although some combinations of process parameters can theoretically avoid the creation of this grain structure, few if any have achieved this economically. However, the use of inter-pass cold working achieves grain refinement in each thermal cycle, resulting in an equiaxed grain structure with isotropic properties. The partners in OAAM will optimise the rolling and process parameters to exceed the static, fatigue and damage-tolerant properties of even beta-annealed forged Ti6Al4V.

NDT

To exploit the near-net-shape capabilities of DED AM for real component geometries, it will often be necessary to deposit geometries where the top surface is not parallel to the base plane. To conduct ultrasonic non-destructive inspection of such geometries, it is likely that phased-array techniques such as total focusing method and full matrix capture together with advanced filtering will be needed. The partners are also addressing this challenge, pushing forward advanced NDT techniques.

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