Dr Neil Calder examines the growing trend for the application of metal additive manufacturing techniques used in the manufacture of aero engine components.
We have all now become quite familiar with seeing the almost artistic freedoms which are afforded by additive manufacturing (AM) processes, and the terminology of ‘3D printing’ has even entered the public vocabulary.
However, the story so far in its uptake within the manufacturing chain for aero engine components has significantly more complexity, like many of the showpiece AM parts which grace press releases and exhibition stands. The niche aspect of additive manufacturing is changing to be more of an accepted portfolio of advanced manufacturing processes. It is appropriate to look beneath the surface, though, in both technical and business contexts at what is increasingly referred to simply as ‘additive’.
It is obvious that there is increasing activity from the main Western aero engine manufacturers in this area, although only a fraction of this is in the public domain. The number of flight-certified pathfinder components is increasing as aero engine primes are buying their way into additive manufacturing capability through partnerships with companies offering equipment, manufacturing capacity or both. Many strategic alignments are forming, between engine companies and makers of additive manufacturing equipment or capabilities to augment internal development and engineering effort. The acceleration of effort in AM appears more rapid than the uptake of fibre-reinforced polymer composite technology in previous decades because the business numbers suddenly make sense and to a certain degree the substitution of existing manufacturing processes can be achieved relatively smoothly.
Beat the heat
Some of the specific materials involved in aero engine manufacturing with high temperature requirements, lend themselves particularly to the solidification metallurgy which is a major constituent part within metal additive manufacturing.
The jury is still out regarding the selection of specific additive manufacturing processes, but for production of metallic aero engine parts there is visible activity in the powder bed processes of direct metal laser sintering (DMLS) and electron beam melting, and in the less size constrained processes of blown powder laser deposition and filler wire-based electric arc deposition methods. The diverse characteristics of each process mean that there is no single panacea. The powder bed processes are characterised by fine detail and the ability to create very complex geometries, although constrained by size and build speed. Added powder and wire processes offer relatively unlimited build volumes and much higher deposition rates, plus the ability to incorporate part-made workpieces within the build strategy.
Some $1.5 billion of investment in GE’s advanced manufacturing capability has included the acquisition of significant majority stakeholdings by GE Additive in AM electron beam melting machine manufacturer Arcam and in Concept Laser with direct metal laser sintering capability. Exemplar parts that have been widely communicated are the T25 sensor housing for the GE90 engine and the LEAP fuel nozzle which are the result of a decade of research. Both of these parts have progressed to series production with a prediction of some 100,000 additively manufactured production parts required by the end of this decade. Each LEAP engine has 19 of the fuel nozzles, which could not be manufactured using conventional processes.
“Additive manufacturing is the new revolution, changing the way we design and manufacture products faster, more sophisticated and more cost efficient,” explains Mohammad Ehteshami, vice-president for additive integration at GE Additive.
Right on the beam
Capability development by Safran is particularly noticeable through the partnerships it has formed, with blown powder laser deposition company, BeAM and more recently with Prodways Group.
The BeAM production equipment marks a step change in thermodynamic and powder efficiencies, by backing off on the powder delivery rate. Development partners in this have included the French laser research institute IREPA.
“We have proved the feasibility during the test series since 2014, and we are now industrialising the process,” states Thierry Thomas, Safran’s additive manufacturing vice-president. “Additive manufacturing is the future of the manufacturing industry, and that is why the Safran group would like to be a leading actor.”
Rolls-Royce has been working behind closed doors within the Manufacturing Technology Centre (MTC) near Coventry on the XWB 97K front bearing housing vanes manufactured using electron beam melting. After tens of thousands of hours and over four years making hundreds of development parts, the company considers that it is the world’s most experienced user of electron beam melting machines.
With each layer of deposition there is a fresh opportunity to get the process wrong. Richard Mellor, chief of manufacturing engineering for the Rolls-Royce Additive Layer Centre of Competence, refers to something like 400 parameters defining thermodynamics and motion which affect the result of laser based AM processes and of which more than 100 are significant. There is a myriad of different ways to produce the same material which means adopting a different way of thinking when it comes to certification. This will force a much tighter relationship between the elements of material, manufacturing, and design both inside the prime customer and the supply chain. He points to industrialisation as the remaining challenge for additive manufacturing capability.
“Everybody gets excited about additive,” Mellor continues, “because there is a perception that you just press the button and get a nice part, but it needs finishing, powder removal, NDT, finish machining, metrology. We were doing iterations on a weekly basis and being a challenging customer for equipment capability suppliers was all part of the development process.”
Technical constraints still remain in the ability to generate finished workpiece material of a known type, specification, and quality. The rigorous classification and understanding of defect types has yet to be matured to the point where the production capability could be reasonably termed robust. Multiscale modelling and the ability to truly understand the statistical distribution of defects, such as inclusions, crystalline dislocations, voids, porosity etc. will pave the way for reduction in the massive non-recurring engineering effort required for all the present production aero engine parts.
Additively manufactured components have become the must-have in any credible demonstration of engineering and structural manufacturing capability. Although it is clear that exemplar parts, many of which are now becoming flight certified, represent the tip of an iceberg of material, process, and design engineering work, the way in which the pioneer companies will make the transition to large-scale series production using these techniques is not yet quite so evident.
Once the process for a particular part has been certified, though, as with the GE fuel nozzle, the journey to series production can be as simple as hitting ‘print’ many times. The supply chain for metallic aero engine parts should take careful note of this and prepare for the inevitable demand for business and human capacity scale up.