Tuesday, 02 September 2014
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Your questions answered: additive manufacturing

Will we ever be able to 3D print an aircraft wing? Our panel of experts answers your questions on what additive manufacturing can and can’t do.

Additive manufacturing (AM) or “3D printing” (3DP) is now frequently referred to as a revolutionary technology that is changing the way products are designed and made. But what exactly is AM capable of beyond producing plastic models and bespoke dental implants?

For the latest in our reader Q&A series, we put your questions to a panel of the UK’s leading experts on AM:

  • Richard Hague, professor of innovative manufacturing at the University of Nottingham and director of the EPSRC Centre for Additive Manufacturing;
  • Stephen Morgan, group leader in materials engineering at BAE Systems Advanced Technology Centre, & Matthew StevensBAE’s team leader for additive manufacturing engineering capability development.
  • Geoff McFarland, group engineering director of Renishaw, producer of additively manufactured medical implants and the UK’s only manufacturer of 3D metal printers;
  • Phil Reeves, managing director of Econolyst, a UK consultancy specialising in additive manufacturing.

What are the key differences between the various types of AM process and what are the benefits/limits of each one?

Geoff McFarland: Fundamentally there are two key types of process: deposition (nozzle-based systems) and powder-bed (electron-beam/laser systems), although there are now emerging hybrid systems that combine deposition and traditional subtractive processes. Powder-bed systems are suitable for a much wider range of applications due to their ability to handle reactive metals (size of chamber allows the creation of a vacuum environment), their ability to create finer detail parts and the ability to create supports during the build process. The main advantage of deposition processes is generally speed and their ability to be combined with subtractive machining to create hybrid machines.

Richard Hague: I think that there should be a distinction made between low-end processes for 3DP applications and systems used by industry for AM. The 3DP based systems are aimed at more of a home/maker/prototyping market and are overwhelmingly dominated by the FDM (material extrusion) type processes with a price-point in the low thousands of pounds. In contrast, the AM systems are predominated by the powder-bed fusion approaches, with systems costing several hundreds of thousands of pounds. It is these processes that give the most design freedoms and are therefore the most interesting currently. However, moving into the next decade or so, I believe that the jetting-based systems will come to the fore as they offer the potential for adding in increasing functionality into a component/product and are more scalable that the powder-bed approaches.

Phil Reeves: The key differentiator is really materials. Some machines process polymers, some metals, other ceramics and some print in organic materials such as waxes, which can be used for investment casting. Of the polymer processes, there are really two different sub-groups. Thermosetting plastics and thermoplastics. The thermosetting plastics tend to be based around UV curable resins, which result in parts made of epoxy. The thermoplastics then split down into two groups – typically amorphous polymers such as ABS which can be extruded and semi-crystalline polymers which are typically melted using a laser beam.

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Source: Econolyst

3D printing differs from additive manufacturing in techniques, materials and uses.

How would you characterise the current state-of-the-art in AM in terms of the quality and complexity of structures and the finish it can produce?

PR: The current state-of-the-art in polymers is still the Stratasys Connex technology, where two different materials are being jetted from a print-head. When these materials combine into a matrix they produce a third material, with improved mechanical properties. Stratasys is using this approach to produce polymers that are actually suitable for making injection mold tools – it sounds illogical making plastic molding tools – but it seems to work.

GM: Focusing on metal systems, complexity is not currently an issue: we are seeing highly complex parts produced for the medical, aerospace and automotive sectors with hidden features, which were simply not possible previously. However, we see a clear need to improve part integrity and geometry, which is being addressed. Whilst part finish is improving, it is still nowhere near to that of subtractive systems and there remains a practical trade-off between layer thickness and process speed to produce cost-effective parts.

RH: One of the difficulties in AM/3DP is the misconception that it is just one thing/process, which is clearly not the case. However, taking the view that “Additive Manufacturing” is currently dominated by the powder bed fusion techniques, it is clear that these systems (for both for metals and polymers) are capable of making significantly more complicated geometries than any other manufacturing technique in a single operation. With the tantalising prospect of moving away from single material to multimaterial, multifunctional AM, then the potential comes to print functionality into parts and produce working systems in one build operation (although this is still a focus of research and not industrial reality at this point).

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What means of optimising the microstructure of additive manufactured metal parts are available now or on the horizon?

Stephen Morgan: We have demonstrated techniques to control the microstructure and are now exploring optimisation and developing the process for production. We have developed and patented a technique to control the development of residual stress and resultant component distortion. The technique involves an established way of making metal parts stronger by rapidly and repeatedly striking them using an ultrasonic tool – a form of peening – applied between deposited layers to relieve stress and refine the component microstructure. 

RH: With the ability to control laser speed, laser power, bed-temperatures, etc, there is some scope in tuning the microstructure of the manufactured part during the production stage. However, it is my view that – in common with many other manufacturing techniques – that metal AM will be reliant, for some time, on post-manufacture heat treatment of parts in order to create the desired microstructure as opposed to creating this in process. 

GM: We are looking closely at what is happening with grain boundary size. With castings, hot isostatic pressing (HIPping) is employed to improve densification, whilst the forging process gives a fine microstructure with high strength and fatigue resistance. Powder bed AM systems are somewhere between the two in performance and in addition to HIPping we are looking at techniques such as bead blasting and shot peening to add compressive stresses.

What are the latest developments in the materials that can now be used in AM? What can be done in multi-material printing?

PR: At present, multi-material printing is limited to dissimilar polymers with different shore harnesses of colors or levels of transparency. In the future it is hoped that we will be able to print with different conductive and insulative materials, but this remains a laboratory exercise at this point in time.

RH: There is very little multi-material printing going on at the moment with commercial processes. Systems that do purport to have multi-material capability, such as the Stratasys Connex system (which is an excellent prototyping platform), are not really multi-material at all (in the case of the Connex, it is simply another photoloymer of similar chemistry, but with different hardness or colour). True multi-material AM is someway off – but is the subject of much industrial interest and current research. We are investigating the contemporaneous deposition of dissimilar materials (both structural and functional) to produce complex electronic, optical and bio-pharmaceautical devices, in one build operation, based on the additive, layer-by-layer principle.

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Source: Econolyst

Direct metal deposition resembles 3D printing but can produce complex metallic structures.

How are alloy metals currently being used in the development of AM techniques and what alloys do you think could be explored in the future?

GM: As well as well proven materials such as aluminium, aluminium alloys and titanium alloy (Ti64), at Renishaw we are also researching very hard materials with partners, such as tungsten carbide and other carbides, which have double the melting point of titanium. Powder bed systems such as ours also allow the creation of new alloys by combining materials during the build process.

The focus for many companies and research establishments is on alloys that can achieve high hardness, toughness, strength, conductivity and temperature. For example, the AMAZE project, in which we are a partner along with the European Space Agency and Airbus, has already showcased tungsten alloy components capable of withstanding temperatures of 3,000ºC, paving the way for commercial use in nuclear fusion reactors and rocket nozzles.

What are the key limitations of AM that are set to remain for the near future?

GM: Whilst AM has significant strengths, there are key areas for improvement including often mentioned issues such as surface finish, process speed and the size of parts that can be produced. However, perhaps as important are the more general infrastructural issues, such as skills and supply chain.

RH: Key current limitations are repeatability, rate of production and cost of produced parts (which is a function of the speed of the machine, the cost of the processes and materials). These are all generally solvable, but some expertise in machine tools is required to take the architectures of the existing AM platforms (which are still predominantly based on the early rapid prototyping platforms) and change them into true manufacturing systems.

PR: Economics is always going to be an issue – irrespective of how low cost the materials drop, we are still using expensive machine tools that have very low productivity. This means the piece part price will always be high. We could reduce the machine cost, but that would be economically detrimental to the vendors. What we need are much higher levels of productivity / throughput – but it is hard to see this happening quickly, as there has only been steady increases in productivity in the past – not step changes.

Matthew Stevens: Education in terms of the extra design freedom, but also what the new limitations are compared to conventional manufacture. To support this new Computer Aided Design toolsets need to be developed to enable full optimisation of the freeform design capability of AM. To compete with conventional mass production processes the speed will need to be increased and surface quality improved. Qualification of materials and verification of the AM processes will be required to demonstrate production consistency. There are currently only a small number of AM companies worldwide, with a limited production capacity.

Do you think AM will ever be able to produce complete large structures e.g. an aircraft wing, and if so what do we need to make this happen?

RH: In theory this would be technically possible but in reality I don’t see this happening for a complete wing for large passenger aircraft. For smaller, drone-type aircraft, I can see some potential. The same would apply to printed buildings: you could probably do it, but why would you want to? The finish of the building would be horrific and the ability to add multiple materials would make the machine tool prohibitively expensive.

MS: Yes I believe this is possible. It would require a different design philosophy to suit the AM process and AM machines with significantly improved capacity and capability, that are able to concurrently build, refine, quality control and surface finish.

PR: I think we are too hung up on AM being a replacement technology for existing products and processes. If AM is going to have value in aerospace it will be for smaller high buy-to-fly ratio parts. If it were ever to be used for a whole wing, then that wing would need to be fundamentally redesigned around AM – e.g. much smaller, modular parts.

GM: There would be a huge up-front cost to purchase raw materials, and the question has to be asked why? This supposes that AM is an ‘all or nothing’ technique. We clearly see AM complementing subtractive machining processes and it will be used where it has a clear design and cost advantage over other techniques. A good example of this is its use within mould tool production to produce conformal cooling channels for complex tools.

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Source: Renishaw

Renishaw is the only UK company manufacturing metal additive manufacturing machines.

As 3D printing improves, to what degree do you think it will change the mainstream manufacturing process beyond designing and prototyping? Where will we see the biggest impact?

PR: AM is just another tool in an engineer’s tool box. It is not the catalyst for an industrial revolution; it is simply something to support technology evolution. I expect as we see costs reduce and productivity increase so more applications in the medical, consumer goods, electronics, fashion, footwear and sports will come to light.

SM: The emergence and growth of “just-in-time manufacturing” could reveal the biggest impact. No longer will manufacturers need to keep vast stockpiles of component parts – they will be able to “print” the part they need at the exact time they need it. This could fundamentally change manufacturing, shrinking the number of and size of manufacturing facilities and reducing the logistics of transporting parts cross country to other facilities. Instead they could be “printed” on site or even in country, with the plans and schematics remaining in the native country and then being printed in the customer country. 

RH: It will not take over from conventional manufacturing, but will become a tool to be used when appropriate. I do however believe that over the coming years, AM will become a “general purpose” approach that will have significant relevance over a wide range of sectors (bio, pharma, consumer, auto, aero, medical, etc) and for a range of products. I think that the exciting things will be the products that we didn’t know we could produce yet, but that are enabled by AM. Customised products are an obvious key area.

GM: The areas that will play to AM’s strengths include the ability to consolidate assembly parts into one build, to customise parts economically (’mass customisation’), to produce lighter parts, and to produce highly complex geometry with hidden features such as battery fuel cells and heat exchangers.

What are the biggest myths about AM? 

PR: That it is new – it’s over a quarter of a century old. That it is the catalyst for an industrial revolution – it’s an evolutionary tool. That you can print anything – all AM processes have their design limitations. That we will all have a printer at home – very unlikely (Amazon works remarkably well at getting product into the home).

GM: The myths that are most misleading are the idea that AM is a one-stop shop solution to the production of all metal parts, and the often-touted notion that you simply upload a 3D design file to the machine, walk away and then return to find a fully finished part waiting to be removed. The reality is that there is significant knowledge required to design the build process and that most AM parts require some form of post-processing once removed from the machine, whether the removal of support structures, or a polishing operation.

RH: That you can make anything on a filament extrusion-based £2000 system!


Readers' comments (4)

  • aircraft wing? Already been done. A drone of course still flying.

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  • It's several levels of complexity below a full-size aircraft wing, though.

  • I have been a Welding Engineer for 30 years and before specialising as a Welding Engineer I was a Metallurgist.

    Even before Cranfield University started working on making parts in weld metal (taken up by Rolls Royce as Shaped Metal Deposition) I was specifying welding to build up features on components as a Metallurgist and then as a Welding Engineer using weld metal to make legacy parts the same way Welding Engineers have for at least 60 years. Our efforts were adding material to repair parts or making replacement legacy parts in a wide range of materials - In my case ranging from C/Mn steels to precipitation hardening Nickel alloys.

    The real difference came with Cranfield's idea of making new parts taken up by Rolls Royce as Shaped Metal deposition

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  • How do you establish the relationship berween thermal dissipation of heat generated during the metlting process, and support structure to maintain the geometric features? How to improve the conductivity of the base material without influencing the final part fuctionality?

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  • Managing heat is a major issue when scaling up of some of the additive layer manufacturig processes (mainly based around wire and powder).
    I seem to remember that GE were, effectively, looking at some ways of doing this when scaling up the manufacture of single crystal castings - which entailed ways of keeping the temperature gradient (in the casting) constant. It would be easiest to do this, I think, for the wire based variant.

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