Lee, PD

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Q&APeter Lee answers questions about additivemanufacturingProfessor Peter D Lee is a materials scientist at the University College London. His group focuses on X-ray imaging andcomputational simulation of materials at a microstructural level for materials design and advanced manufacturing.Tell us a little bit about you and what sparked your interestin additive manufacturing.I came into additive manufacturing from the biomaterials side.About twenty-five years ago I worked with Larry Hench, theinventor of a bioactive material (specifically bioactive glass). Hewanted to structure it to allow bone ingrowth. We had the idea oflooking into additive manufacturing because it has the benefit ofcomplete design freedom. At the same time, I was approached bythe University of Liverpool in tandem with a private biomaterialscompany that wanted to modify their fabrication processes: theywere looking at additive manufacturing to provide joint repla-cements with open cell surface structures for mechanical boneinterlocking, and wanted to achieve more complex and largerinterfaces—essentially lattices on their implants. That’s whatreally took off! For these projects, I used my background inmodelling and in situ X-ray imaging for metrology and optimi-sation of component made by what was then a new additivemanufacturing process—laser powder bed fusion.While I have been doing modelling and imaging of flow inmetals as they solidify for thirty-five years, I have always beenaware of additive manufacturing because, at its core, it is awelding process: metal from a wire or a powder is added into abase plate, which is the definition of welding. What additivemanufacturing brings is essentially whole new scientific chal-lenges to welding. While welding has always been a fast process,the heat transfer rates in additive manufacturing are orders ofmagnitude faster, and four or five orders of magnitude faster thancasting processes. It is a highly non-equilibrium process, whichrequires all new scientific insights. That was the challenge I saw,and that’s what piqued my interested. Unfortunately, 15 years agosynchrotrons weren’t readily available so we had to settle fordoing ex situ imaging or relatively slow in situ imaging usinglaboratory sources.How has additive manufacturing changed your field? Isthere anything specific that it has enabled or made possible?Tell us how it impacted your research.The largest impact of additive manufacturing has been to pushthe boundaries of spatial and temporal resolutions of in situ X-rayradiography. Traditional metal manufacturing is on the timescaleof seconds, so you need 10−100 Hz to capture the phenomena. Inadditive manufacturing, you melt and solidify the weld pool inless than ten milliseconds, so you need to be running a hundredto a thousand times faster than this to capture the phenomena.That’s ten to a hundred thousand frames per second. Being ableto achieve this has enabled us to see inside the weld pool duringadditive manufacturing to obtain all new insights, and thesetechniques and insights have been translated to many otherphenomena in solidification and alloy design.The other way it has impacted the field is in the design free-dom: for polymers and biomaterials, the unparalleled designfreedom enabling topological optimisation (or whole newdesigns) is almost the most important aspect. For metals, additivemanufacturing is being developed first for high-value-addedapplications, such as aerospace and biomedical applicationswhere the value added through designs that more energy efficientor performance enhancing are greatest. The caveat is that in bothof these fields, components need to have an incredibly low failurerate and a long life. One of the weak points of metal additivemanufacturing is the microstructure: because it is cooled sorapidly, it often has a higher strength but lower fatigue perfor-mance. And since in metals, many of the value-added applica-tions are components that undergo cyclic fatigue, it is crucial wecan understand, predict and optimise the microstructure.What does additive manufacturing have to do to be morewidely adopted? Are there any specific hurdles it has toovercome?Additive manufacturing is really a process that is still in itsinfancy, and lessons learned from different approaches (e.g.printing for design freedom in polymers vs printing formechanical and functional properties in metals) will feed intoeach other.One key hurdle that isn’t well-researched is cost. There aremany aspects to this, but two of the key ones are: (1) additivemanufacturing machines are advancing so quickly that industriesfabricating large number of components do not know how longthey should amortise the cost of the machines for, and (2) thecurrent machines are not energy efficient (when producing a largenumber of components, energy costs can be one of the largesthttps://doi.org/10.1038/s41467-020-17732-1 OPENNATURE COMMUNICATIONS |         (2020) 11:3995 | https://doi.org/10.1038/s41467-020-17732-1 | www.nature.com/naturecommunications 11234567890():,;cost components). This is because electricity is converted to laserlight, which is then used to heat up powder material that mayhave a very low absorptivity. You then create spatter, ejectingaway the powder feedstock you’re heating rather than forming thecomponent.Another challenge is understanding how to reuse the feedstockmaterial: is it possible to reuse it multiple times? How do werecycle it? One of the many materials challenges is to determinehow the surface changes to reused powder due to heating andoxidation affect its reuse. Using ultrafast synchrotron imagingwe’re now quantifying this in situ and operando, so that youknow how many times you may safely reuse it, rather thanrecycling.For aerospace or high-value-added markets, where additivemanufacturing is now used commercially, we still need tounderstand the microstructural features that limit life, and haveaccurate models that can predict these microstructures and thetheir impact on component lifing. We also need a better under-standing of in situ heat treatments (which can help homogeniseor modify the microstructure), so that we can use this thermalcycling to improve strength and ductility. As a simple example,the micro-segregation scale in cast and wrought alloys is ten ofmicrons between the centre of the dendrite and the segregatedinterdendritic region, while in additive manufacturing it is on theorder of a couple of microns—and the time required for diffusionto homogenise the composition goes with the square of the dis-tance. This means that you may only need a hundredth of theheat treatment time to produce chemical equilibrium in additivelymanufactured structures.Looking forward: where do you see additive manufacturinggoing next?Additive manufacturing over the next five years… To me,additive manufacturing will remain an expensive process. It hasto go into areas where it will inherently add tremendous value,such as applications with geometrical complexity that can’t beproduced by other methods. However, we have to be able topredict the properties and lifing of these high-value components.But I think we can do this more accurately in additive manu-facturing than other process—we are building in layers that aretens of microns thick, we can observe and record the micro-structural features at this level as we fabricate a component. Wecould create a ‘digital twin’ that embeds a specific type ofmicrostructural features and predict the resulting properties,updating our predictions depending on service conditions.Today, when a component is manufactured via a cast andwrought process, both the alloy and the process are qualifiedusing the average behaviour and a statistical distribution. Thisleads to large safety factors. Instead, we could envisage acomponent-by-component qualification. For example, if youmake two components and measure as building one has featuresthat give a predicted lifing of 2 months and one 20 years, youcould direct the one for an emergency short-term applicationwhere you don’t need it to pass the fatigue life requirements of a20-year application. What if you could more rapidly or cheaplysource this particular part as a way to tie you over until your nextfull servicing? Additive manufacturing allows us to actuallyconsider these types of fundamental questions and could enableus to transform the way we are doing things.We are trying to develop process monitoring techniquesinformed and calibrated using synchrotron imaging, where weobserve the features with the synchrotron and correlating them tolow cost monitoring measurements (such as infrared and opticalcameras). Using these we hope to create ‘digital twins’ which storethe features and service conditions, adding real value to additivemanufacturing, and justifying the extra cost.Going forward, a lot of what we do in my group is ultrafastsynchrotron imaging, and we are still having difficulties seeingmicrostructures because the X-ray attenuation between phases issmall in many industrial alloys. However, fourth-generationsynchrotron sources (such as the recently completed EuropeanSynchrotron Research Facility’s Extremely Brilliant Source, andsimilar upgrades to the Advanced Photon Source and theDiamond Light Source in the next few years) can offer increasedcoherency and the opportunity to better see the differencebetween solid, liquid and the many microstructural features wehope to resolve.For you, is there a difference between additive manu-facturing and 3D printing? If they are not equivalent, how dothey differ?There is no difference for me. I use the term 3D printing whenspeaking to the general public because they immediately under-stand and associate the idea to the manufacturing process. Forresearch I use additive manufacturing: we need to better under-stand the fundamentals and the science, so the term additivemanufacturing, which points to adding value, seems moreappropriate.3D printing is much more prevalent for polymers, which aremuch more consumer integrated. Alloys are more limited in theirapplications, more specialist and with higher value-added, whichis probably why this field has stuck to the term ‘additivemanufacturing’.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, as long as you giveappropriate credit to the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. If material is not included in thearticle’s Creative Commons license and your intended use is not permitted by statutoryregulation or exceeds the permitted use, you will need to obtain permission directly fromthe copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© Springer Nature Limited 2020Q&A NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17732-12 NATURE COMMUNICATIONS |         (2020) 11:3995 | https://doi.org/10.1038/s41467-020-17732-1 | www.nature.com/naturecommunications

Peter Lee answers questions about additive manufacturing

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Peter Lee answers questions about additive manufacturing

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