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QUALITY & INSPECTION
Overcoming metrology bottlenecks in additive manufacturing Metrology is becoming a critical area of focus in additive manufacturing as it moves towards becoming a serious production technology. The nature of the process and the surface characteristics it produces create challenges for many metrology tools. By Eric Felkel of ZYGO Corporation. Undoubtedly there are many benefits associated with the use of additive manufacturing (AM) as a production technology. Across industries, manufacturers exploit the fact that with AM they can not only build complex parts in one piece that were previously impossible, but they can also build stronger, lighter-weight parts, reduce material consumption, and benefit from assembly component consolidation across a range of applications. These advantages have been well documented over the last 1020 years, as AM has emerged as atruly disruptive technology for not just prototyping but also production. They are invariably seen as being enabled by the additive hardware that builds the parts. In reality, this is a partial picture, particularly for serial production applications of AM. The hardware systems are just one part – albeit a vital one – of an extensive ecosystem of technologies that enable AM, both pre- and post-build. Of unique importance today is the role of post-process metrology to validate the integrity of AM builds. One specific reason for this importance is that many parts produced by AM today end up in safety-critical applications where end-use functionality is of vital importance. The nature and relative roughness of AM surfaces, whether analysing individual layers within a build, or the surface of a finished part render conventional metrology solutions somewhat impotent. Recent developments from ZYGO are allowing hitherto unattainable metrology results that are being used to enhance the use of AM as a production technology by making validation protocols more efficient.
Metrology and additive manufacturing Richard Leach, Professor in Metrology at the University of Nottingham in the UK, has been working with ZYGO on a number of projects related to the use of metrology in AM. Leach believes the issue of metrology is crucial to the success of AM as it begins to establish itself as a true production technology. “There is absolutely no doubt that inadequate metrology solutions are a huge obstacle to overcome if AM is to be used as a viable production technology across industry,” says Leach. “Basically, as we stand today, there is a lack of clarity as to the precise nature of defects that you get when undertaking an AM build, and you also have little idea how they may cause problems in terms of part functionality. We don’t have a detailed enough map of how the topography of the defects and the anomalies that you get during the AM process propagate through to the part in an end-use scenario. Leach offers the example of a turbine blade being made in an AM layering process, and where a blip occurs in the topography in layer four. “This layer will in-time be covered up, so its characteristics will be fundamentally different by the time the finished part is complete,” he explains. “And it is at this moment impossible to know – without the clarity that good metrology provides – whether the blip is in fact still there when the build is complete, and if so, if it was actually significant in the first place. Essentially, we are working on, but still haven’t completely solved, the problem of understanding what issues you get on the surface and under the surface when using AM, and how these relate to product functionality. “Therefore, it is difficult to predict the mechanical properties, the thermal processes, the fatigue properties, etc … from the types
AMT AUG/SEP 2020
A sample part made at Nottingham University from Ti6Al4V using selective laser melting (SLM) measured by a ZYGO Nexview CSI microscope.
of structures we are seeing post-process. Defect-function analysis may allow us to move towards controlled AM by just stopping the process when things go wrong, as right now we spend hours building a part that may in fact have a problem in layer one.” Despite these challenges, many companies are already using advanced AM successfully for the production of critical parts and components, often in aerospace applications where part failure is not an option. To ensure that these AM-produced parts conform fully with design intent, part suppliers undertake far more mechanical testing and metrology verification than they would normally employ for conventional manufacturing processes. Manufacturers are forced out of necessity to focus on process development and throw all the validation resources they can to ‘prove’ the integrity of the finished AM part. This latter is effectively a belt-and-braces approach, relying on Gage R&R reproducibility and repeatability as a stand-in for a more rigorous measurement uncertainty approach when evaluating the integrity and functional characteristics of AM parts. The current solution is what could be termed ‘extreme-testing’. “Everyone blames the confusion on a lack of standards for measuring AM parts, but this is not where attention should be focussed,” Leach comments. “You cannot develop standards if you don’t have the correct measurement technology in place to start with. Standards being developed without the technology solution ready to use are actually worse than no standards at all. “That is why the emphasis with ZYGO and other metrology instrument suppliers is on adapting metrology solutions to make them better aligned with the unique characteristics of the AM process and AM end-use parts. In the respect of standards, our focus today is on producing a Good Practice Guide showing OEMs what metrology
