By Eric Felkel, Product Manager (Optical Profilers), Zygo Corp.

Undoubtedly there are many benefits associated with the use of additive manufacturing (AM) as a production technology. Across industries, manufacturers exploit the fact that through the use of AM they can not only build complex parts — in one piece — which 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 during the last 10-20 years as AM has emerged as a truly disruptive technology for not just prototyping but also production, and 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. AM 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 the end-use function is of vital importance. The nature and relative roughness of AM surfaces, whether analyzing individual layers within a build or the surface of a finished part, render conventional metrology solutions somewhat impotent. Here’s a look at a few developments, including work alongside Richard Leach that allows better metrology results for the use with AM in a production capacity.

Metrology and additive manufacturing

Richard Leach, Professor in Metrology at University of Nottingham, Nottingham, U.K., has helped on various projects related to the use of metrology in AM. It is Leach’s view that 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 able to cope with the specific characteristic of an AM produced part are a huge obstacle to overcome if AM is to be used as a viable production technology across industry. 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.”

“Imagine you are making a turbine blade in an AM layering process, and you see that there was a blip in the topography in layer 4. This layer will in-time be covered up, so its characteristics will be fundamentally different by the time the finished part is complete, 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 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.

Necessarily, manufacturers are forced 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.”

Leach comments, “Everyone blames the confusion on a lack of standards for measuring AM parts, but this is not where attention should be focussed. 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 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 along with ASTAM, showing OEMs what metrology solutions are in place today, and how to get the best results from these when applied to AM surfaces, and setting the instrument up in the best way to understand the data.”

Thus, the focus in the area of metrology for AM is to reduce the time and cost inefficiencies inherent of relying on a range of duplicate and often inadequate metrology steps to validate that an end-use part is fit for purpose.

As Leach works on this vital area, he is involved with a number of metrology instrument suppliers, using a variety of measurement technologies.

Leach continues, “For post-process metrology, a number of alternatives exist including confocal and focus variation, and Zygo’s coherence scanning interferometry (CSI). Initially, it was thought that CSI was not suitable to the vagaries of post-process AM parts (with their unusual surface roughness), but Zygo enhanced its CSI instruments by introducing new ways of playing with the optical light source, illumination conditions, and also the company played with the detection conditions which led to the attainment of high-quality results with extremely rough and complex AM surfaces.”

Leach’s initial work with CSI which informed his early view of the inappropriateness of the technology for AM super-rough surface metrology was based on earlier versions of CSI from a number of vendors.

Data from the earlier versions led Leach and his team at Nottingham to conclude that interferometry was fundamentally not suitable for AM metrology because the example instrument failed to capture most of the highly irregular topographic features. The Nexview technology that Leach and Zygo worked on together is a viable AM metrology tool.

The Nexview instrument and its sister product the NewView, included hardware and software upgrades; a package of improvements that made the instrument better suited to AM parts.

Leach says, “We installed a NewView 8300 instrument at Nottingham in October 2016. Measurements made on AM surfaces conclusively demonstrated that this CSI implementation was suited to the task.”

Zygo uses an approach it refers to as “More Data” to improve the baseline sensitivity of CSI and enable high-dynamic-range (HDR) operation, making it valuable for a range of parts, from steeply sloped, smooth parts to rough textures with poor reflectivity. HDR, a feature from Zygo, can measure parts with a range of reflectance, often a struggle for other CSI instruments.

Leach concludes. “My work with Zygo is centered around understanding precisely how the CSI instrument works and accurately modeling it for AM applications. At the moment, the issue is that AM surfaces are so different from what we are used to in terms of the raw surface and the post-processed surface that there is no standardized way of measuring and characterizing these surfaces. We are working with ZYGO to ensure that we continue to optimize metrology solutions for the increasingly important area of AM for production scenarios.”

Zygo Corp., AMETEK, Inc.
www.zygo.com