Beating metrology bottlenecks in additive manufacturing

In this article Eric Felkel, product manager (optical profilers), at the Zygo Corporation discusses

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 and reduce material consumption.

These advantages have been well documented during the last 10-20 years as AM has emerged as a truly disruptive technology not just for prototyping but also production.

Important 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 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 ineffective. In this article we review developments from Zygo and the University of Nottingham that mean improved metrology results and enhance the use of AM as a production technology.

Metrology and additive manufacturing

Zygo has been working with Richard Leach, Professor in Metrology at University of Nottingham, on various projects related to the use of metrology in AM. Professor Leach’s view is that metrology is crucial to the success of AM as it begins to establish itself as a true production technology.

He comments: “There is absolutely no doubt that inadequate metrology solutions able to cope with the specific characteristic of an AM produced part is a huge obstacle to overcome if AM is to be used as a viable production technology across industry.

“Currently, 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 an error in the topography in layer four,” he says. “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 fault is still there when the build is complete, and if so, if it was actually a significant error in the first place.

“Essentially, we are working on 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 build 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 is effectively a belt and braces approach, relying on Gauge R&R (gauge repeatability and reproducibility). It could be described as ‘extreme-testing’.

Professor Leach comments: “You cannot develop standards if you don’t have the correct measurement technology to start with. 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 the end-use parts.

“In the respect of standards, our focus today is on producing a Good Practice Guide along with the ASTAM group, showing OEMs what metrology solutions are in place today, 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.”

Scanning interferometry for AM metrology

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

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

He 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 and illumination.

“They also played with the detection conditions which led to the attainment of high-quality results with extremely rough and complex AM surfaces. I have to admit, that before looking in depth at the Zygo CSI solutions, I thought that they wouldn’t be able to be applied to AM parts, but actually they work extremely well.”

It is the Nexview technology from Zygo that Professor Leach works with today, and which is now accepted to be a strong and viable AM metrology tool, along with its sister product the NewView which the Nottingham research department previously used.

Professor Leach explains: “We installed a NewView 8300 instrument at Nottingham in October, 2016. Measurements made at Nottingham as well as at Zygo’s headquarters in the USA on AM surfaces conclusively demonstrated that Zygo’s CSI implementation was well suited to the task. Today, we work with Zygo’s further technology development, the Nexview optical surface profiler with its ‘More Data’ capability.”

More Data significantly improves the baseline sensitivity of CSI and enables high-dynamic range (HDR) operation making it valuable for a wide range of parts, from steeply-sloped smooth parts to exceptionally rough textures with poor reflectivity. Additionally, HDR is able to measure parts with a high level of reflectance, often a struggle for other CSI instruments.

Today, the focus is on using the Zygo HDR CSI technology to undertake surface texture analysis and to attempt to better understand its links with the AM production process.

Professor Leach concludes: “My work with Zygo is centred around understanding precisely how the CSI instrument works and accurately modelling 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 standardised way of measuring and characterising these surfaces.

“That’s why we are working with Zygo instruments to ensure that we continue to optimise metrology for AM in production scenarios.”

Zygo www.zygo.com