Feature first published in the March/April 2022 issue of 3D ADEPT Mag. The interview and assessments reflect that period; the Porsche piston example refers to the project announced in 2020.
Additive manufacturing enables geometries that are difficult to produce by other routes. That freedom does not remove the need to verify dimensions, materials and potential defects. Metrology and inspection support this work, with methods adapted to the process and application requirements.
The International Vocabulary of Metrology defines metrology as the science of measurement and its application. Inspection uses measurements, examinations and tests to assess conformity with specified requirements. The concepts complement one another: a measurement must also be interpreted with its uncertainty and conditions of use.
Why is that? How important are metrology and inspection to additive manufacturing? What are the different routes a component can go through when operators are looking to perform metrology and conduct an inspection method?
The truth is, metrology and inspection are very large concepts whose advancements often create confusion in the mind of AM users.
“There are a lot of grey areas when it comes to metrology and inspection. Sometimes, these grey areas are reflected on the way each industry understands or defines each concept. Confusion can also arise when you see terms like inspection metrology, a term that does not enable to accurately differentiate the role of each concept. The fact is, both concepts are quite different, especially when we look at the quality management planning of manufacturing”, Dr. Edson Costa Santos, Senior Application Development Manager Additive Manufacturing Process & Control at ZEISS, states from the outset.
Interestingly, for non-experts in the field, the concept of inspection can easily be understood as it is a term that we often use on a daily basis and in different environments. “For instance, the terms ‘home inspection’, ‘police inspection’ give you an idea of what inspection is but when we talk about metrology, a lot of people won’t be able to say what this term means”, Santos adds. When we look at industries, Santos said it is easy to understand the differences between metrology and inspection in the semiconductor industry while in precision engineering for instance, there is a lot of measurement definitions in inspection methods that are close to metrology ones.
The easiest way to avoid confusion between both concepts is therefore to see inspection as “the first step to distinguish between an accepted or rejected component. It’s simply the process of checking something, and see if it meets certain standards. Metrology on the other hand, deals with the definition of units, realization of units, standards, traceability and calibration and also the uncertainties of measurements.”
This exclusive feature therefore ambitions to capture the essence of both concepts for AM as well as draws a landscape that would enable beginners in AM to understand them. While some of the notions will not be new to advanced users, this feature aims to remind them the importance of these concepts as well as all the challenges experts in the field currently address in order to support the advancement of AM.
Why measure components?
Measurement serves several practical purposes across manufacturing:
- To ensure that a part is fit-for-purpose;
- To make assembly work: if you do not understand the dimensions of the components and their tolerances, it’s hard – even impossible – to fit one part to another. The reason is even more important when there are several parts manufactured at different companies that need to be assembled.
- To avoid unneeded scrap material: As the ZEISS expert says, “metrology is pivotal for quality control which allows us to attempt things such as net-shape manufacturing” – getting it right first time as operators dream to achieve with AM.
- To enable control of a manufacturing process: imagine that you need to amend the speed of a cutting tool according to surface texture. You will have to measure the texture during the machining process.
- Build confidence in the product by linking inspection to documented criteria and assembly requirements.
That being said, the aforementioned reasons imply that a number of measurement techniques can be used to address the different needs to meet in the manufacturing value chain. As a matter of fact, there are several areas where metrology can be explored in additive manufacturing. They include but are not limited to on-machine and in-process measurements, metrology of surface texture, defect measurement and control, materials property measurement, process-function correlation studies, AM machine testing and validation, dimensional and geometrical product specifications and verification, Metrological traceability and uncertainty, etc.
To make it easy to understand where they should be applied, Costa Santos explains there are several ways to classify metrology tools, some examples below:
- Contact and non-contact measurement: “contact-type devices must touch the part in order to perform measurement while non-contact devices can measure several points without putting pressure on the part. A non-contact measuring instrument uses for example light while contact-type measuring devices use a stylus. Non-contact measuring tools can also include optical laser or laser scanners.” Hybrid machines can use both type of measurement tools. On another note, as many automotive examples are given throughout this edition, let’s note that contact systems, such as mechanical probe-based coordinate measuring machines (CMMs), are often used in the automotive industry. However, they are often considered as relatively slow and they only measure a limited number of points on an object’s surface.
- One can also classify them by equipment. Such equipment includes for example micrometers, optical scanners, industrial tomography systems, microscopes, etc.
- and measurement volumes which include 2D, 3D or 4D measurement techniques.
The hidden complexities/uncertainties of inspection methods
No matter what advanced manufacturing technologies industrials use, or no matter how well we focus our efforts on the manufacturing size, inspection methods make us rethink our priority which must remain safety and quality.
Furthermore, quality and assurance may often be mentioned as part of the post-processing stage but it is important to keep in mind that they can be used at various levels of the manufacturing value chain. “As far as AM is concerned, we can inspect a powder, and assess if the powder meets all required characteristics before it is used for production. During production, the AM machine and process can be inspected and after production, we can inspect the 3D printed part”, Costa Santos outlines.
Speaking of tools, the same mechanical and nondestructive inspections utilized in most conventional manufacturing processes can also be applied in AM. Your choice for inspection methods will therefore be determined by the type of parts you will produce. The same applies for software solutions.
“Depending on the metrology and inspection tools, you will use, you may have different software solutions for both processes just as you may have the same software solution for both processes”, the expert notes. “Whether we talk about metrology or inspection, most tools can be utilized in both AM and conventional manufacturing processes. However, depending on the AM technology used, there might be a need to further adapt your tool for AM. Two examples are ZEISS in process monitoring systems to see what is happening layer by layer during the process and allows close feedback control; and the need in AM to inspect complex cooling channels non-destructively, both roughness and integrity (powder remaining). There are especially varying degrees to inspection. [At the end of the day, it comes down to choosing the option that delivers the best approach at ensuring part conformance and integrity]”, the ZEISS representative notes.
AM processes often require dimensional, external and internal testing, and sometimes a specific surface roughness. Our guest mentions coordinate measurement machines (CMM), gages, and 3D scanners when it comes to dimensional inspections. A combination of these devices can be used in factories.
Non-destructive methods include visual examination, liquid penetrant testing, eddy-current testing, ultrasound and radiography, including X-ray computed tomography. Penetrant testing detects surface-breaking discontinuities; the depth accessible to eddy currents depends on the test conditions. No method detects every defect: material, geometry, target size and resolution guide the choice.
ASNT’s introductions explain the scope of liquid penetrant and electromagnetic testing.
The debate starts when experts cannot accurately attribute to a specific AM process. Like in most cases in AM, the answer to the question “what inspection method best suits which AM process is” “sometimes” or “assuming that an additional process is performed upfront or later on”. Indeed, sometimes, operators can conduct some surface preparation and smoothing or even additional thermal processing.
Layerwise manufacturing can produce process-specific defects. When a printed part replaces an assembly, its inspection plan must be reconsidered: consolidating functions does not remove their associated requirements.
To these uncertainties, the Senior Innovation Manager replies: “Quality is always aggregated to manufacturing processes. So, the level of metrology and inspection will depend on the aggregate value of your part. Is it a visual prototype? Is it a final part submitted to fatigue loads? It depends on how critical the part is.”
In practice, what does applying measurement and inspection techniques look like in additive manufacturing?
In July 2020, Porsche announced a 3D-printed piston project for the 911 GT2 RS engine with MAHLE and Trumpf, supported by ZEISS measurement technology. This was a development project, not evidence that all production-model pistons were printed.
A load-adapted design reduced piston weight and incorporated a closed cooling channel. According to the partners, this channel geometry exploited the capabilities of additive manufacturing.
MAHLE’s aluminium alloy, already used for cast pistons, was atomised into powder and processed by laser metal fusion at Trumpf. Load adaptation concerned the geometry and distribution of material, not the addition of a second alloy in selected regions.
The July 2020 MAHLE release describes material preparation, finishing and testing.
ZEISS presented a quality-assurance chain covering powder and component. The reported results relate to this project and its test criteria; measuring equipment alone does not guarantee the quality of every part.
The ZEISS case study published in 2021 describes this approach.
In this specific case, the team explains that the powder’s particle size distribution, particle shape, chemical composition and even porosity inside the powder particles are examples of properties which can influence component quality. The fact that the powder’s properties can change each time it is reused must be taken into account, and deviations need to be detected in a manner that is appropriate for the production process.
For example, if the powder’s particle size distribution changes as a result of repeated use, the quality of the applied powder layer may be affected, and simultaneously the risk of pores forming as a result, or of other component defects arising, may increase dramatically.
Light microscopes, scanning electron microscopes and X-ray microscopes from ZEISS are used to analyze the quality of the powder before and after piston production, and the microstructure of the finished component is tested in order to identify defects or property characteristics. Through further processing of the analysis data and additional evaluation processes, it is possible to determine optimized settings for printing. Successful print results must also go through a variety of post-processing steps to ensure that the material and component properties are optimal.
During process development, for example, the component’s structure can be inspected using specially equipped scanning electron microscopes or X-ray computed tomography before and after the heat treatment processes. To analyze the impact of individual production steps on the final quality of the component, an optical 3D scanner or industrial computed tomography are used in addition to a coordinate measuring machine. The ability to seamless combine these procedures is a decisive factor.
The pistons still on the print bed were scanned using a 3D scanner. Once they are removed from the bed, the individual production steps as well as the internal structures can be inspected using computed tomography while defect analysis is performed at the same time. At the end, a final measurement is performed with the coordinate measuring machine. The core capability of ZEISS’ comprehensive quality assurance process is the linking of all data across the various analyses.
Concluding thoughts
This exclusive feature left me with certain certainties and opens up other questions, that I hope we will discuss in other features.
The interview illustrates how metrology and inspection complement one another in AM. Metrology provides a basis for confidence in measurements; inspection uses the necessary evidence to assess product conformity.
On the other hand, the technical standpoint highlights the importance of measurement and characterization of a part shape for quality control of additively manufactured parts. Most importantly, they are important when it comes to characterizing and optimizing AM processes or when new materials and part geometries are developed.
Lastly, while the application with the Porsche part reveals that ZEISS is one of the companies that has built up expertise in the development of comprehensive quality assurance processes, I am keeping some concerns regarding the current metrology and inspection methods used for AM exclusively, for example inline realtime in process monitoring systems as pointed out by Costa Santos. My expectations were probably very high when I started exploring this topic as I didn’t expect to discover very few inspection methods, exclusively designed for AM (or let’s say, I didn’t expect so many inspection methods that can be used for both conventional manufacturing processes and AM processes). When I look across the AM value chain, I see a lot of advancements on the manufacturing processes themselves, I see dedicated post-processing steps developed especially for these manufacturing processes, and it made sense to me to expect the same for metrology and inspection processes.
Using methods also employed in conventional manufacturing is not inherently a weakness. Suitability depends on validation for the relevant defects, geometries and measurement uncertainties.
Maybe I didn’t dive enough (or at all) into the parameter development and optimization for specific AM processes? Maybe I should look at in-process and post-print processes that can further increase productivity and efficiency? Or identify other areas of AM where metrology can be applied?
In any case, there are definitely new inspection methodologies that need to be discussed, evaluated, and developed; and that’s something we (at 3D ADEPT Media) will watch closely.
This feature was first published in the March/April 2022 issue of 3D ADEPT Mag.
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