QMT Features: July 2016
Metrology for additive manufacturing:the key to commercialisation
Professor Richard Leach* looks at the revolution that is going on in manufacturing and what it means for metrology

In additive manufacturing (AM) objects are formed, not by removing material, but by forming the desired shape in a layer-by-layer process. This way of producing a shape has many benefits over the subtractive techniques, but without doubt, the biggest benefit is the ability to produce almost any desired shape. And this design freedom also applies to internal features, allowing advances in areas such as light-weighting for aerospace, internal cooling channels for automotive and designed-in porosity for medical implants.

AM is still at an early stage of development. There are many examples of consumer products using AM with plastics, but, if AM is to be used in earnest in high-value, advanced manufacturing, for example, in the aerospace or medical industries, then it will be metals and ceramics that will be the game-changers. However, right now, the integrity of metal or ceramic parts is not equivalent to that expected from more traditional subtractive manufacturing techniques. AM parts made from metal powders tend to have high surface roughness values and can suffer from undesired material characteristics (for example, high porosity or large numbers of defects). Also, where one would not dream of manufacturing a part with subtractive techniques without a dimensional tolerance scheme, it is still not clear exactly how to apply tolerance principles to AM parts. This is where we get to metrology.

The following are some of the primary reasons why we put so much effort into measuring what we manufacture:
  • To know whether a part is fit-for-purpose.
  • To allow assembly of complex components
  • To allow control of a manufacturing process
  • To avoid unnecessary scrap material and redundant processing time;
  • To improve energy-efficiency
  • To give customers confidence in a product;
From the metrology standpoint, AM is no different to subtractive manufacturing. In fact, I would argue, that a lack of metrology in current AM machines and processes is hindering the commercialisation of the resulting products.
The last bullet point above is especially relevant in this context, for example, an aerospace manufacturer is not going to ‘fly’ a turbine blade made using AM without the high degree of confidence that metrology can supply.

However, for some of us that have been in this game for a while, the current dearth in AM metrology is not a surprise. I have spent my career playing catch-up with all the wonderful machining processes that are developed. Metrology is almost ubiquitously thought of last. So this article will tell you about the significant effort we are putting in at University of Nottingham to address the metrology requirements for AM.

The activity of the Additive Manufacturing and 3D Printing Research Group (3PRG) at Nottingham is perhaps the largest AM group in the world, with around one hundred academics, research fellows and PhD students. I started the Manufacturing Metrology Team in the 3RPG in January 2015 and it is now twenty-five people strong and a significant part of our effort is to solve the metrology issues facing AM.

We spent the first year reviewing what others have done in the field, what existing technologies can be bought to bear on AM and planning our future research.

This has resulted in four critical review articles that are planned to be published in 2016 that cover:
  • Post-process optical form metrology for industrial-grade metal additive manufactured components.
  • X-ray computed tomography for additive manufacture
  • Surface texture measurement and characterisation for additive manufacturing.
  • in-situ process monitoring and in-situ metrology for additive manufacturing.
Form metrology, the measurement and characterisation of a part shape, is critical for quality control of AM products, and for AM machine manufacturers to successfully characterise and optimise their AM processes, when new materials and part geometries are continuously developed.

Shape deformation is one of the most noticeable effects following most metal AM processes due to the relaxation of thermal stresses and hence detailed in-situ and post-process characterisation methods would be highly beneficial in understanding and contributing to the aversion of these effects.

Our review concentrates on the state-of-the-art in non-contact 3D optical metrology applicable to AM industries that have stringent product qualification standards, for example, in the aerospace and automotive industries. Contact systems, such as mechanical probe-based coordinate measuring machines (CMMs), have been used in such industries for many years, and can measure form to high accuracy (usually more accurately than current non-contact systems), but are relatively slow, not ideal for in-line inspection and only measure a limited number of points on an object’s surface.

Our current research into AM form measurement will focus on structured light techniques (fringe projection). We will also take advantage of what I call ‘information-rich metrology’ (IRM). IRM is the combination of accurate modelling of the interaction with the object being measured with all the a priori information that is available (Figure 2).

Making use of existing information

Often when we manufacture something, and especially when we use AM, we have a large amount of information about the object being manufactured, for example, the CAD data gives us the nominal form and we have usually characterised the surface texture to a high degree of confidence. In many cases, the a priori information allows us to solve the complex mathematical problems we encounter when trying to model the interaction with the object being measured (what we call “inverse problems”).

Our research is focusing on accurate mathematical modelling to allow us to optimise a given measurement scenario. IRM can allow us to minimise the measurement time (for example, by optimising the number of views we need to take to capture the form information) and increase the spatial bandwidth in which we measure (for example, by allowing us to measure high slope angles using multiple reflections). Our ultimate goal is to have a form measurement system, based entirely on camera technology that allows us to get the maximum amount of form information with the minimum effort.

As discussed above, AM provides freedom of design that is generally infeasible with other manufacturing methods, particularly regarding the creation of complex internal features that are inaccessible to well-established measurement tools. X-ray computed tomography (XCT) is currently the best method of measurement for these internal features due to the volumetric nature of the XCT process.

As AM and XCT have recently become more viable as methods of production and measurement, respectively, their combined and future work required to further establish both technologies, are research topics for my team. We are starting to apply IRM techniques to use a priori information to reduce the XCT measurement time and allow better detection of otherwise inaccessible surfaces.

Dealing with texture

Whilst the form of a manufactured object is critical, it is often the surface texture that has the biggest impact on its functionality. Surface texture (often called roughness) is often the limiting factor when considering the tolerance of an AM part. Whereas surface texture height structures can be produced on the nanometre scale using precision subtractive textures, due to the nature of powder-based AM techniques, surface texture height structures of tens of micrometres are more normal (Figure 3 shows examples of AM textures).

This throws up a number of metrology questions, some of which include: (i) Can we use conventional surface texture instruments to measure AM surfaces – high slope angles, resulting in multiple reflections and shadowing, cause problems for optical instruments? (ii) Can we use conventional filtering methods and texture parameters with AM surfaces? (iii) Can we examine the surface texture of an AM part to elucidate how the surface was manufactured – AM processes involve some highly complex physics, so this involves a significant amount of experimental and theoretical research? (iv) How can we measure surface texture in-line?

There have been a number of advances over the last decade in in-line AM metrology, mainly using either thermal cameras or optical imaging (2D) methods, often to monitor the melt pool characteristics. Some of these techniques are now available on commercial AM machines. But there is some way to go before we can take advantage of full closed-loop manufacturing – the main bottleneck is measurement speed. Again, IRM can come to the rescue by breaking the measurement process down to its bare minimum and taking advantage of a priori process data. We currently have activities in in-line measurement using optical coherence tomography, focus variation methods, fringe projection, optical scattering and laser-based acousto-optic techniques. This particular area of research is likely to grow significantly over the next couple of years.

Making the revolution a reality

AM is likely to have a significant impact on manufacturing and ultimately on all our lives. But, there needs to be a concerted research effort into AM metrology to allow the predicted revolution to become a reality. AM processes need to be have tolerance and quality control procedures in place, starting with off-line metrology and moving towards closed-loop control using in-line metrology. In parallel with this research effort, there also needs to be international standardisation. ISO and ASTM have officially joined forces and Nottingham is an active part of the normative process. The Manufacturing Metrology Team at Nottingham is addressing many of the metrology demands and is driving its research forward using the concept of information-rich metrology. IRM will be the way to make a difference and a way to leap-frog some of the measurement speed bottlenecks that limit current measurement techniques.
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