Geometrical metrology for metal additive manufacturing
Introduction
There is little doubt that additive manufacturing (AM) will have a large effect across manufacturing, from the home workshop to the most technologically advanced digital factories [304]. Recent review papers describe the state of the art and future trends in the field of AM, and this will not be repeated in depth here [36], [240], [253], [273]. In AM, objects are manufactured, not by removing material, but by generating the desired shape in a process of adding material layer by layer. This way of producing a shape has many benefits over the subtractive techniques, but the biggest benefit is the ability to produce almost any desired shape. There are some caveats to this (for example, care around overhanging features), but AM allows almost infinite design freedom, without the significant constraints on the types of geometries that can be produced using subtractive techniques. 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. The ability to design with such freedom is a rich mathematical discipline in itself and can result in complex and intricate geometries that could not be produced using subtractive techniques (see for example, Fig. 8) [270], [311]. From the metrology point of view, the extra level of design freedom offered by AM with respect to subtractive techniques, results in an extra level of complexity in geometries to be measured; this is why there are so many developments in geometrical metrology for AM and why there are still unsolved measurement issues. These developments and issues are the focus of this paper.
AM is still at an early stage of development. There are many examples of consumer products using AM with plastics, for example, vacuum cleaner casings and even fashionable clothing. 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, although not addressed here) that will be the game-changers. However, right now, the integrity of metal (or ceramic) parts essentially made of fused powders, is not equivalent to that achievable using more traditional subtractive manufacturing techniques [163]. AM parts made from metal powders are commonly characterised by high surface roughness values and can suffer from undesired material characteristics. Also, where a part would not be manufactured with subtractive techniques without a dimensional tolerance scheme, it is still not clear exactly how to apply tolerance principles to AM parts. AM does not currently have the benefit of the over one hundred years of research into the production of components that is the hallmark of precision subtractive techniques.
The following reasons to measure part geometry remind us why so much effort is devoted to measuring manufactured components – all of these also apply to AM, although in many cases, AM can avoid assembly processes:
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To know whether a part is fit-for-purpose; for example, to determine whether or not a shaft will fit within a hole, but still give enough clearance to allow the flow of lubricating fluids
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To allow assembly of complex components; without understanding the dimensions of parts and their associated tolerances, it becomes almost impossible to determine whether a part will fit to another – this is an especially relevant point when assembling parts that have been manufactured at different companies or different parts of a company.
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To allow control of a manufacturing process; for example, to change the intensity of a laser depending on the surface texture that it is producing – the texture (or something from which texture can be inferred) needs to be measured during the production process.
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To avoid unnecessary scrap material and redundant processing time; metrology is essential for quality control which allows things such as net-shape manufacturing – getting it right first time.
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To improve energy-efficiency; the fewer repeat manufacturing processes that are required, the lower the energy required to produce a product.
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To give customers confidence in a product; “customers” in this context could be another manufacturing concern that needs to use your components – without tolerances and quality control, there will be a lack of confidence in the assembly processes down the line.
From the metrology standpoint, AM is no different to subtractive manufacturing. But, a lack of integrated metrology in current AM machines and processes is hindering the commercialisation of the resulting products [28], [177], [246]. 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.
The paper will start with a short history of AM. Section 2 will discuss the infrastructure under development for specification standards in AM, and Section 3 will review the research and developments in the geometrical dimensioning and tolerancing for AM. Section 4 addresses post-process metrology, including the measurement of surface form, surface texture and internal features. Section 5 concerns the in-process requirements and developments in AM and Section 6 reviews the materials metrology that is pertinent to geometrical measurement concerns. Section 7 discusses issues of traceability and reviews the various benchmarking artefacts that are under development. Finally, Section 8 pulls together the information in the technical review sections into a synthesis of current requirements and future research topics.
Note that, to keep the size of the paper to a manageable level, there are some areas of metrology that could arguably be bracketed in with geometrical metrology, but have not been included. This includes the dimensional requirements of the powders and a comprehensive treatment of the multitude of defects found in AM, but see Refs. [257] and [97] respectively for recent reviews in these areas. Another important area of AM that will be increasingly important is the combination of AM with more conventional processes; so-called “hybrid manufacturing”. Again, for the sake of brevity, this area has not been covered in detail, but see Ref. [132].
The triggering event marking the birth of AM is debatable. It is generally associated with the commercial sale of the first industrial fabricator, the 3D Systems stereolithography SLA-1, in 1988 [35], although the issuing of the associated patent two years earlier may be used [112]. The difficulty of the latter is that there is actually a fairly lengthy patent history of AM inventions that predates the 3D Systems founding patent for stereolithography. According to Bourell [35], the development of AM is categorised into three time periods: prehistory, dating to the mid-1860s, in which objects were created in layerwise fashion without part specific tooling and without the use of a computer; precursors, from about 1968 to the mid-1980s, in which a computer was used to assist in fabrication but commercialisation was impeded because computers were not well known or widespread in society; and modern AM, denoted by the commercial marketing and sale of AM fabricators.
Two prehistorical approaches for production of metal parts involve sheet lamination and weld deposition. Blanther in 1890 filed a US patent describing the creation of a mould by cutting and stacking sheets of wax following topographical elevations [31]. As shown in Fig. 1, each sheet was cut into at least two pieces, and by stacking all the pieces separately a full two-piece mould set could the constructed. The mould was used to press paper feedstock to create “3D” topography maps.
Sheet lamination of non-metallic feedstock developed over the next sixty years (for example, [87], [213], [314]). In 1974, DiMatteo proposed to form contours in sheets of metal using milling, followed by stacking to create complex geometric parts [68]. An example of a part built up by a “cut and stack” method is shown in Fig. 2. Nakagawa at Tokyo University advanced layered production of tooling to include blanking, press forming, and injection moulding tools [153], [194], [195].
The use of a moving weld head to build up objects was initially patented by Baker in 1925 [20]. Fig. 3 illustrates a sketch from the patent. Starting in the 1960s, a number of approaches arose for producing tooling using a moving weld head, often to build parts on a moving axis or rotating mandrel [39], [40], [84], [300]. Fig. 4 shows an embodiment from 1964 for creation of pressure rollers using a wire-fed welding head.
For precursor processes, the earliest is the 1972 disclosure by Ciraud in France [56]. As shown in Fig. 5, the process contains many elements of directed energy deposition, including metal powder, an energy source (laser, electron beam or plasma beam), a part built up from a base plate and even powder recycling. The Housholder patent, issued in 1981, describes laser sintering of powdered material [111], eight years before Deckard’s patent [61].
Current AM processes for direct metal processing are divided into three categories according to ISO/ASTM [115]: powder bed fusion (PBF), directed energy deposition (DED), and sheet lamination.
Modern laser PBF (LPBF) is based on Deckard’s invention [61] in the late 1980s, leading to the foundation of DTM Corp. in the early 1990s. DTM was acquired by 3D Systems in 2001. The earliest known metallic parts produced using PBF were copper-tin and copper-(73 Pb/30Sn) solder elemental powder blends [178]. High-energy LPBF systems were developed in the mid-1990s in Germany and Belgium which enabled direct fabrication of parts using metals of commercial interest [150]. These processes have become known as “selective laser melting”, a term first used by Meiners at the Fraunhofer Institute for Laser Technology in 1996. Electron beam-based PBF was developed in the mid-1990s. Based on initial development in collaboration with Chalmers University of Technology in Gothenburg, the company Arcam was founded in 1997 with the first product launched in 2002.
DED was initially developed at Sandia National Laboratories in the United States in the mid-1990s and was termed “laser engineered net shaping” [131]. In DED, metal powder is sprayed into an energy beam, typically a laser.
Modern sheet lamination AM began with Feygin who founded Helisys in 1985, one year before 3D Systems. The first shipment of a fabricator based on Helisys’ Laminated Object Manufacturing technology was in 1991 [23] (later known as sheet lamination [162]). Ultrasonic AM was invented by White and described in a patent that was issued in 2003 [301]. This “stack and cut” process uses a cylindrical sonotrode to solid-state weld a metallic foil onto a previous layer. The layer boundary is machined during the build to produce the part. The technology was commercialised initially by Solidica and now through Fabrisonic, a joint venture between Solidica and the Edison Welding Institute (EWI).
Section snippets
ISO and ASTM specification standards
Advances in metal AM machines and processes in the mid-1990s led to direct manufacturing of functional components (as opposed to prototypes for design validation, tooling for other manufacturing processes, etc.), impacting major industries such as aerospace, defence, medicine, energy, and automotive. Direct manufacturing applications, especially for components with critical functions, created the need for clear communication between designer, manufacturer and the user of such components about:
AM-specific challenges in geometrical dimensioning and tolerancing
Determining geometric tolerances by designers and conveying them to manufacturers is essential to ensure proper functioning of products. Geometric dimensioning and tolerancing (GD&T) specification standards enable the conveying of the design intent to manufacturing process planners. GD&T is also critical for planning the final part inspection process. The methods for such communication are well established for traditional manufacturing processes [16], [120], [128]. However, the high degree of
Surface form and coordinate metrology
This section focuses on coordinate measuring systems (CMS) for off-line measurement of the external shape of metal AM products. Together with surface texture metrology (see Section 4.2) and internal feature metrology (see Section 4.3), coordinate metrology of the external form is fundamental for quality control of metal AM products, as well as for providing feedback for AM process optimisation. The eventual form errors of metal AM parts are the result of complex interactions between the
Materials properties affecting AM part geometry
The main effect of materials on final part shape stems from part distortion during the build, and, for post-processed parts, after the build. In both cases, the distortion arises from residual stress formation and its relief. Residual stress is directly related to the thermally induced strain, which itself is proportional to α ΔT, where α is the coefficient of thermal expansion and ΔT is the change in temperature. Excepting tungsten and lead, α is relatively invariant for metals, changing by
Types of metrology solutions and their role in metal AM
On-machine metrology investigates measurement solutions dedicated to observing the manufacturing process or the part itself executed within an AM machine (see Ref. [85] for recent definitions of the terminology associated with on-machine metrology). Measurement may be used to acquire information while the process is being executed (in-process metrology) either by inspecting the part itself as it is being fabricated, or by observing other process-related variables of interest within the machine.
National measurement institute work
Most of the National Measurement Institutes (NMIs) have programs of research in AM, focussing on metrology and standardisation, but the largest program is The Measurement Science for Additive Manufacturing program at NIST. The NIST program currently focuses on material characterisation, real-time monitoring and control of AM processes, qualification methodologies, and systems integration for the complete AM process chain [204]. Critical geometric measurements include metal powder particle size
Synthesis and future research
In the main body of the paper, the current state of the art, progress, and requirements in industry for geometrical metrology for AM have been reviewed. This section will pull out the main take-home messages and future research topics from each section.
There are significant gaps between existing AM specification standards and the industrial needs for widespread use of AM technology. The most important areas of future AM standards are summarised in Section 2.3. There are many globally- and
Acknowledgements
The authors would like to thank the following for contributions to, and in-depth reviews of, the manuscript: Adam Thompson, Lewis Newton, Ian Maskery, Xiaobing Feng (University of Nottingham); Markus Baier, Filippo Zanini (University of Padova); Peter de Groot (Zygo); Jean-Pierre Kruth (KU Leuven); Stephen Newman (University of Bath); and Christopher Evans (UNCC).
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental
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