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Measurement of the internal surface texture of additively manufactured parts by X-ray computed tomography


Abstract and Figures

Recent advances in X-ray computed tomography (XCT) have allowed for measurement resolutions approaching the point where XCT can be used for measuring surface topography. These advances make XCT appealing for measuring hard-to-reach or internal surfaces, such as those often present in additively manufactured parts. To demonstrate the feasibility and potential of XCT for topography measurement, topography datasets obtained using two XCT systems are compared to those from more conventional non-contact optical surface measurement instruments. A hollow Ti6Al4V part produced by direct metal laser sintering is used as a measurement artefact. The artefact comprises two component halves that can be separated to expose the internal surfaces. Measured surface datasets are compared by various qualitative and quantitative means, including the computation of ISO 25178-2 areal surface texture parameters. Preliminary results show that XCT can provide surface information comparable with more conventional surface measurement technologies, thus representing a viable alternative to more conventional measurement, particularly appealing for hard-to-reach and internal surfaces.
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7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
Measurement of internal surfaces of additively manufactured parts by X-ray
computed tomography
Adam Thompson1, Lars Körner1, Nicola Senin1,2, Simon Lawes1, Ian Maskery1, Richard Leach1
1Manufacturing Metrology Team, University of Nottingham, NG7 2RD, UK e-mail:
2Department of Engineering, University of Perugia, 06125, Italy
Recent advances in X-ray computed tomography (XCT) have allowed for measurement resolutions approaching the point where
XCT can be used for measuring surface topography. These advances make XCT appealing for measuring hard-to-reach or
internal surfaces, such as those often present in additively manufactured parts. To demonstrate the feasibility and potential of
XCT for topography measurement, topography datasets obtained using two XCT systems are compared to those from more
conventional non-contact optical surface measurement instruments. A hollow Ti6Al4V part produced by direct metal laser
sintering is used as a measurement artefact. The artefact comprises two component halves that can be separated to expose the
internal surfaces. Measured surface datasets are compared by various qualitative and quantitative means, including the
computation of ISO 25178-2 areal surface texture parameters. Preliminary results show that XCT can provide surface information
comparable with more conventional surface measurement technologies, thus representing a viable alternative to more
conventional measurement, particularly appealing for hard-to-reach and internal surfaces.
Keywords: additive manufacturing, surface topography, metrology
1 Introduction
Additive manufacturing (AM) is of growing interest to the manufacturing community, particularly for the ability of many AM
technologies to produce parts containing complex geometries that were previously impossible to manufacture [1]. One significant
barrier is the difficulty of applying core principles of quality assurance, such as dimensional and geometric inspection and
verification, to additive parts [2]. In particular, in the inspection of AM surfaces, conventional optical and contact metrology
solutions are often inadequate to measure hard-to-reach surfaces, and inapplicable for measuring internal surfaces. Such
conditions are common with some of the most typical AM geometries, such as complex, hollow parts and lattice structures [3
Over the past decade, X-ray computed tomography (XCT) has become a useful tool in holistic inspection of industrial parts.
Efforts to incorporate XCT technology into the sphere of metrology have begun to make headway, especially in the acceptance
and traceability of XCT machines as measurement instruments [6]. Although much work remains in standardising XCT for
metrology (ISO 10360-11 [7] is still in the draft stages), XCT has begun to show promise for accurate measurement, particularly
for verification of internal geometries present in AM parts [8]. Although the spatial resolutions typically achievable by XCT are
not yet at the level generally required to capture the smaller-scale formations of a surface in addition to the overall shape,
advanced systems are beginning to approach these resolutions in their best-case measurement scenarios. Because of these recent
advances (such as improved detectors, more stable sources, smaller spot sizes), XCT is becoming an appealing option for
measurement of surface topography. When considering AM parts featuring complex, internal geometries, the prospect of using
XCT for surface topography measurement becomes even more appealing, as a method capable of overcoming the access
requirement problems that are inherent with contact and optical measurement. The potential advantage of XCT is highlighted in
a number of recent studies [913]. Specifically, Pyka et al. [911] performed the first investigations into the use of XCT for
surface topography measurement, by extracting profiles from XCT slice data obtained from measurement of lattice struts.
Townsend [12] and Thompson et al. [13] extended this work by initiating a more extensive examination of XCT topography
measurement performance in comparison to conventional optical surface measurement. However, to date, no research effort has
been specifically dedicated to investigate the challenges of measuring internal surfaces. In this paper, a preliminary investigation
into XCT measurement of internal surfaces is presented.
2 Methodology
A hollow artefact fabricated via direct metal laser sintering (DMLS) is measured with two XCT systems as well as by additional
non-contact optical measurement systems. The two industrial XCT systems available at the University of Nottingham are a
Nikon Metrology MCT 225, and a Zeiss XRadia Versa XRM 500, each utilising scanning parameters optimised for each system.
The artefact is fabricated via DMLS using an EOSINT M 280 in two separable parts (see figure 1). The artefact material is
Ti6Al4V, chosen for industrial relevance and because it is known to be well suited to XCT measurement [8]. Once the two parts
are assembled, the internal surfaces become inaccessible to conventional surface measurement solutions, and thus simulate the
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
metrological challenge of internal geometries that are present in many AM parts. When the two parts are separated, they can be
inspected with more established texture measurement technologies, including coherence scanning interferometry (CSI, Zygo
NewView 8300) and focus variation microscopy (FVM, Alicona InfiniteFocus G5). Surface topographies are extracted from
XCT volumetric datasets by using the maximum gradient method [14] and compared with topography data obtained from the
areal and profile topography measurement instruments. The comparison is based on first aligning the surface topographies
obtained from each measurement, i.e. relocating them within the same measurement coordinate system. Topographies are then
cropped to the same size, in order to ensure that the measurements refer to the same surface region. The shapes and sizes of
topographic features of interest for the DMLS process (weld tracks, spatter, unmelted and partially melted particles) as they are
reconstructed from each measurement, are compared using methods introduced in previous work [13]. Topographies are also
subjected to an overall quantitative comparison via the computation of areal texture parameters (ISO 25178-2 [15,16]).
a) b)
Figure 1: a) Artefact for the measurement of internal surface texture. When assembled, cube dimensions are (10 × 10 × 10) mm, b) schematic
diagram of the surface of interest (highlighted by the white square), as the recessed surface on the half of the cube containing three bores.
2.1 XCT measurements of surface topography
XCT measurements on the MCT system were performed using the following setup: voltage 150 kV, current 36 µA, exposure
2829 ms and geometric magnification 35×; yielding a voxel size of 5.7 µm after reconstruction. A warmup scan of approximately
one hour was performed prior to the scan and a 0.25 mm copper pre-filter was used. X-ray imaging and volumetric reconstruction
were performed using Nikon proprietary software (Inspect-X and CT-Pro, respectively), using filtered back projection with a
second order beam hardening correction and a Hanning noise filter.
XCT measurements on the XRadia system were performed using the following setup: voltage 160 kV, current 63 µA and
exposure 6000 ms. A geometric magnification of 5.75× and optical magnification of 0.4× were used, yielding a voxel size of
5 µm after reconstruction. A proprietary Zeiss HE3 pre-filter was also used. X-ray imaging and volumetric reconstruction were
performed using Zeiss proprietary software (Scout-and-Scan and Reconstructor, respectively) using filtered back projection with
no beam hardening correction and a smooth Gaussian reconstruction filter with a kernel size of 0.5. Reconstructed volumetric
data were imported into VolumeGraphics VGStudioMAX 3.0 [17] and surfaces were determined using the maximum gradient
method over four voxels; using the ISO-50 isosurface as a starting point [14].
2.2 Optical measurement of surface topography
Surface topography measurement systems and setups were chosen based on research performed previously by the authors in
understanding a selective laser melted surface [13].
CSI measurements were performed using the CSI system and related proprietary measurement software (Zygo Mx). The 20×
objective lens was used at zoom (numerical aperture (NA) 0.40, field of view (FoV) 0.42 mm × 0.42 mm). Software data
stitching was enabled to acquire a grid of ninety-five FoV, with 10 % lateral overlap. Vertical stitching was also applied, to
merge two measurement z intervals (145 µm and 100 µm wide respectively with 10 µm overlap) in order to maximise vertical
resolution over a large vertical range.
FVM measurements were performed and related proprietary measurement software was used (Alicona MeasureSuite). The 20×
objective lens (NA 0.40, FoV 0.81 mm × 0.81 mm) was used with ring light illumination. Vertical resolution was set at 50 nm
and lateral resolution at 3 µm. Software data stitching was enabled to acquire a grid of twelve FoV.
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
2.3 Topography data processing
XCT surface data were cropped to extract the surface of interest (see figure 1b) in VGStudioMAX, and outputted as triangulated
meshes in .stl format. No simplification was performed in mesh generation. The triangulated meshes were rotated in MeshLab
[18] to align the surface normal to the z axis (surface normal computed via principal component analysis [19] on the mesh point
cloud), and exported again as an .stl. The rotated meshes were then imported into the surface metrology software MountainsMap
by Digital Surf [20] and resampled into height maps, for comparison to topography datasets obtained from the optical
measurement solutions. Resampling into height maps was performed in MountainsMap by projecting rays along the z-axis onto
the triangulated mesh and recording the intersection points.
Height maps obtained by XCT and optical measurement were aligned (i.e. relocated in the same coordinate system) using
MountainsMap. As the Zeiss XRadia system is not a metrology system, XRadia datasets were scaled in reference to CSI data
(keeping proportions constant). All other datasets maintained their original sizes. For all the datasets, the following procedure
was followed. From the aligned height maps, regions of size (1.5 × 1.5) mm were extracted, and levelled by least-squares mean
plane subtraction. The extracted areas were filtered using a Gaussian convolution S-filter with 11 µm cut-off to remove small-
scale surface features. The cut-off value was chosen as the minimum possible for the lowest lateral resolution height map (the
MCT), representative of a grid of 4 × 4 pixels. A region size of (1.5 × 1.5) mm area was chosen as equal to the size of the region
obtained from CSI measurement. At the used 20× magnification, the CSI region was obtained by stitching a large number of
FoVs (ninety-five) and obtaining a larger area was deemed unfeasible due to the prohibitive number of stitching operations,
excessive data sizes and measurement times. An F-operator was applied in the form of a Gaussian convolution filter, with 1.5 mm
cut-off, to remove form error (waviness at larger scales) and obtain the SF surfaces (primary surfaces). Then, an L-filter (again
based on Gaussian convolution) with 0.5 mm cut-off was applied to remove waviness; thus obtaining the SL surfaces (roughness
surfaces). ISO 25178-2 areal texture parameters were calculated for both the SF and SL surfaces [15]. In addition, analyses on
texture direction and power spectrum density were performed.
3 Results and discussion
3.1 Comparison of surface topography features
The comparison was performed on reconstructed top views of the SF height maps. For visual assessment, false colours
(proportional to heights) were used in the reconstructions. Colour scales were homogenised by truncating height points above
and below a common reference vertical range. Truncation was applied for visualisation purposes only, while the original datasets
were maintained for quantitative comparison.
Figure 2: Levelled and truncated surface height maps: a) Zeiss XRadia XCT at 5.75× geometric and 0.4× optical magnification; b) Nikon
XCT at 35× geometric magnification; c) FVM with 20× objective, ring light; d) CSI with 20× objective, 1.0× zoom
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
Visual investigation reveals similarities between the datasets. With the exception of the XRadia data, the topographies feature a
similar rendition of the weld tracks and of larger-scale waviness components. The reconstruction of smaller-scale features,
however, varies greatly between datasets. The FVM and CSI datasets are relatively equivalent, and the MCT system is capable
of reconstructing some of the relevant topographic features (e.g. the weld tracks), although high-frequency noise is increasingly
evident in the data when compared to optical measurement. The XRadia instrument returned the most noise, to the point where
the main topographic features are barely visible.
3.2 Comparison of areal texture parameters
ISO 25178-2 [15] areal texture parameters were computed for the SF and SL surfaces. The results are presented in Tables 1 and
2 respectively. Only one region was analysed per surface type, leading to only one parameter value per measurement. The
reported parameter values are, therefore, only indicative of the differences between the investigated datasets, and may not be
statistically significant indicators of overall performance of a measurement solution compared to another. For the purpose of this
comparison, parameters extracted from CSI data are used here as a reference measurement. Topography datasets were
bandwidth-matched [21] (by cropping to the same sizes and using the same filtering operations with identical cut-offs); therefore,
the differences should be ascribed to different behaviour of each measurement technology when interacting with the same
measured surface region.
First, we examine parameters computed for SF surfaces, which should show trends consistent with what can be seen by visual
observation of figure 2 (as the SF surface is the most similar to the visually reconstructed one). For the SF surfaces, the optical
techniques return results that are the most similar to each other (Sa and Sq parameters differed by 1 % and 0.5 %, respectively).
This is to be expected as both technologies are well established topographical measurement solutions. What is surprising is that
the MCT system returns similar Sa and Sq parameters (within 1.5 % and 1.2 % respectively of the CSI parameters).
Unfortunately, this result does not hold for the XRadia, where despite the scale correction the Sa and Sq parameters are more
different (within 17 % and 20 % respectively of the CSI parameters). This is consistent with the results of visual observation of
the reconstructed topographies (see figure 2) and indicates that the use of XCT for topography measurement should still be
handled with care, as results may not necessarily be reliable. At this point it is not clear why the results from the XRadia system
differ so substantially from the other systems used, and is likely due to a number of factors in the measurement.
The trend observed for Sa and Sq parameters generally holds for Ssk, Sku, Sal and Std parameters. The Ssk parameter of the SF
surfaces varies between all instruments and was negative for the XRadia system and positive for the MCT and optical systems.
The error of the XRadia system in this case has the additional side effect of providing further misleading information, because
the change of sign implies a different balance of peaks and valleys in the topography (despite the effect not being particularly
pronounced, as Ssk is basically zero in the XRadia data). The kurtosis of the topography height distribution (the Sku parameter)
similarly differed noticeably between measurement instruments when compared to the CSI parameters (16 % for the XRadia,
24 % for the MCT and 10 % for the FVM parameters respectively). Data acquired by all instruments reported very similar values
(within 0.1 % of the CSI parameter) for texture direction (the Std parameter). The autocorrelation length (the Sal parameter) for
the XRadia data was within 3.8 % of the value calculated for CSI data as a percentage of the region width (1.5 mm), while Sal
for the MCT system was within 0.5 %. The Sal value calculated for FVM data was within 0.1 % of the CSI parameter as a
percentage of the region width (1.5 mm).
Zeiss XRadia XCT
Nikon XCT
3.85 µm
3.25 µm
3.27 µm
3.30 µm
4.96 µm
4.09 µm
4.16 µm
4.14 µm
85.7 °
85.7 °
85.8 °
85.8 °
0.168 mm
0.119 mm
0.110 mm
0.111 mm
Table 1: ISO 25178-2 [15] surface parameters for SF surfaces.
Following assessment of SF surfaces, we examine parameters computed for SL surfaces. For the SL surfaces, the optical
techniques return results that are the most closely matched (Sa and Sq parameters differed by 3.6 % and 2.0%, respectively). The
MCT in the SF case again system returns similar Sa and Sq parameters (within 1.6 % and 4.3 % respectively of the CSI
parameters). This result again does not hold for the XRadia, where in this case the Sa and Sq parameters are within 36 % and
34 % respectively of the CSI parameters. The effect of the L filter in this case appears to be in exacerbating differences between
calculated parameters, which may be due again to any number of an as-yet unclear reasons.
For SL surfaces, Sa and Sq parameters calculated for the XRadia data were 36 % and 34 % respectively larger than for the CSI
data, while Sa and Sq parameters calculated for the MCT data were 1.6 % and 4.3 % respectively smaller than those calculated
for the CSI data. The Sa and Sq parameters calculated for FVM data differed from CSI parameters by -3.6 % and -2.0%
respectively. Similarly to the SF surface, the skewness of the SL surface varied greatly between instruments, though in this case
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
it was positive in all cases except for the MCT data. The Sku parameter showed greater variations between instruments than for
SF surfaces, with deviations compared to the CSI parameter (-53 % for the XRadia, -46 % for the MCT and -11 % for the FVM
parameters respectively). Std parameters exactly matched those calculated for SF surfaces. The Sal parameter for the XRadia
system was within 1.1 % of the value calculated for the CSI as a percentage of the region width (1.5 mm), while Sal for the MCT
system was within 0.5 %. The Sal value calculated from FVM data was within 0.2 % of the CSI parameter as a percentage of the
region width (1.5 mm) in the SL case.
Zeiss XRadia XCT
Nikon XCT
2.63 µm
1.90 µm
1.86 µm
1.93 µm
3.38 µm
2.42 µm
2.48 µm
2.53 µm
85.7 °
85.7 °
85.8 °
85.8 °
0.0250 mm
0.0403 mm
0.0441 mm
0.0408 mm
Table 2: ISO 25178-2 [15] surface parameters for SL surfaces.
Although Std parameters are very consistent between datasets, surface texture direction analysis (see figure 3) reveals more
information. Each plot represents the values of the angular power spectrum for the SL surfaces as a function of direction. The
angle corresponding to the maximum value is taken as Std. These direction analyses show that, while the position of the primary
peak (i.e. the Std parameter) is consistent between spectra, the ratio between the size of the primary peak and the smaller peaks
(i.e. the signal to noise ratio) varies. This ratio is greatest in the CSI data and smallest in the XRadia data. As measurement noise
is random and, therefore, devoid of direction, this can likely be attributed to greater noise in the XRadia measurement than in
other datasets. It is clear that the values of the angular power spectrum are generally higher in multiple directions in the case of
the noisier XRadia dataset, making it more difficult to isolate the highest peak. Despite the increased noise, however, isolation
of this peak was still possible in the XRadia case. Noise in the Nikon data is much lower than in the XRadia data, but is visibly
more substantial than in either of the optical measurements.
Figure 3: Surface texture direction of SL surfaces: a) Zeiss XRadia XCT; b) Nikon XCT; c) FVM; d) CSI
Further information about the SL surfaces can be provided by analysis of the averaged power spectrum densities (figure 4) of
the surfaces. The plots are truncated at 2.5 µm height for ease of comparison. A number of elements are of interest in the averaged
power spectrum density plots. FVM and CSI plots are very similar; both demonstrate an almost equivalent representation of the
relevant topography frequencies as peaks can be observed corresponding to the main periodic features to be expected in a DMLS
surface (e.g. weld tracks, represented by three peaks between 0.10 mm and 0.15 mm wavelengths). Some spectra carry more
information at smaller scales (i.e. the size of the largest peak between 0.00 mm and 0.10 mm) than others. These peaks are
typically a combination of smaller scale features and high-frequency noise. Interestingly, the position of the smallest-scale peak
is shifted towards slightly larger wavelengths in the FVM dataset compared to the CSI dataset, which indicates a further
attenuation of the smallest scales in FVM measurement. This is presumably due to the averaging mechanisms that implicitly take
place in height determination via contrast, i.e. the way FVM operates (further investigation is in progress to better understand
this observation). The MCT is again capable of capturing many of the same frequencies as the CSI and the FVM, albeit less
strongly. The MCT averaged power spectrum density has a maximum slightly shifted towards the lowest frequencies, suggesting
that the MCT system was not equivalently capable of capturing the highest frequency components of the topography when
compared to CSI and FVM. Finally, consistent with all the previous observations, the XRadia system is the least capable of
capturing the relevant frequencies of the topography.
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
Figure 4: Averaged power spectrum densities of SL surfaces: a) Zeiss XRadia XCT; b) Nikon XCT; c) FVM; d) CSI
4 Conclusions and future work
Visual comparison showed notable similarities between all datasets, with the two optical systems showing the closest similarity
(as could have been expected). However, MCT data were also visibly similar to those outputted by the two optical systems.
XRadia data were not as similar; although some of the key features identifiable in other datasets could be identified in the XRadia
data (i.e. weld track geometry).
Qualitative comparison of areal parameters calculated for SF and SL surfaces also showed similarity between values extracted
from MCT and optical data. The MCT system demonstrates that topography measurement via XCT is viable. The XRadia system,
however, demonstrates that XCT measurement of topography should be handled with care, as results may be unreliable, and
expert assessment, appropriate utilisation, and skilful interpretation of results are still required. In terms of specific parameters,
some are more robust than others (e.g. Std).
Our intention in performing this study was to qualitatively demonstrate the capability of XCT for surface topography
measurement, particularly in reference to measurement of internal or otherwise difficult-to-access surfaces. As such, we have
provided a preliminary assessment of this capability, through comparison of surface data extracted from two XCT systems with
data extracted from conventional optical surface metrology instruments. It is clear that XCT may be a viable method of surface
topography measurement, but performance may be strongly dependent on XCT instrument and set-up, as illustrated by the MCT
and XRadia solutions. Regarding the quality of the XRadia data (in that the XRadia data were not particularly similar to the data
acquired by other systems), it should be noted that these conclusions hold only for the particular measurement setup used. The
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
setup (as opposed to the instrument itself) may be the primary reason that the data were not particularly similar to those acquired
using other systems, and further investigation is required to examine the cause of this problem. The XRadia data in this study
serve to demonstrate the difficulty of acquiring reliable surface data from XCT.
The current limitations of this study should be noted. Primarily, as this study was based upon analyses of single measurements,
significant work is yet to be performed in statistical testing of the methods used here in terms of measurement repeatability and
reproducibility. As such, in their current state, no level of agreement or disagreement can be reported between results. Similarly,
while the MCT system showed some qualitative agreement with the optical setups used, the XRadia system did not exhibit the
same qualitative agreement and much work is yet to be performed in examining why this was the case. As such, a rigorous
assessment of the minimum requirements of an XCT system used for surface topography applications is required. Variables
examined in this assessment will include geometric magnification, sample material, image contrast and any of the myriad of
other variables set during an XCT measurement.
AT, LK and RKL would like to thank the EPSRC (Grants EP/M008983/1 and EP/L01534X/1), 3TRPD Ltd. and Nikon
Metrology for funding this work. NS and RKL would also like to thank the EC for supporting this work through the grant FP7-
PEOPLE-MC METROSURF. The authors would like to thank Martin Corfield of the University of Nottingham, Faculty of
Engineering for performing XRadia scans, and Digital Surf for providing the MountainsMap software.
[1] I. Gibson, D.W. Rosen and B. Stucker, Additive manufacturing technologies: 3D printing, rapid prototyping, and direct
digital manufacturing (New York, NY, USA: Springer), 2014
[2] G. Ameta, R. Lipman, S. Moylan and P. Witherell, Investigating the role of geometric dimensioning and tolerancing in
additive manufacturing J. Mech. Des., Vol 137, 111401, 2015
[3] R.K. Leach, Metrology for Additive Manufacturing, Meas. + Control, Vol 49, 1325, 2016
[4] P.I. Stavroulakis and R.K. Leach, Review of post-process optical form metrology for industrial-grade metal additive
manufactured parts, Rev. Sci. Instrum., Vol 87, 041101, 2016
[5] A. Townsend, N. Senin, L. Blunt, R.K. Leach and J.S. Taylor , Surface texture metrology for metal additive manufacturing:
a review, Precis. Eng., Vol 46, 3447, 2016
[6] L. De Chiffre, S. Carmignato, J.-P. Kruth, R. Schmitt and A. Weckenmann, Industrial applications of computed
tomography, CIRP Ann. - Manuf. Technol., Vol 63, 65577, 2014
[7] ISO 10360-11 DRAFT, Geometrical Product Specifications (GPS) - Acceptance and reverification tests for coordinate
measuring machines (CMM) - Part 11: Computed tomography (International Organization for Standardization)
[8] A. Thompson, I. Maskery and R.K. Leach, X-ray computed tomography for additive manufacturing: a review, Meas. Sci.
Technol., Vol 27, 072001, 2016
[9] G. Pyka, G. Kerckhofs, A. Braem, T. Mattheys, J. Schrooten and M. Wevers, Novel micro-CT based characterization tool
for surface roughness measurements of porous structures, SkyScan User Meeting (Mechelen, Belgium: SkyScan User
Meeting), 15, 2010
[10] G. Kerckhofs, G. Pyka, M. Moesen, S. Van Bael, J. Schrooten and M. Wevers, High-resolution microfocus xray computed
tomography for 3D surface roughness measurements of additive manufactured porous materials, Adv. Eng. Mater., Vol 15, 153
8, 2013
[11] G. Pyka, A. Burakowski, G. Kerckhofs, M. Moesen, S. Van Bael, J. Schrooten and M. Wevers, Surface modification of
Ti6Al4V open porous structures produced by additive manufacturing, Adv. Eng. Mater., Vol 14, 36370, 2012
[12] A. Townsend, L. Blunt and P.J. Bills, Investigating the capability of microfocus x-ray computed tomography for areal
surface analysis of additively manufactured parts, ASPE/euspen Conference: Dimensional Accuracy and Surface Finish in
Additive Manufacturing (Raleigh, NC, USA), 2016
[13] A. Thompson, N. Senin and R.K. Leach, Towards an additive surface atlas, ASPE/euspen Conference: Dimensional
Accuracy and Surface Finish in Additive Manufacturing (Raleigh, NC, USA), 2016
[14] ISO 25178-2, Geometrical product specifications (GPS) -- Surface texture: Areal -- Part 2: Terms, definitions and surface
texture parameters, 2012 (International Organization for Standardization)
[15] R.K. Leach, Characterisation of areal surface texture (Springer-Verlag), 2013
[16] J.J. Lifton, A.A. Malcolm and J.W. McBride, A simulation-based study on the influence of beam hardening in x-ray
computed tomography for dimensional metrology, J. Xray. Sci. Technol., Vol 23, 6582, 2015
[17] Volume Graphics, VGStudio MAX, 2016, (accessed 14th
December 2016)
[18] Visual Computing Lab - ISTI CNR, MeshLab, 2016, (accessed 14th December 2016)
[19] I. Jolliffe, Principal Component Analysis, in Wiley StatsRef: Statistics Reference Online (Wiley), 2014
[20] Digital Surf, Mountains® surface imaging & metrology software, 2016,
(accessed 14th December 2016)
7th Conference on Industrial Computed Tomography, Leuven, Belgium (iCT 2017)
[21] R.K. Leach and H. Haitjema, Bandwidth characteristics and comparisons of surface texture measuring instruments, Meas.
Sci. Technol., Vol 21, 32001, 2010
... by means of CT is also rich in pore statistics compared to the 2D analysis by means of optical microscopy on cross sections. Studies focused on X-ray CT and L-PBF are mostly conducted on low-to mediumabsorbing materials such as aluminium-based alloys or Ti-6Al-4V [38][39][40][41]. The porosity characterisation has been used for optimisation of the process parameters [42]. ...
... Similar to the porosity, the surface topography is currently considered as a source of defects of AM parts [38][39][40]54]. Methods to quantify the surface topography in terms of surface roughness such as coordinate measuring machine (CMM) and optical microscopy (OM) have limitations for small and round-shaped samples as well as complex structures [55]. ...
... Methods to quantify the surface topography in terms of surface roughness such as coordinate measuring machine (CMM) and optical microscopy (OM) have limitations for small and round-shaped samples as well as complex structures [55]. The need for CT regarding the investigation of L-PBF parts is justified because no other non-destructive technique allows the evaluation of complex geometries with inner surfaces [38,39] and re-entrant features [56]. State of the art roughness measurement techniques for conventional materials are tactile and optical surface probing. ...
Full-text available
Additive Manufacturing (AM) in terms of laser powder-bed fusion (L-PBF) offers new prospects regarding the design of parts and enables therefore the production of lattice structures. These lattice structures shall be implemented in various industrial applications (e.g. gas turbines) for reasons of material savings or cooling channels. However, internal defects, residual stress, and structural deviations from the nominal geometry are unavoidable. In this work, the structural integrity of lattice structures manufactured by means of L-PBF was non-destructively investigated on a multiscale approach. A workflow for quantitative 3D powder analysis in terms of particle size, particle shape, particle porosity, inter-particle distance and packing density was established. Synchrotron computed tomography (CT) was used to correlate the packing density with the particle size and particle shape. It was also observed that at least about 50% of the powder porosity was released during production of the struts. Struts are the component of lattice structures and were investigated by means of laboratory CT. The focus was on the influence of the build angle on part porosity and surface quality. The surface topography analysis was advanced by the quantitative characterisation of re-entrant surface features. This characterisation was compared with conventional surface parameters showing their complementary information, but also the need for AM specific surface parameters. The mechanical behaviour of the lattice structure was investigated with in-situ CT under compression and successive digital volume correlation (DVC). The deformation was found to be knot-dominated, and therefore the lattice folds unit cell layer wise. The residual stress was determined experimentally for the first time in such lattice structures. Neutron diffraction was used for the non-destructive 3D stress investigation. The principal stress directions and values were determined in dependence of the number of measured directions. While a significant uni-axial stress state was found in the strut, a more hydrostatic stress state was found in the knot. In both cases, strut and knot, seven directions were at least needed to find reliable principal stress directions.
... Stavroulakis and Leach reviewed optical form metrology for industrial-grade metal additive manufactured parts and emphasized the research needs on new metrology tools, procedures, tolerancing rules and characterization methods for to cope with the complexity of AM parts [12]. Additional works on CT systems were also presented to capture the geometrical characteristics of internal surfaces or porosities of AM produced parts [13], [14]. In depth assessment on application of CT measurement for high quality metal additive manufacturing was studied by Du Plessis and lattice structures were included on top of geometrical features [15]. ...
... The manufactured test parts were then measured by CMM, 3DS and CT precision metrology systems, and the results were compared in terms of different measurement and AM methods. Uncertainty analysis per JCGM 100:2008 were conducted based on the results of repeated CMM measurements [13]. To the authors' knowledge, there is no study comparing FDM and SLS AM methods by three precision measurement systems in terms of dimensional accuracy. ...
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This paper presents a comparative study on precision metrology systems such as Coordinate Measuring Machine (CMM), 3-Dimensional Scanning (3DS) and Computed Tomography (CT) for polymer additive manufacturing. A special test sample was designed and manufactured by Fused Deposition Modeling (FDM) and Selective Laser Sintering (SLS) AM systems. The manufactured parts were then measured by three different precision metrology systems and the results were compared in terms of different measurement and AM methods. Uncertainty analyses were conducted based on the results of CMM measurements. The benchmark highlighted the difference between part characteristics manufactured by FDM and SLS, where FDM part represented higher surface roughness and more deviation to the nominal design. Furthermore, expanded uncertainties computed for the FDM manufactured part were almost three times of the uncertainties computed for the SLS manufactured part. It was also demonstrated that one of the major contributors to the expanded uncertainty occurred because of rougher surface of FDM manufactured part. Similar tendency of part to nominal deviations were observable in all metrology systems including CMM, CT and 3DS. Findings of the study revealed the need of standardized measurement for inspection and control of AM parts.
... The surface roughness of L-PBF parts has recently been discussed in several works. [5][6][7][8][9][10][11] The arithmetic mean roughness value R a is the most common parameter to describe the surface roughness. 12 R a is classically determined by tactile and optical measurement techniques. ...
... This agreement indicates the applicability of XCT for classic line-profile roughness analysis. [14][15] XCT, thanks to its fully 3D capabilities, additionally enables the evaluation of inner surfaces [7][8] , and re-entrant surface features. [16][17] A method for the identification of re-entrant features by considering the direction of the surface normal has been discussed by Pagani et al. 16 , who present a mathematical definition of a reentrant feature. ...
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Layer-by-layer Additive Manufacturing by means of Laser-Powder Bed Fusion offers many prospects regarding the design of lattice structures used for example in gas turbines. However, defects such as bulk porosity, surface roughness, and re-entrant features are exacerbated in non-vertical structures such as tilted struts. The characterization and quantification of these kinds of defects are essential for a correct estimation of fracture and fatigue properties. In this work, cylindrical struts fabricated by Laser-Powder Bed Fusion are investigated by means of X-ray computed tomography (XCT), with the aim of casting light on the dependence of the three kinds of defects (bulk porosity, surface roughness, and re-entrant features) on the build angle. Innovative analysis methods are proposed to correlate shape and position of pores, to determine the angular-resolved surface roughness, and to quantify the amount of re-entrant surface features, q. A meshing of the XCT surface enables the correlation of q with the classical surface roughness Pa. The analysis leads to the conclusions that there is a linear correlation between q and Pa. However, it is conjectured that there must be a threshold of surface roughness, below which no re-entrant features can be build. This article is protected by copyright. All rights reserved.
... Drawbacks are the long acquisition time, limitation of object size and high cost of the system, as well as the requirement of highly specialised operators. However, its potential for a holistic description and understanding of metal AM part quality is undeniable [17,72,73]. ...
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Additive manufacturing technologies enable lightweight, functionally integrated designs and development of biomimetic structures. They contribute to the reduction in material waste and decrease in overall process duration. A major challenge for the qualification for aerospace applications is the surface quality. Considering Ti-64 laser powder bed fusion (LPBF) parts, particle agglomerations and resulting re-entrant features are characteristic of the upper surface layer. Wet-chemical post-processing of the components ensures reproducible surface quality for improved fatigue behaviour and application of functional coatings. The 3D SurFin ® and chemical milling treatments result in smoother surface finishes with characteristic properties. In order to characterise these surfaces, three methods for surface texture measurement (contact and non-contact) were applied, namely confocal microscopy, fringe projection and stylus profilometry. The aim of this work was to show their suitability for measurement of laser powder bed fusion as-built and post-processed surfaces and compare results across the evaluated surface conditions. A user-oriented rating of the methods, summarising advantages and disadvantages of the used instruments specifically and the methods in general, is provided. Confocal microscopy reaches the highest resolution amongst the methods, but measurements take a long time. The raw data exhibit large measurement artefacts for as-built and chemically milled conditions, requiring proper data post-processing. The stylus method can only capture 2D profiles and the measurement was restricted by particle agglomerations and craters. However, the method (process and instrument) is entirely standardised and handheld devices are inexpensive, making it accessible for a large group of users. The fringe projection method was the quickest and easiest regarding measurement and data post-processing. Due to large areal coverage, reproduction of location when performing repeat measurements is possible. The spatial resolution is lower than for confocal microscopy but is still considered sufficiently high to characterise the investigated surface conditions.
... Multiple studies highlight methods to obtain both areal surface roughness measurements [16][17] [18] [19][20] [21], and linear profile surface roughness measurements [22] [23]. The studies on areal roughness measurements explain the challenges using the optimal voxel size and form removal of complex surfaces fabricated via AM. ...
Conference Paper
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Tensile testing is the most prevalent method for characterizing the mechanical properties of additively manufactured (AM) materials. During qualification of metallic AM properties, near-net AM parts are often machined prior to mechanical testing. The aim of this study is to understand the influence of net-shaped tensile coupons without post-AM machining on the accuracy of bulk material properties. The motivation for this study lies in: (1) reducing the qualification time and costs by (2) formulating and validating a correction factor to estimate bulk AM properties from mechanical testing of as-AM coupons. This research focused on the tensile testing of Laser Powder Bed Fusion (LPBF) produced Inconel 718 to isolate the effects of as-AM surface roughness. Six different surface conditions were produced by varying two different laser processing conditions, with and without contour laser scans. Specimens (n = 5 per condition) were tested in both net-shape and post-AM machined conditions. Surface roughness was analyzed using both stylus contact profilometry and micro-computed tomography (micro-CT) non-contact analysis. ANOVA analysis was performed to derive inference on processing conditions and resulting mechanical properties. It was observed that the measurement error in gauge diameter primarily accounts for variability in mechanical properties between machined and net-shape coupons, specifically Ultimate Tensile Strength (UTS). This study presents a methodology to determine corrected gauge diameter based on depth of surface roughness. Findings from this study will enable net-shape tensile data to be compared against machined data for accurately predicting the strength of parts with as-AM surfaces. By accounting for surface roughness depth, tensile strength of net-shape AM coupons was within 1% accuracy to that of machined AM coupons.
... XCT gains significant attention by allowing inspection of internal features and their surface characteristics, without using a destructive procedure, despite the limitations of accuracy, repeatability, resolution and uncertainty [11]. There are many studies conducted regarding its usage in surface characterization [7,[12][13][14][15][16]. Reconstruction of XCT data, resolution, sample material and machine features are some factors affecting the XCT measurement. ...
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Surface characterization becomes an important practice in industry as the nature of metal additive manufacturing (AM) process offers varying resultant surfaces based on different process parameters and geometries. Integrating of X-ray computed tomography (XCT) technology into the characterization of AM surfaces by solving its’ challenges will be a significant leap, especially for internal surfaces. In this paper, data of various AM surfaces obtained using a coherence scanning interferometry is compared with surfaces extracted from XCT measurements. Effects of specimen material and surface characteristics on evaluating different areal surface parameters using XCT are discussed.
... FVM measurement results usually exhibit higher surface roughness, e.g. Sa, Sq, and can capture more surface details than XCT [4][5][6][7][8][9], as shown in Table 1. Senin et al. [6] compared the surface topography characteristics of the particles, and their results show that FVM may not be able to capture the re-entrant feature and such results are therefore labelled as NMPs and noise. ...
Conference Paper
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Focus variation microscopy (FVM) and X-ray computed tomography (XCT) are two popular technologies employed to measure the surface texture of additively manufactured components. The impacts of various influence factors of these two measurement techniques on surface texture measurement are investigated, respectively. A two-stage surface registration program is developed to allow the accurate comparison of surface topographies measured by two instruments at the same location on the surface. The associated surface texture parameters Sa, Sq, Ssk, and Sku are also compared. The results indicate XCT can capture re-entrant features, while FVM can capture more details. However, XCT measurement is limited in its resolution and FVM measurement is restricted by the line-of-sight constraint. The surface parameters of two measurement techniques are closer in the cases where surface texture of the parts is higher.
... Their comparison between 45° and horizontally oriented channels showed that both directions produced similar channels for their material (Inconel 718) and LPBF setup, whereupon channel diameter roundness and internal roughness were investigated. The applicability of CT inspection to derive local surface texture information of internal features was later on assessed by Thompson et al. [26,27]. Their studies concentrated on the comparison between CT analysis and optical surface measurements, showing how CT data can be a viable alternative for topography measurements, even if it needs to be handled with care. ...
Channels and bores in metal components produced by laser powder bed fusion (LPBF) are internal features that are typically affected by defects such as dross and sag formation, dimensional errors and global deformations in different proportions. Such deviations from the ideal geometry may strongly limit the functionality of the channels, but are difficult to prevent, due to complex multi-physical production aspects. Different destructive and non-destructive approaches are available to investigate the geometry of the internal features and possibly correlate their results to the LPBF process parameters; however, such approaches do not offer a systematic method to derive key characteristics of the main contributors for channel deviations. Hence, this work proposes a novel tomographic non-destructive analysis of LPBF channels and bores, focusing on the derivation of sag and dross key parameters. The methodology works on polar-transformed profiles obtained from image stacks which are extracted perpendicularly to the channel axis from the X-ray computed tomography (CT) reconstructed volume. The method allows for the clear determination of surface characteristics and includes the quantitative evaluation of descriptors through an algorithm specifically developed for the purpose. In particular, general form deviations are addressed by fitting sinusoidals on the unwrapped mean surface profile, to tackle deviations induced by thermal residual stresses. Proposed descriptors of sag and dross are the onset angle of protrusions, separation criteria between sag and dross effects, and the peak analysis of the mean profile after approximation with a least squares spline. The developed algorithm is tested in the case study of a LPBF AlSi7Mg0.6 benchmark part comprising hollow cylinders and inter-connecting frusta with different diameters. The resulting evaluation of the benchmark part also corroborates how the proposed methodology can help to obtain more precise information regarding the correlation of LPBF fabrication conditions and obtained channels geometrical deviations. Furthermore, the results show possible routes to enable an a-priori compensation of the nominal channel design for first-time right LPBF manufacturing.
... In the papers of [40][41][42], different measurement systems have been used in order to analyse the measurement technology impact on the obtained results. The authors used both contact and optical measurement methods, considering X-ray computer tomography and multiscale 3D curvature analysis. ...
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Laser technologies for fast prototyping using metal powder-based materials allow for faster production of prototype constructions actually used in the tooling industry. This paper presents the results of measurements on the surface texture of flat samples and the surface texture of a prototype of a reduced-mass lathe chuck, made with the additive technology—powder bed fusion. The paper presents an analysis of the impact of samples’ orientation on the building platform on the surface geometrical texture parameters (two-dimensional roughness profile parameters (Ra, Rz, Rv, and so on) and spatial parameters (Sa, Sz, and so on). The research results showed that the printing orientation has a very large impact on the quality of the surface texture and that it is possible to set digital models on the building platform (parallel—0° to the building platform plane), allowing for manufacturing models with low roughness parameters. This investigation is especially important for the design and 3D printing of microelectromechanical systems (MEMS) models, where surface texture quality and printable resolution are still a large problem.
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X-Ray computed tomography, (XCT) has begun to prove itself as a valid method to quantify internal and external surface metrology of parts. However, the understanding of how a small change in the capture settings of the XCT can affect the surface data extracted from parts needs to be developed so that like all metrology instruments the operating window in which the technology provides repeatable results with minimum uncertainty is understood. For XCT there exists a clear point of uncertainty in the measurement process, this being at the generation point of the X-rays, at this point the electron beam impacts a target material causing the generation of X-rays. As the intensity and size of the electron beam focal spot have direct control of the produced X-rays, it is important to align the filament in such a way that the best quality radiographs are generated. The more optimally focused each individually radiograph is in an XCT scan the better quality the 3D model acquired will be, thus leading to more representative surface data extraction. The paper details an investigative study into the effects of this focusing procedure on the extracted surface data. A further aim is to analyse how any noted changes compare to those noted between filament changes as shown in the author’s previous work. The study found that variance in extracted surface texture parameters exceeded that seen in the author previous work, leading the conclusion that the installation process is likely the main contributing factor to the variance noted in both studies. The findings presented also showed that all hybrid and amplitude parameters from the previous study fell within 2 standard deviations of the means of this study. Spacing parameters showed less variance across all data sets however a larger range of values was still observed in the second study.
Conference Paper
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INTRODUCTION The ability to perform non-destructive areal surface analysis, for example of the internal surfaces of additively manufactured (AM) parts has potential advantages during product development and for production process control. This paper reports on the extraction of areal surface information from microfocus x-ray computed tomography (XCT) data. Using this novel technique a range of areal parameter values were generated from a surface section extracted from XCT scan data of an as-built (no post-processing) AlSi10Mg additively manufactured part. This was then compared with the parameter values generated from a focus variation scan of the same surface section. The data comparison method involving normalisation of data format to allow analysis using industry-standard software, such as MountainsMap (Digital Surf, Besançon, France) or SurfStand (The Centre for Precision Technologies UoH) is demonstrated. Importing the extracted surfaces into these powerful software packages allows one-click data filtering per ISO 25178-3 [1] and the generation of a comprehensive suite of areal surface parameter values. These include feature and field parameters, amplitude, spatial, hybrid and functional parameters, as defined in ISO 25178-2 [2]. A method for characterising the capability of XCT for areal surface measurement is demonstrated by comparing results obtained from samples taken from a Rubert comparator test panel, with sample surface Ra values between 0.8 μm and 50 μm.
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In this review, the use of x-ray computed tomography (XCT) is examined, identifying the requirement for volumetric dimensional measurements in industrial verification of additively manufactured (AM) parts. The XCT technology and AM processes are summarised, and their historical use is documented. The use of XCT and AM as tools for medical reverse engineering is discussed, and the transition of XCT from a tool used solely for imaging to a vital metrological instrument is documented. The current states of the combined technologies are then examined in detail, separated into porosity measurements and general dimensional measurements. In the conclusions of this review, the limitation of resolution on improvement of porosity measurements and the lack of research regarding the measurement of surface texture are identified as the primary barriers to ongoing adoption of XCT in AM. The limitations of both AM and XCT regarding slow speeds and high costs, when compared to other manufacturing and measurement techniques, are also noted as general barriers to continued adoption of XCT and AM.
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Additive manufacturing (AM) has increasingly gained attention in the last decade as a versatile manufacturing process for customized products. AM processes can create complex, freeform shapes while also introducing features, such as internal cavities and lattices. These complex geometries are either not feasible or very costly with traditional manufacturing processes. The geometric freedoms associated with AM create new challenges in maintaining and communicating dimensional and geometric accuracy of parts produced. This paper reviews the implications of AM processes on current geometric dimensioning and tolerancing (GD&T) practices, including specification standards, such as ASME Y14.5 and ISO 1101, and discusses challenges and possible solutions that lie ahead. Various issues highlighted in this paper are classified as (a) AM-driven specification issues and (b) specification issues highlighted by the capabilities of AM processes. AM-driven specification issues may include build direction, layer thickness, support structure related specification, and scan/track direction. Specification issues highlighted by the capabilities of AM processes may include region-based tolerances for complex freeform surfaces, tolerancing internal functional features, and tolerancing lattice and infills. We introduce methods to address these potential specification issues. Finally, we summarize potential impacts to upstream and downstream tolerancing steps, including tolerance analysis, tolerance transfer, and tolerance evaluation.
A comprehensive analysis of literature pertaining to surface texture metrology for metal additive manufacturing has been performed. This review paper structures the results of this analysis into sections that address specific areas of interest: industrial domain, additive manufacturing processes and materials; types of surface investigated; surface measurement technology and surface texture characterisation. Each section reports on how frequently specific techniques, processes or materials have been utilised and discusses how and why they are employed. Based on these results, possible optimisation of methods and reporting is suggested and the areas that may have significant potential for future research are highlighted.
The scope of this review is to investigate the main post-process optical form measurement technologies available in industry today and to determine whether they are applicable to industrial-grade metal additive manufactured parts. An in-depth review of the operation of optical three-dimensional form measurement technologies applicable to metal additive manufacturing is presented, with a focus on their fundamental limitations. Looking into the future, some alternative candidate measurement technologies potentially applicable to metal additive manufacturing will be discussed, which either provide higher accuracy than currently-available techniques but lack measurement volume, or inversely, which operate in the appropriate measurement volume but are not currently accurate enough to be used for industrial measurement.
This book covers in detail the various aspects of joining materials to form parts. A conceptual overview of rapid prototyping and layered manufacturing is given, beginning with the fundamentals so that readers can get up to speed quickly. Unusual and emerging applications such as micro-scale manufacturing, medical applications, aerospace, and rapid manufacturing are also discussed. This book provides a comprehensive overview of rapid prototyping technologies as well as support technologies such as software systems, vacuum casting, investment casting, plating, infiltration and other systems. This book also: Reflects recent developments and trends and adheres to the ASTM, SI, and other standards Includes chapters on automotive technology, aerospace technology and low-cost AM technologies Provides a broad range of technical questions to ensure comprehensive understanding of the concepts covered.
Principal component analysis has often been dealt with in textbooks as a special case of factor analysis, and this tendency has been continued by many computer packages which treat PCA as one option in a program for factor analysis—see Appendix A2. This view is misguided since PCA and factor analysis, as usually defined, are really quite distinct techniques. The confusion may have arisen, in part, because of Hotelling’s (1933) original paper, in which principal components were introduced in the context of providing a small number of ‘more fundamental’ variables which determine the values of the p original variables. This is very much in the spirit of the factor model introduced in Section 7.1, although Girschick (1936) indicates that there were soon criticisms of Hotelling’s method of PCs, as being inappropriate for factor analysis. Further confusion results from the fact that practitioners of ‘factor analysis’ do not always have the same definition of the technique (see Jackson, 1981). The definition adopted in this chapter is, however, fairly standard.