ASTM F3624-23

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F3624 − 23
Standard Guide for
Additive Manufacturing of Metals – Powder Bed Fusion –
Measurement and Characterization of Surface Texture
This standard is issued under the fixed designation F3624; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope ISO 21920 part 2 2022 Geometrical product specifications
(GPS) — Surface texture: Profile. Part 2: Terms, defini-
1.1 This guide is designed to introduce the reader to
tions and surface texture parameters
techniques for surface texture measurement and characteriza-
ISO 21920 part 3 2022 Geometrical product specifications
tion of surfaces made with metal powder bed fusion additive
(GPS) — Surface texture: Profile. Part 3: Specification
manufacturing processes. It refers the reader to existing stan-
operators
dards that may be applicable for the measurement and charac-
ISO 25178 part 1 2016 Geometrical product specifications
terization of surface texture.
(GPS). Surface texture: Areal. Indication of surface tex-
1.2 Units—The values stated in SI units are to be regarded
ture
as the standard. No other units of measurement are included in
ISO 25178 part 2 2012 Geometrical product specifications
this standard.
(GPS). Surface texture. Areal. Terms, definitions, and
1.3 This standard does not purport to address all of the surface texture parameters
safety concerns, if any, associated with its use. It is the
ISO 25178 part 3 2012 Geometrical product specifications
responsibility of the user of this standard to establish appro- (GPS). Surface texture: Areal. Specification operators
priate safety, health, and environmental practices and deter-
ISO 25178 part 600 2019 Geometrical product specifications
mine the applicability of regulatory limitations prior to use. (GPS). Surface texture: Areal. Metrological characteristics
1.4 This international standard was developed in accor-
for areal topography measuring methods
dance with internationally recognized principles on standard- ISO 25178 part 601 2010 Geometrical product specifications
ization established in the Decision on Principles for the
(GPS) — Surface texture: Areal — Part 601: Nominal
Development of International Standards, Guides and Recom- characteristics of contact (stylus) instruments
mendations issued by the World Trade Organization Technical
ISO 25178 part 604 2013 Geometrical product specifications
Barriers to Trade (TBT) Committee.
(GPS). Surface texture: Areal. Nominal characteristics of
non-contact (coherence scanning interferometry) instru-
2. Referenced Documents
ments
ISO 25178 part 606 2015 Geometrical product specification
2.1 ISO Standards (normative):
(GPS). Surface texture: Areal. Nominal characteristics of
ISO 1302 2002 Geometrical product specifications (GPS).
non-contact (focus variation) instruments
Indication of surface texture in technical product docu-
ISO 25178 part 607 2019 Geometrical product specifications
mentation
(GPS). Surface texture: Areal. Nominal characteristics of
ISO 4287 2000 Geometrical product specification (GPS)—
non-contact (confocal microscopy) instruments
surface texture: profile method—terms, definitions, and
ISO/ASTM 52900 2021 Additive manufacturing — General
surface texture parameters
principles — Fundamentals and vocabulary
ISO 4288 1996 Geometrical product specification (GPS)—
2.2 ASME Standards (normative):
surface texture: profile method—rules and procedure for
ASME B46.1 2019 Surface Texture (Surface Roughness,
the assessment of surface texture
Waviness, and Lay)
2.3 National Physical Laboratory (NPL) Guides:
NPL GPG 11 2001 Good Practice Guide No. 11: The
This guide is under the jurisdiction of ASTM Committee F42 on Additive
Beginner’s Guide to Uncertainty of Measurement
Manufacturing Technologies and is the direct responsibility of Subcommittee
F42.01 on Test Methods.
Current edition approved Feb. 15, 2023. Published March 2023. DOI: 10.1520/
F3624-23. Available from American Society of Mechanical Engineers (ASME), ASME
Available from International Organization for Standardization (ISO), ISO International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva, www.asme.org.
Switzerland, https://www.iso.org. Available from NPL.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3624 − 23
NPL GPG 37 2014 Measurement Good Practice Guide No. These factors will also influence the surface features that can
37: The Measurement of Surface Texture using Stylus be observed and measured. As such, some understanding of the
Instruments relative scales of the features produced will be useful to
NPL GPG 116 2010 Measurement Good Practice Guide No understand the limitations of various measurement technolo-
116. The Measurement of Rough Surface Topography gies as well as to assist with measurement planning.
using Coherence Scanning Interferometry
4.2 Metrology:
NPL GPG 129 2013 Measurement Good Practice Guide No.
4.2.1 According to the JCGM 200:2012 International vo-
129: Calibration of the Metrological Characteristics of
cabulary of metrology - Basic and general concepts and
Areal Contact Stylus Instruments
associated terms (VIM), metrology is the science of
NPL GPG 127 2013 Measurement Good Practice Guide No.
measurement, including all theoretical and practical aspects of
127: Calibration of the Metrological Characteristics of
measurement, measurement uncertainty and its field of appli-
Coherence Scanning Interferometers (CSI) and Phase
cation.
Shifting Interferometers (PSI)
4.2.2 Measurement is the process of obtaining quantity
NPL GPG 128 2012 Measurement Good Practice Guide No.
values that can be attributed to a quantity, referred to as a
128: Calibration of the Metrological Characteristics of
measurand, with an associated measurement uncertainty. Mea-
Imaging Confocal Microscopes
surement uncertainty is defined as the parameter that charac-
2.4 JCGM Documents (normative):
terizes the dispersion of the quantity values of the measurand.
JCGM 200:2012 International vocabulary of metrology -
National Physical Laboratory (NPL) Good Practice Guide No.
Basic and general concepts and associated terms (VIM)
11: The Beginner’s Guide to Uncertainty of Measurement is a
JCGM 100:2008 Evaluation of measurement data - Guide to
guide to evaluating measurement uncertainty, and more de-
the expression of uncertainty in measurement (GUM)
tailed descriptions can be found in the JCGM 100:2008
Evaluation of measurement data – Guide to the expression of
3. Significance and use
uncertainty in measurement (known as the GUM).
3.1 Determining optimal strategies for the measurement and
4.3 Surface texture metrology:
characterization of surface texture is necessary to increase
4.3.1 Surface texture metrology (often referred to as surface
confidence in the assessment of surfaces and in any further
metrology) is used to characterize local deviations of a surface
comparisons and correlations sought between manufactured
from a defined form (typically a perfectly flat plane). Simply
surfaces, manufacturing processes, and desired functionality.
put, surface metrology deals with geometrical irregularities
Further, measurement and characterization of surface texture
present at a surface, but not those that contribute to the form or
have implications in the field of tribology and in the determi-
shape of the surface. The ASME B46.1 Surface Texture
nation and specification of part quality. This guide is designed
(Surface Roughness, Waviness, and Lay) covers much of the
to provide users of measurement technologies in both industry
content found in the ISO standards. In this guide, we primarily
and academia with good practice for optimizing measurements
refer to “surface texture” (that is, filtered surface data), over the
of surfaces produced by metal powder bed fusion (PBF)
unfiltered “surface topography” as most characterization pipe-
manufacturing processes. While the focus of this guide is on
lines used for AM surface measurement involve some filtra-
surfaces produced by metal PBF, some of the referenced
tion. However, some pipelines, such as those used in multiscale
methods may also be appropriate for surfaces produced by
topographic characterization (see 4.7.5), do not necessarily
other manufacturing processes.
feature filtering and characterize the whole measured topogra-
phy.
4. General concepts
4.3.2 Measurement technologies—There are seven tech-
4.1 Additive manufacturing:
nologies capable of measuring surfaces that are currently
4.1.1 Additive manufacturing (AM) is defined in ASTM
covered within the ISO 25178-60X series.
52900 as “the process of joining materials to make parts from
4.3.3 Of these, the following are most commonly applied in
3D model data, usually layer upon layer, as opposed to
the measurement of AM surfaces:
subtractive manufacturing and formative manufacturing meth-
4.3.3.1 Contact stylus,
odologies”. The direct creation of fully metal parts, where the
4.3.3.2 Coherence scanning interferometry,
final part is wholly metal as built is currently commercially
4.3.3.3 Focus variation microscopy, and
limited to only a few process categories. While the principles
4.3.3.4 Confocal microscopy.
of this guide could be applied to other processes, the guide is
4.3.4 In addition to these approaches, X-ray computed
focused more specifically on PBF. PBF is divided into two
tomography and other conventional form measurement tech-
sub-technologies, determined by the mechanism by which
nologies have increasingly been used to measure parts to
energy is applied to the powder bed, particularly: laser-based 6
extract surface information (1).
PBF (PBF-LB) and electron beam-based PBF (PBF-EB).
4.4 Surface texture characterization:
4.1.2 In the PBF process, the powder and materials, build
4.4.1 Surface topography refers to the overall surface struc-
parameters, support removal, post-processing and surface fin-
ture of a part, surface form refers to the underlying shape of
ishing operations all contribute to the final surface topography.
The boldface numbers in parentheses refer to the list of references at the end of
Available from BIPM. this standard.
F3624 − 23
that part and surface texture refers to features that remain after covered in ISO 4287, now updated into ISO 21290. It should
the form has been removed. The surface texture can either be be noted that ISO 4287 and 21920 differ in procedure (particu-
separated out by filtering the topography into components of larly with respect to the order of the form removal and
waviness and roughness or assessed as a primary profile or λ /S-filter operations), with ISO 21920-2 and ISO 21920-3 now
s
primary surface. Waviness refers to the lower spatial frequency
following the areal approach found in ISO 25178-2 and ISO
components of the surface texture, and roughness to the higher 25178-3, respectively. Their inconsistencies are a part of the
spatial frequency components. These terms are standardized
standards Profile measurements are acquired in a direction
within the context of profile-based surface texture analysis and perpendicular to the lay of the surface, that is, the direction of
applied to areal (that is, relating to, or involving an area)
the dominant manufacturing process marks. For AM surfaces,
surface texture as well. An example visualization of the
this lay may be the weld tracks on the top surfaces or the build
filtering process is presented in Fig. 1. An example of the
layers on side surfaces. For surfaces where the lay is unclear
filtration process and its effect on an example measured profile
(which can occur on some AM surfaces) profile measurements
is seen in Fig. 2.
should be performed in a number of different directions and
4.4.2 For areal surface texture, there is a different naming
resultant parameters averaged for the surface. Further informa-
convention to that for profile analysis, where the surfaces are
tion and guidance on profile measurement and characterization
defined by the filters applied and how they limit the scale on
can be found in NPL Measurement Good Practice Guide No.
the surface. The equivalent to the roughness profile is the ‘S-L’
37: The Measurement of Surface Texture using Stylus Instru-
scale-limited surface, whilst the primary and waviness profiles
ments.
are referred to as ‘S-F’ and ‘L-F’ surfaces, respectively (for
4.5.2 While the procedures for profile measurement are
clarity, it is important to correctly define filter nesting index
covered in ISO 4288, now ISO 21920, it should be noted that
values). An example visualization of the filtering process is
the filters specified to characterize different surface textures
presented in Fig. 3.
were designed for conventionally machined surfaces, rather
4.5 ISO 21920 profile measurement: than AM surfaces. AM surfaces often have Ra values much
4.5.1 A surface profile is defined as the profile that results higher than 6 μm, which, as specified, would require a 40 mm
from the intersection of a surface with a specified plane, that is, evaluation length (with an λ filter cut-off length of 8 mm).
c
the plane in which the measurement is taken. When character- However, recent research has been used to show that this
izing profiles, the following procedures should be followed, as relatively large evaluation length is not required to successfully
FIG. 1 Effect of filters on the spatial frequencies of a surface texture measurement
F3624 − 23
FIG. 2 Example surface topography filtering process (profile); following ISO 4287
characterize AM surfaces, and a smaller 8 mm evaluation the presence of peaks or pits, and the relationship between the
length (with a λ filter cut-off length of 2.5 mm) is generally height distribution and a Gaussian distribution.
c
sufficient (2). Additionally, it is important to note that the
4.7 ISO 25178 areal measurement:
choice of filter cut-off length should be based on the features of
4.7.1 Areal surface measurement results in a ‘2.5D’ repre-
interest, which can be smaller than 2.5 mm.
sentation of the surface, with height information being a
4.6 ISO 21920 profile parameters: function of the two dimensions of the plane, so that heights are
4.6.1 The roughness parameters, prefixed by an R-, refer to represented as z(x,y), defined within ISO 25178. This data is
those generated only on the roughness surface shown above in commonly considered 2.5D as opposed to 3D, because datasets
Fig. 2. Equivalent parameters, prefixed with P- and W-, refer to are only capable of handling one z value for each value of x, y
those computed on the primary and waviness profiles, respec- (meaning the data cannot account for the presence of undercuts
tively. Most (but not all) R parameters are calculated for each in the surface). The sampling area refers to the xy plane in
sampling length and averaged to calculate a parameter for the which a measurement is performed; typically, this is the size of
evaluated profile. the field of view of an optical measurement system but can also
4.6.2 For profile-based characterization of surface texture, be made of stitched measurement areas or an array of parallel
the most commonly employed parameter is Ra, which is used profiles. Unlike in profile characterization, the components of
to characterize a wide range of engineering surfaces. However, the surface are not defined explicitly as waviness and rough-
there are many more amplitude, spacing, hybrid and curve- ness but are defined by the filtering methods applied to them.
based parameters that can be used to characterize an assessed 4.7.2 As in the profile measurement case, where the λ filter
c
profile. Rq and Rz are often used to give a general assessment cut-off length of 2.5 mm was sufficient for PBF surfaces, for
of roughness and to compare with other conventional surface areal measurement the L-filter nesting index can be set at 2.5
measurements. Parameters that target the peak features on the mm, meaning a (2.5 × 2.5) mm areal measurement would be
profile (Rpk, etc.) may be useful for characterizing particle sufficient to capture enough detail to characterize a PBF
features on the surface. However, profile measurement and surface (2). A measurement size of (2.5 × 2.5) mm is usually
parameters are fundamentally limited by an inability to dis- sufficient for most metal AM surfaces, and L-filter nesting
criminate between some types of features on a surface because indices between 250 μm and 800 μm are commonly employed
they lack areal information. For example, a roughly spherical to extract the weld tracks or smaller features, or both, that exist
particle and an elongated weld track may appear similarly in a on the PBF surface for characterization. It is always recom-
profile measurement. Rsk and Rku may provide indications of mended to set the L-filter nesting index at a value determined
F3624 − 23
FIG. 3 Example surface topography filtering process (areal)
to ensure a valid separation of the features of interest from the comparison. However, these parameters often only quantify a
underlying larger-scale surface components. statistical average or the presence of large height ranges and
4.7.3 Areal parameters for arithmetic mean height Sa, root are limited in assessing the shape of the height distribution. The
mean square height Sq, and maximum height Sz, are commonly skewness parameter Ssk has been applied alongside Sq or Sa to
used to characterize surface texture for general assessment and differentiate upwards facing surfaces from downwards facing
F3624 − 23
surfaces by the nature of the distribution of heights. Functional and have been used as an additional method of characterizing
parameters are applied to assist with the visualization of the the PBF surface, however this lies beyond the scope of this
height range of the surface to identify peak regions on PBF guide.
surfaces, which are related to the presence of particles on
4.8 Surface texture measurement for AM:
upwards facing surfaces.
4.8.1 General Considerations for AM—When measuring
4.7.4 Feature-based characterisation refers to the methods
lattice structures and other complex internal geometries, sur-
that characterize the dimensional and geometric properties of
face texture measurements are commonly limited by either the
individual surface features. These features are any region of the
line-of-sight access requirements of optical systems, or the
surface whose topography is of interest, with common ex-
contact access requirements of contact measurement systems.
amples for PBF ranging down from large particles/spatter to
Another general consideration that should be noted when
individual weld tracks to the weld ripples upon them. Algo-
performing measurements of AM parts is the presence of the
rithmic segmentation is used on the heights of the surface
staircase effect, which is represented by visibly offset layers of
texture to isolate these topographic structures individually,
a fixed height that approximate the 3D model data. Because of
where dimensional characterisation can be performed. Dimen-
this effect (shown in Fig. 4), local surface angle and layer
sional assessment includes step-height assessments in terms of
thickness are often dominant influencing factors on the surface
vertical height (for example, the height of a specific particle, or
topography of side surfaces.
depth of a pit), to contour analysis over the lateral measure-
4.9 General considerations for PBF:
ment (for example, the width of a weld track, or the eccentric-
4.9.1 There are various manufacturing process conditions
ity of spatter) and can be either considered individually, or as
that are specific to the PBF process, most notably the interac-
part of a wider statistic (for example, mean, median).
tion between the energy source and the powder feedstock.
4.7.4.1 These methods can be found in ISO 25178-2 and
Some of the more common challenges presented by PBF
represent additional means of characterizing the PBF surface.
surfaces (shown in Fig. 5) are:
Feature-based characterization methods are generally applied
4.9.1.1 Large measurement ranges.
when a more specific characterization task is to be performed;
4.9.1.2 Sphere-like protrusions.
while texture parameter-based characterization methods pro-
4.9.1.3 Surface pores (also sub-surface).
vide a holistic statistical summary of a surface, feature-based
4.9.1.4 Changing reflectivity.
methods provide specific information about the surface.
4.9.1.5 Large scales of interest.
However, a detailed description of such methods lies beyond
4.9.1.6 Re-entrant features.
the scope of this guide.
4.7.5 Multiscale geometric characterization can be used to 4.9.2 Powder adhered to side surfaces and spatter particles
characterize the fractal complexity for both profile and areal that land on the upper facing surfaces both contribute to the
surface topography, and for establishing scales that are perti- creation of large protrusions on the surface. The surface height
nent to providing value in surface metrology. It is known that range is further increased by the presence of any surface pores
geometric properties, such as surface lengths, surface areas and or valley regions present. Together, these protrusions and
surface slopes, change with respect to the scale of calculation depressions create issues for some measurement techniques
of this fractal complexity. By finding the appropriate aspects of that are limited by their vertical scanning range. Using a larger
geometry and appropriate scale for PBF surface, some corre- measurement range often means that vertical surface topogra-
lations and discriminations have been identified in research. phy repeatability errors are greater than those present when
These methods can be found in ISO 25178-2 and ASME B46.1 measuring comparably flatter test surfaces.
FIG. 4 Graphical representation of the stair-case effect
F3624 − 23
FIG. 5 Example features and challenges of PBF surfaces
4.9.3 Optical properties, such as reflectance, vary signifi- semi-sintered powder and other exogenous particles that ap-
cantly across the materials used in PBF, as well as across single pear as features tens of micrometers in size. There are often
PBF surfaces. Such variations can cause issues for optical also even higher spatial frequency components (such as weld
measurement techniques. Because of the presence of pores and ripples of few micrometers in size) present on the surface. In
deep valleys, there is often a need to use high intensity light the following section, these surface features are examined in
settings to capture light reflected from within these regions. more detail, followed by recommendations for appropriate
Simultaneously, metal PBF surfaces have smooth regions that measurement technologies to overcome the associated chal-
result from the melting and solidification process, which have lenges.
a high reflectance and can easily produce over-saturated
5. Test surface and test surface preparation
images in the optical instrument when high intensity illumina-
tion is employed. Together, these factors make the determina- 5.1 Manufacture of metal parts with PBF:
tion of suitable optical measurement settings difficult. 5.1.1 As-built Surfaces—The as-built condition refers to the
4.9.4 The surfaces typically manufactured by metal PBF are state of the part made by a process before any post processing
very rough, which can be seen through visual inspection as is applied, except for the removal of the part from the build
well as measurement. Metal PBF surfaces generally have a platform, supports or the surrounding powder. The as-built
wide range of spatial wavelengths. These surfaces often have surfaces of PBF parts can be understood in terms of the
large scale components hundreds of micrometers in size (such topographical structure and can generally be split between
as the weld tracks or layers), as well as spatter, un-melted and ‘top’, ‘side’, and ‘bottom’ surfaces with side surfaces also
F3624 − 23
divided between ‘up-facing’ and ‘down-facing’ dependent on particles of a few tens of micrometers to a hundred microm-
surface orientation. It is important to ensure that any aspect of eters in size. Spatter is the result of liquid and partially liquid
the part or surface topography does not cause collision with material that is ejected from the melt pool mid-process, which
optical systems or does not cause damage to the contact probe then flies across the build chamber and lands upon the surface,
tip for the contact stylus measurement. where the material fully solidifies to form an approximately
5.1.2 Top Surfaces Produced by PBF—Top surfaces are spherical particle. This material can be ejected from within and
generally dominated by the final manufacturing processing behind the melt pool (droplet spatter) or as powder that is
cycle (that is, that which forms the top layer) and, therefore, ejected from the powder bed ahead of the melt pool (powder
contain information about the process in the ‘signature’ left by spatter), both of which are influenced by the energy density in
the manufacturing machine. The topography of the top surface the melt pool. Particularly, greater energy density results in
can contain structure and features relating to the specific increased quantities of spatter.
processing parameters (for example, scanning speed, laser 5.1.3 Other defects on the PBF-LB surface include pores,
power, hatch spacing) used during the process, such as ‘weld ‘balling’ effects and cracking.
tracks’, ‘weld ripples’, ‘spatter’ and other features/defects. 5.1.3.1 Pores appear in a variety of sizes as recesses in the
Examples of some of these surfaces are shown in Fig. 6. surface where there has been some lack of fusion. Pores tend to
5.1.2.1 Weld (sometimes ‘scan’ or ‘melt’) tracks are the be a few tens of micrometers to a hundred micrometers in size.
largest observable texture features on the top surface. These Pores appear for a variety of reasons and can appear between
tracks are complex elongated protrusions that approximate the two weld tracks that do not sufficiently overlap, or as a result
scanning strategy of the PBF-LB build process and are just of excessive energy input that causes material vaporization
visible to the naked eye (width of a few hundred micrometers). with a significant source of porosity down to the intrinsic
Weld tracks are characterized by elongated hills separated from porosity present in the starting feedstock material that makes
each other by valley regions that lie between tracks. The size up the powder bed. On any layer, a pore may appear and then
and shape of the tracks are highly influenced by the energy be filled by the process of building up another layer above.
source applied during processing. The pattern of weld tracks However, this ‘healing’ of pores does not always take place,
over the whole surface is influenced by the hatching distance and occasionally results in the formation of sub-surface (and
and hatching style. eventually, with enough subsequent layers, bulk) porosity.
5.1.2.2 Island patterns are a specific topographic structure Sub-surface pores are often of interest to those measuring AM
that occurs because of different scan strategies. Fig. 7 shows an surfaces finished with secondary processes as these pores can
example of these patterns on the AM surface. These features be uncovered by the removal of material and appear as defects
will cause some difficulty in determining the lay for profile on the processed surface.
measurements if the evaluation length exceeds the size of the 5.1.3.2 Balling is another defect, resulting from lack of
‘island’. There is a contour around the edge off these islands fusion or melt-pool instabilities, in the form of large ball-like
which separates these features. structures that take the place of continuous weld tracks. Balling
5.1.2.3 Weld ripples (or ‘chevron patterns’) are smaller features range in size from a few tens to a few hundred
topographic formations (width of a few micrometers) that are micrometers and are sometimes visible to the naked eye,
present along the surface of weld tracks, created during the though in most mature PBF processes, balling defects are
solidification of the weld track. Weld ripples are influenced by usually prevented through process optimization.
the scanning speed of the input energy as the melt pool moves 5.1.3.3 Cracking results from thermal effects and is present
to create the layer. Weld ripples are characterized by chevron- at a wide range of sizes; from large delamination that is a few
shaped hills that pattern along the weld track build direction. millimeters in size (caused by two layers not fusing sufficiently
5.1.2.4 Spatter is also commonly present on the top surface, and then separating as the part cools) to small-scale surface
and generally takes the form of approximately spherical cracks a few micrometers in size.
FIG. 6 Example views of top surfaces produced by PBF, (a) nickel super-alloy PBF-LB surface, (b) titanium alloy PBF-LB surface and
(c) titanium alloy PBF-EB surface
F3624 − 23
FIG. 7 Example magnified image of island pattern produced by PBF-LB
5.1.4 Side surfaces produced by PBF: 5.1.4.3 Sphere-like protrusions made from adhered powder
5.1.4.1 Side surfaces are dominated by the effects of local particles on side surfaces increase in number as the build angle
surface tilt with respect to the build direction, the bonding increases, with upwards facing surfaces possessing less particle
between layers, and the interaction between surfaces and the coverage than downwards facing surfaces. These variations in
surrounding powder bed. The topography of these surfaces can coverage result from multiple melting and re-melting phenom-
contain structure related to the stair-case effect (see 5.1.4.2), ena between the build layer and layers underneath, as well as
and because of the adhesion of particles from the surrounding from heat energy conducting through the part into the sur-
powder to the surface. Up-facing (up-skin) side surfaces may rounding powder bed. In PBF-LB, excess input energy often
show some structure of weld tracks from offset layers at low sinters loose powder adjacent to the build geometry, whilst for
slope angles, whilst down-facing (down-skin) surfaces are PBF-EB the build process pre-sinters a larger region around the
generally dominated by powder adhesion due to the re-melt build into a ‘cake’ prior to melting the layer, which must later
depth (dross formation, see 5.1.5.1) and sometimes contain be removed.
large remnants of support structures. Fig. 8 shows some 5.1.4.4 Defects that can be present on the surface topogra-
examples of surfaces and the features found on side surfaces. phy of side surfaces include pores and smaller thermal cracks,
5.1.4.2 The stair-case effect refers to the presence of visibly as well as large-scale defects (including large recesses) that
offset layers of the build process that approximate the 3D result from delamination and support removal.
model and build orientation (see Fig. 4). Offset layers are 5.1.5 Bottom and supported surfaces produced by PBF:
present as larger scale wave-like components and are most 5.1.5.1 Bottom and supported surfaces possess similar topo-
visibly offset for lower surface slopes, with the adhered powder graphic structure to side surfaces as they are also influenced
becoming more dominant at steeper surface slope angles. mostly by the surrounding powder bed and adhesion due to
FIG. 8 Example views of side surfaces produced by PBF, (a) nickel super-alloy PBF-LB surface, (b) titanium alloy PBF-LB surface and
(c) titanium alloy PBF-EB surface
F3624 − 23
excess energy from the bonding of the layers above. Bottom part or by blowing with compressed air. PBF-EB requires a
surfaces are often made up in their entirety from particles, as more intensive powder removal step, usually performed by grit
energy conducts through the part to the bottom layer as the blasting the part using other loose powder from the build to
build progresses. Dross formations (dross) refers to these large remove the sintered ‘cake’ from around the part.
regions of sintered and melted powder that exceeds the part
5.2.2.2 Powder particles range in size but are generally a
geometry defined by the CAD, which are one of the prevalent
few micrometers to a few tens of micrometers in size.
types of features seen on overhanging regions (including side
Therefore, disturbances of loose powder on the part may cause
surfaces). Dross occurs due to high absorptivity of the powders
airborne contamination, which can manifest as contamination
compared to the solid metal of the bulk part and the mechanism
within mechanisms (including those within measurement in-
of its formation is shown in Fig. 9.
struments) and present a health hazard. To counteract this risk,
5.1.5.2 Bottom surfaces are often heavily affected by the
post-processing and measurement is performed after cleaning
removal of support structures and any remnants from this
of the test surfaces following support removal, either using
process. Build supports can connect the build part to the
compressed air or ultrasonic bathing. Ultrasonic bathing is
building plate or to other underlying surfaces of the part. These
commonly applied and involves the submersion of the whole
supports are necessary to both ensure that the geometry is
part in a bath of a fluid. In the immersion fluid, applied
supported as the process builds overhanging geometries, and to
ultrasound creates cavitation bubbles that can dislodge dust,
ensure that the high residual stresses from the build do not
dirt, and loose powder particles. This process can be applied to
cause excessive warping of the part.
batches of parts and does not require manual interaction, unlike
compressed air cleaning. However, compressed air cleaning
5.2 Preparation of test surfaces for measurement:
uses targeted compressed air and should be performed imme-
5.2.1 Build plate and support removal:
diately prior to measurement (whether an ultrasonic bathing
5.2.1.1 Build supports are often removed manually, using
step is used or not) to dislodge dust and other contaminants
hand tools, and by bandsaws, milling equipment, or wire EDM,
from the surface, which might otherwise be captured in the
or a combination thereof. These operations typically leave
measured topography.
marks on the surface that depend on the method used. These
operations are generally performed to remove support struc-
5.3 Storage and transport of test surfaces—Test surfaces
tures specifically, or in some cases, the entire bottom surface,
should be stored in suitable containers, either within plastic
of the part.
bags or plastic sample cases. To limit contamination and
5.2.1.2 When hand tools (pliers, etc.) are used, scratches can
damage to the test surfaces, it is important to minimize
be seen left on the surfaces surrounding the supports, as can
physical contact with the surfaces of interest, as such contact
any remaining structures that are not completely removed. The
can cause wear and influence the surface topography. For
use of bandsaws or milling equipment also generally leaves
wrapping test surfaces, it is important to ensure that fibrous
some remnant structures that are not completely removed. The
materials (such as cloth and tissue) are not used as these can
surfaces of the supports themselves are akin to those found on
shed and contaminate the surface, leaving fibers that can be
side surfaces, apart from the surface on which the cut has been
captured by the measurement.
made, which will contain deterministic topographical forma-
5.4 Finishing operations for metal AM:
tions (that is, tool marks) from the respective process.
5.4.1 Surface finishing is a large part of post processing in
5.2.1.3 Wire EDM leaves a relatively flat surface, with
AM and refers to the specific steps taken to improve surface
small craters and pits caused by electrical discharges. Wire
texture. There is a large array of finishing operations that can
EDM is most often used to remove the whole bottom surface
be applied to PBF parts. Because each finishing process results
of the part but can be targeted to just remove supports.
in its own characteristic process mark on the surface, it is
5.2.2 Particle removal methods and test surface cleaning:
important to record details of the process inputs as they may
5.2.2.1 The bulk removal of powder from the PBF-LB
relate to the resultant process marks left on the surface. Surface
process is relatively simple, as only a small amount of powder
finishing can include, but is not limited to:
sinters to the part during the build. By lifting the build out of
the build chamber, most of the powder falls away from the part 5.4.1.1 Mass finishing is an example of a commonly applied
and any bulk powder that remains (for example, within finishing operation for metal AM, in which the part is im-
channels) can generally be removed by manually brushing the mersed in a vibratory chamber alongside an abrasive media.
FIG. 9 Dross formation mechanism in the PBF process
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The signature of this process is a smoothed surface (containing measurement principle of the instrument – often related to how
multi-directional scratches from interaction with the media) the instrument deals with vertical scanning and capture. This
and radiused edges. can be taken for each point individually (that is, at each x,y
5.4.1.2 Machining using mills, lathes and drills can be used location) or accumulated as a statistic for the whole measure-
ment.
in order to remove excess material from the near-net shape AM
part, leaving tool marks on the surface that relate to the settings 6.1.2.3 Measurement noise—These are the unwanted com-
used on the machine tool (for example, cutting speeds, tool ponents in the measurement data that can be caused by any
diameters). However, these processes and others that act to number of factors (for example, instrument noise, environmen-
remove material can uncover sub-surface porosity, character- tal noise). Typically, in surface texture, noise is associated with
ized by recesses on the surface a few micrometers to a few tens the unwanted higher spatial frequency components. Some of
of micrometer in size. the measurement noise is removed through application of the
λs/S-filtering operation (see 7.7).
5.4.1.3 Other processes act to mechanically compress re-
gions of the surface whilst preserving some structure (for
6.2 Contact systems:
example, shot peening), while other processes again re-melt
6.2.1 Principle of operation:
surfaces to produce a different topography entirely. Re-melted
6.2.1.1 Contact Stylus—The contact stylus instrument is one
surfaces possess similar characteristics to top surfaces (most
of the oldest and most common technologies used for surface
notably weld tracks resulting from laser polishing and laser
measurement, including in AM. These instruments employ
re-melting). With information about the finishing processes and
mechanical contact between a probe and the surface being
their input parameters, scales of interest can be better deter-
measured. Whilst traversing the surface along a profile, trans-
mined for the measurement process.
ducers convert the vertical movement of the stylus into an
electrical signal that can be encoded by a computer into
6. Instruments for surface texture measurement of PBF
measurement data. As the interaction of the stylus with the
surface
surface can more easily be modelled, contact stylus instru-
6.1 Quality of a measurement: ments are often considered to be traceable (if calibrated).
6.1.1 The quality of the measurement data produced by 6.2.2 Good practice:
contact measurement is relatively straightforward to assess. 6.2.2.1 Good practice in measurement of metal PBF sur-
While the data itself does not often provide an assessment of its
faces using stylus instrument is largely the same as the good
quality (contact measurements will not typically present non-
practice employed in performing stylus measurements of any
measured points), efforts should be made to assess the metro-
surfaces, and NPL Measurement Good Practice Guide No. 37:
logical characteristics of the stylus instrument, assessment of
The Measurement of Surface Texture using Stylus Instruments,
which allows for traceable stylus measurements to be made
presents a comprehensive guide to performing such measure-
(see 6.8). In the metal PBF case, the potential for high slopes
ments.
angles and undercuts should be particularly noted, and the way
6.2.2.2 However, there are some common considerations for
in which measurement data is affected by these surface features
good practice in the metal PBF case, which are covered here.
should be considered (see 6.2). Here, we use the word “points”
The first of these is the consideration of lay (that is, the
to refer to each measured height value present in a topography
direction of the predominant surface pattern) and the direction
height map.
in which to perform stylus measurements for profile measure-
6.1.2 There are a few common metrics that can be used to
ment based on this pattern. While there are often many features
qualify and quantify a good optical measurement. Some of present on metal PBF surfaces, the lay is generally considered
these metrics are directly visible within the dataset (missing
as the pattern of weld tracks on the surface. As such, stylus
points, high levels of noise), whilst others may need to be
profile measurements should be made orthogonal to the direc-
calculated either over the whole dataset or on a point-by-point
tion of the weld tracks on a metal PBF surface. In the absence
basis, that are intertwined with the interactions of the measure-
of clear weld tracks, measurements should be made in many
ment principles, measurement process parameters and the
different directions and results averaged.
surface being measured. Common metrics include the follow-
6.2.2.3 The second consideration to make when using stylus
ing.
instruments for measuring metal PBF surfaces is for the
6.1.2.1 Non-measured points (NMPs)—These are missing
various other difficult-to-measure features present on the
points within the dataset. These points can occur due to there surface, most particularly spatter and other attached particles;
not being a part of the surface within the measurement range
and surface recesses such as pores and cracks. While these
being chosen, but they might also be actively deleted points features are unlikely to directly wear a diamond stylus tip,
that are filtered out by the algorithms within the measurement
metrologists should be aware of the potential for the complex
instrument – they might be either excessively different or
structures of overhangs, step-like transitions, and crevices to
erroneous values beyond a threshold, either within the prin-
damage a stylus itself, for example by removing the tip from its
ciples of the measurement system or with respect to the values
housing. Further, these features have the potential to cause a
of other local points.
phenomenon known as “stylus flight” (where the tip sails off
6.1.2.2 Repeatability error—For surface topographies this is the crest of a peak on the surface and registers erroneous
generally used to refer to the uncertainty in the height datapoints as it falls back to the surface). Contact measurement
determination of a point, which is closely linked to the also has the potential to damage a surface (albeit minimally), in
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that the stylus probe can remove loosely adhered particles and, incoherent with a broadband spectrum (often white light), as
in some cases, cause scratching. the short coherence length reduces ambiguity in determining
the fringe order.
6.2.3 Stylus flight and the risk of damage of stylus and
su
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