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Surface quality parameters for structural components manufactured by DED-arc processes

Authors:

Abstract

Additive manufacturing of structural metallic components is an emerging technology. The DED-arc (direct energy deposition) process combines efficiency with high degrees of freedom and predictable quality. Compared to powder-based processes, the surface is relatively coarse as a result of high heat input and larger molten zones. In industrial applications, the post-treatment of surfaces of DED-arc components cannot be guaranteed for economic reasons or due to geometric constraints. Hence, the surface topography must be evaluated and quality levels need to be defined. Unfortunately, established surface parameters and their determination cannot be directly transferred to DED-arc components and further research on this topic is needed. This article demonstrates surface topography features of representative DED-arc components. Furthermore, waviness profiles were established by applying an alternative cutoff wavelength to better represent the DED-arc surface characteristics. Based on these waviness profiles, 2-dimensional height parameters were calculated. The waviness parameters were determined for four DED-arc specimens made from high strength wire electrodes. The manufacturing parameters were varied in terms of different energy input and interpass temperature, resulting in varying deposition rates and surface qualities. The waviness parameters correlated with manufacturing parameters. A mixed deterministic and random nature of the surface topography was verified.
Surface quality parameters for structural components manufactured by
DED-arc processes
Jonas Hensel
a,
, Anita Przyklenk
b
, Johanna Müller
c
, Markus Köhler
c
, Klaus Dilger
c
a
Chemnitz University of Technology, Chair of Welding Engineering, Reichenhainer Strasse 70, 09126 Chemnitz, Germany
b
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
c
TU Braunschweig, Institute of Joining and Welding, Langer Kamp 8, 38106 Braunschweig, Germany
highlights
Interlayer boundaries and local
minima and maxima of the surface
profile do not generally coincide.
The waviness profile contains
representative features of the surface
topography.
Surface parameters describe the
waviness resulting from different
manufacturing parameters.
graphical abstract
article info
Article history:
Received 9 June 2021
Revised 14 January 2022
Accepted 29 January 2022
Available online 2 February 2022
Keywords:
Additive Manufacturing
Direct Energy Deposition
Metrology
Quality
Surface topography
Construction
abstract
Additive manufacturing of structural metallic components is an emerging technology. The DED-arc (di-
rect energy deposition) process combines efficiency with high degrees of freedom and predictable qual-
ity. Compared to powder-based processes, the surface is relatively coarse as a result of high heat input
and larger molten zones. In industrial applications, the post-treatment of surfaces of DED-arc compo-
nents cannot be guaranteed for economic reasons or due to geometric constraints. Hence, the surface
topography must be evaluated and quality levels need to be defined. Unfortunately, established surface
parameters and their determination cannot be directly transferred to DED-arc components and further
research on this topic is needed. This article demonstrates surface topography features of representative
DED-arc components. Furthermore, waviness profiles were established by applying an alternative cutoff
wavelength to better represent the DED-arc surface characteristics. Based on these waviness profiles, 2-
dimensional height parameters were calculated. The waviness parameters were determined for four
DED-arc specimens made from high strength wire electrodes. The manufacturing parameters were varied
in terms of different energy input and interpass temperature, resulting in varying deposition rates and
surface qualities. The waviness parameters correlated with manufacturing parameters. A mixed deter-
ministic and random nature of the surface topography was verified.
Ó2022 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Advanced architectural and structural design of steel compo-
nents leads to geometric complex and highly individualized struc-
tural members. In construction industry research, additive
manufacturing (AM) has recently been considered as a possible
https://doi.org/10.1016/j.matdes.2022.110438
0264-1275/Ó2022 Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Corresponding author.
E-mail address: jonas.hensel@mb.tu-chemnitz.de (J. Hensel).
Materials & Design 215 (2022) 110438
Contents lists available at ScienceDirect
Materials & Design
journal homepage: www.elsevier.com/locate/matdes
alternative approach to conventional manufacturing for the pro-
duction of such steel components. Buchanan and Gardner summa-
rize the current state of the technology for AM of metal in
construction [1]. They discuss possibilities and challenges of AM
in the context of the construction industry, which must adhere
strictly to engineering standards. One example mentioned here is
the resulting wall thickness under consideration of surface wavi-
ness. Fig. 1 contains examples from construction industry showing
nodal connectors for space frames joining standardized tubes of
different shapes and diameters. These components are tailored to
individual global geometries of spaceframes and are designed to
sustain occurring forces and torque.
1.1. Metal AM in construction industry
Feucht et al. investigated interconnect elements and local rein-
forcements of structural members manufactured by Wire Arc Addi-
tive Manufacturing (WAAM) [2]. Laghi et al. experimentally
investigated AM components of stainless steel with a focus on
geometry and mechanical properties [3]. They also addressed the
effects of layer orientation on tensile properties and investigated
buckling. Furthermore, the desire for sustainable production pro-
cesses and structures yields a minimized use of material, promot-
ing topology optimized design according to individual load bearing
requirements. Reimann et al. presented their approach to topology
optimized design of structural nodes to optimize material usage
[4]. DebRoy et al. presented a comprehensive overview of available
technologies and also discussed the resulting microstructure,
material properties, and approaches for quality assessment [5].
Vafadar et al. provided a review of available AM technologies for
metals and related current standards to emphasize the need for
future standard developments [6]. Emerging AM technologies for
the production of structural steel components are direct energy
deposition (DED) methods using energy from laser (DED-LB), elec-
tron beam (DED-EB), or electric arcs (DED-arc). The processes are
abbreviated in accordance with ISO/ASTM 52900 [7]. The feedstock
material for applications in the construction industry is supplied
either as powder or as filament (wire). As summarized by Frazier,
the material available as powder that can be considered for the
cost sensitive construction industry is currently limited to stainless
steel (e.g. 316L, 1.4404) [8]. Wires are available in greater variety of
grades of unalloyed, low alloyed and stainless steels. The DED-arc
process uses conventional gas metal arc welding processes to melt
wire electrodes in the electric arc to generate components layer-
wise. The process can be highly automated to a degree where the
welding torch is positioned and moved automatically. The process
is easily scalable in size and combines process efficiency with a
high degree of geometric freedom at relatively high deposition
rates. Di Angelo et al. studied the effects of several process param-
eters on the component quality, including surface topography [9].
They made use of their analysis of published research articles to
derive optimal build directions during additive manufacturing
and provide important summaries of the effects of different man-
ufacturing parameters.
The surface quality of AM parts depends on the used processes
and their energy input. The deposition rate and hence the melt
pools of typical DED-arc processes are considerably larger com-
pared to DED-laser processes and laser powder bed fusion (LPBF),
which results in thicker layers and a rougher surface topography.
Gonzáles et al. identified the layer structure on the surface topog-
raphy based on height parameters [10]. Lehmann et al. derived the
waviness profile of a nearly net-shape DED-arc component from
micrographs by image analysis [11]. They also related the compo-
nent quality to process parameters. Furthermore, the deviation of
form and shape of DED-arc components are considered to be larger
compared to LPBF due to the high heat input, inhomogeneous ther-
mal shrinkage, and resulting residual stress and distortion. DED-
arc surfaces are hence often subject to post-processing by milling.
Lopes et al. investigated milling of high strength steels manufac-
tured by DED-arc and found a correlation of milling parameters
and surface roughness as well as the layer-dependent microstruc-
ture [12]. In addition to the aforementioned review articles, Prado-
Cerqueira at al. presented an evolutionary design of a robot-based
and cost-effective hybrid manufacturing system combining DED-
arc and milling which could be used for postprocessing of DED-
arc surfaces [13]. To reduce the effort for post-processing, in a large
number of applications it is desirable to limit the post-processing
to functional surfaces, as described by Webster at in the context
of hybrid manufacturing [14]. On the other hand, the surfaces
may be used as manufactured. Mechtcherine et al. investigated
the use of DED-arc rebars for use in concrete applications and
emphasized the importance of the surface topography for shear
transfer [15]. Müller et al. proposed an advanced testing method
that considers surface geometry, microstructure, and internal
defects for component approval [16]. Accordingly, knowledge of
the resulting surface properties is necessary for the design and
operation of the components.
1.2. Surface topography description of metal AM components
Surface topography of technical components is usually charac-
terized by applying the standards ISO 4287 and ISO 4288 [17,18].
The surface of LPBF components was analyzed by Cabanettes
et al. at all levels of the deviation of form [19]. They proved resolu-
tion issues of certain measurement methods such as coordinate
measurement machines and contact stylus approaches. By analyz-
Fig. 1. Examples of nodal connectors for space frames with topology optimized designs. Design and rendering by Christoph Müller, ITE, TU Braunschweig.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
2
ing roughness profiles, they identified certain process-related
geometry features such as the hatch spacing. On another research,
Cheng at al. compared different measurement methods such as
confocal laser scanning, white light microscopy, and tactile stylus
measurements on LPBF specimens. They found good correlation
between tactile stylus and confocal laser measurements [20].On
the other hand, Leach et al. pointed out that metal AM components
often show high roughness and complex shape, which complicates
the quantitative determination of topography [21]. From their
work, it can be concluded that existing surface measurement tech-
niques and parameters have limited significance due to the mix-
ture of deterministic and random surface geometry of AM parts.
The method of measurement and its spatial resolution affect the
surface topography information included in the measured profile,
as demonstrated by Townsend at al. [22]. The measured profile
contains geometric shape deviations on different geometric levels
as depicted in Table 1. The error of form describes the macroscopic
deviation of the form, for instance the straightness deviation or the
concentricity. The waviness and the roughness describe periodic
shape deviations on smaller scales compared to the error of form.
The waviness describes deviations of shape on longer wavelengths,
while the roughness describes deviations of shape on shorter
wavelengths. The measured surface profile is corrected regarding
the error of form to derive the primary profile (P-profile). The P-
profile contains both waviness and roughness, which are separated
through filtering. A low-pass filter with a cutoff wavelength k
c
is
applied to determine the waviness profile (W-profile). The remain-
ing profile contains shorter wavelengths of the P-profile and is
defined as surface roughness (R-profile). The R-profile is the most
frequently analyzed profile to describe technical surfaces from
milling, turning, blast peening, and other mechanical surface treat-
ments. However, the definition of the cutoff wavelength is very sit-
uational and depends on the measurement method, the surfaces to
be analyzed, and its applications. Liu reported a considerably wide
range of cut-off wavelengths between 0.25 mm and 8 mm applied
for the determination of roughness parameters [23]. The standard
values for the sensing distance l
n
and for k
c
in ISO 4288 are chosen
in dependence of the aperiodic profile height. For coarse surfaces,
these values are l
n
= 40 mm and k
c
= 8 mm. Applying these values
to DED-arc surfaces with layer heights in the typical range of 1 ...
3 mm and waviness in the sub mm-range, it must be noted that the
waviness of the layer structure is not represented accordingly and
the scatter becomes large. Hence, it is conducive to analyze longer
measuring sections and alternatively define the cut-off wave-
length. This explains why LPBF surfaces, with generally small-
scaled surface topography features, may be described by roughness
parameters while the roughness is less meaningful for DED-arc
surfaces.
However, the determination of surface characteristics of AM
components is crucial to fulfil common quality standards. Further-
more, the component properties are affected by the surface shape,
especially in case of cyclic loading where geometric notches pro-
mote the generation of fatigue cracks, often at the surface. While
this is already well known from fusion welding, where weld
notches dominate the fatigue life of welded components (more
details on the importance of weld geometry for fatigue in [24]),
Zerbst et al. approach this issue also for AM components [25].
1.3. General surface characteristics of DED-arc components
Due to the layer by layer material deposition approach, AM
components exhibit characteristic surface properties. Depending
of the deposition strategy, different surface properties can be dis-
tinguished in the spatial directions in DED-arc processes. The sur-
face topography is influenced by the size of the melt pool and
acting process forces, namely the arc force, electromagnetic forces,
gravity, and forces from chemical reactions in the weld bead. Grav-
ity and the arc force are particularly significant for the surface
topography, which act differently on the weld bead depending on
the deposition position and the torch angle. Surfaces of DED-arc
components are provided in this section in order to illustrate typ-
ical surface features. The pictures shown in Figs. 2 to 4 were taken
during own investigations and are presented here for demonstra-
tion purposes only, without going into details of manufacturing
parameters.
As shown in Fig. 2 a), thin walled components exhibit a charac-
teristic surface waviness in building direction evoked by the layer
structure. This waviness is further superimposed by aperiodic
occurring error of form. In the welding direction, the surface
appearance is predominantly characterized by waviness in higher
magnitudes. In contrast, the volumes manufactured without addi-
tional contouring path exhibit significantly differing surfaces, since
the turning points of the deployed deposition pattern are reflected
in the surface structure (Fig. 2 b)). Thereby, a characteristic surface
waviness with the same order of magnitude is formed both in
building and welding direction. Case a) may be described by line
measurements in the building direction, while case b) should be
analyzed by areal measurements and areal surface parameters.
This article focusses on case a) and describes the surface features
on line profile measurements in building direction.
In addition to the deposition strategy, the material (e.g. melt
viscosity, surface tension, solidification behavior) and processing
parameters (e.g. energy input, travel speed, temperature distribu-
tion) can affect the surface formation. Fig. 3 provides an example
for different deposition qualities by means of thin walled alu-
minum samples in cross-sections. In case a) a parallel layer forma-
tion is obtained, while case b) shows a lateral shift of the layers,
resulting in a staggered formation. In order to illustrate the corre-
lation between the layer height and the resulting surface charac-
teristics, the W-profile was graphically derived from the cross-
sections and the profile height was visually magnified to illustrate
local peaks and valleys. Furthermore, the respective layer bound-
aries were marked along the structure. In large parts, minima
and maxima of the profile curve can be assigned to the layer height
Table 1
Deviations of form of technical surfaces (according to [26]).
Level Description Visualization
Error of form Irregularity of form, e.g. straightness deviation.
Describes the coarse shape
Waviness Periodic deviation of shape with intermediate wavelengths. Typically, the waviness
describes irregularities with horizontal spacings larger than the cutoff wavelength k
c
.
Describes the fine shape
Macroscopic roughness
Periodic deviation of shape with short wavelengths. Describes the fine shape
Microscopic roughness
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
3
a) or integer multiples thereof b). Deriving from these qualitative
observations of the DED-arc surfaces, it can be deduced that the
layer height, or integer multiples of the layer height, correlate with
characteristic features of the resulting surface appearance and thus
can be used as an additional input parameter for the determination
of the W-profile. The cross-sections also depict that local minima
may occur at layer boundaries, but also somewhere in the layer
itself. This is a consequence of overlapping weld metal and hence
Fig. 2. Surface appearance of Al-4046 DED-arc samples using different deposition strategies; (a) thin walled sample, (b) zig-zag pattern.
Fig. 3. Graphical determination of surface profiles based on cross-section macrographs of DED-arc aluminum; (a) parallel layer formation, (b) staggered layer formation.
Fig. 4. (a) cross-section macrograph of a steel shell structure, (b) surface with bulge.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
4
interconnected with melt viscosity, gravity and work position as
well as energy input.
As depicted in Fig. 4 a), the surface profile of DED-arc specimens
made from steel does not necessarily reflect the single layers. One
example is marked where a layer interface is located at a local
maximum of the surface topography. Additionally, the height of
the individual layers may vary along the surface profile. The flow
behavior and the weld penetration vary stochastically. In some lay-
ers, the molten material builds bulges in which more material
remains during solidification, see Fig. 4 b). This creates distinct
waviness, especially at higher deposition rates. Reasons for bulge
building are the melt viscosity, the magnetic forces during welding
and irregularly solidifying weld pools.
The DED-arc process results in larger surface features, which
should be associated to the W-profile by definition (fine shape,
deviation of form with larger wavelength). Accordingly, such sur-
face parameters should be determined on the W-profile.
2. Determination of surface profile data on DED-arc specimens
This article focusses on the determination of surface parameters
based on the analysis of the waviness profiles instead of the rough-
ness profiles, which are most frequently used for casting and
machining processes. In contrast to ISO 4287 and ISO 4288
[17,18], the cut-off wavelength for the separation of roughness
from waviness is chosen in this article as a function of the manu-
facturing process, e.g., the layer height, rather than the expected
roughness parameters. Moreover, measuring lengths are consider-
ably longer to account for scatter on the relatively coarse surface
topography of DED-arc components. In addition, this approach
can be applied to surface data obtained by flexible and low-
priced measuring devices such as laser profile sensors with less
restrictions on the measuring range compared to most industry-
typical stylus instruments.
For demonstration purposes, specimens with varying surface
topography were manufactured. This was achieved by a variation
of the melt pool size resulting from varying energy input. The pen-
etration of the molten material was further influenced by a varia-
tion of the interpass temperature between welding two
subsequent layers.
2.1. DED-arc for specimen manufacturing
The experimental setup consisted of a GMAW (gas metal arc
welding) power source (type Fronius TPS500i), a Kuka 6-axis robot
for handling of the welding torch and a mobile fume exhaust (type
Kemper Filter-Master). The welding parameters were monitored
by means of a weld process data scanner (type HKS Weldscanner)
and the temperature of the specimens was measured using an
infrared pyrometer (type optris CTlaser3MH2). The temperatures
were measured on the top surface of the buildup material in the
middle of the last deposition layer and used to control interpass
temperatures between two subsequent layers. The data was fur-
ther processed to determine cooling times t
8/5
(time range during
cooling from 800 °C to 500 °C). The used shielding gas was 18 %
CO
2
and 82 % Argon.
The feedstock used for manufacturing was a high strength wire
electrode with a diameter of 1.2 mm. Table 2 contents the data
sheet information about chemical composition and mechanical
properties of resulting weld metal provided by the supplier. How-
ever, the mechanical properties are highly dependent on the DED-
arc process parameters and only provided as a general reference.
Furthermore, the composition of the deposited material was mea-
sured by means of spectroscopy and is also provided in Table 2.A
good match was found between nominal and measured chemical
contents. The resulting material properties were also determined,
but beyond the scope of this article and are thus not provided here.
However, it was found that the resulting material parameters are
sensitive to interpass temperature, energy input and buildup
direction.
Four specimens with dimensions of 350 mm 160 mm were
manufactured under variation of process parameters. The parame-
ters applied are typical for the DED-arc process of steel and
resulted in different surface qualities. One exemplary specimen is
shown in Fig. 5. 84 layers were deposited on a 15 mm thick sub-
strate plate made from S355N. The applied process parameters
are provided in Table 3. The parameters varied were the energy
input per layer and the interpass temperature. The chosen energy
input levels were 100 kJ and 170 kJ per layer, representing
nominal deposition rates of 136 cm
3
/hour and 407 cm
3
/hour
respectively. Interpass temperatures were varied at levels of
200 °C and 400 °C. The manufacturing parameters resulted in equal
layer heights of h = 1.9 mm, which were set constant. Fig. 5 con-
tains macroscopic metallographic cross-sections of the four speci-
mens. The increased energy input led to a stronger variation of
layer geometry as well as to stronger variations of the wall
thickness.
After manufacturing and before measurement of profile data, all
surfaces were carefully brushed in order to remove slag and sili-
cates without scratching the surface. Fig. 6 provides photographs
of the four specimens with a close-up magnification of the result-
ing surface topography.
A second set of specimens was manufactured using the same set
of parameters, with additional use of promoted cooling to shorten
the waiting time between subsequent layers. Furthermore, the
variation of cooling time was applied to study its effect on the sur-
face topography. A tubular nozzle with pressurized air was used for
cooling. The air flow was aligned so that the last deposited layers
were always cooled, and the flow velocity was 12 m/s. During
the deposition sequence, the cooling was turned off. The setup
and the resulting t
8/5
-times (time during cooling from 800 °Cto
500 °C) are given in Table 4. The t
8/5
-time as a measure of cooling
was chosen because it is commonly related to the mechanical
properties when welding low-alloyed carbon steels. The cooling
times were measured on the top of the last welded layer at mid
length.
2.2. Laser optical profile determination
For determination of surface profile data, a laser triangulation
sensor was used (type Micro-Epsilon optoNCDT1800) with a work
piece reference distance of 50 mm and a vertical measuring range
of 10 mm (50 mm ± 5 mm). The device uses a laser diode with a
laser wavelength of 670 nm (visible / red). The spot size ranges
between 45 mm at 0 mm (focal position) and 320 mmat±5mm
(begin and end of measuring range). The resolution in vertical z-
direction is 2 mm.
The specimen was held in place while the laser sensor was
moved horizontally over the surface. Except brushing, the surface
topography was determined in the as-manufactured condition.
The nominal distance between the specimen surface and the laser
sensor was 50 mm to ensure measurements near the focal position
of the laser beam. The profile data was measured along five lines
with lengths of 150 mm (Fig. 5). The measured profile was cor-
rected regarding its form and position by a linear fit which repre-
sents the nominal linear shape of the wall-shaped specimens. The
primary surface topography profile (P-profile) was determined by
subtracting the form and position fit from the measured profile.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
5
Table 2
Nominal and measured properties of wire electrode Böhler Welding 3Dprint AM 80 HD according to the data sheet.
Data sheet Nominal chemical composition (wt.-%) Nominal mechanical properties of weld metal
after heat treatment
CSiMnCrMoNiR
p0.2
R
m
A
5
0.09 0.4 1.7 0.35 0.6 2 820 MPa 920 MPa 20 %
Measured Determined chemical composition (wt.-%)
CSiMnPS CrMoNiAlCuFe
0.11 0.36 1.7 0.01 0.003 0.38 0.6 2.18 0.01 0.055 bal.
Fig. 5. DED-arc specimen geometry with locations of surface profile measurements (left) and metallographic cross sections made from specimens with varying
manufacturing parameters (compare Table 3).
Table 3
Manufacturing parameters of DED-arc process chosen to vary resulting surface topography.
Specimen Welding travel speed (cm/min) Wire feed
(m/min)
Current
(A)
Voltage
(V)
Energy / layer (kJ) Interpass
temperature (°C)
W-25–2-200 25 2 82 ± 1.9 13.8 ± 0.2 100.8 200
W-25–2-400 25 2 81.4 ± 1.0 13.8 ± 0.3 100.1 400
W-45–6-200 45 6 198.3 ± 3.6 18 ± 0.1 176.8 200
W-45–6-400 45 6 188.8 ± 4.2 18 ± 0.2 168.0 400
Fig. 6. Photographs of DED-arc specimens manufactured with different welding parameters and interpass temperatures (a) to (d), compare Table 3 for manufacturing
parameters.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
6
2.3. Resulting W-profiles of differently manufactured specimens
The DED-arc process, with its relatively large weld pools and
interactions of arc pressure, gravity and surface tension, results
in relatively large magnitudes in the profile height compared to
conventional surface machining or PBF processes. Furthermore,
periodic surface features, as known from milling and turning, are
not common in DED-arc surfaces. The definition of roughness (sur-
face topography with short wavelengths) does not apply to domi-
nant DED-arc surface features which are resulting from layers with
heights in the mm-rage. Additionally, and as described before, the
measurement method affects the resulting P-profile. Typical
roughness parameters in the mm-scale cannot be reliably deter-
mined by the applied laser scanning approach due to resolution
and noise issues. Hence, the surface topography should be ana-
lyzed dominantly regarding its waviness (surface topography with
longer wavelengths).
The P-profile of the surface consists of roughness (R-profile) and
waviness (W-profile), which are separated by Gaussian filtering of
the P-profile. The W-profile will be used to determine surface
parameters (Section 3), while the R-profile will be used to store a
combination of roughness and noise, which will not be analyzed
in the scope of this work. According to ISO 4288, the filter param-
eters for conventional roughness measurements are defined in
dependence of the aperiodic profile deviation, the profile length
and the individual measuring sections. Unfortunately, the parame-
ters used in standards such as ISO 4288 are not directly applicable
to DED-arc components because of the large magnitudes of surface
features. Comparably long measuring sections and single sections
lengths are required to describe the surface topography in a statis-
tically validated manner. Fig. 7 shows the interrelation of P-, R- and
W-profiles for a selected cut-off wavelength of k
c
= h/2 = 0.95 mm.
This cut-off wavelength was chosen after careful investigation of
its smoothening effect while still representing surface topography,
compare Fig. 8. Compared to the approaches defined in ISO 4288,
the W-profile contains local and global waviness, while roughness
and noise are disregarded and not further analyzed.
The cut-off filter length k
c
of the Gaussian filter was varied
between 2 h and h/4 to demonstrate its smoothening effect,
Fig. 8. The profiles are magnified between x = 70 mm and x = 90
mm. It was noted that values of k
c
h (one layer height) led to sig-
nificant trimming of the local minima and maxima. Smaller cut-off
filter lengths of k
c
(h/2) resulted in an accurate description of the
local p-profile peaks on the determined W-profiles. For the follow-
ing analysis, the cut-off filter length was set to k
c
= (h/2)
accordingly.
All surface topography data from differently manufactured
specimens was filtered and W-profiles were derived. Representa-
tive W-profiles of the four specimens are given in Fig. 9.
3. Surface quality parameters on W-profiles
In a first step, the arithmetic mean deviation W
a
and the root
mean square deviation W
q
of the W-profiles of all specimens were
analyzed according to ISO 4287 and Eqs. (1) and (2).
W
a
¼1
l
n
Z
l
n
0
ZðxÞ
jj
dx ð1Þ
Fig. 7. Separation of P-profile into W- and R-profile. Cut off wavelength kc = h/2 with layer height h from DED-arc process.
Table 4
Cooling times from 800 °C to 500 °C and experimental setup used for accelerated cooling.
t
8/5
-time of top layer (s) Experimental setup for accelerated cooling
Specimen Passive cooling Active cooling
W-25–2-200 18 17
W-25–2-400 29 25
W-45–6-200 25 19
W-45–6-400 40 25
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
7
W
q
¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
l
n
Z
l
n
0
Z
2
xðÞdx
sð2Þ
Fig. 10 contains the data from specimens manufactured under
variation of process parameters and cooling strategy. The W-
profiles were determined by application of a cut-off wavelength
of k
c
= (h/2). The results show a general increase of the surface
parameters with increasing energy input. Mean values of W
a
of
specimens manufactured with low energy input range from
approximately 0.16 mm to 0.24 mm. With higher energy input, this
value increases to 0.22 mm to 0.33 mm. Similar observations were
made for W
q
. Active cooling slightly reduced the surface
parameters.
3.1. Segmentation of W-profiles
The W-profile was divided into individual measuring sections
by two segmentation methods. The first method divides the W-
profile into n single measuring sections with constant length l
s
,
Fig. 11. The second method applies a threshold crossing approach,
where single measuring sections l
c
are divided by locations where
the curve crosses the ordinate at Z = 0 mm. Zero-crossing locations
can be defined in different ways (e.g. from left to right, from right
to left, descending, or ascending). Here, the curve is defined by one
local minimum and one maximum from left to right, Fig. 12.In
both cases, the measuring length l
n
defines the maximum number
of single measuring sections that can be analyzed. The non-
Fig. 8. Effect of k
c
on resulting W-profile. A large cut-off wavelength causes inappropriate smoothing of the P-profile.
Fig. 9. Representative W-profiles of four DED-arc specimens manufactured by different parameters (compare Table 3).
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
8
Fig. 10. Arithmetical mean deviation W
a
and root mean square deviation W
q
of the W-profile.
Fig. 11. Curve segmentation through definition of single measuring sections with constant length.
Fig. 12. Curve segmentation through zero-crossing method.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
9
periodic signal shape of W-profiles may lead to quite short or long
single measuring sections in the zero-crossing approach, which
automatically enlarges the scatter. Nonetheless, zero-crossing
was applied since it will become part of ISO 16610 and
ISO 21920 for the analysis of surface topography and is recom-
mended by Seewig et al. for analysis of technical surfaces [27].
Parameters can be defined for the entire W-profile (l
n
) as well as
for single measuring sections (l
s
,l
c
). The global parameter W
t
(maximum waviness) is determined as the height difference
between global maximum and global minimum. It can be expected
that this value increases with increasing measuring length l
n
due to
statistical phenomena. This parameter is independent from the
sectioning method. The local waviness W
z
and W
z,c
describe the
height difference between local maximum and minimum and are
determined on individual measuring sections l
s
and l
c
, respectively.
They vary along the W-profile and must be statistically analyzed.
3.2. Factors affecting the determination of waviness parameters
The waviness parameters were determined on the W-profiles of
specimen a) W-25–2-200 (profiles 1 to 5, measuring length
149 mm), compare Fig. 5 and Fig. 6, to demonstrate the effects of
the main evaluation parameters. The W-profiles were derived from
the P-profiles by Gaussian filtering with a cut-off length of
k
c
= (h/2) = 0.95 mm. The height parameters describing the surface
topography of the W-profile depend on three main influencing
parameters:
the length of the single measuring section l
s
,
the sectioning method to determine single measuring sections,
and
the number of single measuring sections to be analyzed.
The length of the single measuring section l
s
was defined in
dependence of the layer height h on three levels 1 h, 5 h and
10 h. Accordingly, the number of single measuring sections varied
between 78 (l
s
= 1 h), 15 (l
s
= 5 h) and 7 (l
s
= 10 h). Fig. 13 depicts
the effect of l
s
on W
z
.W
z
was determined on the individual W-
profiles 1 to 5, and additionally on all five W-profiles at once for
better comparison. Generally, W
z
increased with increasing length
of l
s
. The mean value
l
of W
z
increased from
l
0.20 mm (1 h) to
l
0.55 mm (5 h) and
l
0.75 mm (10 h). Another parameter is
the standard deviation
r
of the distribution of W
z
, which varied
moderately (1 h and 5 h) and significantly (10 h) between the ana-
lyzed profiles, most likely to be explained by the different sample
sizes. Long single measuring sections on W-profiles, and hence
only few W
z
data led to a higher scatter. Furthermore, the individ-
ual profiles 1 to 5 vary regarding their height profiles. Similar
trends regarding
r
were found on profiles 1 and 5 (
r
is large)
respectively profiles 2, 3 and 4 (
r
is small) when medium and long
single measuring sections (5 h and 10 h) were analyzed. This effect
was less significantly captured when analyzing short single mea-
suring sections (1 h).
The median of W
z
in most cases was slightly smaller compared
to the mean values. The maximum and minimum values also
increased with increasing l
s
.
Fig. 14 compares waviness parameters determined by the two
applied sectioning methods. Additionally, the global parameter
W
t
is given. Mean and median values of W
z
and W
z,c
were similar
for all profiles 1 to 5. The parameters determined on zero-crossing
single measuring sections l
c
were slightly smaller than those deter-
mined from l
s
. This can be explained by the large number of short
single measuring sections after the zero-crossing sectioning
approach, which resulted in smaller W
z,c
values (compare Fig. 12
and Fig. 13). The standard deviation
r
of W
z,c
was significantly lar-
ger than
r
of W
z,
which also resulted from the variation in single
measuring section lengths l
c,i
. The maximum waviness ranged
between 1.12 mm and 2.13 mm.
All parameters W
z
,W
z,c
and W
t
depend on the measuring length
l
n
. To improve the significance of these waviness parameters, a def-
inition for the measuring length is required. Furthermore, single
extreme features on individual single measuring sections may sig-
nificantly affect the waviness parameter. This should be avoided,
since the extreme values are represented by the W
t
parameter
and the standard deviation
r
of W
z
. For better comparison of dif-
ferent surface topography profiles, W
z
should be averaged over a
certain number of single measuring sections. The averaged wavi-
ness parameter W
z
is defined as the arithmetic mean value of
W
z,i
over n single measuring sections by
W
z
¼1
nX
n
i¼1
W
z;i
 ð3Þ
Fig. 15 contents the averaged waviness parameter W
z
for 15
single measuring sections with a length of 5 h for n = 1, 3, 5 and
10. The influence of the single outlier of W
z
(e.g. section 6,
Fig. 13. Determination of W
z
in dependence of single measuring section length l
s
(h: layer height 1.9 mm; 1 to 5, all: 5 measuring sections on specimen W-25–2-200).
Parameters determined on constant total measuring length l
tot
= 149 mm resulting in different numbers of single measuring sections n.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
10
W
z
= 1.59 mm) on W
z
(
l
±
r
) decreases as n increases. In addition
to the mean value, the standard deviation remains stable at n = 10.
In conclusion, the number of single measuring sections chosen to
determine averaged parameters should be chosen with regard to
the influence of possible outliers. Consequently, the total amount
of surface topography data should be increased by long measuring
length l
n
or multiple measurements at different locations.
3.3. Approach for comparable determination of waviness parameters
The studies on surface parameter determination (Sections 3.1
and 3.2) have demonstrated the effects of measuring and evalua-
tion parameters. In accordance with the results shown, and for bet-
ter comparison of profile height parameters between different
manufactures, the measured surface profile should preferably
contain
10 single measuring sections,
with a length of 5xlayer height (h),
and an in- and outlet length for the Gaussian filter (k
c
/2).
The W-profile can be reliably determined from the measured
profile through
the application of a form and position fit that accounts for the
nominally intended shape of the component, which results in
the P-profile,
a separation of the P-profile into R- and W-profiles by Gaussian
filtering with a cut-off wavelength of k
c
(h/2).
The W-profile should be divided into single measuring sections
with constant length. This is less sensitive regarding influences
from the form fit of the measured profile. The length of the single
measuring sections affects the individual W
z
parameter. A good
Fig. 14. Influence of sectioning method on waviness parameters and distribution of maximal waviness W
t
.W
z
determined with single measuring section length of
5 h = 9.5 mm. W
z,c
determined by zero crossing approach. W
t
represents the difference of the global minima and maxima on each measuring section. All parameters
determined on constant total measuring length l
tot
= 149 mm.
Fig. 15. Averaging of W
z
over different averaging lengths studying the effect of outliers on W
z
and its standard deviation.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
11
resolution of W
z
was found when the length l
s
is defined according
to the layer height of the DED-arc process. A single measuring sec-
tion with a length of 5 layers reliably contents local topography
features of the waviness. When determining mean values of W
z
,
the W
z
parameter should be determined on at least 5, preferably
10 single measuring sections to account for outliers.
The determined parameters should be defined in accordance to
the discussed parameters, for instance as
W
t,50h
: Maximal height difference of the entire W-profile over a
length of 50 h (layer height),
W
z
;10;5h: Mean of height differences W
z
on single measuring
sections on the W-profile, determined on 10 subsequent single
measuring sections with lengths of l
s
= 5 h (layer height).
The parameter determination should be conducted on at least
three measured profiles to identify evident trends.
4. Resulting waviness parameters of DED-arc specimens made
from AM80
The waviness parameters were calculated for the eight speci-
mens manufactured with different DED-arc parameters and differ-
ent cooling conditions (Section 2.1). Fig. 16 contains resulting data
from all five laser scan profiles which were used to process the
waviness parameters on single measuring sections W
z
and the
total height of the W-profiles W
t
. A clear effect of the energy input
was proved for both parameters W
z
and W
t
. Low energy input led
to smaller waviness in terms of mean values at smaller standard
deviations. This was likely an effect of the smaller weld bead size
and the lower arc pressure at lower arc energy. An increased inter-
pass temperature also effected the waviness parameters positively.
The accelerated cooling (ac: active cooling) by pressurized air had
no substantial effect on the surface topography. The mean value of
the W
z
parameter was not altered significantly. However, the stan-
dard deviations for both parameters W
t
and W
z
increased for most
applied manufacturing parameters.
The mean values of surface parameters of the individual laser
scan profiles are given in Table 5. Based on the results, homoge-
nous surfaces can be realized at low energy input and high inter-
pass temperatures.
5. Discussion and outlook
The conducted research demonstrated the mixed deterministic
and random nature of surface topography characteristics of DED-
arc components. Layer boundaries do not necessarily correlate
with geometric peaks or valleys on the surface. But the distance
between peaks and valleys can be described to a good approxima-
tion by integers of the layer height. Comparably simple but more
flexible measuring devices such as laser profile sensors may be
used to determine P-profiles of variable length even on curved sur-
faces. The scatter of the Z-data resulting from lower resolution and
reflective surfaces can be larger compared to stylus instruments,
but this may be compensated in the data analysis. The separation
of roughness from waviness with a newly defined cut-off wave-
length as a function of the layer height provides representative
W-profiles for parameter determination and R- profiles containing
a mixture of roughness and noise from measurement. The surface
topography may be described based on height parameters on the
W-profile and the parameters discussed clearly reflect different
surface shapes. The definition of cut-off wavelength and the length
of single measuring sections based on the layer height is hence rec-
ommended. The standard deviation of W
z
values should ideally be
determined to evaluate the inhomogeneity of the surface waviness.
Next to the W-profile, the form fit and its influence on certain sur-
face parameters should be analyzed in greater detail. This informa-
tion could be used, for instance, to identify net sections of near net
shape shell structures. Overall, it was found that the surface
parameters should be determined at different locations of DED-
arc specimens to evaluate the susceptibility of surface features,
such as bulges. However, the total measuring length should not
be chosen based on current technical standards. Short measuring
sections, as recommended for milled surfaces, may be applicable
for investigation on surfaces of LPBF components, but are too short
for DED-arc components.
Furthermore, future work needs to be conducted to estimate
the influence of the measurement method and its resolution on
the quality of surface data. A comparison of optical and tactile
measurement methods should be performed. For this, also 3-
dimensional parameters should be considered. Additionally, the
component behavior under static and cyclic loading conditions
need to be investigated. It is likely that the surface waviness corre-
sponds to static load capacity as it affects the net sections. More-
over, it is widely known that geometric notches are important
Fig. 16. W
z
and W
t
of DED-arc specimens made from steel (AM80) with different manufacturing parameters and cooling conditions.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
12
for fatigue of welded metals. Here, the interaction of internal irreg-
ularities, such as pores and lack of fusion, and the waviness needs
to be investigated, for instance as demonstrated for LPBF by [28].
With regard to fatigue, additional parameters can be derived
from the W-profile in the future. Liinalampi et al. used gradient
based surface features on thin laser welded components and found
correlations with the fatigue properties [29]. They calculated the
ratio of depth/width for laser weld undercuts. Fig. 17 displays gra-
dient based parameters such as the flank angle band the ratio of
(d
y,i
/d
x,i
) on a DED-arc W-profile. These parameters should be
investigated based on experimental and numerical approaches
and correlated to the fatigue strength.
6. Summary
The surface of DED-arc components contains random and deter-
ministic topography features. The layer height and integers thereof
occur deterministically, while bulges and overlapping weld metal
occur randomly. The surface topography can be described by the
W-profiles and height parameters can be derived. The surface
topography can be imaged to a good approximation by using a
laser scanning sensor. Measured and analyzed profiles from differ-
ently manufactured specimens differ significantly. The cut-off
wavelength, the measuring length, and the length of each measur-
ing section affect the surface parameters and should be provided in
addition to the calculated surface parameters. The waviness of
DED-arc components, expressed by height profiles, increases with
deposition rate. A high interpass temperature decreases the
waviness.
CRediT authorship contribution statement
Jonas Hensel: Conceptualization, Formal analysis, Funding
acquisition, Methodology, Project administration, Software, Super-
vision, Visualization, Writing – original draft. Anita Przyklenk:
Data curation, Formal analysis, Methodology, Software, Validation,
Writing – review & editing. Johanna Müller: Data curation, Inves-
tigation, Validation, Writing – review & editing. Markus Köhler:
Data curation, Investigation, Validation, Writing – review & editing.
Klaus Dilger: Resources, Funding acquisition, Supervision, Writing
– review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
The topology optimized nodal connectors presented in Fig. 1 were
designed and rendered by Christoph Müller (ITE, TU
Braunschweig).
Table 5
Surface parameters of DED-arc specimens made from AM80 steel.
Specimen: W-25–2-200 W-25–2-400 W-45–6-200 W-45–6-400
Cooling method: pc ac pc ac pc ac pc ac
W
z,10,5h
(Mean of height differences W
z
on single measuring sections)
0.78
0.58
0.52
0.54
0.62
0.61
0.41
0.47
0.45
0.49
0.60
0.50
0.59
0.49
0.48
0.80
0.82
0.71
0.59
0.76
1.16
0.90
0.91
1.17
1.23
1.27
0.94
1.08
1.10
0.69
0.60
0.70
0.57
0.72
0.74
1.50
1.19
1.51
0.93
0.62
W
t
,
50h
(Maximal height difference of the entire W-profile)
1.58
1.28
1.31
1.12
2.12
1.10
0.78
0.82
0.83
0.85
1.23
1.19
1.06
1.23
1.03
1.46
1.55
1.48
1.26
1.67
2.48
1.54
2.39
2.00
2.58
2.75
2.64
2.09
2.36
1.43
1.69
2.07
1.88
2.4
1.66
3.73
2.68
3.57
2.67
1.66
Fig. 17. W-profile with local minima and maxima to determine wave width d
x,i
, wave height d
y,i
and flank angles.
J. Hensel, A. Przyklenk, J. Müller et al. Materials & Design 215 (2022) 110438
13
Funding acknowledgements
The research presented in this paper is being conducted within
the project ‘‘Wire Arc Additive Manufacturing (WAAM) of Complex
and Refined Steel Components (A07)”. The project is part of the col-
laborative research center ‘‘Additive Manufacturing in Construc-
tion - The Challenge of Large Scale”, funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) -
project number 414265976 - TRR 277.
Data availability
The raw data required to reproduce these findings are available
to download as supplementary files.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.matdes.2022.110438.
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... This process, which is also used in this investigation, enables an automated gas metal arc welding (GMAW) process to generate large parts with comparable coarse surfaces with waviness in the range of 0.5-2.0 mm [11]. The layer thickness typically results in 0.5 to 3 mm depending on the material and manufacturing process parameters. ...
... Additional peaks and valleys with distances of the layer width may occur in the deposition direction when weaving is applied. Irregular bulges may occur due to non-optimal combinations of bead size, arc pressure, deposition speed, working position and material flow behavior [11,27,35]. Moreover, roughness must be expected. ...
... The applied cutoff value for a Gaussian filter is appropriate if the differences are small and close to zero. This approach provides a clear criterion for choosing the cut-off value and isolating waviness and roughness, covering the limitation of the previous study, as the cut-off wavelength was selected after careful investigation of its smoothening effect [11]. The proposed surface topography characterization approach can potentially provide a standard for WAAM specimens, especially since the ISO standard does not currently cover its implementation. ...
... In AM, process parameters directly in uence surface attributes [5]. The surface quality, and consequently the machinability of WAAM-produced parts, can be modulated by deposition parameters. ...
... To determine the surface characteristics, samples are scanned using an ATOS structural laser 3-D scanner. Optical measurement of surface characteristics is already a recognized method for additively manufactured components [5], particularly for WAAM [7]. To eliminate non-uniformities at the beginning and end of each layer and the rst and last layers, only a central area measuring 50mm×75mm of the walls is considered. ...
... This factor depends on the surface's roughness scale and can vary between processes. Typically, different surfaces require the application of distinct cut-off lters to their pro les [5]. However, considering the required accuracy for surfaces in the WAAM process and the resolution of the employed 3D scanner, no lter is applied to the surface pro les in this study. ...
Preprint
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The unique arc-based additive manufacturing process boasts exceptionally high deposition rates, but it comes with a trade-off – the generation of highly irregular surfaces that demand extensive post-machining efforts. These post-processing activities not only consume critical raw materials but also substantial energy resources. To mitigate these challenges and enhance process efficiency, it is crucial to develop predictive capabilities for surface topography based on key process parameters like wire feed speed and travel speed. In this study, a surface topography model is meticulously developed to reconstruct surfaces, utilizing wire feed speed, travel speed, and inter-pass temperature as input parameters. The initial phase involves identifying pertinent surface features that collectively define the surface. Out of the features examined, eight representative attributes emerge, including spatial average roughness, spatial peak height, spatial maximum valley depth, spatial skewness, spatial kurtosis, maximum flatness, and waviness. The surface reconstruction model employs the discrete cosine transform (DCT), necessitating a minimum of 30 DCT parameters for accurate surface reconstruction. Additionally, an ANN model is established to predict DCT parameters based on wire feed speed, travel speed, and inter-pass temperature inputs. Validation using the 309L stainless steel test material demonstrates the model's impressive accuracy in predicting DCT parameters, enabling precise forecasts of overall surface topography and machining allowances. This model lays the foundation for simulation-based additive-subtractive process design by identifying optimal deposition conditions and corresponding machining parameters. Furthermore, it streamlines the integration of realistic surfaces into computational models for additive process simulations, offering significant potential for improving additive manufacturing processes.
... When fabricating voluminous parts with one of the linear patterns, the outer surface of the structure becomes wavy, because, either in the case of raster, the start or stop points are on the edge or, in the case of zigzag, the turning points are on the outside of the structure [11]. Thus, a contour path is necessary when applying linear paths to reduce waviness or unevenness on the WAAM surface, especially if the final part is supposed to be exposed to dynamic loads. ...
... Height differences in a single layer are important for the successful deposition of large structures with a high number of layers. For that, the W t -value (maximum waviness [11]) was determined by digital measurement of the macrosection. The parameter W t is determined as the height difference between global maximum and global minimum. ...
... Examples for force-flowoptimized nodal connectors (credit: Christoph Müller, ITE, TU Braunschweig)[11] ...
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Additive manufacturing with steel opens up new manufacturing possibilities for individual parts or complex supply chains. The direct energy deposition process DED-arc, also known as wire arc additive manufacturing (WAAM), is particularly suited for manufacturing large structures with a high degree of geometrical freedom. The design freedom makes WAAM ideal for creating force-flow-optimized steel nodes and spaceframes for use in the construction industry. Thick-walled parts can be manufactured with manifold strategies and a variety of infill patterns, like hachure, spiral, or meander as well as with and without a contour path. For parts with varying thicknesses, not all of them apply. If the wall thickness changes in increments smaller than the width of a weld bead, the desired shape cannot be made by selecting one of the available deposition strategies but the distance (overlap) between the individual weld beads needs to be varied. A variation of the weld bead overlap can often lead to compromised build quality due to the presence of voids. Thus, utilizing variable bead overlap requires the development of a deposition parameter set that allows deviation from the defined overlap values without running the risk of introducing voids between beads or between layers. In this study, four parameter sets were analyzed regarding their weld bead geometry, and tolerance ranges for the weld bead overlap were identified for each parameter set. They were found to vary between 15 and 50% overlap, depending on the parameter set and its corresponding weld bead geometry. The geometry of the surface of the last deposited layer was evaluated regarding the height difference. It can be stated that, with decreasing wire feed, the surface of the last welded layer becomes smoother. The findings obtained are transferred to the manufacturing of complex parts. As a result, parts with variable wall thickness in building direction were manufactured by variation of weld bead distances.
... Both static and cyclic load-bearing capacity are affected by the coarse surface topography. The surface features of DED-Arc specimens were investigated by [1]. It was pointed out that a mix of deterministic and stochastic features characterizes the surface topography. ...
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The surface characteristics of direct energy deposition (DED)-Arc specimens have been previously explored, revealing the dual effect of the surface topography on fatigue strength. This effect includes a reduction in the nominal load-bearing cross-section due to the waviness of the surface and the initiation of fatigue cracks at combined geometric and metallurgical notches from the surface. To complement these findings, this study focuses on the fatigue life and crack initiation of milled DED-Arc samples, e.g., after removal of the geometric notches. Fatigue tests on DED-Arc specimens validate the impact of surface topography on fatigue strength, emphasizing the significance of surface characteristics in determining structural integrity. The additional tests conducted on milled surfaces provide insights into the failure mechanisms specific to these samples. Fracture surface analysis, microstructure characterization, and hardness measurements are performed. For both surface conditions, cracks originate in the interlayer zone, where local hardness is reduced. In unmilled specimens, this is due to the stress concentration effect, and in milled samples, this is due to softened interlayer zones. The fatigue life of milled specimens is comparatively high, demonstrating the importance of optimizing surface characteristics for improved fatigue resistance. Highlights 1. Both, unmilled and milled specimens, show crack origins in the interlayer areas. For unmilled specimens, this is because of the local stress concentration in the notch; for milled specimens, it was shown that soft zones between the layers act as the weakest link. 2. The fatigue strength of unmilled test coupons is significantly lower than those of milled specimens. 3. Unmilled samples exhibit multiple crack origins.
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Being one of the pillars supporting modern industry, the additive manufacturing (AM) field has gone through significant developments over the years, and that is certainly also the case of deposition processes based on the use of the electric arc. However, although most of the knowledge for that specific purpose comes from the more consolidated arc welding counterparts, inherent and yet critical challenges emerge from arc additive manufacturing (Arc-AM). As main advantage, Arc-AM processes offer the capacity to build metallic parts at very high deposition rates and at accessible prices (given their arc welding roots), but generally at the cost of high heat input levels and of tendency for heat accumulation occurrence. Such a negative facet leads to many complications concerning quality factors of the parts, which are related to thermal effects as in terms of geometry/aspect characteristics, distortions, macro/microstructural features, residual stresses, and mechanical properties, and even of production time, invariably in an intertwined connection. The solution, in this case, depends on a large extent on implementations of proper thermal management of the parts being built, which have been indeed the focus of attention of many research and development efforts in the field of Arc-AM over the years as this review aims to present. Thus, the general intent of this work is to contribute to further developments in Arc-AM via a comprehensive review of the related thermal management approaches that have been attempted. The idea is to facilitate the comparison and assessment for suitability of the different thermal management approaches, helping developers working on Arc-AM processes with the foundations for their application and/or improvement. As the subject of thermal management in Arc-AM tends to a more mature scenario, this review ultimately aims at the establishment of a more direct bridge on that matter between academia and industry. Thus, a scientific and hence more critical perspective of the related knowledge is balanced with a technological/engineering and hence more informational side of it. To be representative, it covers information produced over a decade (2013–2022) on scientific and technological developments that somehow have effects via thermal consequences on the metallic layers during the Arc-AM processing time. And it is organized in three main parts. First, in Section 1, after the introduction to the subject of interest and presentation of the general and specific objectives as well as of the scope and delimitations of the review, the complications related to poor thermal management are compiled and discussed in such a way that its importance is characterized and the scientific bases for understanding the functions/effects of the diverse approaches to it are stablished. Then, in Section 2, a systematic bibliometric survey of related publications is presented in detail for auditability and future consultations, and the various thermal management approaches (in fact applied to different Arc-AM processes, metallic materials, and part shapes within the period of analysis) are categorized and then briefly described and representatively discussed in dedicated subsections, which eventually include preliminary remarks and general guidelines for application. The first main section (specifically the part on complications related to poor thermal management) as well as the subsections of the second one were actually built in such a way that they can be consulted individually depending on specific interests of the audience. At the end, in Section 3, the entire subject is recapitulated with combined graphical data, being the main remarks on the thermal management approaches for Arc-AM outlined and the ways by which they are applied to the different deposition processes, materials, and parts globally quantified and characterized. For closure, within the same section, opportunity windows for further developments on thermal management for Arc-AM are finally drawn. Despite the quantity and diversity of approaches identified, as has always been the case in modern manufacturing, the thermal management technology in Arc-AM is expected to keep evolving in the face of new challenges and different scenarios with constant advancements, for which this review will hopefully serve as a relevant source of consultation.
Chapter
ALSTOM is one of the world's leading rail vehicle manufacturers with several sites worldwide. The potential of AM is utilized even for large metal parts. The article presents some use cases in various technologies and materials with a focus on WAAM (Wire Arc Additive Manufacturing). • Door stopper - the first series part made of metal in one go (FDM, 316L), • Damper brackets (DSLM, mild steel), • cooling water manifold (DSLM, aluminum); Various WAAM (Wire Arc Additive Manufacturing) parts: • Axle bearing cover (mild steel), • damper bracket (mild steel), • safety hook (structural steel). Aspects of production, approval and application are outlined. The outlook focusses on: • the establishment of approval guidelines, • the establishment of specifications, • and the acquisition of valid material data.
Conference Paper
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This work analyzes the effects of different heat treatments on Inconel 718 samples produced with the Wire and Arc additive manufacturing (WAAM) technique and their impact on the machining process of the WAAM products. Since this alloy is a material known for its complex machining procedure, heat treatment is a possibility to improve this manufacturing step. This research was divided into two different parts. First, two distinct heat treatments (solution and homogenization) were performed, and the samples after treatment were compared regarding their microstructure and Vickers hardness. These heat treatments were performed with varying timings to assess possible changes in the original microstructure with those changes. Solution (980 °C) and homogenization (1080 °C), two commonly applied heat treatment steps with known temperature ranges were performed for 1, 4, 8, and 12 h. In the second step of the research, the machinability of the side milling process was assessed regarding the cutting forces, surface roughness, and tool wear, and by comparing the results for those parameters, it was possible to observe and conclude that even though there were significant changes in the microstructure of the samples that the improvement of the machinability of the Inconel 718 samples with the heat treatments was not achieved, since the objectives of reducing cutting forces, improving surface roughness and reducing tool wear was not possible with predictability and for the same parameter.
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The safe fatigue design of metallic components fabricated by additive manufacturing (AM) is still a largely unsolved problem. This is primarily due to (a) a significant inhomogeneity of the material properties across the component; (b) defects such as porosity and lack of fusion as well as pronounced surface roughness of the as-built components; and (c) residual stresses, which are very often present in the as-built parts and need to be removed by post-fabrication treatments. Such morphological and microstructural features are very different than in conventionally manufactured parts and play a much bigger role in determining the fatigue life. The above problems require specific solutions with respect to the identification of the critical (failure) sites in AM fabricated components. Moreover, the generation of representative test specimens characterized by similar temperature cycles needs to be guaranteed if one wants to reproducibly identify the critical sites and establish fatigue assessment methods taking into account the effect of defects on crack initiation and early propagation. The latter requires fracture mechanics-based approaches which, unlike common methodologies, cover the specific characteristics of so-called short fatigue cracks. This paper provides a discussion of all these aspects with special focus on components manufactured by laser powder bed fusion (L-PBF). It shows how to adapt existing solutions, identifies fields where there are still gaps, and discusses proposals for potential improvement of the damage tolerance design of L-PBF components.
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This paper illustrates the comparative results of measuring the surface texture of additive manufacturing parts using optical techniques (Alicona G4) and X-ray Computed Tomography (Nikon XTH225). The results of optical and XCT measurements are analysed respectively, per the surface texture parameters of ISO 25178-2. To target the most suitable measurement configuration, different measurements resulting from the same technique with various configurations are compared. The optical and XCT measured surfaces obtained via the targeted configuration parameters are then matched via surface registration to allow an accurate comparison in respect of both surface topography and its derived surface roughness parameters.
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This paper extends a previously published fatigue strength estimation methodology for additively manufactured metallic bulk material by additionally accounting for effects of as-built surface layers. Interaction of intrinsic defects and surface texture convergently initiates fatigue failure. Holistic consideration of influencing factors significantly contributes to scientific fatigue assessment of structures fabricated by Laser-Powder Bed Fusion (L-PBF). Surface texture is highly dependent on the building parameters and performed post treatments. Three dimensional, optical topography scans form the basis of determining representative areal surface texture parameters. Areal notch valley depth Sv, alongside notch radii ρ evaluation, enables usage of a modified Peterson’s approach. Effects of notch-like roughness features are quantified by a reduction factor ks, as analogously published for bulk material imperfections kb. Superimposition of ex- and intrinsic material characteristics is empirically assessed by an interaction exponent derived from experimental fatigue data. Macroscopic, tensile residual stresses acting as mean stresses are considered by Smith-Watson-Topper’s approach. Unifying presented influencing factors derives a comprehensive model, conceived to estimate fatigue strength of additively manufactured metallic structures. Regardless of post processing condition, sound applicability of developed design approach is substantiated by averaging -7.1%, comparing estimated fatigue strength to experimental results.
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In recent years, Additive Manufacturing (AM), also called 3D printing, has been expanding into several industrial sectors due to the technology providing opportunities in terms of improved functionality, productivity, and competitiveness. While metal AM technologies have almost unlimited potential, and the range of applications has increased in recent years, industries have faced challenges in the adoption of these technologies and coping with a turbulent market. Despite the extensive work that has been completed on the properties of metal AM materials, there is still a need of a robust understanding of processes, challenges, application-specific needs, and considerations associated with these technologies. Therefore, the goal of this study is to present a comprehensive review of the most common metal AM technologies, an exploration of metal AM advancements, and industrial applications for the different AM technologies across various industry sectors. This study also outlines current limitations and challenges, which prevent industries to fully benefit from the metal AM opportunities, including production volume, standards compliance, post processing, product quality, maintenance, and materials range. Overall, this paper provides a survey as the benchmark for future industrial applications and research and development projects, in order to assist industries in selecting a suitable AM technology for their application.
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This study defined hybrid additive manufacturing (AM) as “in-situ or series combination of an additive manufacturing process and secondary energy sources in which physical mechanisms are fundamentally altered/controlled to affect the resulting properties of material and/or part.” This definition includes in-situ secondary processes as well as process chains, and it is anchored in multi-physical mechanisms such that new hybrid-AM processes can be freely and systematically sought or invented through a systems approach epitomized by the “property - mechanism - energy source - hybrid-AM process (PMEH)” thought process. The sequence of driving forces in this framework are as such: desired material properties determine which mechanism is utilized and, in turn, the energy source to be applied, which ultimately defines the hybrid-AM process. The five unifying physical mechanisms that were identified in this study are: melt pool dynamics, microstructure development, stress state, surface evolution, and thermal gradients. Analysis of properties, mechanisms, energy sources, and processes was conducted on more than 100 papers, and the results ultimately show the effect of mechanisms on material properties. Mechanisms are further classified by energy source, which are in turn broken down by hybrid-AM process. Additionally, each mechanism was defined and reviewed in detail, highlighting the PMEH relationship for metal hybrid-AM materials. Further analysis compares reported mechanical property values for hybrid-AM processes to both AM only and wrought properties for 316 L, Alloy 718, and Titanium Gr 5. Finally, future directions of research as well as clear gaps in knowledge are identified, which includes lack of variety in utilized energy sources, lack of material diversity, process chain integration and improvement, and promising hybrid-AM processes. With the presented analysis and PMEH framework, it is determined that metal AM hybrid processes are well suited to address current problems and show promise in creating superior and versatile materials. Further growth in this field is expected to be exponential, and the developed PMEH framework will aid in framing these innovative processes.
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By additive manufacturing technologies, an object is produced deposing material layer by layer. The piece grows along the build direction, which is one of the main manufacturing parameters of Additive Manufacturing (AM) technologies to be setup. This process parameter affects the cost, quality, and other important properties of the manufactured object. In this paper, the Objective Functions (OFs), presented in the literature for the search of the optimal build direction, are considered and reviewed. The following OFs are discussed: part quality, surface quality, support structure, build time, manufacturing cost, and mechanical properties. All of them are distinguished factors that are affected by build direction. In the first part of the paper, a collection of the most significant published methods for the estimation of the factors that most influence the build direction is presented. In the second part, a summary of the optimization techniques adopted from the reviewed papers is presented. Finally, the advantages and disadvantages are briefly discussed and some possible new fields of exploration are proposed.
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Additive Manufacturing (AM) is a novel manufacturing method where a component is fabricated by accumulating material layer-by-layer therefore facilitating customized near-net-shape components while maximizing design freedom. A metal AM technology suited for large-scale component fabrication that has been emerging in recent years and is commonly referred to as Wire and Arc AM (WAAM) uses Gas Metal Arc Welding (GMAW) technology to deposit material and large-scale robotic serial manipulator systems (reach >1 m cubed). One of the requirements for applying GMAW welding technology to AM is to identify and optimize deposition parameters in order to achieve a desired deposition quality measured in terms of geometrical, mechanical and metallurgical consistency. In this work, deposition process parameters qualitatively and quantitatively influencing the geometrical, mechanical and metallurgical consistency are identified and statistically validated. A deposition parameter combination is found that optimizes the quality of single-track wall benchmark components made from low-carbon steel. Moreover, correlations are found between accumulated heat during fabrication and the geometrical variations of the benchmark components due to bead slumping. Additionally, correlations between microstructure variations and geometrical variations are found. Finally, based on the presented analyses, in-situ temperature monitoring methods are proposed in order to achieve optimal component quality.
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Feature characterization of rough surfaces is of growing interest in terms of a function oriented description of technical surfaces. Feature characterization requires a segmentation of significant hills and dales of the measured profile. The segmentation can be done in several ways. One method is the so called crossing-the-line segmentation which will be part of ISO 16610 part 45 and ISO 21920 part 2. The crossing-the-line segmentation described in this publication represents an extension of the algorithm proposed by Scott (Scott P, 2006, Meas. Sci. Technol. 17, 559–564) and is based on new knowledge gathered over the last ten years. As an example, the feature parameters R S m and R c according to ISO 4287:1997 are evaluated.
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Additive manufacturing is considered a motivator for the development of the industrial world. Recent advances show that Wire and Arc Additive Manufacturing (WAAM) has the potential to become a relevant method for the fabrication of large complex-shaped metallic components. In this study, we address one of the most important post-processing methods for WAAM parts to be used in structural applications, milling. In this work, thin-walled high strength low alloy steel parts were manufactured by WAAM and their microstructure was characterized. Then, a milling strategy which considered the microstructure and local mechanical properties of each part was employed. The results show that the mechanical behavior of the as-built parts does not yield a significant influence on the milling process. Overall, it was ascertained that the quality of the milled surfaces improves, that is, has lower roughness, with the increase of cutting speed and with the decrease of feed per tooth. Nevertheless, we highlight the need for more attention to be dedicated on post-process machining operations after WAAM, to establish the best strategies aiming at decreasing tool wear, while maintaining both high surface quality and production rates.