Recoater crashes during powder bed fusion of metal with laser beam: simulative prediction of interference and experimental evaluation of resulting part quality

Abstract

In powder bed fusion of metal with laser beam (PBF-LB/M), repetitive melting and solidification of newly added layers lead to thermal stresses and distortions during part build-up. Particularly at critical component features such as unsupported overhangs, super-elevated edges pose a risk in terms of crashes with the recoating system during powder spreading. Damaged recoater lips lead to irregularities in the form of stripes in the powder bed. These local inhomogeneities cause lack-of-fusion porosity and geometric defects on the part surface. However, quantitative information on important quality aspects, such as tensile properties, dimensional accuracy, roughness, and hardness of parts printed under irregular powder bed conditions is scarce. Here, we show that samples from build jobs with recoater crashes maintain their elastic tensile properties and hardness, but lose elongation at break. Finite-element simulations of in-process distortions are used to design an artifact that intentionally damages the silicone rubber lip of the recoater but does not cause machine breakdown. The lowest mean yield strength of the damage-affected samples is 243 MPa, which is still within the material data sheet limits for AlSi10Mg. Therefore, recoater crashes do not necessarily result in rejects, but users must consider the likely presence of porosity.

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Progress in Additive Manufacturing (2023) 8:759–768
https://doi.org/10.1007/s40964-023-00471-2
FULL RESEARCH ARTICLE
Recoater crashes duringpowder bed fusion ofmetal withlaser beam:
simulative prediction ofinterference andexperimental evaluation
ofresulting part quality
StefanBrenner1,3 · MartinMoser1,3 · LeaStrauß2,3 · VesnaNedeljkovic‑Groha1,3· GüntherLöwisch2,3
Received: 29 October 2022 / Accepted: 6 June 2023 / Published online: 23 June 2023
© The Author(s) 2023
Abstract
In powder bed fusion of metal with laser beam (PBF-LB/M), repetitive melting and solidification of newly added layers
lead to thermal stresses and distortions during part build-up. Particularly at critical component features such as unsupported
overhangs, super-elevated edges pose a risk in terms of crashes with the recoating system during powder spreading. Damaged
recoater lips lead to irregularities in the form of stripes in the powder bed. These local inhomogeneities cause lack-of-fusion
porosity and geometric defects on the part surface. However, quantitative information on important quality aspects, such as
tensile properties, dimensional accuracy, roughness, and hardness of parts printed under irregular powder bed conditions
is scarce. Here, we show that samples from build jobs with recoater crashes maintain their elastic tensile properties and
hardness, but lose elongation at break. Finite-element simulations of in-process distortions are used to design an artifact
that intentionally damages the silicone rubber lip of the recoater but does not cause machine breakdown. The lowest mean
yield strength of the damage-affected samples is 243MPa, which is still within the material data sheet limits for AlSi10Mg.
Therefore, recoater crashes do not necessarily result in rejects, but users must consider the likely presence of porosity.
Keywords Powder bed fusion of metal with laser beam· AlSi10Mg· Recoater crash· Powder bed irregularity· Part quality
1 Introduction
Powder bed fusion of metal with laser beam (PBF-LB/M)
has found acceptance for industrial production of critical
parts in aerospace, medical, energy, and automotive appli-
cations [1]. Drawbacks of the locally focused laser energy
input are the initiation of the temperature gradient mecha-
nism and the associated plastification that cause residual
stresses and part distortions [2]. Unacceptable large distor-
tions can result in build failure or rejection [3].
Part edges warped above the powder bed level, referred
to as super-elevation [4, 5], pose a risk to the stability of
the process in the form of collisions between the recoating
mechanism and the part [6, 7]. Morante etal. [8] sum up
recoater crashes as possible causes of problems like incom-
plete parts, geometric and surface defects, porosity, and
microstructural inhomogeneity. Powder bed irregularities,
such as super-elevated part edges or powder trenches, can
also lead to reduced part properties and therefore rejection
of the parts [9, 10].
To account for these process-related phenomena, the
finite-element (FE) method offers sophisticated approaches.
The simulation of component scale residual stresses, distor-
tions, and recoater collisions are currently the most mature
domains of modeling [11]. Many commercially available
FE simulation tools offer the possibility to predict recoater
crashes [12].
Since process errors still occur, process monitoring is
often used to detect and classify powder bed irregulari-
ties. Comprehensive review articles on in-situ monitoring
methods and the detection of powder bed irregularities in
PBF-LB/M were recently published [13, 14]. To assess the
* Stefan Brenner
stefan.brenner@unibw.de
1 Institute forDesign andProduction Engineering,
Werner-Heisenberg-Weg 39, 85577Neubiberg, Germany
2 Institute forWeapons Technology andMaterials Science,
Werner-Heisenberg-Weg 39, 85577Neubiberg, Germany
3 Department ofMechanical Engineering, University
oftheBundeswehr Munich, Werner-Heisenberg-Weg 39,
85577Neubiberg, Germany
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760 Progress in Additive Manufacturing (2023) 8:759–768
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morphology of powder layers, a method often used is to take
high-resolution images during the process. The detection
and classification of errors allow distinguishing between dif-
ferent types of recoater errors, such as wear and local dam-
age [15] or recoater hopping and streaking [16].
To evaluate the effects of irregularities in the powder
bed on the resulting defects in the printed parts, process
errors are intentionally induced. Grasso [17] pre-damaged
the recoater brush of an Electron Beam Melting machine
to vary the severity of irregularities in the powder bed at
specific locations. In several studies, damage-causing parts
are intentionally printed to draw conclusions about resulting
porosity formation in the printed parts [18], to compare the
acquired images to parts with actual process defects, like
failed support material [19], or to validate recoater crash
simulations [7, 12, 20].
Previous studies focus on the formation of microstructural
defects in parts printed with irregular powder bed conditions
or mention qualitatively increased roughness [15]. Quantita-
tive information on important quality aspects, such as tensile
properties, dimensional accuracy, roughness, and hardness
of parts printed under irregular powder bed conditions, is
scarce.
The objective of this research is to intentionally cause
recoater crashes to evaluate the quality of parts that are
affected by powder bed irregularities during their fabrication.
2 Methods andsamples
2.1 Design ofdamage‑causing artifacts
To intentionally cause and assess powder bed irregularities,
it is possible to pre-damage the recoating device [17] or to
design parts in a way that they will lead to interference with
the recoating device during the build job, damage it, and,
therefore, cause powder bed irregularities [18, 19].
To follow the latter approach, a two-sided overhang was
chosen as a design baseline for the damage-causing artifact,
because such geometries are prone to show super-elevation,
especially if the unsupported overhang angle is 45° or less
[5, 12].
The proprietary PBF-LB/M simulation tool Workbench
Additive within ANSYS Mechanical 2022R1 (ANSYS Inc.,
Canonsburg, USA) was used to produce numeric results for
in-process distortions of unsupported overhangs. The main
abstractions of the FE model include lumped super layers,
which represented eight physical layers in this study, and
the heat application by setting newly added super layers to
the melting temperature, neglecting the scan strategy [21].
The default settings were maintained without calibrating the
Strain Scaling Factor.
The design of the artifact had two goals:
1. To cause interference between the part edges and the
recoater lip during the build job. Local damage at spe-
cific locations is necessary to place adjacent samples
in recoating direction on the build plate, which will be
affected by the resulting powder bed irregularities due
to the damaged lip.
2. To ensure that only the recoater lip is affected and not
the lip mount. This prevents collisions between part
edges and rigid components of the recoater, which
would likely result in wear or more severe machine dam-
age. The design of the artifact is intended to allow the
build job to continue with a damaged recoater lip. There-
fore, the super-elevations of the overhang artifacts must
remain within a certain target range for z-distortions.
A quarter-symmetry FE model was used with the critical
part dimensions, as shown in Fig.1. By varying the over-
hang angle α, the intensity of the super-elevation effect was
studied.
Given the nominal layer thickness of 30µm (Table1),
it might be conceivable to expect recoater crashes as soon
as the maximum z-distortions zdist exceed 30µm. As it is
known from the recent literature, the effective layer thick-
ness (ELT) of powder above a part is significantly higher
Fig. 1 a Quarter-symmetry FE
model and b critical overhang
artifact dimensions
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761Progress in Additive Manufacturing (2023) 8:759–768
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than the nominal value [22]. The ELT is reported to be 4
to 5.5 [23] or even more than 7.5 times [24] the nominal
layer thickness. Reasons for this are shrinkage effects after
solidification [24], formation of spatter and denudation [23].
Considering this behavior, the overhang artifacts have a tar-
get range for z-distortion of 0.2mm < zdist < 0.8mm, with
the lower limit zmin given by ELT ≈0.2mm and the upper
limit zmax given by the free rubber height of 0.8mm (Fig.2)
to avoid collision with the rigid lip mount.
To select an appropriate overhang angle, the maximum
zdist on the top of each super layer is checked before add-
ing the next super layer to determine if the target range is
reached during the simulation.
Based on the above-mentioned characteristics, an appro-
priate artifact design must satisfy the distortion requirement
zmin < zdist < zmax to be selected as a part of the build layouts
which were planned as follows.
2.2 Sample fabrication andbuild layout
The samples used for the experimental tests were printed
from AlSi10Mg powder (20–63µm) on an SLM125HL
machine (SLM Solutions AG, Germany) with a build size
of 125 × 125x125 mm3. The main process parameters used
for all build jobs in this study are listed in Table1.
For each build job, a virgin silicone rubber recoater lip
was used. The schematic in Fig.2 shows super-elevated part
edges that will likely interfere with the recoater lip.
Samples were placed adjacent to the damage-causing
artifacts aligned with the recoating direction in build lay-
outs similar to Bartlett etal. [18] and Foster etal. [19].
Stacks of three horizontal cylinders (diameter = 10mm;
length = 72mm) were printed to evaluate tensile test prop-
erties (see Sect.3.3). Figure3 shows a representative build
layout used in this study. The longitudinal orientation of
the artifact was aligned with the recoating direction and the
dotted line arrow indicates the expected transfer of powder
bed irregularities from the artifact to the adjacent samples.
The cylinders were printed at three different build heights
hbuild, with the axes at z-positions of 18.4mm, 31.4mm, and
44.4mm. This ensured that the samples were positioned
above the artifact and thus printed with a damaged recoater
lip. Block supports were used as a connection between the
cylinders and to the base plate. In a post-processing step, the
cylinders were machined to standardized tensile test samples
Table 1 Process parameters for sample fabrication
Process parameter Value
Laser power [W] 350
Scan speed [mm/s] 1650
Hatch Distance [mm] 0.13
Layer thickness [µm] 30
Base plate temperature [°C] 150
Shielding gas Argon 4.6
Fig. 2 Schematic of the recoater moving toward super-elevated part
edges. The interference of the super-elevated part with the rubber
recoater lip is referred to as recoater crash in this study. A collision
between a part and the rigid lip mount is likely to result in wear or
more severe machine damage
Fig. 3 Build layout with tensile
test samples and a damage-caus-
ing overhang artifact. Vertical
wall samples were positioned
accordingly in recoating direc-
tion, but printed in a separate
build job
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762 Progress in Additive Manufacturing (2023) 8:759–768
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(DIN50125-B5 × 25) with a diameter of 5mm, as used in the
VDI material data sheet for AlSi10Mg [25].
Vertical walls were printed in a separate build job to
determine roughness, flatness tolerance, and hardness (see
Sect.3.4). Four overhang artifacts were located parallel with
two vertical wall samples adjacent to each.
2.3 Recoater crash detection
The SLM125HL machine is equipped with a Layer Con-
trol System (LCS) that takes 8-bit grayscale images after a
new powder layer is spread. Therefore, for each build job, a
stack of images was taken that corresponds to the number
of layers. These images were analyzed to check for appear-
ing powder bed irregularities, indicating local damages to
the recoater lip. The image processing steps are shown in
Fig.4 for an exemplary stack of n + k images. Because of
an uneven illumination within the build chamber (Fig.4a),
the background of the images was removed utilizing the
ImageJ software (National Institutes of Health, USA) to
enable meaningful grayscale value measurements. To deter-
mine the beginning of a recoater crash, the grayscale value
of each pixel was measured along several parallel profiles
(dashed lines in Fig.4b) in the x-direction in each image.
The average value for each column of pixels was formed.
From this, local changes in the grayscale values during a
build job could be recognized and interpreted as appearing
powder bed irregularities.
The control level for deciding whether a recoater crash
occured was set to six times the standard deviation of a pow-
der layer without irregularities at hbuild = 7.5mm, before the
beginning of the overhang. Craeghs etal. [15] have used this
control level successfully for a similar purpose.
The LCS can be used to evaluate if and when a recoater
crash occurred during the build job. This information was
used to validate the super-elevation of the overhang artifacts.
2.4 Static tensile tests
Static tensile tests according to DIN EN ISO 6892–1 were
carried out on a Zwick/Roell testing machine (Z100, Zwick-
Roell GmbH, Germany) to determine Young’s modulus YM,
yield strength YS, ultimate tensile strength UTS and elonga-
tion at break εbreak. A sample size of n = 3 was used for each
combination of hbuild and powder bed condition (irregular or
reference). Fracture surfaces were examined using a digital
microscope (VHX-2000, Keyence Co., Japan) with 50 × and
200 × magnification.
2.5 Surface measurements andhardness tests
The vertical wall samples were scanned with an optical pro-
filometer (VR-5200, Keyence Co., Japan) using 40 × mag-
nification and high-resolution mode. The device was also
used for contour scans and profile measurements on dam-
aged recoater lips.
The average roughness Ra was measured at parallel,
horizontal lines with lengths of 7.5mm on the vertical
wall surfaces (Fig.5a) with filter settings λs = 25µm and
λc = 2.5mm. To compare regular and irregular powder bed
conditions, areas were selected at build heights before and
after the recoater crash.
Regarding the flatness tolerances, the height distance
between the minimum and maximum of the topography at
wall surfaces 1 and 2 (Fig.5b) was measured in each case.
Next, the wall samples were embedded and ground.
Vickers hardness HV0.5 was tested on cross-sections A-A
(Fig.5b) using a hardness tester (ecoHARD XM 1280 A,
AHOTEC e.K., Germany). Measurement points with a
Fig. 4 LCS image processing steps with a stack of original images
and b images after background removal
Fig. 5 Locations of a roughness
and c Vickers hardness meas-
urements. Flatness tolerances
were measured on b Surface 1
and Surface 2
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763Progress in Additive Manufacturing (2023) 8:759–768
1 3
spacing of 1mm were placed in the z-direction long center
and off-center lines (Fig.5c).
3 Results anddiscussion
3.1 Simulation results
The simulation outputs of zdist for every simulated super
layer of the overhang artifacts with 15° < α < 45° are ana-
lyzed. Figure6 shows the curves of zdist of the current
top super layer over hbuild. The curves only differ from
hbuild = 8mm, which is the beginning of the overhangs. The
15° overhang angle shows the steepest increase and crosses
the zmin level at hbuild≈9mm. It reaches the highest zdist of
0.46mm but remains below zmax.
The results show a clear trend toward more pronounced
super-elevation at smaller overhang angles α, which was also
expected. Only the overhang artifact with α = 15° reaches
the target range for zdist, indicating that this variant distorts
sufficiently to damage the recoater lip, but does not distort
enough to cause a collision with the rigid lip mount. There-
fore, the 15° artifact is considered suitable to fulfill the
purpose of causing recoater crashes without damaging the
machine. Despite known limitations in the ability of simu-
lation models to predict recoater crashes [12] and model
abstractions (see Sect.2.1), this design is selected as the
damage-causing artifact for all build jobs in this study.
3.2 Simulation validation
The LCS images from a build job with four parallel over-
hang artifacts and two vertical wall samples adjacent to each
artifact are analyzed to detect recoater crashes and vali-
date the predicted in-process z-distortions from the simu-
lation model. Super-elevated edges are slightly visible at
hbuild≈9mm and clearly protrude from the powder bed at
hbuild = 10.5mm (Fig.7a), but no irregularities are yet pre-
sent (Fig.7b). After the super-elevated edges have interfered
with the recoater lip, the further powder layers are spread
with a damaged lip, resulting in irregularities in the powder
bed. The occurrence of irregularities is detected by LCS in
the range of 10.8mm < hbuild < 11.8mm with some scatter
between the four artifacts. At hbuild = 12.96mm (Fig.7c), the
irregularities are clearly visible and the grayscale measure-
ments show downward spikes to values of about 230, which
are significantly below the control level of 245.7 (Fig.7d).
The white arrow (Fig.7c, middle) points to a torn-out piece
of rubber from the damaged recoater lip.
After build job completion, the recoater lips exhibit
two types of damage. First, there are notches in the lips
(Fig.8a,b) that are up to 1.5mm deep and about 4mm wide
(Fig.8e). Second, there are rubber snippets (Fig.8c, d) that
protrude downward from the lips by up to 1mm (Fig.8f).
The occurrence of super-elevations starting at
hbuild≈9mm corresponds well with the simulated zdist
exceeding the zmin level at about this build height. The
predicted further increase in zdist can also be validated by
the observed powder bed irregularities and the damage to
the lips. Although some notches in the lips are deeper than
the free rubber height (Fig.2), the rigid lip mount remains
unscathed. This suggests that the in-process z-distortions
of the artifacts are less than zmax. The deep notches may be
caused by abruptly torn-out rubber pieces and not by layer-
by-layer wear. During the build job, pieces of rubber were
found in the LCS images of several layers, as exemplified
Fig. 6 Simulation results for in-process z-distortions of artifacts with
overhang angles α of 15°, 25°, 35°, and 45°. Only the artifact with an
overhang angle of 15° reaches the target range for z-distortion
Fig. 7 Original LCS image of build heights 10.5mm a and 12.96mm
c with corresponding grayscale value measurements b and d. Mean
regular = 250.5, control level = mean–6xSD = 245.7. The white ar row
in c points to a torn-out piece of rubber from the damaged recoater lip
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764 Progress in Additive Manufacturing (2023) 8:759–768
1 3
in Fig.7c. The protruding snippets may have been flexible
enough to bend but not be torn out when they passed the
super-elevated edges during the recoater movement.
Based on these findings, the simulative prediction of
interference between the super-elevated edges and the
recoater lips shows good agreement with the experiments.
The overhang artifacts meets the requirements to cause pow-
der bed irregularities, but no damage to the machine.
3.3 Tensile properties
The samples show different behavior depending on the
type of damage to the recoater lip. One representative
stress–strain curve is plotted in Fig.9a each for samples
printed with a regular powder bed as reference (dotted
line), with irregularities from snippet-type damages (black
line) and with notch-type damages (gray line). The sam-
ples with snippet-type damages perfectly follow the refer-
ence curves, but break at low εbreak of 1% to 3%, depending
on their hbuild. Large areas of unmelted powder particles
are found on the fracture surfaces (Fig.9b). The samples
with notch-type damages yield earlier, however, show plas-
tic deformation up to an εbreak of 8.8% at hbuild = 18.4mm.
Only a few unmelted particles are observed on the frac-
ture surfaces (Fig.9c). From the build job with notch-type
damages, only samples from hbuild = 18.4mm are available.
For all samples, a YM of approximately 70GPa is meas-
ured, which is not influenced by the powder bed condition
or hbuild.
YS decreases from 282 to 245MPa as hbuild increases
from 18.4mm to 44.4mm for reference samples and
snippet-type damages. For notch-type damages, a YS of
243MPa is measured at hbuild = 18.4mm, which is the
same as the other samples decrease to at hbuild = 44.4mm.
UTS decreases from 447 to 424MPa as hbuild increases
from 18.4mm to 44.4mm for reference samples. For snip-
pet-type damages, a lower general UTS level of 335MPa
to 361MPa is observed with no clear relationship to hbuild.
For notch-type damages, a UTS of 422MPa is measured
at hbuild = 18.4mm, which again corresponds to the level
of the reference samples at hbuild = 44.4mm.
Fig. 8 Scanned contour of
recoater lips after crash with
notch-type damage a, b and
snippet-type damage c, d.
Height profile of notch-type
damage e and snippet-type dam-
age f. Dashed lines in a-d show
the profile lines of e and f
Fig. 9 a Engineering stress–
strain curves for samples with
regular and irregular powder
bed conditions and fracture sur-
faces for b snippet-type damage
and c notch-type damage. For
better readability, only one rep-
resentative stress–strain curve is
shown each
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765Progress in Additive Manufacturing (2023) 8:759–768
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Table2 provides the results overview given as mean and
standard deviation.
The unmelted particles, which are found on the fracture
surfaces, suggest that the powder bed irregularities cause
lack-of-fusion (LoF) defects. LoF porosity was reported
at overhangs [7] and as a result of incomplete melting of
uneven powder layers [26]. Bartlett etal. [18] described that
both powder pile-up and powder recess cause LoF defects,
the former because of decreased beam penetration and there-
fore insufficient heating, the latter because of poor powder
packing density and altered physical size of the layer that
changes the effective conductivity of the powder bed. Look-
ing at the shapes of the damaged lips in Fig.8, it is conceiv-
able, on the one hand, that powder piles up above the powder
bed level below the notch, forming visible streaks. On the
other hand, downward protruding snippets conceivably dig
into the powder bed and cause powder recess, which forms
visible grooves. From the LCS images, it is not possible to
determine the shape of the irregularities.
Snippet-type damages are found to have no effect on YM
and YS, but to decrease UTS by 15% to 25% depending on
hbuild and a sharp drop is observed for εbreak. It is known
that hbuild affects the mechanical properties in PBF-LB/M
[27, 28]. In the reference build jobs of this study, YS and
UTS decrease as hbuild increases, which is in contrast to
the findings of Weiss etal. [28]. In the case of the build
job with the notch-type damage, the printing process was
stopped after the completion of the horizontal cylinders at
hbuild = 18.4mm. These samples, therefore, experienced a
different thermal history than the samples from other build
jobs, where additional cylinders were printed above them.
The findings reported in this study suggest that certain ten-
sile properties are similarly dependent on the powder bed
condition and the thermal history within the investigated
build height range. Further research is required to gain a
better understanding of these effects.
To put the tensile properties into context, mean values
below or above the limits of the VDI-Standard [25] are
marked in Table2. Only samples from snippet-type dam-
ages fall below the lower limits of UTS and εbreak. All meas-
ured elastic properties (YM and YS) in this study are within
the limits of the VDI-Standard, while the reference samples
and the notch-type damages even exceed the upper end of
the range for εbreak. Therefore, the presence of LoF defects
from powder bed irregularities has no noticeable effect on
the elastic properties.
3.4 Flatness tolerance, roughness, andVickers
hardness
The defects in the sample surfaces are clearly visible
(Fig.10a) and the samples show distinct build heights
hcrash,Surf at which the recoater lips are damaged (Fig.10b).
Large LoF defects are found in the cross-sections (Fig.10c,
d) at hcrash,Cross-sect. The effects are independent of the dam-
age type of the recoater lip.
The flatness tolerances almost double from 0.24mm
to 0.44mm and the roughness Ra increases by 40% from
6.2µm to 8.7µm due to powder bed irregularities. Vickers
hardness remains constant at HV0.5 = 120, both along the
center line and along the off-center line. The measurements
are summarized in Table3.
Table 2 Overview of tensile test
results
All results are given as mean ± standard deviation
For Irregular (notch) only Build height = 18.4mm was tested
a) Mean below limits of material data sheet for AlSi10Mg in VDI standard [25]
b) Mean above limits of material data sheet for AlSi10Mg in VDI standard [25]
*One supposed upwards outlier (YM = 77GPa) due to a clamping error included
Property Powder bed condition Build height [mm]
18.4 31.4 44.4
YM [GPa] Regular 72.3* ± 4.2* 70.2 ± 0.6 67.8 ± 2.0
Irregular (snippet) 69.0 ± 0.4 69.6 ± 1.7 69.8 ± 1.5
Irregular (notch) 69.3 ± 4.6 – –
YS [MPa] Regular 282.4 ± 2.0 274.0 ± 1.4 244.6 ± 1.9
Irregular (snippet) 284.6 ± 1.2 275.0 ± 2.2 245.8 ± 1.0
Irregular (notch) 243.3 ± 0.6 – −
UTS [MPa] Regular 447.1 ± 1.8 441.0 ± 1.5 424.3 ± 2.1
Irregular (snippet) 335.5a) ± 20.2 361.6a) ± 12.1 361.4a) ± 3.0
Irregular (notch) 422.4 ± 1.9 – –
εbreak [%] Regular 10.0b) ± 0.5 10.3b) ± 0.3 11.1b) ± 0.4
Irregular (snippet) 1.0a) ± 0.4 1.8a) ± 0.3 2.7a) ± 0.1
Irregular (notch) 8.8b) ± 0.8 – –
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766 Progress in Additive Manufacturing (2023) 8:759–768
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Several studies report similar phenomena, such as stripes
and swelling defects [15, 17, 29]. Contour defects deteriorate
the dimensional accuracy and roughness of the parts, but can
be removed during post-processing if accessibility is given.
For filigree structures, there is a risk of failure if the contour
defect is large in relation to the feature thickness, e.g., in lat-
tice structures.
Grasso [17] found areas of LoF porosity that vary in size
depending on the severity of the powder bed irregularity below
the swelling defects in Electron Beam Melting. Bartlett etal.
[18] showed a relationship between the severity of powder bed
errors and the formation of microstructural defects in PBF-
LB/M as well. The exemplary LoF defect in Fig.10 is located
just above the crash height and is limited in the z-extension.
In the further printing process with damaged recoater lip, no
more noticeable defects are found in the wall samples. This is
contrary to the tensile samples, where the unmelted powder is
found subsequent to the crash height.
4 Conclusion
To evaluate the resulting quality of parts when recoater
crashes occur in the PBF-LB/M process, FE simulations
are used to design damage-causing artifacts that intention-
ally cause recoater crashes. The experimental results show
the influence of powder bed irregularities on important
quality aspects of samples adjacent to the damage-causing
artifacts.
The FE simulations of in-process distortions are used for
designing efficient artifacts that intentionally damage the
recoater lips but avoid machine breakdown. The design that
shows suitable simulation results is printed and validated.
The predicted target range for the z-distortion is reached.
Therefore, FE process simulations are useful to predict the
interference between parts and the recoater.
The experimental results show only little effect of pow-
der bed irregularities on elastic tensile properties. Young’s
modulus and yield strength of the samples remain within
the limits of the corresponding material data sheet in the
VDI-Standard for AlSi10Mg. In damage-affected samples,
lack-of-fusion defects and decreased elongation at break are
found. In certain samples, fracture occurs with an elonga-
tion at break of only 1% compared to 10% in undamaged
reference samples. In other samples, an elongation at break
of 8.8% is measured, which is higher than the value speci-
fied in the VDI-Standard. It should be emphasized that the
sample size in this experiment is small and the samples
are machined before testing. In the as-built condition, the
observed defects in the sample surfaces will act as notches
and possibly cause earlier failure. No decrease in hardness
due to recoater crashes is observed. Visible defects on the
surfaces deteriorate dimensional accuracy and roughness,
but could possibly be removed in post-processing steps to
improve the usability of the parts.
Fig. 10 a As-built vertical wall
sample with surface topography
b. Cross-section of embedded
and ground sample c with detail
of defect d
Table 3 Overview of the results for flatness tolerance, roughness, and
Vickers hardness
All results are given as mean ± standard deviation
Powder bed
condition
Property
Flatness toler-
ance [mm]
Roughness Ra [µm] Vickers
hardness
[HV0.5]
Regular 0.24 ± 0.03 6.2 ± 1.3 120.2 ± 1.8
Irregular 0.44 ± 0.11 8.7 ± 2.8 120.0 ± 2.6
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767Progress in Additive Manufacturing (2023) 8:759–768
1 3
Overall, even when recoater crashes occur, parts from
these build jobs are not necessarily reject and might be usa-
ble, especially for static loading below yield strength and
considering a reduced margin of safety taking into account
microstructural defects and notch effects of as-built surface
conditions.
Nevertheless, powder bed irregularities are still an area
of concern in PBF-LB/M, especially for dynamic loads. The
detection, classification, and simulative prediction of such
process errors constitute growing research fields. Process
simulation along with process monitoring is useful tool to
optimize part designs and build layouts.
This study contributes quantitative results on important
part quality aspects to evaluate the effects of powder bed
irregularities. With this knowledge available, PBF-LB/M
users can make appropriate decisions about the affected
parts, to reduce reject, avoid machine downtime, and there-
fore increase productivity.
Funding Open Access funding enabled and organized by Projekt
DEAL. As being part of the project FLAB-3Dprint, this work was
partially funded by dtec.bw®–Zentrum für Digitalisierungs- und Tech-
nologieforschung der Bundeswehr (dtecbw.de).
Declarations
Conflict of interest On behalf of all authors, the corresponding author
states that there is no conflict of interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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