Materials & Design 219 (2022) 110790
Contents lists available at ScienceDirect
Materials & Design
journal homepage: www.elsevier.com/locate/matdes
In-situ synchrotron X-ray analysis of metal Additive Manufacturing:
Current state, opportunities and challenges
Chrysoula Ioannidou a,⇑, Hans-Henrik König a, Nick Semjatov b, Ulf Ackelid c, Peter Staron d,
Carolin Körner b, Peter Hedström a, Greta Lindwall a
a
Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellvägen 23, SE-100 44 Stockholm, Sweden
Chair of Materials Science and Engineering for Metals, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Martensstr. 5, 91058 Erlangen, Germany
Freemelt AB, Bergfotsgatan 5A, SE-431 35 Mölndal, Sweden
d
Helmholtz-Zentrum Hereon, Institute of Materials Physics, Max-Planck-Str. 1, 21502 Geesthacht, Germany
b
c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An overview on laser-based metal
powder bed fusion and directed
energy deposition using in-situ
synchrotron X-ray characterization is
given.
The existing instrumentation and the
relevant synchrotron measurement
capabilities are described.
The future challenges on new
equipment development and realtime data collection during additive
manufacturing are highlighted.
Motivation for in-situ synchrotron
characterization of metal electron
powder bed fusion is provided.
a r t i c l e
i n f o
Article history:
Received 20 January 2022
Revised 15 April 2022
Accepted 25 May 2022
Available online 29 May 2022
Keywords:
Metal additive manufacturing
Synchrotron X-ray characterization
In-situ studies
Powder bed fusion
Directed energy deposition
a b s t r a c t
Additive Manufacturing (AM) is becoming an important technology for manufacturing of metallic materials. Laser-Powder Bed Fusion (L-PBF), Electron beam-Powder Bed Fusion (E-PBF) and Directed Energy
Deposition (DED) have attracted significant interest from both the scientific community and the industry
since these technologies offer great manufacturing opportunities for niche applications and complex
geometries. Understanding the physics behind the complex and dynamic phenomena occurring during
these processes is essential for overcoming the barriers that constrain the metal AM development. Insitu synchrotron X-ray characterization is suitable for investigating the microstructure evolution during
processing and provides new profound insights. Here, we provide an overview of the research on metal
PBF and DED using in-situ synchrotron X-ray imaging, diffraction and small-angle scattering, highlighting
the state of the art, the instrumentation, the challenges and the gaps in knowledge that need to be filled.
We aim at presenting a scientific roadmap for in-situ synchrotron analysis of metal PBF and DED where
future challenges in instrumentation such as the development of experimental stations, sample environments and detectors as well as the need for further application oriented research are included.
Ó 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).
⇑ Corresponding author.
E-mail address: cioa@kth.se (C. Ioannidou).
https://doi.org/10.1016/j.matdes.2022.110790
0264-1275/Ó 2022 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Materials & Design 219 (2022) 110790
the stress evolution, and SAXS can be used for quantitative observation of the precipitation evolution.
The experimental approaches for in-situ synchrotron X-ray
measurements during PBF or DED are shown schematically in
Fig. 1. Imaging, diffraction and SAXS can be performed either separately or in combination, offering the possibility for multi-scale
microstructural characterization and for revealing the interaction
between the occurring phenomena. Specialized equipment is necessary for mimicking the real processes, therefore, several setups
have been developed and installed at synchrotron facilities worldwide. AM sample environments are now operated at the synchrotron infrastructures at the Advanced Photon Source (APS) at
Argonne National Laboratory (ANL) [10–14], PETRA III at the
Deutsches Elektronen-Synchrotron (DESY) [15–17], the Diamond
Light Source [18–20], the European Synchrotron Radiation Facility
(ESRF) [21,22], the Swiss Light Source (SLS) at Paul Scherrer Institut
(PSI) [23,24] and the Stanford Synchrotron Radiation Lightsource
(SSRL) [25]. These sample environments are used for in-situ Xray measurements during L-PBF and L-DED, and have provided
unique insights into the laser based AM processes. However, no
efforts on in-situ X-ray characterization during E-PBF or E-DED
have been reported so far. Possible reasons behind that are the following. L-PBF has had a much higher market penetration than EPBF, therefore, the general R&D interest in L-PBF has been higher.
In addition, it is more challenging to create a good experimental
set-up for in-situ synchrotron X-ray studies of E-PBF, because the
E-PBF process involves high vacuum, high process temperatures,
pre-sintering of the powder layers, metal vapor etc.
In regard to the above, the current paper presents an overview
of the existing sample environments developed for in-situ X-ray
measurements on metal L-PBF and L-DED, followed by a review
of the recent progress on revealing the phenomena taking place
during the processing of different metals. The recent developments
and state of the art in terms of knowledge, materials and measurement capabilities are highlighted. Finally, emphasis is given to the
need for in-situ synchrotron research on metal E-PBF as well as on
the future challenges for synchrotron characterization during PBF
and DED.
1. Introduction
Additive manufacturing (AM) of metals is applied to produce
complex components from high performance alloys. Compared to
conventional processes, AM provides more freedom of the geometric design of components by allowing structures to be built up
layer-by-layer from a CAD design without the need of tooling
[1,2]. In addition, unique microstructures and local microstructural
control are possible through alloying and processing strategies [1].
AM thus enables weight reduction, functional integration and optimization of the local mechanical properties within components
[1,3] and may, when reaching its full potential, transform the manufacturing industry.
In the AM powder bed fusion (PBF) processes, a moving heat
source is used to fuse metal powder in selected locations of a powder bed to build up the components layer-by-layer. In the laserpowder bed fusion (L-PBF), a laser is used as heat source, and in
the electron beam-powder bed fusion (E-PBF), the heat source is
an electron beam [2]. AM with powder melting can also be accomplished by directed energy deposition (DED). In DED, the powder is
deposited on a specified surface by a nozzle that is aligned either
with a laser beam (L-DED) or with an electron beam (E-DED),
which melts the powder instantaneously upon deposition [4].
The PBF and DED processing cycles involve repeated heating, melting and cooling, which result in transient temperature profiles at
different locations of the component; thus, many complex physical
phenomena occur simultaneously [5].
The complex interactions between the different physical phenomena as well as the interconnection of the various process
parameters can be described and simulated by computational
models, e.g., [1,5]. With experimental validation, these models
are powerful tools for designing new AM materials and optimizing
the printing process and the quality of components [5] through
Integrated Computational Materials Engineering (ICME) [6]. However, the complex processes and resulting intricate microstructure,
make the collection of experimental data challenging, and postprocess characterization of the printed parts is insufficient to fully
understand the underlying mechanisms governing the microstructure evolution.
In-situ synchrotron X-ray characterization techniques, i.e., Xray imaging, diffraction and small-angle X-ray scattering (SAXS)
allow for real-time measurements of the rapid transient phenomena that occur in and around the melt pool [7–9], and can contribute to the validation and development of the computational
models by providing valuable experimental information. These
capabilities arise from the fact that the X-rays can sufficiently penetrate dense, optically opaque metallic materials and analysis can
be performed with high spatial and temporal resolution. Highspeed X-ray imaging can be used to capture the melt pool dynamics and defect evolution, X-ray diffraction can be used to monitor
the solidification kinetics, solid state phase transformations and
2. Real-time measurements on metal AM
2.1. Current developments in instrumentation
The equipment available at present for in-situ synchrotron Xray measurements of L-PBF are shown in Fig. 2a-f and an overview
of their characteristics is given in Table 1. X-ray imaging, diffraction and SAXS have been performed using monochromatic or
pink/polychromatic X-ray beam energies to follow the melt pool
dynamics, the defect and stress evolution and the phase transformation, solidification and precipitation kinetics during printing. A
pink X-ray beam, i.e., a beam with large energy bandwidth, has
Fig. 1. Schematic illustration of the experimental setups for in-situ simultaneous synchrotron (a) X-ray imaging-diffraction and (b) diffraction-small angle scattering during
PBF or DED AM.
2
Materials & Design 219 (2022) 110790
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
a) APS/ANL – Imaging/Diffraction
b) PETRA III/DESY/Hereon – Diffraction/SAXS
c) Diamond Light Source – Imaging
d) SLS/PSI – Imaging/Diffraction
e) SSRL – Imaging/Diffraction
f) ESRF – Imaging
Fig. 2. Available sample environments for in-situ synchrotron studies on metal L-PBF. (a) ‘‘Schematic of the high-speed X-ray imaging and diffraction experiments on L-PBF.”
by Zhao et al. [10] licensed under CC BY 4.0. (b) Reprinted from [15], with the permission of AIP Publishing. (c) ‘‘Schematic of the LAMPR mounted on a synchrotron beam
line.” by Leung et al. [18] licensed under CC BY 4.0. (d) ‘‘Diffraction geometry during an operando measurement.” by Hocine et al. [23] licensed under CC BY-NC-ND 4.0 and
reprinted from [23] with permission from Elsevier. (e) ‘‘Schematic diagrams of the imaging and diffraction setups and the CAD renderings of the sample holder geometry in
imaging and diffraction mode, respectively.” by Calta et al. [25] licensed under CC BY 4.0. Combination of figures, reprinted from [25], with the permission of AIP Publishing.
(f) Reprinted from [21] with permission from Elsevier.
The first system developed for in-situ X-ray imaging and
diffraction of metal L-PBF was implemented at APS in the US,
beamline 32-ID-B [10] (Fig. 2a). The laser source of this system is
equipped with an Ytterbium single-mode fiber laser (IPG YLR500-AC, USA) and a laser head (IPG FLC 30, USA). The wavelength
is 1070 nm and the maximum power is 520 W. The spot size
been used in many studies as it provides increased photon flux
compared to the monochromatic beam, allowing the measurements at shorter timescales. The alloys that have been studied so
far during L-PBF are Ti-6Al-4 V, Ti-5Al-5 V-5Mo-3Cr, Inconel 625,
Invar 36, Nickel 400, AlSi10Mg, Al10SiMg, Al6061, Al6061 + TiC,
AISI 4140, 316L steel, pure Ti, pure W and CMSX-4.
3
Facility
Detectors
Utilized Beam
Characteristics
Windows
Powder
Bed
Sample
Phenomena
studied
Refs.
APS
Beamline
32-ID-B
Imaging
Frame rate: 50 kHz [10]
Exposure time: 350 ns [10]
Updated frame rate:
up to 6.5 MHz [11] with
exposure time: 50 ns [11]
min exposure time: 100 ps,30 kHz[11]
or Frame rate: 50 kHz [30,31]
Exposure time: 7.5 ls [30]
or Frame rate: 50,000 fps [32]
Exposure time: 1/20 ls [32]
or Frame rate: 20–50 103 fps [33,34]
Exposure time: 1–40 ls [33,34]
Frame rate: 25/50 kHz [35]
Exposure time: 1 ls [35]
Pink beam
24.4 keV
25 keV [32]
24.7 keV [33,34]
24.7–25.3 keV [31]
1000 1000 lm2
Pink beam
24.4 keV
Monochromatic beam
55.6 keV
Kapton
Glassy
Carbon
1 mm
Alloys
Ti6Al4V
[31,32,34]
Ti5Al5V5Mo3Cr
Al-alloys [37]
AISI 4140
316L
17–4 PH SS
Inconel 718 [30]
pure W [32]
Al6061 [31,33,35]
Al6061 + TiC [35]
SS316 [33]
Melt pool & powder
dynamics
Pore evolution
Solidification
Phase
transformation
kinetics
Sample
Environment
[10]
Studies
[10,11,30–34,38–
5235,37]
Studies
[36]
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Table 1
Existing sample environments for in-situ synchrotron X-ray measurements during L-PBF.
Thickness
450 lm
Single layer
height 100 lm
Diffraction
Frame rate: 100 kHz [36]
Exposure time: 5 ls [36]
Diffraction monochromatic
Frame rate: 250 Hz [36]
Exposure time: 2 ms [36]
4
DESY PETRA III
Hereon
Beamline
P07 HEMS
Diffraction
Sampling rate: 10 Hz [53]
Exposure time: 0.1 s [53]/0.2 s [54]
PE-Detector
2048 2048 px
pixel size 200 lm
Sampling rate: 20 Hz in solidified material/
10 Hz in powder layer [55]
Monochromatic beam
98.02 keV, 750 70 lm2
[53]
or
79 keV, 60 60 lm2 [54]
or
103.43 keV, 750 70 lm2
[55]
Kapton
50 lm
BN
0.25 mm
Imaging
Sampling rate: 5.1 kHz
Exposure time: 196 ls
Camera 1280 800 px
Pixel size: 6.6 lm
Field of view: 8.4 3.3 mm
Monochromatic beam
55 keV
SLS/PSI
Beamlines
MicroXAS & MS Diffraction
TOMCAT
- Imaging
Diffraction
Sampling rate: 20 kHz 1 s
Exposure time: 45/50 ls
Eiger-detector
500,000 pixel
Pixel size 75 75 lm2
Monochromatic beam,
9.3, 12, 15 or 17 keV
(reflection)
Sample area:
80 35 lm2 [23]/
130 60 lm2 [23]/
80 140 lm2 [27]
80 80 lm2 [24]
80 105 lm2 [26]
Polychromatic beam
10–55 keV [24]
Imaging
10 kHz
672 512 px, equivalent to a field-of-view of
1.85 1.41 mm2
Alloys
Inconel 625 [50],
CMSX-4 [51]
Pure Ti [55]
Thickness
2.5 mm100 layers
(layer height 50 lm)
or 1.5 mm11 layers
(layer height 100 lm)
BN plates
0.3 mm
Alloys
Invar
316L
Thickness
300 lm
Glassy Carbon
entrance: 100 lm,
exit: 500 lm
–
Alloys
Ti6Al4V33 layers
(layer height 30 lm)
powder blends with Al-0.52Sc and Al0.26Sc-0.26Zr [26]
CM247LC
layer height 40 lm
Stress evolution
[15,16,53–55]
Phase transformations
[55]
Sample
Melt pool & powder
dynamics
Powder dynamics
Pore evolution
Sample
Phase
transformations
Stress evolution
Heating/cooling
rates
Cracking
Sample
Environment
[15]
Studies
[15,16], [53–55]
Environment
[18]
Studies
[18,56,57]
Environment
[23]
Studies
[23,2726]
Studies
[24]
Materials & Design 219 (2022) 110790
Diamond Light
Source
Beamline
I12: JEEP
Glassy
Carbon
1 mm
Materials & Design 219 (2022) 110790
Studies
[28]
Thickness
300 lm
Phase
transformation
Studies
[19,22]
Sample
Alloys
Ti6Al4V
is 220 lm on the powder bed.This machine is equipped with
Kapton windows and the powder bed has a sandwich-type structure with two 1 mm-thick plates made out of glassy carbon. The
powder is evenly spread out on top of a 450 lm wide base plate
in between the walls and single layer build measurements are carried out. A pink beam has been used for X-ray imaging with frame
rates up to 6.5 MHz and various exposure times and diffraction
measurements with frame rate 100 kHz and 5 ls exposure time,
while also monochromatic X-ray beam energy has been used for
diffraction experiments with a frame rate of 250 Hz and exposure
time 2 ms. The frame rates and exposure time were adjusted in
each experiment, depending on the alloy and phenomena investigated (Table 1).
Another system for X-ray diffraction and SAXS experiments
(Fig. 2b) during L-PBF was developed and implemented at the
Hereon beamline P07 HEMS at PETRA III, DESY in Hamburg, Germany [15]. The laser source in this system is a single mode continuous wave Ytterbium fiber laser YLR-400-AC (IPG LASER GMBH,
Burbach, Germany), wavelength 1070 nm with nominal power
output 400 W. The laser fiber is connected to a three-axis deflection unit (Axialscan-30 from RAYLASE) via a collimator. The laser
beam is deflected onto a powder bed with a length 70 mm and a
width 3 mm. The system has been used for measurements on
2.5 mm-thick samples consisting of up to 100 layers. Here, the
powder bed has a sandwich structure confined with 1 mm-thick
glassy carbon walls and the setup is equipped with 50 lm-thick
Kapton windows. A funnel-based powder feeding system is used
for spreading out the powder. The system allows for measurements at varying position due to the linear motion of the powder
bed in both horizontal and vertical directions. A monochromatic
X-ray beam energy was used in different experiments using this
setup (98 keV, 79 keV or 103 keV) and the exposure times were
a few hundreds of ms, depending on the experiment (Table 1).
The Laser AM Process Replicator (LAMPR) [18] (Fig. 2c) is
designed to perform high speed imaging during L-PBF at the Diamond Light Source in UK, beamline I12-JEEP. The laser system consists of an Ytterbium-doped fibre laser (wavelength 1070 nm,
transverse mode TEM00, continuous-wave, beam quality factor
(M2)1.03, power 200 W from SPI Lasers Ltd UK, a beam expander,
IR reflective optics, and a Class 1 laser safety enclosure). The laser
beam is focused down to a 50 lm diameter spot at a focal distance
of 254 mm or at the powder bed surface via an f-theta lens and an
IR reflector. Similar to the systems described above, this machine
also consists of a sandwich-structure powder bed confined with
boron nitride walls. A manual powder feeding is used and the
chamber is equipped with boron nitride windows. With this equipment, in-situ X-ray imaging has been performed on 300 lm-thick
samples, 316L steel and Invar, with a sampling rate of 5.1 kHz and
an exposure time of 196 ls (Table 1), using X-ray beam energy of
55 keV.
A miniature L-PBF machine (MiniSLM) has been developed for
in-situ X-ray diffraction studies at the beamlines MicroXAS and
MS and Imaging studies at the beamline TOMCAT at the SLS, PSI,
in Switzerland (Fig. 2d) [23,24,26]. The laser beam (redPOWER,
SPI Lasers Ltd, UK) has maximum power 500 W and is collimated
as a parallel Gaussian beam into a 2-axis deflection scanning unit
(SuperScan III, Raylase GmbH, Germany). The laser beam is focused
through a F-Theta lens (Sill Optics, Germany, to a minimum spot
size of 25 lm at the focal plane of the lens. The setup allows to
print and measure on a powder bed of 12x12 mm2 with a build
height of 5 mm. This machine is tiltable with respect to the horizontal X-ray beam, allowing measurements either in transmission
or in reflection mode [27] with the X-ray beam passing only
through the 100 lm-thick glassy carbon windows of the printing
chamber, differing from the sandwich-structure powder bed
approach used in the equipment described above. The powder is
Diffraction
Sampling rate: 250 Hz
Exposure time: 0.3 ms.
Beamline ID-31
Monochromatic beam
68.4 keV
50 20 lm2
Imaging
Sampling rate: 40 kHz
Exposure time: 12.5 ls
Camera resolution per px: 4.76 lm
ESRF
Beamline
ID19
Sampling rate: 1 kHz
Exposure time: 997 ls
Eiger-detector
Detection area 77 79.9 mm2
Pixel size: 75 75 lm2
Diffraction
Polychromatic beam
50 keV (peak)
30 keV (mean)
Kapton
Glassy
Carbon
Thickness
2 mm
Single layer
height 600 lm
Melt pool dynamics
Powder dynamics
Pore evolution
Environment
[21]
Studies
[25,58–62]
Sample
Melt pool dynamics
Pore evolution
Phase
transformations
Stress evolution
Heating/cooling
rates
Glassy
Carbon
1 mm
Be
0.5 mm
Imaging
Sampling rate: 1.2–27 kHz
Used sampling rate: 4 kHz
Pixel size 1.1 lm
Camera
2016 2016 px
SSRL
Beamlines
2–2 & 10–2
Monochromatic beam
20 keV
50 100 lm2/
300 50 lm2 [58]
Alloys
Ti6Al4V
316L
Al-alloy
Ni-alloy
Refs.
Phenomena
studied
Sample
Powder
Bed
Windows
Utilized Beam
Characteristics
Detectors
Facility
Table 1 (continued)
Environment
[25]
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
5
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Materials & Design 219 (2022) 110790
b) PETRA III/DESY – Diffraction
a) APS/ANL – Imaging
c) Diamond Light Source – Imaging/Diffraction
Fig. 3. Existing sample environments for in-situ synchrotron studies on metal L-DED. (a) Reprinted by permission from [Springer Nature Customer Service Centre GmbH]:
[Springer Nature Customer Service Centre GmbH] [JOM Journal of the Minerals, Metals and Materials Society] [12], copyright (2021). (b) Reprinted from [17] with permission
from Elsevier. c) Reprinted from [29] with permission from Elsevier.
using an X-ray beam size of 50 100 lm2 and 300 50 lm2,
respectively. The studies are presented in Table 1.
An in-situ and in-operando powder bed process replicator
(ISOPR) has been developed for synchrotron X-ray imaging during
L-PBF at the ESRF facility in France, beamline ID19 (Fig. 2f) [21,22].
The laser system is a Ytterbium-doped fibre laser (SPI Lasers Ltd,
UK) 1070 nm wavelength and 200 W laser power. Kapton windows
are used for the chamber and the powder bed has a sandwich
structure confined with glassy carbon walls. The powder feeding
is realized through a vibration assisted gravity-fed powder hopper
and a blade-type spreader. Images from printing of 5-layers of
Ti6Al4V of 0.3 mm thickness have been captured with X-ray beam
energy of 30 keV (mean), frame rates up to 40 kHz and an exposure
time of 12.5 ls [21,22]. In this setup, a monochromatic beam
of 68 keV is used to monitor the phase transformations during
L-PBF of a BeTi powder with a frame rate of 250 Hz, exposure time
30 ms and beam size 50 20 lm2 [28].
In addition to the in-situ synchrotron studies during metal LPBF, measurements during L-DED have also been performed at
APS, DESY and the Diamond Light Source. The sample environments are shown in Fig. 3a-c and the studies related to these sample environments are listed in Table 2.
The L-DED sample environment for high speed X-ray imaging
(Fig. 3a) at the APS synchrotron facility, beamline 32-ID-B [12]
has been used for studies on Ti6Al4V, MoNbTiV and W(CoCrFeMnNi), using a polychromatic X-ray beam with beam size
1000 1000 lm2, and exposure times 5 ls, 10 ls or 25 ls
(Table 2). For the laser system, a 1070 nm Ytterbium fiber laser
source, a galvanometer laser scanner, a vacuum chamber, and
delivered by a hopper-based design with a doctor blade. The build
plate is mounted on a motorized movable vertical stage, allowing
the monitoring of multiple layers being built. For the diffraction
measurements, a frame rate of 20 kHz and exposure times down
to 50 ls were used at the different beamlines. The beam spot on
the samples was of either 80 35 lm2, 130 60 lm2 or
80 140 lm2, 80 80 lm2 or 80 105 lm2 for diffraction measurements on Ti6Al4V or powder blends with Al-0.52Sc and Al0.26Sc-0.26Zr (Table 1) and the X-ray beam energy was 9.3, 12,
15 or 17 keV monochromatic [23,27]. Recently, diffraction and
imaging measurements were performed in the CM247LC nickelbased superalloy using the same setup [24]. For these diffraction
measurements the X-ray beam spot was 80 80 lm2 while for
imaging the field-of-view of 1.85 1.41 mm2.
A setup for imaging and diffraction studies during L-PBF has
been used for measurements at SSRL in the US, beamlines 2–2
and 10–2 [25], shown in Fig. 2e. A single-mode, 1070 nm wavelength, 500 W power, continuous wave fiber laser (IPG Photonics,
Oxford, MA, USA, YLR-500-WC-Y14) is coupled to a 3-axis galvanometer scanning mirror system (Nutfield Technology, Hudson,
NH, USA, 3XB 3-Axis Scan Head). The laser is focused to a 50 lm
diameter circular Gaussian beam at the sample surface. The powder layer is manually applied. The setup includes a sandwich structure powder bed confined with glassy carbon walls and 0.5 mm
beryllium windows allowing for the X-ray beam to enter and exit
the process chamber. The powder is manually applied. With this
setup, in-situ imaging on several alloys (Ti6Al4V, 316L steel, Alalloy, Ni-alloy) with sample rates of 1.2–27 kHz and diffraction
measurements with a sampling rate of 1 kHz have been performed
6
Materials & Design 219 (2022) 110790
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Table 2
Existing sample environments for in-situ synchrotron X-ray measurements during L-DED.
Facility
Detectors
Utilized Beam
Characteristics
Windows
Sample
Phenomena
studied
Refs.
APS
Beamline
32-ID-B
Imaging
3 / 8 104 fps
exposure time:
10 ls, resol 896x776 px
[12]
25 ls, resol 384x488 px
[12]
5 ls, resol 896x776 px
[63,64]
pixel size 2 lm [63,64]
Polychromatic beam
24 keV
1000 1000 lm2
Kapton
Alloys
Ti6Al4V [12,64]
MoNbTiV [63]Wx(CoCrFeMnNi)
100 x, with
0 x 21 at% [65]
Melt pool dynamics
[12,63]
Pore evolution
[12,63,64]
Mixing of high entropy alloys during
laser remelting [65]
Sample
DESY PETRA III
Hereon
Beamline
P07 HEMS
Diffraction
Sampling rate: 10 Hz
PE-Detector
2048 2048 px
pixel size 200 lm
Monochromatic
beam
97.6 keV
200 1000 lm2
–
Alloys
X40CrMoV5-1
Phase transformation kinetics
Lattice parameter evolution of c-Fe
Diamond Light
Source
Beamline
I12: JEEP
Imaging
frame rates: 200–2000
fps
resolution:
6.67 lm /pixel
Monochromatic
beam
100 100 lm2
Kapton
steelThickness
1.5 mm
53 keV [19,20,66]
70 keV [29]
Alloys
SS316, Ti6Al2Sn4Zr2Mo,
Inconel 718
Thickness
1.5 mm
Environment
[12]
Studies
[12,63–65]
Sample
Environment
[17]
Studies
[17]
Melt pool dynamics
Studies
[19,20,29,66]
Powder dynamics
Phase transformations, stress and
temperature evolution
Diffraction
exposure time each
pattern 6.67 s
spatial resolution:
100 lm
microstructure and the defect formation in the built parts
[50,57], affecting the performance of the final component
[2,7,21,67,68]. Therefore, revealing the relation between the processing conditions and the melt pool and defect dynamics is critical
[5,69].
In L-PBF, the heat-transfer by the incident laser beam plays an
important role in the melt pool morphology as it controls the melt
pool depth, the keyhole formation and evolution [44] and the elemental evaporation [70]. The consequent melt flow velocity and
direction influence the pore motion [57,62,69]. Apart from the process related factors (energy input [71] and scanning strategy), the
defect formation is also affected by the powder characteristics
[56,58,69] and the material [48,68,70,72,73]. Spattering, either by
liquid metal droplets or by unmolten powder particles near the
melt pool, also causes defect formation [51] and surface roughness
[21], and it is dependent on the heat source, scanning speed, pressure conditions and material [46]. Lack of fusion between the
printed layers is also an important concern and is related to melt
pool instabilities and incomplete melting of the material during
processing [5]. Potential vaporization of alloying elements can also
alter the alloy composition, affecting the entire process and
microstructure evolution. Finally, crack initiation and propagation
[48,69,74,75] is a major issue for the quality of the AM
components.
High-speed X-ray imaging has been used to monitor the rapid
transient phenomena of melt pool dynamics [21,32,44,48–
50,58,62]
and
the
defect
evolution
[18,21,22,32,42,43,45,47,48,56–58,60,62,24] in various metal
alloys metals during L-PBF (Table 1). It has been enabled by the
third-generation synchrotron radiation sources [36,76,77] that
allow for X-ray measurements with high temporal (tens of ls
[10]) and spatial (a few lm [11]) resolution, as well as by the direct
and precise subsurface observations [25] due to the good contrast
between the different phases [36]. The in-situ synchrotron X-ray
studies of metal L-PBF this far (Table 1) include measurements of
the melt pool dynamics, visualization of the keyhole evolution
and following the defect evolution. A representative example of
two sets of stepping motors are used. In this setup, the commercial
powder hopper from Powder Motion Labs, LLC is used for powder
feed.
The L-DED system for in-situ X-ray diffraction installed at DESY,
PETRA III, beamline P07 HEMS (Fig. 3b) [17] has been employed for
measurements on multiple layers and specimens (X40CrMoV5-1
and steel) with a thickness of 1.5 mm with a sampling rate of
10 Hz, using an X-ray beam with energy 97.6 keV and size
200 1000 lm2. The laser set-up inludes a solid state YAG-laser
(IPG Photonics), wavelength 1.4 lm, power 300 W and spot size
0.3 mm at the working distance 5 cm below the powder delivery
nozzles. The powder is delivered by two nozzles using pressurized
2.4 bar Ar flow.
The Blown Powder Additive Manufacturing Process Replicator
(BAMPR) was developed to mimic a commercial DED-AM system
and the setup shown in Fig. 3c has been used for imaging and
diffraction
measurements
on
several
alloys
(SS316,
Ti6Al2Sn4Zr2Mo, Inconel 718) (Table 2), during multilayer L-DED
printing at the Diamond Light Source, beamline I12-JEEP [29].
The laser is a 1070 nm Ytterbium-doped fiber laser beam
(continuous-wave mode, power 0–200 W), coupled with tunable
optics to achieve a focused spot size 200–700 lm. The laser is
concentric with the powder delivery stream blown from the nozzle
and normal to the substrate plate. This machine is equipped with
Kapton windows as well. Layers of 1.5 mm thickness have been
printed and monitored with monochromatic X-ray beam energies
of 53 keV or 70 keV. Images with frame rates in the range of
200–2000 fps have been recorded with an imaging resolution of
6.67 lm/pixel. For the diffraction experiments, the exposure time
for each diffraction pattern was 6.67 s. For all experiments, the
beam size was 100 100 lm2.
2.2. Overview of real-time studies to date
2.2.1. Melt pool dynamics and defect evolution
The melt pool profile (size/shape/morphology) and its evolution
during the manufacturing process have a direct impact on the
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Materials & Design 219 (2022) 110790
Fig. 4. Time-series radiographs of a single-layer of Invar 36 during L-PBF (P = 209 W, V = 13 mm s1 and LED = 16.1 J mm1). Evolution of (a) melt track morphology, (b–d)
powder consolidation, e) spatter and f) porosity during L-PBF [57]. Created by Leung et al. [57] and licensed under CC BY 4.0.
in Ti6Al4V originated from keyhole instability and is related to the
keyhole and melt pool depth. Further, Sinclair et al. [22] stated that
the keyhole pore size depends on the scanning speed and the laser
power. Gan et al. [33] developed a scaling law for keyhole stability
and porosity in Al6061 and SS316 in AM. Six mechanisms regarding pore formation have been described [47] and interesting observations on pore motion have been reported [31,48]. In addition,
pore evolution has been captured in single track experiments
[18] and in overhang conditions [57], and a pore elimination mechanism based on a thermocapillary force has been proposed [10,11].
Apart from porosity investigations, five spattering types have been
identified in AlSi10Mg and Ti6Al4V alloys [51] and the initiation
and healing of hot cracking have been observed in Al6061 [36].
Understanding the powder dynamics is also critical in PBF for guiding the design of powder with optimized characteristics. According
to Zhao et al. [45], the powder motion influences the keyhole pore
the melt track morphology and defect evolution in Invar 36 during
L-PBF is shown in Fig. 4 [57], and some of the most interesting findings from X-ray imaging studies are summarized below.
The melt pool flow and the vapor bubble dynamics as a function
of pressure and composition were quantified in 316 L steel, Nickel
400 and Ti6Al4V [61]. Guo et al. [49] mapped the melt pool behavior under different process conditions (energy density, laser power
and scanning speed) and the relation between melt pool geometry
and energy absorption was also studied by Simonds et al. [34]. The
parameters that influence the keyhole profile, such as pressure in
the printing chamber [61] or the laser power [50], have been quantitatively investigated. Porosity and spattering have been correlated to keyhole dynamics and melt pool recovery for a Ti6Al4V
alloy [21]. Pores were detected near the keyhole bottom and
between the built layers and similar keyhole-related features were
found in all layers. Zhao et al. [45] found that the keyhole porosity
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Materials & Design 219 (2022) 110790
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occurring phenomena and for obtaining quantitative data of the
involved physical processes that occur at small length and short
time scales, the beamlines and detector capabilities should fulfill
certain specifications. The X-ray pulse rate should be high enough
so that it does not pose a limit to the otherwise available detection
capabilities of the available detector setups [11]. In-situ X-ray
imaging at the fastest possible temporal resolution comes at the
cost of a lower spatial resolution and thus a reasonable balance
between the two needs to be maintained. The current state of
the art in terms of high speed recording and temporal resolution
was reported by Parab et al. [11]. They performed ultra-fast Xray imaging measurements at the APS synchrotron facility with a
maximum frame rate of 6.5 MHz and an exposure time of 50 ns
during L-PBF of AlSi10Mg and Ti6Al4V. They also reported a minimum exposure time of 100 ps for each image at 30 kHz. The
6.5 MHz image acquisition rate is the fastest X-ray imaging speed
reported so far for in-situ PBF processes. The available cameras
could have offered recording rates up to 10 MHz, however, in order
to avoid mismatch between the X-ray pulses and recording frames,
the final recording rate was limited to the pulse frequency of the
synchrotron source (6.5 MHz). The spatial resolution of these measurements was determined by the camera’s characteristics and settings, and was either 1.9 lm or 3.2 lm per pixel. Parab et al. [11]
also showed that using a frame rate of 30 kHz and an exposure
time of 100 ps in a 800 lm-thick sample, makes it possible to follow the melt pool and keyhole dynamics, the porosity evolution
and the ejection of powder and molten metal from the melt pool
generation in the L-PBF built Ti6Al4V. Guo et al. [46] studied the
powder dynamics and its dependency on pressure and position
during in-situ L-PBF of 316L stainless steel and AlSi10Mg, and
Escano et al. [13,14] investigated the dynamics of powder spreading. Ghasemi-Tabasi et al. [24] recently observed crack formation
during L-PBF of the CM247LC nickel-based superalloy using Imaging, which they combined to diffraction and found evidence of
solidification and liquation cracking.
Compared to L-PBF, less attention has been given to synchrotron characterization of DED-AM, however, studies on metal
L-DED have also been successfully performed with the setups
described in section 2.1 and listed in Table 2. Wolff et al. [12]
reported that the mass flow rate and velocity of the deposited powder particles affect melting and solidification, and the melt pool
dynamics and the powder flow in Ti6Al4V, SS316 steel,
Ti6Al2Sn4Zr2Mo and Inconel 718 were monitored [19,20,29,66].
In another study, Wolff et al. [64] investigated the differences in
the pore formation mechanisms between L-DED and L-PBF in
Ti6Al4V. The melt pool dynamics and pore evolution in the MoNbTiV high-entropy alloy were captured using the same equipment
[63]. Finally, Pegues et al. [65] used high-speed X-ray imaging to
study the mixing of Wx(CoCrFeMnNi)100 x (0 x 21 at%.) high
entropy alloys during laser remelting.
Based on the research mentioned above, it can be concluded
that in-situ synchrotron X-ray imaging is a powerful tool for capturing the melt pool dynamics and the defect evolution in real
time. For a deeper understanding of the dynamic and complex
Fig. 5. In-situ X-ray imaging during L-PBF of a) AlSi10Mg with a frame rate of 30 kHz and exposure time 100 ps in a 800 lm-thick sample, and of b) Inconel 718 of 380 lm
thickness at different exposure times (top to bottom): 1 ls, 5 ls, 10 ls and 20 ls. Created by Parab et al. [11] and licensed under CC BY 4.0.
9
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Materials & Design 219 (2022) 110790
below. A flat field correction is normally applied to the images
which undergo noise removal, background subtraction and segmentation [21,56,57]. Then, the molten pool dimensions and the
spatter characteristics can be determined using customized data
analysis tools [56,57,60]. Noise reduction and contrast enhancement is necessary when the clear identification of the melt pool
boundary and spatter features is required [49,51].
for lighter materials, such as the AlSi10Mg alloy (Fig. 5a). For
Inconel 718, exposure times of 20 ls were used for samples of
380 lm, in order to capture the melt pool (Fig. 5b). For such studies, the probed sample thickness, the X-ray beam energy, the
recording rate and the exposure times to the X-ray beam should
be adjusted so that sufficient temporal and spatial resolutions are
achieved. Regarding the spatial resolution, the best that has been
achieved up to now is 1 lm and has been reported by Martin
et al. [42], who used the same sample environment at APS with a
frame rate of 3.3 MHz and exposure time of 300 ns for each image.
This resolution was limited by the pixel resolution of the detector
(1 lm per pixel) and also by the light emitted by the scintillators,
the size of the X-ray source and its distance from the sample as
explained by Sun et al. [36].
Comparing to L-PBF, the melt pool is larger in L-DED [19], the
scanning speed is normally three orders of magnitude smaller
and the solidification rate is also slower, therefore different acquisition times for imaging the melt pool dynamics in L-DED should
be applied. Nevertheless, these choices should be made based on
each specific experiment and phenomena under investigation.
For instance, Chen et al. [19] used a frame rate of 200 fps, obtained
good contrast between the solid–liquid phases during L-DED on
the SS316 steel, and managed to image the melt pool boundary
with high signal to noise ratio. However, the rapid phenomena
such as the powder behavior and its influence on the melt pool
dynamics could be followed only when using a higher frame rate
of 2000 fps, compromising the signal quality. For L-DED, the fastest
recording speed, 30,000 fps with exposure time 5 ls, was reported
by Wolff et al. [64] and was used for capturing pore evolution in
Ti6Al4V.
The high frame rates used in the above studies make the data
reduction and analysis challenging as large data volumes are produced. A few examples on data reduction and analysis are given
2.2.2. Phase transformation kinetics
The rapid heating and cooling rates, in the order of 105-106 °C/s
for L-PBF [23] and 103-105 °C/s [78,79] for E-PBF, and the repetitive
thermal cycles during PBF AM determine the phase transformation
and solidification kinetics, thus the microstructure evolution.
High-speed X-ray diffraction can provide unique information on
the phase transformation kinetics and can also be used to monitor
the stress evolution as well as the texture/grain structure/dislocation attributes.
Performing in-situ synchrotron diffraction measurements during L-PBF has been accomplished by a few groups worldwide using
the equipment presented in section 2.1. The phase transformation
kinetics in Ti6Al4V [23,25,27,58,59,80], Ti5Al5V5Mo3Cr [58],
CMSX-4 [54], pure Ti (grade 1) [55] and Be-Ti 185 [28] has been
investigated, and indirect information on the heating and cooling
rates through changes in the lattice spacing of these alloys has
been obtained [23,25,27,54,58,59,80]. Fig. 6 shows an example of
in-situ diffraction data obtained during L-PBF of Ti6Al4V, where
the time evolution of the integrated peak intensities shows the
phase fraction evolution during printing [59].
The synchrotron diffraction measurements have been performed with frame rates of hundreds of Hz to kHz and low exposure times. Diffraction patterns of Ti6Al4V with a frame rate of
20 kHz and an exposure time of 45 ls were captured at PSI
[23,27,80], where they managed to follow the phase transforma-
Fig. 6. Diffraction data of Ti6Al4V during L-PBF with 200 W laser power and 144 mm/s scanning speed. a) Diffraction patterns 10 ms before, 4 ms after and 80 ms after
melting. The laser position and the X-ray probed region are shown in the insets. b) Corresponding azimuthally integrated intensities as a function of Q. c) Integrated
intensities of the diffraction peaks as a function of time. Created by Thampy et al. [59] and licensed under CC BY 4.0.
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Materials & Design 219 (2022) 110790
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
being almost identical, the exposure time being short and the c’
phase fraction being small, however, gaining quantitative information on the c’ phase fraction was possible by performing SAXS.
tions, stress evolution and heating/cooling cycles during L-PBF of
Ti6Al4V. At SSRL, the phase transformations, stress evolution and
heating/cooling rates of the Ti6Al4V and Ti5Al5V5Mo3Cr alloys
were studied with 1 kHz frame rate and exposure time of 1 ms
[25,58]. Sun et al. reviewed the recent in-situ synchrotron diffraction studies at APS, which were performed with the highest temporal resolution reported so far [36]. The melting, solidification, and
solid phase transformation processes of Ti6Al4V were revealed. A
pink beam (first harmonic 24 keV) for high flux measurements
was used, and diffraction patterns were obtained with a frame rate
of 100 kHz with 5 ls exposure time, but with q resolution limited
by the large energy bandwidth. In addition to these measurements,
they used a high energy monochromatic beam (55.6 keV) and
recorded diffraction patterns with better q resolution (that allowed
the deconvolution of all existing phases) but with a smaller frame
rate of 250 Hz and exposure time of 2 ms. They showed that
achieving both high temporal and q resolution is challenging and
that these diffraction measurements should be considered complementary. These features could be very useful for probing the phase
transformation kinetics in Ni-based alloys with sufficiently high q
resolution, necessary to separately capture the c΄ precipitates that
have similar lattice parameter as the matrix. For instance, Wahlmann et al. [54] reported that the high heating rate during L-PBF
could not be resolved for exposure times in the range of 200 ms.
Regarding in-situ X-ray diffraction on metal L-DED, Epp et al.
[17] studied the phase transformation kinetics in the
X40CrMoV5-1 steel, and obtained quantitative data on the austenite lattice parameter evolution as well as on the tetragonality and
carbon content in the martensite. Finally, Chen et al. [29] monitored the phase transformation kinetics and the stress development in Inconel 718 with a spatial resolution of 100 lm.
2.2.4. Stress evolution
Cracks and deformation are very common in L-PBF built parts
and they can have a negative effect on the parts’ mechanical properties [2,17,68,83–85]. The highly localized energy from the laser
results in large temperature gradients and a complex stress field
causing tensile and compressive stresses during layer-by-layer
building [53,86], eventually leading to residual stresses [15,87].
The stress magnitude depends on the heat source, the processing
conditions, the material [88] and the geometry of the part [89].
Minimizing the residual stresses and their contribution to cracking
and deformation [16,68] are usually key factors for improving the
resulting part properties, thus, knowledge on their origin and evolution mechanisms and their correlation to the processing parameters is necessary [16].
Ex-situ studies on the stress quantification in parts after PBF
processing can only quantify the remaining stresses but not their
dynamics, origin and type, resulting in a need for time-resolved
measurements [15,16]. In-situ synchrotron measurements offer
an insight into the dynamics of stress generation and temporal
evolution by providing quantitative data on the lattice parameter
dynamics of the different phases involved with sufficient temporal
resolution [15,16,58].
High-speed X-ray diffraction has been used to track the stress
and strain evolution during L-PBF process in Inconel 625
[15,16,53] and Ti6Al4V [27,59,80]. The stress dynamics of Inconel
625 during printing with a sampling rate of 10 Hz was monitored
using the system shown in Fig. 2b [12]. Using this equipment, the
first in-situ X-ray diffraction study on the strain and stress evolution in plane and in build direction was performed [53] and the
absolute stress values in pure Ti were calculated [55]. Moreover,
the diffraction patterns of Ti alloys were recorded with a sampling
rate of 1 kHz using the setup of Fig. 2e [25], following the lattice
evolution of a and b phases and quantifying the residual stress
evolution [58]. In addition to measurements during L-PBF, synchrotron X-ray diffraction in Inconel 718 [29] during multilayer
L-DED was performed with exposure time for each diffraction pattern 6.67 s, managing to quantify the phase transformation kinetics
and the stress evolution.
The quantification of the stesses evolution during L-PBF and LDED processing is achieved either by segmenting the entire diffraction patterns into cake pieces or using azimuthal angles to create
segments, which are being integrated to create 1D profiles from
the 2D diffraction patterns. The 1D profiles are normally fitted by
a Voigt [15,53] or PseudoVoigt [55] function which reveals the
peak shape. This procedure includes many challenges, e.g. defining
a triaxial stress state, fitting of asymmetric peaks caused by the different mechanical stresses between bulk and powder material,
peak shifts due to rapid temperature changes, detector position
dependency of the direction of lattice shifts and uncertainties in
raw data fitting, to name a few. Recent studies prove that these
challenges can be overcome. For instance, Uhlmann et al. [15] indicated that valuable conclusions can be obtained without the
knowledge of the full triaxial stress state because the contribution
of the stress in the longitudinal direction is equal to both transverse and build-up directions and the difference between them is
not affected by the longitudinal direction. In addition, Schmeiser
et al. [53] used only the top 60% of the peak data for the peak fitting
in order filter out the diffracted intensity of the powder and the
resulting asymmetry. In order to determine peak shifting due to
temperature change it is important to decouple the effects of temperature and internal stress on the peak shifting. For the data analysis of each diffraction pattern, an average temperature over the
2.2.3. Precipitation kinetics
SAXS measurements in metals during PBF and DED can provide
quantitative information on the precipitation evolution. Concurrent SAXS and diffraction may offer the possibility to study the
temporal evolution of the precipitate size distribution (by SAXS
[78,81]) as well as the precipitate crystal structure and composition (by diffraction) in the same sample simultaneously [82], yielding a unique insight into the dynamics of precipitation.
Despite the high demand for knowledge on the precipitation
kinetics in AM materials and the unique possibilities that SAXS
provides, only a few in-situ SAXS studies during AM conditions
have been reported. The challenge here lies in enabling the beam
and detectors to offer a small signal-to-noise ratio so that precipitates with small phase fraction can be detected.
Wahlmann et al. studied the c΄ precipitation kinetics in the Nibase super-alloy CMSX-4 using in-situ SAXS during simulated EPBF conditions and reported the temporal evolution of the precipitate dissolution, coarsening, and morphological transition [78].
They detected precipitate sizes in the range of 10–30 nm for a cooling rate of 200 °C/s and slightly smaller particle sizes and faster
kinetics for a cooling rate of 300 °C/s as a result of the higher nuclei
density. Since CMSX-4 is a strong X-ray absorber, an exposure time
of 1 s was chosen to acquire enough counts. They also reported that
potentially fast dissolution of the c’ phase could not be captured
because of the small temporal resolution of the measurements.
They pointed out though that, despite the new insights that could
be gathered during their experiments, the heating and cooling
rates achievable in their setup are still orders of magnitude lower
than predicted by simulation of the E-PBF process.
Recently, the first simultaneous in-situ SAXS and X-ray diffraction measurements during L-PBF were performed by Wahlmann
et al. [54], who measured the c’ phase transformation kinetics of
CMSX-4. They reported that the distinction between the c and c’
phases was difficult by X-ray diffraction due to their peak position
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Materials & Design 219 (2022) 110790
example is the possibility to process crack sensitive materials like
TiAl [98] with E-PBF.
gauge volume can be assumed, so that the lattice spacings obtained
from this pattern, corresponding to different directions (transverse
or build-up), are only affected by the thermal strain and in the
same magnitude [55].
2.4. Challenges in AM sample environment development for
synchrotron studies
2.3. Motivation for in-situ synchrotron characterization of metal E-PBF
Building new equipment or updating existing set-ups is essential for performing advanced in-situ X-ray characterization of PBF
and DED processes. The optimum equipment design for each
experiment should be dependent on the printing process, the Xray characterization techniques and the beamline characteristics.
The equipment should mimic the typical AM features as close as
possible. It should be able to print components under AM conditions similar to the ones that the corresponding industrial machines offer, i.e. vacuum or gas atmosphere depending on the
process, scanning speed, electron or X-ray beam energy, thermal
conditions etc. The printing of complex geometry components in
the equipment specially built for synchrotron X-ray measurements
is challenging because conditions like component thickness, X-ray
transmission through certain materials and windows, space and
weight limitations for equipment in the beamlines need to be
fulfilled.
Future adjustments on the L-PBF and L-DED systems, e.g. X-ray
window dimensions, powder bed geometry and sample thickness,
should consider simultaneous diffraction and SAXS or imaging
measurements, offering the possibility for real-time and multiscale microstructural characterization. This will offer the opportunity to study the residual stresses and their relation to crack formation, or possible interaction between phase transformation and
precipitation kinetics.
Considering the high demand on quantitative knowledge on the
E-PBF process, experimental set-ups suitable for in-situ synchrotron measurements on E-PBF need to be developed. The high
vacuum, the high process temperatures, the pre-sintering of the
powder, the metal vapor and the backscattered electron radiation
involved in the E-PBF process, imply a high technical barrier for
equipment development. Combination of diffraction and SAXS or
imaging studies should also be enabled which put further requirements on the samples environments.
The X-ray techniques and the manufacturing process require the
use of different window materials for the building chambers. When
X-ray techniques are coupled, the window material should satisfy
the requirements of all the involved techniques. The windows
should provide sufficient X-ray transmission and mechanical stability. The windows may require cleaning or replacement due to sputter accumulation, and such costs are important, therefore should be
considered when choosing the window material. They should be
able to resist high temperatures and give a constant and low background signal during the process compared to the specimen. For Xray diffraction, the diffraction peaks of the window material should
not interfere with the sample peaks. For SAXS, the windows should
give a small, constant and uniform contribution to the scattering
pattern. In addition, for E-PBF chambers, the windows should be
leak- and vacuum-tight. A window failure can be catastrophic for
an E-PBF machine as it will create an instantaneous inrush of air
which is likely to destroy the turbomolecular pumps. It may also
cause a risk of fire if hot metal powder is present in the machine.
Furthermore, the window material should not be degraded by bombardment by backscattered electrons and it should be able to withstand the heat created by electron radiation and IR radiation from
the melt pool. The windows should preferably be conductive or
coated with a conductive coating to dissipate electric charge so that
charge accumulation on the window is avoided. An electrically
charged window may affect the electron beam positioning and
can also attract powder particles. A conductive window material
will also help to dissipate the heat that is generated by backscatter
Despite the fact that the E-PBF systems have been available on
the market for over 20 years (with the technology commercialized
by Arcam AB in 1997 for the first time [1]), in-situ synchrotron
experiments have not yet been conducted during E-PBF.
Although L-PBF and E-PBF are conceptually similar, there are
significant differences between these two processes that are
mainly related to the nature of the energy sources, to the processing steps and to the different processing environments. The distance that the photons or electrons can penetrate a material
before being absorbed is, in the case of lasers, in the order of a hundred nanometers [90], whereas for electron beams it is in the order
of a few tens of micrometers [91] (for solid metals). Hence, an electron beam deposits thermal energy in a larger volume of the material, whereas a laser beam heats only close to its surface and as a
result, a different thermal behavior in the powder beds of the
two processes is observed. Another distinction between E-PBF
and L-PBF is the addition of a pre-heating step before the melting
of a layer in E-PBF. This step is used to raise the powder bed temperature to a desirable building temperature and to prevent
‘‘smoke” (electrostatic charge-up and subsequent levitation of
powder particles due to electrostatic repulsion [92,93]), by presintering the powder particles before layer melting, and in this
way increasing the electrical and thermal conductivity of the
power. The high building temperature has proven beneficial for
reducing residual stresses in E-PBF parts [94] (e.g., due to smaller
thermal gradients and a generally hotter process [1,95]). In addition, E-PBF offers the possibility to use the electron beam to maintain a certain temperature in the build part and control the
microstructure evolution during printing [1]. Furthermore, E-PBF
requires a vacuum (e.g. 2 10-3 mbar [96] or 10-4-10-5 mbar [1])
whereas an Argon atmosphere is usually used in L-PBF.
These differences between the two PBF technologies are
expected to result in quite different behavior during melt pool formation (e.g., shape [5]), melt pool evolution over multiple layers
(e.g., re-melting behavior), spattering behavior [56,58] and pore
formation [69]. In addition, the degree of elemental evaporation
is higher in E-PBF [1,5] and the solidification rates [3] as well as
the cooling rates are lower (103-105 °C/s) [78,79] compared to LPBF (105-106 °C/s) [23].
Due to these phenomenological differences between E-PBF and
L-PBF processes, the knowledge gained by previous in-situ highenergy X-ray studies on L-PBF cannot be directly transferred to
E-PBF. Quantitative knowledge on E-PBF by means of in-situ synchrotron techniques is therefore necessary to attain unique
insights into the process. Similar to the synchrotron studies on
metal L-PBF and L-DED, the melt pool dynamics and the defects
and stress evolution can be quantitatively captured with high temporal and spatial resolution. With such a dedicated E-PBF sample
environment, the interaction of the electron beam with the metal
powder, the fluid dynamics and the microstructure evolution can
also be clarified and can be used for validation and further development of existing modeling approaches [97]. In addition, phenomena related to the processing steps unique to E-PBF, for
instance, the effect of high temperature processing on stress- and
microstructure- evolution or the phenomena of ‘‘smoke” (which
can put a limit on what alloys can be processed with E-PBF), can
be investigated. Eventually, understanding the strengths and limitations of each AM technology would allow the development of
new materials specifically designed for these technologies. An
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is obtained. The detectors have also certain capabilities regarding
their recording rates, saturation and resolution. For high quality
measurements, the X-ray beam as well as the detectors characteristics need to be coupled accordingly, while always accounting for
the type of material studied and printing method. A small noise-tosignal ratio, good counting statistics and sufficient temporal, spatial and q-resolution lead to a high quality measurement. These
are dependent on the alloy that is studied and the printing parameters that determine the phenomena taking place during printing
as well as the microstructure evolution.
The X-ray beam characteristics and the settings of the detection
systems employed should be chosen based on the time and length
scales of the phenomena under investigation. For example, the different powder sizes typically used for different AM techniques
need to be considered. The powder sizes are normally, e.g., 5–
65 lm in L-PBF [10,53], 45–105 lm in E-PBF [1] and 50–100
lm in L-DED [17]. The powder size distribution determines the
minimum thickness of the printed layer and consequently, the
appropriate X-ray beam dimensions for the diffraction measurements. Uhlmann et al. [15] performed diffraction measurements
in transmission mode and chose the beam height to be close to
the nominal layer thickness value in order to achieve sufficient
spatial resolution for their diffraction experiments on Inconel
625. In diffraction, the beam should not be too wide to avoid undesired contributions from the surrounding volume [25]. Furthermore, the X-ray beam size should be tuned based on the purpose
and type of each measurement [17]. For imaging, the X-ray beam
is chosen to be larger than the melt pool size, which is determined
by the energy dissipation from the heat source, thus by the AM
process. The melt pool is generally larger in L-DED and E-PBF than
in L-PBF [19], and can vary depending on the scanning strategy [3].
Apart from the layer thickness and the melt pool size, the different
AM processes result in different cooling rates, thermal gradients
and solidification rates and thus, different acquisition times and
recording rates are needed. For instance, higher solidification
velocities are typically observed during L-PBF (in the range of
0.1–1 m/s [42,60]), lower during E-PBF (0.1–1 m/s [96]) and the
lowest during L-DED [3].
In summary, the time and length scales of the occurring phenomena during the processes as well as the beamline capabilities
make the in-situ X-ray characterization during AM challenging
but at the same time very attractive for future research due to
the unique quantitative information that they can provide. Consequently, the development of new equipment as well as the continuous improvement of the existing equipment for more advanced
experiments is of high importance.
electron bombardment. Since smoking effects and metallization
may affect the transparency of the windows, exchangeable windows or window protection are needed.
The materials for X-ray windows that have been used so far in
the AM sample environments are described in section 2.1. and
listed in Tables 1 and 2. These materials have high X-ray transparency and sufficient mechanical strength for the specific experiment. Kapton has been used as a window material for X-ray
imaging [10,12,19–21,29,66] and diffraction [10,15,29], boron
nitride for imaging [18], glassy carbon for diffraction [23], and
beryllium for imaging and diffraction [25]. Beryllium offers excellent X-ray transmission but it is brittle, cancerogenic and expensive. Kapton offers high X-ray transmission [99] and its SAXS
scattering intensity depends linearly on the window thickness
[100]. Finally, glassy carbon is cheap and preferable at lower Xray beam energies and has thus been used for window and powder
bed wall material (see Tables 1 and 2), but gives strong SAXS signal. Depending on the experiment requirements, other window
materials could be considered, such as quartz or aluminum. Quartz
has been used in synchrotron X-ray diffraction under vacuum
[101], however, it is not recommended in SAXS because of its
amorphous scattering and significant background signal at low q
[102]. Aluminum offers relatively high X-ray transparency (but
lower than Kapton or beryllium) and vacuum barrier strength
and has been used in SAXS and X-ray diffraction [103]. If an aluminum alloy is used, the window temperature should be low to
avoid precipitation.
Regarding the powder bed design for the majority of the existing L-PBF equipment, a sandwich-type structure consisting of two
X-ray transparent plates, either from glassy carbon [10,15,21,25] or
from boron nitride [18], has been used, that act as walls holding
the metal powder in between them. This configuration has been
used for measurements in transmission mode. Only Hocine et al.
[23] used a different design approach for the powder bed geometry, i.e., tiltable L-PBF machine, that is suitable for both transmission and reflection measurements. The tilt angle range is
dependent on the X-ray beam energy and the investigated material, and determines the illuminated area and probed volume. Such
a confinement-free powder bed design, without the use of additional X-ray transparent walls, offers the advantages of avoiding
undesirable interactions of the walls with the process or the measurements, and mimics better the real PBF process in respect to the
thermal evolution. However, it adds other challenges. For example,
for transmission measurements, printing and measuring need to be
done close to the edge of the powder bed since thin enough specimen sections are required, i.e., the X-ray beam should be able to
pass through the sample and in the case of imaging measurements
should be able to capture the entire melt pool area so that the critical phenomena during processing can be monitored. Alternative
designs of the raking blades geometry could also be applied instead
of printing close to the powder bed edge, offering the possibility to
shape the powder bed and get the advantages of avoiding undesirable interactions of the walls with the printing process or the measurements as well as achieving more realistic thermal conditions.
For reflection measurements, the accurate determination of the
optimum X-ray beam vertical position with respect to the sample
is challenging due to shrinkage of the material during printing (the
sample volume changes from the sintered powder to the melted
and solidified material).
Finally, the X-ray beam characteristics (size, energy, flux, pulse
frequency) and the measuring capabilities (detectors capacity etc.)
bring their own limitations to the time and spatial resolution with
which the rapid occurring phenomena can be monitored. The available X-ray beam energy range is determined by the flux that the
synchrotron facility and its systems can provide, and it sets the
possible sample thickness for each material so that a good signal
3. Summary and outlook
This article provides an overview of the synchrotron X-ray studies on metal L-PBF and L-DED, summarizing the state of the art in
terms of instrumentation development, applied X-ray techniques,
investigated alloys, advanced beamline and detector characteristics, and experimental findings. The differences between the nature
of the laser- and electron beam- based AM technologies are
described, and emphasis is given on the motivation for in-situ synchrotron measurements on metal E-PBF. In addition, the future
challenges on instrumentation (synchrotron experimental facilities, sample environments and detectors) and measurements are
presented.
PBF and DED are being successfully implemented by many industries worldwide, intensifying the need for high quality part production. However, several challenges still need to be addressed for a
wider application of AM products. Recently, remarkable progress
has been made in process understanding due to the synchrotron
13
C. Ioannidou, Hans-Henrik König, N. Semjatov et al.
Materials & Design 219 (2022) 110790
X-ray technology and thus, many in-situ synchrotron studies on
metal PBF and DED are expected in the coming years. In this direction, designing new and updating existing sample environments
for AM is vital. Sample environments for in-situ X-ray measurements during E-PBF need to be built and future improvements of
the L-PBF and L-DED systems should consider simultaneous diffraction and SAXS/imaging. Improvements on the beamline and detector characteristics are also necessary. It is expected that in the
coming years, the fourth generation of synchrotron facilities will
focus on offering increased brilliance (100 times higher brilliance
than at the current facilities), increased beam flux and enhanced
spatial and temporal resolution for extremely demanding X-ray
experiments [104]. Furthermore, faster area detectors especially
for the high-energy diffraction regime are going to be developed.
These upgrades may open new possibilities for future measurements. The combination of X-ray characterization techniques and
the quantitative results will enable the investigation of the interaction between defect formation, stress evolution, melt pool dynamics, precipitation and phase transformation kinetics in multi-phase
systems for a range of processing parameters, consequently gaining a better understanding of the physical mechanisms and phenomena taking place. Phenomena that were challenging to be
quantified before, like texture evolution and elemental segregation
during solidification may be also explored, and more work is necessary for separating the effects of chemical composition, temperature, and strain. Furthermore, future research should include
studies in a range of alloys, including nickel alloys and steels which
are of industrial interest, and for different AM technologies, aiming
to the design of novel materials for PBF and DED and to the production of parts with desired microstructure and properties that
would not be feasible by a conventional processing route.
Finally, a future focus should be the use of the machine learning
technology for benefiting the applicability of the AM processes.
Quantitative input from real-time experiments and validated models are vital for the development of a closed-loop control. Validated
models can be used for data generation and reduction of experimental effort to obtain a desired set of product properties. Since
the existing models are developed for specific processes and alloys,
in-situ synchrotron studies on more alloys and experimental
parameters should be performed for sufficient model input. Due
to the large number of such experiments expected, another important future challenge is the efficient data acquisition and storage
and additionally possible novel ways and tools for automated
quantitative data analysis.
Overall, despite the significant progress in the in-situ synchrotron X-ray investigations of PBF and DED, there are still many
challenges in this research field that require extensive research in
the next years, hopefully opening new horizons on the manufacturing of AM components that were not possible before.
Acknowledgements
This work was performed within the project ‘‘Real-time tracking of electron beam additive manufacturing”, grant number
2019-06068, funded by the Swedish Reseach Council (VR) and
the Bundesministerium für Bildung und Forschung (BMBF) via
the Röntgen-Ångström Cluster (RÅC).
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CRediT authorship contribution statement
Chrysoula Ioannidou: Writing – original draft. Hans-Henrik
König: Writing – review & editing. Nick Semjatov: Writing –
review & editing. Ulf Ackelid: Writing – review & editing. Peter
Staron: Writing – review & editing, Funding acquisition. Carolin
Körner: Writing – review & editing, Funding acquisition. Peter
Hedström: Writing – review & editing. Greta Lindwall: Writing
– review & editing, Funding acquisition, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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