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A laser powder bed fusion system for in situ x-ray diffraction with high-energy synchrotron radiation.

02 Jul 2020-Review of Scientific Instruments (AIP Publishing LLCAIP Publishing)-Vol. 91, Iss: 7, pp 075104

TL;DR: In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution.

AbstractIn Laser Powder Bed Fusion (LPBF), the highly localized energy input by the laser leads to high-temperature gradients. Combined with the inherent cycles of re-melting and solidification of the material, they can result in high mechanical stresses. These stresses can cause distortion and cracking within the component. In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution. In this work, an LPBF system for the purpose of synchrotron x-ray diffraction experiments during the manufacturing process of multi-layer components with simple geometries is described. Moreover, results from diffraction experiments at the HEMS beamline P07 at PETRA III, DESY, Hamburg, Germany, are presented. Components with a length of ls = 20 mm and a width of ws = 2.5 mm consisting of 100 layers with a layer thickness of Δz = 50 µm were produced using the nickel-base alloy Inconel 625 as the powder material. Diffraction experiments were carried out in situ at sampling rates of f = 10 Hz with a synchrotron radiation beam size of 750 × 70 µm2. The presented experimental setup allows for the observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings. Thus, new possibilities for the observation of the dynamic evolution of strains, stresses, and textures during the LPBF process are provided.

Topics: Synchrotron radiation (55%), Diffraction (55%), Beamline (54%), Synchrotron (54%), DESY (51%)

Summary (2 min read)

Introduction

  • Yet, those are surface related measurement techniques with limited suitability to gather information about bulk properties.
  • They observed dynamic melt pool evolution, keyhole pore formation, and the motion of ejected particles in the process of melting and solidification of Ti6Al4V powder with a frame rate of f = 50 kHz.
  • The setup simulates the thermal behavior of a multi-layer LPBF process but under considerably different conditions than in a common industrial system as no powder material is involved.

II. SYSTEM DESCRIPTION AND SPECIFICATIONS

  • The experimental instrumentation consists of an LPBF system positioned in the x-ray beam path of a synchrotron light source such that a desired gauge volume in the sample is irradiated and the diffracted x rays are detected by a 2D detector, e.g., scintillatorbased.
  • The customized LPBF system, which allows for in situ x-ray diffraction corresponding to the described experimental geometry, is presented below.
  • The third module contains the components for circulation and filtration of the inert gas atmosphere in the process chamber.
  • The sample holder is mounted on a rigid guide rail, which is actuated in the z-axis direction by an EMC electromechanical cylinder, BOSCH REXROTH AG, Lohr am Main, Germany.
  • On its way back, the elastomeric lip attached to the funnel levels the powder layer, as depicted in Fig. 3(c), and scrapes the excess powder to the sides of the sample holder.

B. Measurement modes

  • For the synchrotron x-ray diffraction experiments described here, a two-dimensional area detector is used.
  • Up to this point, three different measurement modes are possible to conduct with the InSituLPBF system at PETRA III (see Fig. 4).
  • In measurement mode 2 (MM2), the gauge volume corresponds to a fixed volume in the sample, which can be monitored during the melting and solidification of all the above layers.
  • This measurement mode is limited to samples with scan tracks longitudinal to the incident synchrotron radiation beam.
  • As a result, the process time increases significantly compared to the other measurement modes.

C. Data preparation

  • The scan path of the laser is provided in a Common Layer Interface (CLI) file, where the scan vectors are listed in sequence for each layer separately.
  • They are defined by their starting and ending coordinates in the working plane.
  • An algorithm to generate CLI-files of simple cuboid samples was implemented in MATLAB 2017b, The MATHWORKS, INC., Natick, USA.
  • The laser scan tracks were oriented either parallel or perpendicular to the incident synchrotron x-radiation beam such that either transverse or longitudinal strains relative to the scan track can be evaluated in situ.
  • Feasible measurement modes with the InSituLPBF system at PETRA III.

A. Sample and system preparation

  • In order to produce a new sample, a substrate plate has to be inserted into the groove of the build plate and fixed via two headless screws.
  • The measuring position can then be set accordingly in relation to the first layer’s edges by adjusting (a) the position of the process chamber to set the vertical distance of the gauge volume to the top layer and (b) the position of the sample holder in the process chamber to set the lateral measuring position.
  • The irradiated volumes Vs,irr and Vp,irr are calculated according to the chosen gauge volume of 750 × 70 μm2, the thickness of the sample ws, and the combined thickness of the powder gaps.
  • The laser scan time tL indicates the time during which the laser emits radiation, i.e., exposes the current powder layer.
  • During the in situ measurements, diffraction patterns are collected continuously by using the post-trigger function of the beamline’s control software.

IV. EVALUATION OF EXPERIMENTAL RESULTS

  • The diffraction patterns are segmented into equal size cake pieces, which correspond to the principal directions of the sample geometry.
  • This figure illustrates three subplots (a)–(c), which show different measuring positions according to Fig. 6(d).
  • For the center and right edge gauge volume, the median strain in TD during the peak is about ε311TD = 0.8%, while in the left gauge volume, it reaches only about 0.4%.
  • The material expands, but the colder material below hinders the thermal expansion due to its lower temperature.
  • The energy input by the laser does still have a noticeable effect, which is indicated by the increase in the strain difference during cooling after the laser passed over the gauge volume.

V. CONCLUSIONS

  • An LPBF system for the realization of in situ x-ray diffraction experiments during the buildup of multi-layer samples was presented.
  • The design of the process chamber allows for observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings.
  • First experiments conducted at PETRA III show promising results and give insight into the dynamics of the lattice spacing during the build-up of Inconel 625 samples consisting of 100 layers.
  • The instrumentation is not limited to the single use of Inconel 625 as the powder material but allows for in situ strain analysis during the manufacturing of other commonly used metals and alloys.

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Rev. Sci. Instrum. 91, 075104 (2020); https://doi.org/10.1063/1.5143766 91, 075104
© 2020 Author(s).
A laser powder bed fusion system for in
situ x-ray diffraction with high-energy
synchrotron radiation
Cite as: Rev. Sci. Instrum. 91, 075104 (2020); https://doi.org/10.1063/1.5143766
Submitted: 07 January 2020 . Accepted: 09 June 2020 . Published Online: 02 July 2020
Eckart Uhlmann, Erwin Krohmer , Felix Schmeiser , Norbert Schell, and Walter Reimers
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Review of
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scitation.org/journal/rsi
A laser powder bed fusion system for in situ
x-ray diffraction with high-energy
synchrotron radiation
Cite as: Rev. Sci. Instrum. 91, 075104 (2020); doi: 10.1063/1.5143766
Submitted: 7 January 2020 Accepted: 9 June 2020
Published Online: 2 July 2020
Eckart Uhlmann,
1
Erwin Krohmer,
1,a)
Felix Schmeiser,
2
Norbert Schell,
3
and Walter Reimers
2
AFFILIATIONS
1
Institute for Machine Tools and Factory Management, Technische Universität Berlin, Pascalstraße 8-9, 10587 Berlin, Germany
2
Institute for Materials Science and Technology, Metallic Materials, Technische Universität Berlin, Ernst-Reuter-Platz 1,
10587 Berlin, Germany
3
Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany
a)
Author to whom correspondence should be addressed: erwin.krohmer@iwf.tu-berlin.de
ABSTRACT
In Laser Powder Bed Fusion (LPBF), the highly localized energy input by the laser leads to high-temperature gradients. Combined with the
inherent cycles of re-melting and solidification of the material, they can result in high mechanical stresses. These stresses can cause distortion
and cracking within the component. In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice
spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution. In this work, an LPBF
system for the purpose of synchrotron x-ray diffraction experiments during the manufacturing process of multi-layer components with
simple geometries is described. Moreover, results from diffraction experiments at the HEMS beamline P07 at PETRA III, DESY, Hamburg,
Germany, are presented. Components with a length of l
s
= 20 mm and a width of w
s
= 2.5 mm consisting of 100 layers with a layer thickness
of Δz = 50 μm were produced using the nickel-base alloy Inconel 625 as the powder material. Diffraction experiments were carried out in
situ at sampling rates of f = 10 Hz with a synchrotron radiation beam size of 750 × 70 μm
2
. The presented experimental setup allows for
the observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings. Thus, new
possibilities for the observation of the dynamic evolution of strains, stresses, and textures during the LPBF process are provided.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5143766
.,
s
I. INTRODUCTION
The market of additive manufacturing systems for metal com-
ponents has been growing rapidly in recent years
1
with a focus on
powder-bed based manufacturing technologies, such as Laser Pow-
der Bed Fusion (LPBF), which is also referred to as Laser Beam
Melting (LBM) or Selective Laser Melting (SLM). LPBF offers the
possibility to design and manufacture components with complex
geometries while maintaining good mechanical properties. How-
ever, the process stability still needs improvement as the limited
reproducibility inhibits the breakthrough of LPBF in fracture-critical
applications.
2
Current challenges include crack-inducing defects
3
or residual stresses
4
and require non-destructive testing methods
and material qualification processes.
5
In order to avoid the for-
mation of defects and to control the residual stress states, further
knowledge considering their origins and a profound understand-
ing of the physical phenomena in melt-pool creation, solidification,
and microstructural evolution is needed. At this point, simulation
models help us to understand and to predict residual stress,
6
defect
formation,
7
and microstructure.
8
However, those models require
experimental validation. Due to the small timescales of melting and
solidification processes during LPBF, highly dynamic monitoring
procedures are required to collect experimental data with sufficient
temporal resolution. Thermal metrology based melt-pool monitor-
ing systems have been widely addressed in research,
9
and some are
state of the art in commercial LPBF systems. They provide impor-
tant insights into the dynamics of the process. Yet, those are surface
related measurement techniques with limited suitability to gather
information about bulk properties. Hence, new in situ measure-
ment methods are necessary, especially in the challenging case of
Rev. Sci. Instrum. 91, 075104 (2020); doi: 10.1063/1.5143766 91, 075104-1
Published under license by AIP Publishing

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the investigation of residual stress evolution and varying metallic
phase structure.
10
With the development of the third-generation
synchrotron light sources, in situ diffraction experiments during
time-critical manufacturing processes became possible.
11
Together
with x-ray imaging data, x-ray diffraction with high-energy syn-
chrotron radiation offers bulk sensitive insights into the melting and
solidification process in LPBF.
Only recently researchers presented their work on in situ x-
ray diffraction and imaging in LPBF. Uhlmann et al.
12
discussed
the complex implementation of such an experimental setup and
the requirements that the LPBF system should meet to simulate an
industrial system. Bidare et al.
13,14
presented a compact system with
access for x rays and suitable for several in situ imaging techniques
such as high-speed schlieren imaging. The first results from in situ x-
ray diffraction and in situ x-ray imaging experiments with a system
that mimics LPBF conditions were reported by Zhao et al.
15
They
observed dynamic melt pool evolution, keyhole pore formation, and
the motion of ejected particles in the process of melting and solid-
ification of Ti6Al4V powder with a frame rate of f = 50 kHz. In
addition, they showed the possibility to investigate phase transfor-
mations by means of x-ray diffraction. Subsequently, the described
LPBF system was upgraded and used in several further investiga-
tions so that high-speed x-ray imaging with a frame rate of up to
6.5 MHz proved possible to give profound insight into the transient
dynamics of the LPBF process.
16
Guo et al.
17
continued the research
on powder spattering behavior and evaluated the dynamics of pow-
der spattering as a function of time, ambient pressure, and location.
Additional research on powder motion was conducted by Escano
et al.
18,19
who designed a device to examine the powder deposition
by a spreading wiper by means of high-speed x-ray imaging. Another
setup that allows the investigation of phase transformations and lat-
tice expansion during cyclic heating and cooling using micro-x-ray
diffraction was introduced by Kenel et al.
20
The setup simulates the
thermal behavior of a multi-layer LPBF process but under consid-
erably different conditions than in a common industrial system as
no powder material is involved. Nevertheless, the advantage of the
presented setup is the in situ gathering of surface temperature infor-
mation.
21
Leung et al.
22
used a custom-built system called LAMPR,
short for Laser Additive Manufacturing Process Replicator, for in
situ x-ray imaging experiments. Research with the LAMPR includes
time-resolved melt track, spatter, and defect formation of different
materials.
22–24
Calta et al.
25
developed a system for the purpose of
in situ x-ray imaging and diffraction of single tracks. They reported
experiments showing pore formation via in situ x-ray imaging and
β-Ti–α-Ti phase transitions upon cooling in Ti6Al4V via x-ray
diffraction. In further studies, utilizing the mentioned experimental
systems, melting and solidification dynamics in single tracks of Alu-
minum 6061 and AISI 4140 steel
26,27
and pore formation and spat-
ter dynamics in the laser–metal-interaction with solid and powdery
Ti6Al4V
28–31
were investigated. Finally, Hocine et al.
32
presented
an advanced experimental instrumentation for in situ diffraction
experiments. Their instrumentation adopts a hopper-based pow-
der feeding system and a particle filter system for the inert gas
circulation and is therefore applicable for multi-layer experiments.
They analyzed the influence of laser processing parameters and
scanning strategies on the phase transformation and microstruc-
tural evolution in several Ti6Al4V samples in the reflection
mode.
In this paper, an experimental system for in situ x-ray diffrac-
tion with high-energy synchrotron radiation is presented, which is
designed for the investigation of the evolution of textures and resid-
ual stresses during the build process of three-dimensional parts.
With the presented setup, it is possible to produce multi-layer parts
with conditions mostly similar to the industrial LPBF process. This
system’s key advantages are the possibility to gather full diffrac-
tion rings to improve measurement accuracy, the free choice of the
measuring position in the sample, and the feasibility of various mea-
surement modes that deliver a range of spatially and temporally
resolved data. Further experimental results using the here described
experimental setup have been discussed in depth by Schmeiser
et al.
33
II. SYSTEM DESCRIPTION AND SPECIFICATIONS
A. Instrument design
The experimental instrumentation consists of an LPBF system
positioned in the x-ray beam path of a synchrotron light source
such that a desired gauge volume in the sample is irradiated and
the diffracted x rays are detected by a 2D detector, e.g., scintillator-
based. The experimental approach is depicted in Fig. 1. The cus-
tomized LPBF system, which allows for in situ x-ray diffraction
corresponding to the described experimental geometry, is presented
below. The specified system was designed to operate at the HEMS
beamline P07 at PETRA III.
34
Hence, any diffraction related speci-
fications or described peripheral equipment refer to the beamline’s
facilities.
One of the main requirements during the design process was to
reproduce the conditions of a commercial LPBF setup as precisely as
possible while accounting for the required x-ray transparency. First,
attenuation of x rays throughout the experimental setup, besides
in the sample, should be avoided in order to prevent noise in the
measurement. Therefore, compared to a state of the art industrial
LPBF system, the powder bed is elevated and unavoidable objects in
the x-ray path are made of material with high x-ray transparency.
Furthermore, the system features an additional linear axis, ensur-
ing linear motion of the powder bed not only vertically but also
horizontally, perpendicular to the incident synchrotron radiation
beam. Since the synchrotron radiation beam and the detector are
stationary, the powder bed, therefore, can be positioned relative to
the measuring instrumentation without moving the whole process
chamber. Thus, free variation of the measuring position is facilitated,
and different measurement modes can be implemented.
The InSituLPBF system presented here is based on the mod-
ular industrial machine AconityMINI manufactured by ACONITY3D
GMBH, Herzogenrath, Germany. The process chamber was designed,
built, and integrated into the industrial system at the INSTITUTE FOR
MACHINE TOOLS AND FACTORY MANAGEMENT of TECHNISCHE UNIVERSITÄT BERLIN
in order to meet the requirements for in situ x-ray diffraction exper-
iments. The system consists of three modules. Control systems for
media and energy supplies as well as the laser source are situated
in the first module, the control cabinet, which also serves as the
human–machine interface. The second module is the process cham-
ber itself. The third module contains the components for circulation
and filtration of the inert gas atmosphere in the process chamber.
For in situ x-ray diffraction experiments, the process chamber is
mounted on a heavy load hexapod from PHYSIK INSTRUMENTE (PI) GMBH
Rev. Sci. Instrum. 91, 075104 (2020); doi: 10.1063/1.5143766 91, 075104-2
Published under license by AIP Publishing

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FIG. 1. Schematic of the experimental approach for x-ray diffraction with synchrotron radiation.
& CO. KG, Karlsruhe, Germany, which is part of the Experimental
Hutch 3 at the HEMS beamline P07. The laser source is a single-
mode continuous wave Ytterbium fiber laser YLR-400-AC from IPG
LASER GMBH, Burbach, Germany, which emits radiation at a wave-
length of λ
L
= 1070 nm with a nominal power output of P
L
= 400 W.
The laser fiber is connected to the three-axis deflection unit
Axialscan-30 from RAYLASE, Wessling, Germany, via a collimator. The
installed deflection unit features a focus compensation and scanning
speeds of v
L
= 3 m/s can be realized. A cooling unit SC11 from GLEN
DIMPLEX DEUTSCHLAND GMBH, Kulmbach, Germany, provides the scan
head with coolant for the galvo mirrors. The laser beam has a Gaus-
sian shape with an adjustable focus diameter between d
L
60 μm and
500 μm (1/e
2
) and is deflected onto a powder bed with a length of
l
b
= 70 mm and a width of w
b
= 3 mm. The optical working dis-
tance of the setup is 455 mm. The powder bed is enclosed by two
glassy carbon plates, HTW HOCHTEMPERATUR-WERKSTOFFE GMBH, Thier-
haupten, Germany, each with a thickness of 1 mm in the x-ray
transmission direction. The samples are built on a replaceable sub-
strate plate, which is sandwiched between the glassy carbon plates.
The material of the substrate is selected according to the powder
material. The sample holder together with the glassy carbon mount
is depicted in Fig. 2. Here, substrate plates with the dimensions of
70 × 10 × 3 mm
3
are mounted in the groove of the stainless
steel build plate. The surrounding material is PAMITHERM 41140,
VON ROLL DEUTSCHLAND GMBH, Augsburg, Germany, a high-temperature
resistant silicone phlogopite mica laminate to thermally insulate the
carriage from the installed heating ceramic, BACH RC GMBH, Seefeld,
Germany, which is in contact with the bottom side of the build
plate. The substrate plate can be heated up to 300
C. If the desired
gauge volume is in a lateral distance of more than 10 mm to either
side of the glassy carbon mount, the sample holder allows for Bragg
angles 2θ 14
without shadowing of the diffracted x rays. The
detailed view in Fig. 2(a) shows a schematic illustration of the
powder bed, which is confined between the two glassy carbon plates
and the substrate beneath during an experiment. The sample holder
is mounted on a rigid guide rail, which is actuated in the z-axis
direction by an EMC electromechanical cylinder, BOSCH REXROTH AG,
Lohr am Main, Germany. Additionally, the sample holder is actu-
ated in the x-axis direction by a C-shaped driver, which is mounted
on a CKK-110 linear axis, BOSCH REXROTH AG, Lohr am Main, Ger-
many. The motion arrangement comprises a sliding contact between
the C-shaped driver and the sample holder such that the move-
ments of the two linear axes are decoupled. As a result, the C-shaped
driver is fixed in the z-direction. This principle assures a constant
working plane, as the glassy carbon plates are mounted on the
driver.
When the sample holder moves in the positive z-direction, the
glassy carbon plates stay in position, leading to a void between the
substrate plate and the edges of the glassy carbon. The fully auto-
matic recoating mechanism fills the void with powder and levels
it. The operating principle is demonstrated in Fig. 3. The powder
recoating mechanism is based on a funnel, which is actuated in the x-
axis direction by another CKK-110 linear motion axis. For recoating,
the sample holder is moved to the recoating position [see Fig. 3(a)].
Then, the funnel is moved over the sample holder up to its turning
point. During motion, the powder flows into the void due to gravity
alone [see Fig. 3(b)]. On its way back, the elastomeric lip attached
to the funnel levels the powder layer, as depicted in Fig. 3(c), and
scrapes the excess powder to the sides of the sample holder. The
gap between the funnel and glassy carbon plates is usually set to
100 μm but can be adjusted manually using the funnel height
adjustment and precision sheet-metal. Returned to its parking posi-
tion, the funnel orifice is sealed by the thick powder layer beneath.
The funnel shape was tested and optimized for titanium and nickel-
base alloys with a particle size distribution of 20 μm–63 μm and a
spherical particle shape. Note that, e.g., finer powder or different
materials could lead to bridging in the orifice due to cohesiveness
of the powder. With the use of small vibration motors, which were
installed on the funnel and are not depicted in Fig. 3, the flowa-
bility of the powder was improved and formerly clogging powders
Rev. Sci. Instrum. 91, 075104 (2020); doi: 10.1063/1.5143766 91, 075104-3
Published under license by AIP Publishing

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FIG. 2. Sample holder: (a) rendering of the sample holder and schematic of the sample during the experiment, (b) linear axis for x-axis motion, and (c) linear axis for z-axis
motion.
could be processed as well, e.g., TNB-V5 powder. Using an exten-
sion, the powder capacity of the funnel can be increased as desired.
With a layer thickness of Δz = 50 μm, parts consisting of 120 lay-
ers or approximately a height of h
s
= 6 mm were produced so far
utilizing such a powder capacity extension. The motion limits of the
z-axis allow for maximum part heights of h
s
= 10 mm or 200 layers
when using a layer thickness of Δz = 50 μm.
For laser processing, the process chamber has to be sealed and
purged with an inert gas, usually argon. For this purpose, the pro-
cess chamber has an inlet and outlet port for inert gas flooding. The
excess pressure during flooding and laser melting process is con-
stantly controlled by the proportional pressure regulator VVPM,
FESTO AG & CO. KG, Esslingen, Germany, while the gas flow is monitored
by the installed flow sensor SFAB, FESTO AG & CO. KG, Esslingen, Ger-
many. Both components are part of the basic AconityMINI system.
The ambient conditions inside the process chamber are monitored
by means of an oxygen sensor and a thermocouple. Oxygen values
and excess pressure and inert gas flow data are logged on the com-
puter besides other data such as axis positions and laser status. Two
further ports in the process chamber walls serve as inlet and outlet
FIG. 3. Functionality of the powder recoating mechanism: (a) positioning, (b) filling with fresh powder, and (c) leveling the powder bed.
Rev. Sci. Instrum. 91, 075104 (2020); doi: 10.1063/1.5143766 91, 075104-4
Published under license by AIP Publishing

Citations
More filters

Journal ArticleDOI
Abstract: Laser powder bed fusion (LPBF) is an additive manufacturing technology to fabricate parts with complex geometries. Residual stresses induced by the rapid heating and cooling processes may cause defects such as cracks, distortion and delamination. This work presents an in-depth study for the understanding of the evolution of residual stresses during LPBF of Ti-6Al-4V. A 3D finite element model was developed to study the spatiotemporal variations of the temperature and the stresses during multi-track and multi-layer LPBF. The results show that the scanning strategies significantly affected the temperature gradients and the resultant stress distributions. The induced residual stresses were higher along the laser scanning direction than that along the perpendicular direction. Monitored at the longitudinal central plane, stresses changed from compressive to tensile along the vertical build direction from the substrate to the top of the build. The Z-component of stress was smaller than the X- and Y-components stresses. The peak tensile Z-component of stress was at the interface between the part and the substrate, which may lead to local warping and cracking. The findings from this work provide insights for the understanding of stress related issues such as the massive longitudinal and transverse solidification cracking during LPBF.

4 citations





Journal ArticleDOI
Abstract: Laser powder bed fusion is an additive manufacturing process that employs highly focused laser radiation for selective melting of a metal powder bed. This process entails a complex heat flow and thermal management that results in characteristic, often highly textured microstructures, which lead to mechanical anisotropy. In this study, high-energy X-ray diffraction experiments were carried out to illuminate the formation and evolution of microstructural features during LPBF. The nickel-base alloy Inconel 625 was used for in situ experiments using a custom LPBF system designed for these investigations. The diffraction patterns yielded results regarding texture, lattice defects, recrystallization, and chemical segregation. A combination of high laser power and scanning speed results in a strong preferred crystallographic orientation, while low laser power and scanning speed showed no clear texture. The observation of a constant gauge volume revealed solid-state texture changes without remelting. They were related to in situ recrystallization processes caused by the repeated laser scanning. After recrystallization, the formation and growth of segregations were deduced from an increasing diffraction peak asymmetry and confirmed by ex situ scanning transmission electron microscopy.

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860 citations


Journal ArticleDOI
Abstract: Lack of assurance of quality with additively manufactured (AM) parts is a key technological barrier that prevents manufacturers from adopting AM technologies, especially for high-value applications where component failure cannot be tolerated. Developments in process control have allowed significant enhancement of AM techniques and marked improvements in surface roughness and material properties, along with a reduction in inter-build variation and the occurrence of embedded material discontinuities. As a result, the exploitation of AM processes continues to accelerate. Unlike established subtractive processes, where in-process monitoring is now commonplace, factory-ready AM processes have not yet incorporated monitoring technologies that allow discontinuities to be detected in process. Researchers have investigated new forms of instrumentation and adaptive approaches which, when integrated, will allow further enhancement to the assurance that can be offered when producing AM components. The state-of-the-art with respect to inspection methodologies compatible with AM processes is explored here. Their suitability for the inspection and identification of typical material discontinuities and failure modes is discussed with the intention of identifying new avenues for research and proposing approaches to integration into future generations of AM systems.

739 citations


Journal ArticleDOI
Abstract: Selective laser melting (SLM) is an attractive technology, enabling the manufacture of customised, complex metallic designs, with minimal wastage. However, uptake by industry is currently impeded by several technical barriers, such as the control of residual stress, which have a detrimental effect on the manufacturability and integrity of a component. Indirectly, these impose severe design restrictions and reduce the reliability of components, driving up costs. This paper uses a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in SLM. A complex interaction between transient thermal history and the build-up of residual stress has been observed in the two laser scan strategies investigated. The temperature gradient mechanism was discovered for the creation of residual stress. The greatest stress component was found to develop parallel to the scan vectors, creating an anisotropic stress distribution in the part. The stress distribution varied between laser scan strategies and the cause has been determined by observing the thermal history during scanning. Using this, proposals are suggested for designing laser scan strategies used in SLM.

347 citations