Laser-assisted post-processing of additive manufactured metallic parts

Abstract

Laser-assisted additive manufacturing (AM) is the process of successively melting thin layers of material using a laser source to produce a three dimensional device or product. From the many technologies available, only a few can produce metallic parts that fulfil the requirements of industrial applications. Ultrafast laser machining is a new and promising technical approach for post-processing AM parts since laser ablation and surface modification processes could be applied with high accuracy for trimming shape and functionality, i.e., edge quality and wettability. The impact of different ultrafast laser parameters is evaluated for AM samples, which are examined for surface roughness before and after the laser-assisted post-processes. For all the parameters tested, the use of ultrafast laser resulted in a homogeneous material ablation of the samples' surfaces. For the investigated parameter range, the AM building tracks were still maintained even after ultrafast laser post-processing. The achieved results showed the formation of self-organized porous structures at low laser scan velocities leading to an enhanced surface roughness. For higher scan velocities characteristic nano ripples might be induced having no significant impact on the measured surface roughness.

Full text

euspen’s 18th International Conference &
Exhibition, Venice, IT, June 2018
www.euspen.eu
Laser-assisted post-processing of additive manufactured metallic parts
Juliana dos Santos Solheid1, Hans Jürgen Seifert1, Wilhelm Pfleging1,2
1Institute for Applied Materials-Applied Materials Physics, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
2Karlsruhe Nano Micro Facility, H.-von-Helmholtz-Platz 1, 76344 Egg.-Leopoldshafen, Germany
juliana.solheid@kit.edu
Abstract
Laser-assisted additive manufacturing (AM) is the process of successively melting thin layers of material using a laser source to
produce a three dimensional device or product. From the many technologies available, only a few can produce metallic parts that
fulfil the requirements of industrial applications. Ultrafast laser machining is a new and promising technical approach for post-
processing AM parts since laser ablation and surface modification processes could be applied with high accuracy for trimming shape
and functionality, i.e., edge quality and wettability. The impact of different ultrafast laser parameters is evaluated for AM samples,
which are examined for surface roughness before and after the laser-assisted post-processes. For all the parameters tested, the use
of ultrafast laser resulted in a homogeneous material ablation of the samples’ surfaces. For the investigated parameter range, the
AM building tracks were still maintained even after ultrafast laser post-processing. The achieved results showed the formation of
self-organized porous structures at low laser scan velocities leading to an enhanced surface roughness. For higher scan velocities
characteristic nano ripples might be induced having no significant impact on the measured surface roughness.
Keywords: Laser finishing, ultrafast laser ablation, additive manufacturing, selective laser melting
1. Introduction
Additive manufacturing (AM) is an established technology
based on the material deposition layer-by-layer to produce a
part or device [1]. It is an alternative for customization and
personalization with little impact on manufacturing complexity.
Also for reducing material waste, time and costs [2]. On the
other hand, the AM process typically results in rough surface
finish which make the parts unsuitable for many applications [3].
The laser post-processing is an alternative of AM post-
processing for being a contactless method and for presenting
great flexibility (wide range of systems, parameters and
technologies available) [4]. By a defined control of laser
parameters such as wavelength, pulse length, laser fluence,
repetition rate, and scan speed, versatile processing for each
type of material becomes possible including thermal processing,
surface modification, and cold ablation.
It is characteristic for ultrafast laser machining that the used
laser energy will induce no, or only a small, heat-affected zone
in comparison to conventional laser with pulse lengths in the
nanosecond regime. Other benefits of this process can be
achieved by sub-micrometer ablation selectivity during
machining. Furthermore, different processing strategies are
available with a single laser source including cutting, drilling,
ablation and surface smoothing [5].
In the present work the surface roughness of AM parts is
examined after a laser-assisted finishing process. The impact on
the surface quality of different laser process parameters, such as
repetition rate and scan velocity, is investigated.
2. Experimental
2.1. Material
The material used in this study was 18 Maraging 300 steel,
manufactured with an EOS M270 SLM machine. The AM samples
have simple cubic geometry with dimensions of 1.5 x 1.5 x 1.0
cm. The laser processing was performed on the top surface of
the samples that have initial Ra roughness of 2.7 ± 0.6 µm. The
chemical composition of the material is shown in Table 1.
Table 1. Chemical composition (wt%) for Maraging steel used in this
study
Ni
Mo
Co
Fe
Ti
Al
O
C
13.5
4.6
6.6
50.3
1.2
0.4
18.8
4.6
2.2. Laser system
For this work an ultrafast fiber laser system (Tangerine,
Amplitude Systèmes, France) was used. The scan velocity (v) and
the repetition rate (f), which can be related to the pulse overlap
and to the energy density of the laser, were varied, while the
wavelength (λ), pulse duration (τ), beam diameter (D), line offset
distance (OD) and average laser power (P) were kept constant.
The process parameters are presented in Table 2.
Table 2. Laser process parameters
wavelength (λ)
1030 nm
pulse duration (τ)
400 fs
beam diameter (D)
0.06 mm
average power (P)
9.4 W
line offset distance (OD)
0.03 mm
scan velocity (v)
200 - 2000 mm s-1
repetition rate (f)
500 - 2000 kHz
2.3. Analytical methods
To measure the roughness of the laser processed parts, a
white light profilometer (MicroProf®, Fries Research &
Technology GmbH, Germany) was used. The profile
measurements were performed in two different directions:

orthogonal to the building tracks (90°) and with a 45° angle (X
and Y). For selected processing parameters areal measurements
will be presented (not shown here).
3. Results and discussion
The change of the surface roughness of the samples as function
of laser parameters is presented in Figure 1.
Figure 1. Roughness Ra as function of scan velocity and repetition rate.
Ra was measured in 45° (a) and 90° (b) to the building tracks.
The highest surface roughness Ra was observed for the lowest
scan velocity (200 mm s-1) and for the repetition rate of
1000 kHz, in both measurement directions 45° and 90°, being
14.9 ± 1.6 µm and 18.8 ± 0.9 µm, respectively. The high values
observed for these parameters are due to laser-induced self-
organized porous surface. The pores observed presented
diameters up to 20 µm. This structure can indicate the
occurrence of selective material removal from the parts. The
comparison of the surface texture and their microstructure is
shown in Figure 2.
Figure 2. SEM of laser processed surface. (top) survey view (3x3mm2)
and (bottom) detail view showing the micro texture: (a) 500 kHz and 200
mm s-1; (b) 500 kHz and 1100 mm s-1; (c) 500 kHz and 2000 mm s-1
The roughness decreased, in all cases, with the increasing of
the scan velocity to 500 mm s-1. Beyond this point, the roughness
tended to vary in the small range of 2 to 3 µm, which is very
similar to the surface roughness of the parts as-built.
The microstructures obtained when scan velocities from 1100
to 2000 mm s-1 are applied, to all repetition rates, are very
similar to each other, thus the low variation on the Ra values in
the mentioned range. They present periodical ripples and
occasional unmelted particles from the building process on the
surface.
The surface modification mechanism observed during the
ultrafast laser post-processing of the AM samples was mainly
ablation, leading to an almost homogenous material removal.
For the used process parameter range the tracks from the
building process could not be planished out, which is indicated
by the similarity of the roughness values when compared to the
initial surface roughness of the part, as mentioned above.
Apart from the roughness values of the lowest velocity, no
significant difference was observed between the two measuring
directions (45° and 90°).
4. Conclusions
The influence of two laser parameters, scan velocity and
repetition rate, on the surface roughness of additively
manufactured parts was presented. By applying material
ablation and surface modification by femtosecond laser
radiation two types of surface roughness formation could be
detected. For small laser scanning velocity the surface roughness
increased due to a selective material ablation. With increasing
scanning speed the surface roughness Ra is reaching values
which are similar to the initial Ra values of the as-built AM part.
Additionally, the formation of nano-ripples, so-called laser-
induced periodical surface structures (LIPSS), could be observed
for high scanning speeds. Due to the cold ablation mechanism,
ultrafast laser processing is a useful technology for edge
processing and selective particle ablation of AM parts.
Furthermore, a combination of laser-assisted thermal polishing
of AM building tracks and subsequent fs-laser for surface
functionalization and edge processing will be studied in
upcoming experiments in order to achieved an enhanced
surface quality and functionality beyond state-of the art AM
parts.
Acknowledgements
We are grateful to our colleagues Alexandra Reif, Heino Besser
and Maika Torge for their contributions and support. This work
has received funding from the European Union’s programme
PAM2 within Horizon 2020 under grand agreement No 721383.
Finally, the support for laser materials processing by the
Karlsruhe Nano Micro Facility (KNMF, http://www.knmf.kit.
edu/) is gratefully acknowledged.
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