Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009
Munich, June 2009
Laser Welding of Polymers – Superior Process Curve
Ulrich A. Russek
Rheinische Fachhochschule Köln, Department of Engineering, Cologne, Germany
Abstract
Goals of industrial manufacturing are clearly defined: increasing product quality and process stability while simultaneously decreasing cycle times and costs. Application of laser technology may
achieve these goals. Today, for setting up production processes with laser welding of polymers
characteristic process curves and partly extensive qualification procedures are considered. Both
tools are limited to react quickly if changes of process conditions occur, such as material fluctuations. Therewith, industrial goals are not fully reachable. On the basis of experimental investigations and process modelling the idea of a superior process curve will be introduced. Additionally,
an outlook will be given concerning using modelling considering industrial demands and goals.
Keywords: laser welding of polymers, process modelling, industrial application
1
The investigated materials are Polyamide 6.6
(Ultramid A3K), Polycarbonate (Makrolon 2405) and
Polypropylene (Novolen 1040). These materials are
joined by means of contour and simultaneous welding.
After laser beam welding the samples with different process parameter settings the weld seam has
been assessed by means of strength testing as well as
microscopic investigation of microtom cuts (in Fig. 1).
Introduction
The commonly applied process for laser welding of
polymers is laser transmission welding [1]. Different
methods are available to apply the process, such as
contour-, quasi-simultaneous-, mask-, simultaneous-,
Globo-, hybrid- and TWIST-welding [2,3,4,5,6].
Each industrial application asks for a certain
process performance, such as laser characteristics, processing parameters, manufacturing conditions and production environment.
Today, the laser process is brought into production starting with feasibility, process and method studies
including quality testing followed by plant construction
and setup and implementation procedures.
Studies and qualification procedures may ask
for extensive effort and time to obtain necessary data.
Even more, changes of the conditions during the production may occur. Polymer properties or product updates have to be taken into account. Polymer manufacturing may change polymer properties. This in turn asks
for fast adaptation of the processing parameters.
Therefore, two tools are necessary to improve
the entire implementation as well as production process.
On the one hand certain data acquisition tools have to be
used to determine characteristics of process influencing
properties. On the other hand computer aided process
modelling facilitates setup procedures as well as allows
quick and qualified reaction if production conditions
change.
In the following, aspects of process modelling
are illustrated and discussed.
2
Fig. 1: Microtom cut of laser welded Polyamide
To obtain deeper process technical insight the
extension of the heat affected zone (haz) for different
parameter settings has been determined via microtom
cuts. The experimentally determined extensions of the
heat affected zone have been compared to those determined via process modelling.
3
Process Modelling
Laser materials processing is connected with the interaction of radiation and matter, while optical energy is
transformed into internal energy (heat) of the specimen.
The interaction leads to an extinction of the laser radiation, while penetrating the specimen. The extinction is caused by scattering and absorption. The
Experimental Setup
The experimental set-up, the performance and the results are described in detail in [7, 8, 9].
1
scattering process leads to a deflection of the beam and
in general to a change of the beam shape without an energy exchange, i.e. elastic scattering. The increase of
internal energy leads to a local heating of the specimen,
phase transitions might occur. The energy conservation
for contour welding reads:
∂E
+ v s ⋅ ∇E = Q L + ∇( K∇T )
∂t
sponding to the morphological change the depth and
width of the heat affected area can be determined.
A comparison of depth and width of the heat
affected area determined by means of the experiment
and the model (Fig. 3) gives a measure for the quality of
the model. The comparison of the heat affected depths
and widths show a good correspondence between experimentally acquired data and the results from the
thermal process model.
(1)
E : energy density,
vs : relative velocity between specimen and laser beam
QL : power input per volume,
K : heat conductivity,
T : temperature.
800
HAD, HAW / µm
WEZ-Breite und -Tiefe / µm
600
The energy conservation (1) includes a convective and a diffusive energy flow. The convective energy
flow is connected to the velocity vs, while the diffusive
energy flow entails to the heat conductivity K.
The Peclet number Pe can be interpreted as the
ratio of both energy flows to an order of magnitude
Pe =
v s rL
κ
PC Konturschweißen,
P = 6,5 W
Contour-Welding
x Experiment
Modell
PC, no gap, 6,5
W
400
200
WEZ-T
HAD
0
-200
WEZ-T
HAD
-400
-600
-800
WEZ-B
HAW
50
70
90
110
130
150
Streckenenergie
J/m
Line
Energy / / J/m
Fig. 3: Comparison of the measured heat affected
depth (HAD) and width (HAW) within the transparent
and absorbing joining part with the calculated heat affected geometry by means of the model vs. the line energy. Contour welding of PC (Makrolon 2405) with PL
= 6.5 W, λ = 808 nm, δOPT = 63 µm, no gap.
(2)
rL : characteristic width of the processing
κ : temperature conductivity
If Pe is larger than one, the power input QL is
mainly compensated by the convective energy flow, in
the opposite case by the diffusive energy flow.
The process model solves equation (1) while
considering different polymer characteristics and processing parameters. The energy density distribution
E(x,y,z), the temperature distribution T(x,y,z) as well as
the volume extension d(x,y) of flat thermoplastic parts
are calculated by the model (Fig. 2).
x
WEZ-B
HAW
The model is limited to consider only thermal
aspects and thermal expansion. Because of the good
correspondence between measured and calculated heat
affected geometries the model may already assists in
process parameter determination, process, method and
plant designing concerning thermal process relations.
From this point of view, interesting question may be answered.
If the feed rate (contour welding) is sufficient
high the energy distribution is mainly caused by the
convective heat flow. In this case the energy distribution
is mainly dependent on the line energy and the power
distribution.
However, line energy is not a global process
describing parameter. Even line energy, material properties and beam radius kept constant the temperature
distribution and the heat affected area changes. At constant line energy the temperature distribution is influenced by the combination of laser power and feed rate.
The existence of an application dependent feed
rate limit is indicated. Therefore, especially for contour
and quasi-simultaneous welding a border concerning
lowering the entire process time seems to be given. E.g.
modelling contour welding of PC at a constant line energy: with a laser power P < 6 W no welding while at P
> 33 W decomposition occurs. In both directions the
limiting link is the thermal conductivity and the small
temperature window between melting and decomposition. In case of low laser powers the thermal conductivity transports the heat sufficiently away from the interaction area so no plastification takes place. In case of
high laser powers the thermal conductivity can not
transport the heat sufficient away to avoid decomposi-
beam axis
z
Fig. 2: Energy density distribution in the x-z-plane
x > 0 : transparent part, x < 0 absorbing part
no gap between the parts
Microtom cuts of the experimentally generated
samples (like Fig. 1) show the heat affected zone (haz)
corresponding to a morphologic change in the interaction area within the joining partners. By this, the depth
and width of the heat affected area can be measured.
Using the model, while considering material
properties, such as optical and thermal properties, leads
to energy density and temperature density distributions.
By considering the course of the isothermal line corre2
tion. This in turn indicates the existence of an application dependent feed rate limit and therefore especially
for contour and quasi-simultaneous welding a border
concerning lowering the entire process time due to increasing laser power.
The first process modelling approach did show
good correspondence with the experimentally obtained
geometry of the heat affected zone as well as allowing
predictions of process parameter influences [8, 9].
However, the handling of the program had to
be improved. Further tools became of interest, such as
obtaining the possibility of easier performance of a
process sensitivity analysis.
Therefore, a new, commercially available process modelling program has been employed to satisfy
actual interests and perform process sensitivity analysis.
The process modelling program is written by
SmartCAE, Munich and based on the software program
Mathematica.
In a first step verifications have been performed, which did indicate a sufficient correspondence
between experimental based microtom cuts and model
calculations of the heat affected zone (Fig. 4, Fig. 5).
calculations. An experimental performance of such a
sensitivity analysis would ask for an extensive effort.
The calculation focuses on the temperature
distribution of a certain combination of power P and
feed rate v within the heat affected zone, like Fig. 5.
The plot in Fig. 6 shows the maximum temperatures within the absorbing part. The lines correspond to P-v-combinations generating the same temperature; especially melting (536 K, red dot line) and
decomposition temperature (700 K, red line) of Polyamide.
Furthermore, Fig. 6 shows for the considered
polymer parts and laser parameters that with a laser
power of 3.6 W and feed rates larger than 2.5 m/min
generate temperatures less than decomposition temperature while feed rates slower than 2.5 m/min always
generate temperatures larger than decomposition temperature in the heat affected zone.
The segment of the depicted horizontal 3.6 W
line (Fig. 6) between cutting the isotherms of melting
and decomposition temperature projected on the abscissa corresponds to the feed rate values generating
temperatures within the heat affected zone between
melting and decomposition temperature.
power / W
6
5
4
3
2
Fig. 4: Microtom cut of a PA sample showing beginning decomposition within the heat
affectted zone
1
1
2
feed rate m/min
4
5
Fig. 6: Graphically depiction of a process study by
means of the model. The lines correspond to
isotherms, P-v-combinations generating the
same temperature; especially melting (red dot
line) and decomposition temperature (red line).
0,1
0
Even another interesting question can be answered using the process model. If the feed rate is zero
which power is necessary to just reaching melting temperature for t → ∞ ? For the considered polymer part
and processing conditions the model calculates this
power to P = 0.23 W.
These calculation features combined with experimental results lead to an interesting idea, the superior process curve.
- 0,1
- 0,3
0
0,3
Fig. 5: Calculated temperature distribution considering the process parameters of Fig. 4. The red
lines indicates the isotherms of melting and
decomposition temperature
The SmartCAE model allows performing a
sensitivity analysis. Fig. 6 shows a graphically depicted
section of a study concerning (contour welding) irradiation of a Polyamide setup with laser powers from 1 up
to 10 W in steps of 0.3 W and feed rates from 0.6 up to
6 m/min in 0.5 m/min steps. This corresponds to 361
4
Superior Process Curve
Contour and simultaneous welding are methods to apply
the laser transmission welding process of polymers. Performed experiments with PA, PC and PP lead to the
following result:
3
Each point in Fig. 7 represents that P-v-combination
which corresponds to the strength maximum of the corresponding characteristic curve for a certain power P.
With the view of strength maximum exists - according
to Fig. 7 - a superior optimal P-v-combination.
The consideration may not only focus on
optimisation of the weld seam strength - the optimal
strength P-v-combination. But also on the common optimisation concerning another or different aspects, such
as strength, intensity distribution, processing and cycle
time, width of process window, process stability, plant
design, investment costs.
Contour welding: the value of optimal line energy as well as the maximum of weld seam strength
tends to smaller line energies when increasing the laser
power [9]. The maximum weld seam strength tends to
decrease with higher laser powers.
Simultaneous welding: like contour welding,
the value of optimal line energy tends to smaller line
energies when increasing the laser power [9]. But, the
maximum weld seam strength tends to increase with
higher laser powers.
The totality of performed and evaluated experiments supports independent on the irradiation method
the following explanations for position and height of the
characteristic curve concerning the line energy.
Since the position of optimal line energy, the
maximum strength as well as the course of the characteristic curve changes with laser power – while laser
beam characteristics stay constant – the line energy is
not a global processing parameter.
There are two energy flows to consider the
convective and the diffusive energy flow. The smaller
(longer) the feed rate (interaction time) the relatively
more the diffusive energy flow becomes (provided no
overheating occurs).
However, a different consideration may lead to
a superior process curve, which may of academic as
well as of industrial interest.
In industrial environment only laser power and
feed rate are easily to tune. Therefore, having a better
understanding of their interplay and generating results
laser power and feed rate may brought into useful relation. This further leads to an interesting task for further
investigations and developments. Namely, deeper understanding leads to easier and quick process optimisation interesting for industrial production. Fig. 7 depicts
a qualitatively course of the reachable strength maxima
parameterized with laser power.
Maximal Strength
5
Today, laser welding of polymer processes are brought
into production on the basis of partly extensive studies
and qualification procedures. Additionally, a quick and
qualified reaction if production conditions change is
often not possible.
Computer aided process modelling may facilitate the determination of processing parameters and
their adaptation if necessary due to changes in processing conditions as well as give a helping hand setting up
procedures, plant design and process performance.
Up to now the developed process models consider and describe only thermal conditions.
However, dependent on the processing parameters the energy and the temperature distribution as well
as the thermal expansion (gap bridging capability) are
computable, while taking several process influencing
properties into account. Even more, to a certain extend
coherences are explicable as well as predictions are possible, but they are still limited.
The process modelling has been improved due
to better process understanding and its transformation
into computer models. From a thermal point of view
they allow to understand processing behaviour and
welding results to certain extend, already.
But, on a long term view simultaneous optimisation concerning different aspects, such as strength,
intensity distribution, processing and cycle time, width
of process window, process stability, plant design, investment costs should be the goal.
P3 > P2 > POPT > P1
(POPT , vOPT)
(P2 , v2)
(P3 , v3)
(P1 , v1)
Bibliography
(PMIN, v0)
[1]
convective
energy flow
(P , v)OPTIMAL
Conclusion
diffusive
energy flow
E S [J/m]
[2]
P [W]
v [m/min]
Fig. 7: Qualitatively course of the reachable strength
maxima parameterized with the laser power.
With increasing laser power the strength maximum is reached with decreased line energy. It
exists an optimal p-v-combination which considers for example strength maximum.
[3]
4
Russek, U.-A.; et. al.; Laser Beam Welding of
Thermoplastics, Proceedings of the LASE 2003,
Conference 4977 B, Laser-Based Packaging in
Microelectronics and Photonics, San Jose, CA,
USA, p. 458 - 472.
Chen, J.W.; Zybko, J.; Laser Assembly Technology for Planar Microfluidic Devices, Proceedings of the Annual Technical Conference, San
Francisco, USA, May 2002
Russek, U.; Simultaneous Laser Beam Welding
of Thermoplastics – Innovations and Challenges,
International Conference on Application of Lasers and Electro-Optics ICALEO 2003, Jacksonville, USA, Paper ID 604, 2003
[4]
[5]
[6]
[7]
[8]
[9]
Chen, J.W.; Hirt, A.; Echtes 3-dimensionales Laserbearbeitungsverfahren für Kunststoff-verbindungen – Neues Einsatzgebiet in der Automobilindustrie, VDI-Berichte 1810, Optische Technologien in der Kunststofftechnik, Optische
Technologien für die Mikrofertigung, S. 87-98,
München, Nov. 2003.
Hofmann, A.; Hybrides Durchstrahlschweißen
von Kunststoffen, Dissertation, Friedrich-Alexander-Universität Erlangen-Nürnberg, 2006
Boglea, A.; et.al.; TWIST - A new method for
micro-welding of polymers with fibre-lasers,
Proceedings of the International Conference on
Application of Lasers & Electro-Optics
ICALEO, 2007, Orlando, USA, M 601, 2007
Russek, U.A.; Aden, M.; Pöhler, J.; Palmen, A.;
Staub, H.; Laser beam welding of thermoplastics
- parameter influence on weld seam quality - experiments and modelling. Proceedings of the
23rd International Conference on Application of
Lasers & Electro-Optics ICALEO 2004, Paper
ID 501, Oct. 2004, San Francisco, CA.
Russek, U.A.; Aden, M.; Pöhler, Laser beam
welding of thermoplastics - experiments, thermal
modelling and predictions. Proceedings of the
International Conference on Lasers in Manufacturing, p. 85-89, June 2005, Munich, Germany.
Russek, U.A.; Prozesstechnische Aspekte des
Laserdurchstrahlschweißens von Thermoplasten,
Shaker Verlag, 2006
Meet the Author
Ulrich A. Russek has been with the Fraunhofer Institute
for Laser Technology, Aachen, Germany from 1998 up
to 2004, working on laser materials processing such as
laser welding of polymers.
From 2005 up to 2008 he has been head of the laser
technology department at Huf Tools, Velbert, Germany
building laser materials processing plants for industrial
applications, such as marking and polymer welding.
Since 2009 he is with the Rheinische Fachhochschule
Köln, University of Applied Science, Cologne, Germany, being responsible for teachings and R&D concerning laser technologies.
5