practical applications of in-process monitoring for laser ... - Prometec

Proceedings of the 23 rd International Congress on Applications of Lasers and Electro-Optics 2004
PRACTICAL APPLICATIONS OF IN-PROCESS MONITORING FOR LASER PROCESSES -
NOT ONLY FOR SINGLE WELDS AND COMMON MATERIALS.
Joerg Beersiek 1 , Thomas Devermann 1 , Klaus Behler 2
1
PROMETEC GmbH, Jülicher Str. 338, 52070 Aachen, Germany
2 University of Applied Sciences, Wiesenstr. 14, 35390 Gießen-Friedberg, Germany
Abstract/Manuscript
Laser welding is by now a conventional industrial
technology. Besides the prevalent processes to
manufacture single welds and endless seams more
complex weld operations involving multiple welds
with differing joint designs in one work piece are
becoming common. Monitoring of such parts require
the weld monitoring systems to offer easily
manageable solutions. For welded parts whose
function is imperative for the secure operation of
systems, the secure in-process judgment of weld
process quality is indispensable.
This paper discusses the functionality of a CMOS
camera based monitoring system for such purposes
with help of case studies from e.g. the automotive
industry. The basic working principle of this system
for the monitoring of laser processes has been
published in former presentations.
The way of solving unique monitoring tasks with the
system is presented using practical examples. These
examples show the capabilities of a camera based
process monitoring system like geometrical resolution,
adaptable algorithms and independence of the welding
direction.
Pos.
Sketch
Description of the
Application
Material
Joint
configuration
Type of
Machine
Type of
Laser
Power
Range
Monitoring
Task(s)
Interface
User
1
Welding of thick
section steel beams
synchronal with two
processing heads in a
T-joint configuration
Mild steel
Double Fillet joint
Special welding
machine with two
processing heads
CO2-laser
12 kW
Laser beam
position with
regard to the
joint; Penetration
depth
Profibus
Power plants
2
Endless welding of
profiles
Mild steel,
galvanized
Lap joint
Tube welding
machine with dual
focus
CO2-laser
2 - 5 kW
Penetration
depth; Lack of
fusion; Focal
position; Laser
power
Profibus
Automotive
3
Circumferential weld
on gear parts
High strength
steel
Butt joint
Special welding
machine with
stationary
processing head
and rotary chuck
Nd:YAG laser 1.5 - 2.5 kW
Penetration
depth; Part
position; Weld
offset; Gas
variations
Profibus
Automotive
4
Various joints on seat
rails
Mild Steel
Fillet joints, Butt
joints, Flare Bevel
joint
Robot laser welding
Nd:YAG laser
cell
3 kW
Weld length,
Laser power,
Focal position,
Gaps, Blow outs
Profibus
Automotive
5
Heat Exchanger
(Var. Types)
Titanium,
Hastaloy,
Nickel etc.
Overlap-Joints
Robot laser
welding cell
Nd:YAG
laser
3-4 kW
Welding
Speed, Laser
Power, Focal
Position, Gas
Supply, Gaps
Profibus
Var.
table 1
Applications overview of the WELDING MONITOR PD2000

Introduction
Today, the use of in process monitoring systems is a
common method to ensure quality in an industrial
laser welding production process. There are lots of
different systems based on analysis of the radiation of
the plasma plume or analysis of the back reflection of
the laser light, or camera based systems, like the
camera system of Prometec, which analyzes plasma
and meltpool coaxial to the laser beam with a high
spatial resolution.
Today, the camera system is used for different
applications and integrated in a number of production
lines. Some examples of different solved applications
are given in table 1.
Additionally, the camera system can be used for
experiments and applications to get interesting view
on welding processes. Therefore trials were executed
in cooperation with Prof. Dr. Behler at the company
Trumpf in Schramberg, Germany.
The aim was to enlarge the working field of laser
processes at the humping border, to find possibilities
for monitoring the arising of humping failures and to
validate theoretical ideas [3-8] with the help of our
camera system[1,2].
In this paper, the resulting images in comparison to
the welding result will be discussed and interpreted.
Experimental setup
In any case our camera system is positioned coaxially
to the laser beam [1,2]. This is an exceptional
position for the camera because it is possible to
obtain information from the inner parts of the
keyhole. The setup of a system using a Nd-YAG laser
is presented in figure 1.
During an industrial process the software of the
camera evaluates different failures and process
parameters online with the help of characteristic
regions within the image of the camera.
However it is possible to use the recorded films to
analyze the process and to improve process
parameters. These possibilities are described using
the example of the humping effect.
figure 1
Laser
Monitoring setup for a Nd-YAG
With this setup a Nd:YAG laser with an output power
of P L = 3,5 kW, was used . During the entire
experiments, the output power and the focal position
was constant.
The used material for all experiments was a standard
mild steel, uncoated partial penetrated. The welding
speed and power distribution (Twin spot optic) will
be changed over the experiments.
The resulting images of the camera are presented in
figure 2.
The camera delivers greyscaled images. In order to
increase the contrast of the picture, the images are
presented in false color. The color table is given in
The two presented images are averaged over a full
weld seam and the zone, which is illuminated by the
laser beam is highlighted by a circle in the image.
They show the interaction zone between the laser
beam and the workpiece. The images show the
temperature radiation of the melt pool and the
keyhole. Differences in the intensity will be
interpreted as differences in temperature.

f=200mm
Laser beam
Laser beam
f=100mm
1 mm
1 mm
Diameter of laser beam: 0,4 mm
Parameters: Nd:YAG-Laser; P L : 3,5 kW; v S : 20 m/min;
Diameter of laser beam: 0,2 mm
Color table:
≅ 100
≅ 150
≅ 200
≅ 255
figure 2
Dimensions of the images of the camera system at different focal length, f
The resolution of the image is controlled by the lens of
the focusing head of the laser beam and by the imaging
optic of the camera. The change of the focal length of
the focusing head from f=200 mm to f=100 mm causes
an increase of the image resolution by a factor of two.
Therefore the number of pixels, which are illuminated
by the interaction zone, are nearly the same. In both
cases the width of the weld was imaged on 40 pixels of
the CCD- chip of the camera. Each pixel had an image
area of 6.7 x 6.7 µm.
The image with a focal length f=200 mm shows a spot
with higher intensity behind the keyhole region. This
spot is interpreted as the result of the melt flow from
figure 3 with the welding speed of 8 m/min. The
image shows a circular intensity distribution and
complies to the region around the keyhole. The
temperature of the melt behind the keyhole seemed to
be too low for the camera. At 15 m/min, the velocity of
the melt flow increases and hot material appears from
the deeper regions of the keyhole to the surface. The
intensity distribution increases and acquires a
cylindrical shape. A comparison of the images between
8 m/min and 15 m/min shows, that the temperature at
the deeper regions of the keyhole. In analogy to a stern
wave of a motorboat the process generated a stationary
wave in the melt pool behind the keyhole. This effect
is often shown in the following chapters.
focal length f=200mm
In the experiments, the welding speed was increased
up to the threshold of humping. The temperature
distribution in the keyhole and in the melt pool was
recorded by the camera. The resulting images are
presented
in
the front of the keyhole was shifted to the back and the
intensity gradient at the front increases for 15 m/min.

8m/min 15 m/min 20 m/min 22 m/min 22,3 m/min 22,5 m/min 23 m/min
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 200 mm
Humping
figure 3
creation of Humping with a focal length f= 200mm
Additionally, the temperature or the intensity at the
center of the keyhole increases. These effects were
observed in former presentations and are theoretically
apprehended [6, 7]. Due to the higher speed of the
welding the laser beam has to increase preheating of
the weld, if the shape of the capillary is to be kept
constant. The increase of the temperature in the center
shows the increase of the evaporation rate in the
keyhole, which is necessary to uphold the shape of the
keyhole.
If the welding speed is increased, up to 20 m/min, the
temperature at the center decreases and the hot spot
behind the keyhole occurs. The whole intensity
distribution acquires a figure eight shape. The hot spot
is interpreted as the result of a hot melt flow coming
from hotter and deeper parts of the keyhole. The
decrease of intensity in the center of the keyhole
indicates a flatter absorption front and a change in the
shape of the keyhole.
When the speed is increased further, a very sharp
threshold for the humping effect occurs at a speed
between 22,3 m/min and 22,5 m/min. This effect in the
melt pool corresponds to a drastic change in the images
of the camera. The whole intensity distribution in the
image decreases. The whole melt pool seemed to
absorb less energy. This is interpreted as an abrupt
change of speed in the melt flow. Due to the higher
speed of the melt flow and the short time of direct
contact with the laser beam, the melt goes not have the
time to absorb more power from the laser and the melt
becomes colder.
The hot spot behind the keyhole is comparable to the
hot spot at lower welding speed. The difference is that
the spot is colder and therefore the melt freezes quicker
and the melt flow has no time for relaxation. This is
the reason for the occurrence of humping.
focal length f=100mm
The shape of the keyhole and the speed of the melt
flow impinge on the occurrence of humping. The shape
of the keyhole and the melt pool are changed using
different focal length. Therefore the results for a focal
length of f=200mm should be compared with another
focal length. According to literature [3-8] it is known,
that the humping effect can be shifted to higher
welding speed by the use of a smaller focal length.
Therefore, a focal length of f=100mm was used for the
comparison.
The resulting images are presented in figure 4.

10 m/min 15 m/min 20 m/min 25 m/min 30 m/min 35 m/min 40 m/min 45 m/min
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 100 mm
figure 4
Dependence of the averaged camera image on welding speed using a focal length of f=100mm
Similar to the results with f=200mm the length of the
melt pool increases at higher welding speed. In
contrast to these results the intensity and therefore the
temperature in the center of the keyhole increases with
higher speed. This result indicates that the shape of the
keyhole is comparable over all welds. Nevertheless the
temperature of the melt decreases again at higher
speed. At a speed of 25 m/min again a hot spot occurs.
Even at 30 m/min a second spot occurs. The
occurrence of humping will be discussed in detail in
figure 5 and figure 6.
V s = 25 m/min
V s = 30 m/min
Camera
image
Detail of
weld seam
Camera
image
Detail of
weld seam
1 mm
1 mm
crossection
crossection
0,5 mm
0,5 mm
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 100 mm
figure 5
occurrence of humping I

Using a focal length of 100 mm the humping effect
occurs at a speed of 30 m/min for the first time. In the
images this effect corresponds with the occurrence of
a second hot spot at the end of the melt pool. This
kind of humping is well known from former
publications [3-7] and corresponds to a bellied shape
in the cross section of the weld seam. From now on
this kind of humping is called humping I.
At 25 m/min the basic approach of the bellied shape
is identifiable but the weld seam is flatter and the
reinforcement is not as distinctive as in the case of
30 m/min.
If the welding speed increases up to 35 m/min
(figure 6) the humping effect vanishes and the
resulting weld seam is smooth again. The difference
in the image of the melt pool is obvious. The melt is
cooler and the image shows only one hot spot.
At 40 m/min the humping effect occurs again and the
effect shows the now expected behavior of a cooler
melt pool. This kind of humping is called from now
on humping II.
V s = 35 m/min
V s = 40 m/min
Camera
image
Detail of
weld seam
Camera
image
Detail of
weld seam
1 mm
1 mm
crossection
crossection
0,5 mm
0,5 mm
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 100 mm
figure 6
occurrence of humping II
Twin spot optic
Another way to change the shape of the keyhole and
therefore the conditions of humping generation is the
use of a twin spot optic.
Under this aim the best results were achieved using a
twin spot optic with a distance of 400µm. The
corresponding images are given in figure 8.
Twin spot optics with different distances between the
two laser spots were tested with regard to their
influence on the occurrence of humping and the aim to
suppress the formation of humping.

Single
Spot
Twinspot
200 µm
Twinspot
300 µm
Twinspot
400 µm
Twinspot
500 µm
10 m/min
10 m/min
10 m/min
10 m/min
10 m/min
Laser beam
Laser beam
Laser beam
1 mm
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 100 mm
figure 7
Influence on keyhole formation using different distances between the two focal spots of twin
spot optic (0 to 500 µm)
The welding results show the humping effect only at
a welding speed of 42 m/min. The so called humping
I effect was suppressed by the twin spot optic and the
corresponding images shows again the effect of
cooling for humping II at 42 m/min.
The image for 20m/min shows a hot bridge between
the two laser spots. With increasing speed this
develops to a hot spot behind the first keyhole. But
this spot is influenced by the keyhole of the second
laser spot. This may be the reason for suppressing the
humping I effect.

20 m/min 25 m/min 30 m/min 35 m/min 40 m/min 42 m/min
Parameters: Nd:YAG-Laser; P L : 3,5 kW; f= 100 mm
Humping II
figure 8 occurrence of humping using a twin spot optic with a spot distance of 400 µm
Conclusion
The humping effect can be suppressed with the use of
a twin spot optic. A detailed description of these
results will be given in the near future [7]. The
occurrence of humping can be observed in-process
using a camera positioned coaxial to the laser beam.
Due to the hot spots behind the keyhole, the presence
and the absence of humping can be monitored
definitely.
With these results it is planned to build up a closed
loop for the welding speed to prevent the welding
process for humping effects.
In addition to that the images can be used in scientific
work to understand the melt flow during welding.
The interpretation of the intensity spots behind the
keyhole as the result of a hot wave must be
investigated further and verified.
References
[1] Beersiek, J. (2002) New Aspects of
Monitoring with a CMOS camera for Laser
Materials Processing
ICALEO 2002
[2] Beersiek J. (2001) A CMOS camera as a
tool for process analysis not only for laser
beam welding
ICALEO 2001, Section F206
[3] Bradstreet, B. (July 1968) Effect of
Surface Tension and Metal Flow on Weld
Bead Formation
Welding Research Supplement

[4] Mendez, P. F; Eagar, T. W (October 2003)
Penetration and Defect Formation in High-
Current Arc Welding
Welding Journal
[5] Beck, M; et al. (1990) Aspects of
Keyhole/Melt Interaction in High Speed
Laser Welding
Proc. 8 th Int. Symp. On Gas Flow and
Chemical Lasers (GCL), Madrid
[6] Schmidt, H. (1994) Hochgeschwindigkeits-
Schweißen von Feinstblechen mit CO 2 -
Laserstrahlung unter besonderer Berücksichtigung
des Humping-Effektes
Diss. RWTH Aachen,
Wissenschaftsverlag Mainz, Aachen
[7] Pirch, N; et al. (1992) Die Humping
Instabilität beim Schweißen mit Laserstrahlung
Proc. Konf. Laser ´91, München
[8] Behler, K. to be published