Design and implementation of a fully automatic robotic laser welding station by Qnet. Here is the relative technical article published at the Proceedings of the Fifth International WLT Conference on Lasers in Manufactoring 2009 in Munich by Davide Kleiner and Geert Verhaeghe

1. Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009
Munich, June 2009
High-power Yb-fibre laser welding of heavy-section tube-to-tubesheet
assemblies
Davide Kleiner1
, Geert Verhaeghe2
1
Mangiarotti S.p.A., Research & Development Department, Udine, Italy
2
TWI ltd., Laser and Sheet Processes Group, Cambridge, UK
Abstract
Mangiarotti S.p.A., a leading pressure vessel, reactor and heat exchanger manufacturer, have
developed, with the help of TWI Ltd, a unique fully-integrated laser processing cell for the
welding of heavy-section tube-to-tubesheet components. Laser welding was identified as a high-
productivity alternative to the conventionally applied TIG welding process, offering speeds at least
an order of magnitude higher. The welding cell is fully autonomous, can be moved around the
workshop, and is capable of welding heat exchangers typically comprising up to 5000 tubes and
tubesheet diameters in excess of 4.5m. An 8kW Yb-fibre laser is manipulated using a 6-axis shelf-
mounted articulated robot arm, and a machine vision operated slide to ensure accurate tube
location and joint alignment. Filler wire addition and scanning optics have been implemented to
compensate for joint gap variations, with real-time (laser) process monitoring to assist in assuring
a weld quality in accordance with relevant product standards (e.g. ASME).
Keywords: pressure vessel, tube, tubesheet, weaving, mechanised
1 Introduction and background
Mangiarotti S.p.A. is based in the North-East of Italy
and operates in the field of pressure, high temperature
as well as heavy-wall and large diameter equipment
engineering and manufacturing. Its typical products
comprise, among others, heat exchangers, condensers,
reactors, columns, finishers, drums, separators and
splitters. Production materials include stainless steels
(solid and clad), non-ferrous metals (e.g. nickel-based
alloys), carbon steels, alloyed steels (e.g. Cr, Mo and
V) and reactive metals, such as titanium. Mangiarotti
operates in the industry fields of oil&gas (onshore and
offshore), chemical, petrochemical, nuclear,
pharmaceutical and power generation, and utilises in its
work all the relevant construction codes such as
ASME, TEMA, API, PD5500, PED, AD Merkblätter,
Stoomvezen and RCC-M.
The large pressurised items that Mangiarotti
manufactures, such as tubular reactors or heat
exchangers, comprise tube-to-tubesheet welding. This
is traditionally carried out using the tungsten inert gas
(TIG) process with the addition of suitably chosen
filler wire. This is a long proven technique, amply
discussed and regulated in the codes relevant to
Mangiarotti’s applications, and is currently carried out
in-house in either manual, semi-automatic (orbital
machines) or fully automatic (CN controlled) modes,
depending on the specific application and associated
welding procedures.
Because in most cases the tube-to-tubesheet
weld is to be a full-strength weld, the TIG procedure
requires chamfering of the tubesheet holes and
multipass welding with the addition of filler wire to
achieve the desired geometrical weld features, above
all the minimum length of the weld throat. The welding
time per tube depends on the component geometry, but
can currently take in excess of one minute. Considering
a heat exchanger can easily comprise a total of 5000
tubes, the total welding time is considerable and
constitutes a large percentage of the total
manufacturing time of a heat exchanger. Moreover, the
heat input for welding this type of joint associated with
a typical TIG procedure can reach values in the order
of 40MJ/m2
on the tubesheet surface. As a consequence
of this, weld metal solidification stresses and
deformation of the tubesheet are always a concern
regardless of its thickness. A great deal of attention is
also given to the quality control of the process. This is
however traditionally limited to the establishment of a
thoroughly tested welding procedure developed on test
samples, trial welds on test samples performed at the
beginning of each work shift. Ultimately, the TIG
welding of the tube-to-tubesheet joint is a blind or
almost blind process, until thorough non-destructive
testing of the fully welded tubesheet is performed.

2. Hence, the need for a shorter processing time and for a
documentable real-time process monitoring was
identified.
In order to stay ahead of its competitors, the
Directorate at Mangiarotti decided to investigate
alternative manufacturing methods that would allow
them to reduce the total processing time per heat
exchanger, whilst maintaining the required weld
quality through process monitoring. In 2006,
Mangiarotti decided to explore the benefits and assess
the feasibility of laser beam welding as an alternative
to TIG welding for the welding of tube-to-tubesheet.
The Research & Development Department, with
the cooperation of the Production and Quality
Departments, carry out some initial studies on the
aforementioned welding technique on full-size
mockups made of carbon, low alloy and stainless
steels. Despite the first welding results being far from
perfect, the technique showed to be very promising in
terms of repeatability and productivity, and it was
decided to proceed with a full project, which would
take laser beam welding into the production
environment.
In early 2007, Mangiarotti called upon the
specific expertise of TWI ltd (Cambridge, UK) to
assists them in designing a dedicated laser facility for
welding their tube-to-tubesheet products.
2 System design
2.1 Philosophy
From the very start of the conceptual design, a number
of key characteristics were identified as being crucial to
the successful completion and operation of the welding
system. These comprised constraints and boundary
conditions from both the product and the process point
of view, as illustrated below.
- Properties of the weld must be acceptable
according to the relevant codes and standards. This
includes the surface appearance, the cross-sectional
weld features, the presence of imperfections and the
local and overall mechanical qualities.
- The cycle time, tubesheet deformation and
joint reject rate must be less than that of equivalent
TIG welded joints.
- Strict quality control procedures must govern
the welding process, and the quality control on the
individual joints must be fully traceable.
- The system must be capable of interpreting the
data from all the on board sensors and take operational
decisions based on that interpretation. The system must
evaluate the reliability of this decision before the start
of any welding.
- The process must be easy to set up, fully
automated be capable of unsupervised operation.
- The envelope within which the system
parameters are flexible must be sufficiently large not to
be a constraint during welding procedure development.
- The system software must be user friendly and
must be able to directly address all the welding
parameters of the system hardware.
- The safety and emergency subsystems must
prevent the occurrence of any situation that could prove
potentially hazardous to the operator or any other
person.
2.2 Hardware
The result of the development is an integrated system
mounted on a transportable steel structure, linked to a
control unit located in a lightproof safety bunker. The
completed tube-to-tubesheet welding system is shown
in Fig. 1.
Fig. 1: processing end of the completed welding
system.
The laser source is an 8kW Yb-fibre laser
manufactured by IPG Photonics. The laser, with a
nominal beam quality of 6mm.mrad, was selected over
alternative options, because of Mangiarotti's
requirement to be able to move the laser system around
the welding shop. The source comprises a built-in 2-
way beam switch transmitting the optical power
through a 30m long 200µm core fibre to a modified
YW-50 Precitec process head, capable of focusing the
power into a 0.40mm minimum beam waist, using a
150/300mm lens combination.
The Precitec process head is equipped with a set
of in-line process monitoring sensors and a scanner
unit, manufactured by ILV, that by employing
oscillating optics achieves laser beam weaving, which
can be precisely modulated both in frequency and in
amplitude. The process head, together with a vision
system and hotwire torch are mounted on the wrist of a
shelf-mounted GE Fanuc 6-axis robot (Fig. 2 and Fig.
3) with an extended reach of 3.5m and a useful wrist
payload of 100kg.
The vision system comprises an industrial
camera equipped with a high quality Sony CCD and
purposely chosen lenses, and a set of two stripe laser
diodes, controlled also by the camera itself. The
hotwire torch is part of a complete Fronius digital
welding set and mechanised wire-feeder unit, all
mounted on the robot arm.

3. Fig. 2: process head, with industrial vision system in
position. The vision system and Precitec welding head
are mounted on pneumatic slides and are toggled
depending on whether the system is performing a
centring or a welding operation.
Fig. 3: process head, with industrial vision system in
position. The setup of the mechanical stops on the
pneumatic slides is such that the axis of the CCD
corresponds to axis number 6 of the robot.
Controls and communication protocols between
the various subsystems take place via analogue, digital
I/O, Ethernet, Profibus and Canbus, all handled by
Beckhoff PLC units. Simplified schematics of the
communications and control logic that run the whole
welding system is shown in Fig. 4.
Festo equipment is employed for the
management of all shielding gas and compressed air
circuits, using, where possible, fireproof pressure
tubing and connecting joints. A dedicated air
compressor feeds all the welding system’s compressed
air lines.
The whole installation is located inside a
laserlight-proof welding cell of 20x61m, controlled
from an annexed safety bunker, where the control
console is located, and accessories, such as warning
indications, emergency and door interlock circuits. The
control console contains the industrial pc which
handles the system software (including the user
interface), various plc controls and the provision of
power for the on-board low voltage subsystems.
The machine is equipped with two on-board
cameras, one wall-mounted PTZ camera and one
special applications camera coaxial to the laser beam
axis. The feed from all of these four cameras is
conveyed to a PC in the safety bunker and recorded on
a Milestone video server. The PTZ camera can also be
accessed from any PC within Mangiarotti by use of a
username and password.
2.3 Process cycle dynamics
As with any automated mechanical cycle, the process
starts with the precise identification of the position and
orientation of the workpiece in 3D space. The
coordinate system of the workpiece is specified with
respect to the robot’s own internal coordinate system
by means of an X,Y,Z translation of the origin and
P,Q,R rotations around the X,Y,Z axes respectively.
The three points which are necessary to create a
coordinate system in 3D space (in this case origin, X-
direction and Y-direction) are identified by the robot’s
vision system itself.
The vision system software performs auto-
setup operations using Montecarlo algorithms for
exposure brightness and contrast optimisation, acting
on the camera shutter time and aperture. It identifies
the tube by applying mathematical image processing
algorithms so as to enhance the geometrical feature of
the tube’s inner circumference. An example of this is
shown in Fig. 5. The numbers that can be seen in the
same figure are the outputs in terms of positioning
error in the X and Y directions. These are then
communicated to the robot, which carries out a
repositioning. The maximum allowed error in both of
these directions is 0.05mm. This is smaller than the
robot’s intrinsic repeatability, hence this cycle is
repeated until convergence to within that value occurs,
which generally takes two or three cycles.
The distance between the welding head and
the tubesheet is measured by means of two stripe laser
diodes projected on the tubesheet in front of the
industrial camera. Being the focal length 300mm, the
tolerance in the Z position is slightly larger, and is set
to 0.2mm.

4. Fig. 4: simplified schematic of the welding system’s communications and control logic.
Once the coordinate system of the workpiece
has been acquired by the robot, the automated welding
cycle can start. The robot positions itself in front of the
tube to be welded at the theoretical X,Y,Z position
according to the CAD/CAM information, aligning the
wrist (and hence the welding head) parallel to the
tubesheet. A fine centring cycle is performed until the
positional tolerances are met. The system toggles a set
of two pneumatic slides, moving the vision system out
of the way (and closing an external protection shutter)
and the welding head into the right position. The radius
of welding around the tube axis is determined by the
position of a mechanical stop mounted on the slides.
The position of this stop is regulated during the
development of the welding parameters. The activation
of the crossjet, the shielding gas and the oscillating
optics follows. Shielding gas can be fed via the
welding head coaxial nozzle, the side nozzle of the
wire-feeder torch, or both.
The circular movement required for welding is
provided by the rotation of axis number 6 of the robot;
no other axis moves during the welding. The laser
source starts emitting as the head starts revolving, with
a preset ramp up time lasting 5° of head rotation. The
system then welds at constant power during 365° of
head rotation followed by a power ramp down lasting
12° of head rotation, for a total of 382°. The crossjet,
the shielding gas and the oscillating optics are at this
point deactivated.
Control system and
human interface
Milestone
videoserver
E
Bunker
Welding cell
Company
network
E
PTZ camera
E
2x onboard
cameras
Welding head
camera
E
Laser source and
chiller unit
A
Door
interlocks
Emergency
circuits
Flashing
lights g/y/r
6-axis robot
D P
Wire feeder
Oscillating
optics control
Welding headQA sensors
control
Pneumatics and
shielding gas control
A
A
Pneumatic and gas
actuators
A
Legend:
A: analog
C: canbus
D: digital
E: ethernet
P: profibus
D
Machine vision
system
C

5. Fig. 5: visualisation of vision system algorithm result.
If filler wire is used in the process, the system
will start feeding it at a rotation angle determined by
the user (>0°), and retracting it at the end of the
constant power section (370°). Before the system
proceeds to the following tube, the system checks
whether the filler wire has become stuck in the
solidified weld pool.
2.4 Software
The software was developed externally by Qnet s.r.l. in
Visual C# .NET and runs on the industrial pc in the
control console. The software is, generally speaking,
the master in the system except while the robot is
carrying out a movement, during which it acts as a
slave.
The software contains a CAD/CAM module as
shown in Fig. 6, so that dxf or dwg files can be
uploaded directly, while the system parameters that can
all be addressed by the software are shown in Fig. 7.
Fig. 6: CAD/CAM interface and automated welding
process commands.
2.5 Quality control
The software structure is that, if there is any doubt
about the reliability of the sensors’ reading, the system
will not weld. Indeed, the machine monitors
continuously all the cycle parameters such as head
position with respect to tube being welded, real tube
position with respect to theoretical tube position, real
tube diameter with respect to theoretical tube diameter,
tubesheet surface deviation from flat, welding head
position with respect to machine vision assembly,
shielding gas flow, compressed air static and dynamic
pressure, laser beam weaving parameters, wire speed
voltage and current, torch position, rotation speed of
axis number 6 of the robot, wire stuck to the weld,
operating temperature of the system subcomponents
and their three separate cooling circuits, and status of
the access doors to the welding cell. Any abnormal
reading in any of the aforementioned parameters
instructs the machine to skip the tube in question or
terminates the process altogether.
Furthermore, the welding head contains sensors
that monitor directly the status of the weld pool at
1kHz frequency, and more specifically the molten
metal temperature, the reflected laser light, the plasma
status over the weld pool and the effective laser power
output. This data, together with the wire speed voltage
and current, rotation speed of axis number 6 of the
robot and images used by the machine vision system,
is collected live and recorded. The data from the
welding sensors is then compared against
“acceptability curves” created during welding of the
mockups. Any deviation from the optimum readings
gives rise to an error message and stops the process.
All the monitoring data is recorded for post-weld
analysis, and any noticeable event that is “cause for
concern” is communicated to the operator via SMS.
Fig. 7: welding parameters selection.
Where possible, more than one sensor monitors
a specific system. This is especially true for the
machine vision system, which is critical for the correct
positioning of the welding head in relation to the centre
of the tube. All critical subsystems dealing with system
status and information exchange and management are
provided with battery backup.
A very rigid structure exists for data
management on the control console pc. Welding
parameters, their revision number, CAD/CAM data
(and their association with the welding parameters),
machine vision images, sensors’ readings etcetera are
given unique IDs for traceability reasons.

6. 3 Production trials and results
The complete system has undergone long rigorous
testing from the point of view of all hardware, software
and welds produced. It is immediately clear that
mechanised laser welding (or cutting) is an intrinsically
highly repeatable process. All tubes welded on the
same tubesheet with the same parameters look identical
both visually and in cross-section. Fig. 8 shows a
typical surface finish for a stainless steel joint, with its
microsection shown in Fig. 9.
Fig. 8: surface finish of a typical autogenous stainless
steel joint (Ø19.05x1.65mm 321 tube to 347 tubesheet).
Fig. 9: microsection of the same joint. The effect of the
scanning optics on the weld profile is very visible.
The last two figures represent one of two
configurations that have been so far successfully
completed with a WPS (welding procedure
specification) in accordance to the ASME code. Initial
pull-out destructive tests have performed three times
better than what was required in the joint specification
(60-65kN against 20kN), and no defects where
detected by non-destructive testing.
4 Comparison against equivalent TIG
tube-to-tubesheet welding procedures
The system is currently occupied in the development of
welding procedures for the welding of real assemblies,
to be compared with the traditionally used TIG
procedures. The data obtained from the testing of both
the laser and the TIG samples will provide the grounds
for further validation of the system and subsequent full
integration into production on the shop floor.
The welding procedures developed so far
exhibit a cycle time of between 50 and 10% that of
equivalent TIG welding. Furthermore, there is a time
saving on the tubesheet preparation as well, as the joint
requires no chamfer.
The resulting heat input of the laser beam
welding process is approximately 10% of that of
equivalent TIG welding. As a result, deformation of the
tubesheet after welding and asymmetries of the tube in
the tube opening are minimal.
The resulting chemical composition and hence
the metallurgical properties of the weld metal have
been successfully controlled through the application of
small quantities of appropriate filler wire, generally for
non-stainless materials.
5 Conclusion
Validation work on the system continues for the tube-
to-tubesheet welds, as new materials, material grades,
joint geometries or mechanical requirements are
investigated.
At the time of this article, Mangiarotti had
started publicising this welding system with its clients,
a number of which responded positively asking for
further information and for a live demonstration.
Welding procedure specifications have already been
produced for the product of a specific client, which is
being manufactured at Mangiarotti.
More unusual materials have also been
successfully welded under the ongoing research
programme, such as for example 99.9% pure copper to
70/30 CuNi alloy.