Svetsaren 1/2007 - Esab

circumferential welds on vacuum chamber
helium vessel
Figure 1. Sketch of the housing of a superconducting
magnet, indicating circumferential joints on the helium
vessel and the outer vacuum chamber. Longitudinal
welds are part of the prefabricated subassembly which is
plasma welded according to SMT welding specifications.
Figure 2. Superconducting magnet coil to be contained
in the helium vessel.
vacuum chamber
circumferential weld on helium vessel
Figure 3. Motoman robot equipped with the Aristo TM
robot package and internal clamp/manipulator.
patient
aperture
narrow, due to the requirement of a perfect
magnetic field in the middle of the scanner.
Robot stations
Apart from a substantial amount of manual TIG
welding on smaller parts, including tacking of the
major parts, the important circumferential joints are
welded by robotic MIG welding – three or four per
vessel, depending on the magnet type (Figure 1).
For these welds, SMT uses two robot stations with
Motoman UP50 6-axis robots on 2-axis
manipulators. Both robots are equipped with the
ESAB Aristo TM robot package consisting of an
Aristo TM 500 water-cooled inverter power source, a
robot mounted Robofeed 30-4 encapsulated wire
feeder, cable assembly and torch and the U8
control unit (Figure 3). Communication with the
programming software of the robot is through
Device Net. The robot head is equipped with a
laser sensor for joint tracking and a camera to
monitor the welding process on a screen.
The first robot to be equipped with the Aristo TM
process package was installed by a UK robot
integrator, Bauromat, about 12 months ago, when
SMT started manufacturing a new product. The 2nd
was a retrofit of an older robot, about 6 months ago.
Weld requirements and welding
The number one requirement for the welds – on
the helium vessel and the outer vacuum chamber
– is absolute leak tightness. Helium is the second
lightest gas known and a “very searching” element
that can escape through the most microscopic of
apertures, thereby determining the lifecycle of the
complete magnet. One complication of this kind of
closed construction, are the limited possibilities to
perform NDT on the welds. The method applied by
SMT is 100% penetrant testing of all welds, the
reason being that any unseen defects will only
become apparent at final testing, when the magnet
is filled with helium and brought under vacuum,
and can only be repaired at tremendous costs.
A stable MIG welding process and a consistent
and repeatable weld quality are therefore
paramount. These are provided by carefully
designed and tested welding procedures for the
various joints, in combination with the digital programming
and arc control features of the Aristo TM
robot package. Parameter settings for each layer
are stored at memory positions in the U8 control
box, using the standard synergic lines with some
slight adjustments. During the start-up phase of
each product, a coupon plate was tested for each
of the first 50 vessels requiring 100% success.
Now confidence, backed-up by experience, is so
great that only with every 50th magnet a coupon
plate is welded, X-rayed and mechanically tested.
As an example, the circumferential joint closing the
helium vessel is a 60-70° V-preparation in 4mm to
8mm thick stainless steel welded onto a stainless
backing strip. It is welded in two layers – a root
pass and a capping pass (Figure 5). The torch is in
a fixed position – slightly over 12 o’ clock – while
the magnet is rotated counter clockwise by a
manipulator arm on the robot station (Figure 3).
The filler material is an 1.2mm diameter 308LSi
solid wire. The Si-type, in combination with the
shielding gas, is chosen to obtain flat welds with a
perfect wetting onto the plate edges. The shielding
gas is an Ar/He/O 2
mixture, where both the He and
the O 2
promotes flatter welds and improves
wetting. The finishing touch is given, making use of
the SuperPulse facility in the U8 control unit.
SMT uses Pulse-Pulse for the root run – a high
current pulse gives the required penetration, while
a pulse at a lower current avoids overfilling of the
joint. Altogether it gives an excellent weld pool
control and the required security in terms of
penetration. The capping run is done with
traditional pulse with a weaving action.
User-friendly equipment does the job
SMT is very pleased with the performance of the
Aristo TM process package in general and the userfriendliness
of the U8 control box in particular,
according to Malcolm Faithfull, Process Welding
Engineer: “Our production is an environment where
consistent quality is the number one requirement. In
that sense it very much resembles the aerospace
industry. Everything is produced to very narrow
tolerances. The functionality of the Aristo robot
equipment enables us to have complete control
over the welding process and obtain a consistent
weld result. All you need is there in terms of welding
intelligence and all very easily accessible. The whole
set-up of the control unit is designed to give you the
shortest route to an optimum arc condition”.
30 - Svetsaren no. 1 - 2007