FIELD OF INVENTION
The invention generally relates to an automatic welding of small diameter, higher
thickness titanium tube to tube butt welding required for tube built up for
producing a tube coil bundle assembly used in Special Heat Exchanger. More
particularly, the invention relates to a device and method for Gas Tungsten Arc
Welding (GTAW) for carrying out welding of Titanium tubes.
BACKGROUND OF THE INVENTION
Manufacturing of tubes bundles used in Special Heat Exchanger involves
thousands of tube-to-tube welded joints of Titanium tubes. Titanium is a reactive
metal that interacts readily with oxygen, making it prone to oxidization and
contamination, and making it difficult to weld. However, meticulous preparation,
a steady hand, and lots of practice lead to successful welds. Welding Titanium
tubes requires extreme cleanness in respect of the base metal, filler metal, and
in addition, the welding environment must be very clean and pure.
Contamination by natural body oils, oils from the forming and drawing process,
shop dust, paint, dirt, cutting fluids, and lubricants all can lead to embrittlement
and weld failure. To protect the weld from getting contaminated, a shielding gas
coverage is necessary. Normally ultra pure Argon is used for this purpose. An
inert gas secondary trailing shield of high purity argon or helium is used during
all such weldings. The trailing gas protects the solidified weld metal and the
associated heat affected zones (HAZ) until the temperature falls below 400 °C.
Purging, a process that eliminates the oxygen contained within the pipe, is also
carried-out during welding of the Titanium tubing.

The Non Destructive Examination (NDE) of Titanium welds involves 100% visual
inspection for contamination and coloration in as welded condition and 100%
examination of the welds by radiography (RT), ball test, hydro test and Helium
leak test prior to tube coiling. Any crack, lack of fusion (penetration), porosity on
the root or cap surface, is not permitted. The color of the final weld requires to
be of Silver color, both at weld and base metal including the inside and outside
of the weld joint. According to the standards, the weld geometry should limit the
root penetration (P) to 0.5 mm, weld reinforcement (R) within 0.2 mm to 0.5
mm, and the weld width (W) between 5 to 7 mm for all the joints with nil
distortion.
According to the prior art, the tube to tube welding of Titanium tubes involved
two steps, namely, (i) root welding by an orbital TIG welding machine without
filler, and (ii) filler pass welding by manual TIG welding with filler material. The
disadvantage is that the welds made with this process generally produce
inconsistent weld geometry even though they normally pass all the other tests.
The required weld geometry is very important with respect to the application.
More than 75% of the rejected welds showed variations in weld reinforcements
from 0.1mm to 1.00 mm.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device for carrying-out a
fully automatic GTAW of Titanium tube to tube butt welding, which eliminates
the disadvantages of the prior art.

A further object of the invention is to propose a process for welding of Titanium
tube to tube butt welding which produces acceptable weld geometry.
These and other objects and advantages of the invention will be apparent from
the ensuing description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
1. Figure - 1 Shows weld geometry in a tube to tube butt welding
2. Figure - 2 Schematically illustrates an automatic welding device
according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in Figure 2, the fully automatic GTAW machine comprises at least two
holding means (16) supported on a base (02). The holding means (16) in turn
have a pair of rotating chucks (03). At least one pair of pneumatically operated
tube clamping means (04) is provided. The rotating chucks (03) and the
clamping means (04) ensure that both the butting ends of the tubes (01, 01) are
clamped in such a way to have a minimum root-gap and zero mismatch. A
plurality of adjustable screws are optionally provided to take care of any
mismatch if the tubes have bends. A prime mover (05) provided at the base (02)
rotates the chucks (03) at a required RPM. A TIG torch (06) and a filler wire

feeding means (07) are mounted on a vertical stand (08). The torch (06) can be
moved in both vertical and horizontal directions. The torch (06) is surrounded by
an annular space for maintaining a gas flow. One each pressure gauge (09) and
flow meter (09) is provided for precisely controlling individual gas for welding as
well as for inner and outer shielding. The butting ends of the tubes (01) to be
welded are enclosed in a gas shielding chamber (10) provided with a view glass
in the front. The filler wire is fed to the welding zone through a nozzle (15).
A Central Processing Unit (CPU) (11) including a programmable Logic controller
(PLC), suitably programmed, is provided to precisely control all the sequential
operations of the various components of the device including monitoring the
parameters such as tube inner gas pre flow, purging chamber pre flow, torch gas
pre flow, current upslope, tube rotation, welding root pass, current down slope,
torch gas post flow and tube inner and outer post flow for the root passes, which
are co-related with the filler wire. A console panel (12) is provided with a
plurality of switches to control the operation of the device. The Argon gas is
supplied to the device from a gas supply unit (13). Electrical power for welding is
drawn from a power source (14).
As shown in Figure-1, the edges of the tubes (01) to be welded are prepared,
the prepared edges and the adjacent surface at the inner and outer diameters of
the tube, are cleaned by acid pickling and subsequently by using rectified high
pure spirit. The tubes (01) are loaded and clamped by the chucks (03) and by
the automatic clamping means (04) in such a way that the edges of the tubes

(01) are butted against each other maintaining a gap of less than 0.1 mm and
zero mismatch. The circumference of the tube (01) is divided into at least eight
segments and at least ninety-nine technical parameters are fed into the
programmable logic control (PLC) of the welding device to carry out the first pass
and second pass welding. The Argon gas flow and timing at various locations of
the weld are optimized. The voltage, current and gas flow rates are set by the
program. The argon gas is passed through the ID of the tubes (01) as well as
into the purging chamber. The tubes (01) are rotated by the chucks (03) and the
first pass welding is carried out in down hand position by fusion between the
tubes (01) without using any filler wire. The second pass welding with filler wire
is carried out in the similar way. After the welding is over, the tube (01) is
declamped and removed from the device. Thus a consistent weld geometry as
per the requirement is achieved automatically by this process for all the joints
welded.

WE CLAIM
1. A device for joining titanium tubes by adapting an automatic Gas
Tungsten Arc Welding (GTAW), the device comprising a base (02) having a
prime mover (05) to impart rotational movement to a pair of rotatable
chucks (03), the base (02) being adjoined to a vertical stand (08) which
accommodates a movable TTG torch (06) including a filler wire feeding
means (07); a pair of clamping means (04) enabling the two prepared
edges of a pair of joinable tubes (01, 01) to be clamped on the rotating
chucks (03) so as to maintain a minimum root-gap including a zero
mismatch, the edges of the joinable tubes (01) being enclosed in a gas-
shielded chamber (10); and a central processing unit (11) incorporated
with a programmable logic controller (PLC) which being programmed with
data relating to the technical parameters of the tubes and the welding
process enabling the device to automatically and correspondingly carry-out
a first and a second pass of welding the first pass welding being
implemented by fusion without using the filler wire and the second pass
welding with the filler wire.
2. The device as claimed in claim 1, wherein at least one each pressure
gauge and flow meter (09) is provided for controlling the pressure and
flow of individual gas for welding including shielding.
3. The device as claimed in claim 1, wherein the filler wire is fed to the filler-
wire feeding means (07) via a nozzle (15).

4. The device as claimed in claim 1, comprising an operator console panel
(12) provided with a plurality of switches to control the operation of the
device.
5. The device as claimed in claim 1, comprising a gas supply unit (13) to
supply argon gas.
6. The device as claimed in claim 1, comprising a power source (14) for
supply of electrical power to the device.
7. A process for joining titanium tubes by automatic Gas Tungsten Arc
Welding (GTAW) using the above device comprising the steps of:
clamping the tubes on the chucks by clamping means to maintain a gap of
less than 0.1 mm and zero mismatch;
passing inert as through the tubes and purging chamber;
rotating both the tubes at the same speed;
carrying out the first pass welding by fusion without the filler wire; and
carrying out the second pass welding with the filler wire;
8. The process as claimed in claim 7, wherein the weld geometry is
characterized in that the root penetration (P) is limited to 0.5 mm, weld
reinforcement (R) is limited to 0.2 mm to 0.5 mm, and weld width (W) is
limited to 5 to 7 mm for all the joints.
9. The process as claimed in claim 7, wherein the weld geometry is
characterized with nil distortion.
10.The process as claimed in claim 7, wherein the weld and base metal side
of the Titanium tubes at inside and outside of the joint are produced with a
bright Silver color.
11. The process as claimed in claim 7, wherein the welding is carried out in
down hand position.
12.The process as claimed in claim 7, wherein the circumference of the tube
is divided into at least eight segments.
13. A device for joining titanium tubes by adapting an automatic Gas Tungsten
Arc Welding (GTAW) as substantially described and illustrated herein with
reference to the accompanying drawings.
14. A process for joining titanium tubes by adapting an automatic Gas
Tungsten Arc Welding (GTAW) as substantially described and illustrated
herein with reference to the accompanying drawings.

The invention relates to a device for joining titanium tubes by adapting
automatic Gas Tungsten Arc Welding (GTAW) comprising a base (02) having a
prime mover (05) to impart rotational movement to a pair of rotatable chucks
(03), the base (02) being adjoined to a vertical stand (08) which accommodates
a movable TIG torch (06) including a filler wire feeding means (07); a pair of
clamping means (04) enabling the two prepared edges of a pair of joinable tubes
(01, 01) to be clamped on the rotating chucks (03) so as to maintain a minimum
root-gap including a zero mismatch, the edges of the joinable tubes (01), being
enclosed in a gas-shielded chamber (10); and a central processing unit (11)
incorporated with a programmable logic controller (PLC) which being
programmed with data relating to the technical parameters of the tubes and the
welding process enabling the device to correspondingly carry-out a first and a
second pass of welding wherein the first pass welding by fusion without filler
wire and the second pass welding with the filler wire. The invention further
relates to a process in a device for joining titanium tubes by adapting a GTAW.