FIELD OF THE INVENTION:
The present invention relates to the welding method of fuel tank upper body
by using CMT MIG welding of hot dip galnannealed steels. This invention
further relates to the copper backing material design and joint fit up design.
It belongs to the field of metal processing.
BACKGROUND OF THE INVENTION:
Two wheeler fuel tanks designed and manufactured with the considerations
of various factors, for example, containing automobile gasoline, the
manufacturing materials require corrosion resistant to the environment of
the fuel tank. Hence the materials of fuel tank body require to have
corrosion resistance property. For these applications, galvannealed steel
sheets are generally used as the material for manufacturing fuel tank.
Galvannealed steels are hot dip coated steel in molten Zn bath, in which the
steel sheet has a base steel sheet cold-rolled from a material containing
0.005 wt % or less of C, 0.005 to 0.05 wt % of Ti, 0.01 to 0.1 wt % of Al,
0.005 to 0.015 wt % of Nb and 0.0002 to 0.002 wt % of B. In the process of
making, the hot-dip plating layer applied after the alloying heat treatment,
has a Fe content of from 9 wt % to 12 wt %. They exhibit superior corrosion
resistance and hence, are broadly used as the material of automobile body
parts.
From Japanese patent Laid-Open No. Hei 6-47572, it is known that, two
wheelers fuel tank body consists of upper and lower body aligned in vertical
direction to form a sealed space for storing fuel in the tank body. The upper
part of the fuel tank is made by joining two halves by means of solid state
welding or brazing based on the joint design i.e. butt joint and lap joint. In

general, resistance seam welding method is used for lap joining and MIG
brazing method is used for butt joining for the fuel tank upper body joining.
At present arc welding process like MIG welding of these steels is highly
challenging because of the differences in the melting points of iron (1534°C),
zinc (420°C), and their intermetallic phases like gamma (900°C), delta
(750°C), zeta (550°C), hence arc welding results in fumes generation, arc
instability and burn through of material.
OBJECTS OF THE INVENTION:
• It is therefore an object of the present invention to propose a method
for welding of fuel tank upper body by using cold metal transfer metal
inert gas (CMT MIG) welding of hot dip galnannealed steels, which is
adaptable to automotive industry in particular for manufacturing of
fuel tanks.
• Another object of the present invention is to propose a joining method
using CMT MIG welding for hot dip galvannealed material without any
welding defects.
• Still another object of the present invention is to propose a welding
technique using minimum heat input of 60J/mm, which provides
desired weld penetration.
• A further object of the present invention is to propose a copper
backing support element used for joining of hot dip galvanized steels,
which helps in reducing burn through.

SUMMARY OF THE INVENTION:
The invention discloses butt welding of galvannealed steel fuel tank upper
part components of thickness 0.8mm and material composition 0.005 wt %
or less of C, 0.005 to 0.05 wt % of Ti, 0.01 to 0.1 wt % of Al, 0.005 to 0.015
wt % of Nb and 0.0002 to 0.003 wt % of B, using CMT MIG welding power
source. A 500mm long welding torch was used to MIG weld fuel tank
components. The welding filler wire used was specified as per AWS SFA5.1
ER70S-6 of 0.8mm diameter solid wire and shielding gas used was mixture
of argon and carbon dioxide in a ratio of 82% and 18% respectively. The
heat input used during welding was in a range of 55J/mm to 65J/mm. A
copper block with a grove used as backing material during welding. Under
these conditions, the material was found to be weldable without any burn
through.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 – Lap Joint
Fig. 2 – Butt Joint
Fig. 3 – Copper Block Support
Fig.4 – Fixture with copper backing for fuel tank upper components
Fig. 5 – 500mm long welding torch for fuel tank welding application.
DETAILED DESCRIPTION OF THE INVENTION:
This invention relates to the defect free galvannealed sheets welding, which
is only possible with very low heat input welding, precise square butt joint
fit up with rigid clamping arrangement, copper backing with a slot
arrangement to avoid burn through.

A. Low heat input: The galvannealed sheets welding require very low heat
input as the thickness of the material is very low i.e. 0.8mm. Trials were
carried out starting from 120J/mm and gradually reduced to 60 ± 5J/mm to
get consistent quality welds. The welding parameters which contribute to get
low heat input are welding current (amperes), arc voltage (Volts), Welding
travel speed (mm/min). The formula used for calculating heat input (J/mm)
when the thermal efficiency 0.9 is used, as follows.
heat input (E)
= 0.9 X Welding current) X (Arc voltage) X60 / Welding travel speed
In the present invention, the heat input was restricted to 60±5 J/mm, also a
contact tip to work distance of 10mm maintained during welding. In order
to maintain consistency in weld quality and precision control on heat input
advanced welding power source, CMT MIG welding used.
Cold Metal Transfer (CMT) MIG welding:Thefuel tank MIG welding requires
the invention relates to cold-metal transfer metal inert gas (CMTMIG
)welding process, in which the welding process is defined by cyclic variation
of arcing and short-circuit phase, The CMT process also includes a
completely new type of droplet detachment, combined with reversing wire
electrode movement. i.e., droplet detachment and re-ignition are controlled
by means of backwards movement of the wire electrode. In this process the
wire is fed to the work piece until a short circuit occurs and the arc goes
out. The direction of the wire movement is then reversed, i.e. the wire is
withdrawn from the workpiece. This breaks the short circuit and the arc is
reignited. The wire movement is subsequently reversed again and the
process described here starts again. Depending on the characteristics for the
filler metal, shielding gas and electrode diameter, this reversing motion
occurs within a frequency range of 50 to 130 Hz.

In addition, the welding filler wire, specified as per AWS SFA5.1 ER70S-6 of
0.8mm diameter solid wire and shielding gas used was mixture of argon and
carbon dioxide in a ratio of 82% and 18% respectively.
B. Butt joint fit-up: The joint design used in this invention was square butt
joint without any root gap. As the fuel tank material has 0.8mm thick,
during experimental trials any root gap caused burn through while welding.
Fig.2 represents the butt joint fit up used during welding. It also requires a
clamping arrangement to hold the sheets close to the joint. A pneumatic
clamping arrangement used to hold the sheets without any root gap.
C. Copper backing: This welding also requires a copper backing
arrangement having a slot in it. Fig.3 represents the copper block design,
which has a slot depth of 0.5mm and 2.8mm width. This slot acts as zinc
fumes vent generated. The copper block also acts as heat sink during
welding.
Application:
Since the invention main objective is to develop welding method for fuel tank
material, a fixture was made to accommodate fuel tank upper components;
Fig.4 represents the fixture model used for fuel tank welding. As mentioned
butt joint fit-up, copper backing arrangement used during welding. Also a
narrow width and long neck welding torch of 500mm length (Fig.5), which is
connected to a robotic arm, was used for fuel tank welding.
Using these arrangements following welding parameters (Table.1) used to
carry out welding experiment on the fuel tank upper components.


In the above experiment the heat input used was in between 58J/mm to
64J/mm and the welding quality obtained was without any burn through.

We claim:
1. A method for welding fuel tank upper body, the method comprising
steps of: placing fuel tank upper body from inner side over a copper
block, the fuel tank being galvannealed, the fuel tank comprising left
and right component; maintaining zero root gap between left and right
components of fuel tank for butt joining; adjusting CMT MIG welding
parameters to minimum heat input of 60 J/mm; and welding the left
and right component.
2. The method as claimed in claim 1, wherein welding current (A) is 78-
82 A.
3. The method as claimed in claim 1, wherein wire feed rate (m/min) is
5.5-6.0mm/min.
4. The method as claimed in claim 1, wherein Arc Voltage (V) is 11-11.5.
5. The method as claimed in claim 1, wherein welding travel speed 800
mm/min.
6. The method as claimed in claim 1, wherein the copper block
comprisesU groove with slot depth 0.5mm and 2.8mm width.
7. The method as claimed in claim 1, wherein the welding is done by
means of 500mm long welding torch, and where the welded product is
devoid of burn through.
8. The method as claimed in claim 1, wherein the average thickness of
the tank upper part components is at least 0.8 mm with a material
composition 0.005 wt% of Ti, 0.01 to 0.1 wt% of Al, 0.005 to 0.015
wt% of Nb and 0.002 to 0.003 wt% of B.

9. The method as claimed in claim 1, wherein a controllable butt
welding procedure is applied by using CMT MIG power source.
10. The method as claimed in any of the preceding claims, wherein a
contact dp to work distance during the welding is maintained at least
at 10 mm, and wherein the welding filter wire is selected as per AWS
SFA 5.1 ER705.6 of 0.8 diameter solid wire.
11. The method as claimed in any of the preceding claims, wherein the
shielding gas used is a mixture of argon and carbon dioxide in a ratio
of 82% and 18% respectively.
12. The method as claimed in any of the preceding claims, wherein a
pneumatic clamping means is used to hold the joining components
with zero gap, and wherein the copper block acts as a heat sink
during the welding.