By Dr Fuad M. Khoshnaw and Idrees A. Hamakhan
Abstract
In
order to emphasise the automation ability of the manual welding
process, two types of alloys have been selected, austenitic stainless
steel and wrought iron. In this study two types of welding processes
have been selected, which are the manual Tungsten Inert Gas TIG model
AE-300 and the semi-automatic Metal Inert Gas MIG model CPT-350.
Wrought iron 1mm thick and austenitic stainless steel type AISI 316L
2mm thick have been selected to show the automation capability of these
two welding machines.
Introduction
Two common but
advanced types of welding are Gas Tungsten Arc Welding (GTAW), often
called Tungsten Inert Gas TIG welding, and Gas Metal Arc Welding
(GMAW), often called Metal Inert Gas MIG. A lot of TIG and MIG
equipment is used in many workshops and factories in the world. In
spite of its high efficiency in welding most metals, there are some
difficulties associated with it during welding. The source of these
difficulties is attributed to the fact that most of the old generation
of TIG and MIG equipments are manual or semi-automatic. Therefore they
are unable to provide a uniform heat input along the welded line.
Accordingly reflection on the automation of this equipment is necessary.
Traditionally
TIG uses welding rods, whereas MIG uses a spool of continuously fed
wire that allows the welder to join longer stretches of metal without
stopping to replace the rod. TIG is mainly used for welding stainless
steel and aluminium alloys. However, MIG has the ability to weld metals
with a similar composition to the filler metal.
In TIG welding, the
welder holds the welding rod in one hand and an electric torch in the
other. The torch is used simultaneously to melt the rod and the
workpiece. In MIG welding the welder holds the wire feeder, which
functions like the alligator clip in arc welding. Instead of using gas
flux surrounding the rod, TIG and MIG protect the initial weld from the
environment by blowing inert gas onto the weld.
The TIG welding
process joins metals by heating them with a tungsten electrode, which
should not become part of the completed weld. Filler metal is mainly
used with thick metals and argon inert gas or inert gas mixtures are
used for shielding. The consumables are: tungsten electrode, filler
metal and shielding gas.
In the TIG process a torch is used to
control the position of the electrode, to transfer current to the arc
and to direct the flow of the shielding gas. The tungsten is placed in
contact with the workpiece; when the tungsten is lifted from the
workpiece an arc is established. Tungsten is a rare metallic element
with an extremely high melting point, 3410oC, and is used for
manufacturing TIG electrodes.
MIG is an arc welding process that
joins metals by heating them with an arc. The arc is between a
continuously fed filler metal (consumable) electrode and the workpiece.
Externally supplied gas or gas mixtures provide shielding. Common MIG
welding is also referred to as short circuit transfer. Metal is
deposited only when the wire actually touches the work. Another method
of MIG welding, spray transfer, moves a stream of tiny molten droplets
across the arc from the electrode to the weld puddle. The consumables
are contact tips, shielding gas and welding wire (1,2).
The values
of σu and HB are directly determined from the ABI test. However, an
accurate determination of yield strength should be based on the entire
load-displacement curve from the ABI test (3,4).
The following equation is used for calculating the heat input (5):
Heat
Input HI = (V * I)/
S
Eq. 1
where : V: Voltage (Volt) , I: Current (Ampere) and S: Speed (Sec.)
Materials and experimental methods
For
demonstrating the automation ability of TIG welding, the austenitic
stainless steel plate type AISI 316L was used in this investigation.
The chemical composition of the alloy was 0.03%C, 16%Cr, 11%Ni, 2.7%Mo,
1.45%Mn, and Fe%Bal. The mechanical properties for received alloy (AISI
316L) were σy=260MPa, σu=490MPa and HB=150. Five heat inputs were
selected which are equal to 213, 225, 237, 305, 385 Joules/mm. Table 1
shows the details of calculating the heat input values.
Table: 1 The heat input calculations for austenitic stainless steel
| No. |
Voltage (volt) |
Current (Amp) |
Speed (mm/Sec) |
H I (J/mm) |
| 1 |
18.5 |
100 |
8.7 |
213 |
| 2 |
17.5 |
100 |
7.8 |
225 |
| 3 |
18.5 |
100 |
7.8 |
237 |
| 4 |
19.0 |
125 |
7.8 |
305 |
| 5 |
20.0 |
150 |
7.8 |
385 |
For
demonstrating the automation ability of MIG welding, wrought iron of
1mm thickness is used in this investigation. The chemical composition
of the alloy was 0.01%C, 0.2%Mn, 0.01%Si, and Fe%Bal. The mechanical
properties for received alloy (wrought iron) were σy=70MPa, su=270MPa,
and HB=80.
Five heat inputs were selected which are equal to 118,
118, 235, 209 and 270 Joules/mm (Table 2). All the heat input values
are mentioned in Table 1 and 2 and are calculated according to Eq. 1.
Table 2: The heat input calculations for wrought iron
No. |
Voltage (volt) |
Current (amp) |
Speed (mm/Sec) |
H I (J/mm) |
| 1 |
16.0 |
50 |
6.795 |
118 |
| 2 |
17.0 |
75 |
6.795 |
188 |
| 3 |
18.5 |
100 |
8.805 |
209 |
| 4 |
17.5 |
100 |
7.56 |
235 |
| 5 |
19.0 |
125 |
8.805 |
270 |
The
filler metal type wrought iron and filler metal type 316L are used for
welding wrought iron and austenitic stainless steel respectively, i.e.
both the metals and their fillers have high similarities in their
chemical composition. The shielded inert gas used in this study for
both welding processes, TIG and MIG, was argon with 99.9% purity.
For
preparing the specimens for the welding process, specimens were
prepared with dimensions 150 mm x 70 mm. After welding, to prepare
specimens for mechanical tests, samples were prepared with dimensions
22 x 20 mm from the welded specimens. The milling process was used to
leave (cut) the pool bed (reinforcement) on the welded metals to give a
flat and smooth surface during the calculation of the mechanical
properties by ABI tests as shown in Fig. 1. The welded specimens are
firstly mounted vertically (the edge appears) in a circular plastic
holder, so that it can be easily grounded and polished. Different
grades of emery papers are used subsequently such as 220, 320, 400 and
600, with water used for lubrication.
Fig. 1. Indented plates using ABI Technique
Welding process machines
For
this study, two types of welding process machines were selected, the
manual TIG model AE-300 (AC/DC arc welder), S.No. P7146 Y0797086, 1979
and the semi-automatic MIG model CPT-350 constant potential DC power
supply, Se.No. P 7161Y A11134204, 1979.
In this study attempts
were made to convert both of them to automatic processes, constant
torch speed and feeding rate. In order to achieve these changes and/or
modifications, all the variables that relate to the welding process
were recognised to give a proper result. The related variables were
current, voltage, welding speed, electrode extension, angle of the
filler, distance between the electrode tip to the workpiece, the exit
distance of filler from the nozzle, and the speed of the filler. These
variables were calculated by a trial and error approach till the
sufficient welding processes were obtained.
The flat (down hand)
position used for both welding processes achieves the best weld
uniformity and penetration. The gas sprayed on the weld bed is used
with 6 lit/min for both types.
TIG welding process
In
gas tungsten arc welding for austenitic stainless steel, the heat
required to join the two workpieces is generated through an electric
arc applied at the joint. The arrangement of fixtures, weld joints,
tools and equipment is best arranged. The type of current used in TIG
welding was a direct current straight polarity DCSP, in which the torch
serves as the negative electrode (cathode) and the workpiece as the
positive electrode (anode). The 2% (as marked) thoriated tungsten
electrodes selected for TIG provide arc stability and increase
electrode service life in comparison to that of standard tungsten
electrodes. Welding was carried out with high purity argon shielded
gas, which provides good penetration without oxidation or contamination.
The
filler metal was added at a constant rate using the torch of the MIG
welding process. The gap between the two workpieces was arranged as
1mm. Filler metal of a 1mm diameter was used for welding austenitic
stainless steel. The speed of the filler metal and the speed of the
welding torch equal 42 mm/sec and 1.395 mm/sec respectively. The
distance between the electrode tip and the workpiece is 4.7mm. The
filler wire was added to the base metal at an angle of 19o to the
horizontal position. The distance between the electrode tip and the
filler wire is 2mm(1,2,5,6).
MIG welding process
The
gap between the two workpieces was arranged as 0.8 mm. Direct current
reverse polarity DCRP was used. The filler metal of a 1.2 mm diameter
was used for welding the wrought iron. The speed of filler metal was
45mm/sec. Moreover, the distance between the nozzle tip and the
workpiece, and the angle of the torch with horizontal surface, were
10.4 mm and 80o respectively (5,8).
Automation of welding processes
Conventionally,
the speed of filler metal or feeding rate in the MIG welding process
can be controlled at a constant rate. However, the torch is moved
manually, i.e. it is a semi-automatic process. However, in the TIG
welding process, both the torch and the filler metal are moved
manually. Despite the welder being very skilful, achieving an equal
heat input along the welding line is very difficult. As a result, the
welding process is completed with an unequal heat input from point to
another. Different heat inputs mean different cooling rates and
frequently various microstructures developed. Finally, different
mechanical properties would be achieved along the welded line, and that
is undesirable in design approaches. Therefore, automating these
welding machines is highly desirable for both types of welding to
maintain weld heat input uniformity by controlling the torch speed,
feeding rate, distance between the work piece and the nozzle, the
distance of the electrode from the nozzle, etc. For this purpose
certain gear train systems were established from the turning machine
bed speeds and used to control the torch speed and feeding rate of
filler metal.
In TIG welding with filler, the two torches TIG and
MIG are installed on the supported plate on the turning machine as
shown in Fig. 2. The torch on the MIG process is utilized as an
automatic feeder machine (tool) of the filler metal to combine with the
major TIG torch. In using a wire feed motor, several speeds on the
turning machine were selected to provide an optimal adjustment between
the two speeds; arc speed (TIG torch) and filler-feeding rate (MIG
torch) to produce fully automatic TIG welding (1,2).
Conventionally,
in the MIG process, the speed of the filler metal (feeding rate) could
be controlled at a constant rate but the torch itself moved manually as
a semi-automatic process. Thus the same problems that were previously
mentioned in manual TIG welding occurred again. Bu, in this method, by
supporting the torch on the supported plate on the turning machine bed,
the process can be converted to a fully automatic welding process, as
shown in Fig. 3.
Fig. 2. Automation of TIG welding process.
Fig. 3. Automation of MIG welding process with filler.
Results and discussion
The
mechanical properties of the welded and heat-affected zones showed that
both the welding machines used in this study of TIG and MIG have a good
capability of being automated. The results showed a variation of
mechanical properties for austenitic stainless steel with the different
heat inputs used in this study. The heat inputs were varied from 213
J/mm to 385 J/mm. However, the mechanical properties for both the
welded region and HAZ with all the heat input values were higher than
the base metal. Heat input of 305 J/mm is considered to be the optimal
value that provides maximum strengths su and sy and Hardness HB in
welded region and HAZ. The results showed that the mechanical
properties in HAZ were σu=490 MPa, σy=288 MPa, and HB=151.
However, the mechanical properties of welded region were σu=560 MPa,
σy=287 MPa, and HB=168.
Similarly, the mechanical properties in
wrought iron weldment using the MIG process varied with changes of the
heat input, which varied from 118 to 270 J/mm. The greatest value in
both the welded region and HAZ with the heat input value is 209 J/mm,
which is considered as an optimal value that provides maximum strength
and hardness. This is apt to increase the properties of the welded
region to 680 MPa, 219 MPa, and 196 for UTS, σy, and HB respectively.
For HAZ the mechanical properties were σu=360MPa, σy=70MPa, and
HB=105. However, the mechanical properties for both the welded region
and HAZ with all heat input values were higher than the base metal.
Each
value of the mechanical properties shown in this study is the average
of five runs on the welded line. The results showed that there is
little difference from one point to another. This means that the
automation process helps to provide the welded line with more uniform
heat. Achieving this uniformity with a manual or semi-automatic welding
process is more difficult (1).
Finally, it should be mentioned that
the aim of this study was to emphasize the ability of both TIG and MIG
welding to be automated with simple connections and fixtures. For
further studies, suitable sensors and connectors could be used and
controlled by computer.
Conclusions
- TIG and MIG welding processes have shown suitability for automation.
- Automated TIG and MIG welding processes provides good mechanical properties.
- Automation helps to create more uniformity in heat input along the welded line.
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Authors
Dr. Fuad M. Khoshnaw
Mechanical Department, College of Engineering, University of Salahaddien-Hawler, Kurdistan, Iraq
Dr Khoshnaw is currently studying at Loughborough University, UK
Idrees A. Hamakhana
Mechanical Department, College of Engineering, University of Salahaddien-Hawler, Kurdistan, Iraq