Welding technologies

Question 1

Define the Fusion Welding Processes

Answer 1

Fusion welding is defined as melting together and coalescing materials by means of heat (with or without the application of pressure ).

Coalescence is the process by which two or more droplets droplets or particles merge during contact to form a single droplet. Coalescence is a term used in welding, where two (or possibly more) metals are welded together.

The thermal energy required for these welding operations is usually supplied by chemical or electrical means.

Filler metals: metals added to the weld area during welding of the join:

• They are available as rods or wire.
• These consumable filler rods may be bare or coated with flux.

The purpose of the flux is:
-Retard oxidation of the surface of the parts being welded by generating a gaseous shield around the weld zone.

• Dissolve and remove oxides and other substances from a workpiece, resulting in a stronger joint.
• The slag developed protects the molten puddle of metal against oxidation as it cools.

Autogenous welds: fusion weld made without the addition of filler metals.

These processes include oxyfuel, arc, thermit, high-energy beam.

Question 2

Explain Oxyfuel Gas welding and identify its safety components.

Answer 2

Oxyfuel Gas Welding:

Welding that uses a fuel gas combined with oxygen to produce a flame.

The most common gas welding process uses acetylene fuel (C2H2) and O2.

The combustion process is as follows:
C2H2 + 2.5(02) -> 2(CO2) + H20 +heat

The temperature may reach 3300°C at a neutral flame and become less otherwise.

The use of safety equipment such as goggles with shaded lenses, face shields, gloves, and protective clothing is important.

Proper connection of hoses to the cylinders is also an important factor in safety.
Oxygen and acetylene cylinders must have a different thread, so hoses cannot be connected to the wrong cylinders.
The gas cylinder should be anchored securely and should not be dropped or mishandled.

Question 3

Be able to produce a diagram of Oxy-acetylene welding equipment and identify its components

Answer 3

Refer to the schematic on page 8 of the Welding processes lecture.
The components shown in the diagram should be:

1. Oxygen Cylinder
2. Acetylene Valve
3. Welding Tip
4. Oxygen Valve
5. Oxygen Hose
6. Acetylene Hose
7. Acetylene Cylinder
8. Regulators
9. Working Pressure Gauges
10. Cylinder Pressure Gauges
11. Main Oxygen Valve

Question 4

Dicuss the types of Flame used in welding.

Answer 4

Other gases such as hydrogen and methyl acetylene propadiene can be used in oxyfuel gas welding.

The temperatures developed are low ,and hence they are used for welding metals with low melting points, such as lead, parts that are thin and small.

The flame with hydrogen gas is colorless, making it difficult to adjust the flame by eyesight.

Other gases ,such as natural gases ,propane ,and butane ,are not suitable for oxyfuel welding because of the low heat output or because the flame is oxidizing.

When using Oxy Acetylene Flames there are three distinct flame settings that are used:

1. Neutral Flame
2. Oxidizng Flame
3. Carburizing Flame

Question 5

Describe the Neutral flame used in Oxy Acetylene

Answer 5

Neutral Flame
The neutral flame is produced when the ratio of oxygen to
acetylene, in the mixture leaving the torch, is almost exactly one-to-one.
It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld.

Question 6

Describe the Oxidizing flame used in Oxy Acetylene

Answer 6

Oxidizing Flame
With a greater oxygen supply, it becomes as an Oxidizing flame.

This flame is harmful, especially for steel, because it oxidizes the steel.

Only in copper and copper-base alloys is an oxidizing flame desirable because of a thin protective layer of slag forms over the molten metal.

Question 7

Describe the Carburizing flame used in Oxy Acetylene

Answer 7

Carburizing Flame

If the supply of oxygen is lowered, it becomes a Reducing or Carburizing flame.

The temperature of a reducing flame is lower.

It is suitable for applications requiring low heat, such as brazing, soldering, and flame hardening.

Question 8

What is a Weld Pool?

Answer 8

Weld pool commonly refers to the molten metal portion of a weld where the base metal has reached its melting point and is ready to be infused with filler material.

The weld pool is central to the success of the welding process.

The weld pool must be carried along the joint in a consistent width and depth, and the motion used to carry the weld pool has a direct effect on the quality of the weld bead.

A weld made by starting and carrying a weld pool, without the addition of a filler material, is called an autogenous weld

Question 9

identify the advantages and disadvantages of using Oxy-Acetylene Welding.

Answer 9

Advantages:

Relatively easy to learn.

The equipment is cheaper than most other types of welding rigs (e.g. TIG or MIG welding).

The equipment is more portable than most other types of welding rigs (e.g. TIG welding). It is used typically for repair work in location.

Oxy-Acetylene equipment can also be used to “flame-cut” large pieces of material.

Disadvantages:

Oxy-Acetylene weld lines are much rougher in appearance than other kinds of welds and require more finishing if neatness is required.

Small joints may consist of a single weld bead

Joints are made in multiple passes (layers).

Cleaning the surface of each weld bead prior to depositing the second layer is important for joint strength and avoiding defects.

Hand or power wire brushes may be used for this purpose.

Oxy-Acetylene welds have large heat affected zones (areas around the weld line that have had their mechanical properties adversely affected by the welding process)

Question 10

List some approximate material power densities required for certain welding processes.

Answer 10

Welding process = Approximate Power Density (W/mm^2)

• Oxyacetylene Welding = 10 W/mm^2
• Arc Welding = 50 W/mm^2
• Laser Welding = 9000 W/mm^2

Question 11

What are the three main factors that affect the quantity of heat required to melt a given volume of metal?

Answer 11

The heat to raise the metal from room temperature to its melting point.

The melting point of the metal

The heat required to transform the metal from a solid to a liquid state

Question 12

Present the Equation used to find an approximation of heat required to melt metal.

Answer 12

Um = K x (Tm)^2

Um - the unit energy for melting ie. The quantity of heat
to melt a unit volume of metal.

Tm – Melt temperature of the metal ˚K
Um - units J / mm^3
K - constant = 3.33 x 10^-6

Question 13

Present the equation for discovering the amount of heat available for welding.

Answer 13

Hw = f1 x f2 x H

Hw = Heat available (Joules)
f1 = Heat transfer factors (0 < f1 > 1)
f2 = Melting factor (0 < f2 > 1)
H = Total heat generated by the welding process.

There are TWO heat transfer mechanisms both of which
reduce the amount of generated heat that is used in the
welding process.

The first is the heat transfer mechanism between the heat source and the workpiece.
Heat transfer factor f1

The second mechanism involves the conduction of heat away from the weld area
Heat transfer factor f2

Question 14

Present the Balance equation - The energy Input and Energy needed for welding.

Answer 14

Hw = Um x V

Hw = Net energy for Welding (J)
Um = Unit energy for welding (J/mm^3)
V = The volume of metal melted (mm^3)

Question 15

Present the equations utilized to calculate the rate of heat energy delivered when the welding processes are assumed to be Steady state.

Answer 15

Assuming welding processes are Steady state.
The rate of heat input for welding is constant
and the Weld bead moves at a constant
velocity.

Rhw = Um x Rwv

• Rhw - Rate of heat energy delivered to the weld (J/sec = W)
• Rhv - Volume rate of metal welded (mm^3/sec)

Hw = f1 x f2 x H

Rhw = f1 x f2 x Rh = Um x Aw x V

Aw - Weld Area
V - Weld travel velocity
Um - Unit energy for welding (J/mm^3)

Question 16

Identify the 8 specific ARC welding processes

Answer 16

1. Carbon Arc Welding (CAW)
2. Manual Metal Arc Welding (MMAW)
3. Gas Metal Arc Welding (GMAW)
4. Gas Tungsten Arc Welding (GTAW)
5. Flux Cored Arc Welding (FCAW)
6. Submerged Arc Welding (SAW)
7. Plasma Arc Welding (PAW)
8. Stud Arc Welding (SW)

Question 17

Define Arc Welding

Answer 17

Arc welding is a process utilizing the concentrated heat
of an electric arc to join metal by fusion of the parent
metal and the addition of metal to joint usually provided
by a consumable electrode or without electrode
(nonconsumable).
Using either a consumable or non-consumable electrode.
Either direct (DC) or alternating (AC) current may be
used for the arc, depending upon the material to be
welded and the electrode used.

Question 18

What is Manual Metal Arc Welding (MMAW)

Answer 18

About 50% of all industrial and maintenance welding is
performed by this process.
The electric arc is generated by touching the tip of a
coated electrode against the work piece and then with
drawing it quickly to distance sufficient to maintain the
arc.
The electrodes are in the shape of thin ,long sticks, so
this process is also known as stick welding.
The heat generated melts a portion of the tip of the
electrode, its coating and the base metal in the immediate
area of the arc .
A weld forms after the molten metal –mixture of the base
metal (work piece) , electrode metal, and substances from
the coating on the electrode –solidifies in the weld area .
The electrode coating deoxidizes and provides a shielding
gas in the weld area to protect it from oxygen in the
environment .

Electrical current usually ranges between 50A and 300A.
The current may be AC or DC.
With DC the polarity of the electrode may be positive or
negative.
The choice depends on the type of electrode ,type of
metals to be weld, and the depth of the heated zone
Too low a current causes incomplete fusion
Too high a current can damage the electrode coating
and reduce its effectiveness

When an arc is struck between the metal rod (electrode) and the workpiece, both the
rod and workpiece surface melt to form a weld pool.
Simultaneous melting of the flux coating on the rod will form gas and slag which
protects the weld pool from the surrounding atmosphere. The slag will solidify and
cool and must be chipped off the weld bead once the weld run is complete (or before
the next weld pass is deposited).
The process allows only short lengths of weld to be produced before a new electrode
needs to be inserted in the holder. Weld penetration is low and the quality of the
weld deposit is highly dependent on the skill of the welder.

Question 19

Identify and describe the types of Flux/Electrodes used in Manual Arc Welding (MMAW) and the porperties they affect.

Answer 19

1. Cellulosic:
   Cellulosic electrodes contain a high proportion of cellulose in the
   coating and are characterised by a deeply penetrating arc and a
   rapid burn-off rate giving high welding speeds. Weld deposit can be
   coarse and with fluid slag, deslagging can be difficult. These
   electrodes are easy to use in any position and are noted for their
   use in the ‘stovepipe’ welding technique.
   Welding Features
   deep penetration in all positions
   suitability for vertical down welding
   reasonably good mechanical properties
   high level of hydrogen generated - risk of cracking in the heat
   affected zone (HAZ)
2. Rutile:
   Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating.
   Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter.
   These electrodes are general purpose electrodes with good welding properties. They
   can be used with AC and DC power sources and in all positions. The electrodes are
   especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.
   Features:
   moderate weld metal mechanical properties
   good bead profile produced through the viscous slag
   positional welding possible with a fluid slag (containing fluoride)
   easily removable slag
3. Basic:
   Basic electrodes contain a high proportion of calcium carbonate (limestone) and
   calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid
   than rutile coatings - this is also fast-freezing which assists welding in the vertical and
   overhead position. These electrodes are used for welding medium and heavy section
   fabrications where higher weld quality, good mechanical properties and resistance to
   cracking (due to high restraint) are required.
   low hydrogen weld metal
   requires high welding currents/speeds
   poor bead profile (convex and coarse surface profile)
   slag removal difficult

Arc stability, depth of penetration, metal deposition rate and positional
capability are greatly influenced by the chemical composition of the flux
coating on the electrode.

Question 20

Identify the purpose of Metal powder electrodes

Answer 20

Metal powder electrodes contain an addition of metal
powder to the flux coating to increase the maximum
permissible welding current level. Thus, for a given
electrode size, the metal deposition rate and efficiency
(percentage of the metal deposited) are increased
compared with an electrode containing no iron powder in
the coating. The slag is normally easily removed. Iron
powder electrodes are mainly used in the flat and H/V
positions to take advantage of the higher deposition
rates. Efficiencies as high as 130 to 140% can be
achieved for rutile and basic electrodes without marked
deterioration of the arcing characteristics but the arc
tends to be less forceful which reduces bead penetration.

Power source
Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can
be operated on AC power sources, however AC electrodes may be used on either AC
or DC.
Welding current
Welding current level is determined by the size of electrode - the normal operating
range and current are recommended by manufacturers. Typical operating ranges for
a selection of electrode sizes are illustrated in the table. As a rule of thumb when
selecting a suitable current level, an electrode will require about 40A per millimetre
(diameter). Therefore, the preferred current level for a 4mm diameter electrode
would be 160A, but the acceptable operating range is 140 to 180A.

Question 21

What purpose does a transistor have in Arc Welding.

Answer 21

Transistor (inverter) technology is now enabling very
small and comparatively low weight power sources to be
produced. These power sources are finding increasing
use for site welding where they can be readily
transported from job to job. As they are electronically
controlled, add
-on units are available for TIG and MIG
welding which increase the flexibility. Electrodes are now
available in hermetically sealed containers. These
vacuum packs obviate the need for baking the
electrodes immediately prior to use. However, if a
container has been opened or damaged, it is essential
that the electrodes are redried according to the
manufacturer’s instructions.

Question 22

What is Gas Metal-Arc Welding (GMAW) and explain the process.

Answer 22

Also know as “MIG” (metal inert gas).
Weld pool shielded by an external inert gas source such
as argon.
Inert gas could be :argon Ag,helium H2
,CO2
Gas metal arc welding generates an electrical arc
between a continuous filler metal (consumable)
electrode and the work piece.
Shielding is provided by an externally supplied shielding
gas.

With MIG (Metal Inert Gas) welding also called Gas-Shielded Metal Arc Welding
(GMAW) an arc is maintained between a continuous solid wire electrode and the
work piece.
The arc and weld pool are shielded by a stream of inert or active gas. The process is
suitable for most materials and filler wires are available for a wide range of
metals.
MIG/MAG welding is inherently more productive than SMAW, where productivity
losses occur each time a welder stops to replace a consumed electrode.
Material losses also result from SMAW welding when the stub of each electrode is
thrown away. For every kilogram of coated stick electrode purchased, about 85
per cent becomes part of the weld (the rest being discarded). The use of solid
wire and flux cored wire has increased this efficiency to typically 95 per cent.
MIG/MAG welding is a versatile process, which can deposit weld metal at a very high
rate and in all positions. The process is widely used on light to medium gauge
steel fabrications and on aluminium alloy structures particularly where high-rate
manual operator production is required.
The introduction of flux cored wires is finding increased application in heavy steel
structures.

Question 23

Identify the advantages and disadvantages of MIG welding.

Answer 23

Advantages:
1. Because of continuously fed electrode, GMAW process is much faster as compared to TIG or stick electrode
welding.
2. It can produce joints with deep penetration.
3. Thick and thin, both types of work pieces can be welded effectively.
4. The process can be easily mechanized.
5. No flux is used (reducing total welding cost).

Disadvantages:
1. The process is slightly more complex as compared to
stick electrode welding because a number of variables
(like electrode stick out, torch angle, welding
parameters, type and size of electrode, welding torch
manipulation, etc.) are required to be controlled
effectively to achieve good results.
2. Welding equipment is more complex, more costly and
less portable.

Question 24

Why are shielding gases important for arc welding.

Answer 24

In addition to general shielding of the arc and the weld pool, the shielding gas performs a
number of important functions:
forms the arc plasma
stabilises the arc roots on the material surface
ensures smooth transfer of molten droplets from
the wire to the weld pool.

Thus, the shielding gas will have a substantial effect on the stability of the arc and metal
transfer and the behaviour of the weld pool, in particular, its penetration. General purpose
shielding gases for MIG welding are mixtures of argon, oxygen and CO
2, and special gas
mixtures may contain helium. The gases which are normally used for the various materials
are:

• steels
• CO2
• argon +2 to 5% oxygen
• argon +5 to 25% CO2
• non-ferrous
• argon
• argon / helium

Argon based gases, compared with CO2, are generally more tolerant to parameter settings

Question 25

Identify and describe the three principla metal transfer modes for MIG welding.

Answer 25

The manner, or mode, in which the metal transfers from the electrode to the weld pool
largely
determines the operating features of the process. There are three principal metal transfer
modes:

• Short circuiting:
  Short-circuiting and pulsed metal transfer are used for low current operation while
  spray metal transfer is only used with high welding currents. In short-circuiting or
  ‘dip’ transfer, the molten metal forming on the tip of the wire is transferred by the
  wire dipping into the weld pool. This is achieved by setting a low voltage; for a
  1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care
  in setting the voltage and the inductance in relation to the wire feed speed is
  essential to minimise spatter. Inductance is used to control the surge in current which
  occurs when the wire dips into the weld pool.
• Droplet/spray:
  For droplet or spray transfer, a much higher voltage is necessary to ensure that the
  wire does not make contact i.e. short-circuit, with the weld pool; for a 1.2mm
  diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A).
  The molten metal at the tip of the wire transfers to the weld pool in the form of a
  spray of small droplets (about the diameter of the wire and smaller).
  However, there is a minimum current level, threshold, below which droplets are not
  forcibly projected across the arc. If an open arc technique is attempted much below
  the threshold current level, the low arc forces would be insufficient to prevent large
  droplets forming at the tip of the wire. These droplets would transfer erratically
  across the arc under normal gravitational forces.
• Pulsed:
  The pulsed mode was developed as a means of stabilising the open arc at low current
  levels i.e. below the threshold level, to avoid short-circuiting and spatter. Metal
  transfer is achieved by applying pulses of current, each pulse having sufficient force
  to detach a droplet. Synergic pulsed MIG refers to a special type of controller which
  enables the power source to be tuned (pulse parameters) for the wire composition
  and diameter, and the pulse frequency to be set according

Question 26

What are the applications of MIg welding?

Answer 26

MIG is widely used in most industry sectors and
accounts for more than 50% of all weld metal
deposited. Compared to MMA, MIG has the
advantage in terms of flexibility, deposition rates
and suitability for mechanisation. However, it
should be noted that while MIG is ideal for
‘squirting’ metal, a high degree of manipulative
skill is demanded of the welder.

Question 27

What is Gas Tungsten Arc Welding?

Answer 27

Know as “TIG” (Tungsten Inert Gas).

Non-consumable electrode.

A constant and stable arc gap is maintained at a constant current level.

The filler metal is supplied from a filler wire .

The shielding gas is usually argon or helium or mixture of two.

Welding with TIG may be done without filler metals.

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replacement for gas and manual metal arc welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structuralvapplications.

Question 28

Describe the process of Gas Tungsten Arc Welding

Answer 28

In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.

TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately onethird at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the ‘lift arc’ technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source. Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding. HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.

Question 29

What type of electrodes are required for TIG welding

Answer 29

Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption).

It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or ‘ball’ profile.

Question 30

What type of shielding gas is used in TIG Welding?

Answer 30

Shielding gas is selected according to the material being welded. The following guidelines may help:

Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.

Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

Question 31

What are the applications of TIG welding?

Answer 31

TIG is applied in all industrial sectors but is especially suitable for high
quality welding. In manual welding, the relatively small arc is ideal
for thin sheet material or controlled penetration (in the root run of
pipe welds). Because deposition rate can be quite low (using a
separate filler rod) MMA or MIG may be preferable for thicker
material and for fill passes in thick
-wall pipe welds. TIG is also
widely applied in mechanised systems either autogenously or with
filler wire. However, several ‘off the shelf’ systems are available for
orbital welding of pipes, used in the manufacture of chemical plant
or boilers. The systems require no manipulative skill, but the
operator must be well trained. Because the welder has less control
over arc and weldpool behaviour, careful attention must be paid to
edge preparation (machined rather than hand-prepared), joint fit-up
and control of welding parameters.

Question 32

Identify the advantages and disadvantages of TIG Welding.

Answer 32

Advantages:

1. Superior quality welds.
2. Welds can be made with or without filler metal.
3. Precise control of welding variables (heat).
4. Low cost in comparison with processes such as electron beam and laser welding.

Disadvantages:

1. Requires greater welder skills than MIG or stick `welding.
2. Lower deposition rates (slow).

Question 33

Produce diagrams related to MIG and TIG welding as seen throughout the welding lecture.

Answer 33

Refer to lecture slides.