Chapter VI of ASME B31.1 contains the qualification of NDE Personnel and the acceptance/rejection criteria for the weld defects by the following examination methods;

1. Visual Examination (Para 136.4.2)
2. Magnetic Particle Examination (Para 136.4.3)
3. Liquid Penetrant Examination (Para 136.4.4)
4. Radiography (Para 136.4.5)
5. Ultrasonic Examination (Para 136.4.6)
6. In-Process Examination (Para 136.4.7)

Weld defect acceptance/rejection criteria for visual Inspection :

The following indications are unacceptable as per ASME B31.1;

(1) Cracks — external surface.

(2) Lack of fusion on the surface.

(3) Incomplete penetration (applicable only when the inside surface is accessible).

(4) Weld reinforcement more than specified in the below table.

(5) Undercut — Greater than 0.8 mm (1∕32 in.) deep, or if it exceeds the minimum required section thickness.

(6) Undercut on the surface of longitudinal butt welds

(7) Any other linear indications: Greater than 5mm (3∕16 in.) in length

(8) Surface porosity with rounded indications having dimensions greater than 3∕16 in. (5 mm) or four or more rounded indications separated by 1∕16 in. (1.5 mm) or less edge to edge in any direction.

(9) Arc strikes outside of the weld joint.

To know the acceptance criteria as per ASME Se9ction VIII Divison 1, Please click here.

Aluminum is found on the earth in its oxidized form known as Bauxite. Bauxite is refined to produce Alumina (Aluminium Oxide – Al2O3). Approximately 40% – 60% Alumina i.e. Aluminium Oxide (Al2O3) can be extracted from Bauxite.

The Hall-Héroult electrolysis process (invented in 1886) further processes Alumina (Aluminum Oxide-Al2O3) to separate it from oxygen. This process made possible the production of Aluminium on a large scale and significantly reduced the production cost too.

Some of the very important properties of Aluminum that makes it a very lucrative metal are;

• It is highly resistant to corrosion
• It is non-toxic
• Possesses good electrical and thermal conductivity
• It is nonmagnetic.
• Retains good ductility at sub-zero temperatures

Aluminum and aluminum alloys are very suitable for critical applications such as;

• Space, aircraft, and marine industries
• Vessels and storage tanks
• Automotive industries
• Structural applications and many such consumer products.

[NOTE: ‘Aluminum’ and ‘Aluminium’ are essentially the same. The word ‘Aluminium’ is very popular in Non-American English.]

Melting point of Aluminium:

The melting point of pure Aluminum is 660°C (1220°F).  However, the melting point of the Aluminum alloys varies from  480°C to 660°C (900°F to 1220°F), depending on the alloying element.

Density of Aluminium:

The density of aluminum is;

• 2.7 g/cm³ (2,710 kg/m³) 

• 168.56 lbs/ft³ (0.0975 lbs/in³)

Major alloying elements (with Aluminium):

Major alloying elements are copper, manganese, silicon, magnesium, and Zinc, etc.

Aluminum alloys are identified by a 4 digit number. The first digit represents the major alloying element, please see the table below;

As explained above, the first digit represents the major alloying elements. However, the last three digits represent the following;

For 1XXX series (Pure Aluminum group):

• The second digit indicates a consecutive modification of an original alloy
• The last two digits indicate the minimum percentage of Aluminium

For alloys in the 2XXX – 9XXX series (Aluminum with major alloying elements):

• The second digit indicates a consecutive modification of an original alloy
• The last two digits have no significance, it only shows that this particular series of metal belongs to the Aluminium alloy group.

1XXX Series:

• This series is often regarded as the pure aluminum group. These alloys have the following characteristics;
• The aluminum content shall be more than 99.0% in this group of alloys
• The ultimate tensile strength for this group of alloys is between 69 MPa to 186 MPa (10 ksi to 27 ksi)
• They have superior corrosion resistance and hence suitable for specialized chemical tanks and pipings
• Good electrical conductivity hence suitable for bus bar applications
• Comparatively poor mechanical properties hence not suitable for structural applications
• These alloys are not heat treatable
• They have a very narrow melting range but are weldable
• Can be welded with a matching filler metal (1XXX series) or with the filler alloys of 4XXX

Most common alloys in this group are the following;

• 1060 (99.60% minimum aluminum) – Used in chemical process equipment, tanks, and piping.
• 1100 (99.00% minimum aluminum) – Used in architectural and decorative applications, furniture, deep drawn parts, etc.
• 1350 (99.50% minimum aluminum) – Used in electrical conductor wire, bus, and cable

2XXX Series:

The major Alloying element in this group of materials is copper along with aluminum, also called as Aluminium-Copper (Al-Cu) group.

• The amount of copper varies from 0.7% to 6.8%. These alloys have the following characteristics;
• Possesses high strength over a wide range of temperatures hence suitable for aerospace and aircraft applications
• Some of the alloys in the 2XXX series are not considered weldable by arc welding processes because of their susceptibility to hot cracking and stress-corrosion cracking; however, other alloys in this series can be welded by arc processes.
• These alloys are heat treatable
• Can be welded with a matching filler metal (High strength filler of 2XXX series) or with the filler alloys of 4XXX series, which contain silicon (Si) or silicon and copper.

Most common alloys in this group are the following;

• 2014 – Used in structures, structural and hydraulic fittings, hardware, and heavy-duty forgings for aircraft or automotive uses.
• 2017 – Usage similar to 2014 and screw machine parts.
• 2024 – Used in structural, aircraft sheet construction, truck wheel and used for cladding for good corrosion resistance
• 2036 – Used in the automotive body sheet.
• 2090 – Used in Structural work, high strength, and damage tolerant aerospace applications.
• 2218 – Used in pistons, engine cylinders, forging alloy and parts which require good strength and hardness at a higher temperature.
• 2219 – Used in structural work, aerospace tanks

3XXX Series:

The major Alloying element in this group of materials is manganese along with aluminum, also called as aluminium-manganese (Al-Mn) alloys group.

• The amount of manganese ranges from 0.05% to 1.8%. These alloys have the following characteristics;
• The ultimate tensile strength for this group of alloys is between 110 MPa to 283 MPa (16 ksi to 41 ksi).
• These alloys are not heat treatable
• Moderate strength, good corrosion resistance, excellent formability, suitable for use at elevated temperatures hence used for manufacturing of heat exchangers.
• Due to moderate, not suitable for structural applications
• Can be welded with filler alloys from the 1XXX, 4XXX, and 5XXX series, depending on their chemical compositions

Most common alloys in this group are the following;

• 3003 – Used in Process and food handling equipment, chemical, and petroleum drums and tanks, and general-purpose applications where slightly higher strength than 1100 is required.
• 3004 – Used in sheet metal which requires higher strength than 3003.

4XXX Series:

The major Alloying element in this group of materials is silicon along with aluminum, also called as aluminum-silicon (Al-Si) alloys group.

The amount of silicon varies from 0.6% to 21.5%. These alloys have the following characteristics;

• The ultimate tensile strength for this group of alloys is between 172 MPa to 379 MPa (25 ksi to 55 ksi)
• This series of alloys are mainly used as filler metals for both fusion welding and brazing

5XXX Series:

The major Alloying element in this group of materials is magnesium along with aluminum, also called as aluminum-magnesium (Al-Mg) alloys group.

• The amount of magnesium varies from 0.2% to 6.2%. These alloys have the following characteristics.
• The ultimate tensile strength for this group of alloys is between 124 MPa to 352 MPa (18 ksi to 51 ksi)
• They have the highest strength among the not heat treatable alloys
• This series of alloys have good weldability
• These alloys are used in a wide variety of structural applications, such as components of ships, vehicles, bridges and pressure vessels, etc.

Most common alloys in this group are the following;

• 5005 – Used in the electrical conductor and architectural applications
• 5050 – Similar to 3003 and 5005 but stronger. Good finishing qualities
• 5052, 5652 – Used in Sheet metal applications that require higher strength than 5050. Good corrosion resistance, Storage tanks, boats, appliances.
• 5083 – Used in cryogenics structures, unfired pressure vessels, marine components, tanks, railroad cars, drilling rigs.
• 5086 – Used in marine components, tanks, tankers, truck frames.
• 5154, 5254 – Used in unfired pressure vessels and tankers
• 5454 – Used in structural applications and tanks for sustained high-temperature service
• 5456 – Used in structures, tanks, unfired pressure vessels, marine components

6XXX Series:

Major Alloying element in this group of materials is magnesium and silicone along with aluminum, also called as Aluminium-Magnesium-silicon (Al-Mg-Si) alloys group.

• The amount of magnesium and silicone is around 1.0%. These alloys have the following characteristics.
• These alloys are heat treatable
• The ultimate tensile strength for this group of alloys is between 124 MPa to 400 MPa (18 ksi to 58 ksi).
• These alloys can be welded with filler metals from the 4XXX and 5XXX series

Most common alloys in this group are the following;

• 6009 – Used in the automotive body sheet
• 6010 – Used in the automotive body sheet
• 6013 – Used in general structural applications, it has improved strength over 6061
• 6061 – Used in structural work, architectural, automobile, railway, and marine applications, pipe, and pipe fittings. This alloy possesses good strength, corrosion resistance, good formability, and good weldability
• 6063 – Used in Pipe, railings, hardware, and architectural applications
• 6070 – Used in structural applications and piping
• 6101 – Used for electrical conductors
• 6262 – Used for Screw machine products and fittings
• 6351 – Usage similar to 6061
• 6951 – Used in brazing sheet core alloy

7XXX Series:

The major Alloying element in this group of materials is zinc along with aluminum, also known as Aluminum-zinc (Al-Zn) alloys group.

The amount of zinc varies from 0.8% to 12.0%. These alloys have the following characteristics;

• These alloys are among the highest-strength aluminum alloys and very suitable for high-performance applications, such as aircraft and aerospace components.
• The common base metals in this series are 7003 and 7005 and can be welded with the 5XXX series filler metal.

Most common alloys in this group are the following;

• 7004 – Used in truck trailer and railcar extruded shapes
• 7005 – Used in truck trailer and railcar extruded shapes
• 7039 – used in Armor plate
• 7075 – used in High-strength aircraft and other applications; cladding gives good corrosion resistance
• 7079 – used in massive parts for aircraft and allied construction
• 7178 – Used for Aircraft construction, strength slightly higher

Plasma Arc Welding is essentially an extension of Gas Tungsten Arc Welding (GTAW). Like GTAW, a non-consumable electrode is used in Plasma Arc Welding (PAW). However, a different technique is used to deliver the heat for welding in Plasma Arc Welding (PAW).

The welding torch used in Plasma Arc Welding (PAW) contains two nozzles an inner nozzle for orifice gas and an outer nozzle for shielding gas (See Figure 1).

Figure 1

The inner nozzle contains orifice gas which surrounds the electrode. The orifice gas is a neutral gas that gets converted into a plasma state (the fourth state of matter) when an arc is ignited in the chamber. The arc heats the orifice gas to a temperature at which the electrons present in the atoms of orifice gas leave their orbit, due to which, the orifice gas becomes ionized. The ionized gases come out from the orifice of the nozzle as a “plasma jet stream”. Plasma is a good conductor of electricity.

Plasma emanates from the nozzle of the orifice at a temperature of about 16,700°C (30,000°F), creating a narrow, constricted arc pattern that provides excellent directional control and produces a very favorable depth-to-width weld profile.

The outer nozzle contains shielding gas like Gas Tungsten Arc Welding (GTAW). The shielding gas covers the area of arc plasma impingement on the workpiece to avoid contamination of the weld. Shielding gas may be the same as the orifice gas or it may be different from orifice gas.

Some important terms used in Plasma Arc Welding:

Electrode Setback: The distance between the tip of the electrode and the face of the constricting nozzle is known as electrode setback (see Figure 1).

Torch Standoff Distance: The distance between the outer face of the constricting nozzle and the workpiece is known as the torch standoff distance (see Figure 1).

Plenum or Plenum Chamber: The space between the inside wall of the constricting nozzle and the electrode is known as the plenum or plenum chamber (see Figure 1).

Equipment:

Plasma arc welding can be performed in manual, mechanized, or robotic operations. However, for manual plasma arc welding following items are used:

1. A Power Source
2. A Welding Torch
3. A Plasma Control Console,
4. Gases (Orifice and Shielding gas)
5. Torch coolant
6. Other accessories such as remote control for current, gas flow timers and an on-off switch

Power Source: The power source used for plasma arc welding is similar to that used for TIG welding (GTAW). Both GTAW as well as PAW processes use constant-current power sources and a high-frequency source for arc starting.

Welding Torch: A Plasma Arc Welding (PAW) torch has the following features:

• It holds the electrode and allows current to pass through the electrode
• Inner nozzle for the supply of orifice gas or plasma gas
• Outer nozzle for the supply of shielding gas

Plasma Control Console: The crucial control systems of plasma arc welding are encompassed in a plasma console also known as plasma control console or console. The console is generally integrated with the primary power source, but it can also be available as a separate stand-alone unit. A typical plasma control console includes controls for the following;

• Plasma gas flow
• shielding gas flow
• the pilot arc current

Gases (Orifice gas/Shielding gas): Selection of gases for plasma arc welding depends on the following criteria;

• welding position
• joint configuration
• Base metal

Shielding gas is often the same as the orifice gas for many plasma arc welding applications. However, some advantages can be observed when a different gas is used for certain applications.

Orifice Gas: The orifice gas should be inert with respect to the electrode to avoid the rapid deterioration of the electrode. To enhance the electrode life 99.99% pure orifice gas must be used. Flow rates for orifice gases are generally between 0.1 liters per minute (L/min) to 5 L/min. The most commonly used orifice gases are;

• Argon
• Argon – Hydrogen Mixture

Shielding Gas: Generally inert gases are used as shielding gas. However, an active gas can also be used for shielding if it is not considered to adversely affect weld properties. Following gases are used for shielding the weld pool;

• Argon
• Argon – Hydrogen mixture
• Argon-helium mixture
• Carbon Dioxide

Flow rates for shielding gases are usually in the range of 5 L/min to 15 L/min for low-current applications. For high-current welding, flow rates of 15 L/min to 32 L/min are used.

Coolant System: Plasma Arc Welding requires a cooling system. A cooling system should consist of a coolant reservoir, radiator, pump, flow sensor, and control switches. Corrosion-resistant materials are used for the construction of the liquid-contacting surfaces.

Electrodes: Like GTAW, tungsten electrodes are used in Plasma Arc Welding (PAW). Tungsten electrodes with small additions of thorium, lanthanum, or cerium can be used for PAW with straight polarity (DCEN). Pure tungsten and zirconiated electrodes are seldom used in plasma arc welding because the electrode tip geometry cannot be maintained. To learn more about tungsten electrodes, Please click here.

Filler Metal: The filler metal is added externally (if required). In the case of manual welding, filler metal in form of rods is used. Whereas, filler metal in wire form is used for mechanized or robotic welding. Filler metal specification is the same as that used in gas Tungsten Arc Welding (GTAW). To learn more about tungsten electrodes, Please click here.

ARC MODES:

Two types of arc modes are used in plasma arc welding, these are;

• Transferred Arc Mode
• Non-Transferred Arc Mode.

In transferred arc mode, the electrode is connected to one terminal of the power source (Generally with Negative polarity) and the workpiece is connected with the other terminal (Positive terminal). Hence the workpiece becomes a part of the electrical circuit (the nozzle remains intact), and heat is obtained from the anode spot on the workpiece and the plasma jet.

In the non-transferred arc mode, the electrode is connected to one terminal of the power source (generally with negative polarity) and the nozzle is connected with the other terminal (positive terminal). Hence, the arc is established and maintained between the electrode and the constricting orifice. The workpiece remains out of the electrical arc circuit. non-transferred arc mode is suitable for cutting and joining nonconductive materials.

Advantages of Plasma Arc Welding (PAW):

1. The constricted opening yields high heat concentration in a smaller area
2. It gives deeper penetration and produces a sound weld
3. Less current input as compared to another welding process
4. The distance between torch and workpiece (standoff distance) does not affect the arc formation
5. Can achieve faster travel speeds
6. Less heat-affected zone (HAZ) if compared to GTAW (Gas tungsten arc welding)
7. It is more stable and does not gets deflected from the base metal

Limitations of Plasma Arc Welding (PAW):

1. PAW equipment are relatively costly, hence start-up costs are high
2. The safety concern is high due to harmful radiations
3. PAW is a very noisy process
4. A highly skilled operator is required

• Air Acetylene welding (AAW)
• Oxy Hydrogen welding (OHW)
• Pressure gas welding (PGW)

The required heat for welding is generated by a flame caused by the chemical reaction between oxygen and the fuel gas (Acetylene). Fuel gas and Oxygen are combined in a mixing chamber, provided in the welding torch itself. Additional filler metal can be used with this process. A flux may be used to protect the molten weld pool. Flux deoxidizes and cleanses the weld metal. The flux melts, solidifies, and forms a slag on the weld metal. A typical Oxy-Acetylene welding (OAW) setup contains the following basic items (Figure – 1):

1. Oxygen Cylinder
2. Acetylene cylinder
3. Hose pipes
4. Gas torch
5. Filler Metal (Optional)
6. Flux (Optional)
7. Safety valves (Flashback arrestor/Non returning valve or Check valve)

Three different types of flames can be obtained (Figure – 2), depending upon the ratio of Oxygen and Acetylene, these flames are known as;

1. Neutral flame

2. Reducing flame

3. Oxidizing flame

Neutral Flame: When oxygen and Acetylene are mixed in equal proportions, Neutral flame is obtained. This type of flame is characterized by a short inner cone and a longer outer cone.

In neutral flame combustion takes place in two stages, Primary combustion takes place at the inner core, when the Oxygen (O2) and Acetylene (C2H2) meets. The heat produced by this reaction accounts for two thirds of the total heat generated. Following chemical reaction takes place at this stage:

2O2 + 2C2H2  –  4CO + 2H2

Secondary combustion takes place at the outer envelope. In secondary combustion CO and H2, obtained from primary combustion reacts with surrounding air (O2) and forms CO2 and H2O. One third of total heat is generated during this combustion. Chemical formula for this reaction is as follows:

4CO  +  2O2  –  4CO2

2H2  +  O2  –  2H20

CO and H2 present in the Outer envelope consume the additional oxygen coming from surrounding and hence the molten weld pool remains protected and Oxidation doesn’t take place. This is why outer envelope is also called as the protection envelop. Neutral flame is used to weld most of the metals.

Reducing Flame: In Reducing flame, excess acetylene is used. Due to excess amount, the combustion of acetylene remains incomplete. This flame is characterized by a greenish Acetylene feather between the inert cone and the outer envelope. This excess Acetylene makes this flame reducing in nature and it’s suitable for welding aluminium alloys because aluminium oxidizes easily. It is also good for welding high-carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal.

Oxidizing Flame: When excess amount of Oxygen is used then this type of flame is occurred. Due to the presence of unconsumed oxygen, the flame becomes oxidizing. This type of flame is characterized by a short white inner cone. This flame is suitable for welding brass, because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool.

Safety Valves: In Oxyfuel welding, chances of reverse flow of flame or gas into supply line (or even into the cylinder) is very high which may cause a flashback, fire, or explosion in any part of the apparatus. To prevent such reverse flow of flame a flashback arrestor and Reverse-flow check valves should be located at the torch inlet. An additional check valve may be used at the regulator outlet.

The purpose of a check valve is to help prevent the reverse flow of gases into the hose, regulator, or cylinder. A flashback arrestor at the torch inlet offers additional protection to the welder and the hosepipe.

ADVANTAGES OF OXY-FUEL GAS WELDING:

1. Equipment is cheaper than other welding processes, easy to learn and use
2. It’s very portable and can be transported anywhere very easily
3. Since no electricity is required, hence can be used at locations where power sources are not available.
4. The equipment is very versatile and can also be used for metal cutting, preheating, postheating and surfacing.
5. The welder can control the heat input, temperature, weld bead size and shape very efficiently.
6. Very useful for maintenance related work

DISADVANTAGES OF OXY-FUEL GAS WELDING:

1. Slow welding if compared to other arc welding processes, hence less productive
2. Not good for reactive metals and thick metals
3. Large heat affected zone (HAZ)
4. Due to the presence of combustible gas (Acetylene or Hydrogen), lots of Safety precautions needed because these gases are highly flammable and can explode if catches fire.

Also read Submerged Arc Welding (SAW)

Also read Gas Tungsten Arc Welding (GTAW/TIG)

Also read TIG welding (GTAW) Electrodes

Also read Flux Cored Arc Welding

Also read Types of weld joint

Also read How to write a welding procedure specification (WPS)

Submerged arc welding is typically used to weld flat position, however horizontal position welding can also be achieved by using proper tooling and fixtures. This process is highly suitable for automated operation. It is used extensively in pressure vessel fabrication, pipe manufacturing, ship and barge building, railroad car fabrication and the fabrication of structural members where long welds are required.

Fundamentals of SAW:

Initially, the bare electrode (filler wire) is inserted into a heap of flux that covers the joint to be welded. Then an arc is initiated and a wire feeding mechanism begins to feed the electrode (filler wire) in the direction of the joint at a predefined rate. The feeder can be moved manually or the entire system can be automated. In automated welding, the workpiece moves under a stationary wire feeder or the welding head is moved over the stationary workpiece. Additional flux is continuously fed around the electrodes and gets evenly distributed over the weld joint.

The heat generated during welding, melts some of the flux. These liquid flux floats over the molten metal and completely shields the molten weld pool from the atmosphere.

Following factors should be considered before choosing submerged arc welding process for a particular application:

• Chemical composition and mechanical properties required of the final weld deposit,
• Frequency or volume of welding to be performed
• Accessibility of the joint and the position in which the weld is to be made
• Thickness of base metal and alloy to be welded,
• Length of the joint to be welded
• Cost and profitability of the project

(Figure – 1 illustrates the basic submerged arc welding process)

The basic components of a SAW processes are:

• Power source
• Welding head
• Flux delivery system (Flux hopper)
• Electrode
• Flux

Power Source: The power source for a Submerged arc welding process is of great significance and should be able to produce high current at high duty cycle. Both alternating current power sources as well as direct current power sources can be used to supply power for this welding process.

Welding Head: A typical welding head consists of the following:

• The gun assembly and contact tip
• Wire feed motor and feed-roll assembly
• Accessories for mounting and positioning the head

Flux delivery system (Flux hopper): The flux is stored in a hopper and keeps on feeding sufficient amount of flux on the joint. A nozzle is usually mounted on the welding head for the delivery of flux to the weld joint.

Flux: Role of flux is very important in this process, because of the following three reasons:

• Stability of arc depends on the flux
• Mechanical and chemical properties of the deposited weld metal can be controlled by the flux
• The control and handling of the flux impacts the soundness of weld joint

The flux becomes conductive, flow of current takes place between the electrode and the work, During molten state. Flux consists of Silica, Lime, Manganese Oxide, Calcium fluoride and other compounds.

Electrode: Bare solid wire or strip is commonly used as electrode of submerged arc welding. However composite metal cored electrodes (similar to flux cored arc welding electrodes) can also be used. Following base metals can be welded with submerged arc welding process:

• Carbon steels
• Low-alloy steels
• Stainless steels
• Nickel-based alloys
• Chromium-molybdenum steels

Advantages of SAW:

1. Excellent weld quality and high productivity
2. high deposition rates
3. controllable penetration (deep or shallow)
4. adaptability to automated operation
5. Most of the flux can be reused
6. Very less fumes

Limitations of SAW:

1. Can be used in only flat and horizontal positions
2. Requires post weld slag removal
3. Limited to materials that are long and straight or are rotated pipe
4. Residuals from the flux possess a great risk to health and safety

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Submerged Arc Welding Practical Video;

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Also read: Flux Cored Arc Welding

Also read: Types of weld joint

Also read: TIG welding (GTAW)

Flux Cored Arc Welding are mainly of two types:

• Self-Shielded (FCAW – S)
• Gas Shielded (FCAW – G)

In case of FCAW – S, flux present at the core of the filler metal, acts as shielding agent and protects the molten weld pool. Whereas, in FCAW – G, shielding gas is also used along with the flux (in the filler metal) hence FCAW – G is a dual shielded welding (Primary shielding – Flux, Secondary shielding – gas), which adds greater flexibility to the process.

FCAW process is very much similar to GMAW, Both the welding processes use a continuous feeding filler metal (electrode). Basic difference between the two welding processes is the composition of their filler metal. A solid wire is used as a filler metal (electrode) in GMAW. Whereas, a tubular or hollow filler metal with flux at the centre is used in FCAW (Figure 1). Second difference is the shielding gas, GMAW requires a shielding gas to protect the molten weld pool. Whereas, in FCAW shielding gas is optional due to the presence of primary shielding by flux at the core of the filler metal (electrode). Power source used for FCAW and GMAW is same.

Flux cored arc welding is capable of producing high quality weld at a high deposition rate. It’s a semiautomatic process and can be automated too. FCAW is a very efficient welding process and high-integrity welds can be achieved with less effort and lower cost, that’s why industries are preferring this welding process.

The Equipment needed for FCAW set up are (Figure 2):

1. Power Source
2. Filler metal feeding Mechanism
3. Welding Gun
4. Shielding Gas
5. Welding cables / leads and gas hoses

Power Source: The power source used for FCAW is Direct Current (DC) constant voltage type, similar to power sources for GMAW. There should be a voltage regulator, capable of adjusting the volts at an increment of one volts or less.

Filler metal Feeding Mechanism: Feeding mechanism supply the filler metal at a constant speed (pre-set) to the welding gun from filler wire spool. Feeding mechanism should be such that filler metal doesn’t get damaged or distorted during rolling.

Welding Gun: Welding gun facilitates and controls the filler metal as well as provides smooth passage for the shielding gas. Electrode is controlled by a switch, which is mounted on the welding gun. Welding guns are either Gas (or air) cooled and water cooled.

Welding cables / leads and gas hoses: Welding cables are used to connect the welding gun with power source and work piece with the power source. Shielding gas is supplied through a hose pipe.

Advantages of FCAW:

1. High deposition rate and higher productivity is one of the main advantage of the FCAW
2. Suitable for all positions
3. Can be used for a variety of materials
4. Consistent welding with lesser defect
5. Yields high quality weld with less effort and lower cost.
6. Provides excellent penetration (especially for thick materials)
7. Excellent weld appearance
8. Less distortion compared to SMAW

Limitations of FCAW:

1. FCAW process is limited to ferrous metals and Nickel based alloy welding.
2. Produces a slag cover on the weld metal which must be removed
3. Equipment is costlier and complex compared to SMAW
4. Produces more smoke and fumes than GMAW and SAW
5. Higher filler metal cost than solid wire used in GMAW

Also read: Submerged Arc Welding (SAW)

Also read: Gas Tungesten Arc Welding (TIG)

Also read: Welding Symbols

Also read: Welding Electrodes

Also read: Oxy-Fuel Welding (Oxy-Acetylene/Gas Welding)

In TIG welding, the arc is generated between the tungsten electrode and the workpiece to produce the necessary heat required for the welding. Shielding gas is also required, in TIG welding, for the protection of the molten weld pool from atmospheric contamination. Inert gases such as Argon or Helium can be used as shielding gas. Additional filler metal may be added in this process.

The tungsten (or tungsten alloy) electrode is held in a torch. Shielding gas is fed through the torch to provide an inert atmosphere that protects the electrode and the molten weld pool. An electric current flows from the tip of the electrode to the workpiece through conductive ionized shielding gas and the arc is generated. Additional filler metal may also be added to the molten weld pool to fill the joint. The TIG welding process is illustrated in the following figure:

Welding current:

TIG welding can be used with direct current (DC) as well as alternating current (AC), Choice of the current largely depends upon the metal to be welded. Direct current with electrode negative (DCEN) also known as “straight polarity” is commonly used in GTAW since it facilitates deeper penetration and fast welding speed. Direct current with electrode positive (DCEP) also known as “reverse polarity” is avoided in GTAW because it causes overheating of electrodes.

Components of GTAW:

The basic components of the TIG Welding process are:

1. Power source

2. Electrode

3. Welding Torch

4. Shielding Gas

1. Power Source: A constant current power source is the most common and appropriate power source for the TIG welding process. A transformer-rectifier power source is used to supply the required power.

2. Electrode: Either pure tungsten or tungsten alloys are used as GTAW electrodes. The reason for using tungsten as an electrode is its high melting point (3410⁰C or 6170⁰F). In TIG welding, the electrode acts as an electrical terminal and generates the required heat for melting the base metal, unlike the other conventional welding process such as SMAW/GMAW the electrode doesn’t get consumed during the welding. Additional filler metal is fed if required.

The TIG welding electrodes come in a variety of sizes and lengths. Classification of tungsten electrodes is done based on chemical composition. These electrodes are color-coded to distinguish them from others. The color appears at the tip of the electrode. Different types of TIG welding electrodes are as follows;

1. Pure Tungsten (Color Code: Green)
2. Thoriated (Color Code: Red)
3. Ceriated (Color Code: Orange)
4. Lanthanated (Color Code: Gold)
5. Zirconiated (Color Code: Brown)
6. Rare Earth (Color Code: Gray)

To know more about color coding and its uses of tungsten electrodes please click here.

3. Welding Torch: The GTAW welding torch holds the tungsten electrode and provides the means to convey shielding gas to the molten weld pool through a nozzle. A clamping device called the collet is provided in the torch for holding the tungsten electrode. Collets are generally made up of copper alloys. Nozzles are provided onto the head of the torch to provide direction for the shielding gas flow. Nozzles are generally made up of ceramic materials.

4. Shielding Gas: Shielding gas is supplied through the nozzle of the torch to prevent the atmospheric contamination of the molten weld pool. Inert gases such as Argon or Helium or a combination of both can be used as shielding gas.

Argon provides a smoother and quieter arc with reduced penetration, apart from this, the cost of argon is less as compared to Helium. Due to its reduced penetration Argon is used for the joining of thinner metals. On the other hand, Helium provides higher heat penetration hence useful for thicker materials and materials with high heat conductivity such as Aluminium and Copper.

Advantages of TIG Welding:

1. TIG welding produces high-quality weld, most suitable for industries requiring precision weldings such as aerospace industries and nuclear industries.

2. No flux is required due to the presence of shielding gas.

3. Produces spatter-free joints.

4. Welding can be made in all positions.

5. Dissimilar metals may also be welded with this process.

6. TIG welding process can easily be automated.

7. Excellent root pass weld penetration can be obtained with TIG welding.

Limitations of TIG Welding:

1. Lower deposition rate as compared to other consumable arc welding processes.

2. Slightly more skilled welders required as compared to Shielded metal arc welding (SMAW) or gas metal arc welding (GMAW).

3. Very low tolerance for contaminants on filler or base metals.

4. TIG welding is quite expensive than other conventional welding processes such as Shielded Metal Arc Welding (SMAW) or Gas Metal Arc Welding (GMAW).

5. Shielding of the molten weld pool is quite difficult during windy and drafty environments.

Also read: MIG Welding

Also read: Types of Welding

Also read: Plasma Arc Welding (PAW)

Also read: Flux Cored Arc Welding (FCAW)

Also read: Submerged Arc Welding (SAW)

Please watch the following video for a better understanding of GTAW/TIG Welding:

Whenever Carbon dioxide is used as a shielding gas in Gas Metal Arc Welding (GMAW). The term MAG (Metal Active Gas) welding is used because Carbon dioxide is an active gas. Whereas, other shielding gases such as Argon and Helium are inert in nature. Hence, whenever these gases are used as shielding gas, the term MIG (Metal Inert Gas) welding is used

Selection of shielding gas should be done with lots of care. A mixture of CO2 and Argon is considered a good combination. 75% Argon + 25% CO2 (very popularly known as “75/25” or “C25”) works as the best “all-purpose” shielding gas for carbon steel. It results in the least amount of spatter and proper weld bead.

100% carbon dioxide yields deeper penetration, but extra caution is needed since it may increase the amount of spatters.

The equipment needed for GMAW set up are (Figure 1):

1. Power Source
2. Filler metal feeding Mechanism
3. Welding Gun
4. Shielding Gas
5. Welding cables or leads and gas hose pipes

Figure – 1

In MIG welding, Voltage, Current, and wire feed speed must be controlled for producing sound welding.

Voltage controls the weld profile. Whereas, Current controls the heat input, size of weld, and depth of penetration.

The following table (Table – 1) gives the relationship between the current range and the filler wire feed speed, based on the filler wire diameter. This table is just a guide and the ranges may vary depending upon the shielding gas and parent metal.

Table – 1

Mig welding wire speed and Current chart:

The following figure (figure – 2) shows a GMAW welding gun:

Figure – 2

Advantages of GMAW (MIG Welding)

1. GMAW can be used to weld all commercial metals and alloys
2. No restriction of limited electrode length as we face in shielded metal arc welding (SMAW).
3. Due to the gas shielding, no additional flux is required for the protection of the molten pool
4. It can be used to weld in all positions.
5. Higher deposition rates than shielded metal arc welding (SMAW)
6. Due to the continuous electrode feeding mechanism, we can achieve higher welding speeds and higher filler metal deposition rates than shielded metal arc welding (SMAW)
7. Due to the continuous electrode (wire feed) feed, longer welds can be deposited without intermediate stops and starts;
8. Very less post-weld cleaning is required due to the absence of any heavy slag;
9. Comparatively easy to learn and less skilled welders can also do the MIG welding.
10. This process can be easily automated

Limitations of GMAW (MIG Welding)

1. The GMAW equipment is more complex, costlier, and less portable than that for shielded metal arc welding (SMAW)
2. Due to the absence of flux covering, chances of faster cooling rates exists which may affect badly
3. Shielding of the molten weld pool is quite difficult during windy and drafty environments. High chances of porosity in the weld metal during windy environments.
4. MIG welding requires moderate cleaning of joints before welding

Please watch this video lecture for a better understanding of MIG welding:

The basic components of SMAW (Figure – 1) are the following;

• Power Source (Figure – 2)
• Electrode Holder (Figure – 3) 
• Electrodes (Figure – 4)
• Cables/Lead

Please see the below schematic diagram (Figure – 1) of SMAW for a better understanding.

(Figure – 1)

In SMAW, an electric arc is used to produce the necessary heat to melt the base metal, this arc is generated by striking the electrode with the workpiece. Soon after the generation of arc, the electrode is withdrawn from the workpiece but it remains in close contact with the workpiece. The gases present between the gap (i.e the gap between electrode and workpiece) get ionized and a smooth flow of electrons takes place hence, despite the gap between the electrode and the workpiece the circuit remains closed (or energized), and hence arc doesn’t get extinguished.

A power source for SMAW welding is shown in figure – 2.

(Figure – 2)

The output of the power source can be either AC (alternating current) or DC (Direct current). In AC the polarity of the terminals keeps on changing (almost 100 times in a second) but in DC one terminal always acts as a Positive terminal and the other acts as a negative terminal.

If the electrode is connected with the positive terminal and the workpiece with the negative terminal then it is known as DCEP (i.e. direct current electrode positive or Reverse Polarity). Whereas if the electrode is connected with the negative terminal and the workpiece with positive, then it is known as direct current electrode negative (ie. DCEN or Straight Polarity).

In DCEP, the electricity flows into the tip of the electrode and concentrates about two-thirds of the heat, which gives good penetration. Hence it is usually used on thicker steels.

In DCEN, the electricity flows out of the rod, concentrating about one-third of the heat on the electrode. Hence the penetration is less, this a very good choice for thinner steels.

An electrode holder is shown in figure – 3.

(Figure – 3)

The electrodes of Shielded metal arc welding are always covered with flux, Purpose of flux is as follows;

• During welding, the flux material decomposes and produces fumes. These fumes shield the molten weld pool from the atmosphere. In the absence of those fumes, molten weld pool will be exposed to the atmosphere and may react with oxygen present in the environment and thus oxidation may take place, which is very dangerous and may lead to failure. (i.e prevents oxidation of molten weld pool).
• The density of flux material is less than the weld material hence the flux comes upwards and floats on the weld pool and thus protects the weld pool. During the solidification of the molten weld pool, flux also solidifies and forms a thin layer on the surface of the weld called slag. This thin layer of slag helps in the slow cooling of weld material. In the absence of this thin layer of slag, rapid cooling of the molten weld pool will occur, which is very dangerous because rapid cooling results in microstructural changes of weld metal (martensite formation).
• Flux also helps in the initiation of arc. Especially when welding is done in AC (Alternating Current), polarity keeps on changing at a constant rate, i.e. it attains a positive value then a negative value and it happens within a fraction of a second. Polarity changes more than 100 times in a second. While going from positive to negative it attains a value ‘0′ and whenever it goes to zero, the arc has to be re-initiated and the flux plays its role. It happens very fast (i.e. within a fraction of seconds) hence we can not see this with our eyes.

SMAW electrodes are shown in figure – 4.

(Figure – 4)

The typical current (Amperage) range for shielded metal arc welding (SMAW) electrodes is given below (Table 1);

Table 1

Advantages of SMAW:

1. This process is suitable for most of the commercially available metals and alloys.

2. The equipment is comparatively inexpensive and portable.

3. Equipment is relatively simple.

4. This process can be used in all welding positions.

5. This welding process is flexible and can be applied to a variety of joint configurations and positions.

6. No need for separate gas shielding.

7. Less sensitive to wind and drafts if compared to gas shielded arc welding processes.

8. It can be used in areas where access is limited (electrodes can be bent and even a mirror can be used in tight spaces).

Disadvantages of SMAW:

1. Low deposition rate than GMAW and FCAW

2. More skilled welding operators are required than many other welding processors.

3. Not suitable for reactive metals such as Titanium, Zirconium, Tantalum, and Niobium.

4. Not suitable for metals with low melting temperature such as Lead, Tin, and Zinc and their alloys.

5. This process cannot be automated.

6. More slag due to flux shielded electrode.

7. Repeated changing of consumed electrodes with a new one makes this process quite slow if compared to GMAW.

Please watch the following videos for a better understanding of Shielded Metal Arc Welding. In the first video, you will see the practical setup and the second video is about the theory behind SMAW.

Please leave your valuable comments.

Thank you

The American Welding Society (AWS) has recognized more than 50 different types of welding processes, some of them are very popular among the industries such as Shielded metal arc welding (SMAW) or stick welding, Gas metal arc welding (GMAW) also known as MIG/MAG welding process, Gas tungsten arc welding (GTAW) or TIG Welding, Submerged arc welding (SAW) and Flux-cored arc welding (FCAW), etc. Whereas, some are very specific and limited to a few industries such as Ultrasonic welding (USW) and Diffusion welding (DFW), etc.

Welding processes are broadly divided into two types;

• Pressure Welding Process

• Fusion Welding Process:

In the pressure welding process, joining is done by the application of external pressure or force at the area of contact, which causes more or less plastic deformation of both the contact surfaces. The facing surfaces are heated to some extent in order to permit or to facilitate permanent bonding.

Usually, the heat used during this welding process is much less than the melting point of the base metal. Most of the time additional filler metal is not required during these types of welding techniques. Resistance welding and Diffusion welding are the examples of the pressure welding process.

Whereas, In the Fusion welding process, joining is done by melting the base metals in the area of contact. No external pressure or force is required during fusion welding processes and very often filler metal is also used.  The arc welding process is one of the most popular welding processes, which belong to the family of the fusion welding process. Commonly used fusion welding processes are;

1. Arc Welding

• SMAW (Shielded Metal Arc Welding or Stick Welding)
• GMAW (Gas Metal Arc Welding or MIG welding)
• GTAW (Gas Tungsten Arc Welding or TIG Welding)
• SAW (Submerged Arc Welding)
• FCAW (Flux Cored ARC Welding)
• PAW (Plasma Arc Welding)
• Carbon Arc Welding 
• Stud Welding

2. Oxyfuel Gas Welding

3. Electron beam welding

4. Laser beam welding

5. Thermit Welding

6. Electroslag Welding

Some of the pressure welding processes are:

1. Resistance Welding

• Spot Welding
• Seam Welding
• Projection Welding
• Flash Welding
• Upset Welding
• Percussion Welding

2. Diffusion welding (DFW)

3. Friction welding (FRW)

4. Ultrasonic welding (USW)

5. Cold Pressure Welding

6. Forge welding

7. Explosion welding (EXW)

This is how the welding processes are classified to read more about these welding processes please click on the link given below;

• SMAW (Shielded Metal Arc Welding or Stick Welding)
• GMAW (Gas Metal Arc Welding or MIG welding)
• GTAW (Gas Tungsten Arc Welding or TIG Welding)
• SAW (Submerged Arc Welding)
• FCAW (Flux Cored ARC Welding)
• PAW (Plasma Arc Welding)
• Oxyfuel Gas Welding

For theoretical and practical lectures on welding please click the link below;