Welding Technology or Techniques

Welding technology or techniques

Arc welding process 

Welding is a process of joining two or more pieces of metal by producing a localized union through fusion. The mandatory requirements for the union through fusion are:

• Clean the metallic surface
• Heat for raising the temperature of the local area to the melting point.
• The two metal to be joint do not have wide differential properties, such as aluminum and steel

Other optional requirements are:

• Filler metal - the purpose is to fill up the gap and this is not used in all fusion welding processes,e.g. no filler is used in resistance welding
• Pressure - this is used to bring the two surfaces close to each other.

In solid-state welding, two technique is employed.

• Deformation welding 
• Diffusion welding

Deformation welding:-In this, two surfaces are brought close to very contact by applying high pressure which also deforms them. the surface temperature may or may not be very high e.g. Forge welding, Roll welding, Extrusion welding.

Diffusion welding:-Diffusion welding takes place by atomic diffusion between the two surfaces. The surfaces are usually heated to high temperature and pressure may be employed.

Brazing:-Brazing is also a process of joining of two metals. In this process, the two surfaces are to be joint not melted. Molten filler metal is used to fill up the gap. In than 450 deg. Celsius but less than that of either base metal. The filler metal is distributed between closed fit surfaces of joint by capillary action. e.g. by use of EutectRod 1801 to join copper to steel. Copper-based, silver-based alloys are most commonly used in brazing as filler material.

Braze welding:- It is a joining process like brazing where the filler metal melts but the base metal does not.The difference is, in braze welding, the filler metal is deposited in a groove and capillary action does not a factor.

Soldering:- Soldering is a process in which two metal pieces are joined together by melting or putting a filler metal into the joints, the filler metal having a lower melting point than the adjoining metal. In soldering the melting point of filler alloy is lesser than brazing, in soldering is less than 450 deg Celsius.  

Fusion Welding (Electrical Energy)

Several processes are in the process by which fusion welding is carried out. Since the primary factor for welding is the source of heat by which the welding process is differentiated.

Processes may be:-

• MMAW (Manual Metal Arc Welding) or SMAW (Shield Metal Arc Welding)
• GTAW (Gas Tungsten Arc Welding) or TIG (Tungsten Inert Gas)
• GMAW (Gas Metal Arc Welding) or MIG (Metal Inert Gas)
• SAW (Submerged Arc Welding)
• FCAW (Flux Cored Arc Welding)
• Gas Welding

MMAW:-

Shield or Manual metal arc welding is a process, in which the heat for welding is generated by an electric arc, established between a flux covered consumable electrode and the workpiece. The electrode rod is made of a material that is compatible with the base material being welded and covered with a flux that gives off vapors that serve as a shielding gas, and provides a layer of slag both of which the protect the weld area by atmospheric contamination.

                                                                                                                                                                       

Advantages:-

• This is the simplest of all arc welding process.
• Portable
• Cost economical
• A big range of metals and their alloys can be welded.
• Welding can be carried out in any position.
• This process founds countless application.

More about MMAW Welding

                                               

GTAW (Gas Tungsten Arc Welding) or TIG (Tungsten Inert Gas) -     

TIG is also an arc welding process in which a non-consumable tungsten electrode is used to produced weld. The weld area is protected from atmospheric contamination by inert shielding gas such as helium and argon, and filler metal is fed separately to weld. TIG is commonly used to weld a thin section of stainless steel and non-ferrous metal such as aluminum, magnesium, and copper alloy.

It is a relatively difficult welding method. 

GTAW process normally requires two hands, since most application requires that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in other. Maintaining a short arc length while preventing contact between electrode and workpiece, is also important.

Once the arc is struck, welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. while maintaining a constant separation between the electrode and workpiece, the operator then moves the torch back slightly and tilts it backward about 10-15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.

GMAW OR MIG (Metal inert gas welding):

This process is known as MAG (Metal active gas welding).

GMAW is an arc welding process in which an electric arc forms between a consumable wire electrode and the workpiece.
Along with the wire a shielding gas also feeds through the welding torch to protect the weld pool from atmospheric contamination.
This process can be automatic or semi-automatic.
MIG welding can be used to weld steels, aluminum alloys, copper alloys(not with high zinc), magnesium alloys. It is suitable for position welding but works best in a down hand position.
The deposit efficiency is much higher than in manual arc welding with a very small amount of slag.
MIG Welding, however, requires a more expensive power source than MMAW. Normally a constant voltage power source is used. The equipment is also less portable.
Since the welding gun has to be close to the job, the process is less adaptable than SMAW in difficult to reach access.

SAW (Submerged Arc Welding):-

This is an arc welding process in which the heat for welding is supplied by an arc developed between a consumable solid wire or flux-cored wire and the job. The arc is shielded by a layer of granular fusible flux which covers the molten weld metal and the base metal near the joint. The arc is submerged influx is not seen easily from outside.

Submerged Arc Welding is either used in semi-automatic or fully automatic mode.
The process gives very high deposition rates (up to 40 kg/hr.) and can be used for welding very thick sections. It is used for both joining and overlaying. Due to the high current employed, the heat input is large and the flux cover gives a slow cooling rate. The combination of high heat input and low cooling rate makes this process more suitable for high hardenability metals like Ni-Hard than manual arc welding. As the flux is deposited in the granular form prior to arcing, SAW is suitable only for down hand welding.
SAW is also not suitable for welding plates less than 5mm in thickness as it might result in burn through.

FCAW  (Flux-cored arc welding):-

In this process, welding is done by producing an arc between the tubular electrode and base metal.
In the FCAW where auxiliary shielding gases are used, shielding gases are normally CO or a mixture of argon and CO are used. The process becomes similar to GMAW (MIG) except that the electrode is tubular and not solid. Because of the flux in the electrode, more slag is present in FCAW as compared to GMAW but less than in SMAW.
FCAW with shielding gases gives radiography quality welds and can also be used for position welding if used with a small diameter electrode. The self-shielding FCAW requires a simpler welding torch and does not need a consumable inert gas. Since it is self-shielded, so it can be used in the outdoor process also. The quality weld made by this process is slightly inferior to that made by FCAW with auxiliary shielding gases like CO or Argon. That's why this process is not used for radiography welding quality.
FCAW process is faster than manual arc welding. Also the deposit efficiency is also much higher. One advantage of FCAW over GMAW is that Terocote deposit with high hardness etc. can be had with FCAW process. This cannot be achieved with a solid wire bare electrode. 

MIG

GMAW OR MIG (Metal inert gas welding):

Sometimes this process is refers as MAG (Metal active gas welding).

GMAW is a arc welding process in which an electric arc forms between a consumable wire electrode and the work piece.

Along with the wire a shielding gas also feed through the welding torch to protect the weld tool from atmospheric contamination.

This process can be automatic or semi-automatic.

MIG welding can be used to weld steels, aluminium alloys, copper alloys(not with high zinc), magnesium alloys. It is suitable for position welding but works best in downhand position.

The deposit efficiency is much higher than in manual arc welding with a very small amount of slag.

MIG Welding however requires a more expensive power source than MMAW. Normally constant voltage power source is used. The equipment is also less portable.

Since the welding gun has to be close to the job, the process is less adaptable than SMAW in difficult to reach access.
Shielded gases plays a vital role in MIG Welding, it prevents the welding bead from atmospheric gases e.g. nitrogen and oxygen, which may causes porosity, fusion defects, and weld metal embrittlement if they come in contact during welding.

Required Equipment for MIG:

• Power source
• Electrode
• Shielding gases
• Welding gun Or wire feeder unit 

These are the basic requirements, except this operator should take acre of necessary PPE`s.

From quality point of view two most common quality issue are there First is DROSS and second is POROSITY. During welding process if they are not in control of operator then they lead weaker weld, less ductile, and slag inclusion. In order to prevent weld bead should be clean by wire brush and perfect control on gap.

Second is porosity because of gas entrapment in weld pool, which occurs when the metal solidify before gases escape from there. It may because of impurities in shielding gases or work piece or arc length variation.
And the cooling rate is related to gases entrapped.

TIG

TIG Welding (Tungsten Inert Gas Welding)

 TIG is technically stands for GAS Tungsten Inert Gas Welding.

TIG is also a type of welding process, which is known as tungsten inert gas welding. In this Process arc is produced between non-consumable tungsten electrode and base metal and simultaneously consumable filler metal filled at the welding spot. Filler metal are consumable TIG filler in the forms of rods in different different diameter.
The weld area is protected is protected from atmospheric contamination by an inert shielding gases such as helium and argon, and filler metal is feeded separately to weld .TIG is commonly used to weld thin section of stainless steel and non ferrous metal such as aluminium, magnesium, and copper alloy.

TIG process requires continuous movement of two hand, first hand to feed the filler metal in weld area and other to manipulate the torch in required direction. The important factor in to be maintain arc length and preventing the contact between electrode and work piece

Once the arc is struck, welder moves the torch in a small circle to create welding pool, size of which depends on size of electrode and amount of current. while maintaining a constant separation between the electrode and workpiece, the operator then moves the torch back slightly and tilts it backward about 10-15 degree from vertical. Filler metal is added manually to the front end of weld pool as it is needed.

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TIG always gives a better result in quality aspect, because it affords better control over the time of welding process thru the welding area. For better quality always it requires a very skill operator, Cleanness, Surfaces & equipment's should free from oil, grease, dirt, impurities and contamination for a better result in the form of welding strength.

Shield gases always plays a vital role in welding process, it covers the welding pool by shield gas that prevent the welding pool from atmospheric contamination such gases are nitrogen and oxygen. Argon is most commonly used gas for GTAW for high quality welding and HELIUM is often used for high penetration.

Submerged Arc Welding (SAW)

Submerged Arc Welding (SAW)

SAW is a type of welding process that means submerged arc welding process. This welding process remains continuous in which Arc is submerged in flux and welding take place.

This is an arc welding process in which the heat for welding is supplied by an arc developed between a consumable solid wire or flux cored wire and the job. The arc is shielded by a layer of granular fusible flux which covers the molten weld metal and the base metal near the joint. The arc being submerged in flux is not seen easily from outside.

Required basic equipments for SAW are:

1. Power Source
2. Wire Feeder
3. Wire Reel
4.  Wire feeding control system
5. Flux Hopper
6. Flux Torch
7. Welding Holder
8. Welding Wire
9. Base Plate

Submerged Arc Welding is either used in semi-automatic or fully automatic mode.

The process gives very high deposition rates (upto 40 kg/hr.) and can be used for welding very thick sections. It is use for both joining and overlaying. Due to high current employed, the heat input is large and the flux cover gives a slow cooling rate. The combination of high heat input and low cooling rate makes this process more suitable for high hardenability metals like Ni-Hard than manual arc welding. As the flux is deposited in granular form prior to arcing, SAW is suitable only for downhand welding. SAW is also not suitable for welding plates less than 5mm in thickness as it might result in burn through.

Every welding process has their own advantage and limitations, If we talk about Submerged arc welding the major advantage of this process is massive buildup. It is suitable for big HRC roller and for that component which required continuous  heavy buildup.

Advantage of Submerged Welding Are:

• Very high deposition rate (40-50 kg/hr).
• No spatter of welding because Arc is covered with flux
• Deep penetration of arc 
• Can be use in indoor and out door activity
• High operation feature in mechanised applications.
• Welding fumes and are light emission is very low.
• 70% or more than 70% flux recoverable.

Limitation:

• Limited to some nickle base alloy and limited to (ferrous) steel and stainless steel.
• Limited to high thickness end sheets.
• Backing strips required for good penetration.
• Limited to Down-hand welding process.

Manual Metal Arc Welding

MMAW (Manual Metal Arc Welding)
or
SMAW(Shield Metal Arc Welding)

Shield or Manual metal arc welding is a process, in which the heat for welding is generated by an electric arc, established between a flux covered consumable electrode and the workpiece. The electrode rod is made of a material that is compatible with the base material being welded and covered with a flux that gives off vapors that serve as a shielding gas and provides a layer of slag both of which the protect the weld area by atmospheric contamination.

Types of welding:-

• Autogenous Welding- It is a kind of welding in which only similar metal parts can be welded. e.g. mild steel to mild steel, brass to brass, copper to copper.  
• Heterogeneous Welding- It is a kind of welding in which only dissimilar parts can be welded. e.g. mild steel to copper, cast iron to mild steel, stainless steel to mild steel, etc.

Advantage of Shielded Metal Arc welding-

1. This is the simplest among all arc welding processes.
2. It is portable.
3. This process is the benchmark for all arc welding processes.
4. Cost economical.
5. This can be used outdoor and indoor.
6. Welding can be carried in any position.
7. This process finds countless applications because of the availability of a wide range of electrode. 
8. A big range of metal and their alloys can be welded.
9. The power cable of SMAW can be extended, that's why it can be used for long-distance and height process.

Application:-

SMAW is used both in fabrication and maintenance repair job. This process finds application in 

• Tank, boilers, and pressure vessel fabrication.
• Shipbuilding
• Pipes and penstock joining
• Building and bridge fabrication
• automotive and aircraft industry etc.

Equipment required for shielded metal arc welding:-

• Arc welding power source 
• Welding cables 
• Electrode holder
• Earth clamp
• Welding electrode
• Welding helmet and hand shield
• Protective coating including hand gloves
• Chipping hammer, wire brush, etc.
• PPE (Personal protective equipment)

Power sources for SMAW:
In SMAW both AC and DC power sources can be used. All of them supply the high current used for welding at a sufficient safe lower voltage. The output requires is either AC or DC. Welding current plays a significant role in deposit quality and deposition rate.
In manual metal arc welding, short circuit voltage can fluctuate due to arc length changes. Hence if a constant voltage power source is used, current will fluctuate with arc gap in manual arc welding.
To overcome this problem a constant current power source is used in manual arc welding. In direct current, the current flows continuously in one direction and hence welding arc is relatively steady and smooth. DC is superior to AC especially while welding with lower currents.

Polarity in DC welding:- 

Polarity indicates the direction of current flow in the welding circuit.

The flow of electrons is always from negative to positive.

Kinds of polarity

1.  Straight polarity
2. Reverse polarity

Straight polarity:- In straight polarity, the electrode is connected to the negative and the work to the positive terminal of the power source.

Reverse polarity:- In reverse polarity, the electrode is connected to the positive and work to the negative terminal of the power source.

Why polarity is important in welding:

In DC welding, 2/3 of the heat is liberated from the positive end and 1/3 from the negative end. To have this advantage of unequal heat distribution in the electrode and base metal, the polarity as an important factor for successful welding.

Straight polarity is used for:

• Welding with bare light coated and medium coated electrodes.
• Welding the thicker sections to obtain more fusion and penetration.

Reverse polarity is used for:

• Welding of non-ferrous metals.
• Welding with heavy and super-heavy coated electrodes.
• Position welding 
• Sheet metal welding

ARC Lenght:-
Arc length is known as the gap or distance between the tip of the electrode and the workpiece during welding.

Types of arc length:

1. Standard arc length
2. Short arc length
3. Long arc length

Advantage of standard arc length:

1. Good weld joint
2. Good penetration
3. Maximum strength
4. Less chance of porosity
5. Metal deposition as per standard
6. Maximum ductility
7. Lessor no oxidation