Welding - Made Easy

WELDING

Welding: It is the process of joining similar dissimilar metals with / without application of heat, with / without application of pressure and with / without addition of filler material.

Weldability: It is the capacity of being welded into

inseparable joints having specified properties such as

definite weld strength, proper structure etc. Weldability

depends on : (1) Melting point (2) Thermal conductivity (3)

Thermal expansion (4) Surface condition (5) Change in

Micro structure etc.

These characteristics may be controlled / corrected by proper shielding atmosphere, proper fluxing material, proper filler material, proper welding procedure, proper heat treatment before and after deposition.

Metallurgically there are 3 distinct zones in a welded part namely. In the weld, the metal solidities from the liquid state. Hence fusion welds are considered as castings and

the crystalline structure will usually be columnar

(Dendritic). The metallurgical changes are due to the

heating and subsequent cooling of the weld and the heat

affected zone of the parent materials. A random grain

growth take place in the melt boundary. Within the heat affected zone, the grains become coarse due to heat input and a partial recrystallization takes place. With increasing distance from the melt boundary, the grains become liner until the heat unaffected zone with original grains is reached.

Grain-Coarsened-HAZ:

The peak temperatures reached in the grain-coarsened-HAZ region range extends from much above the upper critical transformation temperature to just below the solidus

temperature (2000 to 2700oF). The microstructure is austenite

(for the most part). Any carbides, which constitute the main obstacle to growth of the austenite grains, dissolve resulting in coarse grains of austenite and the likelihood of martensite can be considered. It depends on the carbon content of steel.

Grain-Refinement-HAZ: This region comprises temperature from just above the lower critical transformation

temperature and up to 200oF higher. Austenite is still

produced and the likelihood of martensite can be considered. It depends on the carbon content of steel.

Intercritical-HAZ: The temperatures in this region include

the intercritical ranges, between the lower and upper critical temperatures. Some austenite is produced in this partially transformed range, such that very high potential for martensite transformation exists. In medium and high

carbon steels, this austenite can contain large amounts of carbon which has a higher tendency to produce martensite on cooling.

Subcritical-HAZ: The subcritical-HAZ includes the

tempered area of the Fe-Fe₃C phase diagram(since the heat

of welding may be sufficient for further tempering). There are no phase transformations which take place in this area since the lower critical transformation temperature is not exceeded.

Types of Welds & Welded joints:

The different types of joints are Lap, Butt, Corner, etc. Butt Joints require edge preparation like V, U, J, Bevel.

V – Joints are easier to make but amount of metal to be filled increases with thickness. Hence other preparations are preferred for higher thicknesses.

EDGE PREPERATIONS:

U V

J

Gas Welding:

It is the process of generating the heat required for melting the joint by burning a combustible gas with air/oxygen in a

concentrated flame at high temp. It can weld most

common materials.

Fuel Gases for welding operations:

Commercial fuel gases have one common property: they all require oxygen to support combustion. To be suitable for welding operations, a fuel gas, when burned with air/oxygen, must have the following:

1. High flame temperature

2. High rate of flame propagation 3. Adequate heat content

4. Minimum chemical reaction of the flame with base and filler metals

Among the commercially available fuel gases hydrocarbon gases such as propane, butane, LPG, natural gas are NOT suitable for welding ferrous materials due to their oxidizing characteristics and are suitable for heating, bending, cutting. MAPP gas is a liquefied petroleum gas mixed with methylacetylene-propadiene (acetylene + propane) and has a heat value a little less than acetylene and suitable for welding and cutting. Hydrogen also produces low-temperature flame and is best for aluminium. Hydrogen flame is non-luminous, commonly used for underwater welding and cutting.

Acetylene most closely meets all the above requirements Acetylene is also a hydrocarbon gas and when it reaches its kindling temperature; the bond breaks and releases energy. In other hydrocarbons, the breaking of the bonds between the carbon atoms absorbs energy.

(a) Oxy-Acetylene Welding:

This is suitable for joining metal sheets and plates having thickness of 2 to 50 mm. Additional metal called filler metal is added to the weld in the form of welding rods whose composition is same as the part being welded. Oxygen is stored at a pressure of 14 MPa. Acetylene decomposes in to carbon and hydrogen if stored as a gas and increases the pressure which may cause explosion. Hence Acetylene cylinders are packed with porous material (balsa wood, charcoal, corn pith, or portland cement) that is saturated with acetone to allow the safe storage of acetylene. These porous filler materials help in the prevention of high-pressure gas pockets forming in the cylinder. Acetone is a liquid capable of absorbing 25 times its own volume of acetylene gas at normal pressure without changing the nature of the gas.

Video 1,2

Chemistry of Oxy Acetylene Process

The most common fuel used in welding is acetylene. It has a two stage reaction;

(1) The first stage primary reaction involves the acetylene disassociating in the presence of oxygen to produce heat, carbon monoxide, and hydrogen gas.

2C₂H₂ + 2O₂ = 4CO + 2H₂ + Heat --- (1)

(2) A secondary reaction follows where the carbon monoxide and hydrogen combine with more oxygen to produce carbon dioxide and water vapour.

4CO + 2H₂ + 3O₂ = 4CO₂ + 2H₂O + Heat--- (2)

When you combine equations (1) and (2) you will notice that about 5 parts of oxygen is necessary to consume 2 parts of acetylene

Hence it can be seen that 2.5 volumes of oxygen is required for consuming of 1 volume of acetylene. In the first reaction 35.6% of total heat is generated at the inner cone by burning one volume of Oxygen and one volume of acetylene supplied from the cylinders. The remaining 1.5 volumes of oxygen is supplied from

atmosphere.

Types of Flames:

1. Neutral Flame: When oxygen and acetylene are supplied in nearly equal volumes, this is produced having a max.

temperature of 3200oC. This is desired in most welding

operations. It has sharp brilliant Inner cone and outer cone faintly luminous with bluish colour. Used for most welding applications for many metals like Mild steel, Stainless steel, Cast Iron, Copper, Aluminium etc.

Carburizing Flame:

There is excess of acetylene. This has 3 zones, sharp inner

cone, intermediate whitish cone, bluish outer cone. The

length of the intermediate cone is an indication of the

proportion of excess acetylene. If little excess of acetylene is used it is called reducing condition and is used for welding High carbon steel, Ni, non-ferrous Alloys, low alloy steel etc. If more excess of acetylene is used it is called carburizing condition and is used for low carbon steels for carburizing heat treatment purpose.

Oxidizing Flame:

There is excess oxygen. It has inner cone with purplish tinge and outer cone. This is used for non-ferrous alloys. Such as Cu-base and Zn-base alloys like Brass (Cu-Zn) and bronze (Cu-Sn). The oxidizing atmosphere, in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away. The formation of a covering copper oxide prevents the zinc from dissipating.

(b) Air Fuel Gas Welding:

This process uses a torch similar to a Bunsen burner and operates on Bunsen burner principle. The air is drawn into

the torch as required and mixed with fuel gas. The gas is

then ejected and ignited, producing an air-fuel flame. The

common fuels used are natural gas, propane & Butane.

This type of welding has limited application because of low

temp. This is suitable for low melting point metals and

alloys such as lead etc.

c) Oxy Hydrogen

Welding:-This was once used for welding low temperature metals such

as Al, lead, Mg. The process is similar to oxygen –

acetylene system with the only difference being a special regulator used in metering the hydrogen gas.

Gas Welding procedures:

a) Leftward / Forward welding: The weld is made working from right to left. This is found most advantageous on plates up to about 3 mm.

b) Right ward / back ward welding: The weld is made

working from left to right. This method provides better shielding against oxidation and slows down its cooling.

Hence the weld metal is denser, stronger and tougher.

Welding speed is 20% to 25% higher and fuel

consumption is 15% to 25% lower in this procedure

suitable for over 12mm thick plates.

c) Vertical welding: This is often advantageous for

thickness of 6mm and above. It does not require edge preparation up to 15mm thickness. Here the operator starts at the bottom and proceeds to the top.

• Leftward Welding

Gas cutting / Oxygen cutting:

This used for cutting plates of large thickness and also when cut is to be made along a specified contour. The equipment is similar to that of Gas Welding, but with a different tip the site of the hole depends on thickness to be cut. The metal is heated to ignition / kindling temp. the Jet of Oxygen causes

rapid oxidization and blows away the oxide and molten

metal particles thus creating the cut (Kerf) (Kindling

Temperature – Kindling temperature is the lowest

temperature at which a substance bursts into flame)

Video 3

Oxygen Lance cutting: It is the process by which holes are pierced in heavy blocks of metal by a jet of oxygen passing thro’ a consumable steel pipe (M.S) called lance creating

very high temperatures (45000C) due to reaction of oxygen

with hot metal. The pipe is packed with mixed metal wires of iron, Al, Mg etc. Pure oxygen gas is passed through the pipe from one end from an oxygen cylinder and regulator. The other end of the pipe is preheated to its kindling temperature with an oxy-acetylene torch. The wires in the pipe burns in the oxygen coming down the pipe to produce enormous heat and a liquid slag of iron oxides and other materials, which dribbles and splashes out to longer distances depending on oxygen flow rate. The flow of gas creates a

combustion-friendly environment and the high-temperature flame

Application: Also used for opening of tap holes in blast furnace, making centering holes in heavy shafts, Cutting large

metal castings or frozen metal spills in foundries,

cutting concrete slabs and large steel beams in demolition and renovation of buildings etc.

Video 4

Arc Welding:

It is a process of generating the heat required for melting the joint by means of an electric arc. This is most widely used than Gas welding because of the ease of use and high production rates.

Principle of Arc:

An Arc is generated between two conductors of Electricity, Cathode and Anode, when they are touched to establish the flow of current and then separated by a small distance. An arc is a sustained electric discharge through the ionized gas

column called plasma between the two electrodes. The

electrons liberated from the cathode strike the anode at high

velocity, generating large amount of heat (6000oC). About

It should be noted that Arc temperature depends upon the energy density of the arc column.

With AC the cathode and anode change continuously and as

a result temp. across the arc would be more uniform

compared to a DC arc.

Straight Polarity / DCEN (Direct Current Electrode – ve) is used for thick sheets. Here the W.P. is anode, thus more heat is liberated at the anode which gives deeper penetration. Reverse Polarity / DCEP (Direct current Electrode +ve) is used for thin sheets. Here penetration is small.

In AC welding, the penetration obtained is medium.

DC welding is more expensive and is used for difficult tasks.

Electrodes:

The electrodes used can be consumable (same base material) (or) Non-consumable (Tungsten, Carbon or Graphite). The consumable electrode can be either coated (stick electrode) or uncoated (bare electrode). The coatings serve a No. of purposes.

1. To facilitate establishment and maintenance of arc 2. To produce shield gas around arc & weld pool

3. To provide formation of slag to reduce rapid cooling.

4. To introduce alloying elements not contained in core wire.

Shielded Metal Arc Welding: (SMAW)

Here a metal rod is used as electrode. The temp. is about

2400oc on -ve and 2600oc on +ve electrodes respectively.

This is called Shielded Metal Arc Welding (SMAW) when stick (coated) electrodes are

used. This is a manual process and used for general purpose welding. A.C is the current source. D.C also can be used. This can be used for

thicknesses above 3mm.

The main disadvantages are slow speed, slag inclusion,

moisture pick up by coatings,

wastage of electrode material etc.

Flux – cored Arc Welding: (FCAW)

This is a variant of GMAW, where a consumable tubular electrode wire containing flux at the centre is fed from a reel. DC is used. It is limited to steel and some types of S.S.

Video 2

Carbon Arc Welding:

Here, one or two rods of

carbon are used as –ve

electrodes and work is +ve.

The temp. is about 3200oc

on –ve and 3900oc on +ve

electrodes respectively.

Here DC is always used as

fixed polarity is not

obtained with A.C. This is used where no addition of

filler metal is required.

Used for welding sheet

steel, Al, Cu alloys like Brass, Bronze etc.

Video 3,4

Atomic Hydrogen Welding: (AHW)

Here an arc is maintained between two non-consumable

tungsten electrode, while a stream of hydrogen gas under

pressure is passed through the arc and around the

electrodes. As the molecules of H₂ pass thro’ the arc, they

change into atomic state absorbing considerable amount of

energy. Just outside the arc, the atoms of H₂ recombine into

molecules liberations large amount of heat and produces a

temp. of the order of 4000oC. This process removes all

oxygen and other gases which form oxides and impurities and thus produces smooth, uniform, strong and ductile weld. This is used for welding alloy steel, stainless steel and most non-ferrous metals. This method is now obsolete after development of MIG and TIG.

Video 5

Submerged Arc Welding: (SAW)

This is an automatic process developed for high quality butt welds in steel plates like large container manufacturing, bridges construction, ship building, penstocks, pressure vessels, other structural applications etc. The arc is formed under the layer of flux (Granular flux of coarse size) and is not visible. The bare electrode is fed from a reel through a

gun/nozzle. Speeds up to 80 mm/s on thin plates and

deposition rates up to 45 Kg/hr on thick plates are possible. Plate thicknesses up to 25 mm can be welded in a single pass without edge preparation using DCEP. Deep penetration with high quality weld is possible. gouge

Video 6

Stud Arc Welding (SW)

It is a process for faster joining of the studs to the work pieces such as M/C assemblies, motor assemblies, automobile assemblies, structural assemblies etc. The equipment consists of a Gun similar to GMAW torch which holds the stud to weld. An are is initiated between the stud and the work piece which melts the end of the stud and contact area of

work piece. The stud is pushed into the weld pool and

current is switched off simultaneously and thus the stud gets welded to the work piece.

Video 7

Gas Metal Arc Welding (GMAW) (or) Metal Inert Gas (MIG) Welding:

This is a gas shielded, metal are welding process, where, the consumable electrode wire is continuously fed from a reel and the welding area is flooded with a inert gas which will not combine with metal. The wire is often bare (or) very lightly coated. This is advantageous

because of high welding speeds, No flux requirement, welds many metals The welding gun is either Air cooled / Water cooled.

D.C is the current source and

Inert gases used:

1. CO₂ is used for steel,

2. Ar (or) Ar – He mixture is used for Al (or) Cu

3. Ar – O₂ [1 to 5 % of Oxygen is added for better fluidity

and improved arc stability] (or) He – Ar mixture is used for stainless steel

4. Pure Ar gas is used for Titanium

5. Ar – He mixture is used for Cu-Ni and high-Ni alloys.

Helium has higher thermal conductivity. So it gives higher arc voltage for a given current and higher heat input. However, helium being lighter (than argon and air) rises in turbulent manner and tends to disperse into air. So higher flow rate will be required in the case of helium shielding.

Modes of metal transfer in GMAW Welding:

In GMAW, the filler metal is transferred from the electrode to the joint. Depending on the current and voltage used, different ways of transfer occurs.

1. Short circuit / Dip Transfer: Here the electrode tip melts and forms a Globule of molten metal at tip. As the

electrode advances it touches the W.P. short circuit

occurs. The tip is pinched by electromagnetic forces and transferred by surface tension into the weld pool. This is used up to thicknesses of 5mm with small diameter wires (up to 0.9 mm). Best for vertical welding and overhead welding.

1)

2)

2. Globular / Drop Transfer: It occurs at higher currents than the first. The melted tip forms a big size drop (twice the wire dia) at tip which is pinched by electromagnetic forces and pulled by gravity in to the weld pool. It causes excessive spatter hence usually avoided mode of transfer. It may sometimes cause short circuit also.

3. Spray Transfer: It occurs at higher currents than the second. Here the molten metal is detached from tip by the increased electromagnetic pull irrespective of gravity force. It produces very little spatter & used for thick plates (>6 mm) in flat and horizontal positions only. Wire diameters are more. 4. Pulsed Spray Transfer: The current is pulsed between spray transfer range and nearer to globular range cyclically so that it is suitable for all positions of welding. It is mainly used for S.S as it reduces distortion and inter granular corrosion.

Tungsten Inert Gas (TIG) welding:

This process was invented for welding Al as Al forms an oxide layer immediately on exposing to atmosphere. DCEP was used in welding Al as it causes peeling of oxide layer (Cathode cleaning process). A.C. was later found to give better result. Filler material can be used if required in TIG welding by feeding as if in Gas welding. Pure tungsten is used for DCEN for welding most of the metals. Thoriated tungsten or Zirconated tungsten is used for A.C and DCEP for welding Al and Mg alloys.

This process is being widely used for thin sheets for precision

welding in nuclear, air craft, space craft, chemical industries.

Video 9

Plasma Arc Welding: (PAW)

It is extension a TIG. Difference is constriction of arc

column. Plasma is a high-temp. ionized gas and occurs in any electric are between two electrodes. The ionized gas gets hotter by resistance heating from the current passing through it. If the arc is constrained by an orifice, the proportion of ionized gas increases and plasma are welding is created. A non-consumable tungsten electrode with a

water-cooled nozzle is enveloped by a gas. The gas is

forced past an electric arc thro’ the constrained opening of the nozzle. The gas passing thro’ the arc is dispersed and

temp. raises to the order of 11000oC to 14000oC.

Application is in electronic, instrumentation, aero space industries. It can also weld Carbon steels, S.S, Cu, Brass, Al, Ti, Monel, Inconel, Mo, Tantalum, Haste Alloys etc.

Video 10

A lower flow rate of the orifice inert gas is maintained, as excessive flow rate may cause turbulence in the weld pool. This flow rate is insufficient to shield the weld pool effectively. Hence inert gas at higher flow rate is also passed through outer gas nozzle to protect the weld pool.

Arc Blow:

Due to fixed polarity in D.C. Welding, magnetic lines form in the W.P. When welding at the centre of W.P. these lines are equally distributed on both sides so Arc will be straight. But while welding at the edges, the magnetic lines will try to pull back the arc and it

will be deflected

towards the W.P., as these lines will be formed only in the material. This

phenomenon is called arc blow and causes spatter and improper bead geometry.

• Keeping metal plates at entry and exit of the arc.

• Holding as short an arc as possible to help the arc force counteract the arc blow.

• Reducing the welding current - which may require a reduction in arc speed

• Changing the ground positions.

• Inclining the electrode with the work opposite to the direction of arc blow as shown:

Arc cutting:

This is based on melting the metal by the heat of an

electric arc and blowing molten metal by a jet of air

supplied along the electrode and into the cut. This is used for cutting small sections like pipes, angle channels, separation of gating system from castings, etc.

Power sources in Arc Welding:

Selection of power source is mainly dependent on type welding process. The open circuit voltage normally ranges between 70-90 V and short circuit current ranges between 600-1000A in any welding transformer. Welding voltages and welding currents are lower as compared to open circuit voltage of the power source.

a) Constant current type transformer (Non-Linear):

In manual arc welding since are length cannot be

controlled, the arc current is controlled by the transformer. It has the drooping V-I characteristic curve as shown. It can be observed that a major change in Arc voltage causes in significant change in Arc current.

b) Constant Voltage Transformer (Linear):

It has a flat V-I characteristic with a slight droop. This is

used for continuous electrode wire welding like GMAW,

SAW and other automatic welding processes. It can be

observed that a major change in Arc current causes in

Note:

1. Voltage required to generate arc at no load condition is

called Open Circuit Voltage (V_OC )

2. Current required during arc generation is called Short

Circuit Current (I_SC).

Duty Cycle:

Duty cycle is the ratio of arcing time to the weld cycle time expressed as percentage. If arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle

and 50% as per American standard. At 100% duty cycle

minimum current is to be drawn. The welding current which

can be drawn at a duty cycle can be evaluated from the

D_R x I_R2 = I2 x D₁₀₀

Where I = Current at 100% duty cycle

D₁₀₀ = 100 % Duty cycle

I_R = Current at required duty cycle

D_R = Required duty cycle

Duty cycle and associated currents are important as it ensures that power source remains safe and its windings are not getting damaged due to increase in temperature beyond specified limit. The maximum current which can be drawn from a power source depends upon its size of winding wire, type of insulation and cooling system of the power source.

Expressions:

1.For a linear power source characteristic, the arc voltage is given by :

V = V_oc – ((V_oc / I_SC) x I)

Where I = Arc current

2.For a stable arc, in a constant voltage transformer, V_arc = V_transformer

3.For a stable arc, in a constant current transformer, I_arc = I_transformer

3.For a linear power source, the Arc length – Voltage characteristic is given by

V = a + bl

4. The equation of the line can also be written as (V-V₁) = {(V₂-V₁) / (I₂-I₁)} (I-I₁)

5. Heat required for melting =

Volume melted x rate of melting

Volume melted = Area of Joint x welding speed

6. Net heat supplied = η_HT x V x I

η_HT = Heat transfer Efficiency

Resistance welding: (RW)

This is a fusion welding process where both heat and pressure are applied on the joint, but no filler metal (or) flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint. Here a low voltage (typically 1 V) and very high current (typically 15000 A) is passed thro’ the joint for a very short time (typically 0.25 sec.). This heats the Joint due to the contact resistance at the joint and melts it. The pressure on the Joint continuously maintained fuses the metal parts.

Electrodes:

Copper in alloyed form is used for making electrodes.

Cu - Cd Alloys  for non-ferrous materials like Al & Mg. Cu – Cr Alloy  for mild steels and low alloy steels

Note: The transformer in the machine converts low amperage, 240V shop line current in to high secondary amperage, low voltage welding current, safe from electrical shock. Proper earthing is also important. (Range: 1–25V, 1000–100,000 A)

Heat Balance:

Proper fusion is obtained only when proper heat balance is there. This can be provided by increasing or decreasing the

contact areas of the electrodes as follows for different

combinations.

1.

sheet.

2. Large contact area is required for high electrical

conductivity and small contact area for low electrical

conductivity (Dissimilar metals)

3. Smaller contact area is required for higher thermal

conductivity and large contact area for low thermal

Upset Butt welding: (UW)

The parts to be welded are clamped edge to edge in Copper Jaws of welding M/c and brought together in Solid contact, which forms a locality of high electric resistance. As the current flows here, the joint gets heated us and the pressure applied upsets the parts together.

This is used for non-ferrous

materials and is used for welding bars, rods, wires, tubes, pipes etc.

Video 1

Flash Butt welding: (FW)

Here the edges are brought together in light contact. A high current starts a flashing action between the two surfaces and continues as reached. The

upsetting action will cause melted metal to flash out through the

joint and forms like a fin around the joint. This is used for ferrous materials and is used for welding bars, rods, wires, tubes, pipes etc. This is not suitable for

materials like lead, Tin, Zinc, Antimony, Bismuth etc.

Video 2

Percussion Welding: Here one part is held stationary, and other part is held in a clamp mounted on slide which

is backed up against pressure from a heavy spring. During

welding, the movable clamp released rapid carries the part forward. When the distance between the parts is approx. 1.5mm, a sudden discharge of electrical energy is released, causing intense Arc between the two surfaces. To complete the weld it takes about 0.1 sec. No upset / flash occurs at the weld. This is a automatic process and is limited to small

areas of 144 mm2 max. and is suitable for welding small

wires to electrical components.

Video 3

Spot Welding: (RSW)

This is employed to join overlapping strips, sheets or plates at small areas. This is widely used in electronic, electrical, air craft, automobile, home appliance industries for body constructions.

Projection Welding: (RPW)

This is modification of spot welding. One (or) both of the

work pieces are embossed to produce projections. The

current and pressure employed on the embossing flattens out this projection resulting in good welds at point of contact. By this process fastening attachments like nuts, brackets handles etc. can be welded to sheet metal in electrical, electronic, domestic equipment.

•

RSW

RPW

Video 4,5,6,7

Seam Welding: (RSEW)

This is a method of making a continuous joint between two

overlapping pieces of steel metal. The work is placed

between wheels which serve as conductors for producing

continuous welds. Used for pressure tight / leak proof fuel tanks in automobiles, seam welded tubes, drums, small containers etc.

Expressions:

1. Heat required for melting = Vol melted x rate of melting

= mL + mC_p (T_m –T_a)

Where m = mass of metal melted = (vol melted x ρ) L = Latent heat of fusion of the metal

C_p = Sp. Heat of metal

T_m = Melting temp. of metal

T_a = Ambient temp.

ρ = Density of metal

2. Net heat supplied = I2 RT = V2T / R (Since V = IR)

Where I = Current (Amp)

R = Resistance (Ω)

T = Time for welding (sec)

3.Melting η =

Thermit Welding: (TW)

This is used for the welding of very thick plates, like ship hulls, broken large castings, rail sections etc. Thermit is a

mixture of finely divided Al (1 part) and Iron oxide (3

parts). The Process is based on the chemical reaction where Oxygen leaves Iron oxide and combines with Al, producing Al. oxide and superheated thermit

steel. [8Al + 3Fe₃O₄  4 Al₂O₃ + 9 Fe]

The temperature is around 3000oC.

A wax pattern is first shaped around the parts to be welded. A sand mould is prepared around it. Pre heating is done and wax is drained out. The

thermit mixture is poured in to the mould and then pressure is applied after welding temp. is reached.

Electro slag welding : (ESW)

This is developed to weld very large plates (200 mm section) without any edge preparation. Here a consumable electrode is used for filling the gap between the two heavy plates. The heat required for melting the plates and electrode is obtained initially by means of an arc so that the flux will form the molten slag. Then further heating is obtained by the resistance heating of slag itself. For effective welding, vertical welding is done to maintain a continuous slag pool, which is contained in the gap with the help of water cooled copper dam plates which move along with the weld.

Appln: Frames of heavy presses, rolling mills, Locomotives

etc. _Video

•

Electron beam Welding: (EBW)

Here a focused beam of electrons are accelerated towards

the anode from the electron gun which forms the cathode. This is done with the help of a electro magnetic lens. The

material in the path on the beam gets melted. Larger

penetrations are possible here. No filler material / flux is needed here. Here the welding zone is narrow and hence weld distortions are eliminated. (0.25mm – 1mm dia beam can be possible).

Appln: Specially suitable for welding dissimilar metals and super alloys, turbine and air craft engine parts where distortion is unacceptable, Air plane, automobile, farm equipment etc.

Video 3

Laser Beam Welding (LBW)

Here a laser beam is directed on to the joint to be welded. Narrow. Heat Zones (0.05 mm to 0.1mm wide) are possible here and hence very small wires used in electronic devices can be welded. This is called Micro welding. They can also be used for joining multi layer materials with differing thermal properties. It can weld dissimilar metals and difficult to weld metals like, Cu, Ni, S.S, Ti, Columbium etc. Widely used in Aerospace and electronic industries.

The lasers used for welding are:

Solid–state lasers like Ruby - Neodymium (Nd); Nd - Glass; YAG (Yttrium - Aluminium – Garnet) etc.

The chief gas laser is CO₂ laser.

Video 4

Forge Welding:

This is a oldest method. The ends to be joined are heated to a temperature slightly below the solidus temperature and pressure is applied so that a fusion joint is obtained. The force can be applied by machines / continuously rotating rolls / manually.

ART METAL HORSE SHOE

Video 5

Friction Welding (FRW)

One of the parts to be joined is axially aligned and pressed tightly against another part and rotated at a high speed (3000 rpm). The friction between the parts rises the temperature of both ends. The rotation is stopped abruptly and pressure on

fixed part is increased so that joining takes place. Even

dissimilar metals can be joined. This process is limited to parts with

rotational symmetry.

Video 6

Friction welded parts in production applications span over wide products for aerospace, agricultural, automotive, defense, marine and oil industries.

Right from tong holds to critical aircraft engine components are friction welded. Automotive parts like gears, engine valves, axle tubes, driveline components, strut rods, shock absorbers are friction welded.

Hydraulic piston rods, track rollers, gears , bushings, axles and similar parts are commonly friction welded for agricultural equipment.

Diffusion Welding: (DFW)

Also called Diffusion bonding is the process of joining two

parts purely by diffusion, which can be achieved by

keeping the two pieces in intimate contact under pressure. This does not necessarily need heat. But its temperature is raised, the diffusion rate is increased. The joint is formed without any filler metal and the microstructure and composition at the interface are the same as those of the base metals. Pressure is applied which will cause local plastic and creep deformation at the temperature of operation. Bonding will take place due to diffusion and will depend on temperature, time and the pressure applied. An interlayer foil

or coating may be used to improve the bonding

characteristics. This process makes it possible to join metal to metal as well as metal to ceramic also.

Appln: Used by gold smiths to bond gold over copper, most

suitable for joining dissimilar

metals like Ti, Be, Zr, refractory materials, composite materials etc.

Diffusion bonding with super

plastic forming is widely used in aero space (Wing Structures).

Video 7

Explosion welding: (EXW)

Here, detonation of explosives is used to accelerate a part to

move towards the other plate at a fast rate, so that the

impact creates the joint. As the plate moves at high

velocity and meets the other plate with a massive impact, very high stress waves (of order thousands of MPa) created between the plates makes a clean joint. Application is for cladding of metals for the purpose of corrosion prevention. Used for joining of dissimilar metals like Titanium to steel,

Al. to steel, Al to Cu etc. Tantalum can be explosively

welded to steel though the welding point is higher than vapourisation temperature of Steel.

•

Brazing:

Here a filler material also called spelter is used, whose melting point is less than the melting point of parts to be joined. The parts to be welded are cleaned properly Flux (usually Borax) is applied and then filler material is placed in between and the parts are heated which melts the filler material and it flows into the space by capillary action. The filler materials are copper-base alloys / silver base alloys. Brass is more commonly used filler metal.

Eg: Small LPG cylinders, Hydraulic Fittings, Heat

Exchangers, Tube Manipulations, Machined Assemblies Pressed Assemblies etc

Video 10

•

BRAZING OF

WATCH ASSEMBLIES, CONNECTORS IN AUTOMOBILES

Bronze Welding / Braze Welding:

This process requires more heat than Brazing and Tin is added in filler metal for better flowing of melted filler metal. This process is intermediate between true welding and true brazing. Here the parts are heated to a temp. of melting point of the bronze – filling rod which contains 60% Cu and 40% Sn. During the operation, the edges of the parent metal are heated by oxy-acetylene flame or some other suitable heat source. Here the filler metal reaches the Joint without the capillary action since the Joint gap is more. The filler metal enters the joint by gravity.

Eg: Carbide inserts in tool shanks, carbide drill bits, repair

works etc Video

Soldering:

This is a method of joining metal parts by means of a fusible alloy called solder, applied in the molten state. Fluxes used in soldering are ammonium chloride, zinc chloride etc. The solder is composed of Pb and Sn with a melting point of

150 to 350oC

Soft soldering: is used for sheet metal works that are not subjected to excessive loads.

Hard Soldering: employed solders whose melting temp. is higher than soft solders.

Soft solder - lead 37%, tin 63%

Medium solder - lead 50%, tin 50%

Plumber solder - lead 70%, tin 30%

Electricians solder - Lead 58% , tin 42%

Video 12

Note: (1) During brazing or soldering flux is used for: Dissolving oxides from the surfaces to be joined, Reduce surface tension of molten filler metal i.e. increasing its wetting action or spreadability, Protect the surface from oxidation during joining operation. (2) Any metal which has a

melting point of < 4500C cannot be used as filler material in

brazing or braze welding and can only be used in soldering.

SOLDERING OF COPPER TUBES, SEAT BELT BRACKETS, STEEL VALVE

Weld Defects:

The defects in the weld can be defined as irregularities in

the weld metal produced due to incorrect welding

parameters or wrong welding procedures or wrong

combination of filler metal and parent metal.

Weld defect may be in the form of variations from the intended weld bead shape, size and desired quality.

Defects may be on the surface or inside the weld metal. Certain defects such as cracks are never tolerated but other defects may be acceptable within permissible limits.

Welding defects may result into the failure of components under service condition, leading to serious accidents and causing the loss of property and sometimes also life.

1) Poor Fusion  Lack of thorough and complete union

between the deposited and

present metal this appears as a discontinuity in the weld zone.

Lack of fusion is because of

failure to raise the temperature of

base metal or previously

deposited weld layer to melting

point during welding. Lack of

fusion can be avoided by

properly cleaning of surfaces to

be welded, selecting proper

current, proper welding technique and correct size of electrode.

2) Under cut 

This appears as a small

notch in the weld

interface. Main reasons for undercutting are the

excessive welding

currents, long arc

lengths and fast travel speeds.

3) Porosity  Porosity results when

the gases are entrapped in the

solidifying weld metal. These gases are generated from the flux or coating

constituents of the electrode or

shielding gases used during welding or from absorbed moisture in the coating. Rust, dust, oil and grease present on the surface of work pieces or on electrodes

are also source of gases during

welding. Porosity can also be

controlled if excessively high welding currents, faster welding speeds and long arc lengths are avoided, flux and coated electrodes are properly baked.

4)Slag Inclusion  These may be in the form of slag or any other nonmetallic material entrapped in the weld metal as these may not able to float on the surface of the solidifying weld metal. However, if the molten weld metal has high viscosity or too low temperature or cools rapidly then the slag may not be released from the weld pool and may cause inclusion. Slag inclusion can be prevented if all the slag from the previously deposited bead is removed, low welding current are avoided.

5) Cracks  Cracks occur

when localized stresses

exceed the ultimate tensile

strength of material. These

stresses are developed due to

shrinkage during

solidification of weld metal.

Cracks may be developed

due to poor ductility of base

metal, high sulphur /

phosphorous and carbon

contents, high arc travel

speeds i.e. fast cooling

rates, high hydrogen contents in the weld metal etc.

6) Distortion 

Bending of components due to improper thermal expansions and contractions. Hence proper clamping and preheating is to be done to avoid distortion.

7) Miscellaneous Defects  Multiple arc strikes, spatter, grinding & chipping marks, misalignment of weld beads, un removed slag, etc.

Design Considerations:

The selection of a welded joint and a welding process

involves the following considerations:

1. The configuration of the component or structure to be welded, their thickness and size.

2. The service requirements, such as type of loading and the stress generated.

3. The location, accessibility and ease of welding. 4. The effects of distortion and appearance.

5. The costs involved in the edge preparation, the welding, post – processing of weld including machining and finishing operations, Heat treatment etc.

Design guide lines:

1. Product design should minimize the number of welds. 2. Components should fit properly before welding.

3. Select designs that can avoid (or) minimize the need for edge preparation.

4. Weld bead size should be kept to a minimum to conserve weld metal.

5. Weld location should be selected so as not to interfere with further processing of the part.

Note:

1.The correct sequence in ascending order of their weldability for most common metals is : Al < Cu < CI < MS

2. Due to improper surface cleaning, hydrogen may enter in to weld pool and get dissolved in the weld metal. During cooling it diffuses in to HAZ developing cracks due to the residual stresses assisted by hydrogen coalescence (growing together). This is called hydrogen embrittlement.

Welding of Cast Irons:

They are difficult to weld because of high carbon contents and poor ductility.

Massive carbon deposits have a tendency to form in the areas adjacent to the weld.

Thus a high carbon martensite tends to form in the HAZ which has very brittle micro structure that may lead to cracks during welding or after welding under load application.

CI is joined by Oxy Acetylene welding and SMAW. Proper pre heating and post heat treatment may be required.

Welding of Stainless Steel:

Stainless steel is difficult metal to weld because it contains both Ni and Cr. The best method for welding stainless steel is TIG welding. SMAW is also used but requires use of a heavily coated electrode. Low current setting with fast travel speed is preferred for stainless steel as certain stainless steels are subjected to carbide precipitation.

Ferritic stainless steels are generally less weldable than austenitic stainless steels and require both preheating and post weld treatments. Welding ferritic stainless steels can be done autogenously (or) with an austenitic stainless steel (or) using a high nickel filler alloy (or) type 405 filler containing low % Cr (11%), low % C(0.08%) and small % Al (0.2%). It can be welded by TIG, MIG, SMAW, PAW.

WELD DECAY

Weld decay is a form of intergranular corrosion, usually found in corrosion-resistant alloys like stainless steels or certain nickel-base alloys and occurs as the result of sensitization in the HAZ during the welding operation. The corrosive attack is restricted to the HAZ. Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is possible to visually recognize weld

decay if parallel lines are already

formed in the HAZ along the weld as shown.

In this case, the precipitation of chromium carbides is induced by the welding operation when the HAZ experiences a

particular temperature range (550oC~850oC). The precipitation

of chromium carbides will consume the alloying element – chromium, from a narrow band along the grain boundary and this makes that zone anodic to the unaffected grains. The chromium depleted (consumed) zone becomes the preferential path for corrosion attack or crack propagation if under tensile stress. Weld decay can be prevented through:

• Using low carbon (e.g. 304L, 316L) grade of S.S electrodes. • Using stabilized electrode grades alloyed with Ti (type 321)

or Nb (type 347). Ti and Nb are strong carbide- formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion.