Production Technology

K. Srinivasulu Reddy

Department of Mechanical EngineeringSreenidhi Institute of Science and Technology

Welding

Name AWS Characteristics Applications

Air acetylene welding AAW Chemical welding process, not popular Limited

Oxyacetylene welding OAWCombustion of acetylene with oxygen

produces high-temperature flame, inexpensive equipment

Maintenance, repair

Oxygen/Propane welding

Gas welding with oxygen/propane flame

Oxyhydrogen welding OHW Combustion of hydrogen with oxygen produces flame Limited

Pressure gas welding PGW Gas flames heat surfaces and pressure produces the weld

Pipe, railroad rails (limited)

GAS WELDING -TYPES

Oxy-Fuel Gas Welding

Oxy-fuel gas welding (OFW) is a fusion welding processes wherein the joint is completely melted to obtain the fusion.

The heat produced by the combustion of gas is sufficient to melt any metal and as such is universally applicable

In oxy-Acetylene welding(OAW) which is an oxy-fuel gas welding process that uses Acetylene as fuel gas.

In pressure gas welding – application of pressure without filler metal.

Oxy-Acetylene Welding

Consists of high temperature flame produced by the combustion of Acetylene with oxygen and directed by torch.

The intense heat of the flame (3100 °C) melts the surface of the base metal to form a molten pool.

Filler metal is added to fill gap or grooves.

As the flame moves along the joint, the melted base metal and filler metal solidifies to produce the weld.

The temperature of oxy-Acetylene flame is not uniform throughout, its length and the combustion is also different in different parts of the flame.

The temperature is highest just beyond the inner cone and decreases gradually towards the end of the flame.

Oxy-Acetylene Welding cont..

The heat generated by the combustion is in accordance with a pair of chemical reactions

- The primary combustion process which occurs in the inner core of the flame is

C2H2 + O2 2CO + H2 + Heat(18.75 MJ/m3 )

- However both CO and H2 are combustible and will

react with oxygen from air

2CO + H2 + 1.5O2 2CO2 + H2O + Heat(35.77 MJ/m3 )

Oxy-Acetylene Welding cont..

- High heat produced in 2nd stage is distributed over a large area, the temp. achieved is small(1200 to 20000C), used for pre heating the steels.

- The inner white cone temp. is of the order of 31000C, which is used for directly melting the steel joint.

- The first reaction is dissociation of Acetylene to CO and H2.

It produces about 1/3rd of the total heat generated in the flame.

- The second reaction in which further burning of the H2 and

the CO.- The second reaction produces about 2/3rds of the total

heat.

Oxy-Acetylene Flame Types

Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame (b) oxidizing flame (c) carburizing, or reducing flame.

The gas mixture in (a) is basically equal volumes of oxygen and acetylene.

33000C 29000Cwhite

Blue

(reddish)

Certain amount of oxygen is required for complete combustion of fuel gases(acetylene)

Based on the amount of supply of oxygen three types of flames are formed viz. neutral, oxidizing and carburizing (reducing) flames.

When equal amount of oxygen is provided, neutral flame is obtained.

When less oxygen is provided part of the combustible matter is left as it is and it results in a reducing flame or carburizing flame.

If more oxygen is provided, oxidizing flame will result.

Oxy-Acetylene Flame Types cont..1.Neutral Flame:

- Ideal condition i.e., complete combustion(1:1)- All the Acetylene is completely burned. - All the available heat in the Acetylene is released.- Most desirable flame to be used in oxy-Acetylene flame.- Temp. in inner white cone 31000C

When less oxygen is provided. Similar to neutral flame, only with the addition of a third phase in between outer envelope and inner cone and is called Acetylene feather.

Length of feather is an indication of excess acetylene present.

The temperature of reducing flame is lower, so it is suitable for applications requiring low heat such as brazing, flame hardening.

2.Carburising or reducing flame

Since unburned carbon goes into the weld pool, the metal appears to boil – this excess carbon causes the steel to become extremely hard and brittle.

Not suggested for general use

Useful for those materials which are readily oxidised

Ex: Oxygen free copper alloys,High carbon steels,High Speed Steels, Cemented carbides &Cast Irons

Reducing flame contd…

When oxygen is in excess it is called oxidizing flame. This is similar to neutral flame except that the inner

white cone is somewhat small, giving rise to higher tip temperatures(33000C)

This flame is harmful especially for steels because it oxidizes steel.

3.Oxidizing Flame

Oxy-Acetylene Flame Types cont..

Because of the burning of the metal, the weld pool foams and sparks, produces a loud noise.

Oxidizing flame is desirable for non-ferrous alloys such as copper and zinc base alloys.

A thin protective layer of slag forms over the molten metal.

Other fuel gas such as hydrogen, propylene, propane can be used in OFW, but temperatures developed by these gases are low.

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

Flame with pure H2 is colourless & hence difficult to adjust.

Oxy-Acetylene Torch

General view

cross-section of a torch used in oxyacetylene welding.

Basic equipment used in oxyfuel-gas welding

To ensure correct connections

1.All threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed.

2. Oxygen regulators are usually painted green, and

acetylene regulators red.

Acetylene Generation Methods

• An acetylene generator is used instead of an acetylene cylinder.

• Acetylene is produced by a reaction between calcium carbide and water which is instantaneous as shown below.

CaC2 + 2 H2O C2H2 + Ca(OH)2

• The acetylene generator consists of a cylinder which is partially filled with water.

• CaC2 is stored in an hopper near the top of the generator.• A pressure regulated valve controls the flow of CaC2

into water, depending on the pressure of the acetylene in the generator.

Calcium carbide

+ =

LimeCoke

CaC2 Hopper

Water

Automatic pressure release valve

Acetylene pipe

ethanol is an expensive alternative to carbide to ripen the fruits

•Calcium carbide (Carbide is known to be a carcinogenic- cancer inducing substance) used to artificially ripen the fruits

•Used in the desulfurisation of iron

•Calcium carbide is produced industrially in an electric arc furnace from a mixture of lime and coke at approximately 2000 °C. This method has not changed since its invention in 1888

•CaO + 3 C → CaC2 + CO

•The reaction of calcium carbide with water, producing acetylene and calcium hydroxide, was discovered by Friedrich Wöhler in 1862.

•CaC2 + 2 H2O → C2H2 + Ca(OH)2

•This reaction is the basis of the industrial manufacture of acetylene, and is the major industrial use of calcium carbide.

Welding Technique

1. Acetylene valve on the torch is opened slightly and lighted with the help of a spark lighter.

2. The flame draws oxygen from atmosphere air and thus results in reducing flame.

3. Now acetylene valve is opened to get the required flow of acetylene.

4. The oxygen valve is slowly opened till the intermediate flame feather of the reducing flame recedes into the inner white cone.

5. The actual adjustment of the flame depends on the type of material to be joined.

1. The choice of the torch depends on the thickness of the metal to be joined.

2. Large torch tip sizes cause higher amount of oxygen and fuel to flow out, causing the release of more heat.

3. Except for outside corners, all other joints require a filler metal to fill the joint.

4. The torch tip should be positioned above the metal plate so that the white cone is at a distance of 1.5 to 3 mm from the plate.

5. The torch should be held 30 to 45 degrees from the horizontal plane.

Other Aspects

6. The torch movement along the joint should be either oscillating or circular.

7. When welding rod is used to provide filler material, it is necessary to hold it at a distance of 10mm from the flame and 1.5 to 3.0 mm from the surface of the puddle

8. Can be used for all types joints and in all positions

9. Thicker plates require more than one pass of the gas torch along the length, to complete the joint, which is called multi pass welding

Forehand welding:

Torch is moved in the direction of the tipPreheat the metal before the white cone of the tip melts it.

Back hand welding

The torch moves backwards

Outer blue flames are now directed on the already welded joint

Continuously annealed by relieving the welding stressesBetter penetration and bigger weld beadUsed for thicker materials

Disadvantages

Slower and can’t compete with other welding methods such as electric arc welding

Advantages

1.Versatality of the equipment2.Same equipment with a range of torches can be used be for gas cutting, brazing and braze welding3.Useful for small shops

Flame Cutting• To separate piece of material into two or more pieces or

into various contours.• Heat source – torches, electric arcs, lasers.

Types of flame cutting- Arc cutting- Oxy fuel (acetylene) gas cutting

GAS CUTTING

• Produces temperatures about 8700 C while cutting steel. • Cutting occurs mainly by oxidation of steel.• Maximum thickness cut by oxy-acetylene gas is 300-350mm.• The flame leaves drag lines on cut surface which are very

rough.• Basic reactions with steel

Fe + O FeO + Heat

3Fe + 2O2 Fe3O4 + Heat

4Fe + 3O2 2Fe2O3 + Heat

Shearing is used to cut plates in straight line and also upto a thickness of 40 mm.

To cut along a specified contour and thickness more than 40 mm and upto 2 metres oxy fuel gas cutting(OFC) is useful.

pre-heating flames

Differences in torch tips for gas welding and gas cutting

Slag + Molten metal

Direction of travel

Position of cutting torch in oxy-fuel gas cutting

Drag Kerf

1. Oxidise(burn) the iron and steel by heating to a temp of 800 to 10000C

2. When a high pressure oxygen jet with a pressure of the order of 300 kPa is directed against a heated steel plate, the oxygen jet burns the metal and blows it away causing the cut(kerf)

Equipment is simple and can be carried anywhere

Gas cutting outfit is similar to that of welding except for the torch tip.

Here torch tip has a provision for preheating the plate as well as providing the oxygen jet.

The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark.

If a large size of orifce is used than that required, the kerf width is wider and larger volumes of oxygen are consumed.

Tip has a central hole for oxygen jet with surrounding holes for preheating.

Tip size is dependent on the thickness of the plate, which determines the amount of preheating as well as the oxygen jet flow required for cutting

The heat generated causes the metal to melt and get blown away by the oxygen pressure.

About 30 to 40% of the metal in the kerf(cut) is simply blown away, while the rest is oxidised.(60-70%)

It gets readily combined with oxygen giving iron oxide.

After the steel plate has reached the kindling temperature, which is about 8700C, the operator should release the oxygen jet to start the cutting, moving in the forehand direction to achieve the desired cut.

Melting point of cast iron is less than that of iron oxide. So cutting of cast iron is difficult. Preheating and post heating should be done. If not, white cast iron will form resulting hard edges. If chromium and nickel are present in ferrous alloys, these will interfere with the oxidation and difficult to cut without special provisions.

Drag: the amount by which the lower edge of the drag line trails from the top edge.

Torch tip can held vertically or slightly inclined in the direction of travel. The torch should be positioned above the metal at a distance of about 1.5 to 3 mm

A good cut is characterized by very small or negligible drag. Forward cutting speed should not be high.

More the speed, more the drag, which leads to turbulence conditions for the oxygen jet, which gives very rough and irregular shaped cut edges.

If the torch moved slowly, starting on and off of the oxygen jet takes place and leads to irregular cut.

Widely used for those materials which readily get oxidised and the oxides have low MP than metals.

Hence widely used for ferrous materials. Not used for materials such as Auminium, bronze, stainless steel etc. since they resist oxidation.

Shielded gas cutting processes (Gas Metal Arc Cutting & Gas Tungsten Arc Cutting) are used for materials like Aluminium, stainless steel, Nickel alloys.

Plasma Arc Cutting(PAC) will produce very high temperatures of the order 14,0000C. Any metal can easily be melted and blown away by this process.

Gas cutting can be done manually or by a machine. Microprocessor controlled flame cutting machines are also available.

ELECTRIC ARC WELDING

+

Ammeter

Voltmeter BatteryResistance

Simple electrical circuit

+

I

V

Welding machineElectrode

Welding electrical circuit (Straight polarity)

-Work

Arc

Arc WeldingName AWS Characteristics Applications

Atomic hydrogen welding AHW Two metal electrodes in hydrogen atmosphere Historical

Bare metal arc welding BMAW Consumable electrode, no flux or shielding gas Historical

Carbon arc welding CAW Carbon electrode, historical Copper, repair (limited)

Flux cored arc weldingFCAWFCAW-S

Continuous consumable electrode filled with flux

Industry, construction

Gas metal arc welding 

GMAW Continuous consumable electrode and shielding gas Industry

Gas tungsten arc welding

GTAW Nonconsumable electrode, slow, high quality welds Aerospace

Plasma arc welding PAW Nonconsumable electrode, constricted arc

Tubing, instrumentation

Shielded metal arc welding 

SMAWConsumable electrode covered in flux, can weld any metal as long as they have the right electrode

Construction, outdoors

Submerged arc welding SAW Automatic, arc submerged in granular flux

Electric Arc

An arc is generated between two electrodes

An arc is a sustained electric discharge through the ionized gas column called plasma between the two electrodes

Accelerated electrons – strike anode at high velocity – conversion of KE to heat – ionization of the column - arc column – temperature generated is about 6000°C

In order to produce the arc, the potential difference between the two electrodes should be sufficient

The arc formation is similar for both the AC or DC welding

In AC the polarity changes continuously and hence the temperature across the arc is uniform

In DC the polarity is fixed and hence the heat is more concentrated at one of the electrodes

Heat produced by the electric arc to fusion weld metallic pieces

Electrons liberated from the cathode move towards the anode and are accelerated in their movement. When they strike the anode at high velocity, large amount of heat is generated.

Electrons are moving through the air gap between the electrodes, also called the arc column, they collide with the ions in the ionised gas column between the electrodes

In DC welding

Striking of electrons on anode from cathode: 65 to 75% heat liberated at anode(60000C)

Positively charges ions, moving from anode to cathode, and liberating 25 to 35% heat at cathode.

The larger air gap requires, higher potential differences. If the air gap becomes too large for the voltage, the arc may be extinguished.

In order to produce the arc, the potential difference between the two electrodes should be sufficient to allow them to move across the air gap.

50% of all industrial and maintenance welding is currently performed by this process (Shielded metal arc welding –SMAW, also know as stick welding)

Current ranges: 50A to 500AVoltage: 20 to 40 VPower requirements: less than 10 kWCurrent : AC or DCFor sheet metal work: DC preferred as it produces steady arc

Arc welding Contd.. In the normal operation of a transformer as amperage is

increased, the voltage decreases, and vice-versa.

Electrical arc welding power supply characteristics are constructed so that either the voltage or the amperage is relatively constant as the other factor changes.

Arc gap is proportional to voltage

Heat generated is proportional to current

Polarity of DC SupplyDirect current straight polarity(DCEN)

Direct current reverse polarity(DCEP)

+

I

V

Welding machine (DC power source)

Electrode -

WorkArc

Electron flow

+

I

VElectrode -

WorkArc

Electron flow

Welding machine (DC power source)

Deep penetration

Shallow penetration but good deposition

I

VElectrode

WorkArc

Electron flow

Electrode Work

AC Welding Circuit

DC arc welding is more expensive than AC welding. But DC is preferred because of the control of the heat input offered by it.

In DC, 70% of heat is liberated near anode. Hence if more heat is required and for thicker sheets, DCEN (straight polarity)is preferred for more penetration.

For thinner sheets where less heat is required, reverse polarity or DCEP is preferred for small penetration.

In case of AC, the weld bead is some where in between DCEP and DCEN.

DC welding is generally preferred for difficult tasks.

Comparison of Penetration Contours

Arc blow in DC, deflection of the arc by means of magnetic fields setup due to the flow of welding current

Arc blow in DC arc welding

Arc welding equipmentThis allows two different types of power supply characteristics:

i. Constant current(droop curve machines or droopers) In a constant current power supply, the current (amperage)

stays relatively constant when the voltage is changed.

- Shield Metal Arc Welding (SMAW)- For a large change in o/p voltage, the corresponding change in current is so small that the quality of the weld

can be maintained.- This is essential for manual arc-welding processes, since

the maintenance of constant arc is nearly impossible by a human welder.

Amperes

Vol

ts

100

80

60

40

20

0

0 25 50 75 100 125 150

25

115

Characteristics of constant current power supply.• The machine provides a high voltage for striking the arc.

• Open circuit voltage (OCV)• When the arc is struck, the voltage drops to the welding voltage.

• Arc voltage• Arc voltage varies with the arc length.

• As the welding proceeds, the current will not vary much as the arc length changes.

• Increasing the voltage from 20 to 25 volts (25%) only decreases the amperage from 125 to 115 Amp (8%).

ii. Constant Voltage

-To maintain constant arc gap.

- In a constant voltage power supply, the voltage stays relatively constant when the amperage is changed.

- Automatic welding (self corrective) which will maintain constant arc gap

When the electrode comes a bit closer to the work, the arc voltage drops raising the output current to a very high value. This current instantly melts the electrode and thus maintains the arc gap. And vice versa.

Amperes

Vol

ts

100

80

60

40

20

0

0 25 50 75 100 125 150

Characteristic curves of a constant Voltage arc welding machine

I : 100 to 200V: 30 to 25 only

ISCCOCVOCVV

OCV= Open circuit voltageSCC= Short circuit current

For stable arc in constant voltage transformer, V arc = V transformer

For stable arc in constant current transformer, I arc = I transformer

Power P = VI

For maximum power, dP/dL = 0 Where L= Arc length

For linear power source characteristics, voltage (V) is

The constant-current or drooping type of power source is preferred for manual metal arc welding since it is difficult to hold a constant arc length.

The changing arc length causes arc voltage to increase or decrease, which in turn produces a change in welding current.

The welding voltages range from 20 to 30 V depending upon welding current i.e. higher the current, higher the voltage.

Welding current depends on the size of the electrode i.e. core diameter.

The approximate average welding current for structural steel electrodes is 35xd (where d is electrode diameter in mm) with some variations with the type of coating of electrode.

The output voltage of the power source on ‘no load' or ‘open circuit' must be high enough to enable the arc to be started.

A value of 80 V is sufficient for most electrodes but certain types may require more or less than this value.

A manual welding power source is never loaded continuously because of operations such as, electrode changing, slag removal etc.

Most MMA welding equipment has a duty cycle of around 40% at maximum welding current.

Arc welding machine specifications:

1.Max. rated open-circuit voltage: The voltage between the o/p terminals when no welding is done, normally fixed at 80V. Normally 40 to 50V is enough for starting an arc. For continuous welding 20 to 30 V is sufficient.

Min. welding load voltage Vm = 20 + 0.04 I, where I= load current in amp

2. Rated current in amp: The max. current in amp that a welding machine is capable of supplying at a given voltage. Preferred current ratings are 150, 200, 300, 400, 500, 600 and 900 A

3. Duty cycle: The % of time in a 10 min period, that a welding machine can be used at its rated o/p without overloading. Normally 60% duty cycle is suggested.(remaining time for setting up, metal chipping, cleaning and inspection). For automatic welding machines, 100% duty cycle.

I2D = ConstantWhere D= Duty cycleI= Current

1. The arc length characteristic is given by expression V=24 + 4L (L= arc length in mm). The volt ampere characteristic of a power source can be approximated by a straight line with open circuit voltage 80V and short circuit current is 600A. Determine optimum arc length for maximum power.

OCV=80VSCC=600A

V= 80-[80/600]IStable Varc =V transformer

V=24 +4L = 80 –[80/600]I

I = [56-4L] [60/8]

We know power P= VI={24 +4L}{56-4L}{60/8}=[60/8] {(56)(24) + 128L -16 L2

For maximum power, dp/dL = 0128-32L =0L=4 mmThe arc length for maximum power is 4 mm

2. Arc length voltage characteristic can be represented by V=20 + 4 L. If the arc length in the welding operation varies between 4 mm to 6mm and the welding current between 450 amp to 550 amp, assuming a linear power source characteristic, calculate 1. Open circuit voltage(OCV) 2. Short circuit current (SCC)

At arc length L1=4mmV1= 20 +(4x4) =36 voltsAt arc length L2=6mmV2= 20+(4x6)=44 volts

I (Amp)

V (volts)

(550,36)

(450,44)

Equation of the line:

V-36 = {(44-36)/(450-550)} x (I – 550)

V= 80 – (8/100) I

y - y1 = [(y2 - y1) / (x2 - x1)]·(x - x1)

V-V1 = {(V2-V1)/(I2-I1)} x (I - I1)

ISCCOCVOCVV

By comparison,

OCV = 80V

OCV/SCC = 8/100

SCC= 1000 Amp

V= 80 – (8/100) I

3. A DC welding machine with a linear power source characteristic provides OCV of 80 V and SCC of 800 A. During welding with the machine, the measured arc current is 500 A corresponding to an arc length of 5 mm and the measured arc current is 460 A corresponding to an arc length of 7 mm. What is the linear voltage (V) – arc length (L) characteristic of the welding arc.

ISCCOCVOCVV

V= 80-(80/800)I = 80 –(1/10)I

Let linear voltage(V)-arc length(L) characteristic of welding arc be V= a + b (L)Where a, b are constants.

For arc current (I)=500 A, Arc length L = 5 mm

From (1), V= 80-(1/10) 500 = 30 VFrom(2), V= a + 5 b

---(1)

---(2)

Equating these two, a + 5b = 30

Similarly , for arc current(I) , 460 A, arc length (L) is 7 mm

From (1), V= 80-(1/10)460= 34 V

From (2), E= a+ 7b

Equating these two, a + 7b = 34

Solving these equations, a= 20 , b= 2

So, V= 20 + 2 L

Electrodes in Arc WeldingTypes1. Consumable Electrodes:

a) Bare electrodes: MIG, SAW

b) Coated electrodes: MMAW (SMAW)

2. Non-Consumable Electrodes: TIG, Atomic Hydrogen welding, Plasma Arc Welding, Carbon(graphite) Arc Welding.

High Energy Beam

a) Electron Beam Welding

b) Laser Beam Welding

If Metal is included in the title of welding process, it uses consumable electrode.

If Metal name like carbon, Tungsten is used, It is non consumable electrode. So Filler metal is used separately.

Basic functions of Electrodes

1. Strike the arc with work, Stabilizes and directs the arc

2. Formation of slag, which protects solidified hot metal from atmospheric gases

3. Filler metal

4. Shielding of weld pool- Forms a gaseous shield

5. Alloying with certain elements such as Cr, Ni, Mo to improve weld metal properties

Welding of dissimilar metals: Forge welding, Resistance welding, Friction welding, Laser beam welding, Explosive welding (cladding-for corrosion prevention), Soldering, Brazing, Adhesive bonding etc.

Applications

Weld Bead Geometry

Figure shows the important parameters of the weld bead geometry for a butt weld.

Manual Metal Arc Welding (Shielded Metal Arc Welding SMAW)

-Most extensively used manual welding process, with stick(coated electrodes)-Least expensive than most of the arc-welding processes-For all jobs in any position-Currents: 50 to 500A; Voltages 20 to 40-Dis Adv: slow speed(1 to 8 kg/hr for flat position)

Shielded Metal Arc Welding (SMAW) SMAW is a process in which an electric arc is established between the

electrically grounded work piece and a 9”-18” length of covered consumable metal rod, called the electrode.

The heat of the arc melts the base metal in the immediate area, the electrode’s metal core, and any metallic elements in the coating of the electrode.

It also melts, vaporizes or breaks down chemically non-metallic substances in the coating to shield the arc, protect the weld, and add alloys or properties to the weld deposit.

Stick is the most commonly used name for SMAW. This is because the electrode resembles a stick. It is also, often referred to as manual welding

The SMAW process uses constant current power sources.

Section view of arc welding with a coated electrode

SMAW-Shielded Metal Arc Welding

(Co2)

Coatings give off inert gases such as carbon dioxide under the arc heat, which shields the molten metal pool and protects it from the atmospheric oxygen, hydrogen and nitrogen pick-up, thus reducing the contamination of the weld metal.

Coatings provide flux to the molten metal, which mixed with the oxides and other impurities present in the puddle, forms slag. The slag being lighter, floats on the top of the puddle and protects it against the surrounding air during the weld-bead solidification. The slag covering also helps the metal to cool slowly preventing the formation of a brittle weld. When the weld is sufficiently cooled, the slag can be removed exposing the shiny weld underneath.

Inert-Gas Shielded Arc Welding

To complete exclusion of oxygen and other gases, which interfere with the weld pool to the detriment of the weld quality.

In manual metal arc welding, the use of stick electrodes does this job to some extent but not fully.

In Inert gas shielded arc welding process, a high pressure inert gas flowing around the electrode while welding, would physically displace all the atmospheric gases around the weld metal to fully protect it.

Ex: Carbon dioxide(1890-Higher currents but economical gas), Helium & Argon gases with non consumable electrodes in 1930. Argon is preferred(low arc voltage, longer arc)

No separate flux

Tungsten Inert Gas Arc Welding (TIG, GTAW)Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is

a welding process in which heat is generated by an electric arc struck between a tungsten non-consumable electrode and the work piece.

The weld pool is shielded by an inert gas (Argon, Helium, Nitrogen) protecting the molten metal from atmospheric contamination.

The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, if required(for thick sheets). Generally a bare wire. Difficult to use for thick sheets.

Nozzle(shield) size, gas flow rate, filler rod size, electrode diameter, current to be chosen depending on position of the weld & metal thickness.

Nozzle

Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used in the process.

Preferred for Al, Mg, stainless steel, cast iron, High carbon steel of thin sheets.

Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): 

Weld composition is close to that of the parent metalHigh quality weld structureSlag removal is not required (no slag)Thermal distortions of work pieces are minimal due to concentration of heat in small zone.Suitable for all types of metals

Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): 

1.Low welding rate2.Relatively expensive3.Requires high level of operators skill4.For thin sheets only

Metal Inert Gas Welding (MIG, GMAW)

Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in which the weld is shielded by an external gas (Argon, helium, CO2, argon + Oxygen or other gas mixtures).

For more thickness sheets than in TIG welding

Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic.

The operator controls only the torch positioning and speed. 

Power supply: Constant voltage type only. Normally with DCEP. With DCEN arc is highly unstable and results in spatter.

Shielding gas is used and hence no flux

MIG

Advantages of Metal Inert Gas Welding (MIG, GMAW): 

1.Continuous weld may be produced (no interruptions)2.High level of operators skill is not required3.Slag removal is not required (no slag)

Disadvantages of Metal Inert Gas Welding (MIG, GMAW): 

1.Expensive and non-portable equipment is required2.Outdoor application are limited because of effect of wind, dispersing the shielding gas.

Submerged Arc Welding is a welding process, which utilizes a bare consumable metallic electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld.

The arc heats and melts both the work pieces edges and the electrode wire. The molten electrode material is supplied to the surfaces of the welded pieces, fills the weld pool and joins the work pieces.

Since the electrode is submerged into the flux, the arc is invisible.

Flux partially melts and forms a slag protecting the weld pool from oxidation and other atmospheric contaminations. For faster welding jobs.

Consumable electrode wire: 1.5 mm to 12 mm in diameterCurrent: 300A to 4000A(very high currents)Metal deposition rates: more than 20 kg/hrHigh welding speeds: 5m/ minPlate thickness: 75 mm in single pass

No shielding gas and hence flux is used separately

Submerged Arc Welding

A part of the flux melts and forms the slag, which covers the weld. The unused flux is collected and reused.

Advantages of Submerged Arc Welding (SAW): 

1.Very high welding rate2.The process is suitable for automation3.High quality weld structure

Disadvantages of Submerged Arc Welding (SAW): 

1.Weld may contain slag inclusions2. Limited applications of the process - mostly for welding horizontally located plates.

Forge Welding

Forge welding is a welding process of heating two or more pieces of metal and then hammering them together.

The process is one of the simplest methods of joining metals and has been used since ancient times.

Forge welding is versatile, being able to join a host of similar and dissimilar metals.

With the invention of electrical and gas welding methods during the Industrial Revolution, forge welding has been largely replaced.

Forge Welding cont.. Forge welding between similar materials is caused by solid-state

diffusion. This results in a weld that consists of only the welded materials without any fillers or bridging materials.

Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. Due to this the weld is often stronger than the individual metals.

The temperature required to forge weld is typically 50 to 90% of the melting temperature.

The oldest welding process in the world. Oxides must be removed by flux or flames

Resistance welding

Fusion welding Process where both heat and pressure are applied on the joint but no filler metal or flux is added.

Heat necessary for melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name resistance welding.

This is one of the most commonly used process in sheet-metal fabrication and in automotive-body assembly.

No consumables.

For joining dissimilar materials(different thicknesses)

A low voltage (typically 1 V) and very high current (Typically 15,000 A) is passed through the joint for a very short time(typically 0.25 s)

Resistance Spot Welding

(a) Sequence in resistance spot welding. (b) Cross-section of a spot weld, showing the weld nugget and the indentation of the electrode on the sheet surfaces.

Resistance Spot Welding cont..Heat generated in resistance welding in JoulesH = I2 R t kt = time in secR = resistance (ohms )I = current (Amp)k = factor which represents energy losses through conduction

and radiation (<1)

The total resistance in this process consists of a) Resistance of the electrodes.b) Electrode-workpiece contact resistance.c) Resistance of individual parts to be weldedd) Workpiece-workpiece contact resistance.

Principle of Resistance spot Welding

Electric resistance welding Schedule

One of the very important characteristics of the resistance welding process is the transfer of heat to the two parts being joined differently so that proper fusion obtained even when the plates are dissimilar from the stand point of material or thickness.

Proper fusion can be obtained only if there is proper heat balance, by providing an electrode with a smaller contact area at the thinner sheet and a thicker electrode at the thicker sheet together with very high current densities for short times.

If two dissimilar metals with different electrical conductivities or thermal conductivities are to be joined,

1.Use large contact area electrode for the one which has higher electrical conductivity

2.Use small contact area electrode for the one which has higher thermal conductivity

Electrode Shapes for Spot Welding- Heat Balance

• Very little skill is required to operate the resistance welding machine.

• These are very well suited for mass production, as they give a high production rate.

• There are no consumables used in this process except for the electrical power and a relatively smaller electrode wear.

• Heating of the workpiece is confined to a very small part, which results in less distortion.

• It is possible to weld dissimilar metals as well as metal plates of different thicknesses.

Advantages

• The resistance welding machine is highly complex with various elements such as a heavy transformer, electrodes and heavy conductors for carrying the high currents, the electrode force applying mechanism such as a pneumatic cylinder and its supply, the heavy machine structure to support the large forces and an expensive timing arrangement.

• Certain resistance welding processes are limited only to

lap joints.

Disadvantages

Spot Welding Configurations

Schematic illustration of an air-operated, rocker-arm, spot welding machine

Spot Welding

(a) and (b) Spot-welded cookware and muffler. (c) An automated spot-welding machine with a programmable robot; the welding tip can move in three principal directions. Sheets as large as 2.2 m 0.55 m (88 in. 22 in.) can be accommodated in this machine.

Resistance Seam Welding

(a) Resistance seam-welding process in which rotating rolls act as electrodes; (b) overlapping spots in a seam weld; (c) roll spot welds; (d) mash-seam welding.

Modification of spot welding – bottom and top electrodes replaced by rotating wheels.

The electrically conducting rollers produce spot weld, when the current reaches a high value.

This process can be carefully controlled to produce continuous seam.

Applications: Mufflers, gasoline tanks.

Resistance Projection Welding

High electrical resistance at the joint is developed by embossing one or more projections on the surface to be welded.

High localized temperatures are generated at projections which are in contact with flat making part.

Process produces number of weld in one pass, extended electrode life.

(a) Schematic illustration of resistance projection welding. (b) A welded bracket. (c) and (d) Projection welding of nuts or threaded bosses and studs. (e) Resistance-projection-welded grills.

Resistance Projection Welding cont..

Another variation of spot welding. Where of the sheets to be joined is provided with a number of

projections to help localize the current at a predetermined spot. The projection are very small (0.8 mm) and are obtained by means

of embossing. Because of the localization of the current by the use of projections it

is no more necessary to use small size electrodes. As the welding current from this projections they often get melted

and a fusion joint is made under pressure applied from the electrodes.

Applications: Metal Baskets, grills, over drags, shopping carts.

Name AWS Characteristics Applications

Resistance

spot weldingRSW

Two pointed electrodes

apply pressure and current

to two or more thin

workpieces

Automobile

industry,

Aerospace

industry

Resistance

seam

welding

ERW

Two wheel-shaped

electrodes roll along

workpieces, applying

pressure and current

Aerospace

industry, steel

drums, tubing

Upset

weldingRSEW

Butt joint surfaces heated

and brought together by

force

Thermit Welding(Exothermic Welding) Thermit welding is the process of igniting a mix of high energy materials, also

called thermite, that produce a metallic slag that is poured between the working pieces of metal to form a joint.

Commonly utilizing the composition of iron oxide red powder (Fe3O4) with aluminium powder(Al) giving aluminium oxide powder (Al2O3) and iron(Fe).

To heat the metal, it requires no external source of heat or current.

Aluminium dust reduces the oxide of another metal, most commonly iron oxide, because aluminium is highly reactive.

The complete reaction takes place in a total of 1 minute, irrespective of the amount of thermit mixture present in the crucible.

Thermit Welding cont… A violent exothermic reaction occurs. Aluminium is a strong reducing

agent, and combines with the oxygen from the iron oxide, the iron oxide being reduced to iron and intense heat will be released.

Ignition temp. 12000C Produces a white hot iron slag and vapors of aluminium oxide.

8Al + 3Fe3O4 9Fe + 4 Al2O3 + Heat Temperature of this exothermic reaction is around 2500 °C. Because of large difference in densities, aluminium oxide floats on the

top with molten steel settling below. Sometimes the mixture contains other materials(like manganese) to

impart special properties to the weld. Used for aligning the parts to be joined Gap is usually filled with wax, around which a sand or ceramic mold is

buit. If the parts are thick, the mold cavity may be preheated to improve

welding and dry the mold.

Other thermit mixtures: Aluminium and copper oxide(for welding copper cables)

Similar to casting process. Molten metal obtained by thermit reaction is poured into the refractory cavity made around the joint.

1.Wax is poured in the joint and wax pattern is formed where the weld is to be obtained.2. A molding flask is kept around the joint and sand is rammed carefully around the wax pattern3.Pouring basin, sprue and riser are made4.A bottom opening is provided to run off the molten wax5.The wax is melted through the opening at the bottom, which is used to preheat the joint and make it ready for welding6.The igniting mixture ( barium peroxide or magnesium) is placed at the top of the thermit mixture and is ignited by means of a heated rod of acetylene gas7.Complete reaction takes place & molten metal is produced.8.Strength of thermit welded joint is same as forged metal without any defects.

Applications

Welding and repairs of large forgings and broken castings. Welding of thick structural sections. Rail road repairs, joining tracks on site. Welding cable conductors

FRICTION WELDING

• It is solid state joining process.

• Mechanical friction between a moving work piece (1500 to 3000 rpm) and a stationary component.

• Lateral force (upset) is applied (40 MPa to 450MPa) to plastically displace and fuse the materials

• Similar to lathe machine• Power requirements:

25KVA to 175 KVA

Major Parameters: Rotational speed, axial pressure applied & time( 2-30 sec)

Types of Friction Welding

• Spin Welding:A rotating chuck along with flywheel.

Linear Friction Welding:

Oscillating Chuck is used.

Used for non-round shapes as compared to Spin welding.

Material should be of high shear strength.

• Friction Surfacing:

It is a surface coating process.

Coating material Mechtrode is rotated under pressure over substrate.

• Friction Stir Welding:

A cylindrical shouldered tool, with a profiled probe (nib or pin) is used.

Friction is between tool shoulder, nib and workmetal.

Advantages of Friction

• Simplicity of operation and simple equipment.• Less time requirement.• Low Surface impurities and oxide films.• Heat affected zone is small as compare to conventional

flash welding.• There is no flux, gas, filler metal or slag present to cause

imperfections in welds. • Dissimilar metals can be joined• No distortion and warping• Because of high quality of the weld obtained, widely

accepted in aerospace and automobile industry for critical parts

Disadvantages of Friction

• Process is restricted to flat and angular butt welds.• Used only for joining small parts.• It require heavy rigid machine due to high Thrust

pressure.• In case of tube welding process becomes complicated.• In case of high carbon steels it is difficult to remove

flash.

Materials used in Friction welding process:

Variety of metals can be joined by this process as well as it gives variety of metals combination which cannot join by conventional process.

• Aluminum • Brass • Cast iron• Ceramic• Copper• Lead• Bronze

• Aluminum Alloys• Steel Alloys• Magnesium • Magnesium Alloys• Tungsten• Vanadium

Cu + Al

Weldable Material Combinations

Applications

• Automobile: Bimetallic engine valve, universal joint yoke, gear hub etc.

• Aerospace: Turbine blade joining, seamless joining etc.

• Consumer: Hand tools, sports equipment • Industrial: Spindles, tapers, tools• Military • Medical: Stainless steel joining of containers• Marine: Shipping Industry• Mining/Drilling: Twist drill etc.• Hydraulic equipments

Applications

Drill Pipe

Since dissimilar metals can often be joined, a significant cost savings can be realized by designing bimetallic parts that use a minimum of expensive metals only where needed.

Expensive forgings and castings can sometimes be replaced with less expensive forgings welded to bar stock, tubes, and plates, or with components created solely by welding together bar stock, tubes, and plates.

Process diagram for Plasma arc welding

Power source

+- Plasma gas

Tungsten electrode

Shielding gas

Orifice to constrict arcPlasma stream

Nozzle

Circulating Coolant

Plasma-Arc Welding Process

Two types of plasma-arc welding processes:

(a) transferred, (b) non-transferred.

Deep and narrow welds can be made by this process at high welding speeds.

Plasma-Arc Welding Process (PAW) Plasma is a state of matter reached by gases when heated to

temperatures above 1000 °C

PAW is an arc welding process that uses a constricted arc between a non-consumable electrode and the work or between the non-consumable electrode and the constricting nozzle

Compared to TIG, PAW has higher energy concentration – higher temperature, constricted cross-sectional area, velocity of plasma jet

Stiff columnar plasma

Torch – to – work distance less critical

Complete penetration single pass welding, small heat affected zone

High welding speeds

Transferred arc mode for welding applications

Non -Transferred arc mode for thermal spraying and heating non-conducting materials

Laser Welding (LW)Laser Welding (LW) is a welding process, in which heat is generated by a high energy laser beam targeted on the work piece. The laser beam heats and melts the work pieces edges, forming a joint.

Energy of narrow laser beam is highly concentrated: 108-1011 W/in2 (108-1010 W/cm2), therefore diminutive weld pool forms very fast (for about 10-6 sec.). Solidification of the weld pool surrounded by the cold metal is as fast as melting. Since the time when the molten metal is in contact with the atmosphere is short, no contamination occurs and therefore no shields(neutral gas, flux) are required.

The joint in Laser Welding (Laser Beam Welding) is formed either as a sequence of overlapped spot welds or as a continuous weld.

Laser Welding is used in electronics, communication and aerospace industry, for manufacture of medical and scientific instruments, for joining miniature components.

Advantages of Laser Welding: 

Easily automated process;Controllable process parameters;Very narrow weld may be obtained;High quality of the weld structure;Very small heat affected zone;Dissimilar materials may be welded;Very small delicate work pieces may be welded;Vacuum is not required;Low distortion of work piece.

Disadvantages of Carbon Arc Welding: 

Low welding speed;High cost equipment;Weld depth is limited.

Name AWS Characteristics ApplicationsCoextrusion welding CEW Dissimilar metals are extruded through the

same dieJoining of corrosion resistant alloys to cheaper alloys

Cold pressure welding CW Joining of soft alloys such as copper and

aluminium below their melting point Electrical contacts

Diffusion welding DFW No weld line visible Titanium pump impellor wheels

Explosion welding EXW Joining of dissimilar materials, e.g. corrosion resistant alloys to structural steels

Transition joints for chemical industry and shipbuilding. Bimetal pipelines

Electromagnetic pulse welding

Tubes or sheets are accelerated by electromagnetic forces. Oxides are expelled during impact

Automotive industry, pressure vessels, dissimilar material joints

Forge welding FOW The oldest welding process in the world. Oxides must be removed by flux or flames. Damascus steel

Friction welding FRW Thin heat affected zone, oxides disrupted by friction, needs sufficient pressure

Aerospace industry, railway, land transport

Friction stir welding FSW A rotating consumable tool is traversed

along the joint line

Shipbuilding, aerospace, railway rolling stock, automotive industry

Hot pressure welding HPW

Metals are pressed together at elevated tempeartures below the melting point in vacuum or an inert gas atmosphere

Aerospace components

Hot isostatic pressure welding HPW A hot inert gas applies the pressure inside a

pressure vessel, i.e. an autoclave Aerospace components

Roll welding ROW Bimetallic materials are joined by forcing them between two rotating wheels Dissimilar materials

Ultrasonic welding USW High-frequency vibratory energy is applied to foils, thin metal sheets or plastics.

Solar industry. Electronics. Rear lights of cars.

Diffusion: Process of movement of atoms from one location of higher concentration to another of lower concentration or to a vacant place.

Diffusion of atoms would be faster at high temperatures and in liquid phase. It is also a time dependent phenomenon as the atoms have to physically travel from one site to the other.

No plastic deformation occurs.

Diffusion welding can be used for joining metals to metals and metals to nonmetals.

Liquidus temperature: It specifies the temperature above which a material is completely liquid

Solidus temperature: Temperature on a phase diagram below which a given substance is completely solid (crystallized)

Brazing & Soldering Metal Joining Processes

Brazing & Soldering

• Filler metal distributed by capillary action

• Only filler metal is melted, not base metal

• Strength of joint typically

– Can join dissimilar metals

– Less heat - can join thinner sections (relative to welding)

– stronger than filler metal itself– weaker than base metal

– Excessive heat during service can weaken joint

• Pros & Cons

• Lower temperatures than welding

– gap at joint important (0.001 – 0.010”)

• Metallurgical bond formed between filler & base metals

Brazing: A joint with the help of a filler metal whose liquidus temperature is above 4500C and is below the solidus temperature of the base metal.

The filler metal is drawn into the joint by means of capillarity.

Dissimilar metals such as stainless steel to cast iron can be joined by brazing.

Except Aluminium and Magnesium, brazing can join all metals.

Not useful for high-temperature service because of low melting temp. of the filler metal.

Joints need to be extremely clean. Fluxes are added into the brazed joint to remove any of the oxides present or prevent the formation of oxides so that base metal and filler metal remain pure during brazing.

Ex: 75% borax + 25% boric acid

Number of filler metals are available.

For brazing ferrous materials, copper based filler metals with less zinc content are used.

For brazing Aluminium, Al-Si filler material is used

Silver brazing is used for applications requiring high strength (upto 900 MPa tensile strength)and high temperature service.

Heat source: Molten bath of brazing filler metal, oxyacetylene torch, controlled atmospheric furnace.

Braze Welding: Similar to brazing. In braze welding the filler metal reaches the joint without the capillary action since the joint gap is bigger.

Edge preparation will be done.Filler metal enters the joint by gravity.Similar to oxyacetylene gas welding. Filler metals: Brasses(Cu + Zn) with zinc content upto 40%.Joint is obtained by diffusion.

Explosive Welding: In explosive welding (EXW), 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 the order of thousands of MPa) are created between the plates, which clear all the oxide and scales present in the interface and make a clean joint.

Produce sound welds without any shock effects on the plates joined. The detonation velocity changes with the thickness of the plate being welded(2.4 to 3.6 km/s)

Applications: Cladding of metals for the purpose of corrosive prevention. Very large plates can be cladded.

Joining of dissimilar metals such as titanium to steel, aluminium to steel, aluminium to copper can be successfully carried out.

Brazing

Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action

Metal Joining Processes

Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys (melt point typically above 4500C)

• Automotive - joining tubes

• Electrical equipment - joining wires• Jewelry Making

• Flux also used

• Types of brazing classified by heating method:

– Torch, Furnace, Resistance

• Joint can possess significant strength

Soldering: Joining similar or dissimilar metals by means of a filler metal whose liquidus temperature is below 4500C

Used for obtaining a neat leak-proof joint or a low-resistance electrical joint. Not suitable for high-temperature service because of the low melting temperatures of the filler metals used.

Similar to brazing as filler metal enters the joint by capillary action.

Soldered joint is weaker compared to a brazed joint.

To remove oxides from the joint surfaces, fluxes are used.

For electrical soldering work, Rosin and Rosin plus alcohol based fluxes are used

For nonelectrical soldering work, organic fluxes such as zinc chloride and aluminium chloride are used.

Filler metals are normally called solders, which are alloys of lead and tin (lower liquidus temp.)

Soldering iron is a copper rod with a thin tip, which is heated by keeping in a furnace or by means of an internal electrical resistance , whose power rating may range from 15W for electronic applications and of 200W for sheet metal joining.

Soldering

Solder = Filler metal

Metal Joining Processes

Soldering

Applications:

• Printed Circuit Board (PCB) manufacture• Pipe joining (copper pipe)• Jewelry manufacture

Easy to solder: copper, silver, gold

Difficult to solder: aluminum, stainless steels

(can pre-plate difficult to solder metals to aid process)

• Alloys of Tin (silver, bismuth, lead)

• Melt point typically below 4500C

Flux used to clean joint & prevent oxidation

• Typically non-load bearing

Tinning = pre-coating with thin layer of solder

• separate or in core of wire (rosin-core)

PCB Soldering

• Soldering Iron & Solder Wire

Metal Joining Processes

Manual PCB Soldering

• Heating lead & placing solder

• Trim excess lead

• Heat for 2-3 sec. & place wire opposite iron

PTH - Pin-Through-Hole connectors

Flame Cutting• To separate piece of material into two or more pieces or into various

contours.• Through heat source – torches, electric arcs, lasers.

Types of flame cutting- Arc cutting- Oxy fuel (acetylene) gas cutting

Arc cutting• Same principles as that of arc welding processes.• Air carbon – Arc cutting, plasma arc cutting, lasers and electron beams.• Process leaves ‘HAZ’.

Oxy-Acetylene Gas Cutting

(a) Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate, showing drag lines.

• Produces temperatures about 870 C while cutting steel. • Cutting occurs mainly by oxidation of steel.• Maximum thickness cut by oxy-acetylene gas is 300-350 mm.• The flame leaves drag lines on cut surface which are very rough.• Distortion may be a problem

Basic reactions with steelFe + O FeO + Heat

3Fe + 2O2 Fe3O4 + Heat

4Fe + 3O2 2Fe2O3 + Heat

pre-heating flames

Differences in torch tips for gas welding and gas cutting

Slag + Molten metal

Direction of travel

Position of cutting torch in oxy-fuel gas cutting

Drag Kerf

Water-Jet Cutting Process

Water-Jet Cutting Process

Nonmetallic Parts Made by Water-Jet Cutting

Brazing is a method of joining two metal work pieces by means of a filler material at a temperature above its melting point but below the melting point of either of the materials being joined.Flow of the molten filler material into the gap between the work pieces is driven by the capillary force. The filler material cools down and solidifies forming a strong metallurgical joint, which is usually stronger than the parent (work piece) materials. The parent materials are not fused in the process.

Brazing is similar to Soldering. The difference is in the melting point of the filler alloy: brazing filler materials melt at temperatures above 840°F (450°C); soldering filler materials (solders) melt at temperatures below this point.

The difference between brazing and welding processes is more sufficient: in the welding processes edges of the work pieces are either fused (with or without a filler metal) or pressed to each other without any filler material; brazing joins two parts without melting them but through a fused filler metal. 

Surface cleaning and brazing fluxesCapillary effect (Fundamentals of adhesive bonding#Wetting|wettability) is achieved by both: a proper Surface preparation and use of a flux for wetting and cleaning the surfaces to be bonded.Contaminants to be removed from the part surface are: mineral oils, miscellaneous organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides, scale, smut, rust.The work pieces are cleaned by means of mechanical methods, soaking cleaning and chemical cleaning (acid etching).

A brazing flux has a melting point below the melting point of the filler metal, it melts during the heating stage and spreads over the joint area, wetting it and protecting the surface from oxidation.It also cleans the surface, dissolving the metal oxides.It is important that the surface tension of the flux is: 1. Low enough for wetting the work piece surface; 2. Higher than the surface tension of the molten filler metal in order to provide displacement of the flux by the fused brazing filler. The latter eliminates the flux entrapment in the joint.The flux is applied onto the metal surface by brushing, dipping or spraying.

Advantages of brazingLow thermal distortions and residual stresses in the joint parts;Microstructure is not affected by heat;Easily automated process;Dissimilar materials may be joined;High variety of materials may be joined;Thin wall parts may be joined;Moderate skill of the operator is required.

Disadvantages of brazingCareful removal of the flux residuals is required in order to prevent corrosion;No gas shielding may cause porosity of the joint;Large sections cannot be joined;Fluxes and filler materials may contain toxic components;Relatively expensive filler materials.

Soldering

Soldering is a method of joining two metal work pieces by means of a third metal (solder) at a relatively low temperature, which is above the melting point of the solder but below the melting point of either of the materials being joined.Flow of the molten solder into the gap between the work pieces is driven by the capillary force. The solder cools down and solidifies forming a joint. The parent materials are not fused in the process.

Soldering is similar to Brazing. The difference is in the melting point of the filler alloy: solders melt at temperatures below 840°F (450°C); brazing filler materials melt at temperatures above this point.

The difference between soldering and welding processes is more sufficient: in the welding processes edges of the work pieces are either fused (with or without a filler metal) or pressed to each other without any filler material; soldering joins two parts without melting them but through a soft low melting point solder.

Soldering joints have relatively low tensile strength of about 10000 psi (70 MPa).

Surface cleaning and soldering fluxesCapillary effect (wettability) is achieved by both: a proper Surface preparation and use of a flux for wetting and cleaning the surfaces to be bonded.Contaminants to be removed from the part surface are: mineral oils, miscellaneous organic soils, polishing and buffing compounds, miscellaneous solid particles, oxides, scale, smut, rust.The work pieces are cleaned by means of mechanical methods, soaking cleaning and chemical cleaning (acid etching).

A soldering flux has a melting point below the melting point of the solder, it melts during the preheating stage and spreads over the joint area, wetting it and protecting the surface from oxidation. It also cleans the surface, dissolving the metal oxides.

It is important that the surface tension of the flux is: 1. Low enough for wetting the work piece surface; 2. Higher than the surface tension of the molten solder in order to provide displacement of the flux by the fused solder. The latter eliminates the flux entrapment in the joint.

The flux is applied onto the metal surface by brushing, dipping, spraying, in form of a gas-flux foam or by a flux wave (flowing flux forms a wave and the printed circuit board moves over the apex of the wave).Flux is acidic therefore its residuals may cause corrosion if not removed.

Advantages of soldering

Low power is required;Low process temperature;No thermal distortions and residual stresses in the joint parts;Microstructure is not affected by heat;Easily automated process;Dissimilar materials may be joined;High variety of materials may be joined;Thin wall parts may be joined;Moderate skill of the operator is required.

Disadvantages of soldering

Careful removal of the flux residuals is required in order to prevent corrosion;Large sections cannot be joined;Fluxes may contain toxic components;Soldering joints can not be used in high temperature applications;Low strength of joints.

Welding 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.Various welding defects can be classified into groups such as cracks, porosity, solid inclusions, lack of fusion and inadequate penetration, imperfect shape and miscellaneous defects.

1. CracksCracks may be of micro or macro size and may appear in the weld metal or base metal or base metal and weld metal boundary. Different categories of cracks are longitudinal cracks, transverse cracks or radiating/star cracks and cracks in the weld crater. Cracks occur when localized stresses exceed the ultimate tensile strength of material. These stresses are developed due to shrinkage during solidification of weld metal.

Fig 1: Various Types of Cracks in WeldsCracks may be developed due to poor ductility of base metal, high sulpher and carbon contents, high arc travel speeds i.e. fast cooling rates, too concave or convex weld bead and high hydrogen contents in the weld metal.

2. 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 may be easily prevented if work pieces are properly cleaned from rust, dust, oil and grease. Futher, 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.

Different Forms of Porosities

3. Solid Inclusion

Solid inclusions 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. During arc welding flux either in the form of granules or coating after melting, reacts with the molten weld metal removing oxides and other impurities in the form of slag and it floats on the surface of weld metal due to its low density. 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 proper groove is selected, all the slag from the previously deposited bead is removed, too high or too low welding currents and long arcs are avoided.

Slag Inclusion in Weldments

4. Lack of Fusion and Inadequate or incomplete penetration:

Lack of fusion is the failure to fuse together either the base metal and weld metal or subsequent beads in multipass welding 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.

Types of Lack of Fusion

Incomplete penetration means that the weld depth is not upto the desired level or root faces have not reached to melting point in a groove joint. If either low currents or larger arc lengths or large root face or small root gap or too narrow groove angles are used then it results into poor penetration.

Examples of Inadequate Penetration  

5. Imperfect ShapeImperfect shape means the variation from the desired shape and size of the weld bead.During undercutting a notch is formed either on one side of the weld bead or both sides in which stresses tend to concentrate and it can result in the early failure of the joint. Main reasons for undercutting are the excessive welding currents, long arc lengths and fast travel speeds.Underfilling may be due to low currents, fast travel speeds and small size of electrodes. Overlap may occur due to low currents, longer arc lengths and slower welding speeds.

Excessive reinforcement is formed if high currents, low voltages, slow travel speeds and large size electrodes are used. Excessive root penetration and sag occur if excessive high currents and slow travel speeds are used for relatively thinner members.Distortion is caused because of shrinkage occurring due to large heat input during welding.

6. Miscellaneous DefectsVarious miscellaneous defects may be multiple arc strikes i.e. several arc strikes are one behind the other, spatter, grinding and chipping marks, tack weld defects, oxidized surface in the region of weld, unremoved slag and misalignment of weld beads if welded from both sides in butt welds.

Welding ProcessesFusion Welding Processes

GMAW – Gas Metal Arc Welding

SMAW – Shielded Metal Arc Welding

Non-Consumable Electrode

GTAW – Gas Tungsten Arc Welding

Electron Beam Welding

SAW – Submerged Arc Welding

Consumable Electrode

PAW – Plasma Arc Welding

High Energy Beam

Laser Beam Welding

Welding ProcessesSMAW – Shielded Metal Arc Welding

• Slag keeps oxygen off weld bead during cooling

• Consumable electrode

• Flux produces protective gas around weld pool

• Flux coated rod

Power = VI 10 kW

Power... Current I (50 - 300 amps)Voltage V (15 - 45 volts)

• General purpose welding—widely used

• Thicknesses 1/8” – 3/4”• Portable

Welding ProcessesElectric Arc Welding -- Polarity

SMAW - DC Polarity

Straight Polarity

Shallow penetration Deeper weld penetration(thin metal)

Reverse Polarity

(+)(+)

(–)(–)

(–)(–)

(+)(+)

AC - Gives pulsing arc

- used for welding thick sections

Welding ProcessesGMAW – Gas Metal Arc Welding (MIG)

• DC reverse polarity - hottest arc

• MIG - Metal Inert Gas

• Consumable wire electrode

• AC - unstable arc

Groover, M., Fundamentals of Modern Manufacturing,, p. 734, 1996

Gas Metal Arc Welding (GMAW) Torch

• Shielding provided by gas

• Double productivity of SMAW

• Easily automated

Welding ProcessesSAW – Submerged Arc Welding

• 300 – 2000 amps (440 V)

• Consumable wire electrodeGas Metal Arc Welding (GMAW) Torch

• Shielding provided by flux granules

• Automated process (limited to flats)

• Low UV radiation & fumes

• Flux acts as thermal insulator

• High speed & quality (4 – 10x SMAW)

• Suitable for thick plates http://www.twi.co.uk

Welding ProcessesGTAW – Gas Tungsten Arc Welding (TIG)

• Non-consumable electrode

• a.k.a. TIG - Tungsten Inert Gas

• Shield gas usually argon

• Used for thin sections of Al, Mg, Ti.

• With or without filler metal

Power 8-20 kW

Current I (200 A DC) (500 A AC)

• Most expensive, highest quality

Welding ProcessesLaser Welding

Typical laser welding applications :

•Catheters & Other Medical Devices •Small Parts and Components •Fine Wires •Jewelry •Small Sensors •Thin Sheet Materials Down To 0.001" Thick

• Laser beam produced by a CO2 or YAG Laser

• High penetration, high-speed process

• Concentrated heat = low distortion

• Laser can be shaped/focused & pulsed on/off

• Typically automated & high speed (up to 250 fpm)

• Workpieces up to 1” thick

Welding ProcessesSolid State Welding Processes

Friction Welding

Ultrasonic Welding

Resistance Welding

Diffusion Welding

Welding ProcessesFriction Welding (Inertia Welding)

• One part rotated, one stationary

• Stationary part forced against rotating part

• Friction converts kinetic energy to thermal energy

• Metal at interface melts and is joined

• When sufficiently hot, rotation is stopped & axial force increased

Welding ProcessesResistance Welding

Resistance Welding is the coordinated application of electric current and mechanical pressure in the proper magnitudes and for a precise period of time to create a coalescent bond between two base metals.

• Heat provided by resistance to electrical current (Q=I2Rt)

• Force applied by pneumatic cylinder

• Typical 0.5 – 10 V but up to 100,000 amps!

• Often fully or partially automated

- Spot welding

- Seam welding

Welding ProcessesResistance Welding

Resistance Welding is the coordinated application of electric current and mechanical pressure in the proper magnitudes and for a precise period of time to create a coalescent bond between two base metals.

• Heat provided by resistance to electrical current (Q=I2Rt)

• Force applied by pneumatic cylinder

• Typical 0.5 – 10 V but up to 100,000 amps!

• Often fully or partially automated

- Spot welding

- Seam welding

Welding ProcessesDiffusion Welding

• Parts forced together at high temperature (< 0.5Tm absolute) and pressure

Kalpakjian, S., Manufacturing Engineering & Technology, p. 889, 1992

• Atoms diffuse across interface

• After sufficient time the interface disappears

• Good for dissimilar metals

• Heated in furnace or by resistance heating

• Bond can be weakened by surface impurities

Soldering & Brazing Metal Joining Processes

Soldering & Brazing

• Filler metal distributed by capillary action

• Only filler metal is melted, not base metal

• Strength of joint typically

– Can join dissimilar metals

– Less heat - can join thinner sections (relative to welding)

– stronger than filler metal itself– weaker than base metal

– Excessive heat during service can weaken joint

• Pros & Cons

• Lower temperatures than welding

– gap at joint important (0.001 – 0.010”)

• Metallurgical bond formed between filler & base metals

Soldering

Solder = Filler metal

Metal Joining Processes

Soldering

Applications:

• Printed Circuit Board (PCB) manufacture• Pipe joining (copper pipe)• Jewelry manufacture

Easy to solder: copper, silver, gold

Difficult to solder: aluminum, stainless steels

(can pre-plate difficult to solder metals to aid process)

• Alloys of Tin (silver, bismuth, lead)

• Melt point typically below 840 F

Flux used to clean joint & prevent oxidation

• Typically non-load bearing

Tinning = pre-coating with thin layer of solder

• separate or in core of wire (rosin-core)

PCB Soldering

• Soldering Iron & Solder Wire

Metal Joining Processes

Manual PCB Soldering

• Heating lead & placing solder

• Trim excess lead

• Heat for 2-3 sec. & place wire opposite iron

PTH - Pin-Through-Hole connectors

PCB Reflow Soldering Metal Joining Processes

Automated Reflow Soldering SMT = Surface Mount Technology

Printed solder paste on a printed circuit board (PCB)

• Solder Paste serves the following functions:– supply solder material to the soldering spot, – hold the components in place prior to soldering, – clean the solder lands and component leads – prevent further oxidation of the solder lands.

• Solder/Flux paste mixture applied to PCB using screen print or similar transfer method

• PCB assembly then heated in “Reflow” oven to melt solder and secure connection

Brazing

Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action

Metal Joining Processes

Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F)

• Automotive - joining tubes

• Electrical equipment - joining wires• Jewelry Making

• Flux also used

• Types of brazing classified by heating method:

– Torch, Furnace, Resistance

• Joint can possess significant strength

Brazing

Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action

Metal Joining Processes

Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F)

• Automotive - joining tubes

• Electrical equipment - joining wires• Jewelry Making

• Flux also used

• Types of brazing classified by heating method:

– Torch, Furnace, Resistance

• Joint can possess significant strength

Brazing Metal Joining Processes

Brazing

Figuring length of lap for flat joints.

X = Length of lap

              

T = Tensile strength of weakest member

W = Thickness of weakest member

C = Joint integrity factor of .8

L = Shear strength of brazed filler metal

Let’s see how this formula works, using an example.Problem: What length of lap do you need to join .050" annealed Monel sheet to a metal of equal or greater strength? Solution:

C = .8 T = 70,000 psi (annealed Monel sheet)

W = .050"

L = 25,000 psi (Typical shear strength for silver brazing filler metals)

X = (70,000 x .050) /(.8 x 25,000) = .18" lap length

Soldering & Brazing Metal Joining Processes

Brazing

Figuring length of lap for tubular joints.

          

 

X = Length of lap area

W = Wall thickness of weakest member

D = Diameter of lap area

T = Tensile strength of weakest member

C = Joint integrity factor of .8

L = Shear strength of brazed filler metal

Again, an example will serve to illustrate the use of this formula. Problem: What length of lap do you need to join 3/4" O.D. copper tubing (wall thickness .064") to 3/4" I.D. steel tubing?

Solution: W = .064"

D = .750"

C= .8

T = 33,000 psi (annealed copper)

L = 25,000 psi (a typical value)

X = (.064 x (.75 – .064) x 33,000)/(.8 x .75 x 25,000)

X = .097" (length of lap)

Coated Electrodes are specified based on core wire diameter.

Commonly used electrode diameters are 2, 2.5, 3.18, 4, 5 and 6 mm.

Length of electrodes may depend on diameter of core wire ranging from 250 to 450 mm i.e. larger the core diameter larger the length. However, special electrodes may be of 8-10 mm diameter

The electrodes are also specified based on ratio of diameter of coated portion of electrode to core wire diameter.

If this ratio is lesser than 1.2 then electrodes are thin coated, if ratio ranges between 1.2 to 1.5 then medium coated and if ratio exceeds 1.5 then electrodes are heavy coated or thick coated. This ratio may vary slightly in different codes.The ingress of oxygen and nitrogen from the atmosphere to the weld pool and arc environment would cause embrittlement and porosity in the weld metal and this must be prevented.

The Actual method of arc shielding from atmospheric nitrogen and oxygen attack varies with different type of electrodes

Electrode coating performs many functions depending upon coating constituents, during welding to improve weld metal properties.

The important functions are as follows:1.Improve the electric conductivity in the arc region to improve the arc ignition and stabilization of the arc.

2. Formation of slag, which;(a) Influences size of droplet.(b)Protects the droplet during transfer and molten weld pool from atmospheric gases.(c) Protects solidified hot metal from atmospheric gases.(d) Reduces the cooling rate of weld seam.

3. Formation of shielding gas to protect molten metal.4. Provide deoxidizers like Si and Mn in form of FeSi and FeMn.5. Alloying with certain elements such as Cr, Ni, Mo to improve weld metal properties.6. Improve deposition rate with addition of iron powder in coating.

Various constituents of electrode coating are cellulose, calcium fluoride, calcium carbonate, titanium dioxide, clay, talc, iron oxide, asbestos, potassium/sodium silicate, iron powder, ferro-maganese, powdered alloys, silica etc.

Each constituent performs either one or more than one functions.

Electrode metallic core wire is the same but the coating constituents give the different characteristics to the welds. Based on the coating constituents, structural steel electrodes can be classified in the following classes

1.Cellulosic ElectrodesCoating consists of high cellulosic content more than 30% and TiO2 up to 20%. These are all position electrodes and produce deep penetration because of extra heat generated during burning of cellulosic materials. However, high spatter losses are associated with these electrodes

2.Rutile ElectrodesCoating consists of TiO2 up to 45% and SiO2 around 20%. These electrodes are widely used for general work and are called general purpose electrodes.

3.Acidic ElectrodesCoating consists of iron oxide more than 20%. Sometimes it may be up to 40%, other constituents may be TiO2 10% and CaCO3 10%.

Such electrodes produce self detaching slag and smooth weld finish and are used normally in flat position.

4.Basic ElectrodesCoating consist of CaCO3 around 40% and CaF2 15-20%. These electrodes normally require baking at temperature of approximately 250 ° C for 1-2 hrs or as per manufacturer's instructions. Such electrodes produce high quality weld deposits which has high resistance to cracking.

This is because hydrogen is removed from weld metal by the action of fluorine i.e. forming HF acid as CaF2 generates fluorine on dissociation in the heat of arc.

Coating Constituent FunctionsMain Functions Other Functions

Cellulose Gas former Coating Strength and Reducing agent

Calcium Fluoride (CaF2) Slag basicity and metal fluidity, H2removal

Slag former

Clay (Aluminum Silicate) Slag former Coating strengthTalc (Magnesium Silicate) Slag former Arc stabilizer

Rutile (TiO2 ) Arc stabilizer, Slag former, Fluidity

Slag removal and bead appearance

Iron Oxides Fluidity, Slag former Arc Stabilizer, improved metal transfer,

Calcium Carbonate Gas former, Arc stabilizer Slag basicity, Slag formerAsbestos Coating strength Slag former

Quartz (SiO2 ) Slag fluidity, Slag former Increase in current carrying capacity.

Sodium Silicate / Potassium Silicate

Binder, Arc stabilizer Slag former

FeMn / FeSi Deoxidizer -Iron Powder Deposition Rate -

Powdered Alloys Alloying -

Coating Constituents and Their Functions

Classification of Electrodes as per Indian Standard:Structural steel electrodes were classified as per IS 814:1974 and this code was revised and the revised code is IS 814:1991.

As per IS 814:1974, electrodes are designated with letters and digits.P X X X X X X SPrefix (P) is either E or R which indicates solid extruded (E) or reinforced extruded (R) Electrode.

1 st digit – Indicates type of coating.2 nd digit – Indicates weld positions in which electrode can be used.3 rd digit – Indicates welding current conditions.4 th and 5 th digit – Indicate UTS and YS of all weld metal.6 th digit – Requirement of minimum % elongation and absorbed energy in charpy V- notch impact test of weld metal.Suffix (s) – P – Deep penetration electrodeH – Hydrogen controlled electrodeJ, K and L – Amount of metal recovery in case of iron powder electrodeSuffix (s) are optional and may or may not be given if not applicable.

IS 814:1991As per IS 814:1991 electrodes are designated with letters and digits as given below:E L X X X X SE indicates extruded solid electrode,L is a letter to designate type of coating,

First digit indicates UTS and YS of deposited weld metal,Second digit gives percentage elongation and impact values of weld metal depositedThird digit gives welding positions in which electrode can be used and Fourth digit gives the current conditions for the use of electrode.

Suffix(s) are optional and indicate special characteristics of electrode such as H1, H2, and H3 indicate hydrogen controlled electrodes with different amount of diffusible hydrogen J, K, L indicate different amount of metal recovery in weld pool in case of iron powder electrodes and X means radiographic weld quality

Electrodes: used for providing heat input in arc welding are 2 types. Consumable and non-consumable.Once the arc is initiated, the electrode is continuously consumed and hence, the electrode should be moved continuously towards the w/p to maintain the constant arc length. Since the electrode melts continuously, it also act as the filler rod to provide the filler metal into the joint.

Functions of providing a filler metal and heat are both built into a single electrode.

Consumable electrodes: steel, CI, copper, brass, bronze, Al.Welding process is called metal arc welding.Electrode may be bare or coated(stick electrode)Generally used in manual arc welding process.

Non-consumable electrodes: Carbon, graphite, Tungsten. Carbon & graphite used only in DC welding and Tungsten for both AC & DC.Separate filler metal rod is used. Can control heat input as well as the amount of filler metal deposited, since both are separately controlled.Welding process is termed by the electrode material used.Ex: Carbon arc welding, Tungsten arc welding

When starting the arc, strike the electrode like a match on the work surface by gently and quickly moving it along the weld metal.

Next, withdraw the electrode to form an arc length from the work piece of approximately 1/8”.

Reduce the arc length to the approximate length required to produce the desired weld.

Starting The Arc

Tapping technique

Electrode is brought straight down to work piece then lifted slightly to start arc

Scratch start technique

For AC welding

Drag electrode across work piece, immediately lift electrode after touching

1) Electrode2) Work piece3) Arc