Welding Process Note

7/28/2019 Welding Process Note

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Welding


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Introduction

JoiningWelding,Brazing,

Soldering andAdhesive bondingIn these process a permanent joint between theparts is formed and cannot be separated easily

Assembly usually refers to mechanical methodsof fastening the parts together. Some of these

methods allow easy disassembly, while others donot
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Welding

Welding is a material joining process in whichtwo or more parts are coalesced (joinedtogether) at their contacting surfaces by asuitable application of heat and/or pressure.In some welding process a filler material is

added to facilitate coalescence.Welding is most commonly associated withmetallic parts but for plastics also it is used.
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Types of Welding Processes

Solid state welding processesLiquid state welding processes

Solid / Liquid state bonding processes


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Solid State Welding Processes

In solid state welding the surfaces to be joined arebrought into close proximity by:

Heating the surfaces without causing melting and applyingnormal pressureProviding relative motion between the two surfaces andapplying light normal pressureApplying high pressure without heating

In these processes the materials remain in solidstate and welding is achieved through theapplication of heat and pressure, or high pressureonly


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Forge WeldingForge welding is the oldest method of welding in thecategory of solid state welding.Surfaces to be joined are heated till they are red hot and then forced together by

hammering.It is a crude method of welding and quality dependsupon t

he skill of the welder.
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Friction Welding
http://c/Windows/system32/Welding/Friction%20welding.flv


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Friction Stir WeldingFriction-stir welding joinsmetal with heat and highpressure.

1. The bobbin's shoulders, working like

a vise, stabilize 1-inch-thick aluminumalloy plates.

2. The rotating head, spinning at 275rpm, generates friction heat thatsoftens the metal along the path of theweld.

3. The pressure that merges the metalis absorbed by the bobbin instead of byseparate machinery.

4. The rotation of the tool leavesconcentric circles on both sides of the

joint
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Explosive Welding

Very high contact pressure developed by detonating athin layer of explosive.

The detonation imparts high kinetic energy to the piecewhich on striking the other piece causes plasticdeformation.No filler material is used and no diffusion takes place.
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Liquid State Welding

Arc WeldingResistance weldingOxyfuel gas weldingOther processes


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Arc WeldingIn Electric Arc Welding a sustained arc provides the heatrequired for melting the parent as well as filler material.

The workpiece and the electrode are connected to the twomaterials of the power source. The arc is started by momentarilytouching the electrode on to the workpiece and then withdrawingit to a short distance (a few mm) from the workpiece.
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Arc WeldingWhen the electrode and workpiece are in contact, current flowsand when they are separated an arc is generated and the currentcontinues to flow.

The arc is generated by the electrons liberated form cathode andmoving towards anode.

The arc changes electrical energy into heat and light.


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Arc WeldingAbout 70% of the heat liberated due to striking of electrons at anode raises the anode temperature to avery high values (5,000 to 30,000 oC). This heat melts thebase metal as well as tip of the electrode in the area

surrounding the arc.A weld is formed when the mixture of molten base andelectrode metal solidifies in the weld area.Since 70% heat is generated at anode a workpiececonnected to anode will melt 50% faster as comparedto if connected with cathode. This is why work piece isusually made positive and electrode as negative and istermed as straight polarity.


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Shielded Arc Welding

Consumable electrodesusually have a coating on itsouter surface which on

melting release gases likehydrogen or carbon dioxideto form a protective coveringaround the molten pool.

The electrode coating alsoreacts to from slag which isa liquid, lighter than themolten metal.


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Shielded Arc Welding The slag therefore rises to thesurface and on solidificationforms a protective covering overthe hot metal.

This also slows down the rate of cooling of the weld. The slaglayer can be removed by lightchipping.Electric arc welding of this type isknown as Shielded Metal ArcWelding . More than 50%industrial arc welding is done bythis method.


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Submerged Arc WeldingFor continuous arc welding operations , the consumableselectrode is bare wire in the form of a coil and the flux is fed intothe welding zone, or the weld area is covered by an inert gas. InSubmerged Arc Welding the base electrode is shielded bygranular flux supplied from a hopper, while in Gas Metal ArcWelding shielding of the area is provided by an inert gas such asargon, helium, carbon dioxide , etc.
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Tungsten Inert Gas WeldingNon consumable arc welding processes use tungsten electrodesand shielding is provided by an inert gas around the weld area.Once such process, the Gas Tungsten Arc Welding (GTAW) isalso called as Tungsten Inert Gas (TIG) welding.

It uses tungsten alloy electrode and helium gas shield. Becauseof inert gas atmosphere tungsten is not consumed. Fillermaterials supplied by a separate rod or wire.
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Resistance Spot Welding

Two opposing solid cylindrical electrodes arepressed against the lap joint and two metallic sheetsto be welded.

Current ranges 3,000 to 40,000 Ampere dependingon the requirement causes a weld nugget of sizevarying from 6 to 10 mm diameter to be formed atthe metallic interface.

The current is switched on for a duration lasting 0.1to 0.5 sec .

At low pressures , the resistance and heat are highand melted metal tend to squeeze out of the weld.


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http://c/Windows/system32/Welding/ResistanceSpotWelding.flvhttp://c/Windows/system32/Welding/ResistanceSpotWelding1.flv


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At high pressure , the resistance decreases andheat is less and smaller weld formed provides lowerweld strength. Thus, for a given set of conditions,optimum electrode current and electrodepressure are indicated.

The time duration of current flow should not bebeyond certain critical, because the heat then has achance to spread out and harm work piece andelectrode.Optimum values of current, pressure and theirapplication timing are dependent on weld size andmaterial.


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Seam Welding
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Oxy fuel Welding (OFW)

OFW is the term to describe the group of fusion operations that burn various fuelsmixed with oxygen to perform welding.

The OFW processes employ several type of gases, which is the primary distinction amongthe members of this group.

The most important OFW process isoxyacetylene welding .


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Oxy Acetylene Welding
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Oxy Acetylene Welding


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Oxy Acetylene WeldingFiller materials are used tosupply additional material tothe weld zone.Flux is often used to clean thesurfaces and to retard oxidationby providing inert gas shieldaround the weld area.Flux also helps in removing

oxide and other impurities.Borax , is the most commonflux, but sometimes othersubstances are added to

improve its effectiveness .


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Salient points aboutoxyacetylene welding

The heat is obtained by combustion of acetyleneand oxygen . Here primary combustion occurring inthe inner zone gives:

and the second reaction in the outer zone gives

The maximum temperature at the tip of inner cone

reaches up to 3000-3500

C . Therefore, most gaswelding is performed by keeping this inner zone tip just above the metal to be welded so thatmaximum temperature is available for welding.

HeatHCOOHC +++2222

2

HeatOHCOOHCO ++++ 2222 25.12


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Temperature distribution alongthe flame


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Flame Types

A neutral flame is obtained when the ratio of is oxygenand acetylene is 1. Most gas welding operations arecarried out by this flame.An oxidizing flame is obtained when this ratio is morethan 1. This type of flame is not suitable for welding of steels since excess oxygen present reacts with carbon insteel and is generally used for welding of copper and itsalloys.When the ratio in mixture is less than 1 a

carburizingflame is obtained. In this type of flame acetylenedecomposes into carbon and hydrogen and the flametemperature gets reduced. J oining operations such asbrazing and soldering which require lower

temperature generally use this flame .


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Flame Types
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Flame Types


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Flame Cutting

Metal is melted by the flame of the oxyfuelgas torch and blown away to form a gap orkerf.When ferrous metal is cut, actually burning of iron takes place according to one or more of the following reactions.

QOFeOFe

QOFeOFe

QFeoOFe

++

++

++

322

432

234

23


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Flame CuttingBecause, these reactions cannottake place below 815 C .Oxyfuel flame is first used toraise the metal temperature where burning can be initiated.

Then a stream of pure oxygen is added to the torch (or theoxygen content of the oxyfuelmixture is increased) to oxidizethe iron.

The liquid iron and iron oxides are then expelled from the jointby the kinetic energy of theoxygen gas stream.
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Flame CuttingLow rate of heat input, and need of preheating ahead of the cut, oxyfuel produces a relatively large heataffected zone and thus associated distortion zone.

The process is suitable when edge finish or tolerance is

not critical. Theoretically heat generated due to burning of Fe issufficient to continue cutting, however due to lossesadditional heat supply is needed.If the work is already hot from the other processes,

supply of oxygen t hrough a small diameter pipe isneeded to continue cut. This is called Oxygen LanceCutting . A workpiece temperature of 1200C is neededto sustain the cutting.


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Brazing

In brazing the joint ismade by heating thebase metal red hotand filling the gapwith molten metal whose melting

temperature is typicallyabove 450 C but belowmelting temperatureof base metal.
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Brazing

The filler metals aregenerally copper alloys .Cu-Zn and Cu-Ag alloys are used forbrazing because they

form alloy with iron and have goodstrength.


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Brazing

Brazing methods Torch and fillerrodsRing of filler metalat entrance of gapFoil of filler metalbetween flat partsurfaces


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Brazing Vs Welding

Welding joins metals bymelting and fusing them,usually adding a fillermaterial.Fusion requires concentratedheat directly at the joint, andtemperatures must exceedthe melting point of themetals and filler.Welded joints are usually asstrong or stronger than thebase materials.

Brazing differs from weldingin that the temperature isconsiderably lower and doesnot melt the base metals.Rather, the heat source meltsa filler metal and draws it intothe joint by capillary action.It creates a metallurgicalbond between the filler metaland part surfaces.


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Brazing Vs Welding

Welding is usually moresuited for joining largeassemblies.

Brazing applies heat to abroad area, often the entireassembly. Larger assembliestend to dissipate heat andcan make it difficult to reachthe flow point of the fillermetal.

Assembly Size

The intense heat of weldingwill likely burn through, or atleast warp, the thin section.

Brazing is the better optionon thin sections

Thickness


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Brazing Vs Welding

Welding and brazing both readily produce spot joints. Butlinear joints are usually easier to braze than weld.

Joint Configuration

Because welding melts thebase metals, attempting to

join copper to steel, forinstance, would requiresophisticated and expensivewelding techniques.

Brazing holds a significantadvantage when joining

dissimilar metals. These can form a strong jointwith minimal alteration of base metal properties.

Types of materials


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Brazing Vs Welding

Welding produce irregularbead after joint.

Requires final finishing

Brazing typically produces atiny, neat fillet, versus the

irregular bead of a welded joint. This is especially importanton consumer products-where

appearance is critical.Brazed joints can almostalways be used as is, withoutadditional finishingoperations.

Appearance
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Soldering

Soldering is verysimilar to brazingexcept that filler

material is usually alead-tin based alloy which has muchlower strength and

melting temperaturearound 250 C .
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Soldering

In this process lessalloying actionbetween base metal

and filler material ascompared to brazingtakes place hence thestrength of joint is

lesser.It is carried out usingelectrical resistanceheating .
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Joints in Soldering

a) Crimped lead wire PC boardb) Plated through hole on PC board to maximize solder contact areac) Hooked wire on flat terminalsd) Twisted wires
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Some common solder alloycompositions


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Welding Defects

Cracks This causes significant reduction in the strength of weldment. Welding cracks are caused by embrittlement orlow ductility of the weld and/or base metal combined with

high restraint during contraction.

Various forms of Welding Cracks


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Welding Defects

Cavities These includes porosity and shrinkage voids.


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Welding Defects

Solid inclusions These are metallic or non-metallic solid materialparticles entrapped in the weld metal. The most

common form is slag inclusion or metallic oxides.


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Welding Defects

Incomplete fusionlack of penetration.

Several form incomplete fusion


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Welding Defects

Imperfect shape

Welding Process Note

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