Different Types of Metal Joining

8/10/2019 Different Types of Metal Joining

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Different techniques ofDifferent techniques ofmetal joiningmetal joiningSubjects of Interest

Conventional joining techniques

Oxyacetylene welding

Shielded metal arc welding

Gas-tungsten arc welding

Gas-metal arc welding

Flux-core arc welding

Plasma arc welding

Submerged arc welding

Electroslag welding

Electron beam welding

Laser beam welding

Resistance welding

Friction (stir) welding

Suranaree University of Technology Sep-Dec 2007Tapany Udomphol


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ObjectivesObjectives

This chapter provides information on metal joiningprocesses, for example, fusion welding and other highenergy beam welding techniques.

Students are required to indicate advantages anddisadvantages of different methods of metal joining.

Students should be able to select appropriate methods ofmetal joining for the required applications.



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Introduction to metal joining

Types of metal joining

Power density of heat source

Type of joint and weld position Welding processes and materials



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Types of metal joining Types of metal joining

Fusion welding

Gas welding

Arc welding

High energy beam welding

Other joining techniques


Shielded metal arc welding (SMAW)Gas-tungsten arc welding (GTAW)Gas-metal arc welding (GMAW)Flux-core arc welding (FCAW)Plasma arc welding (PAW)Submerged arc welding (SAW)Electroslag welding (ESW)

Electron beam welding (EBW)Laser beam welding (LBW)

Friction (stir) weldingSuranaree University of Technology Sep-Dec 2007

BoltingRiveting

Brazing

Soldering

Adhesive joining

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Power density of heat sources As the power density of the heat sourceincreases, the heat input to the workpiece thatis required for welding decreases.

Variation of heat input to the workpiece with power density of the heat source.

Power density Deeper weld penetration Higher welding speeds Better weld quality Less damage to the workpiece

Variation of weld strengthwith unit thickness ofworkpiece heat input perunit length of weld.

Heat input

Weld strength



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Welding processes and materials



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Types of joints and welding positions

Five basic typesof weld jointdesigns.

Typical weld joint variations.

The surface of the weld is called the face . The two junctions between the face and theworkpiece surface are called the toes .

The portion of the weld beyond the workpiecesurface is called the reinforcement .Suranaree University of Technology Sep-Dec 2007

Four welding positions

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Suranaree University of Technology Sep-Dec 2007

Bolting

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A fastening method using a threadedpin or rod with a head at one end (bolt),designed to be inserted through holes inassembled parts and secured by amated nut , that is tightened by applyingtorque .

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Riveting

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Two or more pieces are jointed byinserting a headed shank through ahole and closed by forming a head

on the projecting part of the shank.

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Soldering

A joining method using a low melting point lead-tin alloy (solder) to

fill in a small gap between sheets. The solder wets the metal and produce an intermetallic bond. T ~ 183275 oC depending on compositions.

www.seas.upenn.edu

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Blazing

Blazing is similar to soldering but uses fillers ofhigher T m (450-800 oC).

Give better strength than soldering but might getoxidation problems or discoloration.

Heating methods such as gas torch, furnace, molten-flux bath, induction heating resistance heating are used.

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Gas welding


Gas welding is a welding process that

melts and joints metals by heating themwith a flame caused by a reaction of fuelgas and oxygen.

The most commonly used method isOxyacetylene welding , due to its highflame temperature.

The flux may be used to deoxidize andcleanse the weld metal.

The flux melts, solidifies and forms aslag skin on the resultant weld metal.

Oxyacetylene welding (a) overall process, (b) welding area.Tapany Udomphol


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There are three types of flame in oxyacetylene welding: Neutral flame

Reducing flame

Oxidizing flameThree types of flame in oxyacetylene welding

Acetylene ( C2H2) and O2 are mixed inequal amounts and burn at the tip of thewelding torch. The inner cone gives 2/3of heat whereas the outer envelopeprovides 1/3 of the energy.

The excess amount of acetylene is used,giving a reducing flame. The combustion ofacetylene is incomplete (greenish) betweenthe inner cone and the outer envelope .

Good for welding aluminium alloys, highcarbon steels.

The excess amount of O 2 is used, giving an

oxidizing flame. Good for welding brass.Tapany Udomphol


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Chemical reactions and temperaturedistribution in a neutral oxyacetylene


The secondary combustion is also called the protection envelope since COand H 2 here consume the O 2 entering from surrounding air, thereby

protecting the weld from oxidation.

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Advantages and disadvantages


Advantages Disadvantages

Simple equipment Portable

Inexpensive

Easy for maintenance and repair

Limited power density

Very low welding speed

High total heat input per unit length

Large heat affected zone Severe distortion

Not recommended for welding reactivemetals such as titanium and zirconium.

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Shield metal arc welding


Shield metal arc welding ( SMAW ) is a process that melts and joins metalsby heating them with an arc established between a sticklike coveredelectrode and the metals .

Shield metal arc welding

The core wire conducts the electriccurrent to the arc and provides filler metal for the joint.

The electrode holder is essentially a metalclamp with an electrically insulated outside

shell for the welder to hold safely. The heat of the arc melts the core wire andthe flux covering at the electrode tip intometal droplets .

Molten metal in the weld pool solidifies intothe weld metal while the lighter molten fluxfloats on the top surface and solidifies as aslag layer .

Shielding gas Mixture of H 2 , CO, H 2 O and CO 2 Tapany Udomphol


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Functions of electrode (flux) covering


Protection Provide the gaseous shield to protect the molten metal from air.

Cellulose-type electrode (C 6H10 O5)x , providing gas mixture ofH2, CO, H 2O and CO 2.

Limestone-type electrode (CaCO 3) low in hydrogen and it isused for welding metals that are susceptible to hydrogencracking such as high-strength steels.

DeoxidationProvide deoxidizers and fluxing agent to deoxidizeand cleanse the weld metal. The solid slag alsoprotects the weld metal from oxidation.

Arc stabilization Provide arc stabilizers which are compounds such aspotassium oxalate and lithium carbonate. They readilydecompose into ions in an arc, which increaseelectrical conductivity.

Metal addition Provide alloying elements (for composition control) andmetal powder (increase deposition rate) to the weld pool.

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Advantages and disadvantages Advantages and disadvantages



Simple welding equipment

Portable

Inexpensive

Used for maintenance, repair,and field construction

Not clean enough for reactive metalssuch as aluminium and titanium.

The deposition rate is limited becausethe electrode covering tends to overheatand fall off. The electrode length is ~ 35 mm andrequires electrode changing lower theoverall production rate.

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GasGas -- tungsten arc weldingtungsten arc welding


Gas-tungsten arc welding (GTAW) is a process that melts and joins metals byheating them with an arc established between a nonconsumable tungstenelectrode and the metals .

Gas tungsten arc welding

The tungsten electrode is normallycontacted with a water cooled copper tube,which is connected to the welding cable.prevent overheating .

The shielding gas ( Ar, He ) goes throughthe torch body and nozzle toward the weldpool to protect it from air.

Filler metal (for joining of thicker

materials) can be fed manually orautomatically to the arc.

Also called tungsten inert gas (TIG)welding.

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Electrodes and shielding gasesElectrodes and shielding gases


Electrodes Tungsten electrodes with 2% cerium or thorium give betterelectron emissivity, current-carrying capacity, and resistance tocontamination than pure electrodes. the arc is more stable.

Shielding gases

Ar is heavier and offers more effective shielding and cheaper than He .

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Suitable for joining thin sectiondue to its limited heat inputs.

Can weld metals without fillers(autogenous welding).

Very clean welding process, whichcan be used for welding reactivemetals such as titanium, zirconium,aluminium and magnesium.

Deposition rate is low.

Excessive welding current causesmelting of the tungsten electrode andresults in brittle tungsten in the weld

pool. Multiple passes

Low welding speed

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Keyholing Keyholing


Keyholing is a positiveindication of full penetration andallow higher welding speed (forthickness 2.5-6.4 mm).

Requires proper combinationsof the orifice gas flow, the travelspeed, and the welding current.

A wine cup shape is common inPAW .

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The electrode is recessed in thenozzle, therefore no contamination tothe weld metal.

Less sensitive to arc length variation.

Full penetration. High welding speed.

More complicated. Require proper electrode tipconfiguration and positioning andorifice selection.

Equipment is more expensive.

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GasGas -- metal arc weldingmetal arc welding

Suranaree University of Technology Sep-Dec 2007Gas metal arc welding

Gas metal arc welding (GMAW) is a process that melts and joins metals byheating them with an arc established between a continuously fed filler wireelectrode and the metals.

Ar and He are also used as inertshielding gases to protect the moltenweld pool. often called metal inert gas(MIG). However, non-inert gases, i.e.,CO 2 are also used for carbon and low

alloy steels. Ar , He or Mixtures of (25%) Ar ,(75%) He are used for non-ferrous(mostly Al ) as well as stainless and alloysteels.

The Ar arc plasma is stable andbeneficial for transferring metaldroplets through the arc plasma.

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Modes of metal transfer Modes of metal transfer


The molten metal at the electrode tip can be transferred to the weld pool by

2. Spray transfer

Metal droplets travel across the arc gap under theinfluence of gravity . Often not smooth and cause spatter . Low welding current (180A). Globular transfer

Spray transfer

Occur above a critical current level (280-320A). Small metal droplets travel across the gap under theinfluence of the electromagnetic force at much higherfrequency and speed than in the globular mode. More stable and spatter free .

Molten metal droplets are transferred from electrode tip to the weld pool when ittouches the pool surface ( short-circuit ).

Require very low current and electrode diameter, giving small and fast- freezing weld pool desirable for welding thin section or out of position welding.

1. Globular transfer

3. Short-circuit transfer

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Ar shielding 75%He-25%Arshielding

Very clean due to inert shielding gasused.

Much higher deposition rate.

Can weld thicker weld sections at

higher weld speeds. Can use dual-touch and twin-wireprocesses to further increase thedeposition rate.

Short and stable arc make it easier toweld (skill is not required).

The GMAW gun is quite bulkyand difficult to reach small areasor corners.

Papillary (nipple) type penetration pattern

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Submerged arc weldingSubmerged arc welding


Submerged arc welding (SAW) is a process that melts and joins metalsby heating them with an arc established between a consumable wireelectrode and the metals , with the arc being shielded by a molten stagand granular flux .

Submerged arc welding

The arc is submerged and invisible.

The flux is supplied from a hopper,which travel with the torch.

The shielding gas may not berequired because the molten metal isseparated from the air by the moltenslag and granular flux .

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Clean welds are obtained due toprotecting and refining action of the slag.

At high welding current, spatter and heatloss are eliminated because the arc issubmerged.

Alloying elements and metal powderscan be added to the granular flux tocontrol the composition and increase thedeposition rate respectively.

The deposition rate can be increased byusing two or more electrodes in tandem.

Can weld thick section.

Cannot weld in a flat-position andcircumferential (pipe).

High heat input can reduce theweld quality and increasedistortions.

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ElectroslagElectroslag weldingwelding


Electroslag welding (ESW) is a process thatmelts and joins metals by heating them with apool of molten slag held between the metals andcontinuously feeding a filler wire electrode into it.

Electroslag welding

The weld pool is covered with molten slagand move upward as welding progresses.

A pair of water-cooled copper shoes keepsthe weld pool and the molten slag frombreaking out.

The molten slag protects the weld metalfrom air and refine it.

The arc is only used during the initiation process .

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Transverse crosssection of electroslag weld in 70-mm thicksteel.

Extremely high deposition rate.

One single pass no mater howthick the workpiece is.

No angular distortion because theweld is symmetrical with respect toits axis.


Very high heat input weld qualitycould be poor due to coarse grain inthe fusion zone or heat affectedzone.

Strict to vertical position due to very

large weld pools.

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Comparison of deposition rate in differentComparison of deposition rate in differentarc welding processes arc welding processes



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Electron beam weldingElectron beam welding


Electron beam welding (EBW) is a process that melts and joinsmetals by heating them with an electron beam.

Electron beam welding

When the filament in the electron gun is negatively charged, it emits electronswhich is accelerated by the electric field .

These electrons go through the anode and are focused by an electromagnatic coil to point at the workpiece surface ( ~ 0.3-0.8 mm, due to a high powerdensity (~10 10 W/m 2).

The high intensity electron beam

can vapourise the metal and form adeep penetrating keyhole .

A single pass electron beam canbe obtained.

EBW is not intended forincompletely degassed materialssuch as rimmed steels because highspeed welding does not allow gas

bubbles to escape in time.

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Electron beam welding Electron beam welding


Dispersion of electron beam at various ambient pressures.

Welds in 13 mm-thick 2219 aluminium (a) Electronbeam weld and (b) gas tungsten arc weld.

Vacuum operation

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High power density to produce a full-penetration keyhole even in thick

specimens. Produce a single pass welding at a highwelding speed.

Very narrow heat affected zone (HAZ)

and little distortion due to lower heat inputper unit length than in arc welding.

Reactive and refractory metals can bewelded in vacuum (no contamination).

Very rapid cooling can prevent coarse,brittle intermetallic compounds.

Dissimilar metals can be welded.

Very high equipment cost.

High vacuum (10 -3 10 -6 torr) isinconvenient. medium vacuum (10 -3 25 torr) and non vacuum (1 atm) EBWhave also been developed.

Precise alignment of the joints and the

gun is required due to small beam size. Missed jointed of dissimilar metal canbe obtained due to beam deflection.

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Laser beam weldingLaser beam welding


Laser beam welding (LBW) is a process that melts and joins metals by heatingthem with a laser beam.

The laser beam can be produced by a solidstate laser ( YAG -Yttrium-aluminium garnetcrystal) or a gas ( CO 2) laser, which can befocused and directed by optical means toachieve high power densities.

CO 2 laser gives higher power than YAG . Laser is produced when excited electronsreturn to their normal state.

A plasma (an ionic gas) produced during

welding can absorb and scatter the laserbeam, which reduces the weld penetration .

Inert gas (He, Ar) is required to increaseweld penetration.

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Produce deep and narrow weldsat high welding speeds.

Narrow heat-affected zone.

Little distortion.

Can be used for weldingdissimilar metals in varying sizes.

Very high reflectivity of a laserbeam by the metal surface.

High equipment cost.

Require precise joint and laserbeam alignment.

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Resistance weldingResistance welding


Resistance welding is used to join two or more metal parts together in alocalised area by the application of heat and pressure . The heat isproduced by the resistance of the material to carry a high amperage current.

Flash welding High-frequency resistance welding Percussion welding Projection welding

Resistance seam welding Resistance spot welding Upset welding

There are at least seven differenttypes of resistance welding:

Spot welding

Projection welding

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Workpiece

Electrode

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Friction weldingFriction welding


Friction welding is a mechanical solid-phasewelding process in which heat generated by frictionis used to create high-integrity joint between similar

or dissimilar metals.

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Can be used for round andrectangular sections.

Inertia friction welding Tapany Udomphol

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Inertia Inertia Friction welding Friction welding


One piece is attached to a flywheel and the other isrestrained from rotating. The flywheel is accelerated by a motor to apredetermined rotational speed, storing the energy. The motor is disengaged and the work pieces areforced together by the friction welding force.

The energy required to make the weld is supplied primarily bythe stored rotational kinetic energy of the welding machine.

www.mtiwelding.com The surfaces rub together underpressure producing heat throughfriction at the weld interface. The flywheel speed decreases

and stop when the stored kineticenergy has used up. Forge force may be applied beforethe rotation ceases.

p g

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Friction weldingFriction welding


Friction welding for rectangular sections

Weld structure obtained

from friction welding.Friction welding process.

The heat generated due to frictional motions of the two workpieces, creatinga columnar microstructure as shown in figure.

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Different Types of Metal Joining