Lecture 8 Welding Processes

7/28/2019 Lecture 8 Welding Processes

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Lecture 8Welding Processes

Successful students will be able tounderstand:

Types of weld Power densities of several fusion

welding processes

Melting temperatures of selected metals

Typical fusion welded joint

Properties of electrodes


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Welding ProcessesPlug Welds and Slot Welds

Plug welds and slot welds are used forattaching flat plates using one or more holesor slots in the top part and then filling with

filler metal to fuse the two parts together.


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Welding ProcessesPlug Welds and Slot Welds


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Welding ProcessesSpot Weld and Seam Weld

Spot welds and seam welds, used for lapjoints.

A spot weld is a small fused section between

the surfaces of two sheets or plates.

Multiple spot welds are typically required tojoin the parts.


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It is most closely associated with resistancewelding.

A seam weld is similar to a spot weld except it

consists of a more or less continuously fusedsection between the two sheets or plates.


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Welding ProcessesFlange Weld and Surface Weld

Aflange weldis made on the edges of two (ormore) parts, usually sheet metal or thin plate,at least one of the parts being flanged.

Asurfacing weldis not used to join parts, butrather to deposit filler metal onto the surfaceof a base part in one or more weld beads.


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The weld beads can be made in a series ofoverlapping parallel passes thereby coveringlarge areas of the base part.

The purpose is to increase the thickness ofthe plate or to provide a protective coating onthe surface.


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Welding ProcessesComparison of Power Densities ofseveral Fusion Welding Processes


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Welding ProcessesMelting Temperatures for SelectedMetals


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Welding ProcessesFusion Welded Joint

4 Typical Zones of Fusion Welded Joint1. Fusion Zone

The fusion zone consists of a mixture of filler

metal and base metal that has completelymelted.

This zone is characterized by a high degreeof homogeneity among the componentmetals that have been melted duringwelding.


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Welding Processes1. Fusion Zone The mixing of these components is

motivated largely by convection in themolten weld pool.

The resulting structure in the solidifiedfusion zone tends to feature coarsecolumnar grains.


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Welding Processes1. Fusion Zone

The grain structure depends on variousfactors, including welding process, metalsbeing welded (for example, identical metals

versus dissimilar metals), whether a fillermetal is used, and the feed rate at whichwelding is accomplished.


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Welding Processes2. Weld Interface

The second zone in the weld joint is theweld interface, a narrow boundary thatseparates the fusion zone from the heataffected zone.

The interface consists of a thin band ofbase metal that was melted or partiallymelted (localized melting within the grains)

during the welding process, but thenimmediately solidified before any mixingwith the metal in the fusion zone.


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Welding Processes2. Weld Interface

Its chemical composition is thereforeidentical to that of the base metal.

3. Heat Affected Zone (HAZ)

The third zone in the typical fusion weld isthe heat affected zone (HAZ).


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Welding Processes3. Heat Affected Zone (HAZ)

The metal in this zone has experiencedtemperatures below its melting point, yethigh enough to cause microstructuralchanges in the solid metal.

The chemical composition in the heataffected zone is the same as the base metal,but this region has been heat treated due tothe welding temperatures so that its

properties and structure has been altered.


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Welding Processes3. Heat Affected Zone (HAZ) The amount of metallurgical damage in the

HAZ depends on factors such as the amountof heat input and peak temperatures

reached, distance from the fusion zone,length of time the metal has been subjectedto the high temperatures cooling rate, and

the metal's thermal properties.


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Welding Processes3. Heat Affected Zone (HAZ)

The effect on mechanical properties in theheat affected zone is usually negative. and itis in this region of the weld joint that

welding failures often occur.


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Welding Processes4. Unaffected Base Metal Zone As the distance from the fusion zone

increases, the unaffected base metal zoneis finally reached, in which no metallurgical

change has occurred.

The base metal surrounding the HAZ islikely to be in a state of high residual stress,

the result of shrinkage in the fusion zone.


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Welding ProcessesTypical Fusion Welded Joint


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Welding ProcessesArc Welding (AW)

Arc welding (AW) is a fusion welding processin which coalescence of the metals is achievedby the heat from an electric arc between an

electrode and the work.An electric arc is a discharge of electric

current across a gap in a circuit.

It is sustained by the presence of a thermallyionized column of gas (called a plasma)through which current flows.


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Welding Processes To initiate the arc in an AW process, the

electrode is brought into contact with thework and then quickly separated from it by ashort distance.

The electric energy from the arc thus formed

produces temperatures of 5500 degree C orhigher, sufficiently hot to melt any metal.

A pool of molten metal, consisting of basemetal and filler metal (if one is used), is

formed near the tip of the electrode.


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In most arc welding processes, filler metal isadded during the operation to increase thevolume and strength of the weld joint.

As the electrode is moved along the joint, the

molten weld pool solidifies in its wake. Movement of the electrode relative to thework is accomplished by either a humanwelder (manual welding) or by mechanicalmeans (machine welding or automaticwelding, or robotic welding).


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One troublesome aspect of manual arcwelding is that the quality of the weld joint isvery dependent on the skill and work ethic ofthe human welder.

Productivity is also an issue in arc welding. Formanual welding, arc time is usually around20%.


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Frequent rest periods are needed by thewelder to overcome fatigue in manual arcwelding, which is demanding in hand-eyecoordination under stressful conditions.

Arc time increases to about 50% (more orless, depending on the operation) formachine, automatic, and robotic welding.


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Welding ProcessesElectrodes

Electrodes used in AW processes are classified as1. consumable or

2. Nonconsumable

1. Consumable Electrodes Consumable electrodes provide the source of the

filler metal in arc welding.

These electrodes are available in two principal

forms: rods (also called sticks). and wire. Welding rods are typically 9 to 18 in. long (225 to

450 mm) and 3/8 in. (1.5 mm) or less in diameter.


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Welding Processes1. Consumable Electrodes

Problem with consumable welding rods inproduction welding operations, is that theymust be changed periodically, reducing arctime of the welder.

Consumable weld wire has the advantagethat it can be continuously fed into the weldpool from spools containing long lengths ofwire, thus avoiding the frequentinterruptions that occur when using welding

sticks.


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Welding Processes1. Consumable Electrodes In both rod and wire forms, the

electrode is consumed by the arc duringthe welding process and added to theweld joint as filler metal.


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Welding Processes2. Nonconsumable Electrodes

Nonconsumable electrodes are made oftungsten (or carbon, rarely), which resistsmelting by the arc.

Despite its name, a nonconsumable

electrode is gradually depleted during thewelding process (vaporization is the principalmechanism), analogous to the gradualwearing of a cutting tool in a machiningoperation.


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Welding Processes2. Nonconsumable Electrodes For AW processes that utilize

nonconsumable electrodes, any fillermetal used in the operation must be

supplied by means of a separate wirethat is fed into the weld pool.

Lecture 8 Welding Processes

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