lecture 8 welding processes
TRANSCRIPT
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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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Welding ProcessesSpot Weld and Seam Weld
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 ProcessesSpot Weld and Seam Weld
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Welding ProcessesSpot Weld and Seam Weld
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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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Welding ProcessesFlange Weld and Surface Weld
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 ProcessesFlange Weld and Surface Weld
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Welding ProcessesFlange Weld and Surface Weld
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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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Welding ProcessesArc Welding (AW)
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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Welding ProcessesArc Welding (AW)
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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Welding ProcessesArc Welding (AW)
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 ProcessesArc Welding (AW)
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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.