Unit-III-Welding_MEB 212.pdf welding important

B. Tech (Mech.) – 2nd Year
(Manufacturing Sciences-I)
Unit-III: Joining Processes
By
Dr. Leeladhar Nagdeve
Mechanical Engineering Department
National Institute of Technology Delhi - 110036 INDIA

 Definition: Welding is a materials joining process in which two or more parts are coalesced at their
contacting surfaces by a suitable application of heat and/or pressure.
 Many welding processes are accomplished by heat alone, with no pressure applied; others by a
combination of heat and pressure; and still others by pressure alone, with no external heat
supplied.
 In some welding processes a filler material is added to facilitate coalescence.
 The assemblage of parts that are joined by welding is called a weldment.

Welding
Welding
Fusion
Fusion welding
welding
Fusion
Fusion welding
welding
coalescence
coalescence is accomplished by
is accomplished by
melting the two parts to be joined,
melting the two parts to be joined,
in some cases adding filler metal to
in some cases adding filler metal to
the joint
the joint
Solid
Solid-
-state welding
state welding
heat and/or pressure are used to achieve
heat and/or pressure are used to achieve
coalescence, but no melting of the base
coalescence, but no melting of the base
metals occurs and no filler metal is added.
metals occurs and no filler metal is added.

Fusion-welding processes use heat to melt the base metals. In many fusion welding operations, a filler
metal is added to the molten pool to facilitate the process and provide bulk and strength to the
welded joint. A fusion-welding operation in which no filler metal is added is referred to as an
autogenous weld. The fusion category includes the most widely used welding processes, which can be
organized into the following general groups:
 Arc welding (AW) : Arc welding refers to a group of welding processes in which heating of the
metals is accomplished by an electric arc. Some arc- welding operations also apply pressure
during the process and most utilize a filler metal.
Fusion Welding
Fusion Welding
during the process and most utilize a filler metal.
 Resistance welding (RW) : Resistance welding achieves coalescence using heat from electrical
resistance to the flow of a current passing between the faying surfaces of two parts held
together under pressure.
 Oxy-fuel gas welding (OFW) : These joining processes use an oxy-fuel gas, such as a mixture of
oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal, if one
is used.
 Other fusion-welding processes : Other welding processes that produce fusion of the metals
joined include electron beam welding and laser beam welding.

Solid-state welding refers to joining processes in which coalescence results from application of
pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process
is below the melting point of the metals being welded. No filler metal is utilized. Representative
welding processes in this group include:
 Diffusion welding (DFW) : Two surfaces are held together under pressure at an elevated
temperature and the parts coalesce by solid-state diffusion.
 Friction welding (FRW) : Coalescence is achieved by the heat of friction between two surfaces.
 Ultrasonic welding (USW) : Moderate pressure is applied between the two parts and an
Solid
Solid-
-State
State Welding
Welding
 Ultrasonic welding (USW) : Moderate pressure is applied between the two parts and an
oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting
surfaces. The combination of normal and vibratory forces results in shear stresses that remove
surface films and achieve atomic bonding of the surfaces.

The Welded Joint
The Welded Joint
Welding produces a solid connection between two pieces, called a weld joint. A weld joint is the
junction of the edges or surfaces of parts that have been joined by welding.
TYPES OF JOINTS
a. Butt joint. : In this joint type, the parts lie in the same plane and are joined at their edges.
b. Corner joint. : The parts in a corner joint form a right angle and are joined at the corner of the angle.
c. Lap joint. : This joint consists of two overlapping parts.
d. Tee joint. : In a tee joint, one part is perpendicular to the other in the approximate shape of the letter ‘‘T.’’
Fig. Five basic types of joints: (a) butt, (b) corner, (c) lap, (d) tee, and (e) edge.
d. Tee joint. : In a tee joint, one part is perpendicular to the other in the approximate shape of the letter ‘‘T.’’
e. Edge joint. : The parts in an edge joint are parallel with at least one of their edges in common, and the joint
is made at the common edge(s).

TYPES
TYPES OF WELDS
OF WELDS
Fillet weld Groove welds Plug and slot welds
spot and
seam weld
Flange and
surfacing welds

Fig. Various forms of fillet welds: (a) inside single fillet corner joint; (b) outside
single fillet corner joint; (c) double fillet lap joint; and (d) double fillet tee joint.
Dashed lines show the original part edges.
Fillet and Groove Welds
Fillet and Groove Welds
Fig. Some typical groove welds: (a) square groove weld, one side; (b) single
bevel groove weld; (c) single V-groove weld; (d) single U-groove weld; (e)
single J-groove weld; (f) double V-groove weld for thicker sections. Dashed
lines show the original part edges.

Fig. (a) Plug weld; and (b) slot weld.
Plug and Spot Welds
Plug and Spot Welds
Fig. (a) Spot weld; and (b) seam weld.

Flange and Surfacing Weld
Flange and Surfacing Weld
Fig. (a) Flange weld; and (b) surfacing weld.

Physics of Welding
Physics of Welding
 Although several coalescing mechanisms are available for welding, fusion is by far the most common
means. We consider the physical relationships that allow fusion welding to be performed like power
density and its importance, and the heat and power equations that describe a welding process.
 To accomplish fusion, a source of high-density heat energy is supplied to the faying surfaces, and the
resulting temperatures are sufficient to cause localized melting of the base metals. If a filler metal is
added, the heat density must be high enough to melt it also. Heat density can be defined as the power
transferred to the work per unit surface area, (W/mm2).
Where P = power entering the surface (W), and A = surface area over which the energy is entering
(mm2)

Q. A heat source transfers 3000 W to the surface of a metal part. The heat impinges the surface in a
circular area, with intensities varying inside the circle. The distribution is as follows: 70 % of the power
is transferred within a circle of diameter = 5mm, and 90 % is transferred within a concentric circle of
diameter = 12 mm. What are the power densities in (a) the 5-mm diameter inner circle and (b) the 12-
mm-diameter ring that lies around the inner circle?
Question
Question

The quantity of heat required to melt a given volume of metal depends on
(1) the heat to raise the temperature of the solid metal to its melting point, which depends on the
metal’s volumetric specific heat,
(2) the melting point of the metal, and
(3) the heat to transform the metal from solid to liquid phase at the melting point, which depends on
the metal’s heat of fusion. To a reasonable approximation, this quantity of heat can be estimated
by the following equation
Heat Balance in Fusion Welding
Heat Balance in Fusion Welding
Where Um = the unit energy for melting i.e. the quantity of heat required to melt a unit volume of
metal starting from room temperature (Jmm-3); Tm = melting point of the metal on an absolute
temperature scale (K); and K = constant whose value is 3.33 × 10-6 when the Kelvin scale is used

Not all of the energy generated at the heat source is used to melt the weld metal. There are two heat
transfer mechanisms at work, both of which reduce the amount of generated heat that is used by the
welding process.
 The first mechanism involves the transfer of heat between the heat source and the surface of the
work. This process has a certain heat transfer factor f1, defined as the ratio of the actual heat received
by the workpiece divided by the total heat generated at the source.
 The second mechanism involves the conduction of heat away from the weld area to be dissipated
throughout the work metal, so that only a portion of the heat transferred to the surface is available for
melting. This melting factor f is the proportion of heat received at the work surface that can be used
melting. This melting factor f2 is the proportion of heat received at the work surface that can be used
for melting. The combined effect of these two factors is to reduce the heat energy available for
welding as follows:

 The balance equation between the energy input and the energy needed for welding:

Q. The power source in a particular welding setup generates 3500 W that can be transferred to the work
surface with a heat transfer factor = 0.7. The metal to be welded is low carbon steel, whose melting
temperature, is 1760 K. The melting factor in the operation is 0.5. A continuous fillet weld is to be made
with a cross-sectional area = 20 mm2. Determine the travel speed at which the welding operation can be
accomplished.
Let us first find the unit energy required to melt the metal Um
Question
Question

Features of a Fusion
Features of a Fusion-
-weld Joint
weld Joint
A typical fusion-weld joint in which filler metal has been added consists of several zones:
(1) Fusion zone,
(2) Weld interface zone,
(3) Heat-affected zone, and
(4) Unaffected base metal zone.
Fig. Cross section of a typical fusion-welded joint: (a) principal zones in the joint and (b) typical grain
structure.

The fusion zone consists of a mixture of filler metal and base metal that have completely melted. This
zone is characterized by a high degree of homogeneity among the component metals that have been
melted during welding.
The second zone in the weld joint is the weld interface, a narrow boundary that separates the fusion zone
from the heat-affected zone. The interface consists of a thin band of base metal that was melted or
partially melted (localized melting within the grains)during the welding process but then immediately
solidified before any mixing with the metal in the fusion zone. Its chemical composition is therefore
identical to that of the base metal.
The third zone in the typical fusion weld is the heat-affected zone (HAZ). The metal in this zone has
experienced temperatures that are below its melting point, yet high enough to cause microstructural
changes in the solid metal. The chemical composition in the heat-affected zone is the same as the base
metal, but this region has been heat treated due to the welding temperatures so that its properties and
structure have been altered.
As the distance from the fusion zone increases, the unaffected base metal zone is finally reached, in
which no metallurgical change has occurred. Nevertheless, the base metal surrounding the HAZ is likely
to be in a state of high residual stress, the result of shrinkage in the fusion zone.

Fusion Welding
Fusion Welding
Arc welding
(AW)
Resistance welding
(RW)
Oxy-fuel gas welding
(OFW)
Other fusion
welding processes

General Technology of Arc Welding
General Technology of Arc Welding
Electrodes:
Electrodes used in AW processes are classified as consumable or non- consumable.
• 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 225 to 450 mm (9–18 in)
long and 9.5 mm (3/8 in) or less in diameter.
• Non-consumable electrodes are made of tungsten (or carbon, rarely), which resists melting by the arc.
Despite its name, a non-consumable electrode is gradually depleted during the welding process
Despite its name, a non-consumable electrode is gradually depleted during the welding process
(vaporization is the principal mechanism), analogous to the gradual wearing of a cutting tool in a machining
operation.

Arc Shielding:
• At the high temperatures in arc welding, the metals being joined are Chemically reactive to
oxygen, nitrogen, and hydrogen in the air. The mechanical properties of the weld joint can be
seriously degraded by these reactions. Thus, some means to shield the arc from the surrounding
air is provided in nearly all AW processes.
• Arc shielding is accomplished by covering the electrode tip, arc, and molten weld pool with a
blanket of gas or flux, or both, which inhibit exposure of the weld metal to air.
• Common shielding gases include argon and helium, both of which are inert. In the welding of
ferrous metals with certain AW processes, oxygen and carbon dioxide are used, usually in
ferrous metals with certain AW processes, oxygen and carbon dioxide are used, usually in
combination with Ar and/or He, to produce an oxidizing atmosphere or to control weld shape.
Flux:
• A flux is a substance used to prevent the formation of oxides and other unwanted contaminants,
or to dissolve them and facilitate removal. During welding, the flux melts and becomes a liquid
slag, covering the operation and protecting the molten weld metal.
• The slag hardens upon cooling and must be removed later by chipping or brushing.
• Flux is usually formulated to serve several additional functions:
a. provide a protective atmosphere for welding,
b. stabilize the arc, and
c. reduce spattering.

Power Source:
• Power Source in Arc Welding both direct current (DC) and alternating current (AC) are used in arc
welding.
• AC machines are less expensive to purchase and operate, but are generally restricted to welding of
ferrous metals.
• DC equipment can be used on all metals with good results and is generally noted for better arc
control.
In all arc-welding processes, power to drive the operation is the product of the current I passing through
the arc and the voltage E across it. This power is converted into heat, but not all of the heat is transferred
to the surface of the work. Convection, conduction, radiation, and spatter account for losses that reduce
to the surface of the work. Convection, conduction, radiation, and spatter account for losses that reduce
the amount of usable heat.
The resulting power balance in arc welding is defined by:

Q. A gas tungsten arc-welding operation is performed at a current of 300 A and voltage of 20 V. The
melting factor f2 = 0.5, and the unit melting energy for the metal Um = 10 Jmm-3. Determine (a) power in
the operation, (b) rate of heat generation at the weld, and (c) volume rate of metal welded.
Question
Question
Table. Heat transfer
factors for several arc-
welding processes.

CONSUMABLE ELECTRODES
 Shielded Metal Arc Welding
 Gas Metal Arc Welding
 Flux-Cored Arc Welding
 Electro-gas Welding
Arc Welding PROCESSES
Other Arc-Welding and
Related Processes
 Carbon arc welding
 Stud welding
 Electro-gas Welding
 Submerged Arc Welding
NON-CONSUMABLE ELECTRODES
 Gas Tungsten Arc Welding
 Plasma Arc Welding

Arc Welding
Arc Welding
 Arc welding (AW) is a fusion-welding process in which coalescence of the metals is achieved by the heat of an
electric arc between an electrode and the work.
 To initiate the arc in an AW process, the electrode is brought into contact with the work and then quickly separated
from it by a short distance.
 The electric energy from the arc thus formed produces temperatures of 5500 ℃ or higher, sufficiently hot to melt
any metal.
Fig. The basic configuration and electrical circuit of an arc- welding process.

 Shielded metal arc welding(SMAW) is an AW process that uses a consumable electrode consisting of a filler metal
rod coated with chemicals that provide flux and shielding.
 The welding stick (SMAW is sometimes called stick welding) is typically 225 to 450 mm long and 2.5 to 9.5 mm in
diameter.
 The filler metal used in the rod must be compatible with the metal to be welded, the composition usually being
very close to that of the base metal. The coating consists of powdered cellulose mixed with oxides, carbonates, and
other ingredients, held together by a silicate binder. Metal powders are also sometimes included in the coating to
increase the amount of filler metal and to add alloying elements.
Shielded
Shielded Metal
Metal A
Arc
rc W
Welding
elding
Fig. Shielded metal arc welding (SMAW).

Gas
Gas Metal Arc Welding
Metal Arc Welding
 Gas metal arc welding (GMAW) is an AW process in which the electrode is a consumable bare metal wire, and
shielding is accomplished by flooding the arc with a gas. The bare wire is fed continuously and automatically
from a spool through the welding gun.
 Wire diameters ranging from 0.8 to 6.5 mm are used in GMAW, the size depending on the thickness of the
parts being joined and the desired deposition rate.
 Gases used for shielding include inert gases such as argon and helium, and active gases such as carbon
dioxide. Selection of gases (and mixtures of gases) depends on the metal being welded, as well as other
factors.
Fig. Gas metal arc welding (GMAW).

Flux
Flux-
-Cored
Cored Arc Welding
Arc Welding
 This arc-welding process was developed in the early 1950s as an adaptation of shielded metal arc
welding to overcome the limitations imposed by the use of stick electrodes.
 Flux-cored arc welding (FCAW) is an arc-welding process in which the electrode is a continuous
consumable tubing that contains flux and other ingredients in its core. Other ingredients may include
deoxidizers and alloying elements.
 The presence or absence of externally supplied shielding gas distinguishes the two types:
I. self-shielded, in which the core provides the ingredients for shielding; and
II. gas shielded, in which external shielding gases are supplied.
Fig. Flux- cored arc welding.

Electro
Electro-
-gas
gas Welding
Welding
Fig. Electro-gas welding using flux-cored electrode wire: (a) front view with molding shoe
removed for clarity, and (b) side view showing molding shoes on both sides.

Submerged
Submerged Arc Welding
Arc Welding
 This process, developed during the 1930s, was one of the first AW processes to be automated.
 Submerged arc welding (SAW) is an arc-welding process that uses a continuous, consumable bare wire
electrode, and arc shielding is provided by a cover of granular flux.
 The electrode wire is fed automatically from a coil into the arc. The flux is introduced into the joint
slightly ahead of the weld arc by gravity from a hopper.
Fig. Submerged arc welding (SAW).

 The blanket of granular flux completely submerges the welding operation, preventing sparks, spatter, and
radiation that are so hazardous in other AW processes. Thus, the welding operator in SAW need not wear
the somewhat cumbersome face shield required in the other operations.
 The portion of the flux closest to the arc is melted, mixing with the molten Weld metal to remove
impurities and then solidifying on top of the weld joint to forma glass- like slag. The slag and unfused flux
granules on top provide good protection from the atmosphere and good thermal insulation for the weld
area, resulting in relatively slow cooling and a high-quality weld joint, noted for toughness and ductility.
 The unfused flux remaining after welding can be recovered and reused. The solid slag covering the weld
must be chipped away, usually by manual means.
 Applications:
 Applications:
• Submerged arc welding is widely used in steel fabrication for structural shapes (e.g., welded I-beams);
longitudinal and circumferential seams for large diameter pipes, tanks, and pressure vessels; and
welded components for heavy machinery.
• In these kinds of applications, steel plates of 25-mm thickness and heavier are routinely welded by
this process. Low-carbon, low-alloy, and stainless steels can be readily welded by SAW; but not high-
carbon steels, tool steels, and most nonferrous metals.
 Because of the gravity feed of the granular flux, the parts must always be in a horizontal orientation, and a
backup plate is often required beneath the joint during the welding operation.

Gas
Gas Tungsten Arc Welding
Tungsten Arc Welding
 Gas tungsten arc welding (GTAW) is an AW process that uses a non-consumable tungsten electrode and an inert
gas for arc shielding. The term TIG welding (tungsten inert gas welding) is often applied to this process.
 GTAW can be implemented with or without a filler metal.
Fig. Gas tungsten arc welding (GTAW).

 When a filler metal is used, it is added to the weld pool from a separate rod or wire, being melted by
the heat of the arc rather than transferred across the arc as in the consumable electrode AW processes.
 Tungsten is a good electrode material due to its high melting point of 3410 ℃. Typical shielding gases
include argon, helium, or a mixture of these gas elements.
 Applications:
• GTAW is applicable to nearly all metals in a wide range of stock thicknesses. It can also be used for
joining various combinations of dissimilar metals. Its most common applications are for aluminum
and stainless steel. Cast irons, wrought irons, and of course tungsten are difficult to weld by GTAW.
• In steel welding applications, GTAW is generally slower and more costly than the consumable
electrode AW processes, except when thin sections are involved and very-high-quality welds are
required.
required.
 When thin sheets are TIG welded to close tolerances, filler metal is usually not added. The process can
be performed manually or by machine and automated methods for all joint types.
 Advantages:
• Advantages of GTAW in the applications to which it is suited include high-quality welds, no weld
spatter because no filler metal is transferred across the arc, and little or no post weld cleaning
because no flux is used.

Plasma
Plasma Arc Welding
Arc Welding
 Plasma arc welding (PAW) is a special form of gas tungsten arc welding in which a constricted plasma
arc is directed at the weld area.
 In PAW, a tungsten electrode is contained in a specially designed nozzle that focuses a high-velocity
stream of inert gas (e.g., argon or argon–hydrogen mixtures) into the region of the arc to form a high-
velocity, intensely hot plasma arc stream.
Fig. Plasma arc welding (PAW).

 Carbon arc welding (CAW) is an arc-welding process in which a non-consumable carbon
(graphite) electrode is used.
 It has historical importance because it was the first arc-welding process to be developed, but
its commercial importance today is practically nil.
 The carbon arc process is used as a heat source for brazing and for repairing iron castings. It
can also be used in some applications for depositing wear-resistant materials on surfaces.
 Graphite electrodes for welding have been largely superseded by tungsten (in GTAW and
PAW).
Carbon
Carbon Arc Welding
Arc Welding

Stud
Stud Arc Welding
Arc Welding
 Stud welding (SW) is a specialized AW process for joining studs or similar components to base parts.
 In SW shielding is obtained by the use of a ceramic ferrule.
Fig. Stud arc welding (SW): (1) stud is positioned; (2) current flows from the gun, and stud is pulled
from base to establish arc and create a molten pool; (3) stud is plunged into molten pool; and (4)
ceramic ferrule is removed after solidification.

 To begin with, the stud is chucked in a special weld gun that automatically controls the timing
and power parameters of the steps shown in the sequence.
 The worker must only position the gun at the proper location against the base work part to
which the stud will be attached and pull the trigger.
 Applications:
• SW applications include threaded fasteners for attaching handles to cookware, heat
radiation fins on machinery, and similar assembly situations.
 Advantages:
 Advantages:
• In high-production operations, stud welding usually has advantages over rivets, manually
arc-welded attachments, and drilled and tapped holes.

Resistance
Resistance Welding
Welding
Fig. Resistance welding (RW), showing the
components in spot welding, the
predominant process in the RW group.

Question
Question
The heat generated in the operation is given by
The volume of the weld nugget (assumed disc-shaped) is
The heat required to melt this volume of metal is

Resistance
Resistance
Welding
Welding
Resistance
Resistance
Spot
Spot
Welding
Welding
Resistance
Resistance
Projection
Projection
Welding
Welding
Resistance
Resistance
seam
seam
welding
welding
Other
Other
Resistance
Resistance-
-
Welding
Welding
Operations
Operations

Fig. Steps in a spot-welding cycle and plot
of squeezing force and current during
cycle. The sequence is:
1) parts inserted between open
electrodes,
2) electrodes close and force is applied,
3) weld time—current is switched on,
Resistance Spot Welding
Resistance Spot Welding
3) weld time—current is switched on,
4) current is turned off but force is
maintained or increased (a reduced
current is sometimes applied near the
end of this step for stress relief in the
weld region), and
5) electrodes are opened, and the
welded assembly is removed.

Resistance Seam Welding
Resistance Seam Welding
 In resistance seam welding (RSEW), the stick-shaped electrodes in spot welding are replaced by rotating
wheels, and a series of overlapping spot welds are made along the lap joint.
 The process is capable of producing air-tight joints, and its industrial applications include the production
of gasoline tanks, automobile mufflers, and various other fabricated sheet metal containers.
 Technically, RSEW is the same as spot welding, except that the wheel electrodes introduce certain
complexities.
Fig. Different types of seams produced by electrode wheels: (a) conventional resistance seam welding, in which
overlapping spots are produced; (b) roll spot welding; and (c) continuous resistance seam.

Resistance
Resistance Projection Welding
Projection Welding
 Resistance projection welding (RPW) is an RW
process in which coalescence occurs at one or
more relatively small contact points on the
parts.
 These contact points are determined by the
design of the parts to be joined, and may consist
of projections, embossments, or localized
intersections of the parts.
 A typical case in which two sheet-metal parts
are welded together is described in Figure.
 The part on top has been fabricated with two
Fig. Resistance projection welding (RPW): (1) at start of operation,
contact between parts is at projections; and(2) when current is
applied, weld nuggets similar to those in spot welding are formed at
the projections.
 The part on top has been fabricated with two
embossed points to contact the other part at
the start of the process.
 It might be argued that the embossing
operation increases the cost of the part, but this
increase may be more than offset by savings in
welding cost.
 There are variations of resistance projection welding, two of which are shown in figure. In one variation, fasteners with machined
or formed projections can be permanently joined to sheet or plate by RPW, facilitating subsequent assembly operations.
 Another variation, called cross-wire welding, is used to fabricate welded wire products such as wire fence, shopping carts, and
stove grills. In this process, the contacting surfaces of the round wires serve as the projections to localize the resistance heat for
welding.

Variations of Resistance Projection Welding
Variations of Resistance Projection Welding
 There are variations of resistance projection welding, two of which are shown in figure.
 In one variation, fasteners with machined or formed projections can be permanently joined to sheet or plate by
RPW, facilitating subsequent assembly operations.
 Another variation, called cross-wire welding, is used to fabricate welded wire products such as wire fence,
shopping carts, and stove grills. In this process, the contacting surfaces of the round wires serve as the
projections to localize the resistance heat for welding.
Fig. Variations of resistance projection welding: (a) welding of a machined or formed fastener onto a
sheet-metal part; and (b) cross-wire welding.

Other
Other
Resistance
Resistance
Welding
Welding
Flash
Flash
Welding
Welding
Percussion
Percussion
Welding
Welding
Welding
Welding
Upset
Upset
Welding
Welding
High
High-
-
frequency
frequency
resistance
resistance
welding
welding

Flash
Flash Welding
Welding
 In flash welding(FW), normally used for butt joints, the two surfaces to be joined are brought into contact or near
contact and electric current is applied to heat the surfaces to the melting point, after which the surfaces are forced
together to form the weld. The two steps in flash welding is shown in figure.
 In addition to resistance heating, some arcing occurs (called flashing, hence the name of the welding process),
depending on the extent of contact between the faying surfaces, so flash welding is sometimes classified in the arc-
welding group. Current is usually stopped during upsetting. Some metal, as well as contaminants on the surfaces, is
squeezed out of the joint and must be subsequently machined to provide a joint of uniform size.
 Applications of flash welding include butt welding of steel strips in rolling-mill operations, joining ends of wire in wire
drawing, and welding of tubular parts. The ends to be joined must have the same cross sections. For these kinds of
high-production applications, flash welding is fast and economical, but the equipment is expensive.
Fig. Flash welding (FW): (1) heating by electrical resistance; and (2) upsetting—parts are forced together.
high-production applications, flash welding is fast and economical, but the equipment is expensive.

 Upset welding (UW) is similar to flash welding except that in UW the faying surfaces are pressed
together during heating and upsetting. In flash welding, the heating and pressing steps are separated
during the cycle.
 Heating in UW is accomplished entirely by electrical resistance at the contacting surfaces; no arcing
occurs. When the faying surfaces have been heated to a suitable temperature below the melting point,
the force pressing the parts together is increased to cause upsetting and coalescence in the contact
region.
 Thus, upset welding is not a fusion-welding process in the same sense as the other welding processes
we have discussed. Applications of UW are similar to those of flash welding: joining ends of wire, pipes,
Upset and Percussion Welding
Upset and Percussion Welding
we have discussed. Applications of UW are similar to those of flash welding: joining ends of wire, pipes,
tubes, and so on.
 Percussion welding (PEW) is also similar to flash welding, except that the duration of the weld cycle is
extremely short, typically lasting only 1 to 10 ms.
 Fast heating is accomplished by rapid discharge of electrical energy between the two surfaces to be
joined, followed immediately by percussion of one part against the other to form the weld.
 The heating is very localized, making this process attractive for electronic applications in which the
dimensions are very small and nearby components may be sensitive to heat.

High
High-
-frequency Resistance
frequency Resistance Welding
Welding
 High-frequency resistance welding (HFRW) is a resistance-welding process in which a high-frequency alternating current
is used for heating, followed by the rapid application of an upsetting force to cause coalescence.
 The frequencies are 10 to 500 kHz, and the electrodes make contact with the work in the immediate vicinity of the weld
joint. In a variation of the process, called high-frequency induction welding (HFIW), the heating current is induced in the
parts by a high- frequency induction coil. The coil does not make physical contact with the work.
 The principal applications of both HFRW and HFIW are continuous butt welding of the longitudinal seams of metal pipes
and tubes.
Fig. Welding of tube seams by: (a) high-frequency resistance welding, and (b) high-frequency induction welding.

Oxyacetylene Welding
Oxyacetylene Welding
 Oxyacetylene welding (OAW) is a fusion-welding process performed by a high-temperature flame from
combustion of acetylene and oxygen. The flame is directed by a welding torch.
 A filler metal is sometimes added, and pressure is occasionally applied in OAW between the contacting
part surfaces.
 When filler metal is used, it is typically in the form of a rod with diameters ranging from1.6 to 9.5 mm
Fig. A typical oxyacetylene welding operation (OAW).

Oxyacetylene Flame
Oxyacetylene Flame
 When the mixture of acetylene and oxygen is in the ratio 1:1, the resulting neutral flame is shown in figure.
 The first-stage reaction is seen as the inner cone of the flame (which is bright white), while the second-stage
reaction is exhibited by the outer envelope (which is nearly colorless but with tinges ranging from blue to
orange).
 The maximum temperature of the flame is reached at the tip of the inner cone; the second-stage
temperatures are somewhat below those of the inner cone.
 During welding, the outer envelope spreads out and covers the work surfaces being joined, thus shielding
them from the surrounding atmosphere.
 Total heat liberated during the two stages of combustion is 55 × 106 Jm-3 of acetylene.
Fig. The neutral flame from an oxyacetylene torch, indicating temperatures achieved.
 Total heat liberated during the two stages of combustion is 55 × 10 Jm of acetylene.

An oxyacetylene torch supplies 0.3 m3 of acetylene per hour and an equal volume rate of oxygen for an OAW
operation on 4.5-mm-thick steel. Heat generated by combustion is transferred to the work surface with a heat
transfer factor f1 ¼ 0.20. If 75 % of the heat from the flame is concentrated in a circular area on the work
surface that is 9.0 mm in diameter, find (a) rate of heat liberated during combustion, (b) rate of heat
transferred to the work surface, and (c) average power density in the circular area.
Question
Question

Alternative Gases for Oxy
Alternative Gases for Oxy-
-fuel
fuel Welding
Welding
Gases used in oxy-fuel welding and/or cutting, with flame temperatures and heats of combustion.
aNeutral flame temperatures are compared since this is the flame that would most commonly be used for welding.
bMAPP is the commercial abbreviation for methylacetylene-propadiene.
cPropylene is used primarily in flame cutting.
dData are based on methane gas (CH4); natural gas consists of ethane (C2H6) as well as methane; flame temperature
and heat of combustion vary with composition.

Pressure Gas Welding
Pressure Gas Welding
 This is a special OFW process, distinguished by type of application rather than fuel gas.
 Pressure gas welding (PGW) is a fusion-welding process in which coalescence is obtained over the entire contact
surfaces of the two parts by heating them with an appropriate fuel mixture (usually oxyacetylene gas) and then
applying pressure to bond the surfaces. Parts are heated until melting begins on the surfaces.
 The heating torch is then withdrawn, and the parts are pressed together and held at high pressure while
solidification occurs.
 No filler metal is used in PGW.
Fig. An application of pressure gas welding: (a) heating of the two parts, and (b) applying pressure to form the weld.

Other Fusion
Other Fusion
Welding
Welding
Processes
Processes
Electron
Electron-
-Beam
Beam
Welding
Welding
Electro
Electro-
-slag
slag
Welding
Welding
Processes
Processes
Laser
Laser-
-Beam
Beam
Welding
Welding
Thermit
Thermit
Welding
Welding

Electron
Electron-
-Beam Welding
Beam Welding

 The process had its beginnings in the 1950s in the atomic power field. When first developed, welding
had to be carried out in a vacuum chamber to minimize the disruption of the electron beam by air
molecules. This requirement was, and still is, a serious inconvenience in production, due to the time
required to evacuate the chamber prior to welding.
 The pump-down time, as it is called, can take as long as an hour, depending on the size of the
chamber and the level of vacuum required. Today, EBW technology has progressed to where some
operations are performed without a vacuum.
 Three categories can be distinguished:
• high-vacuum welding (EBW-HV), in which welding is carried out in the same vacuum as beam
generation;
generation;
• medium-vacuum welding (EBW-MV), in which the operation is performed in a separate
chamber where only a partial vacuum is achieved; and
• non vacuum welding (EBW-NV), in which welding is accomplished at or near atmospheric
pressure.
 The pump-down time during work part loading and unloading is reduced in medium-vacuum EBW
and minimized in non-vacuum EBW, but there is a price paid for this advantage. In the latter two
operations, the equipment must include one or more vacuum dividers (very small orifices that
impede air flow but permit passage of the electron beam) to separate the beam generator (which
requires a high vacuum) from the work chamber. Also, in non-vacuum EBW, the work must be
located close to the orifice of the electron beam gun, approximately 13 mm or less.

 Finally, the lower vacuum processes cannot achieve the high weld qualities and depth-to-width ratios
accomplished by EBW-HV.
 Applications: Any metals that can be arc welded can be welded by EBW, as well as certain refractory
and difficult-to-weld metals that are not suited to AW. Work sizes range from thin foil to thick plate.
EBW is applied mostly in the automotive, aerospace, and nuclear industries. In the automotive
industry, EBW assembly includes aluminum manifolds, steel torque converters, catalytic converters,
and transmission components.
 Advantages: Electron-beam welding is noted for high-quality welds with deep and/ or narrow
profiles, limited heat-affected zone, and low thermal distortion. Welding speeds are high compared to
other continuous welding operations. No filler metal is used, and no flux or shielding gases are
other continuous welding operations. No filler metal is used, and no flux or shielding gases are
needed.
 Disadvantages: EBW include high equipment cost, need for precise joint preparation and alignment,
and the limitations associated with performing the process in a vacuum, as we have already
discussed. In addition, there are safety concerns because EBW generates X-rays from which humans
must be shielded.

Laser
Laser-
-Beam
Beam Welding
Welding
 Laser-beam welding (LBW) is a fusion-welding
process in which coalescence is achieved by
the energy of a highly concentrated, coherent
light beam focused on the joint to be welded.
 The term laser is an acronym for light
amplification by stimulated emission of
radiation.
 This same technology is used for laser-beam
machining. LBW is normally performed with
shielding gases (e.g., helium, argon, nitrogen,
shielding gases (e.g., helium, argon, nitrogen,
and carbon dioxide) to prevent oxidation.
 Filler metal is not usually added.
 LBW produces welds of high quality, deep
penetration, and narrow heat-affected zone.
These features are similar to those achieved
in electron-beam welding, and the two
processes are often compared.

 Advantages: There are several advantages of LBW over EBW: no vacuum chamber is
required, no X-rays are emitted, and laser beams can be focused and directed by optical
lenses and mirrors.
 Disadvantages:
• LBW does not possess the capability for the deep welds and high depth-to-width
ratios of EBW.
• Maximum depth in laser welding is about 19 mm, whereas EBW can be used for
weld depths of 50 mm or more; and the depth-to-width ratios in LBW are typically
limited to around 5:1. Because of the highly concentrated energy in the small area
limited to around 5:1. Because of the highly concentrated energy in the small area
of the laser beam, the process is often used to join small parts.

Electro
Electro-
-slag
slag Welding
Welding
 This process uses the same basic equipment as in some arc-welding operations, and it utilizes an arc to
initiate welding. However, it is not an AW process because an arc is not used during welding.
 Electro-slag welding (ESW) is a fusion-welding process in which coalescence is achieved by hot, electrically
conductive molten slag acting on the base parts and filler metal.
Fig. Electro-slag welding (ESW): (a) front view with molding shoe removed for clarity; (b) side view showing
schematic of molding shoe. Setup is similar to electro-gas welding except that resistance heating of molten
slag is used to melt the base and filler metals.

 The general configuration of ESW is similar to electro-gas welding. It is performed in a vertical
orientation (shown here for butt welding), using water-cooled molding shoes to contain the molten
slag and weld metal.
 At the start of the process, granulated conductive flux is put into the cavity. The consumable
electrode tip is positioned near the bottom of the cavity, and an arc is generated for a short while to
start melting the flux.
 Once a pool of slag has been created, the arc is extinguished and the current passes from the
electrode to the base metal through the conductive slag, so that its electrical resistance generates
heat to maintain the welding process.
 Since the density of the slag is less than that of the molten metal, it remains on top to protect the
 Since the density of the slag is less than that of the molten metal, it remains on top to protect the
weld pool. Solidification occurs from the bottom, while additional molten metal is supplied from
above by the electrode and the edges of the base parts.
 The process gradually continues until it reaches the top of the joint.

Thermit
Thermit Welding
Welding
 Thermit is a trademark name for thermite, a mixture of aluminum powder and iron oxide that produces an
exothermic reaction when ignited. It is used in incendiary bombs and for welding.
 Thermit welding (TW) is a fusion-welding process in which the heat for coalescence is produced by
superheated molten metal from the chemical reaction of thermit.
Fig. Thermit welding: (1) Thermit ignited; (2) crucible tapped, superheated metal flows into mold; (3)
metal solidifies to produce weld joint.

 Filler metal is obtained from the liquid metal; and although the process is used for joining, it has more in
common with casting than it does with welding.
 Finely mixed powders of aluminum and iron oxide (in a 1:3 mixture), when ignited at a temperature of
around 1300 ℃, produce the following chemical reaction:
 The temperature from the reaction is around 2500 ℃, resulting in superheated molten iron plus
aluminum oxide that floats to the top as a slag and protects the iron from the atmosphere.
 In thermit welding, the superheated iron (or steel if the mixture of powders is formulated accordingly) is
contained in a crucible located above the joint to be welded.
contained in a crucible located above the joint to be welded.
 After the reaction is complete (about 30 s, irrespective of the amount of Thermit involved), the crucible is
tapped and the liquid metal flows into a mold built specially to surround the weld joint.
 Because the entering metal is so hot, it melts the edges of the base parts, causing coalescence upon
solidification. After cooling, the mold is broken away, and the gates and risers are removed by
oxyacetylene torch or other method.
 Thermit welding has applications in joining of railroad rails, and repair of cracks in large steel castings and
forgings such as ingot molds, large diameter shafts, frames for machinery, and ship rudders. The surface of
the weld in these applications is often sufficiently smooth so that no subsequent finishing is required.

 In solid state-welding, coalescence of the part surfaces is achieved by
(1) pressure alone, or
(2) heat and pressure.
 For some solid-state processes, time is also a factor. If both heat and pressure are used, the amount
of heat by itself is not sufficient to cause melting of the work surfaces.
 In other words, fusion of the parts would not occur using only the heat that is externally applied in
these processes.
 In some cases, the combination of heat and pressure, or the particular manner in which pressure
SOLID
SOLID-
-STATE WELDING
STATE WELDING
 In some cases, the combination of heat and pressure, or the particular manner in which pressure
alone is applied, generates sufficient energy to cause localized melting of the faying surfaces.
 Filler metal is not added in solid-state welding.

 In most of the solid-state processes, a metallurgical bond is created with little or no melting of the base metals.
To metallurgically bond two similar or dissimilar metals, the two metals must be brought into intimate contact so
that their cohesive atomic forces attract each other.
 In normal physical contact between two surfaces, such intimate contact is prohibited by the presence of chemical
films, gases, oils, and so on. In order for atomic bonding to succeed, these films and other substances must be
removed. In fusion welding, the films are dissolved or burned away by high temperatures, and atomic bonding is
established by the melting and solidification of the metals in these processes.
 But in solid-state welding, the films and other contaminants must be removed by other means to allow
metallurgical bonding to take place. In some cases, a thorough cleaning of the surfaces is done just before the
welding process; while in other cases, the cleaning action is accomplished as an integral part of bringing the part
General Considerations in Solid
General Considerations in Solid-
-state Welding
state Welding
welding process; while in other cases, the cleaning action is accomplished as an integral part of bringing the part
surfaces together.
 To summarize, the essential ingredients for a successful solid-state weld are that the two surfaces must be very
clean, and they must be brought into very close physical contact with each other to permit atomic bonding.
 Welding processes that do not involve melting have several advantages over fusion- welding processes.
 If no melting occurs, then there is no heat-affected zone, and so the metal surrounding the joint retains its original
properties.
 Many of these processes produce welded joints that comprise the entire contact interface between the two parts,
rather than at distinct spots or seams, as in most fusion-welding operations. Also, some of these processes are
quite applicable to bonding dissimilar metals, without concerns about relative thermal expansions, conductivities,
and other problems that usually arise when dissimilar metals are melted and then solidified during joining.

Solid State
Solid State
W
Welding
elding
Forge
Forge
Welding
Welding
Cold
Cold
Welding
Welding
Friction
Friction
Welding
Welding
Friction
Friction
Stir
Stir
Welding
Welding
Ultrasonic
Ultrasonic
Welding
Welding
W
Welding
elding
Welding
Welding
Explosive
Explosive
Welding
Welding
Diffusion
Diffusion
Welding
Welding
Hot
Hot
Pressure
Pressure
Welding
Welding Roll
Roll
Welding
Welding

 Forge welding is of historic significance in the development of manufacturing technology.
 The process dates from about 1000 BCE, when blacksmiths of the ancient world learned to join two
pieces of metal.
 Forge welding is a welding process in which the components to be joined are heated to hot working
temperatures and then forged together by hammer or other means. Considerable skill was
required by the craftsmen who practiced it in order to achieve a good weld by present-day
standards.
 The process may be of historic interest; however, it is of minor commercial importance today
except for its variants that are discussed below.
Forge Welding
Forge Welding
except for its variants that are discussed below.

 Cold welding (CW) is a solid-state welding process accomplished by applying high pressure between
clean contacting surfaces at room temperature. The faying surfaces must be exceptionally clean for
CW to work, and cleaning is usually done by degreasing and wire brushing immediately before
joining.
 Also, at least one of the metals to be welded, and preferably both, must be very ductile and free of
work hardening. Metals such as soft aluminum and copper can be readily cold welded. The applied
compression forces in the process result in cold working of the metal parts, reducing thickness by as
much as 50%; but they also cause localized plastic deformation at the contacting surfaces, resulting
in coalescence.
Cold Welding
Cold Welding
in coalescence.
 For small parts, the forces may be applied by simple hand-operated tools. For heavier work,
powered presses are required to exert the necessary force.
 No heat is applied from external sources in CW, but the deformation process raises the temperature
of the work somewhat.
 Applications of CW include making electrical connections.

 Hot pressure welding (HPW) is another variation of forge welding in which coalescence occurs from
the application of heat and pressure sufficient to cause
 Considerable deformation of the base metals.
 The deformation disrupts the surface oxide film, thus leaving clean metal to establish a good bond
between the two parts.
 Time must be allowed for diffusion to occur across the faying surfaces. The operation is usually
carried out in a vacuum chamber or in the presence of a shielding medium.
 Principal applications of HP Ware in the aerospace industry.
Hot
Hot Pressure
Pressure Welding
Welding

 Diffusion welding (DFW) is a solid-state welding process that results from the application of heat
and pressure, usually in a controlled atmosphere, with sufficient time allowed for diffusion and
coalescence to occur. Temperatures are well below the melting points of the metals (about 0.5 Tm
is the maximum), and plastic deformation at the surfaces is minimal.
 The primary mechanism of coalescence is solid-state diffusion, which involves migration of atoms
across the interface between contacting surfaces.
 Applications of DFW include the joining of high-strength and refractory metals in the aerospace
and nuclear industries.
 The process is used to join both similar and dissimilar metals, and in the latter case a filler layer of
Diffusion Welding
Diffusion Welding
 The process is used to join both similar and dissimilar metals, and in the latter case a filler layer of
a different metal is often sandwiched between the two base metals to promote diffusion.
 The time for diffusion to occur between the faying surfaces can be significant, requiring more than
an hour in some applications

Roll
Roll Welding
Welding
 Roll welding is a variation of either forge welding or cold welding, depending on whether external heating of the
work parts is accomplished prior to the process.
 Roll welding (ROW) is a solid-state welding process in which pressure sufficient to cause coalescence is applied by
means of rolls, either with or without external application of heat. If no external heat is supplied, the process is
called cold-roll welding; if heat is supplied, the term hot-roll welding is used.
 Applications of roll welding include cladding stainless steel to mild or low alloy steel for corrosion resistance, making
bimetallic strips for measuring temperature, and producing ‘‘sandwich’’ coins for the U.S. mint.
Fig. Roll welding (ROW).

Explosive
Explosive Welding
Welding
 Explosion welding (EXW) is a solid-state welding process in which rapid coalescence of two metallic
surfaces is caused by the energy of a detonated explosive.
 It is commonly used to bond two dissimilar metals, in particular to clad one metal on top of a base metal
over large areas. Applications include production of corrosion-resistant sheet and plate stock for making
processing equipment in the chemical and petroleum industries.
 The term explosion cladding is used in this context. No filler metal is used in EXW, and no external heat is
applied. Also, no diffusion occurs during the process (the time is too short).
Fig. Explosive welding (EXW): (1) setup in the parallel configuration, and (2) during detonation of the explosive charge.

Friction
Friction Welding
Welding
Fig. Friction welding (FRW): (1) rotating part, no contact; (2) parts brought into contact to generate
friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created.

Friction
Friction Stir Welding
Stir Welding
 Friction stir welding (FSW), is a solid state welding process in which a rotating tool is fed along the joint line
between two workpieces, generating friction heat and mechanically stirring the metal to form the weld seam.
 The process derives its name from this stirring or mixing action. FSW is distinguished from conventional FRW by
the fact that friction heat is generated by a separate wear-resistant tool rather than by the parts themselves.
Fig. Friction stir welding (FSW): (1) rotating tool just prior to feeding into joint and
(2) partially completed weld seam. N = tool rotation, f = tool feed.

Ultrasonic
Ultrasonic Welding
Welding
 Ultrasonic Welding Ultrasonic welding (USW) is a solid-state welding process in which two
components are held together under modest clamping force, and oscillatory shear stresses of
ultrasonic frequency are applied to the interface to cause coalescence.
Fig. Ultrasonic welding (USW): (a) general setup for a lap joint; and (b) close-up of weld area.

 The rapid heating and cooling in localized regions of the work during fusion welding, especially arc welding,
result in thermal expansion and contraction that cause residual stresses in the weldment. These stresses, in
turn, can cause distortion and warping of the welded assembly.
 The situation in welding is complicated because
I. heating is very localized,
II. melting of the base metals occurs in these local regions, and
III. the location of heating and melting is in motion (at least in arc welding).
 The operation begins at one end and travels to the opposite end. As it proceeds, a molten pool is formed
from the base metal (and filler metal, if used) that quickly solidifies behind the moving arc. The portions of
Residual Stresses and Distortion
Residual Stresses and Distortion
from the base metal (and filler metal, if used) that quickly solidifies behind the moving arc. The portions of
the work immediately adjacent to the weld bead become extremely hot and expand, while portions
removed from the weld remain relatively cool.
 The weld pool quickly solidifies in the cavity between the two parts, and as it and the surrounding metal
cool and contract, shrinkage occurs across the width of the weldment.
 The weld seam is left in residual tension, and reactionary compressive stresses are set up in regions of the
parts away from the weld. Residual stresses and shrinkage also occurs along the length of the weld bead.
Since the outer regions of the base parts have remained relatively cool and dimensionally unchanged, while
the weld bead has solidified from very high temperatures and then contracted, residual tensile stresses
remain longitudinally in the weld bead.

Fig. (a) Butt welding two plates; (b) shrinkage across the width of the welded assembly; (c) transverse
and longitudinal residual stress pattern; and (d) likely warping in the welded assembly.

 Following are some techniques to minimize warping in a weldment:
(1) Welding fixtures can be used to physically restrain movement of the parts during welding.
(2) Heat sinks can be used to rapidly remove heat from sections of the welded parts to reduce distortion.
(3) Tack welding at multiple points along the joint can create a rigid structure prior to continuous seam
welding.
(4) Welding conditions (speed, amount of filler metal used, etc.) can be selected to reduce warping.
 The net result of these residual stresses, transversely and longitudinally, is likely to cause warping in the welded
assembly.
 The arc-welded butt joint in our example is only one of a variety of joint types and welding operations.
Thermally induced residual stresses and the accompanying distortion are a potential problem in nearly all fusion-
welding processes and in certain solid-state welding operations in which significant heating takes place.
(4) Welding conditions (speed, amount of filler metal used, etc.) can be selected to reduce warping.
(5) The base parts can be preheated to reduce the level of thermal stresses experienced by the parts.
(6) Stress relief heat treatment can be performed on the welded assembly, either in a furnace for small
weldment, or using methods that can be used in the field for large structures.
(7) Proper design of the weldment itself can reduce the degree of warping.

Welding Defects In addition to residual stresses and distortion in the final assembly, other defects can occur in
welding. Following is a brief description of each of the major categories, based on a classification:
 Cracks. Cracks are fracture-type interruptions either in the weld itself or in the base metal adjacent to the
weld. This is perhaps the most serious welding defect because it constitutes a discontinuity in the metal
that significant reduces weld strength. Several forms are defined in Figure 30.32.Welding cracks are caused
by embrittlement or low ductility of the weld and/or base metal combined with high restraint during
contraction. Generally, this defect must be repaired.
Welding Defects
Welding Defects
Fig. Various forms of welding cracks.

 Incomplete fusion. Several forms of this defect are illustrated in figure. Also known as lack of fusion, it is
simply a weld bead in which fusion has not occurred throughout the entire cross section of the joint. A
related defect is lack of penetration which means that fusion has not penetrated deeply enough into the
root of the joint.
Fig. Several forms of incomplete fusion.

 Imperfect shape or unacceptable contour. The weld should have a certain desired profile for maximum
strength, for a single V-groove weld. This weld profile maximizes the strength of the welded joint and
avoids incomplete fusion and lack of penetration.
 Cavities. These include various porosity and shrinkage voids. Porosity consists of small voids in the weld
metal formed by gases entrapped during solidification. The shapes of the voids vary between spherical
(blow holes) to elongated (worm holes).
 Porosity usually results from inclusion of atmospheric gases, sulfur in the weld metal, or contaminants
on the surfaces. Shrinkage voids are cavities formed by shrinkage during solidification. Both of these
cavity-type defects are similar to defects found in castings and emphasize the close kinship between
casting and welding.
Solid inclusions. These are nonmetallic solid materials trapped inside the weld metal. The most
 Solid inclusions. These are nonmetallic solid materials trapped inside the weld metal. The most
common form is slag inclusions generated during arc-welding processes that use flux. Instead of
floating to the top of the weld pool, globules of slag become encased during solidification of the metal.
Another form of inclusion is metallic oxides that form during the welding of metals such as aluminum,
which normally has a surface coating of Al2O3.

Mechanical
Mechanical Tests used in Welding
Tests used in Welding
 A variety of inspection and testing methods are available to check the quality of the welded joint.
Standardized procedures have been developed and specified over the years by engineering and trade
societies such as the American Welding Society (AWS).
 For purposes of discussion, these inspection and testing procedures can be divided into three
categories:
(1) Visual,
(2) Non-destructive, and
(3) Destructive.
Fig. Mechanical tests used in welding: (a) tension–shear test of arc weldment, (b) fillet break test, (c)
tension–shear test of spot weld, (d) peel test for spot weld.

 Visual inspection is no doubt the most widely used welding inspection method. An Inspect or
visually examines the weldment for (1) conformance to dimensional specifications on the part
drawing, (2) warping, and (3) cracks, cavities, incomplete fusion, and other visible defects. The
welding inspector also determines if additional tests are warranted, usually in the nondestructive
category. The limitation of visual inspection is that only surface defects are detectable; internal
defects cannot be discovered by visual methods.
 Non-destructive evaluation (NDE) includes various methods that do not damage the specimen being
inspected. Dye-penetrant and fluorescent-penetrant tests are methods for detecting small defects
such as cracks and cavities that are open to the surface. Fluorescent penetrants are highly visible
Inspection and Testing Methods
Inspection and Testing Methods
such as cracks and cavities that are open to the surface. Fluorescent penetrants are highly visible
when exposed to ultraviolet light, and their use is therefore more sensitive than dyes.
 Several other NDE methods should be mentioned.
 Magnetic particle testing is limited to ferromagnetic materials. A magnetic field is established
in the subject part, and magnetic particles (e.g., iron filings) are sprinkled on the surface.
Subsurface defects such as cracks and inclusions reveal themselves by distorting the magnetic
field, causing the particles to be concentrated in certain regions on the surface.

 Ultrasonic testing involves the use of high-frequency sound waves (>20 kHz) directed through the
specimen. Discontinuities (e.g., cracks, inclusions, porosity) are detected by losses in sound
transmission.
 Radiographic testing uses X-rays or gamma radiation to detect flaws internal to the weld metal. It
provides a photographic film record of any defects.
 Destructive testing methods in which the weld is destroyed either during the test or to prepare the test
specimen. They include mechanical and metallurgical tests. Mechanical tests are similar in purpose to
conventional testing methods such as tensile tests and shear tests. The difference is that the test
conventional testing methods such as tensile tests and shear tests. The difference is that the test
specimen is a weld joint. Metallurgical tests involve the preparation of metallurgical specimens of the
weldment to examine such features as metallic structure, defects, extent and condition of heat-affected
zone, presence of other elements, and similar phenomena.

 Weldability is the capacity of a metal or combination of metals to be welded into a suitably designed
structure, and for the resulting weld joint(s) to possess the required metallurgical properties to perform
satisfactorily in the intended service. Good weldability is characterized by the ease with which the
welding process is accomplished, absence of weld defects, and acceptable strength, ductility, and
toughness in the welded joint.
 Factors that affect weldability include
 welding process,
Weldability
Weldability
 welding process,
 base metal properties,
 filler metal, and
 surface conditions.
 The welding process is significant. Some metals or metal combinations that can be readily welded by one
process are difficult to weld by others. For example, stainless steel can be readily welded by most AW
processes, but is considered a difficult metal for oxy-fuel welding.

 Properties of the base metal affect welding performance. Important properties include melting point, thermal
conductivity, and coefficient of thermal expansion.
 One might think that a lower melting point would mean easier welding. However, some metals melt too easily for
good welding (e.g., aluminum). Metals with high thermal conductivity tend to transfer heat away from the weld
zone, which can make them hard to weld (e.g., copper). High thermal expansion and contraction in the metal
causes distortion problems in the welded assembly.
 Dissimilar metals pose special problems in welding when their physical and/or mechanical properties are
substantially different. Differences in melting temperature are an obvious problem. Differences in strength or
coefficient of thermal expansion may result in high residual stresses that can lead to cracking.
 If a filler metal is used, it must be compatible with the base metal(s). In general, elements mixed in the liquid state
DESIGN CONSIDERATIONS IN WELDING
DESIGN CONSIDERATIONS IN WELDING
 If a filler metal is used, it must be compatible with the base metal(s). In general, elements mixed in the liquid state
that form a solid solution upon solidification will not cause a problem. Embrittlement in the weld joint may occur
if the solubility limits are exceeded.
 Surface conditions of the base metals can adversely affect the operation. For example, moisture can result in
porosity in the fusion zone. Oxides and other solid films on the metal surfaces can prevent adequate contact and
fusion from occurring.
If an assembly is to be permanently welded, the designer should follow certain guidelines:
 Design for welding. The most basic guideline is that the product should be designed from the start as a
welded assembly, and not as a casting or forging or other formed shape.
 Minimum parts. Welded assemblies should consist of the fewest number of parts possible. For example, it is
usually more cost efficient to perform simple bending operations on a part than to weld an assembly from
flat plates and sheets.

Cont.
Cont.
The following guidelines apply to arc welding:
 Good fit-up of parts to be welded is important to maintain dimensional control and minimize
distortion.
 Machining is sometimes required to achieve satisfactory fit-up.
 The assembly must provide access room to allow the welding gun to reach the welding area.
 Whenever possible, design of the assembly should allow flat welding to be performed, since this is
the fastest and most convenient welding position. The possible welding positions are defined in
figure. The overhead position is the most difficult.
Fig. Welding positions (defined here for groove welds): (a) flat, (b) horizontal, (c) vertical, and (d) overhead.

The following design guidelines apply to resistance spot welding:
 Low-carbon sheet steel up to 3.2 mm is the ideal metal for resistance spot welding.
 Additional strength and stiffness can be obtained in large flat sheet metal components by:
• spot welding reinforcing parts into them, or
• forming flanges and embossments into them.
 The spot-welded assembly must provide access for the electrodes to reach the welding area.
 Sufficient overlap of the sheet-metal parts is required for the electrode tip to make proper contact in spot
welding. For example, for low-carbon sheet steel, the overlap distance should range from about six times
stock thickness for thick sheets of 3.2 mm to about 20 times thickness for thin sheets, such as 0.5 mm.
Cont.
Cont.

Unit-III-Welding_MEB 212.pdf welding important

Unit-III-Welding_MEB 212.pdf welding important

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