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Welding Method

Topics: Welding, Arc welding / Pages: 12 (2772 words) / Published: Mar 20th, 2013
CHAPTER 27&29
INTRODUCTION TO WELDING & FUSION
WELDING PROCESS
Dr. Tasnim Firdaus Ariff

Welding Fundamentals
Overview of Welding Technology
2. The Weld Joint
3. Features of a Fusion Welded Joint
1.

2

Joining and Assembly Distinguished
Joining - welding, brazing, soldering, and adhesive bonding
 These processes form a permanent joint between parts
Assembly - mechanical methods (usually) of fastening parts together  Some of these methods allow for easy disassembly, while others do not

3

Welding
Joining process in which two (or more) parts are coalesced at their contacting surfaces by 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
 Still others by pressure alone with no external heat
 In some welding processes a filler material is added to facilitate coalescence

4

Limitations and Drawbacks of Welding
Most welding operations are performed manually and are expensive in terms of labor cost
 Most welding processes utilize high energy and are inherently dangerous  Welded joints do not allow for convenient disassembly
 Welded joints can have quality defects that are difficult to detect 

5

Types of Welding Processes
Some 50 different types of welding processes have been catalogued by the American Welding Society (AWS)
 Welding processes can be divided into two major categories:


 Fusion welding
 Solid state welding

6

Five Types of Joints
1.
2.
3.

4.
5.

7

Butt joint
Corner joint
Lap joint
Tee joint
Edge joint

Butt Joint
Parts lie in same plane and are joined at their edges

Figure 30.2 Five basic types of joints: (a) butt

Corner Joint
Parts in a corner joint form a right angle and are joined at the corner of the angle

Figure 30.2 (b) corner

Lap Joint
Consists of two overlapping parts

Figure 30.2 (c) lap

Tee Joint
One part is perpendicular to the other in the approximate shape of the letter "T"

Figure 30.2 (d) tee

Edge Joint
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) Figure 30.2 (e) edge

Types of Welds
Each of the preceding joints can be made by welding
 Other joining processes can also be used for some of the joint types  There is a difference between joint type and the way it is welded - the weld type


13

Fillet Weld


Used to fill in the edges of plates created by corner, lap, and tee joints

 Filler metal used to provide cross section in approximate shape of a right triangle
 Most common weld type in arc and oxyfuel welding
 Requires minimum edge preparation

Figure 30.3 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.
14

Groove Welds


Usually requires part edges to be shaped into a groove to facilitate weld penetration  Edge preparation increases cost of parts fabrication
 Grooved shapes include square, bevel, V, U, and J, in single or double sides
 Most closely associated with butt joints

Figure 30.4 Some 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 original part edges.
15

Spot Weld
Fused section between surfaces of two plates
 Used for lap joints
 Closely associated with resistance welding

Figure
30.6 (a)
Spot
weld

Welding Methods and Procedures
1.
2.
3.

4.
5.
6.
7.
8.

17

Arc Welding
Resistance Welding
Oxyfuel Gas Welding
Other Fusion Welding Processes
Solid State Welding
Weld Quality
Weldability
Design Considerations in Welding

Two Categories of Welding Processes


Fusion welding - coalescence is accomplished by melting the two parts to be joined, in some cases adding filler metal to the joint
 Examples: arc welding, resistance spot welding, oxyfuel gas

welding


Solid state welding - heat and/or pressure are used to achieve coalescence, but no melting of base metals occurs and no filler metal is added
 Examples: forge welding, diffusion welding, friction welding

18

Arc Welding (AW)
A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work
 Electric energy from the arc produces temperatures ~
10,000 F (5500 C), hot enough to melt any metal
 Most AW processes add filler metal to increase volume and strength of weld joint

19

What is an Electric Arc?
An electric arc is a discharge of electric current across a gap in a circuit
 It is sustained by an ionized column of gas (plasma) through which the current flows
 To initiate the arc in AW electrode is brought into contact
,
with work and then quickly separated from it by a short distance 20

Arc Welding
A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake

Figure 31.1 Basic configuration of an arc welding process.
21

Two Basic Types of AW Electrodes
 Consumable – consumed during welding process

 Source of filler metal in arc welding
 Welding rods (a.k.a. sticks) are 9 to 18 inches and 3/8 inch or less

in diameter and must be changed frequently
 Weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions
 Nonconsumable – not consumed during welding process
 Filler metal must be added separately
 Made of tungsten which resists melting
 Gradually depleted during welding (vaporization is principal

mechanism)
 Any filler metal must be supplied by a separate wire fed into weld pool 22

Power Source in Arc Welding


Direct current (DC) vs. Alternating current (AC)
 AC machines less expensive to purchase and operate, but

generally restricted to ferrous metals
 DC equipment can be used on all metals and is generally noted for better arc control

23

Consumable Electrode AW Processes







24

Shielded Metal Arc Welding
Gas Metal Arc Welding
Flux-Cored Arc Welding
Electrogas Welding
Submerged Arc Welding

Shielded Metal Arc Welding (SMAW)
Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding
 Sometimes called "stick welding"
 Power supply, connecting cables, and electrode holder available for a few thousand dollars

25

Welding Stick in SMAW
Composition of filler metal usually close to base metal  Coating: powdered cellulose mixed with oxides, carbonates, and other ingredients, held together by a silicate binder
 Welding stick is clamped in electrode holder connected to power source
 Disadvantages of stick welding:


 Sticks must be periodically changed
 High current levels may melt coating prematurely

26

SMAW Applications
Used for steels, stainless steels, cast irons, and certain nonferrous alloys
 Not used or rarely used for aluminum and its alloys, copper alloys, and titanium


27

Gas Metal Arc Welding (GMAW)
Uses a consumable bare metal wire as electrode and shielding accomplished by flooding arc with a gas
 Wire is fed continuously and automatically from a spool through the welding gun
 Shielding gases include inert gases such as argon and helium for aluminum welding, and active gases such as CO2 for steel welding
 Bare electrode wire plus shielding gases eliminate slag on weld bead - no need for manual grinding and cleaning of slag
28

Gas Metal Arc Welding

31.4 Gas metal arc welding (GMAW).
29

GMAW Advantages over SMAW


Better arc time because of continuous wire electrode
 Sticks must be periodically changed in SMAW



Better use of electrode filler metal than SMAW
 End of stick cannot be used in SMAW

Higher deposition rates
 Eliminates problem of slag removal
 Can be readily automated


30

Flux-Cored Arc Welding (FCAW)
Adaptation of shielded metal arc welding, to overcome limitations of stick electrodes
 Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g., alloying elements) in its core
 Two versions:
 Self-shielded FCAW - core includes compounds that produce

shielding gases
 Gas-shielded FCAW - uses externally applied shielding gases

31

Flux-Cored Arc Welding

Figure 31.6 Flux-cored arc welding. Presence or absence of externally supplied shielding gas distinguishes the two types: (1) self-shielded, in which core provides ingredients for shielding, and (2) gas-shielded, which uses external shielding gases.

32

Electrogas Welding (EGW)
Uses a continuous consumable electrode, either flux-cored wire or bare wire with externally supplied shielding gases, and molding shoes to contain molten metal
 When flux-cored electrode wire is used and no external gases are supplied, then special case of self-shielded FCAW
 When a bare electrode wire used with shielding gases from external source, then special case of GMAW

33

Electrogas Welding

Figure 31.7 Electrogas 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.

34

Submerged Arc Welding (SAW)
Uses a continuous, consumable bare wire electrode, with arc shielding provided by a cover of granular flux
 Electrode wire is fed automatically from a coil
 Flux introduced into joint slightly ahead of arc by gravity from a hopper
 Completely submerges operation, preventing sparks, spatter, and

radiation

35

SAW Applications and Products







36

Steel fabrication of structural shapes (e.g., I-beams)
Seams for large diameter pipes, tanks, and pressure vessels
Welded components for heavy machinery
Most steels (except hi C steel)
Not good for nonferrous metals

Nonconsumable Electrode Processes
Gas Tungsten Arc Welding
 Plasma Arc Welding
 Carbon Arc Welding
 Stud Welding


37

Gas Tungsten Arc Welding (GTAW)
Uses a nonconsumable tungsten electrode and an inert gas for arc shielding



A.k.a. Tungsten Inert Gas (TIG) welding
 In Europe, called "WIG welding"



Used with or without a filler metal
 When filler metal used, it is added to weld pool from separate rod or wire



38

Melting point of tungsten = 3410 C (6170 F)

Applications: aluminum and stainless steel most common

Advantages / Disadvantages of GTAW
Advantages:
 High quality welds for suitable applications
 No spatter because no filler metal through arc
 Little or no post-weld cleaning because no flux

Disadvantages:
 Generally slower and more costly than consumable electrode

AW processes

39

Plasma Arc Welding (PAW)
Special form of GTAW in which a constricted plasma arc is directed at weld area




40

Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream

Temperatures in PAW reach 28,000 C (50,000 F), due to construction of arc, producing a plasma jet of small diameter and very high energy density

Advantages / Disadvantages of PAW
Advantages:
 Good arc stability
 Better penetration control than other AW
 High travel speeds
 Excellent weld quality
 Can be used to weld almost any metals

Disadvantages:
 High equipment cost
 Larger torch size than other AW
 Tends to restrict access in some joints
41

Resistance Welding (RW)
A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence  Heat generated by electrical resistance to current flow at junction to be welded
 Principal RW process is resistance spot welding
(RSW)

42

Advantages / Drawbacks of RW
Advantages:
 No filler metal required
 High production rates possible
 Lends itself to mechanization and automation
 Lower operator skill level than for arc welding
 Good repeatability and reliability
Disadvantages:
 High initial equipment cost
 Limited to lap joints for most RW processes
43

Resistance Spot Welding (RSW)
Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes
 Used to join sheet metal parts using a series of spot welds  Widely used in mass production of automobiles, appliances, metal furniture, and other products made of sheet metal
 Typical car body has ~ 10,000 spot welds
 Annual production of automobiles in the world is

measured in tens of millions of units

44

Spot Welding Cycle

Figure 31.13 (a) Spot welding cycle, (b) plot of squeezing force & current in cycle (1) parts inserted between electrodes, (2) electrodes close, force applied, (3) current on,
(4) current off, (5) electrodes opened.
45

Resistance Seam Welding (RSEW)
Uses rotating wheel electrodes to produce a series of overlapping spot welds along lap joint
 Can produce air-tight joints
 Applications:
 Gasoline tanks
 Automobile mufflers
 Various other sheet metal

containers
46

Resistance Projection Welding (RPW)
A resistance welding process in which coalescence occurs at one or more small contact points on parts
 Contact points determined by design of parts to be joined
 May consist of projections, embossments, or localized intersections of parts

47

Cross-Wire Welding

Figure 31.18 (b) cross-wire welding.
48

Oxyfuel Gas Welding (OFW)
Group of fusion welding operations that burn various fuels mixed with oxygen
 OFW employs several types of gases, which is the primary distinction among the members of this group
 Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts
 Most important OFW process is oxyacetylene welding

49

Oxyacetylene Welding (OAW)
Fusion welding performed by a high temperature flame from combustion of acetylene and oxygen  Flame is directed by a welding torch
 Filler metal is sometimes added
 Composition must be similar to base metal
 Filler rod often coated with flux to clean surfaces and prevent oxidation

50

Acetylene (C2H2)
Most popular fuel among OFW group because it is capable of higher temperatures than any other - up to 3480 C (6300 F)
 Two stage chemical reaction of acetylene and oxygen:


 First stage reaction (inner cone of flame):

C2H2 + O2 2CO + H2 + heat
 Second stage reaction (outer envelope):
2CO + H2 + 1.5O2 2CO2 + H2O + heat

51

Oxyacetylene Torch


Maximum temperature reached at tip of inner cone, while outer envelope spreads out and shields work surfaces from atmosphere

Figure 31.22 The neutral flame from an oxyacetylene torch indicating temperatures achieved.
52

Safety Issue in OAW
Together, acetylene and oxygen are highly flammable
 C2H2 is colorless and odorless


 It is therefore processed to have characteristic garlic odor

53

Other Fusion Welding Processes
FW processes that cannot be classified as arc, resistance, or oxyfuel welding
 Use unique technologies to develop heat for melting
 Applications are typically unique
 Processes include:
 Electron beam welding
 Laser beam welding
 Electroslag welding
 Thermit welding

54

Electron Beam Welding (EBW)
Fusion welding process in which heat for welding is provided by a highly-focused, high-intensity stream of electrons striking work surface
 Electron beam gun operates at:
 High voltage (e.g., 10 to 150 kV typical) to accelerate electrons
 Beam currents are low (measured in milliamps)

55

Three Vacuum Levels in EBW


High-vacuum welding – welding done in same vacuum chamber as beam generation
 Highest quality weld



Medium-vacuum welding – welding done in separate chamber with partial vacuum
 Vacuum pump-down time reduced



Non-vacuum welding – welding done at or near atmospheric pressure, with work positioned close to electron beam generator
 Vacuum divider required to separate work from beam

generator
56

EBW Advantages / Disadvantages
Advantages:
 High-quality welds, deep and narrow profiles
 Limited heat affected zone, low thermal distortion
 High welding speeds
 No flux or shielding gases needed
Disadvantages:
 High equipment cost
 Precise joint preparation & alignment required
 Vacuum chamber required
 Safety concern: EBW generates x-rays
57

Laser Beam Welding (LBW)
Fusion welding process in which coalescence is achieved by energy of a highly concentrated, coherent light beam focused on joint
 Laser = "light amplification by stimulated emission of radiation"  LBW normally performed with shielding gases to prevent oxidation  Filler metal not usually added
 High power density in small area, so LBW often used for small parts
58

Comparison: LBW vs. EBW
No vacuum chamber required for LBW
 No x-rays emitted in LBW
 Laser beams can be focused and directed by optical lenses and mirrors
 LBW not capable of the deep welds and high depth-to-width ratios of EBW


 Maximum LBW depth = ~ 19 mm (3/4 in), whereas EBW

depths = 50 mm (2 in)

59

Thermit Welding (TW)
FW process in which heat for coalescence is produced by superheated molten metal from the chemical reaction of thermite  Thermite = mixture of Al and Fe3O4 fine powders that produce an exothermic reaction when ignited
 Also used for incendiary bombs
 Filler metal obtained from liquid metal
 Process used for joining, but has more in common with casting than welding

60

Thermit Welding

Figure 31.25 Thermit welding: (1) Thermit ignited; (2) crucible tapped, superheated metal flows into mold; (3) metal solidifies to produce weld joint.

61

TW Applications
Joining of railroad rails
 Repair of cracks in large steel castings and forgings
 Weld surface is often smooth enough that no finishing is required 

62

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