1

CHAPTER 1

INTRODUCTION

Today is the 21st century and in this century the competition has increased to many

folds. Over the last decade or so there has been considerable demand for cost effective

and high turnaround machine tool for use in industries. The demand for composites in

aerospace and industrial applications has been skyrocketing in the past decade and the

same trend will continue for the next two decades. Conventional machine tools are

often not suitable for machining composites and so are lasers. This method of abrasive

waterjet machining involves entraining abrasive particles carried in air into a high

velocity waterjet. This stream is directed by means of a suitably designed nozzle on to

the work piece to be machined [4]. Metal removal occurs due to erosion caused by the

abrasive particles impacting the work surface at high speed. AWJs have several

inherent merits that are unmatchable by most other machine tools:

Preservation of structural/chemical integrity – No HAZ and minimum surface

hardening and no tearing with minimum fraying.

Fatigue performance enhancement by combining AWJ and low-cost dry-grit

blasting.

Material independence – Even for nanomaterials that are integrated seamlessly at the

molecular level.

No contact tool to wear and break when machining extremely hard and tough

materials

For example- silicon carbide ceramic matrix composites.

2

Cost-effective with fast turnaround (no tooling or mask needed) for ones and twos

(R&D) and/or for thousands (production) – Complete a part from design to finish in

minutes to several hours, saving manufacturing jobs from outsourcing.

Minimum limitation in part size – from macro to micro.

Multi-machining mode - roughing, parting, drilling, turning, milling, and grooving,

etc. in a single setup with no need for tool change and part transfer No contact tool to

break when machining extremely hard and tough materials.

Compatible with “Just-In-Time” practice for lean manufacturing

The method is very inefficient with less than 3% of a waterjet’s energy being

transferred to abrasive particles. The process of entraining abrasive carried in air 2

becomes increasingly ineffective at jet diameters under 500 micro meters and

ceases to operate at jet diameters of 300 micro meters. As jet diameters less than 100

micro meters are required for micromachining the current generation of abrasive

waterjets cannot be used to micromachine. Since the introduction of AWJs there has

been no paradigm shift in the way abrasive waterjets are generated but there have been

very substantial improvements in AWJ cutting performance. Improved cutting

performance is the result of incremental developments in ultra high pressure pumps,

cutting heads, software and control systems. Improving cutting performance, combined

with advances in machine tool design and innovative marketing and sales activities, has

resulted in AWJs becoming one of the three major non contact cutting methods; the

others being lasers and wire electric discharge machining (WEDM). Probably the most

important development leading to widespread commercialization of AWJ based

machine tools was the adoption of reacted tungsten carbide for cutting head focus tubes

– a paradigm shift in super hard materials technology by a major chemical company

(Dow Chemical Company), exploited by a nozzle manufacturer (Boride Products Inc,

now part of Kennametal Inc). A twenty times improvement in focus tube life to 50 to

100 hours transformed the prospects of abrasive waterjets from a niche market to a

main stream machine tool [6].

3

4

CHAPTER 2

PRINCIPLE

Waterjets are fast, flexible, reasonably precise, and in the last few years have become

friendly and easy to use. They use the technology of high-pressure water being forced

through a small hole (typically called the “orifice” or “jewel”) to concentrate an

extreme amount of energy in a small area. The restriction of the tiny orifice creates high

pressure and a high-velocity beam, much like putting your finger over the end of a

garden hose. The inlet water for a pure waterjet is pressurized between 20,000 and

60,000 Pounds per Square Inch (PSI) (1300 to 6200 bar). This is forced through a tiny

hole in the jewel, which is typically 0.007" to 0.020" in diameter (0.18 to 0.4 mm). This

creates a very high-velocity, very thin beam of water (which is why some people refer

to waterjets as "water lasers") travelling as close to the speed of sound (about 600 mph

or 960 km/hr) [3].

An abrasivejet starts out the same as a pure waterjet. As the thin stream of water leaves

the jewel, however, abrasive is added to the stream and mixed. The high-velocity water

exiting the jewel creates a vacuum which pulls abrasive from the abrasive line, which

then mixes with the water in the mixing tube. The beam of water accelerates abrasive

particles to speeds fast enough to cut through much harder materials. The cutting action

of an abrasivejet is two-fold. The force of the water and abrasive erodes the material,

even if the jet is stationary (which is how the material is initially pierced). The cutting

action is greatly enhanced if the abrasivejet stream is moved across the material and the

ideal speed of movement depends on a variety of factors, including the material, the

shape of the part, the water pressure and the type of abrasive. Controlling the speed of

the abrasivejet nozzle is crucial to efficient and economical machining. The most

commonly used abrasive is garnet because of its optimum performance of cutting

power versus cost and its lack of toxicity. It is also a good compromise between cutting

power and wear on carbide mixing tubes. There are two types of garnet that are

5

generally used: HPX® and HPA®, which are produced from crystalline and alluvial

deposits, respectively.13 HPX garnet grains have a unique structure that causes them to

fracture along crystal cleavage lines, producing very sharp edges that enable HPX to

outperform its alluvial counterpart. There are other abrasives that are more or less

aggressive than garnet [2].

WORKING

Intensifier, shown in Fig. 1 is driven by a hydraulic power pack. The heart of the

hydraulic power pack is a positive displacement hydraulic pump. The power packs in

modern commercial systems are often controlled by microcomputers to achieve

programmed rise of pressure etc. The hydraulic power pack delivers the hydraulic oil to

the intensifier at a pressure of ph . By using direction control valve, the intensifier is

driven by the hydraulic unit. The water may be directly supplied to the small cylinder

of the intensifier or it may be supplied through a booster pump, which typically raises

the water pressure to 11 bar before supplying it to the intensifier. Sometimes water is

softened or long chain polymers are added in “additive unit”. Thus, as the intensifier

works, it delivers high pressure water. As the larger piston changes direction within the

intensifier, there would be a drop in the delivery pressure. To counter such drops, 4

a thick cylinder is added to the delivery unit to accommodate water at high pressure.

This is called an “accumulator” which acts like a “fly wheel” of an engine and

minimises fluctuation of water pressure. High-pressure water is then fed through the

flexible stainless steel pipes to the cutting head. It is worth mentioning here that such

pipes are to carry water at 4000 bar (400 MPa) with flexibility incorporated in them

with joints but without any leakage. Cutting head consists of orifice, mixing chamber

and focussing tube or insert where water jet is formed and mixed with abrasive

particles to form abrasive water jet.

6

Fig-1 Intensifier-Schematic

Fig. 2 shows a cutting head or jet former both schematically and photographically.

Typical diameter of the flexible stainless steel pipes is of 6 mm. Water carried through

the pipes is brought to the jet former or cutting head. The potential or pressure head of

the water is converted into velocity head by allowing the high-pressure water to issue

through an orifice of small diameter (0.2 – 0.4 mm). The velocity of the water jet thus

formed can be estimated, assuming no losses as using Bernoulli’s equation, pw is the

water pressure and ρw is the density of water. The orifices are typically made of

sapphire. In WJM this high velocity water jet is used for the required application where

as in AWJM it is directed into the mixing chamber. The mixing chamber has a typical

dimension of inner diameter 6 mm and a length of 10 mm. As the high velocity water is

issued from the orifice into the mixing chamber, low pressure (vacuum) is created

within the mixing chamber. Metered abrasive particles are introduced into the mixing

chamber through a port.

7

Fig-2 Schematic and photographic view of the cutting head

Fig. 3 schematically shows the mixing process. Mixing means gradual entrainment of

abrasive particles within the water jet and finally the abrasive water jet comes out of the

focussing tube or the nozzle. During mixing process, the abrasive particles are

gradually accelerated due to transfer of momentum from the water phase to abrasive

phase and when the jet finally leaves the focussing tube, both phases, water and

abrasive, are assumed to be at same velocity. The mixing chamber is immediately

followed by the focussing tube or the inserts.

8

Fig-3 Schematic view of mixing process

The focussing tube is generally made of tungsten carbide (powder metallurgy product)

having an inner diameter of 0.8 to 1.6 mm and a length of 50 to 80 mm. Tungsten

carbide is used for its abrasive resistance. Abrasive particles during mixing try to enter

the jet, but they are reflected away due to interplay of buoyancy and drag force. They

go on interacting with the jet and the inner walls of the mixing tube, until they are

accelerated using the momentum of the water jet.

9

CHAPTER 3

PROCESS PARAMETERS

The variables that influence the rate of metal removal and accuracy of machining in this

process are:

Carrier Gas:

Carrier gas, to be used in abrasive jet machining, must not flare excessively when

discharged from the nozzle into the atmosphere. Further, the gas should not be

nontoxic, cheap, easily available and capable of being dried and cleaned without

difficulty. The gasses that can be used are air, carbon dioxide or nitrogen. Air is most

commonly used owing to easy availability and little cost.

Types Of Abrasive: The choice of abrasive depends upon the type of machining operations, for example,

roughing, finishing etc., work material and cost. The abrasive should have a sharp and

irregular shape and be fine enough to remain suspended in carrier gas and should also

have excellent flow characteristics.

Grain Size: The rate of metal removal depends on the size of the abrasive grain. Finer grains are

less irregular in shape, and hence, possess lesser cutting ability. Moreover, finer grains

tend to stick together and choke the nozzle. The most favourable grain size ranges from

10-50 μ. 7

10

Jet Velocity: The kinetic energy of the abrasive jet is utilized for the metal removal by erosion.

Finnie and Sheldon have shown that for erosion to occur, the jet must impinge the work

surface with a certain minimum velocity. Figures 4 and 5 shows the effect of nozzle

pressure on the rate of metal removal.

Fig-4 Pressure vs. MRR

Fig-5 Pressure vs. MRR

(Grain Size- 40μ, 24μ, 10μ) (Abrasive- AL 2O3

11

Work material- cemented carbide

Grain Size- 40μ)

Mean Number Of Abrasive Grains Per Unit Volume Of Carrier Gas: An idea about mean number of abrasive grains per unit volume of the carrier gas can be

obtained from the mixing ratio M. A large value of M should result in higher rates of

metal removal but a large abrasive flow rate has been found to adversely influence jet

velocity, and may sometimes even clog the nozzle. Thus, for the given conditions, there

is an optimum mixing ratio that leads to a maximum metal removal rate.

Work Material: AJM is recommended for the processing of brittle materials, such as glass, ceramics,

refractories, etc. Most of the ductile materials are practically unmachinable by AJM. 8

12

Stand Off Distance: A large SOD results in the flaring up of the jet which leads to poor accuracy. Fig 6

shows the relationship between the SOD and the rate of material removal. Small metal

removal rates at a low SOD is due to a reduction in nozzle pressure with decreasing

distance, whereas a drop in material removal rate at large SOD is due to a reduction in

the jet velocity with increasing distance.

Fig-6 SOD vs. MRR

Abrasive- AL2O3, Grain Size- 40μ, Work Material- Glass,

Pressure- 0.03)

Nozzle Design: The nozzle has to withstand the erosive action of abrasive particles, and hence, must be

made of materials that can provide high resistance to wear. The common materials for

nozzle are sapphire and tungsten carbide. The nozzle should be so designed that the

pressure loss due to bends, friction, etc. is as little as possible.

13

NOZZLE WEAR IN AWJM:

As shown in Fig. 7 the focusing nozzle, which is the most critical part in AWJ cutting

systems, is subjected to two modes of wear: 9

Impact erosion beginning at the entry cone down to approximately one third of the

nozzle length;

Sliding erosion in the downstream area, where particles travel parallel to the wall and

the wear mode shifts from shallow impact to abrasion.

As the bore diameter of the nozzle increases owing to wear of the material, the

coherence of the jet beam decreases, which ultimately leads to failure of the nozzle:

The widening jet beam increases the kerf width, i.e. the width of the cut, and decreases

the cutting efficiency. The material of a brittle work piece is removed during AWJ

cutting owing to a network of cracks created by the direct impact of erodent particles

and by adjacent impacting particles. The crack network model Fig. 8, assumes a

14

vertical impact of erodent particles, which fragment during the impact. During impact

two types of cracks are produced:

Median and radial cracks normal to the surface,

Lateral cracks which are parallel to the surface.

15

Fig-8 Brittle material removal by impact of a waterjet containing abrasive particles

The interaction of lateral and radial cracks is considered to result in material removal,

i.e. the spallation of tiny chips off the surface. Boron carbide displayed the highest

calculated wear resistance as compared with hard metals, alumina-based ceramics,

silicon nitride based ceramics and some grades of silicon carbide ceramics. However, in

erosion experiments boron carbide showed best resistance only at low impact angles

( <20º), followed by a “brittle response” regime of erosion wear, in which wear rates

peak at 90º erodent impact angle (see Fig. 9). The technical superiority of boron carbide

as a blast nozzle material is well established; therefore its poor erosion resistance at

high impact angles does not compromise its effectiveness in this particular application.

16

Fig-9

However, owing to the extreme speeds of abrasive particles, when entrained in a

waterjet (700 m s-l), the bore entry zone of waterjet nozzles is heavily loaded at impact

angles of 15-45º. For very brittle materials such as boron carbide it is well known, that

the critical load for generating lateral cracks (“cracking threshold” P,) is orders of

magnitude lower than for hard metals or “tough ceramics” [1].

COMPARISON BETWEEN WJM, AWJM AND AJM:

17

18

19

20

3.1.2 Gas Metal Arc Welding (GMAW)

21

Gas metal arc welding (GMAW), sometimes referred to by its

subtypes metal inert gas (MIG) welding or metal active

gas (MAG) welding, is a welding process in which an electric arc forms

between a consumable wire electrode and the workpiece metal(s), which

heats the workpiece metal(s), causing them to melt, and join. Along with

the wire electrode, a shielding gas feeds through the welding gun, which

shields the process from contaminants in the air. The process can be semi-

automatic or automatic. A constant voltage, direct current power source is

most commonly used with GMAW, but constant current systems, as well

as alternating current, can be used. There are four primary methods of

metal transfer in GMAW, called globular, short-circuiting, spray, and

pulsed-spray, each of which has distinct properties and corresponding

advantages and limitations. Originally developed for

welding aluminium and other non-ferrous materials in the 1940s, GMAW

was soon applied to steels because it provided faster welding time

compared to other welding processes. The cost of inert gas limited its use

in steels until several years later, when the use of semi-inert gases such

as carbon dioxide became common. Further developments during the

1950s and 1960s gave the process more versatility and as a result, it

became a highly used industrial process. Today, GMAW is the most

common industrial welding process, preferred for its versatility, speed and

the relative ease of adapting the process to robotic automation. Unlike

welding processes that do not employ a shielding gas, such as shielded

metal arc welding, it is rarely used outdoors or in other areas of air

volatility.

22

Figure.3.2 Gas Metal Arc Welding

3.1.3 Gas Tungsten Arc Welding

GAS TUNGSTEN ARC welding (GTAW) uses a non

consumable tungsten electrode which must be shielded with an inert gas.

The arc is initiated between the tip of the electrode and work to melt the

metal being welded, as well as the filler metal, when used. A gas shield

protects the electrode and the molten weld pool, and provides the required

arc characteristics. The process may employ direct current with positive or

negative electrode or alternating current. In general, ac is preferred for

welding aluminium and magnesium. Direct current electrode negative is

preferred for welding most other materials and for automatic welding of

thick aluminium. Thin magnesium sometimes is welded with direct current

electrode positive. When ac is used with argon shielding, an arc cleaning

action is produced at the joint surfaces on aluminium and magnesium. This

cleaning action removes oxides and is particularly beneficial in reducing

weld porosity when welding aluminium. When using dc, helium may be

used as the shielding gas to produce deeper penetration. However,

stringent precleaning of aluminium and magnesium parts is required with

helium shielding. Argon and helium mixtures for gas shielding can provide

some of the benefits of both gases. Regardless of polarity, a constant

current (essentially vertical volt-ampere characteristic) welding power

23

source is required. In addition, a high-frequency oscillator is generally

incorporated in power sources designed for GTAW. High-frequency can

be employed with dc to initiate the arc instead of touch starting to

minimize tungsten electrode contamination. Normally, the high frequency

is turned off automatically after arc ignition. The high frequency power is

normally operated continuously with ac to maintain ionization of the arc

path as the arc voltage passes through zero. Some special power sources

provide pulsating direct current with variable frequency. This provision

permits better control of the molten weld pool when welding thin sections,

as well as when welding in positions other than flat. Several types of

tungsten electrodes are used with this process. Thoriated and zirconiated

electrodes have better electron emission characteristics than pure tungsten,

making them more suitable for dc operations. The electrode is normally

ground to a point or truncated cone configuration to minimize arc wander.

Pure tungsten has poorer electron emission characteristics but provides

better current balance with ac welding. This is advantageous when welding

aluminium and magnesium. The equipment needed consists of a welding

torch, a welding power source, a source of inert gas with suitable pressure

regulators and flowmeters, a welding face shield, and protective clothing.

Electric power requirements depend upon the type of material and the

thicknesses to be welded. Power requirements range from 8 kw for a 200 A

unit to 30 kw for a 500 A unit. Portable engine- driven power sources are

also available. A small 200 A welding equipment setup will cost about

$1000, while a simple automatic unit of 500 A capacity may cost about

$5000. The addition of arc voltage control, slope control, and other

accessories will materially increase the cost. Gas tungsten arc welding

requires more training time, manual dexterity, and welder coordination

than does SMAW or GMAW. The equipment is portable, and is applicable

24

to most metals in a wide range of thickness and in all welding positions.

Sound arc welds can be produced with the GTAW process when proper

procedures are used. The process can be used to weld all types of joint

geometries and overlays in plate, sheet, pipe, tubing, and other structural

shapes. It is particularly appropriate for welding sections less than 3/8-in.

(10mm) thick and also 1-to 6-in. (25.4-to 152.4-mm) diameter pipe.

Thicker sections can be welded but economics generally indicate the

choice of a consumable electrode process. This combination of GTAW for

root pass welding with either SMAW or GMAW is particularly

advantageous for welding pipe. The gas tungsten arc provides a smooth,

uniform root pass while the fill and cap passes are made with a more

economical process. Gas tungsten arc welding is generally more expensive

than SMAW due to the cost of the inert gas, and is only 10 to 20 percent as

fast as GMAW. However, GTAW will provide the highest quality root

pass, while accommodating a wider range of thicknesses, positions, and

geometries than either SMAW or GMAW.

Figure.3.3 Gas Tungsten Arc Welding

25

3.2 DRY WELDING

Welding in the dry environment produces high quality weld

joints. It needs a pressurized enclosure having controlled atmosphere.

Weld metal is therefore is not indirect contact with water. The gas-tungsten

arc welding process is used for pipe works. This welding is used at depths

of 200ft (61m) for joining pipe. Gas Metal Arc welding is the best process

for this welding. It is an all position process.

Figure.3.4 Dry Welding Process

There are two basic types of dry welding

1) Hyperbaric welding

2) Cavity welding

26

3.2.1 Hyperbaric Welding

Hyperbaric welding is carried out in chamber sealed around the

structure be welded. The chamber is filled with a gas (commonly helium

containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is

sealed onto the pipeline and filled with a breathable mixture of helium and

oxygen, at or slightly above the ambient pressure at which the welding is

to take place. This method produces high-quality weld joints that meet X-

ray and code requirements. The gas tungsten arc welding process is

employed for this process. The area under the floor of the Habitat is open

to water. Thus the welding is done in the dry but at the hydrostatic pressure

of the sea water surrounding the Habitat. Hyperbaric welding is the process

of welding at elevated pressures, normally underwater. Hyperbaric welding

can either take place wet in the water itself or dry inside a specially

constructed positive pressure enclosure and hence a dry environment. It is

predominantly referred to as "hyperbaric welding" when used in a dry

environment, and "underwater welding" when in a wet environment. The

applications of hyperbaric welding are diverse—it is often used to

repair ships, offshore oil platforms, and pipelines. Steel is the most

common material welded. Dry hyperbaric welding is used in preference to

wet underwater welding when high quality welds are required because of

the increased control over conditions which can be exerted, such as

through application of prior and post weld heat treatments. This improved

environmental control leads directly to improved process performance and

a generally much higher quality weld than a comparative wet weld. Thus,

when a very high quality weld is required, dry hyperbaric welding is

normally utilized. Research into using dry hyperbaric welding at depths of

up to 1,000 metres (3,300 ft.) is on-going. In general, assuring the integrity

27

of underwater welds can be difficult (but is possible using various non-

destructive testing applications), especially for wet underwater welds,

because defects are difficult to detect if the defects are beneath the surface

of the weld. Underwater hyperbaric welding was invented by the Russian

metallurgist Konstantin Khrenov in 1932.

This type of welding has certain limitations:-

1) Increase in pressure as depth increases introduces problem both for the

welding process and for divers weld chemistry, are physics are all affected.

2) Air as the habitat atmosphere soon becomes objectionable as pressure

increases because it presents a potential fire hazard at ambient pressures.

Moreover, oxygen becomes narcotic. Also, both nitrogen and oxygen

affect the weld metal. For these reasons, air as an atmosphere is not

recommended for depths above 15 meters. Habitat atmospheres of helium-

oxygen and argon-oxygen are under trials.

28

Figure.3.5 Hyperbaric Welding

3.2.2 Cavity Welding

Cavity welding is another approach to weld in water free

environment. In this process, the conventional arrangement for feeding

wire and shielding gas are surrounded by a means for introducing a cavity

gas and the whole is surrounded by a trumpet shaped nozzle through which

a high velocity conical jet of water passes. This process was used in the

late 1990’s in Japan and in the early years of this century. Cavity method

avoids the need for a habitat chamber and it lends itself to automatic and

remote control. The process is very suitable for flat structures where butt

welds with a backup strip can be weld in the flat or overhead positions. But

it does not appear to be suitable for un backed pipe line butt joints not for

lap joints which have accounted for the majority of underwater welding

29

work. The chamber is filled with a gas (commonly helium containing 0.5

bar of oxygen) at the prevailing pressure. The habitat is sealed onto the

pipeline and filled with a breathable mixture of helium and oxygen, at or

slightly above the ambient pressure at which the welding is to take place.

This method produces high-quality weld joints that meet X-ray and code

requirements. The gas tungsten arc welding process is employed for this

process. The area under the floor of the Habitat is open to water. Thus the

welding is done in the dry but at the hydrostatic pressure of the sea water

surrounding the Habitat.

Figure.3.6 Cavity Welding

30

CHAPTER 4

ADVANCED UNDERWATER WELDING TECHNIQUE

4.1 FRICTION WELDING (FRW)

Friction welding is a solid state welding process which

produces coalescence of materials by the heat obtained from

mechanically-induced sliding motion between rubbing surfaces . The

work parts are held together under pressure. This process usually involves

rotating of one part against another to generate frictional heat at the

junction. When a suitable high temperature has been reached, rotational

motion ceases and additional pressure is applied and coalescence occurs.

Fig. 4 shows the schematic of friction welding process. The start of the

new millennium will see the introduction of friction welding for

underwater repair of cracks to marine structures and pipelines. There are

two variations of the friction welding process. In the original process one

part is held stationary and the other part is rotated by a motor which

maintains an essentially constant rotational speed. The two parts are

brought in contact under pressure for a specified period of time with a

specific pressure. Rotating power is disengaged from the rotating piece and

the pressure is increased. When the rotating piece stops the weld is

completed. This process can be accurately controlled when speed,

pressure, and time are closely regulated.

31

The other variation is called inertia welding. Here a flywheel is

revolved by a motor until a preset speed is reached. It, in turn, rotates one

of the pieces to be welded. The motor is disengaged from the flywheel and

the other part to be welded is brought in contact under pressure with the

rotating piece. During the predetermined time during which the rotational

speed of the part is reduced the flywheel is brought to an immediate stop

and additional pressure is provided to complete the weld. Both methods

utilize frictional heat and produce welds of similar quality. Slightly better

control is claimed with the original process. Among the advantages of

friction welding is the ability to produce high quality welds in a short cycle

time. No filler metal is required and flux is not used. The process is

capable of welding most of the common metals. It can also be used to join

many combinations of dissimilar metals. It also produces a fine-grained

forged weld without any weld dilution, or weld inclusions. Since there is

never a liquid weld pool, hydrogen enrichment and hydrogen

embrittlement are eliminated. Similarly nitrogen enrichment cannot occur.

No shielding gasses or fluxes are required and it is possible to join

dissimilar and exotic materials impossible to weld by any other means –

including aluminium to ceramic.

Figure.4.1 Schematic of Friction Welding Process

32

Friction welding requires relatively expensive apparatus similar

to a machine tool. There are three important factors involved in making a

friction weld:

1. The rotational speed which is related to the material to be welded and

the diameter of the weld at the interface.

2. The pressure between the two parts to be welded. Pressure changes

during the weld sequence. At the start it is very low, but it is increased to

create the frictional heat. When the rotation is stopped pressure is rapidly

increased so that forging takes place immediately before or after rotation is

stopped.

3. The welding time. Time is related to the shape and the type of metal and

the surface area. It is normally a matter of a few seconds. The actual

operation of the machine is automatic and is controlled by a sequence

controller which can be set according to the weld schedule established for

the parts to be joined. Normally for friction welding one of the parts to be

welded is round in cross section; however, this is not an absolute necessity.

Visual inspection of weld quality can be based on the flash, which occurs

around the outside perimeter of the weld. Normally this flash will extend

beyond the outside diameter of the parts and will curl around back toward

the part but will have the joint extending beyond the outside diameter of

the part. If the flash sticks out relatively straight from the joint it is an

indication that the time was too short, the pressure was too low, or the

speed was too high. These joints may crack. If the flash curls too far back

on the outside diameter it is an indication that the time was too long and

the pressure was too high. Between these extremes is the correct flash

shape. The flash is normally removed after welding.

33

4.2 LASER WELDING

Laser as a source of coherent and monochromatic radiation, has

a wide scope of application in materials processing. Laser assisted

welding, because of the sheer volume/proportion of work and advancement

over the years, constitutes the most important operations among the laser

joining processes. Figure.4.9 shows the front view of the schematic set up

for laser underwater welding with a filler rod. The focused laser beam is

made to irradiate the work piece or joint at the given level and speed. A

shroud gas protects the weld pool from undue oxidation and provides with

the required oxygen flow. Laser heating fuses the work piece or plate

edges and joins once the beam is withdrawn. In case of welding with filler,

melting is primarily confined to the feeding wire tip while apart of the

substrate being irradiated melts to insure a smooth joint. In either case, the

work piece rather than the beam travels at a rate conducive for welding and

maintaining a minimum heat affected zone (HAZ).

There are two fundamental modes of laser welding depending on

the beam power/configuration and its focus with respect to the work piece:

(a) conduction welding and (b) keyhole or penetration welding shown in

figure.4.10. Conduction limited welding occurs when the beam is out of

focus and power density is low/insufficient to cause boiling at the given

welding speed. In deep penetration or keyhole welding, there is sufficient

energy/unit length to cause evaporation and hence, a hole forms in the melt

pool. The ‘keyhole’ behaves like an optical black body in that the radiation

enters the hole and is subjected to multiple reflections before being able to

escape. The transition from conduction mode to deep penetration mode

34

occurs with increase in laser intensity and duration of laser pulse applied to

the work piece. Combination of laser beam with metal inert gas (MIG) or

tungsten inert gas (TIG) arc (so-called hybrid technique) seems to be

promising from the viewpoint of bead , but occurrence of large blowholes

and voids still remains an important problem for further research.

Figure.4.2 Schematic of laser welding with a filler rod.

Argon shroud removes heat and prevents undue oxidation and

displaces water. The relative position of the laser focus determines the

quality and configuration of the weld.

35

Figure.4.3 Schematic view of (a) conduction melt pool, and (b) deep

penetration welding mode.

36

CHAPTER 5

ADVANTAGES AND DISADVANTAGES OF WET WELDING

5.1 ADVANTAGES

Wet underwater MMA welding has now been widely used for

many years in the repair of offshore platforms. The benefits of wet welding

are: -

1) The versatility and low cost makes highly desirable.

2) Other benefits include the speed. With which the operation is carried

out.

3) It is less costly compared to dry welding.

4) The welder can reach portions of offshore structures that could not be

welded using other methods.

5) No enclosures are needed and no time is lost building. Readily available

standard welding machine and equipments are used. The equipment

needed for mobilization of a wet welded job is minimal

37

5.2 DISADVANTAGES

Although wet welding is widely used for underwater fabrication

works, it suffers from the following drawbacks: -

1) There is rapid quenching of the weld metal by the surrounding water.

Although quenching increases the tensile strength of the weld, it decreases

the ductility and impact strength of the weldment and increases porosity

and hardness.

2) Hydrogen Embrittlement- Large amount of hydrogen is present in the

weld region, resulting from the dissociation of the water vapour in the arc

region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld

metal, which causes Embrittlement, cracks and microscopic fissures.

Cracks can grow and may result in catastrophic failure of the structure.

3) Another disadvantage is poor visibility. The welder sometimes is not

able to weld properly.

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CHAPTER 6

ADVANTAGES AND DISADVANTAGES OF DRY WELDING

6.1 ADVANTAGES

1) Welder/Diver Safety – Welding is performed in a chamber, immune to

ocean currents and marine animals. The warm, dry habitat is well

illuminated and has its own environmental control system (ECS).

2) Good Quality Welds – This method has ability to produce welds of

quality comparable to open air welds because water is no longer present to

quench the weld and H2 level is much lower than wet welds.

3) Surface Monitoring – Joint preparation, pipe alignment, NDT

inspection, etc. are monitored visually.

4) Non-Destructive Testing (NDT) – NDT is also facilitated by the dry

habitat environment

39

6.2 DISADVANTAGES

1) The habitat welding requires large quantities of complex equipment and

much support equipment on the surface. The chamber is extremely

complex.

2) Cost of habitat welding is extremely high and increases with depth.

Work depth has an effect on habitat welding. At greater depths, the arc

constricts and corresponding higher voltages are required.

40

CHAPTER 7

RISKS INVOLVED IN UNDERWATER WELDING

There is a risk to the welder/diver of electric shock. Precautions

include achieving adequate electrical insulation of the welding equipment,

shutting off the electricity supply immediately the arc is extinguished, and

limiting the open-circuit voltage of MMA (SMA) welding sets. Secondly,

hydrogen and oxygen are produced by the arc in wet welding. Precautions

must be taken to avoid the build-up of pockets of gas, which are potentially

explosive. The other main area of risk is to the life or health of the

welder/diver from nitrogen introduced into the blood steam during

exposure to air at increased pressure. Precautions include the provision of

an emergency air or gas supply, stand-by divers, and decompression

chambers to avoid nitrogen narcosis following rapid surfacing after

saturation diving. For the structures being welded by wet underwater

welding, inspection following welding may be more difficult than for

welds deposited in air. Assuring the integrity of such underwater welds

may be more difficult, and there is a risk that defects may remain

undetected.

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CHAPTER 8

SCOPE FOR DEVELOPMENT

Wet welding has been used as an underwater welding technique

for a long time and is still being used. With recent acceleration in the

construction of offshore structures underwater welding has assumed

increased importance. This has led to the development of alternative

welding methods like friction welding, explosive welding, and stud

welding. Sufficient literature is not available of these processes. Wet

MMA is still being used for underwater repairs, but the quality of wet

welds is poor and are prone to hydrogen cracking. Dry Hyperbaric welds

are better in quality than wet welds. Present trend is towards automation.

THOR – 1 (TIG Hyperbaric Orbital Robot) is developed where diver

performs pipefitting, installs the trac and orbital head on the pipe and the

rest process is automated.

Developments of diverless Hyperbaric welding system is an

even greater challenge calling for annexe developments like pipe

preparation and aligning, automatic electrode and wire reel changing

functions, using a robot arm installed. This is in testing stage in deep

waters. Explosive and friction welding are also to be tested in deep waters.

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CHAPTER 9

APPLICATIONS

1) Offshore construction for tapping sea resources.

2) Temporary repair work caused by ship’s collisions, unexpected

accidents.

3) Salvaging vessels sunk in the sea.

4) Repair and maintenance of ships.

5) Construction of large ships beyond the capacity of existing docks.

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CHAPTER 10

CONCLUSION

International interests to develop and utilize oceans which cover

70% of the earth and its resources such as development of offshore gas and

oil field, fisheries multiplication, large offshore construction and mineral

resources, mining in the sea bottom have let to the development of

underwater welding. Underwater welding has been used for temporary

repair work caused by ships collisions, unexpected accidents, corrosion

and other maintenance works. So under water welding process is a highly

advantageous process that can be used to repair and reduce high risk

situations and thereby reduce destruction of life threat situations to a great

extent.

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CHAPTER 11

REFERENCES

1) Ogawa Y. (1998): Proceedings of Eighth International Offshore and Polar Engineering Conference, vol. 4.

2)Peng, Y., Chen, W., Wang, C., Bao, G. and Tian, Z.(2001): J. Phys. D: Appl. Phys. Vol. 34pp. 3145–3149

3) Schmidt, H.-Blakemore, G. R. (2000): UnderwaterIntervention 2000 – Houston, Jan 24-26.

4) Chen W, Zhang X, et al. (1998): Proc SPIE, Vol. 3550,pp. 287–297.Case Study on Underwater Welding- Present Status and Future Scope 75

5)Dawas, C. (ed.) (1992): Laser Welding, Mc. Graw-Hill,N. York.

6)Duley W. W. (ed.) (1999): Laser Welding, John Wiley& Sons, Inc., N. York, pp. 1

7)DuttaMajumdar, J. And Manna, I. (2003): Sadhana, Vol.28, pp. 495.

8)Farson D, Ali A, Sang Y. (1998): Weld Res Suppl., Vol

9) D. J Keats, Manual on Wet Welding.

10) Annon, Recent advances in dry underwater pipeline welding, Welding Engineer, 1974. 11) Lythall, Gibson, Dry Hyperbaric underwater welding, Welding Institute.