ALUMINIUM ALLOYS IN THIRD MILLENNIUM...

The Fifth International Forum on Aluminum Ships Tokyo, Japan 11-13, October, 2005

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_______________________________________________________________________________________________________________________________

* Manager of Ship Platform Basic Design

** In charge of teaching activities at several Universities; involved in the choice of materials and technologies for welded products

ALUMINIUM ALLOYS IN THIRD

MILLENNIUM SHIPBUILDING: MATERIALS,

TECHNOLOGIES, PERSPECTIVES.

Stefano FERRARIS*

Fincantieri Cantieri Navali Italiani S.p.A., Naval Vessel Business Unit, Genoa, ITALY

Luis Mario VOLPONE**

Istituto Italiano della Saldatura, Genoa, ITALY

KEY WORDS: strength, lightness, speed, alloys,

technologies, laser, FSW, production, cost-effectiveness,

opportunities.

INTRODUCTION

The Shipbuilding sector, particularly the branch devoted to

the transport of passengers on high speed ferries, is

continuously overwhelmed with demands for the increase of

speed and for contemporary energy spare.

It is a paradox that the global market, so rich of innovative

materials and technologies, gives so few suitable

combinations of them for the design and production of high

speed craft. So few, in fact, within such a strongly

competitive market as shipbuilding is, where the costs for

ship structure must be “kept as low as possible” and

generally not exceed the 10% of the total price. Therefore

expensive materials and technologies are not very much

appreciated by the shipbuilders.

At the same time, innovations inside the design are not easy

to introduce because they often have to cope with rigid

reference rules, like those of Classification Societies, where

new materials and technologies are not easily implemented,

sometimes for conservatism, sometimes for excessive

caution, more often because of uncertainties, which include:

• limited knowledge and experience of the properties of

new materials, particularly after fabrication, and of

joints achieved by means of new joining methods. Low

confidence levels in the statistical data (arising from

limitations in the quantity of data available) have often

to be compensated by increased safety factors, which

effectively penalise the introduction of new solutions;

• insufficient data on the long-term degradation of new

materials and joints because of both the loads they are

subjected to and the prolonged exposure to the marine

environment.

This paper aims to highlight some arguments about

materials and technologies, which may represent valid

solutions for quality improvement of manufactured

structures as well as possible cost reduction and

productivity gain.

BRIEF HISTORICAL BACKGROUND

Since its industrialisation, about 120 years ago, aluminium

began to be considered as a very attractive material for

marine applications. In the last decade of the Nineteenth

Century, aluminium was adopted for the shell plating of

some sailing boats (one of which, the “Defender”, won the

America Cup in 1895) and for minor “fast” naval vessels.

These applications were followed by a rather long period of

stagnation: severe corrosion phenomena, due partly to the

chemical composition of the alloys and partly to wrong

production methods, which had made galvanic corrosion

much easier to occur, had not allowed to ensure a

reasonably long service life. Furthermore, the ratio between

the cost of aluminium and the cost of steel was much higher

than it is today.

The strength / lightness ratio was anyway too attractive to

prevent aluminium alloys from being considered an

important class of materials for shipbuilding. Hence, as the

aluminium-magnesium alloys became available, they found

immediate application both in the civil and the naval field.

However, the actual success of aluminium alloys in

shipbuilding came clear only after the Second World War,

when the development in arc welding methods gave a real

alternative to riveting as the joining technique for the

material. The availability of a reliable, industrial and rather

cheap assembly technology, together with the increasing

demand for higher speed performances, made aluminium

alloys one of the best choices as the structural material for

several kinds of boats, vessels and components.


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Nowadays, better alloys, capable of ensuring improved

corrosion resistance and higher mechanical properties, even

in HAZ, well established industrial solutions, like extrusion,

new joining techniques, like laser welding, FSW and

adhesive bonding, give aluminium the chance to be not only

the reference material for many marine applications but also

to look for further developments.

AN IDEAL COMBINATION OF LIGHTNESS AND

STRENGTH: ALUMINIUM ALLOYS AND THE BLUE

RIBBON

Lightness, strength, easy production: the ideal recipe for

speed at sea. It was not at all surprising when an aluminium

wave-piercing catamaran, the Hoverspeed Great Britain

(Fig 1), conquered the Hales Trophy in 1990, crossing the

Atlantic Ocean at an average speed of almost 37 knots. The

Trophy was then won, eight years later, by another similar

but larger vessel, the CAT-LINK V (Fig 2), which sailed

from America to Europe at more than 41 knots.

Fig 1 The Hoverspeed Great Britain

Fig 2 The CAT-LINK V

Apart from commercial vessels, the only entitled to win the

Hales Trophy, the Blue Ribbon for the fastest crossing with

no intermediate refuelling is owned by another well known

aluminium ship, the monohull Destriero (Fig 3), which

reached Bishop Rock in 1992 at an average speed exceeding

53 knots.

Fig 3 Destriero

ALUMINIUM ALLOYS IN PRESENT SHIPBUILDING

Aluminium alloys play an important role in many sectors of

present shipbuilding.

Many pleasure boats and large yachts are made of

aluminium, either entirely or partially (superstructures,

deckhouses, funnels, masts, etc.). Several minor, light and

fast vessels, having customs, police or coast guard

patrolling purposes, are aluminium ones.

The material is currently used in the offshore field for the

construction of specific portions of oil platforms: that is the

case of living quarters and landing pads, generally made of

extruded profiles.

Fig 4 Living quarters and landing pad

on an offshore platform


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As regards civil ships, aluminium superstructures are widely

used for cruise vessels, due to the need for light, efficient

structures with reduced impact on vessel stability.

In the HSC field, aluminium is again an essential:

catamarans, swaths, wave-piercers, SES, monohulls, etc.,

are made, partially or entirely, of its alloys, which grant a

significantly high strength/weight ratio, giving the chance,

case by case, of increased speed, reduced fuel consumption

or increased deadweight. The argument has been widely

treated in the ISSC 2000 and ISSC 2003 Specialist

Committees dedicated to the Structural Design of High

Speed Vessels.

Fig 5 Midship section of an aluminium alloy monohull

The use of aluminium in naval vessels deserves special

consideration: up to the Nineteen Seventies, many military

vessels were designed and built with aluminium alloy

superstructures, which granted a remarkable spare of weight,

even taking account of the higher amount of insulation

needed for fire protection purposes, with low impact on

both intact and damaged vessel stability.

The Falkland Islands war, at the beginning of the Eighties,

changed the feeling of most navies about the use of

aluminium alloys for structural applications: nine British

ships were attached and sunk by Argentine aircraft and the

media suggested those vessels had been lost because the

aluminium used for superstructures had caught fire. There

was no actual evidence of a direct relationship between

aluminium and those accidents, but the loss of mechanical

properties, which aluminium alloys undergo when exposed

to temperatures exceeding 200 °C, was strongly emphasized

and considered a sufficient reason for banishing the

structural use of the material.

After more than twenty years, that belief is still present even

though some specific applications have been re-introduced

(i.e. small deckhouses, funnels, masts). A recent tender for

few corvettes in Eastern Europe has anyway explicitly

asked for aluminium superstructures. Nevertheless, the use

of aluminium for structural applications is not permitted by

all Navies and/or Classification Societies: for instance,

while DNV and RINA allow the use of light alloys, Lloyd’s

Register practically does not and ABS limits the use to

AA5xxx series alloys.

ALUMINIUM VESSEL DESIGN: THE NEED FOR A

CHANGE OF MENTALITY

The peculiarities of aluminium alloys with respect to steel,

in terms of mechanical properties, loss of strength in HAZ,

higher notch sensitivity and consequent lower capability of

withstanding cyclic loads, reduced fire resistance, higher

production constraints but, at the same time, availability of

tailored solutions, push to the application of different

concepts and design approaches.

The designer, who believes it possible to treat an aluminium

vessel as a conventional steel one, will probably undergo a

severe failure: aluminium does not allow mistakes and

makes it necessary to adopt more sophisticated structural

details so as to keep stress concentration factors as low as

possible. This is particularly important for structures that

can experience sudden high loads (e.g. slamming pressure)

or are constantly subjected to vibrations or other cyclic

loading, which can easily induce damages and fatigue

failures. Not only such concepts are to be applied to

structures but also to outfitting components and their

integration with structures themselves.

On the other hand, the possibility of using tailored sections

and purpose-designed components with a high strength on

weight ratio makes extrusions a cost-effective solution for

aluminium structures. This can be obtained both by the

suitable mechanisation of traditional processes, like MIG

welding, and by the use of more recent joining techniques,

like laser and FSW, which give the chance of a significant

improvement for every designer, who is ready to study their

best application inside production processes.

STRUCTURAL MATERIALS

The general outline has shown aluminium alloys as the best

technical choice for the construction of ship structures with

particular speed requisites. There are also other light

materials suitable for minor ships, specially mass-produced

ones, such as various kinds of GRP or titanium alloys, but

they are very expensive and do not ensure high productivity.

Aluminium alloys own a lot of characteristics that are very

interesting for high speed craft designers and builders:

lightness, good corrosion resistance, good attitude to

welding, cutting and shaping, in other words an excellent

predisposition towards manufacturing technologies.

Table 1 gives typical mean values of some physical

characteristics of specific interest for design and production.

With reference to same characteristics, the table also

highlights possible implications or consequences that may

derive from the use of aluminium alloys instead of steel.


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Table 1 Comparison between physical properties of aluminium alloys and steel

Physical property Aluminium

alloys

Steel for

construction Related matters

Density

[kg/m3]

2700 7850 Lighter structures, hence opportunity to reduce the installed

power and spare money with respect to steel solutions.

Young modulus

[MPa] 72000 205000

Aluminium alloys are much more deformable in the elastic

field. This requires the adoption of more stiffening elements

to keep strain down to acceptable values.

Thermal conductivity

[W/m °K] 235 79

Welding processes based on thermal sources are less efficient

in aluminium alloys with respect to steel.

Melting temperature

[°C] 550 ÷ 650 ~ 1500

The fusion bath is much more fluid and tends to breaking.

Fire resistance is worse and penalized.

Oxides melting

temperature

[°C]

2060

(Al2O3)

800 ÷ 900

(FeO, Fe2O3,

Fe3O4)

The presence of alumina in way of joints causes remarkable

inconveniences to welding (difficult management of the arc

and possible sticking).

Electrical resistivity

[Ohm cm] ~ 2.65 10

-6 ~ 10 10

-6 Resistance welding is more difficult in aluminium alloys.

Relative magnetic

permeability

< 1

(paramagnetic)

80 ÷ 160

(ferromagnetic)

Aluminium alloys do not magnetize as a consequence of

working processes and can not be controlled by means of

magnetic crack detection.

Crystalline structure

(elementary cell)

single-phase

CFC

two-phase

CBC - CFC

In general, aluminium alloys do not undergo transformations

of phase but only precipitation phenomena.

The aluminium universe includes some hundreds of alloys,

grouped in families, where they are classified as a function

of alloy elements inside them.

Which are the alloys of major interest for the shipbuilding

world? The answer seems quite clear. Those alloys that

grant:

• availability of semi-finished products (sheets, profiles,

etc.), having shapes and sizes corresponding to design

and fabrication requirements;

• a good attitude towards all manufacturing technologies,

in particular welding;

• good marine corrosion resistance;

• costs compatible with shipbuilding economy.

All these requisites are largely satisfied by:

• aluminium-magnesium alloys (series AA5xxx)

• aluminium-magnesium-silicon alloys (series AA6xxx)

The former are mainly found as sheets and rolls, while the

latter are generally used for extruded profiles.

Table 2 shows a list of chemical and mechanical

characteristics of some of the alloys most frequently used in

shipbuilding.

Alloys AA5083 and AA5059, and other similar ones, like

for instance AA5383, are single-phase alloys, whose

mechanical properties are determined both by Mg content in

solid solution and grain size.

The zirconium contributes to the grain refinement in alloy

AA5059 especially in the heat-affected zones (HAZ) of

welded joints.

Table 2 also offers an overview of the mechanical

properties of same alloys, but the real problem is the

decrease of same properties after the execution of a welded

joint.

AA5xxx series alloys are known as work-hardened alloys,

while AA6xxx series ones are suited for thermal treatment.

It is therefore understandable that the heat coming from a

welding process can modify the metallurgical grade of

supplied wrought materials.

Table 3 shows the drop in strength levels of the AA5xxx

series alloys considered in Table 2. It refers to butt welds on

grade H321 sheets, MIG-welded with AA5183 filling wire.

The loss of mechanical properties of AA5083 is such that

the HAZ of the joint reaches the state 0, characterized by

the lowest strength level for this alloy, while the loss for

AA5059 is contained.

AA6xxx series alloys undergo a similar drop in mechanical

properties, which can be compared to a reduction to state T4

(hardened or solubilized) with a decrease of strength

sometimes equal to half of the RP0,2 value corresponding to

state T6. These alloys are anyhow capable to subsequently

recover by natural ageing, reaching values around 70% of

the original RP0,2 value for the wrought material after some

weeks.

The phenomenon of resistance drop, particularly in way of

H.A.Z., the area of the welded joint coinciding with the

structural notch between the filling material of the seam and

the parent metal, has a disruptive effect on the fatigue

properties of welded joints. It is worth underlining that the

fatigue resistance of aluminium alloys, like those mentioned

in present paper, is about three times lower than that of

structural steels.


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Table 2 Chemical and mechanical properties of some aluminium alloys typically used in shipbuilding

Alloy Nominal chemical

composition [%]

Mechanical strength

minimum levels Metallurgical grades

For thickness ! 12.5 mm

RP0.2 " 125 MPa

Rm " 275 MPa

A50 " 14%

0

AA5083 (1)

Mg: 4.0 ÷ 4.9

Mn: 0.4 ÷ 1.0

Cr: 0.05 ÷ 0.25

Cu + Fe + Si + Zn: ! 1.15

Ti: ! 0.15

For thickness ! 10 mm

RP0.2 " 215 MPa

Rm " 305 MPa

A50 " 10%

H321


RP0.2 " 160 MPa

Rm " 330 MPa

A50 " 24%

0

AA5059 (2)

Mg: 5.0 ÷ 6.0

Mn: 0.6 ÷ 1.2

Cr: 0 ÷ 0.3

Zr: 0.05 ÷ 0.25

Cu + Fe + Si + Zn: ! 2.9

Ti: ! 0.15


RP0.2 " 270 MPa

Rm " 370 MPa

A50 " 10%

H321

AA6005A

Si: 0.50 ÷ 0.90

Mg: 0.40 ÷ 0.70

Cu + Fe + Zn: ! 0.85

Mn + Cr: 0.12 ÷ 0.50

Ti: ! 0.10


RP0.2 " 215 MPa

Rm " 280 MPa

A50 " 11%

T5 / T6 (3)

AA6082

Si: 0.70 ÷ 1.30

Mn: 0.40 ÷ 1.0

Mg: 0.60 ÷ 1.2

Cu + Fe + Cr + Zn: ! 1.05

Ti: ! 0.10


RP0.2 " 250 MPa

Rm " 310 MPa

A50 " 9%

T6 (3)

Notes about Table 2: (1) the values may be also representative of similar and better AA5383

(2) alloy patented by CORUS Aluminium Rolled Products – Karl Später strasse 10,

56070 Koblenz, Germany

(3) Grade T6 is here generically indicated: extruded profiles manufacturers often

apply thermal treatments together with stretching or other more complex methods

Table 3 Mechanical properties of AA5xxx series alloys after welding

Alloy

RP0.2 [MPa]

Rm [MPa]

AA5083 125 275

AA5059 160 300

Fig 6 shows the results of fatigue tests carried out on

various types of welded joints.

From fatigue resistance point of view, alloy AA5059 shows

a certain advantage in comparison with traditional alloys

like AA5083: for instance, the fatigue resistance of

non-levelled butt welded joints at 107 cycles is about 20

MPa higher than that of alloy AA5083, thanks essentially to

the grain refinement in H.A.Z..

Certainly above mentioned alloys represent a great potential

for the fabrication of ship structures, but often unfit welding

procedures could cancel all benefits: excessive heating or

the use of either too little or unfit filling material will

unavoidably lead to poor results.

The use of innovative and, at the same time, cost-effective

technologies surely implies a significant change in the

mental approach to welded joints.

INNOVATIVE WELDING TECHNOLOGIES

In traditional practice, aluminium alloys are welded by

means of G.T.A.W., G.M.A.W. and P.A.W. arc processes


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but, within the shipbuilding sector, only G.M.A.W. welding

procedures, in their MIG version, are generally adopted.

Fig 6 Fatigue behaviour of MIG and FSW

welded joints in aluminium alloys

The reason is that G.T.A.W. is a slow and not very

productive process, while P.A.W. is particularly good for

thin sheets, is more expensive and does not grant the same

versatility like G.M.A.W. does. G.M.A.W. technique is

undoubtedly cheap, highly productive and can be easily

automated. Depending on the type of vessel, most of its

major structural parts (decks, sides, bulkheads) can be built

by using this procedure.

Anyway the technique shows quite clearly the metallurgical

problems already pointed out in previous paragraph and is

affected by diffuse porosity in the joint as well as

remarkable shrinking deformation.

Arc-welding techniques require the complete removal of

alumina (Al2O3), the passivating oxide that spontaneously

covers the surfaces of all aluminium alloys, as it is

refractory and dielectric and creates serious problems

because of its difficult melting and the transfer of electric

charges.

Since a few years, industry makes use of continuous filling

wire gas metal welding systems, which weld by means of

two separately fed synergic wires (“twin arc”). Such a

technique allows the reduction of overall heating, which

means a decrease of welded structures deformation level.

Nevertheless, big efforts have still to be made to leave

behind the traditional way of welding.

Two are the innovative welding methods, one of them being

really revolutionary:

• laser welding (or the hybrid laser-MIG process);

• friction stir welding (F.S.W.).

Such procedures have an elevated level of automation, high

productivity, and can carry out high-quality welded joints

with very low or null deformation levels (FSW).

The peculiarities of these welding techniques require a

change of mentality, as the structural design must be

reviewed to obtain the utmost benefits from the use of the

joining process.

Next pages describe in detail the two methodologies and

illustrate some of the possible results.

Laser

“Laser” (light amplification by stimulated emission of

radiations) sources are heat ones consisting of a

monochromatic light beam at elevated density of energy,

capable to concentrate, in only one cm2, from 7·10

5 up to

1.2·106 W.

Laser techniques mainly used in welding processes are

Nd:YAG (laser pumped by diodes), with a light beam

having a wave length # = 1.06 µm and power of about 4 kW,

and CO2 (laser slab), with a light beam of a wave length # =

10,6 µm at variable intensities up to more than 25 kW.

In latter method the light is driven by optical glasses and

mirrors, which allow to obtain a “dot-like” beam with a

section between 0.4 and 0.6 mm in the focal area. Energy

density can carry out a hole, known as “keyhole”, in the

metallic surface, in which overheated plasma is generated.

The walls of the keyhole melt and the molten metal fills the

hollow step by step along the laser track. The welding has

very small dimensions in comparison with a traditional


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arc-welded joint, approximately 1/5 with respect of a MIG

joint.

Fig 7 Operating principle of laser welding

It is worth pointing out that the welding process takes place

without any filler material. The edges are straight and the

gap between them is to be max 0.1 ÷ 0,2 mm.

However, the use of filler metal is sometimes suggested in

order to correct the chemical composition of the molten

material (excessive evaporation of Mg) or avoid hollow

spots in case of excessive gaps.

The impact of the already exposed metallurgic problems is

reduced: for instance the width of the H.A.Z. can reach 1/10

in comparison with MIG welded joints, but the drop of the

mechanical properties in such an area takes place anyway.

Significant advantages of laser methodology are that

caulkers and filling wires are not necessary and welding

speed, for single-pass welds and a thickness from 2 to 6 mm,

can reach about 2.5 ÷ 4 metres/minute.

Typical welded joint types are:

• butt welding (a)

• 100% penetrating fillet welding (b)

• lap welding (c)

These joints are shown in Fig 8.

Up to now, only a summary description of laser

methodology has been given. The use of such a welding

technique is obviously not limited to aluminium alloys, but

ensures also many advantages for what concerns the

welding of structural steel, stainless steel, etc..

But which is the real level of interest for the shipbuilders

towards laser technology? Is laser welding actually

applicable to shipbuilding?

The significant industrial application of laser within the

shipbuilding sector has started about fifteen years ago, but

its practical use is more recent.

Laser technology gives many benefits but it imposes a lot of

severe requisites, such as:

• constant and extremely reduced gap between welding

edges (0.1 ÷ 0.2 mm);

• flatness of welding edges;

• firm clamping of the parts to weld.

a

b

c

Fig 8 Three typical laser welding joints

Who is acquainted with shipbuilding production activities

surely knows the difficulties one has to cope with in case of

a butt weld of up to 18 meters on rather thin sheets (3 to 8

mm). The problem has been faced in various manners,

among which the most successful, so far, is a combination

of two processes, Laser + GMAW, into a method better

known as “Hybrid-Laser”. Fig 9 shows schematically the

basic operating principle.

Hybrid-laser is a technological solution that allows the laser

beam to produce a keyhole, in which a MIG type

continuous filling wire is put. The arc itself is facilitated by

the plasma created by the laser source.

An advantage is that the method accepts gap values between

welding edges significantly higher than those tolerated by

the classical laser technique. In fact, compared with the 0.1

÷ 0.2 mm of the “pure” laser process, the "hybrid" laser

process tolerates gaps of even 0.8 ÷ 1.0 mm, can weld

sheets from 5 mm up to 10 mm thick and grant speeds from


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1 metre/min up to 2.5 metres/min, which are indeed

interesting for shipbuilders!

Fig 9 Hybrid-Laser functioning scheme

There are many kinds of panel lines for the production of

flat panels, which carry out automatic butt welds of steel

sheets and fillet welds of stiffening elements. Same

methodology could be used for the production of aluminium

alloy flat panels, where Nd:YAG laser technique, supported

by a glass fibre, may provide an additional benefit, that is

the reduction of reflection losses and of absorption of the

beam by the plasma created by the laser beam itself.

Another interesting application of laser in shipbuilding is

the manufacturing of “sandwich” panels via overlapping or

“transparency” welding. Fig 10 shows one of the most

typical “sandwich” panels with the basic variables that

concur to its design definition.

Fig 10 A kind of sandwich panel

Table 4 Advantages offered by sandwich(1)

solutions

(1) CORALDEC ! is a Trade Mark of CORUS

WALZPRODUKTE GmbH

It is interesting to point out that three components of the

sandwich panel, i.e. top and bottom sheets as well as the

corrugated surface, can be of different thickness and even

be made of three different materials.

In order to illustrate “sandwich” structure benefits, Table 4

shows the weight saving factor of different structural

“sandwiches”, made of laser-welded AA5059, destined to

two typical marine applications: passenger decks and car

decks.

Friction Stir Welding

The F.S.W. methodology has found interesting applications

all around the world, from north to south, from east to west.

It may be considered the natural evolution of welding

processes with the intent to overcome the metallurgic

problems related to hardening of molten metal.

As well known to everybody, F.S.W. can produce welded

joints by softening metallic material via heat deriving from

the friction produced by a circular-moving tool. Such a tool

performs the welding with its translational motion along the

contact line between two straight edges without any

particular caulk. Fig 11 shows the operating principle of this

process: the scheme has become the symbol of a radical

change.

Fig 11 F.S.W. operating principle

The benefits offered by this process, related to aluminium

alloys, can be summarily described as follows:

• solid-state welding process, free from re-crystallisation

phenomena of the liquid metal;

• it allows to obtain optimum mechanical characteristics

of the welded joint, comparable with those of the parent

material or even slightly better thanks to grain

refinement;

• flat welds, levelled as the parent material. The

substantial absence of notches between the seam and the

parent metal determines a significant increase of fatigue

characteristics, as clearly shown by Fig 6. Moreover,

due to its flatness, the welded joint can weigh down to

12% less than traditional MIG seams;


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• it produces the lowest level of distortion among all

known welding processes;

• it requires neither any filling metal nor, in general, any

inert gas, reaching a maximum temperature of about

80% of the melting temperature of the parent metal and

a welding speed up to 2.7 metres/min. (for a reference

thickness of 5 mm) in one single pass;

• it can weld up to 35 mm thick sheets in one single pass;

• the process does not produce light radiation, dangerous

smoke, sparkles or noise;

• it allows to weld practically all metals (with melting

temperature < 1800 °C) and even combinations of

different kinds of materials, which are practically

impossible to join by using traditional welding

processes.

It is absolutely clear what a revolutionary way of welding

this process represents!

Fig 12 Example of deck panel welded by using F.S.W.

Fig 13 Panel welded by using F.S.W. and then rolled up

This methodology was born in 1991, patented by T.W.I.

(Cambridge, UK) and only a few years later, in 1994, a

huge machine built by ESAB began to produce ship

structural parts (decks, bulkheads, side panels, landing pads,

etc.) at (today) Hydro Marine Aluminium in Haugesund,

Norway.

Approved by the most important Classification Societies, it

has got its place among the pioneering technologies for the

production of ship structural parts in aluminium alloys, but

its industrial use to weld steel structures is forthcoming.

Fig 12 and Fig 13 are good, even though limited, examples

of what can be done by using F.S.W..

Anyhow, the range of application will grow in so far as the

mind of the designer will grow too.

Up to now the FSW process offers remarkable benefits in

the automated construction of flat panels, therefore a “panel

line” equipped to produce panels for decks, bulkheads and

sides.

But we ought to exit this contexts and project towards a

wider and feasible one. Fig 14 suggests in a fanciful manner

what could become a reality in a few years: the use of

aluminium alloy panels, suitably moulded from flat ones,

for the construction of portions of the external shell as well

as of superstructure side bulkheads.

The diffusion of these kinds of aluminium alloy products

has still to be broadened.

OTHER OPPORTUNITIES

Combined structural solutions for sandwiches

There are numerous solutions to connect sandwich panels

among each-other, either on top or sides, as well as simple

solutions by using of lot of ad hoc extruded profiles to

connect a sandwich panel to the ship conventional structures,

or to carry out deck passages for various applications

(piping, cables, etc.).

Adhesive bonding

Adhesive bonding offers considerable potential benefits.

This kind of joining technique allows the use of thinner

plating and higher strength alloys, thanks to the lack of heat

affected areas, giving the chance for significant reductions

in scantling and structural weight.

The fabrication process can be more flexible, as there is the

strong opportunity to paint the single parts before joining

them together.


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Fig 14 Possible new structural solutions by adopting F.S.W.

Rework and fairing of surfaces by means of fillers can be

indeed minimised, as there is much less distortion than

with traditionally welded structures: a significant reduction

in production costs may be consequently achieved.

The main advantages of adhesive bonding with respect to

traditional welding connections are that:

• mechanical properties of parent material do not

decrease due to the joining technique;

• lack of heating implies no distortions;

• high quality surface finish can be easily achieved;

• rework is very limited.

The factors which still prevent the wide use of such a

technique on vessels are the lack of information about

long-term behaviour in the marine environment and

strength retention in case of fire, the substantial lack of

reference rules and the need for precise application,

inspection and repair procedures.

Honeycomb

Adopted for mezzanines and movable car decks,

honeycomb is a possible way towards a further reduction

of secondary structure weight. All-aluminium products

(sheets + core) fulfil IMO requirements in terms of

toxicity, smoke generation and low flame spread.

Honeycomb solutions, even though in competition with

FRP ones, can be used on high speed vessels as separating

divisions or other secondary structures, wherever their

excellent rigidity/weight ratio can be properly exploited.

In the last decade, constant development in sandwich

construction has led to the production of honeycomb

materials with higher peel strength, capable of

withstanding complex processes, such as folding, pressing

and forming, and thus giving chances of a wider

utilization.

CONCLUSIONS

New aluminium alloys, capable of higher corrosion

resistance, tailor-made aluminium products, the fantastic

improvement in joining methodologies: a whole world of

opportunities for every shipbuilder, who wants to improve

his designs and fully exploit cost-effective and

production-friendly solutions.


11

The advantages achievable in some fields of shipbuilding

scenario, like high speed craft and light naval vessels, as

well as those related to the design and construction of

portions of major ships and offshore platforms, are quite

evident. But it is also clear that there is the need for a

further change of mentality in the praxis of ship structural

composition.

Not only the designer must understand the benefits offered

by mentioned materials and methods, but he has also to

carefully study the procedures for the correct integration of

the “new” structures with traditional ones, in order to

avoid distressing damages to the former when the latter are

implemented on them, generally by means of traditional

joining techniques like MIG welding.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. Eng. Stefanie Mueller

(Istituto Italiano della Saldatura) and Mrs. Hennie Van der

Waard (Fincantieri S.p.A., Naval Vessel Business Unit)

for their fundamental contribution.

A special thank is reserved to Prof. Enrico Evangelista

(Marche Polytechnic University), who provided the

opportunity for the submission of present paper.

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