Alloy Design

7/31/2019 Alloy Design

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ALLOY DESIGN AS A MEANS

TO COUNTER FAILURES INMANUFACTURING ANDSERVICE


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Alloys are made by combining elements in varying ratios

strength, hardness and other mechanical properties

corrosion resistance

electrical properties

ALLOYS


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Alloy%Design%

Preven/ve%

Alloy%Deign%

Improving%

Proper/es%


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Func%on'Specifica%on'

Geometric'Environment'Of'Opera%on'

Flows'and'Currents'

What'are'the'goals?'

PreASelec%on'Of'Material'Class'

Material'Types'that'apply'are'selected'

Manufacturing'Method'may'be'

iden%fied'too'

Discrimina%ng'Material'Selec%on'

Op%misa%on'In'Material'Selec%on'


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Constraints on the physical properties, often one sided

Could involve properties that are not numerical such asmachinability or surface appearance

Problem may be over-constrained

No existing material satisfies all specified parameters?

FUNCTION SPECIFICATION


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Intui&ve)ways)of)Material)

Selec&on)

First)best)material)

Same)material)as)for)a)similar)

part)

Problem)solving)material)

selec&on)

Searching)material)

selec&on)


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select base-line alloy

commercial alloy

Modify:

Composition

Process

Rules help move in the correct direction, example for an Aluminum alloy:

Composition:

If an element with low atomic number is added then density will decrease

If Mg is added then strength will increase

Microstructure:

If the aging process is done for a long time then equilibrium precipitates will form

If equilibrium precipitates are present then they are usually incoherent

If incoherent precipitates are present then they may form on the grain boundary

If precipitates are on the grain boundary and strength is medium or high then elongation and fracture-toughness arelow

ALLOY SELECTION


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ALUMINUM ALLOYS SHOULD NOT BE USED WHEN:

If the service temperature exceeds 200C or if it exceeds 100C in combination with significant mechanical loads.

If the part is in contact with water or placed underground for a longer period of time without protection.

In solutions with higher or lower pH, since the protective oxide layer is not intact.

When thermal or electric insulation is required.

When thermal expansion should be kept low.

When strength requirements exceed 500MPa.

When fatigue limit requirements exceed 230MPa.

When low elastic deflections are anticipated.

When wear is expected to be critical.

NEGATIVE RULES


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Usage Requirements

Max use temp = 160C

Good heat conductivity

Corrosion resistance in household chemicals

Non toxic

Manufacturing

Conventional Methods Available

Availability

Low Price

Conventional Material

EXAMPLE OF A PRE-SELECTION


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Usage Requirements

Max use temp = 160C

Good heat conductivity

Corrosion resistance in household chemicals

Non toxic

Manufacturing

Conventional Methods Available

Availability

Low Price

Conventional Material

Aluminum Alloys

Yes

Yes

Yes

for Cu-free alloys

Spinning, Deep Drawing

Yes

Yes

EXAMPLE OF A PRE-SELECTION


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UsageRequirements

Aluminum Alloys C-SteelC-Fibre

Composite

Max Use Temp (>50C) Yes Yes Yes

Yield Strength (>100MPa)

Yes Yes Yes

Elastic Modulus (>50GPa)

Yes Yes Yes

Corrosion inatmosphere

Yes Must be painted Yes

Low Density Yes Yes Yes

Low Price Yes Yes No

Conventional Material Yes Yes No

PRE-SELECTION EXAMPLE


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Clearly testing each possible combination individually is going to take a long time!

Designing and optimizing metallic alloys using combinatorial principles

Rapid synthesis and evaluation of a large number of samples to the development of new engineeringmaterials

Develop techniques that can be used to fabricate an alloy specimen with a continuous distribution ofbinary and ternary alloy compositions across its surface

spatially resolved probing techniques to characterize the structure, composition, and relevantproperties of the library

COMBINATORIAL MATERIALS

SCIENCE


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Corrosion Prevention

High Temperature Applications

Low Temperature Applications

Fatigue

SPECIFIC SCENARIOS, AND

HOW TO DEAL WITH THEM


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CORROSION PREVENTION


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Structure comprising extrusions and plate in aluminium alloy2014A

Superior strength properties, but susceptible to atmosphericcorrosion

Significant, and unacceptable, atmospheric corrosion on thestructures after a relatively short service time

EXAMPLE


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Low temperature precipitation of CuAl2 occurs along the grainboundaries

Strength increases but corrosion resistance decreases

Galvanic situation created between Cu-rich zones (cathodic) andCu-depleted zone (anodic)

Susceptible to intergranular attack

WHY?


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REGION OF DAMAGE

Fig 1: Cu depletedregion along the

grain boundaries


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Cladding with a layer of commercially pure aluminium or a lowmagnesium-silicon alloy

Protective anodising

Switching to a different grade of aluminium alloy, with comparableproperties but higher corrosion resistance, eg. 6xxx series alloy(6082)

SOLUTION


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Important where corrosion might be a problem-corrosive environments

Corrosion resistance is important, but not the only factor

Also strength, ductility, fabricability, availability, cost- compromise required

Selection:

Need to understand type and amount of corrosion expected to occur,

and to what level it is tolerable

Selection based on corrosion resistance data-previous application or

testing data

MATERIAL SELECTION


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MajorConsiderations

Corrosionvariables

Mainconstituents

and impurities

(identity andamount)

Temperature,pressure, pH,

velocity oragitation

Degree ofaeration

Estimatedrange of each

variable

Type ofapplication

Function of part orequipment. Desired

service life?

Compatibility of designwith the corrosion

characteristics of the

material

Effect of environment andservice conditions.

Effect will different typesof corrosion have on

serviceability andseriousness of the

problem

Experience

Use of the materialin similar/identicalsituation. Results?

Any plantcorrosion-test

data

Laboratorycorrosion test

data

Available reports


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Stems from the tendency to form a passive layer on top of the base material

Reduces diffusion of corrosive elements

Base element itself may form passive layer; affected by alloying elements

Eg. Al alloys

Alloying element forms passive layer

Eg. Cr addition to Fe to make steels and stainless steels

Nature of alloying element and oxide film decides extent of corrosion resistance

EFFECT OF ALLOYING

ELEMENTS


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Depletion of protective alloying element by precipitation

Eg. depletion of Cr by precipitation of Cr23C6

Galvanic couples must be prevented, within the alloy and external contacts

Presence of elements that make the alloy susceptible to attack by corrosive

agents, especially chloride ions

Fe additions in Al alloys that are to be used in chloride/marine

environments

ISSUES TO BE AVOIDED


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Precipitation that causes corrosion:

Matrix is less noble with respect to the precipitate causing matrix to corrode

Eg. CuAl2 precipitates in Al-Cu alloys

Matrix is more noble with respect to precipitate, causing corrosion of

precipitates, especially at grain boundaries (segregation)

Eg. AI-Mg alloys and Al-Zn-Mg base alloys with Mg2AI3 and MgZn2intergranular precipitates, respectively

ISSUES TO BE AVOIDED


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Pure Al has very good corrosion resistance due to passivation; Alspontaneously passivates

Some alloying additions form harmful precipitates that aidcorrosion

Galvanic Corrosion (Al and steel contact)

Prone to pitting and crevice corrosion, where corrosive (chloride)species compromise the integrity of the passive layer locally

ALUMINUM


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Close-up of galvanic corrosion in

an aluminium rail post (25 years

use). The rectangular hollow profile

was held in place by a carbon steel

bolt. The contact surfaces between

the steel and the aluminium were

often wet and attack was

aggravated by wintertime salting.


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Main species affecting Al alloy properties: Cu, Mn, Si, Mg, Zn

Less significant: Fe, Cr, Ni, Ti, others

ALLOYING ADDITIONS


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Mg gives good overall corrosion resistance, but increases susceptibility to SCC

Cu causes decrease in general and pitting corrosion resistance, but provides resistance to SCC

Zn-increases susceptibility to SCC

Fe (impurity) increases pitting corrosion, especially in aqueous chloride solutions

C- impurity, bad, forms Al4C3, which decomposes in presence of moisture, can cause pitting

H- impurity, bad, causes de-cohesion of grain boundaries during SCC

EFFECT OF ALLOYING

ADDITIONS IN AL ALLOYS


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Cu is known to reduce the corrosion resistance of Al alloys

Strength is lower with respect to Al-Mg alloys

Crack propagation is slower

Hence, although general corrosion resistance of these alloys islower, the stress corrosion resistance is higher

CONTRADICTION?


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Helicopter main rotor blades underwent severe corrosion attack

and rivet heads were severed at shoulder in service

Rivet failure due to stress corrosion cracking

Rivets were not only corroded, but also brittle

Rotor blades underwent extensive surface corrosion- aluminumoxide formation due to atmospheric corrosion

CASE STUDY: RIVET FAILURE


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DAMAGE

Fig 1: Cracks emanatingfrom the severely cold

worked region of the rivet

head shoulder radius (x200)

Fig 2: Severe corrosionattack on the surface of

the propeller blade (x1)


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Material used for rivet heads was Al-5% Mg alloy (AG5),susceptible to SCC:

Under severe cold working conditions

Under marine conditions

Riveting produced severe cold working in rivet-shank shoulderradius zone

Helicopters flew in coastal areas, under marine conditions

CAUSE: MATERIAL FAULT


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Riveting and marine atmosphere cannot be eliminated

Change of material: AG5 to AU 4G (Al-4% Cu-1% Mg)

Stabilizing/stress relief treatment of AG5 rivets to reducesusceptibility to stress corrosion cracking

SOLUTION


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HIGH TEMPERATUREAPPLICATIONS


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Major reason for failure : Creep

Other issues with high temperature

Corrosion rate increases

Increases diffusion rate

Increased solubility in the material

HIGH TEMPERATURE

APPLICATIONS


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CREEP:STAGES


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Bulk diffusion (Nabarro-Herring creep)

Climb

Grain boundary diffusion (Coble creep)

Thermally activated glide ie via cross-slip

CREEP: MECHANISM


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Low alloy 2 Cr 1 Mo steels were used in the design of high

temperatures reactors previously

Properties improved substantially by addition of right amounts ofVanadium (V) and Colombium(Cb)-Kobe steel

Now in ASME standards for materials in use for making Hightemperature de-sulphurization reactors ,Ammonia convertersetc.

CASE STUDY


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Hydrogen attack resistance limit on the Nelsons curve is nearly atOperating conditions of these vessels(454 C)

Low Hydrogen embrittlement resistance

Creep resistance was also just about sufficient and generallyborderline

ISSUES WITH THE OLD


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The enhanced precipitation of carbides of V and Cb realizeshigher strength compared with the existing 2 1/4Cr-1Mo steel,leading to a reactor weight reduction of about 10%.

HIGH STRENGTH HELPS MINIMIZE

THE REACTOR WEIGHT


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The precipitation of stable vanadium carbide and columbiumcarbonitride suppress the methanization reaction (hydrogenattack), and the trapping of the hydrogen in the steel by the finevanadium carbide suppresses the hydrogen concentration atcrack tips (hydrogen embrittlement).

IMPROVED RESISTANCE TO HYDROGEN ATTACKAND HYDROGEN EMBRITTLEMENT


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The trapping of hydrogen in the steel by fine vanadium carbidesuppresses the hydrogen concentration at the boundary ofoverlay and base-metal.

HIGHER RESISTANCE TO DISBONDING OFSTAINLESS STEEL WELD OVERLAY


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Conventional21/4Cr-1Mosteel(SA-336-F22)

Modified21/4Cr-1Mo-VsteelHighestworkingtemperature(ASMEVIII,Div.2design) 482C 482CHydrogenattackresistancelimit(NelsonCurve)

454C 510C

Hydrogenembrittlement Higher resistance than conventional steelOverlaydisbondlimit 200 bar at 454C 300 bar at 600CImpacttesttemperature(Av.40ft-lb/min.35ft-lb)

-30C 300 bar at 600C

Temperembrittlement(Stepcooltest) vTr40+3vTr40


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HED department at L&T noticed that Vanadium modified 214 Cr1Mo 14 V steels having higher creep strength that was suppliedsuffered damage and had to be repaired due to nitride formationand minor cracking within 2-3 years of service.

The main cause of this is believed to be Nitridation and study onthis is an ongoing project.

ISSUE WITH THE NEW


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DUCTILE TO BRITTLETRANSITION


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Bri$le'Fracture'

Low'Temperature'

High'Strain'Rate'

Triaxial'State'Of'

stress'


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Disloca(on*pile-up*at*obstacle*

Building*of*shear*stress*to*nucleate*

microcrack*

Propoga(on*of*microcrack*across*

obstacle*

STEPS INVOLVED


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k#is#high#(Fe,#Mo)#=>#tendency#for#bri2le#fracture#

D#increases#=>#tendency#for#bri2le#fracture#increases#

"$#increases,#stress#for#yielding#is#high#and#so#bri2leness#increases#

COTTRELL THEORY FOR

BRITTLE FAILURES


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In BCC metals increases as temp falls

Increasing strain rate increases

Alloying affects all the parameters on LHS

FCC and HCP metals have active slip systems at all temp

At low temp BCC metals have limited active slip system

DEPENDENCE ON

TEMPERATURE


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TOUGHNESS DEPENDENCE

ON TEMPERATURE


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Failure mode transforms from ductile to brittle

Occurs abruptly at a critical temperature (Tc)

Temperature depends on composition, microstructure andmechanical history

Occurs predominantly in BCC metals

Control of critical temperature is critical

WHAT IS DBT


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Largest increases in Tc by increasing C

Increase of S & P increases Tc

O increases Tc drastically

Mo increases Tc (Mo has high k)

Si Increases Tc (increases D and )

Nitrogen increases Tc

Increase of Mn reduces Tc (reduces k and D)

Nickel lowers Tc

EFFECT OF ALLOYING

ELEMENTS ON TC OF STEELS


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Was the largest and most luxurious ship made

Set sail from Southampton on April 10, 1912

Two days later hit an iceberg, hull damaged

Six forward compartments ruptured

Stern and bow separate

Ship sinks in less than 160 minutes

CASE STUDY - TITANIC


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STEEL COMPOSITION


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Steel used made in acid open hearth furnace

Steel was partially deoxidised (semikilled)

The Si content was quite high even though it was only semikilled

High C and P equivalents

Very Low Mn:S ratio

Longitudinal section Tc = 32 C

Transverse Section Tc = 56 C


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The US Navy built 2,751cargo ships under the liberty

fleet

During World War II, therewere nearly 1,500 instances of

significant brittle fractures

Twelve ships broke in twowithout warning

LIBERTY FLEET

MODERN SHIP STEELS (TC


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MODERN SHIP STEELS (TC >

0 C)

LOWTEMPERATURE SHIP


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American Bureau for Shipping has detailed standards for ship steels

For very low temperature applications (polar icebreakers), addition of Ni and Nb are used

Sometimes austenitic steels are used

Tc can be as low as -196 C (A353, A553)

Upto 36% Ni is used (A658)

LOW TEMPERATURE SHIP

STEELS


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FATIGUE PREVENTION


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SAE 0030-NT

SAE 0050A-NT

C-Mn(NQT)

Mn-Mo(NQT)

AISI 8630(NQT)

CAST STEELS


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Fatigue cracks initiated from the surface and at regions containing porosity and inclusions.

Multicracks were often initiated in specimens subjected to smaller strain amplitudes

The poor fatigue resistance at larger strain amplitudes for 8630 cast steel is attributed to

excess microshrinkage in the room temperature specimens

The room temperature fatigue strengths of the five cast steels at 106 reversals ranged from

30 ksi (208 MPa) for 0030 steel to 53.7 ksi (370 MPa) for 8630 steel. The five steel roomtemperature fatigue strengths were within 30 to 40 percent of the ultimate tensile strength.

This range was 32 to 46 percent at -50F (-45C).

CONCLUSIONS


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Low cycle fatigue concepts, which were principally developed andproven with wrought steels, are applicable to these five cast steelsand would appear to be quite realistically applicable for additionalcast steels.

The cyclic stress-strain curves and the cyclic yield strength at-50F (-45C) increased an average of about 10 percentcompared to room temperature results except for 8630 steelwhich had substantial microshrinkage in the room temperature

specimens. The increases were similar to increases found in Syand Su from monotonic tests.

L l f i b h i 50F ( 45C) l


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Low cycle fatigue behavior at -50F (-45C)was equal to orbetter than at room temperature for lives greater than 5x105reversals;however,mixed behavior existed at shorter lives.The

fatigue strengths at 106 reversals were from zero to 30 percentbetter at -50F (-45C).

C-Mn and Mn-Mo cast steels had the least low temperature

crack sensitivity while 8630 cast steel was the most sensitive tocracks at low temperature.

0030, C-Mn, Mn-Mo and 8630 cast steels are suitable for lowclimatic temperature conditions

the three martensitic cast steels 8630, Mn-Mo and C-Mn hadbetter fatigue resistance than the ferritic-pearlitic 0030 and0050A cast steels.

CASE STUDY ON PRESSURE


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Objectives:

To help create an understanding of how material properties, costs of materials and of their

fabrication,required product life,and product liability interact in different ways depending onthe product.

Prerequisites:

Student with some knowledge of fabricating processes and the significance of such factors

as fatigue,fracture toughness and environmental performance on material selection.

CASE STUDY ON PRESSURE

VESSELS


75/85

High pressure cylinders for the storage and transport of gaseshave been available in steel for over one hundred years and were

widely used before aluminium became a viable engineeringmaterial.

Aluminium and the aircraft industries have spurred each others

growth since the time when they were both born at thebeginning of the 20th century.


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AIRCRAFT FUSELAGE


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The cabin must be capable of withstanding the internal pressure in flight(0.6 kg/cm2)as wellas loads transmitted by wings and undercarriage, but it must withstand many repetitions of

these loads without catastrophic failure

Despite the many alloys available to select from, the almost universal choice of aluminium

alloy for pressure cabins is 2024-T3.This alloy is used for both skin and stringers.

Although parts of the frame might include 2014-T6 for parts where both strength and

fatigue resistance are required.

Other parts where strength is the only important criteria can be made from 7075-T6.

HIGH PRESSURE GAS


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In general, aluminium cylinders are lighter than steel

for sizes which can be lifted by an individual, say up to 20litrecapacity

Carbon Dioxide, Oxygen and Air are the gases most commonly

packed

good corrosion resistance and attractive appearance

HIGH PRESSURE GASCYLINDERS


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Approximately 2.0 million aluminium high pressure gas cylindersare made each year and probably 90 % of them are made fromone or two alloys and one process

aluminium/magnesium/ silicon group i.e. 6351 (6082) and 6061

better deformation characteristics and better toughness,

corrosion and stress corrosion resistance, as well as higherspecific fatigue strength


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The easy opening end, which to date has only been possible with

aluminium

Although alloys very similar to that now used for the can-end(5182) were available when the development began, there have

been necessary changes in composition and in the rollingsequence in the production of the end stock to provide acombination of strength and formability

BEVERAGE CANS


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Alloy Design

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