2007-09 Aluminum Fabrication

8/10/2019 2007-09 Aluminum Fabrication

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FABRICATION OF

ALUMINUM STRUCTURES

ROEBERT A SIELSKI

9-18-07


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9/18/ 2007 Aluminum Fabrication 1

Fabrication of Aluminum StructurePresented to the Southwest Section

The Society of Naval Architects

and Marine EngineersTuesday, September 18, 2007

Robert A. Sielski

Naval Architect Structures

40391 Camino MontecitoIndio, California 92203

760-200-3193

[email protected]


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Background on Aluminum at Sea

Aluminum used for ships and craft for morethan a century

Early vessels (yachts, patrol craft) Severe corrosion problems

Use of copper-strengthening alloys

Mixed with steel frames

Practically dissolved at the pier Lesson learnedTest new alloys and systemsbefore using


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Background on Aluminum at Sea (Cont.)

US Navy deckhouses

Aluminum used since 1930s

Relatively good service

Some corrosion (exfoliation)

Fatigue problems sometimes

severe


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High-speed vessels

Beginning in 1950s

Crew boats, fishing vessels, pleasure craft

US Navy derivative craftSwift boats

US Navy high-speed vessels

Beginning in 1960s Hydrofoils, air-cushion vehicles, surface effects ships

Vessels didnt always find use in fleet, but aluminum

made them possible


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Commercial high-speed vessels since 1980s

Mostly ferries

Increasingly larger

Japanese superliner Ogasawara Surface Effects Ship

LBP 126.8 m

14,500 GT

Speed 39 kts


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US Navy adaptation of commercial HSVs

Commercial vessels designed for coastal and inland

waters High sea states are not encountered

US Navy requires unlimited service

30-year lifetime

Little service experience for such conditions

Require knowledge of operating envelopes

New ship designs are extrapolation of existing designs


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Material Property And Behavior

Research on marine applications of

aluminum

Began in 1930s, including corrosion studies

US Navy extensive research 1960s to early

1980s

Declined after then

Some new alloys developed and newapplications recently


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Chemical Composition of Aluminum Alloys

0.100.200.250.60-1.200.40-1.000.100.500.70-1.306082

0.100.100.100.45-0.900.100.100.350.20-.606063

0.150.250.04-0.35.80-1.200.15.15-.400.70.40-.806061

0.100.200.30.40-.700.500.300.35.50-.906005A

0.200.250.05-0.204.70-5.500.50-1.000.100.400.255456

0.200.250.05-0.202.40-3.000.50-1.000.100.400.255454

0.150.400.254.00-5.200.70-1.000.200.250.255383

0.150.250.05-0.253.50-4.500.20-0.700.100.500.405086

0.150.250.05-0.254.00-4.900.40-1.00.100.400.405083

0.200.40-0.900.255.00-6.000.60-1.200.250.500.455059

TiZnCrMgMnCuFeSiAlloy


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Aluminum Alloys

Used in Marine Service 5xxx-series

Magnesium is principal alloying agent

Many have a significant amount of manganese

Work hardening Number followed by H and up to 3-digit number

H1 only strain hardened

H2 strain hardened and then slightly annealed

H3 strain hardened and then has the properties stabilized byeither low-temperature treatment, or by heat introduced duringfabrication

O Annealed condition

Example: 5083-H116


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Aluminum Alloys

Used in Marine Service ASTM B 928

Developed because of stress-corrosion cracking of 5083-H321

5059-H116 5059-H321 5083-H116

5083-H321 5086-H116 5383-H1165383-H321 5456-H116 5456-H321

Need not be B-928 if:

5xxx alloys containing less than 3 percent Mg

Tempers not susceptible to sensitization Annealed (-O temper)

Must order to B 928

alloy and temper not sufficient


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Aluminum Alloys

Used in Marine Service 6xxx series

Magnesium and silicon as principal alloying

agents

Heat-treatable

T and following number indicates type of heat

treatment T6 is most common

Example: 6061-T6


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Material Property And Behavior (Cont.)

Corrosion Resistance Sensitization

Can be problem with high-magnesium 5xxx-series

alloys

Can lead to stress corrosion cracking and

exfoliation

Can occur from:

Thermal and mechanical processing at mill

Welding during production

Exposure to higher temperatures in service.


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Sensitized Aluminum

Sensitized Material Unsensitized Material


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Sensitized 5083 Plate Showing both Stress

Corrosion Cracking and Pitting Corrosion


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Effective Diffusion Rate Diagrams for

Sensitization of 5083 and 5456

(Catherine Wong, NSWCCD)

360

280

140

180

F


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Diffusion Rate Diagrams for

Sensitization of 5454-H117

(Vassilaros and Czyryca, 1979)


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Yield Strength of Selected

Welded Aluminum Alloys (ksi)

262419AWS HullWelding

2622US Navy

191814Aluminum

Association

1713DNV

262419ABS

5456-H1165083-H1165086-H116Alloy/Source

262419AWS HullWelding

2622US Navy

191814Aluminum

Association

1713DNV

262419ABS

5456-H1165083-H1165086-H116Alloy/Source


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Comparison of Stress-Strain Behavior

of Aluminum and Steel


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Similarities between Fabricating with

Aluminum and Steel

Transition from steel to aluminum

No significant changes in facilities or personnel

Many shipyards that build with both materials at the same time

Significant differences

Welding in an enclosed area is a necessity for aluminum

Electromagnetic devices for material handling and for holdingwork in place are useless

Oxyacetylene, gas or carbon arc cutting not used on aluminum

Workers need to learn new procedures

Same welders generally dont work on both at the same time

Closer link required between design and fabrication

Design often changed to fit the advantages and limitations ofconstruction capabilities


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Fabrication Facilities

Should be fabricated in enclosed conditions Temporary shelters or permanent buildings

Coefficient of thermal expansion about twice as great

as the coefficient of steel

Dimensions will vary greatly as the temperatures change

Localized changes in temperature (direct sunlight) induce

warping of structural assemblies

Protection from wind for shielding gas when

welding Moisture and high humidity has serious effect on

the quality of aluminum welds


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Shipyard Facilities for Fabricating Aluminum

(How not to do it)


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(How not to do it)


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(www.Austal.com)


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(www.Austal.com)


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(www.Austal.com)


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Cutting and Forming Aluminum

Aluminum is softer than steel

Easily cut with steel cutting tools

Sawing, machining, and other mechanical means of cutting performed

with ease

Sawing performed with blades that have relatively coarse teeth Blades should have a high speed

Band saws and hand-held or stationary rotary saws

Jigsaws and saber saws are used for cutting curved shapes

Hole saws for circular openings

Saw-cut edge is generally suitable for welding Smooth first by filing, planing, routing, sanding, polishing, or milling

prior to solvent cleaning


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Cutting and Forming Aluminum (Cont.)

Hacksaw not recommended except for small, thin pieces

Time-consuming

Does not present a very smooth edge

Shears for cutting plate up to 4.8 mm (0.188 in) thick

Edge should be dressed and cleaned prior to welding Do not shear exposed edges on alloys with magnesium content

greater than 3 percent

5083, 5086, or 5456, 5383, 5059, etc.

Edge can become sensitive to stress-corrosion cracking

Nibbler (similar in action to a shear) is used for curvededges


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Numerically Controlled Cutting

Numerically controlled cutting machines

Fastest and most accurate method of cutting aluminum

Cut edge is ready for welding, with only cleaning

required

Intricate shapes can be easily obtained

Cut outs through which structural shapes can be passed

Used today in even small boatyards

Economical to have an outside shop prepare plates

Requires additional advanced planning All openings and cutouts made at one time

Not when workers are fitting systems such as piping and

electrical systems.


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Numerically Controlled Cutting (Cont.)

Plasma-arc cutting Either dry or wet

Dry cutting has plate usually positioned above a pond of water

Wet cutting the plate is submerged

Fluid Jet Cutting

Jet of water includes abrasive particles

Very high pressure stream from a nozzle

Very clean and accurate cut

No heat-affected zone

Plates from 1 mm to 100 mm (0.04 in to 4 inches) thick

Cut at rates of 3,500-mm/min. (140 in/min.) for the thinner sheet

30-mm/min. (1.2 in/min.) for the thicker plate

683 sources listed at www.Thomas net.com


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Water Jet Cutting

(www.kustomwaterjet.com)


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Water Jet Cutting

(www.kustomwaterjet.com)


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Water Jet Cutting

(www.calypsowaterjet.com)


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Bending Aluminum Plates 5xxx-series aluminum alloys are work hardened

Overworking in forming operations can have a deleterious effect onmechanical properties

Plates can be easily bent in a press- brake

Minimum bend radii should be no less than those recommended by theAWS guide

Minimum Bend Radii for Cold Bends in Aluminum Alloys as a

Multiple of Plate Thickness, t (AWS, 2004)

5t4.5t3.5t3t2.5t6061-T6

4t3t2.5t2t1.5t5456-H116

4t3t2.5t2t25454-H344t3t2.5t2t1.5t5083-H116

4t3t2.5t2t1.5t5086-H116

13 / 0.509.5 / 0.3756.4 / 0.254.8 / 0.1883.2 / 0.125

Base Metal Thickness (mm / in)Alloy


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Forming Plate

Plates can be curved with rollers

Warped shapes with different curvatures at opposite ends

Forming compound curvature is extremely difficult

If small amount of cross-curvature is needed

Plate is rolled to the principal direction of curvature

Forced into position against hull framing members

Difficult operation

Scantlings of framing members may have to be selected for

forming forces


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Forming Plate (Cont.)

Roll forming compound curvature

Loose filler material such as sawdust or soft wood shavings

applied at rolls

Curved rollers also used to form compound curvature

Either process requires a great deal of skill

Orange peel sections Triangular plates

Given single curvature or

Some compound curvature using a press

Joined together to form approximation of the desired shape Compound curvature usually avoided in the hull form

Limitation of aluminum


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Forming Plate by Furnacing (Cont.)

1240.025.03.0014.000

1241.029.01.5013.000

1244.031.01.2511.500

1246.033.00.5001.250

1046.033.00.0630.449

Elongation (%)Ultimate

Strength (ksi)

Yield Strength

(ksi)

Thickness

Range (in)

Reduction of Mechanical Properties of 5456-H116 with

Increased Thickness (Hay and Holtyn, 1980)

Strength of thicker plates closer to specified values.


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Forming Plate by Furnacing (Cont.)

Above 400 F the Mg solubility is so high that the

beta phase starts to dissolve so there is no problem

with sensitization from the process. May disrupt the stability of the Mg in solution so the

material may sensitize more quickly in service.

Cooling rate between 400 and about 100 F must

be fast enough to lock in the supersaturated Mg.


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Line Heating to Form Plate

Not an approved process Would require research program to develop

Temperature controls similar to flame straightening wouldbe used

If extensive shaping of plates is be required May be better with annealed temper

5083-0 plate

less strength than the work-hardened tempers

design calculations should reflect the reduced strength.

Careful control of the shaping process is required because it couldlead to sensitization of material previously not sensitized.


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Forming Structural Shapes

Aluminum structural shapes are easily formed inlight sections

Deeper sections are more difficult to bend without

causing buckling of flanges or webs

Bend tee stiffeners by cutting V-notches in the websor cutting the flanges

Holes should be drilled at the ends of the notches prior

to forming to prevent cracking Not used if design is based on the unwelded strength

of the shape


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Forming Structural Shapes (Cont.)

Special rollers with slots to support webs of teesand angles

Heating may be necessary

When there is a considerable amount of curvature

(transverse frames) Cut plate to the shape of the hull to form the web

Inside edge straight or curved for welding a flange

Can also bend edge to form flanged plate

Least expensive alternative

Cost of numerical cutting is low

Shipyard labor is saved


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Structural Assembly

Pre-outfitted structural assemblies

Aluminum compared to steel

Larger subassemblies can be built of the same weight

More care required in handling larger assemblies

Temporary welded handling pads a necessity for handling

aluminum subassemblies

Lighter scantlings

Ineffectiveness of electromagnetic handling devices

Distortion of aluminum subassemblies can be greater than withsteel subassemblies


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Structural Assembly (Cont.)

Temporary stiffening members to hold thesubassembly prior to its being welded into theother structure

Aluminum welding not tolerant of gaps, especiallyuneven gaps

Greater care must be taken with the fit-up of joints priorto welding

Punch marks or scribe marks can cause problems

Sites of fatigue crack initiation

Should be welded over


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Panel Construction

Special aluminum panel lines for this purpose

Plates are butt welded to form large panels Stiffeners welded first in mechanized stations

Frames are fitted over the stiffeners and welded

Curved hull sections formed by laying the plates

in jigs Butt weld plate

Fit the stiffeners and frames

Stick construction

Bulkheads and frames are first laid up and tack weldedtogether

Stiffeners are then fitted to the frames

Entire assembly is welded


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Stick Construction (Cont.)

Plating laid over the stiffening Welded to the frames and stiffeners

Avoids having to construct jigs to handle curvedsections

More advantageous for one-off designs

Better access to the details of stiffener-frameintersections

Details easier to weld

Small stiffeners typical of smaller craft Alignment of intercostal members easily accomplished


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Egg Crate Detailing

Good through-thickness properties

No concern for innerlaminar exclusions

Cruciform welds will not to fail by splitting the intervening plate

Better suited for stick-type construction


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Guidance on Welding Aluminum American Welding Society Committee D.3 on Welding in

Marine Construction

Guide for Aluminum Hull Welding (AWS, 2004)

Thoroughly reviews welding aluminum for marine fabrication

The Aluminum Association Welding Aluminum: Theory and Practice

American Bureau of Shipping Part 2, Aluminum andFiber Reinforced Plastics Part of the Rules for Materials and Welding

Military Standard MIL-STD-1689

David W. Taylor Naval Ship Research and DevelopmentCenter

Guide for the Use of Aluminum Alloys in Naval ShipConstruction (Beach et al., 1984)

Volume on fabrication


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Welding Distortion of Aluminum

Comparison of aluminum to steel

Elastic modulus of aluminum is one-third that of steel

Coefficient of thermal expansion is about twice as

much Strains from cooling of welds and surrounding areas producelower residual stress

Reduced elastic modulus

When residual stresses do occur

Tend to produce greater distortion than in steel structure

Buckling of plating


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Comparison of Aluminum Distortion to Steel

(Cont.)

Aluminum conducts heat anywhere from 2.5 to

9 times faster than steel

Area heated during welding processes is greater Not as intense

Aluminum structure tends to distort more

during welding

Tolerances for ship construction reflect this


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Comparison of distortion at a fillet weld

(Masubuchi, 1990)

0

10

20

30

40

50

60

70

0 10 20 30

Thickness (mm)

AngularChange(R

adiansx10

-3)

Aluminum Steel


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Tolerances (Cont.)

Fairness of frames and stiffeners

Primary strength structure Subject to dynamic loading, such as bottom slamming

MIL-STD 1689 and the ABS aluminum rules have thesame tolerance

Unfairness < 530 l / dw (mm) l is the span in meters

dw is the depth of the web in mm

Tolerances for plate and stiffeners determined bysurveys

Tolerances achieved in normal shipbuilding practice

Effects of tolerances on structural strength not asextensively studied for aluminum as for steel


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MIL-STD 1689 Plate Tolerances

Aluminum plate in critical areas Aluminum plate in secondary areas


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Flame-Straightening(Hay and Holtyn, Naval Engineers Journal,1980)

Generally not permitted in aluminum Special permission required

Classification society

U.S. Navy.

5xxx-series aluminum should not be heated to above

288 degrees Celsius (550 degrees Fahrenheit)

should not be permitted to remain at that temperature

for any length of time.

Aluminum does not glow when it is heated

Use temperature-sensitive crayons (Temple sticks)


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Flame-Straightening (Cont.)

Two operators generally required

first operator has crayons that melt at 288 C (550 F) and anoxy-acetylene torch to heat the plate.

The second individual has a device for providing a fine sprayof water and air.

Constantly check temperature with the crayon

When it melts

immediately cool the plate to 66 C (125 F)

Extreme care to prevent overheating required Lowers mechanical strength

Reduces the corrosion resistance Neither can be easily determined by quality control means.

Should have effective diffusion rate diagram forspecific alloy to determine temperature and time

limits.


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Other Means of Straightening Plate

Weld beads to straighten

Lay weld beads in a pattern on the surface of the plate

Not generally permitted as it reduces strength of plate

Radical distortions in plating Cut slit in plate

Straighten plate or possibly distort it in the opposite direction

Reweld

Special permission may be required for such anoperation


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Minimizing Distortion During Welding

Plate should not be free to rotate about the axis ofthe weld during welding

Design of the joint should be symmetrical

Welding procedures should be symmetrical

Minimum welding heat should be used

Excessive filler material should be avoided

Fillet welds should be made with minimum heat

input Fillets should be no greater than required for

strength


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(Cont.)

Fit-up should be made as accurate as possible tominimize weld size

Minimize root gaps and irregularities in the root gaps

Sequence of welding is very important

Butts and seams in plating should progress outwardfrom the center

Butts in strakes of plating welded before thelongitudinal seams

Beneficial to weld only small portions at a time Welding short intermittent beads

Returning to the weld seam after structure farther away hasbeen welded


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Recommended Welding Sequence for Butt

Welds (AWS, 2004)


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(Cont.)

Intermittent fillet welds

Smaller craft

Non-critical structure


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Summary

Fabricating structure with aluminum is similar tosteel construction

More difficulties involved

Cutting aluminum generally not as fast as steel

cutting Faster with plasma arc or water jet

Aluminum can be formed into different shapes

Heating is very difficult Compound curvature of plates should be avoided


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Summary (Cont.)

Welding aluminum more expensive thanwelding steel

More joint preparation and cleanliness required

Need for shielding gas Somewhat slower welding speeds

Aluminum more prone to distortion during

welding


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Summary (Cont.)

More care needed with welding procedures toreduce distortion

When distortions occur

More difficult to remove

Limitations on the use of heat on aluminum

Extruded panels reduce construction cost

Many welds of stiffeners to plate are eliminated.

Less distortion

2007-09 Aluminum Fabrication

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