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FABRICATION OF
ALUMINUM STRUCTURES
ROEBERT A SIELSKI
9-18-07
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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
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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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Background on Aluminum at Sea (Cont.)
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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Background on Aluminum at Sea (Cont.)
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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Background on Aluminum at Sea (Cont.)
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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Shipyard Facilities for Fabricating Aluminum
(How not to do it)
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Shipyard Facilities for Fabricating Aluminum
(www.Austal.com)
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Shipyard Facilities for Fabricating Aluminum
(www.Austal.com)
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Shipyard Facilities for Fabricating Aluminum
(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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Minimizing Distortion During Welding
(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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Minimizing Distortion During Welding
(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
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