2007-09 Aluminum Fabrication

FABRICATION OF ALUMINUM STRUCTURES ROEBERT A SIELSKI 9-18-07

Fabrication of Aluminum Structure Presented to the Southwest Section The Society of Naval Architects and Marine Engineers Tuesday, September 18, 2007

Robert A. Sielski Naval Architect — Structures 40391 Camino Montecito Indio, California 92203 760-200-3193 [email protected]

9/18/ 2007

Aluminum Fabrication

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Background on Aluminum at Sea • Aluminum used for ships and craft for more than a century – Early vessels (yachts, patrol craft) • • • • •

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Severe corrosion problems Use of copper-strengthening alloys Mixed with steel frames Practically dissolved at the pier Lesson learned—Test new alloys and systems before using Aluminum Fabrication

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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 craft—Swift boats – US Navy high-speed vessels • Beginning in 1960s – Hydrofoils, air-cushion vehicles, surface effects ships – Vessels didn’t always find use in fleet, but aluminum made them possible 9/18/ 2007

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Background on Aluminum at Sea (Cont.) – Commercial high-speed vessels since 1980s – Mostly ferries – Increasingly larger • • • • •

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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 9/18/ 2007

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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 new applications recently 9/18/ 2007

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Chemical Composition of Aluminum Alloys Alloy

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

5059

0.45

0.50

0.25

0.60-1.20

5.00-6.00

0.25

0.40-0.90

0.20

5083

0.40

0.40

0.10

0.40-1.0

4.00-4.90

0.05-0.25

0.25

0.15

5086

0.40

0.50

0.10

0.20-0.70

3.50-4.50

0.05-0.25

0.25

0.15

5383

0.25

0.25

0.20

0.70-1.00

4.00-5.20

0.25

0.40

0.15

5454

0.25

0.40

0.10

0.50-1.00

2.40-3.00

0.05-0.20

0.25

0.20

5456

0.25

0.40

0.10

0.50-1.00

4.70-5.50

0.05-0.20

0.25

0.20

6005A

.50-.90

0.35

0.30

0.50

.40-.70

0.30

0.20

0.10

6061

.40-.80

0.70

.15-.40

0.15

.80-1.20

0.04-0.35

0.25

0.15

6063

0.20-.60

0.35

0.10

0.10

0.45-0.90

0.10

0.10

0.10

6082

0.70-1.30

0.50

0.10

0.40-1.00

0.60-1.20

0.25

0.20

0.10

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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 by either low-temperature treatment, or by heat introduced during fabrication • 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-H321 5086-H116 5383-H321 5456-H116 • Need not be B-928 if:

5083-H116 5383-H116 5456-H321

– 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 9/18/ 2007

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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 9/18/ 2007

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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 9/18/ 2007

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

°F 280

180

140 9/18/ 2007

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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) Alloy/Source

5086-H116

5083-H116

5456-H116

ABS DNV Aluminum Association AWS Hull Welding

19 13 14

24 17 18

26

19

24

26

US Navy

22

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26

17

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 holding work in place are useless – Oxyacetylene, gas or carbon arc cutting not used on aluminum

• Workers need to learn new procedures – Same welders generally don’t work on both at the same time

• Closer link required between design and fabrication – Design often changed to fit the advantages and limitations of construction capabilities 9/18/ 2007

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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 9/18/ 2007

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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 curved edges

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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. 9/18/ 2007

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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 • • •

–

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

–

Very high pressure stream from a nozzle Very clean and accurate cut No heat-affected zone 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 on mechanical properties – Plates can be easily bent in a press- brake – Minimum bend radii should be no less than those recommended by the AWS guide

Minimum Bend Radii for Cold Bends in Aluminum Alloys as a Multiple of Plate Thickness, t (AWS, 2004) Alloy

Base Metal Thickness (mm / in)

3.2 / 0.125

4.8 / 0.188

6.4 / 0.25

9.5 / 0.375

13 / 0.50

5086-H116

1.5t

2t

2.5t

3t

4t

5083-H116

1.5t

2t

2.5t

3t

4t

5454-H34

2

2t

2.5t

3t

4t

5456-H116

1.5t

2t

2.5t

3t

4t

6061-T6

2.5t

3t

3.5t

4.5t

5t

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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 9/18/ 2007

Aluminum Fabrication

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Forming Plate by Furnacing (Hay and Holtyn, Naval Engineers Journal,1980) • If thick plates are to be heated for shaping – Must be soaked in a furnace for several hours at desired temperature

• Maximum temperature should be less than 260 ºC (500 ºF) • 70-mm (2.75-in) plate of 5456-H116 – Held at a temperature of 245 ºC (475 ºF) • No significant affect on condition of the matrix and grain boundary precipitate • Yield strength decreased to 165 MPa (23.6 ksi) – Compared to the specified minimum yield strength of 200 MPA (29.0 ksi). 9/18/ 2007

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Forming Plate by Furnacing (Cont.) •Strength of thicker plates closer to specified values. Reduction of Mechanical Properties of 5456-H116 with Increased Thickness (Hay and Holtyn, 1980) Thickness Range (in)

Yield Strength (ksi)

Ultimate Strength (ksi)

Elongation (%)

0.063–0.449

33.0

46.0

10

0.500–1.250

33.0

46.0

12

1.251–1.500

31.0

44.0

12

1.501–3.000

29.0

41.0

12

3.001–4.000

25.0

40.0

12

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Forming Plate by Furnacing (Cont.) • Above 400 ºF 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 º 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 would be 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 could lead to sensitization of material previously not sensitized.

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Forming Structural Shapes •

Aluminum structural shapes are easily formed in light sections –

Deeper sections are more difficult to bend without causing buckling of flanges or webs – Bend tee stiffeners by cutting V-notches in the webs or 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 9/18/ 2007

Aluminum Fabrication

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Forming Structural Shapes (Cont.) • Special rollers with slots to support webs of tees and 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 • • • • • 9/18/ 2007

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 Aluminum Fabrication

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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 with steel subassemblies

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Structural Assembly (Cont.) • Temporary stiffening members to hold the subassembly prior to its being welded into the other structure • Aluminum welding not tolerant of gaps, especially uneven gaps – Greater care must be taken with the fit-up of joints prior to welding

• Punch marks or scribe marks can cause problems – Sites of fatigue crack initiation – Should be welded over 9/18/ 2007

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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 welded together • Stiffeners are then fitted to the frames • Entire assembly is welded 9/18/ 2007

Aluminum Fabrication

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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 curved sections • More advantageous for one-off designs

– Better access to the details of stiffener-frame intersections • 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 and Fiber Reinforced Plastics – Part of the Rules for Materials and Welding

• Military Standard MIL-STD-1689 • David W. Taylor Naval Ship Research and Development Center – Guide for the Use of Aluminum Alloys in Naval Ship Construction (Beach et al., 1984) – Volume on fabrication 9/18/ 2007

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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 produce lower 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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70

Angular Change (Radians x 10

-3

)

Comparison of distortion at a fillet weld (Masubuchi, 1990) 60 50 40 30 20 10 0 0

10 20 Thickness (mm) Aluminum

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Tolerances • Aluminum more prone to distortion from welding than steel – Tolerances for fairness in aluminum structure not as strict as for steel

• Unfairness in aluminum plating – – – –

ABS Rules for Materials and Welding (Aluminum) Similar to MIL-STD-1689 Greater tolerances are permissible in aluminum 12.7 mm (0.50 in) strength deck plate on 610 mm (24 in) stiffener spacing: • Tolerance = 9.5 mm (0.375 in) for aluminum • Tolerance = 6.4 mm (0.25 in) for 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 the same 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 by surveys – Tolerances achieved in normal shipbuilding practice – Effects of tolerances on structural strength not as extensively studied for aluminum as for steel 9/18/ 2007

Aluminum Fabrication

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MIL-STD 1689 Plate Tolerances

Aluminum plate in critical areas 9/18/ 2007

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 an oxy-acetylene torch to heat the plate. • The second individual has a device for providing a fine spray of 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 for specific alloy to determine temperature and time 9/18/limits. 2007 Aluminum Fabrication

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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 an operation

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Minimizing Distortion During Welding • Plate should not be free to rotate about the axis of the 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 9/18/ 2007

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Minimizing Distortion During Welding (Cont.) • Fit-up should be made as accurate as possible to minimize weld size – Minimize root gaps and irregularities in the root gaps

• Sequence of welding is very important – Butts and seams in plating should progress outward from the center – Butts in strakes of plating welded before the longitudinal seams – Beneficial to weld only small portions at a time • Welding short intermittent beads • Returning to the weld seam after structure farther away has been welded 9/18/ 2007

Aluminum Fabrication

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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 to steel 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 than welding 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 to reduce 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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