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Aluminium Alloy Construction (Page 4) Page 1 2 3 |4| 5 6 7 8

In a paper read to R.I.N.A. in 1982 Messrs Y Kaneco and E Baba discussed the long term appraisal carried out by the Japanese Committee of Light Metals for Shipbuilding. Part of this apprasial was carried out on the 15m patrol boat Arakaze, built in 1954, and involved the removal and testing of sections of her plating after 20 years of hard rescue service. Analysis of the chemical composition of the material and tests of tensile strengths showed that the alloy remained in its original condition and still met the standards of JIS H 4000.A 5083 for aluminium alloy. The same tests taken again 1981 after 27 years of service, produced substantially the same results. These examinations demonstrate the reliability of aluminium alloy as a hull material.

Aluminium does not require painting for protection and it is preferable that the interior of the hull is left completely unpainted. The exterior is painted for decorative purposes only and the underwater to prevent weed growth. Bearing in mind the electrolytic effects discussed above, it is vital to avoid paints and coatings that contain any of the metals that are cathodic to aluminium. Several of these are used in conventional coatings. Copper in antifouling is the most obvious example, but many primers and other coatings can also contain some of these metals.

Provided a painting system is properly carried out in the right conditions of temperature and humidity, and using the correct materials, it will have a long life. Blistering of the filler and paint coatings does occur with new aluminium boats but this is usually associated with incorrect proceedures being adopted or faulty preparation; it can also be caused by items being fixed to the painted surface without proper insulation. It has to be said that it can be difficult with an object as large as a boat, to achieve all the right conditions all the time while the filling and coating system is being carried out. Later damage to the paint systems, while possibly unsightly, will cause no deterioration of the aluminium alloy structures.

It is of the utmost importance to be certain of the composition of the materials that are being applied to the vessel, whether originally or for repair, and to keep to epoxy and non-metallic coatings intended for use with aluminium.

The aluminium alloys have several other properties that are not easily found together in other materials. They are non-magnetic, non-inflamable and non-sparking. They do not therefore affect the ship's compasses; while they will melt in a fire, they do not contribute to the fire by burning; and they can be used in areas where sparking could be a hazard.

3. Construction

Careful handling and storage of the aluminium alloy materials is important to prevent damage, water staining and embedded foreign matter, which can cause later corrosion or poor welding. Benches and plate preparation areas need to be covered in aluminium or chipboard. Machinery should preferably be reserved for aluminium, or, if it has previously been used for steel, carefully cleaned and freed from burrs and particles of steel that could become embedded in the aluminium.

MiG welding in a pure argon shield is the most suitable for aluminium up to 25mm. Above this size some pre-heating may be required, or the gas shield changed to a helium/argon mix. Machine MiG welding can be employed with advantage on butt welds to build up larger plates for areas such as decks.

TiG welding will only be necessary for small or intricate items, where th erun of weld is too short for successful MiG welding, or the change of angle too much. This is the case when welding seacock stubs to hull inserts, for example.

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