design considerations for aluminum hull structures - Ship Structure ...

-59-Presentlyj sawing is the most common way of cutting aluminum since thefinished edges need the least amount of dressing-up. Some amount of re-toolingfor large quantities of thicker material would be necesssry. Where modulesof the hull are to be butted together, an allowance would be left to establishthe “final cut” which couldbe done by a power saw mounted on a trackway.Another common method of cutting, generally used for preparing access orlightening holes, is by plasma arc. ThLs does not leave as smooth a finish assawing and sometimes requires dressing.MIG seems tobe the preferred choice for welding, with a mixture of 75per cent Argon and 2S per cent Helium. Small quantities of aluminum ship constructiondo not necessitate extensive use of automatic welding but it wasfelt that more automatic welding, perhaps as high as 70 per cent, would be inorder for a large aluminum hull. A higher content helium-gas mixture was alsoproposed as a means of speeding up the welding process, but this would have iObe evaluated against the additional cost of the helium.A training program to qualify additional aluminum welders would benecessary for any shipyard undertaking the project, but no major problemswere foreseen by any of the yards in either upgrading steel welders or intraining new welders for this particular skill.The general consensus of opinion seemed to dictate flat panel corlstructioninitially rather than three dimensional assemblies. The heavyscantlings proposed would in part minimize distortion problems and curvedshapes would tend to remain as rolled. When fabricating aluminum, morecare must be exercised since it has a shrinkage rate of two to three timesthat of steel. This is an area where a careful study should be made in thedesign stage to minimize any problem areas, particularly in way of shafting.&pansion tables could be developed for various combinations of plate thicknessesand weld sizes which would be an invaluable tool during construction.Heavy stress should be placed on the development of the minimal weldingsizes required. This will not only minimize distortion and expansion butreduce the total overall cost of welding. The sizes of aluminum fillet weldspresently required by Navy welding specifications are considered excessive bythe shipyards, and result in excessive distortion.htermittent welding is not reco~ended. Continuous welding is pre.ferred since it results in smaller, better quality welds and hence lowerrates of rejection, and minimizes cratering at the end of beads.Experience has shown it is very difficult to meet an acceptable weldquality X-Ray standard when welding outside with high humidity. Furtherexamination of possibly reducing this standard was proposed. Possible reductionsin porosity standards were also viewed as another cost saving item.ATTACHMENTSAttachment of ferrous materials was another area which it was feltdeserved special attention. Presently utilized methods Yor installingdeck fittings and other hardware in the weather do not provide a completelysatisfacto~ installation. It was proposed that flexible plastic sealerSbe used to cover all of these exposed joints over and above the normaltape and paint methods presently employed.