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

-33-electrode in the welding tool ati the aluminum being welded. The weld areais protected from oxidation by a shield of inert gas such as argon, heliumor a mixture of the two. Neither method requires flux, thus minimizing thepossibility of porosity or corrosive residue. The methods are applicableto flat, horizontal, vertical and overhead welding, though downhand weldingis fastest ati of highest quality, as with steel. The fundamental differencesbetween MIG and TIG welding is that with llIGwelding, the aluminum fillerwire serves as the electrode and is consumed, whereas the TIG process usesa non-consumable tungsten electrode, with filler metal provided in rod form,either manually or automatically. The MIG process is somewhat faster andmore economical, but the TIG process produces a smoother beady which isvery important in maintaining high fatigue strengths.Based upon discussions with aluminum fabricator and review of availablelitera:ttie, it appears that the weldability of JOOO series alloysimproves as alloy magnesium content is reduced. The shipbuilding and boatbuildingindustg presemtlyfavor S086 alloys in preference to S083 and5456alloys, which have approximately one-half and one per cent higher magnesiumcontents respectively.Aluminum is somewhat more susceptible to weld distortion than steel,due to greater thermal conductivity. Heat fairing can be used to eliminatethis distortion. However, when ~000 series aluminum alloys are heat treatedto 150 to 200 degrees F for a sufficiently long period to allow heat fairing,the magnesium in the alloy migrates within the metal, which becomessusceptible to exfoliation. Heat Tairing at higher temperatwes for morelimited periods may be used, providing the critical range is passed throughquicldy for both heatiingand cooling. However, this will result in thealuminum’s properties being reduced to those of the annealed material,which may not be acceptable. Proper weld techniques (speed, heat input,number of passes, edge preparation, etc.) can mitimize distortion problems=It is generally preferable to use continuous welding for aluminumstructures, even if strength considerations would permit the use of intermittent’welding.Although intermittent welding reduces the quantity ofelectrode used and minimizes heat distortion, it can lead to crater crackingat the ends of beads ard crevice corrosion if moisture is present. Thereforeintermittent welding should be used only for secondary structures whichare reasonably free from wetness, and are lightly loaded.The problem of shrinkage in way of aluminum welds requires particularattention, since improper welding techniques can produce high residualstresses. These high stresses can lead to cracking, particular~y when theYoccur in way of a poorly designed structural detiails. ThTs is perhaps themost serious production problem to be overcome in the fabrication of largealuminum hulls, because experience in fabricating large aluminum weldmentswith thick plates is relatively limited. In nearly all cases of residualstress cracking of aluminum weldments, solutions can be found throughimproved design, testing and personnel training. Even with these precautions,however, it is vitally important that all structural details be verycarefully designed and fabricated to eliminate sources of stress concentration.Among other things, this would require careful development of cuts inthe hull girder, proper stiffener endings> elimination of excessive concentrationof welding, including triaxial welds at intersections of threemutually perpendicular surfaces, avoidance of details which are difficult toproperly fabricate, elimination of notches and hard sPots~ and location ‘fwelds away from highly stressed areas.