ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 24 over the boat’s lifetime. Fiberglass boats are far from maintenance-free and suffer from delamination and structural fatigue. Fiberglass is brittle and more likely than either aluminum or steel to rupture upon impact, and more difficult to repair than metal when damaged or cracked. The 5xxx and 6xxx aluminum alloy products used for marine construction normally do not need painting or other coatings to protect against corrosion. By contrast, steel boats rust quickly in saltwater and thus require corrosion protection inside and out. Prior to coating, the steel surface must be sandblasted to prepare for flame spraying or epoxy paints. The expense of corrosion protection for steel boats offsets some of their lower raw material cost compared to aluminum[4,5]. Cost savings for aluminum boatbuilders has come from improvements in computer- aided manufacturing technologies: design, robotic plasma or water jet cutting, and welding. Both large and small boatbuilders can design with 3D computer-aided design (CAD) software. The file is sent to a computer-controlled cutting table, saving time and increasing quality and uniformity. Because aluminum boats are not built from molds, design changes can be made on the computer and implemented quickly. Aluminum alloys used for small boat hull construction, commonly 5086 and 5052 sheet and 6061 or 6082-T6 for extruded ribs or reinforcements, are amenable to fusion welding (Fig. 1). Product thicknesses of approximately 1.5-3 mm (0.06-0.125 in.) are easily joined by metal inert gas (MIG) welding for much of the basic construction, while tungsten inert gas (TIG) welding can be used for more detailed work. Improvements in welding equipment have made the process more robust, using less power input to reduce distortion in the welded structure. Pulse MIGs, which use a computer to create a controllable square wave pulse, provide smoother welds with a more appealing surface appearance[5]. ALLOYS FOR SHIPBUILDING The 5xxx (Al-Mg-Mn) plate and sheet alloys have gained wide acceptance for construction of marine vessels due to their attractive combination of properties. These non-heat treatable (NHT) alloys derive their strength from additions of magnesium (up to ~6%), manganese (up to ~1.0%), and chromium (up to ~0.2%) along with special deformation processing. The most widely used shipyard alloys for larger vessels are 5086 and 5083, which are commercially produced in a range of thicknesses. The strength of marine alloys is directly related to the alloy composition and the amount of cold work used for strengthening the H tempers (Table 1). Alloy 5052-O has the lowest strength of the products typically used for smaller boat construction. The aluminum portions of pontoon boats are often made of 5052-O or H32. The alloy is quite weldable, easily formable, and extremely resistant to corrosion. The remaining alloys, particularly 5083 and 5383, are most often used for thicker sheet and plate that need to withstand higher stresses. Because the magnesium content is higher, the strength of the parent plate and the weld zone are correspondingly higher than 5052. The magnesium content for alloy 5456 pushes the limits of commercial rolling capabilities due to the high flow stress—and is not popular with the mills asked to produce it. Consequently, its usage has been restricted mostly to naval vessels. CORROSION Corrosion is an important issue with the 5xxx shipbuilding alloys. The magnesium in these alloys is primarily in solid solution. However, when the alloy is exposed for long service times at temperatures between about 50° and 100°C, the magnesium will combine with aluminum atoms to form precipitates on the grain boundaries (Fig. 2). In this condition, the microstructure is referred to as “sensitized.” A continuous network of grain boundary precipitates, a corrosive environment (sea water), and stresses due to welding or other causes result in perfect conditions for stress corrosion cracking (SCC). Normally, SCC in aluminum products is an issue Fig. 1 — Small boat construction showing: (a) cross members and supporting structure for the deck; and (b) longitudinal ribs and hull sheets. (a) (b) TABLE 1 — COMPOSITIONS OF COMMON 5XXX MARINE-USE ALLOYS Alloy Year Si Fe Cu Mn Mg Cr Zn Ti 5052 1935 0.25 0.40 0.10 0.10 2.2-2.8 0.15-0.35 0.15 --- 5086 1955 0.40 0.50 0.10 0.2-0.7 3.5-4.5 0.05-0.25 0.25 0.15 5186 1996 0.40 0.45 0.25 0.2-0.5 3.8-4.8 0.15 0.40 0.15 5083 1954 0.40 0.40 0.10 0.40-1.0 4.0-4.9 0.05-0.25 0.25 0.15 5383 1995 0.40 0.25 0.20 0.7-1.0 4.0-5.2 0.25 0.40 0.15 5456 1956 0.25 0.40 0.10 0.50-1.0 4.7-5.5 0.5-0.20 0.25 0.20
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