ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 25 for high-strength aircraft alloys such as 7075-T6, but this particular exception exists for sensitized 5xxx alloys with more than 3% magnesium content. In 2003, The Aluminum Association revised the temper specification to be applied for 5xxx alloys used in shipbuilding. The need for the new spec arose when problems were discovered with nearly 200 vessels produced by shipbuilders in the Pacific Northwest[6]. Newly launched vessels were leaking and heavy pitting was discovered along with SCC cracks adjacent to welds (Fig. 3). A team including the U.S. Coast Guard, ship designers, and aluminum producers was formed to investigate the problem. After the team eliminated issues with material identification, design, and welding practices, they considered the condition of the 5083H321 temper sheet. The microstructures resembled those shown in Fig. 2. Inquiries at the rolling mill revealed that a change in the thermal practices to achieve the H321 temper had been mistakenly applied to the problematic material. In other words, the material was delivered in a severely sensitized condition. It was clear that this mistake should never be repeated within the industry. The task group drafted a new standard for the temper employed for marine construction: ASTM standard B928, “Standard Specification for High Magnesium Aluminum Alloy Products for Marine Service.” The standard replaced B209, which had been in effect since the Vietnam War era. Certification for the new H321 marine temper requires that metal pass a laboratory test to demonstrate it will not experience severe grain boundary attack in a sensitized state. ALLOYS FOR OUTBOARD ENGINES Another issue encountered in the marine environment is the potential for corrosion or damage to aluminum outboard engines. Motors and propellers can be subject to impact with underwater objects and will be immersed in whatever water chemistry exists at a particular locale. Unlike car engines, motors for boats are cooled by water intake from the river, lake, or ocean. Mercury Marine, a division of Brunswick Corp., has been producing castings for marine engines and components for nearly 70 years. The company’s research into aluminum castings resulted in several highperformance “Mercalloy” brand products with improved toughness, fatigue strength, and corrosion resistance that can be produced at competitive costs using a high volume of recycled metal. Mercury Marine has successfully produced these alloys over the years for marine engines and other parts[7]: • Mercalloy 368 for high-ductility propellers and ease of damage repair by welding • Mercalloy 367 for drivesha housings, which connect the outboard engine to the propeller, with superior impact resistance during collision with an underwater object • Mercalloy 362 with high fatigue strength characteristics to facilitate engine weight reduction • Mercosil 391, a hypereutectic Al-Si alloy used for marine engine applications as well as various bearing surfaces requiring good wear resistance Low limits on copper and other impurities promote excellent corrosion resistance without the need for expensive coatings. FRICTION STIR WELDING Friction stir welding (FSW) has been used commercially in the shipbuilding industry since its invention by TWI in the 1990s[8,9]. The early applications of FSW panels were made by Scandinavian extruders and shipbuilders. Today, the technology is used in shipyards in Australia, North America, Europe, Japan, and China. Wide FSW panels can be prefabricated and then loaded onto the ship for assembly, saving total construction time— especially useful for multistory vessels such as cruise ships. The wide panels avoid problems such as porosity and weld cracking and offer minimal heat distortion and low residual stress levels. Thus, the flatness and tolerances of FSW panels are superior to their fusion-welded counterparts. Further, FSW joints are stronger and more consistent than common fusion-welded joints. The FSW joints are neat and clean and normally do not need any dressing or rework (Fig. 4). Friction stir welding has been applied Fig. 2 — Aluminum ship corrosion showing 5083 plate microstructure in sensitized condition. Fig. 3 — Stress corrosion cracking adjacent to welds in a ship structure. Fig. 4 — Wide FSW panels used for shipbuilding. Courtesy of Hydro Aluminum.
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