Marine Applications of Aluminum Alloys
haomeialu > 02-10-2018, 01:23 AM
Aluminum is used in hulls, deckhouses, and hatch covers of commercial ships, as well as in equipment items, such as ladders, railings, gratings, windows, and doors.
The major incentive for employing aluminum is its weight saving compared to steel. Because it is common practice to use weldable aluminum alloys having strengths approaching or comparable to mild steel, equal-strength structures can be designed to a weight saving of 55 to 67%. However, to compensate for the lower modulus of elasticity of aluminum and to conform to normal deflection limitations, a somewhat lower, but substantial, reduction in weight is usually obtained.
Aluminum is used in hulls, deckhouses, and hatch covers of commercial ships, as well as in equipment items, such as ladders, railings, gratings, windows, and doors. The major incentive for employing aluminum is its weight saving compared to steel. Because it is common practice to use weldable aluminum alloys having strengths approaching or comparable to mild steel, equal-strength structures can be designed to a weight saving of 55 to 67%. However, to compensate for the lower modulus of elasticity of aluminum and to conform to normal deflection limitations, a somewhat lower, but substantial, reduction in weight is usually obtained.
The principal advantages of weight saving in many types of marine vessels are to increase payload, to expand capacity for equipment, and to decrease the power required. With other types of vessels, the chief benefit is to permit better distribution of the weight, improving stability and facilitating efficient hull design. The use of aluminum normally results in initial cost premiums that are justified over the life of the application by the benefits of lightweight and low maintenance cost.
The weight saving achieved depends on the approach to design, which varies with different applications. Where known or rule loadings exist for specific structures, normal design principles are applied along with consideration of the mechanical properties of specific alloys.
Alloys. The 5xxx series alloys used for the majority of commercial marine applications have weld yield strengths of 100 to 200 MPa. These aluminum-magnesium alloys retain good weld ductility without postweld heat treatment, and they can be fabricated with normal shipyard techniques and equipment. The weldable aluminum-magnesium-zinc alloys are also receiving attention in this field. The corrosion resistance of the 5xxx series alloys is another major factor in the selection of aluminum for marine applications. Tensile strength reductions in 10-year sea-water corrosion tests of 1.62mm (0.064-in.)-thick bare sheet specimens are only 2 to 5%. The 6xxx series alloys, widely used for pleasure boats, show a 5 to 7% decrease in similar tests.
Alclad aluminum alloys are seldom required in construction of marine vessels. They are used, however, in a few applications, such as piping, for maximum assurance against excessive depth of pitting. Also, alclad 2xxx and 7xxx series alloys are selected where tensile strengths of 70,000 to 80,000 psi (482.6-551.2 MPa) are required, considerably higher than now available in the 5xxx series alloys.
The high-strength alloys are employed where welding is not required, and where their higher strengths can be used to advantage. Because of their lower resistance to corrosion by sea water, protective measures such as cladding, painting, or cathodic protection must be used for satisfactory life in marine service.