锻造和铸造铝合金的一些特性概述(英文科普)

Afeng · 发表于 2016-11-29 22:17:15

锻造和铸造铝合金的一些特性概述(英文科普)www.mejxw.com Afeng
锻造铝合金的优点

▓ Corrosion Resistance（耐蚀特性）. As a result of a naturally occurring tenacious surface oxide film, many aluminum alloys provide exceptional resistance to corrosion in many atmosphere and chemical environments. Alloys of the 1xxx, 3xxx, 5xxx, and 6xxx systems are especially favorable in this respect and are even used in applications where they are indirect contact with seawater and antiskid salts. With some electrocoating enhancements （e.g., anodizing）, the oxide coating can be thickened for even greater protection.

▓ Thermal Conductivity（导热特性）.Aluminum and its alloys are good conductors of heat, and, while they melt at lower temperatures than steels, about 1000ºF （about 535ºC）,they are slower to reach very high temperatures than steel in fire exposure.

▓ Electrical Conductivity（导电特性）. Pure aluminum and some of its alloys have exceptionally high electric conductivity（i.e., very low electrical resistivity）, second only to copper among common metals as conductors.

▓ Strength/Weight Ratio（比强度）. The combination of relatively high strength with low density means a high strength efficiency for aluminum alloys and many opportunities for replacement of heavier metals with no loss （and perhaps a gain） in load-carrying capacity.This characteristic, combined with the excellent corrosion resistance and recyclability, has led to aluminum’s broad use in containers, aircraft, and automotive applications.

▓ Fracture Toughness and EnergyAbsorption Capacity（断裂韧度和能量吸收）.Many aluminum alloys are exceptionally tough andexcellent choices for critical applications where resistance to brittle fracture and unstable crack growth are imperatives. Alloys of the 5xxx series,for example, are prime choices for liquified natural gas （LNG） tankage. And special high-toughness versions of aircraft alloys, such as 2124, 7050, and7475, replace the standard versions of these alloys for critical bulk head applications.

▓ Cryogenic Toughness（低温韧性）.Aluminum alloys, especially of the 3xxx, 5xxx, and 6xxx series, are ideal for very low temperature applications because the ductility and toughness as well asstrength of many alloys at subzero temperatures are as high as or higher thanat room temperature, even down to near absolute zero.

▓ Fatigue Strength（疲劳强度）.On an efficiency basis （strength to density） the fatigue strengths of many aluminum alloys are comparable to those of steels.

▓ Modulus of Elasticity（弹性模量）.Aluminum alloys have elastic moduli about one third those of steels （about 10,000,000 psi vs. about 30,000,000 psi）, so they absorb about three times as much elastic energy upon deformation to the same stress. They also deflect three times moreunder load.

▓ Workability（加工特性）.Aluminum alloys are readily workable by a great variety of metal-working technologies andespecially amenable to extrusion （the process of forcing heated metal through shaped dies to produce specific shaped sections）. This enables aluminum to be produced in a remarkable variety of shapes and forms in which the metal can be placed in locations where it can most efficiently carry the applied loads.

▓ Ease of Joining（易于联接）.Aluminum alloys may be joined by a very broad variety of commercial methods including welding, brazing, soldering, riveting, bolting and even nailing in addition to an unlimited variety of mechanical procedures. Welding, while considered difficult by those familiar only with joining steel and who try to apply the same techniques to aluminum, is particularly easy when performed by proven techniques such as gas–metal arc welding （GMAW or MIG） or gas–tungsten arc welding （GTAW）.

▓ Recyclability（可回收）. Aluminum and its alloys are among the easiest to recycle of any structural materials. And they are recyclable in the truest sense, unlike materials that are reused but in lower quality products: Aluminum alloys may be recycled directly backinto the same high-quality products like rigid container sheet （cans） and automotive components.

2铸造铝合金的优点

The desirable characteristics of wrought alloys are also generally applicable to cast alloys, but in fact the choice of one casting alloy or another tend to be more often made on the basis of their relative abilities to meet one or more of the following characteristics:

▓ Ease of Casting（易于铸造）.Many aluminum casting alloys have relatively high-silicon contents that provide excellent flow characteristics during casting, enabling them to be utilized forlarge and complex castings （even complete engines, for example）. Relatively minute details in the shape of the casting can be accurately and reliably replicated.

▓ High Strength（高强度）. Many aluminum casting alloys respond to heat treatment following casting and achieve relatively high levels of strength and excellent strength/weight ratios. With careful design of molds, high chill rates can be assured and both high strength and high toughness can be achieved.

▓ Quality of Finish（表面光洁）. By proper selection of aluminum casting alloy, extremely fine surface quality canbe achieved. While such alloys typically require more attention to casting practice typically, they are widely used in applications where the finished casting surface mirrors the finish needed in surface.

▓ Some Points（其它要点）.Unfortunately few casting alloys possess all three characteristics. The high-silicon 3xxx series are outstanding with respect to ease of casting as the relatively high silicon contents lend a characteristic of good flow and mold filling capability.

As aresult, the 3xxx series are the most widely used and especially chosen forlarge and very complex casting. The 2xxx alloys typically provide the very highest strengths but are more difficult to cast and lack good surface characteristics.

Therefore their use is usually limited to situations where expert casting techniques can be applied and where strength and toughness areat a premium, as in the aerospace industry. The 5xxx and 7xxx series are noteworthy for the fine finish they provide, but they are more difficult to cast than the 3xxx series and so are usually limited to those applications where that finish is paramount. A good example is the use of 7xxx alloys for bearings.

3铸造和锻造铝合金的缺点

There are several characteristics of aluminum alloys that require special attention in alloy selection or design:

▓ Moduli of Elasticity（弹性模量）.As noted earlier, the elastic moduli of aluminum alloys are about one third those of steel. In applications such as bridges, where somedesigns may be deflection critical, consideration should be given to the fact that aluminum alloys will deflect about three times more than comparably sized steel members; in order to compensate for this aluminum members subject to bending loads are usually made deeper or thicker in their upper and lower extremities to reduce stresses and/or deflections.

▓ Melting Temperature（熔点）.Aluminum alloys melt at about 1000ºF （535ºC）, well below where steels melt, and so they should not be selected for applications such asflue pipes and fire doors where the low melting point may result in unsatisfactory performance. The useful limit of high-temperature structural application of aluminum alloys is about 500ºF （about 260ºC） for conventional alloys or about 600ºF （315ºC） for alumina-enhanced alloys. It is useful to note that even in the most intense fires, aluminum alloys do not burn; they are rated noncombustiblein all types of fire tests and achieve the highest ratings in flamespread tests.Further, as noted earlier, they are slower to reach high temperatures in fires than other metals such as steels because of their higher thermal conductivity and emmissivity. Nevertheless, they should not be used where service requirements include structural strength above 500ºF or exposure above 1000ºF.

▓ Stress–Corrosion Susceptibility of Some Alloys（一些合金具有应力腐蚀敏感性）. Some aluminum alloys, notably the 2xxx and 7xxx alloys, when stressed perpendicular to the major plane of grain flow （i.e., in the short-transverse direction） may be subject to intergranular stress–corrosion cracking （SCC） unless they have been given a special thermal treatment to reduce or eliminate this type of behavior. If short transversestresses are anticipated in relatively thick components, 2xxx alloys should only be utilized in the T6- or T8-type tempers （not T3- or T4-type tempers）, and 7xxx alloys should only be used in the T73-type tempers （not T6- or T8-typetempers）. Similarly, 5xxx alloys with more than 3% Mg should not be used in applications where a combination of high stress and high temperature will beexperienced over a long period of time （more than several hundred hours）,because some susceptibility to SCC may be encountered （i.e., the alloys maybecome ‘‘sensitized’’）; for applications where temperatures above about 150ºF are likely to be encountered for long periods, the use of 5xxx alloys with 3% or less Mg is recommended.

▓ Mercury Embrittlement（汞脆）. Aluminum alloys should never be used when they may be indirect contact with liquid or vaporized mercury; severe grain boundary embrittlement may result. This is an unlikely exposure for the vast majority of applications, but in any instance where there is the possibility of mercury being present, the use of aluminum alloys should be avoided.