Aluminium alloy
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper,magnesium, mangane, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is ud for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminium alloys are widely ud in engineering structures and components where light weight or corrosion resistance is required.[1]
wonderful mamaAluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups,[3] and ASTM International.
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Engineering u
somewhat-higher tensile strengths than the commonly ud kinds of steel, simply replacing a steel part with an aluminium alloy might lead to problems.
With completely new metal products, the design choices are often governed by the choice of manufacturing technology. Extrusions are particularly important in this regard, owing to the ea with which aluminium alloys, particularly the Al-Mg-Si ries, can be extruded to form complex profiles.
In general, stiffer and lighter designs can be achieved with aluminium alloys than is feasible with steels. For instance, consider the bending of a thin-walled tube: the cond moment of area is inverly related to the stress in the tube wall, i.e. stress are lower for larger values. The cond moment of area is proportional to the cube of the radius times t he wall thickness, thus increasing the radius (and weight) by 26% will lead to a halving of the wall stress. For this reason, bicycle frames made of aluminium alloys make u of larger tube diameters than steel or titanium in order to yield the desired stiffness and strength. In automotive engineering, cars made of aluminium alloys employ space frames made of extruded profiles to ensure rigidity. This reprents a radical change from the common approach for current steel car design, which depend on the body shells for stiffness, that is a unibody design.
Aluminium alloys are widely ud in automotive engines, particularly in cylinder
blocks and crankcas due to the weight savings that are possible. Since aluminium alloys are susceptible to warping at elevated temperatures, the cooling system of such engines is critical. Manufacturing techniques and metallurgical advancements have also been instrumental for the successful application in automotive engines. In the 1960s, the aluminium cylinder
heads of the Corvair earned a reputation for failure and stripping of threads, which is not en in current aluminium cylinder heads.
An important structural limitation of aluminium alloys is their lower fatigue strength compared to steel. In controlled laboratory conditions, steels display a fatigue limit, which is the stress amplitude below which no failures occur - the metal does not continue to weaken with extended stress cycles. Aluminum alloys do not have this lower fatigue limit and will continue to weaken with continued stress cycles. Aluminium alloys are therefore sparly ud in parts that require high fatigue strength in the high cycle regime (more than 107 stress cycles). Heat nsitivity considerations
Often, the metal's nsitivity to heat must also be considered. Even a relatively routine workshop procedure involving heating is complicated by the fact that aluminium, unlike steel, will melt without first glowing red. Forming operations where a blow torch is ud therefore require some experti, becau no visual signs reveal how clo the material is to melting.quarter
Aluminium also is subject to internal stress and strains when it is overheated; the tendency of the metal to creep under the stress tends to result in delayed distortions. For example, the warping or cracking of overheated aluminium automobile cylinder heads is commonly obrved, sometimes years later, as is the tendency of welded aluminium bicycle frames to gradually twist out of alignment from the stress of the welding process. Thus, the aerospace industry avoids heat altogether by joining parts with adhesives or mechanical fasteners. Adhesive bonding was ud in some bicycle frames in the 1970s, with unfortunate results when the aluminium tubing corroded slightly, looning the adhesive and collapsing the frame.
Stress in overheated aluminium can be relieved by heat-treating the parts in an oven and gradually cooling it—in effect annealing the stress. Yet the parts may still become distorted, so that heat-treating of welded bicycle frames, for instance, can result in a significant fraction becoming misaligned. If the misalignment is not too vere, the cooled parts may be bent into alignment. Of cour, if the frame is properly designed for rigidity (e above), that bending will require enormous force.
Aluminium's intolerance to high temperatures has not precluded its u in rocketry; even for u in constructing combustion chambers where gas can reach 3500 K. The Agena upper stage engine
ud a regeneratively cooled aluminium design for some parts of the nozzle, including the thermally critical throat region; in fact the extremely high thermal conductivity of aluminium prevented the throat from reaching the melting point even under massive heat flux, resulting in a reliable lightweight component.crowds
Houhold wiring
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Main article: Aluminium wire
Becau of its high conductivity and relatively low price compared with copper in the 1960s, aluminium was introduced at that time for houhold electrical wiring in North America, even though many fixtures had not been designed to accept aluminium wire. But the new u brought some problems:
∙The greater coefficient of thermal expansion of aluminium caus the wire to expand and contract relative to the dissimilar metal screw connection, eventually looning the connection.
∙Pure aluminium has a tendency to "creep" under steady sustained pressure (to a greater degree as the temperature ris), again looning the connection.
∙Galvanic corrosion from the dissimilar metals increas the electrical resistance of the connection.
All of this resulted in overheated and loo connections, and this in turn resulted in some fires. Builders then became wary of using the wire, and many jurisdictions outlawed its u in very small sizes, in new construction. Yet newer fixtures eventually were introduced with connections designed to avoid looning and overheating. At first they were marked "Al/Cu", but they now bear a "CO/ALR" coding.
Another way to forestall the heating problem is to crimp the aluminium wire to a short "pigtail" of copper wire. A properly done high-pressure crimp by the proper tool is tight enough to reduce any thermal expansion of the aluminium. Today, new alloys, designs, and methods are ud for aluminium wiring in combination with aluminium terminations.
Wrought alloys
The International Alloy Designation System is the most widely accepted naming scheme
for wrought alloys. Each alloy is given a four-digit number, where the first digit indicates the major alloying elements.
∙1000 ries are esntially pure aluminium with a minimum 99% aluminium content by weight and can be work hardened.
∙2000 ries are alloyed with copper, can be precipitation hardened to strengths comparable to steel. Formerly referred to as duralumin, they were once the most common aerospace alloys, but were susceptible to stress corrosion cracking and are increasingly replaced by 7000 ries in new designs.
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圆脸适合什么样的发型∙3000 ries are alloyed with mangane, and can be work hardened.
∙4000 ries are alloyed with silicon. They are also known as silumin.
∙5000 ries are alloyed with magnesium.
∙6000 ries are alloyed with magnesium and silicon, are easy to machine, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach.
∙7000 ries are alloyed with zinc, and can be precipitation hardened to the highest strengths of any aluminium alloy.
∙8000 ries is a category mainly ud for lithium alloys.[citation needed]
Cast alloys
The Aluminum Association (AA) has adopted a nomenclature similar to that of wrought alloys. British Standard and DIN have different designations. In the AA system, the cond two digits reveal the minimum percentage of aluminium, e.g. 150.x correspond to a minimum of 99.50% aluminium. The digit after the decimal point takes a value of 0 or 1, denoting casting and ingot respectively.[1] The main alloying elements in the AA system are as follows:[citation needed]
∙1xx.x ries are minimum 99% aluminium
∙2xx.x ries copper
自我介绍英文∙3xx.x ries silicon, copper and/or magnesium
∙4xx.x ries silicon英汉词典手机版
∙5xx.x ries magnesium
∙7xx.x ries zinchometown
∙8xx.x ries lithium
Aerospace alloys
Scandium-Aluminium
Parts of the Mig–29 are made from Al-Sc alloy.[9]
The addition of scandium to aluminium creates nanoscale Al3Sc precipitates which limit the excessive grain growth that occurs in the heat-affected zone of welded aluminium components. This has two beneficial effects: the precipitated Al3Sc forms smaller crystals than are formed in other aluminium alloys[9] and the width of precipitate-free zones that normally exist at the grain boundaries of age-hardenenable aluminium alloys is reduced.[9] Scandium is also a potent grain refiner in cast aluminium alloys, and atom for atom, the most potent strengthener in aluminium, both as a result of grain refinement and precipitation strengthening.
However, titanium alloys, which are stronger but heavier, are cheaper and much more widely ud.[10]
