2024年6月11日发(作者：濮阳怜南)
Aluminium alloy
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical
alloying elements are copper,magnesium, manganese, 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 used 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 used in engineering structures and components
where light weight or corrosion resistance is required.
[1]
Alloys composed mostly of aluminium have been very important in aerospace
manufacturing since the introduction of metal skinned aircraft. Aluminium-magnesium alloys
are both lighter than other aluminium alloys and much less flammable than alloys that contain
a very high percentage of magnesium.
Aluminium alloy surfaces will keep their apparent shine in a dry environment due to the
formation of a clear, protective layer of aluminium oxide. In a wet environment, galvanic
corrosion can occur when an aluminium alloy is placed in electrical contact with other metals
with more negative corrosion potentials than aluminium.
Aluminium 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, and ASTM International.
[3]
[2]
Engineering use
Overview
Aluminium alloys with a wide range of properties are used in engineering structures. Alloy
systems are classified by a number system (ANSI) or by names indicating their main alloying
constituents (DIN and ISO). Selecting the right alloy for a given application entails
considerations of its tensile strength,density, ductility, formability, workability, weldability,
and corrosion resistance, to name a few. A brief historical overview of alloys and
manufacturing technologies is given in Ref. Aluminium alloys are used extensively in aircraft
due to their high strength-to-weight ratio. On the other hand, pure aluminium metal is much too
soft for such uses, and it does not have the high tensile strength that is needed
for airplanes and helicopters.
[4]
Aluminium alloys versus types of steel
Aluminium alloys typically have an elastic modulus of about 70 GPa, which is about one-third
of the elastic modulus of most kinds of steel and steel alloys. Therefore, for a given load, a
component or unit made of an aluminium alloy will experience a greater elastic deformation
than a steel part of the identical size and shape. Though there are aluminium alloys with
somewhat-higher tensile strengths than the commonly used 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
ease with which aluminium alloys, particularly the Al-Mg-Si series, 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 second moment of
area is inversely related to the stress in the tube wall, i.e. stresses are lower for larger values.
The second moment of area is proportional to the cube of the radius times the 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 use 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 represents 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 used in automotive engines, particularly in cylinder
blocks and crankcases 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 seen
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 afatigue 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 sparsely used in parts
that require high fatigue strength in the high cycle regime (more than 10 stress cycles).
7
Heat sensitivity considerations
Often, the metal's sensitivity 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 used therefore
require some expertise, because no visual signs reveal how close the material is to melting.
Aluminium also is subject to internal stresses and strains when it is overheated; the tendency
of the metal to creep under these stresses tends to result in delayed distortions. For example,
the warping or cracking of overheated aluminium automobile cylinder heads is commonly
observed, sometimes years later, as is the tendency of welded aluminium bicycle frames to
gradually twist out of alignment from the stresses of the welding process. Thus, the aerospace
industry avoids heat altogether by joining parts with adhesives or mechanical fasteners.
Adhesive bonding was used in some bicycle frames in the 1970s, with unfortunate results
when the aluminium tubing corroded slightly, loosening the adhesive and collapsing the frame.
2024年6月11日发(作者：濮阳怜南)
Aluminium alloy
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical
alloying elements are copper,magnesium, manganese, 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 used 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 used in engineering structures and components
where light weight or corrosion resistance is required.
[1]
Alloys composed mostly of aluminium have been very important in aerospace
manufacturing since the introduction of metal skinned aircraft. Aluminium-magnesium alloys
are both lighter than other aluminium alloys and much less flammable than alloys that contain
a very high percentage of magnesium.
Aluminium alloy surfaces will keep their apparent shine in a dry environment due to the
formation of a clear, protective layer of aluminium oxide. In a wet environment, galvanic
corrosion can occur when an aluminium alloy is placed in electrical contact with other metals
with more negative corrosion potentials than aluminium.
Aluminium 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, and ASTM International.
[3]
[2]
Engineering use
Overview
Aluminium alloys with a wide range of properties are used in engineering structures. Alloy
systems are classified by a number system (ANSI) or by names indicating their main alloying
constituents (DIN and ISO). Selecting the right alloy for a given application entails
considerations of its tensile strength,density, ductility, formability, workability, weldability,
and corrosion resistance, to name a few. A brief historical overview of alloys and
manufacturing technologies is given in Ref. Aluminium alloys are used extensively in aircraft
due to their high strength-to-weight ratio. On the other hand, pure aluminium metal is much too
soft for such uses, and it does not have the high tensile strength that is needed
for airplanes and helicopters.
[4]
Aluminium alloys versus types of steel
Aluminium alloys typically have an elastic modulus of about 70 GPa, which is about one-third
of the elastic modulus of most kinds of steel and steel alloys. Therefore, for a given load, a
component or unit made of an aluminium alloy will experience a greater elastic deformation
than a steel part of the identical size and shape. Though there are aluminium alloys with
somewhat-higher tensile strengths than the commonly used 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
ease with which aluminium alloys, particularly the Al-Mg-Si series, 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 second moment of
area is inversely related to the stress in the tube wall, i.e. stresses are lower for larger values.
The second moment of area is proportional to the cube of the radius times the 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 use 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 represents 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 used in automotive engines, particularly in cylinder
blocks and crankcases 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 seen
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 afatigue 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 sparsely used in parts
that require high fatigue strength in the high cycle regime (more than 10 stress cycles).
7
Heat sensitivity considerations
Often, the metal's sensitivity 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 used therefore
require some expertise, because no visual signs reveal how close the material is to melting.
Aluminium also is subject to internal stresses and strains when it is overheated; the tendency
of the metal to creep under these stresses tends to result in delayed distortions. For example,
the warping or cracking of overheated aluminium automobile cylinder heads is commonly
observed, sometimes years later, as is the tendency of welded aluminium bicycle frames to
gradually twist out of alignment from the stresses of the welding process. Thus, the aerospace
industry avoids heat altogether by joining parts with adhesives or mechanical fasteners.
Adhesive bonding was used in some bicycle frames in the 1970s, with unfortunate results
when the aluminium tubing corroded slightly, loosening the adhesive and collapsing the frame.