| Materialen in de metaalbewerking
ANODISEREN VAN ALUMINIUM [engelstalige bron] |
Aluminum is used for many parts because of its desirable properties, which
include low density (light weight), high specific strength (which is
commonly known as strength-to-weight ratio), its resistance to corrosion,
its thermal conductivity. Aluminum has approximately one third the density
of copper or steel, which is why its used where weight is an important consi-
deration. While it has just 50% of copper's thermal conductivity, it has four
times that of low-carbon steel.
Aluminum is extracted electrolytically from bauxite ore. It is made by the
electrolysis of aluminum oxide which is found in larger concentrations within
bauxite ore. Bauxite is a mixture of the hydroxides of aluminum, together
with other impurities such as oxides of iron, titanium, and silicon. Bauxite
is produced by the weathering and change of aluminum silicate rocks usually
found in tropical and semitropical regions where climate has produced an
accelerated weathering process. Bauxite is not a rare ore and is widely
available in the US, the Caribbean, and Europe. Approximately 4 pounds of ore
are required to produce 1 pound of aluminum. The process used almost
universally to purify bauxite is the Bayer process, which separates aluminum
hydydrate from the bauxite and then uses a calcination process to convert it
to oxide of aluminum, which has 2 aluminum and 3 oxygen atoms.
The aluminum oxide is dissolved in electric furnaces, (resembling melting
pots) in a molten bath of sodium-aluminum fluoride at 940 to 980 degrees
centigrade (1725 to 1800 degrees fahrenheit). Using a method developed by
Charles M. Hall in 1886, the furnace pots are made of carbon lined steel.
The carbon is also an electrode, in this case an anode, and current intro-
duced through it electrolytically separates the aluminum and also provides
the heat necessary to keep the bath molten. With electricity applied, the
oxygen in the ore combines with the carbon in the pot, leaving at the bottom
of the pot or vessel 99.9% pure molten aluminum. The molten aluminum is
removed periodically from the bottom of the furnace or cell as it collects.
It takes approximately 9 to 10 kilowatt hours of electricity to make each
pound of aluminum, for that reason aluminum refining is done in areas of
the world where electricity is relatively cheap. The molten aluminum, once
siphoned off, is poured into molds to form what is known as a primary ingots.
If alloying with other metals is desired, the molten aluminum is sent to a
remelt furnace where pure alloying elements or master alloys (concentrated
alloys within an aluminum base) are added to produce the desired aluminum
alloy. The alloyed aluminum is then poured into molds to make primary alumi-
num ingots.
| #3.Aluminum Casting / Wrought |
The ingots can be remelted to make cast aluminum products, using various
methods of casting including, die casting where molten aluminum is injected
under high pressure into the cavity of a metal die. Aluminum alloys have a
reasonably low melting point which makes a dense, fine-grain surface struc-
ture with excellent wear and fatigue properties when die cast. Also permanent
mold casting may be used, which uses a metal mold repeatedly for producing
many castings of the same form. These casting techniques are the way many
crank arms, pedal bodies, hub shells, seatpost head pieces, stems, and some
headset parts are commercially made in volume.
The ingot can also be mechanically worked to make wrought aluminum products.
The designation wrought indicates that the aluminum, when it leaves the mill,
takes the form of a worked product, which includes, sheet, foil or plate
aluminum. It also includes rod, bar, or tubing, and includes extrusions, and
some forgings. The transformation from ingot to wrought product is known as
working the metal. The working operations and thermal treatments transform
the cast ingot's metallurgic structure into a wrought structure whose grain
and crystalline structure may range from fully recrystallized (re-melted) to
fibrous depending on the metallurgic characteristics of the alloy, the work
techniques employed, and the product manufactured. This metallurgic structure
influences the strength, corrosion resistance, and several other properties
of the finished good. We find wrought aluminum must generally be machined
again and assembled into the finished part. This is the basic material, in
its many forms, that is used by small run, or one-off manufacturers parts.
The wrought alloy pieces are what the more expensive after market replacement
parts are engineered and machined from, including, hubs, seatposts, stems,
bottom bracket cups, headset parts, some pedal bodies, nearly all aluminum
handlebars, all rims, many expensive crank arms, and the alloy rail assembly
found in a few saddles.
Another technique for aluminum alloy manufacture is known quot or Powder
Metallurgy. This process involves compressing under great pressure and heat,
a powdered form of the alloy within a shaped die. The compressed powders at
high temperature densify into a solid, shaped piece. This technique could be
used effectively to mass produce headset cups.
| #4.Alloy Composition/Designation |
Aluminum alloy composition designations, in the United Sates, are made under
the guidance of the Aluminum Association. The major aluminum producers have
agreed upon a four digit numerical system for designating specifically the
composition of wrought aluminum and aluminum alloys, while a three digit
system is used for casting aluminum alloys. The first of the four digits in
the alloy number indicates the alloy group which identifies the primary
alloying element or elements within the alloy.
The 1XXX group is at least 99% pure aluminum
The 2XXX group is alloyed primarily with Copper, which is added for higher
strength but reduces the corrosion resistance
The 3XXX group is alloyed primarily with Manganese, for moderate strength
The 4XXX group is alloyed primarily with some Silicon to lower the melting
point and increase fluidity for casting
The 5XXX group is alloyed primarily with Magnesium to make a moderate
strength alloy
The 6XXX group is alloyed primarily with both Magnesium and Silicon to make
a moderate strength alloy
The 7XXX group is alloyed primarily with Zinc to make a high strength alloy.
The 8XXX group is used to indicate an alloy whose primary alloy element is
other than those above
The 9XXX group has not so far been used or assigned, but with the cold war
over it wouldn't be surprising to find that the defense industry may have
privately had something in the 9XXX group held secret
In the alloys designated from the 2XXX to the 7XXX group, the last two digits
identify the uniquely different alloys in the series. As new alloys become
commercially available, the last two digits are assigned consecutively
beginning with XX01, (wouldn't you think they would be running short on
these by now?). The two digits bring with them a specific chemical
composition range, which must be adhered to.
The second digit indicates any modifications in the composition of the
original alloy used. If the second digit is 0, zero, there have been no
modifications made to the original assigned and designated alloy. If
modifications are made to the original alloy, integers 1 to 9, which are
assigned consecutively, are used to indicate the modification, and note
commonly its existence. New, experimental aluminum alloys are grouped with
the appropriate 1XXX to 9XXX series above but are prefixed with an X. The
X is discontinued when the alloy becomes standard.
Do not make the mistake of believing that a higher alloy designation number
means the aluminum alloy is stronger, harder, or more resistant to failure.
Sometimes that is true but the alloy's temper also says something about its
strength, and hardness.
Aluminum alloys are sometimes tempered. Tempering is the process of raising
the alloy's hardness. The vernacular term for temper in the industry is to
say that it's heat treated, which is borrowed term from the aluminum industry,
but the words don't reflect the actual process which might better be called
solution heat treatment or thermal treatment.
Temper designations for aluminum and its alloys fall into two general
categories, which depend on the alloy's responsiveness to heat treatment,
the heat-treatable and the non-heat-treatable. The non-HT type generally
fall into the 3XXX and the 5XXX series, which owe their strength to the
hardening effects of either manganese or magnesium. They can be strengthened
however by cold working. The 2XXX, 6XXX, and 7XXX series which have copper,
magnesium, and zinc along with small percentages of other elements are known
as heat treatable alloys.
These alloying elements show increasing solubility in aluminum at raised
temperatures, the alloys can be strengthened through solution heat treatment,
also known as thermal or heat treatment. The alloy can also be strengthened
through strain hardening.
Greater strength can be achieved through cold working, where the alloy can be
subjected to a strain hardening or work hardening process. Cold working
involves mechanically changing the shape through rolling, drawing,
straightening, or flattening the aluminum into another shape, without
remelting it. When the alloy is remelted, it is said to be recrystallized.
(Strain hardening increases the strength and hardness of a metal, and
correspondingly decreases its ductility, making it less able to stretch or
bend, in essence, making it more brittle.)
Remember, metals, like many other compounds, are made from a crystal
structure. The sides of the crystal lie next to one another with common flat
surfaces or planes. As the crystals of metal grow, while cooling, they clump
into smaller masses of cooling metal, until the piece reaches a common lower
temperature. The clumps take the form of grains within the metal. The grains
have crystallographic planes in various orientations. The grain of the metal,
when cooled, establishes the reasonably stable structure of the metal. When
the metal is cold worked, the crystal forming the grain shears or slips along
the crystallographic plane, against one another, leading to an even more
stable structure. With increased cold working the grains elongate in the
direction of the work.
The greater the metal is squeezed to reduce its original cross section the
more elongated the grains become. This increases the density, squeezing the
grain, reducing what ever space exists microscopically. The crystals are
forced to slip against one another to further compress out microscopic space.
Extreme cold working leaves very few crystallographic planes unaffected by
this slip as the hardness, yield strength, and tensile strength increase.
Remember, increased strength decreases ductility. Cold working, or strain
hardening of metal changes its internal grain characteristic. The metal
parallel to the direction of cold working, (the direction that the metal
squeezed, elongating its grain), exhibits an increase in tensile strength,
yield strength and hardness. The opposite occurs when these properties are
measured in the perpendicular or transverse direction.
The concept of metal having grain directional properties is called anistropy
Anisotropy is desirable if the cold worked metal is loaded (having applied
force), in a way that uses the increased strength developed in the cold
working direction. Metal that has this increased strength in all directions,
not just the worked grain, is said to be isotropic and is generally achieved
through annealing heat treatment. This process consists of heating the metal
to a suitable temperature, holding it at temperature, and then cooling it at
a suitable rate. During the annealing, the metal reverts back to a softer
condition with three major changes occurring to the crystal and grain
structure, which are known as recovery, recrystalization, and then grain
growth. In essence you've begun the process over again.
But what is solution heat treatment and this solubility? This process
involves heating the aluminum alloy to a temperature where the alloying
element or elements are dissolved into what is known as a solid solution
Solution heat treatment, referred in some cases to as heat treated is really
the fine melding of the alloy metal at a specific temperature. Not all alloy
compositions are capable of combining this way, when alloy elements are known
to not dissolve at elevated temperatures to a solution this way, they are
said to be insoluble.
Pure aluminum, for example, can't be strengthened through thermal treatment
because there are no alloying elements. The solution heat treatment process
consists of heating the aluminum and the alloying elements up to an appropri-
ate temperature to dissolve the elements in the aluminum. Time and temperature
control are an important part of solution heat treatment. During the heating
the alloy must approach, but not quite reach, the melting point. The beginning
of melting temperature is known as the alloy's solidus. To obtain the full
effect of solution heat treatment, the importance of temperature regulation
can't be understated, if the alloy temperature is not controlled well, there
is the possibility of localized melting, which is referred to as burning.
When an area of the alloy actually melts, you find that area of the has now
expanded from the heat. Later as the metal cools, the tiny areas where the
localized expansion occurred through melting, shrink. This shrinking leaves a
tiny void in the metal which will be very effective in causing a failure from
stress, or fatigue in the future finished product. For this reason, burned
metal is near valueless, good only for its foundry scrap value. In this
combining process, the more sluggish the alloying elements are to dissolve,
the higher the temperatures must be, some solution treatment temperatures
reach up to 1,000 degrees fahrenheit. When the aluminum has decomposed the
alloying elements, the structure is called a solid solution, which usually
takes between 2 and 3 hours to form.
Solid solutions are formed when the base or solvent metal (aluminum)
incorporates atoms of the alloy element or solute metal. The final step of
solution heat treatment is to quench; or cool the material (rod, bar plate,
extrusion) rapidly with hot water to lower the material to room temperature.
Lowering the temperature arrests the structure of the metal obtained from the
thermal treatment. Small parts may be dipped in hot water, while large pieces
can be sprayed with hot water. Hot water is used because cold water has been
found to develop small cracks in some wrought aluminum pieces. The required
rate of cooling is more critical for some alloys than others, also the
thickness in cross section of the piece affects the cooling rate, in some
instances just still air may be adequate. At this point, the cooled alloy in
the solution treated condition, is in most cases fully annealed, rendering
the metal both less brittle and more soft.
Cold working, (discussed above), may be carried out after the solution heat
treatment to increase the metal's strength, however, properties of the
solution heat treatment alloy may spontaneously change with the passage of
time at room temperature. The solution treated alloy may noticeably increase
in strength due to a phenomenon called precipitation hardening. The alloy,
on its own, will naturally harden and strengthen from this solution treatment
process through this precipitation hardening or age hardening process.
|
Precipitation Hardening or Aging
|
Some aluminum alloys begin the aging process almost immediately after they
are quenched in the solution heat treatment process. After a period of a few
days aluminum alloys become considerably stronger. Those alloys that contain
magnesium and zinc or magnesium and silicon continue to age harden over a
long period of time at room temperature. As soon as the alloy reaches room
temperature, the elements added to the aluminum in making the alloy, begin to
form fine particles within the crystal and at the grain boundaries.
Technically the added alloy elements are known as phases and the action of
their separating out of the alloy microscopically is called precipitation.
More technically, for chemistry students, the alloy has become supersaturated
meaning at the lower room temperature the alloy is less capable of carrying
the dissolved phases and the phases, in attempting to separate, or precipitate
out, are trying to reduce the saturation content. In most alloys, precipi-
tation occurs very slowly at room temperature and becomes even slower at lower
temperatures where molecular movement is more difficult. In fact, this
precipitation action in most alloys can be completely stopped or arrested by
lowering the metal to temperatures. The converse is also true, if the
temperature of the solution treated alloy is raised, the action is greatly
increased and the process can be completed in relatively short time.
When the precipitation or aging process is allowed to be completed at room
temperature, over the necessary period of time, it is known as natural aging.
When the aging process is performed in heated or elevated temperature
conditions, it is said to be artificially aged. Whether the aging process is
performed naturally or artificially, the metal's structure goes through a
similar change and a submicroscopic precipitate will form throughout the
grain structure. The larger the precipitate formation, the less opportunity
there will be for atoms to dislocate into a cluster of slip planes. With
fewer common planes for the slip to occur, the metal structure becomes more
stable, which to the outward examiner would be noted as increased hardness,
tensile strength, and maximum yield strength.
|
Actual Temper Designations
|
Now that we have you up to speed on what the aluminum alloy hardening
processes actually are, and the terms are all defined, we can now tell you
what the most used temper designations are and what they really mean. Temper
designations occur as a suffix at the end of the alloy's numeric designation,
an example would be 6061-T6, the T6 is the temper designation.
Remember when we started this process, you probably thought heat treated had
something to do with flame or a torch.
- means the alloy is as fabricated, no special control over strain hardening
is noted
O - means that is has been annealed only, the alloy has been recrystallized,
this is the softest temper
H1 - means that it has strain hardened only
H2 - means that it has been strain hardened and partially annealed
H3 - means that it has been strain hardened and thermally stabilized
W - means that is has been solution heat treated
T1 - means that it has been partially solution heat treated (cooled from an
elevated-temperature shaping process such as extrusion), and naturally aged
T2 - means that it has been cooled from an elevated-temperature shaping
process, (casting), cold worked, and naturally aged
T3 - means that it has been solution heat treated, then cold worked and
naturally aged
T4 - means that it has been solution heat treated, and naturally aged, it
applies to alloys not cold worked after solution treatment, or where the
effect of cold working may not be recognized in applicable specifications
T5 - means that it has been partially solution heat treated and artificially
aged, the temper is produced after an elevated temperature, rapid cool
fabrication process, (like extrusion)
T6 - means that it has been solution heat treated and then artificially aged,
without cold working
T7 - means that it has been solution heat treated and stabilized to control
characteristics such as grain growth, distortion, or residual stresses
T8 - means that it has been solution heat treated, then cold worked, and
artificially aged
T9 - means that it has been solution heat treated, artificially aged, and
then cold worked
T10 - means that it has been partially solution treated (cooled from an
elevated shaping process, such as extrusion), cold worked, then artificially
aged.
| #6.Aluminum Surface Treatment - Anodizing |
Very few people actually know what anodizing is, and because so many bicycle
parts are made of aluminum and then anodized, it bears some discussion. We'll
first overview, and then detail the process.
Several metals are capable of being anodized, aluminum is the most common but
the process may also be applied to magnesium, titanium, and tantalum.
Anodizing is an electrochemical conversion process that changes the outer
structure of the metal, rather than an applied coating, like paint.
The anodizing of aluminum is performed by making the part that is to be
anodized, the anode or positive end of an electrical circuit within an acid
electrolyte. With electricity applied through the acid from the cathode an
oxide layer develops in and on the outer layer of the metal. This outer layer
can be formed so that it has a porous quality and the aluminum oxide layer
can be dyed in many colors.
Now the details. Aluminum, as we have mentioned, on exposure to air develops
a thin aluminum oxide film that seals the aluminum from further oxidation.
For most purposes, this thin oxide layer doesn't enhance the protection or
surface hardness enough, anodizing makes a much thicker oxide coating, up to
several thousandths of an inch thick. The anodizing process was developed in
the 1930's. Because it adds surface hardness, it has permitted aluminum to be
used in applications where it wouldn't have been considered before. The
hardness of the anodized aluminum oxide coating rivals that of a diamond, so
aluminum's abrasion resistance is enhanced. The added depth of the oxide layer
improves the aluminum's corrosion resistance, while making cleaning of the
surface easier, and potentially if the metal is dyed, more attractive.
The aluminum part is hung on metal racks which are designed to hold many of
the same parts spaced an even distance from one another. The racks can be made
of aluminum, but after each use they must be stripped of the anodized coating
that forms on them so they will continue to make good electrical contact, or
they may be made of more expensive commercially pure titanium, which needn't
be stripped after each use. The racks with the parts affixed are suspended in
a series of tanks that can be made of lead lined steel, stainless steel, lead
lined wood, fiberglass lined concrete or plastic. The yet to be anodized
aluminum part must first be cleaned and have its natural oxide coating
removed. The part on the rack is dipped in a tank for cleaning usually
containing a cleaning agent of the non-silicated, inhibited soak cleaner type
at 140 degrees to 160 degrees F temperature to remove all surface dirt.
After cleaning, the part is rinsed to avoid contaminating the solution in
subsequent tanks.
The next tank is used for de-oxidizing the part with an acid solution at a
temperature generally between 120 degrees and 160 degrees F. This acid bath
removes the natural, thin, non-uniform aluminum oxide surface on the aluminum.
The de-oxidizing agents are typically mixtures of chromic, sulfuric, nitric,
or phosphoric acids. Again the part is rinsed to avoid contaminating future
tanks.
The part is now ready for etching step. Etching is carried out to remove the
natural shine of the metal and provide a soft, matte, textured appearance.
Etching is performed by suspending the racked part in a tank containing a
5% sodium hydroxide solution at a temperature between 90 degrees and 120
degrees fahrenheit for a period of 3 to 5 minutes.
The part is now ready for the anodizing tank. Still mounted on the rack with
other like parts, the rack is suspended in the anodizing tank, which contains
a diluted acid and water mixture that is capable of permitting electrical
current flow. The acid is generally a 15 % solution of sulfuric (165 g/L),
with the temperature of the solution maintained between 70 degrees and 80
degrees F depending on whether the final coating is to be left clear or dyed
a color. Temperature control is important so the coating process and
properties will be consistent from batch to batch.
The negative leg of the electrical circuit is connected to the rack of parts
and the positive side of the circuit is connected to one or more cathodes made
generally of aluminum. The cathodes introduce the electricity into the tank,
there placement and number may vary on the size and shape of the part as well
as the total square footage of aluminum surface to be treated, and the surface
's distance from the cathode. Occasionally a metal stainless steel tank itself
may be connected as the cathode leg of the circuit. Those surfaces closest to
the cathode will receive a thicker anodic coating. For normal sulfuric
anodizing a Direct Current power source capable of producing up to 24 volts is
used, with the voltage held generally between 18 and 24 volts, variables in
the needed voltage include, the amount of surface to be treated, the
temperature of the solution (electrolyte), and the acid dilution and balance.
The amount of current applied to the anodizing tank will vary depending on the
amount of surface to be treated, as a rule between 12 and 16 amps are required
for each square foot of coverage. The electrolyte solution is agitated during
the anodizing process to provide uniform solution temperature . The anodizing
tank process, under normal conditions, takes between 12 and 60 minutes.
The anodizing process conditions have a great influence on the properties of
the oxide film formed. If low temperatures and acid concentrations are used,
it yields a less porous and harder coating. Higher temperatures, higher acid
content, and longer immersion times will produce softer, more porous, and
even powdery coatings. Changing just one of these parameters will influence
all the others because they are interrelated.
Even the specific aluminum alloy itself introduces changes in these
relationships, for this reason many anodizers keep a log of the process
parameters, so that replication of a particular finish is possible.
Adding color to the aluminum part, is called dyeing. The parts are dipped in
a tank with a diluted from concentrate, water soluble, organic dye. Each dye
varies in the length of time and temperature for this immersion. It is this
process which forms the Blue, Black, Lavender, Red, and many shades of other
colors, on so many of the aluminum bike parts we sell.
The final consideration in the anodizing process is sealing the now dyed
outer surface so it doesn't sunlight bleach, or stain. Unsealed the porous
outer surface has a lowered corrosion resistance. For clear coatings, ( the
aluminum remains un-dyed or Silver), the anodized aluminum part is put into
boiling de-ionized water, which will convert the amorphous, unstructured form
of the aluminum oxide to a more stable crystalline hydrate form. This
conversion reaction closes off the pores in the aluminum oxide surface. If the
anodized parts are dyed, the sealing process is performed in a tank with a
nickel acetate solution. The sealing times are between 3 and 5 minutes for
nickel acetate sealings and 20 to 30 minutes for water sealing.
Hard Anodizing
Many want to know the difference between the hard anodizing surface on rims
and the colored anodized surface on, say, a cantilever brake arm. The hard
anodized rim may be treated in one of two ways depending on the country of
origin. Japanese hard anodizing is performed in a tank filled with, not
sulfuric, but oxalic acid in a 3% solution with water. It's carried out at a
temperature between 75 degrees and 95 degrees F, using between 10 and 20 amps
per square foot of surface treated, frequently with Alternating Current (AC)
voltage for a period of time between 30 and 40 minutes.
In the United States, a mixed acid solution of sulfuric and oxalic acid with
water is used to hard anodize aluminum. It's performed at low temperature,
between 30 degrees and 50 degrees F, using 24 to 36 amps of current for each
square foot of surface treated at a much higher voltage, between 75 and 100
volts.
Both of these produce an oxide layer that is about 3 thousandths of an inch
thick, in the Grey color we have come to recognize as hard anodized, that is
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