Extrusion
Extrusion is the forcing of a metal at the right temperature and pressure through a die creating a consistent cross-section of any desired length into any shape imaginable. The extrusion process can be compared to squeezing toothpaste from a tube with the extruded metal assuming the shape of the shape of the die through which it is pressed. Extrusion enables man to manufacture products that take advantage of the unique properties of a metal, such as aluminum. Aluminum is the most abundant and versatile of the Earth’s elements. It can be cast, rolled, or forged. It is the most common metal that is extruded. Aluminum is lightweight, which increases the load trucks can carry. It is resistant to corrosion, is stronger than steel, and has great electrical and thermal conductivity. Log-shaped aluminum billets are preheated and fed into an extrusion press where pressure is applied to force the aluminum through the die onto a runoff table where the extrusion is straightened by stretching and then cut to length. All this being said, whenever metal is mentioned, it is usually aluminum that is being spoken of.
There
are many benefits to extrusion in many manufacturing processes. For example, in one step a part can be
produced, unlike other processes where welding is needed. Extrusion produces parts that have great
strength and performance, which decreases post-production cost. Extrusion parts can be designed to create
the most simple assembly parts possible resulting in variability of the parts such
as interlocking, grooved, and hinge type parts. Inexpensive changes to the extrusion die allow the manufacturer
to eliminate material without melting it.
A pre-production amendment for a post-production process saves on
material and fabrication. Metal can
also be added through the same fashion of a design change. This leaves the end product with no
additional fabrication costs and eliminates center waste. While basic products such as rod, bar, and
tube may be sent for further fabrication, the extrusion process can produce
products ready for placement in the finished product without further
fabrication. Aluminum extrusions are
used as structural components in automobiles, aircraft, bridges, windows, and
doorframes. Extrusion provides a choice
of alloy, finish, design, and quality.
Powder metallurgy was practiced before the art of melting and casting iron. Egyptians made iron tools using powder metallurgy techniques from at least 3000 BC. Ancient Inca Indians made jewelry and artifacts from precious metal powder as well. The first modern powder metallurgy, or P/M, product was the tungsten filament for electric light bulbs developed in the early 1900s.
Powder
metallurgy is the process in which particle are heated and fused into a desired
part. This is also known as
atomization. This is when selected
materials are melted together for purity and uniformity and therefore resulting
in a powder with defined properties.
The most common metals available in powder form are iron and steel, tin,
nickel, copper, aluminum, and titanium.
The mixing of the powders allows for more controlled alloys and their
properties. P/M adds to the high
melting point materials, which gives extraordinary design flexibility.
The
basic P/M processes use pressure and heat to form precision metal parts and
shapes. Powder is squeezed at room
temperature in a rigid precision die into an engineered shape like a gear. After the mass of powder is squeezed into a
shape and ejected from the press, it is fed slowly through a special
high-performance controlled atmosphere furnace to bond the particles
together. They are metallurgically
fused together without melting.
By
the process of P/M unusual designs not able by other processes are done,
high-volume parts are completed without waste, and small complex components can
be made. P/M ranges from coarse to fine
depending on the application. P/M is
used in cars and Earth-moving equipment.
Also P/M is able to resist grueling situations and high traffic use.
The
advantage P/M is that it is a cost effective method of forming precision
net-shape metal components that allows for more efficiently designed consumer
and industrial products. In short,
powder metallurgy saves in scrap loss, saves natural resources through
recycling, and saves on time and labor.
P/M results in dimensionally accurate components and moderate to high
volume production and most important, P/M is reliable.
Rolling is the process of using rotating rolls to reduce the thickness of the stock. Rolling utilizes a process called coldworking. Coldworking is plastic deformation below decrystallization temperature. Deformation takes place between the input thickness and the output thickness. In the rolling process, aluminum ingots are passed between rolls under pressure and rolled to the desired thickness. The products at the end of the rolling process are classified by their thickness, either as plate, sheet, or foil. In rolling the labor is intensive because most of the rolling takes several passes. Waste materials are created at the coin and trim. There are three types of rolling: through rolling, pinch and roll, and cold ring rolling. Cold ring rolling is the most popular.
In
pinch and roll, the slug or stock is extruded and then preformed. Second, up to six roll operations are
done. Finally the slug is coined and
trimmed.
In
cold ring rolling, the stock is cold worked then welded. Many rolling passes are done for desired
shape. Lubrication is necessary to
ensure die life and proper finish. Rolled dies are always used and stored in
pairs. Human error maybe more prone in
this process due to the need to measure between rolls.
The
advantages of rolling include less cost and accuracy.
Forging is the manufacturing process where metal is pressed, pounded, or squeezed under great pressure into high strength dies known as forgings. It is the controlled deformation of a metal under pressure. The process is normally performed hot by preheating the metal to a desired temperature before it is worked and deformed between the dies, where one die is stationary. The pressures that are used include hammers, hydraulic, and mechanical.
Before
forging an ultrasonic machine is used to detect voids in the material. The slug is then heated up to 1400°F and
rolled through a forged press (the extrusion process). The forging temperature is usually
1700-1950°F. The slug then undergoes
upset forging. This is where it is
heated and then put into an “upset” form.
The third step is block forging.
In conjunction to or before this step, the slug is dipped in a ceramic
coating. The dies are then lubricated,
the parts are heated, and then placed in a rotary furnace. The final step is called final forging. This is where deburring, trim for surface
finish, and heat treating takes place.
The Chem Mill is done with hydrofluoric acid to take away access
material if the forging part is oversized.
The slug is then prepared for machining.
Common
applications of forging include the automobile industry, agriculture machining
and equipment, valves, hand tools, and aerospace.
Casting is the oldest process known to man. The basic process of casting is that a solid melted into a molten material. The molten metal is then poured into a cavity, or mold, which contains it in proper shape during solidification. There are eight main elements in casting: the crucible, pattern, mold, core/core print, pour cup, sprue, gate, and runner.
The
crucible is the vessel with molten metal made of ceramic material or metal if
the melting points are not the close.
The pattern is made of wood, wax, etc.
The mold is where the molten metal is poured into to create a desired
shape. Molds are either expendable or
permanent. The shrinkage factor is
usually 2%. The core and/or core print
helps shape the parts with a hole in them when the molten metal is being
poured. The pouring cup is the portion
of the gating system that initially receives the molten metal from the pouring
vessel and controls its delivery to the rest of the mold. From the pouring cup, the metal travels down
the sprue, then along horizontal channels called runners, and finally through
controlled entrances, or gates, into the mold cavity. The gating system is the network of channels used to deliver the
molten metal from outside the mold into the mold cavity.
There
are four main casting processes: investment, sand, permanent mold, and
die. All casting processes begin with a
part’s design requirement.
Modern investment casting is
the equivalent of the ancient “lost wax” casting. Investment casting starts with tooling of the die(s) and the wax
pattern. The toolmaker designs a tool
that will produce a wax pattern of the specifications of the part. This tool is usually made from a block of
aluminum. A cavity is machined into the
block and an injecting port from the surface to the cavity is added. Wax is injected into the tool and then
removed and sent to an assembly area.
The wax patterns are welded onto a runner system. These wax patterns,
called clusters, are cleaned and dipped into a ceramic slurry. This shell is known as investment. The slurry is a mixture of water, sand, and
clay. The clusters are stuccoed with
sand as many as 9 times. The clusters
are then dried. The next step is
firing, where the molten metal is poured into the mold. Firing gets rid of small amounts of wax that
are left in the small crevices. Upon
cooling, the remaining ceramic is removed.
This step is called knockout. It
can be physically removed with a hammer and chisel and water blaster. Also grit blast is done as a final way of
removing small particles that are left behind.
Finally before finishing, machining, deburring, and heat treating is
done. The finishing includes
inspection, which is done by visual inspection, x-ray, and fluorescent
penetration.
Investment
casting is the most expensive form of casting.
Its advantages are that it casts intricate parts, very thin parts, has
close tolerance, good surface finish, and no risk of thermal shock.
Sand
casting uses sand as the mold material.
There are two parts of the part to be cast. They are both placed, at different times, in a drag or core with
sand covering them tightly. Manual
ramming or CO2 hardens the sand. The
pattern is taken out and molten metal is poured into the cavity using the
gating system. The advantages of sand
casting are that part and tooling costs are low. The disadvantage is that production rate is low and there is poor
surface finish.
Permanent
mold casting is a cost-effective approach for near net shape castings. The permanent mold process utilizes a metal
casting die in conjunction with metal or sand cores. The basic casting sequence
is as follows: the metal mold is preheated prior to production to elevate the
mold temperature to operable conditions. The mold, consisting of two or more
parts, is then assembled and closed. Molten metal is introduced at the top of
the mold with steel cores, if present, being removed shortly thereafter. After
solidification, the mold is opened and the casting ejected. The mold is
reassembled and the cycle repeated. The
advantages are that complex shapes can be done cheaply and more precise, less
machining and tooling wear.
The
defects that can happen with casting include shrinkage voids; gas porosity,
where gas is trapped in metal during solidification and there is a presence of
hydrogen; inclusions, where pieces of foreign material is trapped in the part
during solidification like sand or slag; surface finish; misrun, where there is
a dimension discrepancy; and parting line, where there is a “line” one can see
where the two sides met and solidified.