Metal Casting Processes
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Considered to be the sixth largest industry in the USA
copper smelting technique around 3000 BC
the ancient Egyptians invented the ‘lost-wax’ molding process
the Chinese developed certain bronze alloys
in 1340 - cast iron
in 1826 - malleable iron
in 1948 - nodular cast iron
– It is among the oldest methods of net-shape and near net-shape
manufacturing
– The important factors are:
• solidification and accompanying shrinkage
• flow of the molten metal into the mold cavity
• heat transfer during solidification and cooling of the metal in
the mold
• influence of the type of mold material
• Solidification of metals
• Casting Alloys
– Ferrous alloys
• cast irons: wear resistance hardness, and good machinability
• a family of alloys: gray cast iron (gray iron), nodular (ductile
or spheroidal) iron, white cast iron, malleable iron, and
compacted-graphite iron
• magnesium base alloys - good corrosion resistance and
moderate strength
• cast steels - high temperatures required up to 1650 degree C
• cast stainless steels - have a long freezing range and high
melting temperatures, high heat and corrosion resistance
– Nonferrous alloys
• aluminum base alloys
• copper base alloys
• zinc base alloys
• high temperature alloys
• Cast irons
– This is a family of ferrous alloys composed of iron, carbon (from
2.11% to 4.5%), and silicon (up to 3.5%). They are classified
according to their solidification morphology as:
– Cast iron:
• gray, white, ductile (nodular), and malleable 2.25% to 4.4%C
and 1.15% to 3% Si
• used as structural material (structures and frames of machine
tools, presses, and rolling mills, the housings of water turbines
and of large diesel engines
– Ingot casting and continuous casting
• shaping of the molten metal into a solid form - an ingot - for
further processing by rolling it into shapes, casting it into
semifinished forms, or forging
• ingots may be square, rectangular, or round in cross-section,
and their weight ranges from a few hundred pounds to 300 tons
• Ferrous alloy ingots:
– certain reactions take place during solidification
» significant amounts of oxygen and other gases can
dissolve in the molten metal during steelmaking
» much of these gasses are rejected during solidification
of the metal
» the rejected oxygen combines with carbon, forming
carbon monoxide, which causes porosity in the
solidifies ingot
» depending on the amount of gases evolved during
solidification, three types of steel ingots can be
produced: killed, semi-killed, and rimmed
– Liquid metals have much greater solubility for gases than do
solids. Gases either accumulate in regions of existing porosity,
such as interdendritic areas, or they cause microporosity in the
casting, particularly in cast iron, aluminum, and copper. Dissolved
gases may be removed from the molten metal by flushing or
pouring with an inert gas or by melting and pouring the metal in
vacuum.
– Ingots
• 10-40 tons for rolling
• up to 300 tons for open die forging
• oxygen, hydrogen, nitrogen are dissolved in molten steel
• depending on the measure to deoxidize the steel, different
kinds of steel are produced: the steel, different kinds of steel
are produced: killed, semikilled, capped, or rimmed steel
• The amount of oxygen dissolved in molten steel increases with
the decreasing %C
• in the low carbon steels deoxidizing elements are: Al, Mg, Si,
they are rimmed or capped
• steels with C>3% are produced as killed or semikilled
• segregation - different components of steel in different parts of
the ingot purer metal solidifies first
• killed steels are the least segregated
• rimmed steels with 0.06 - 0.15%C
• 0.15 - 0.3%C semikilled steels
• >0.3%C - fully killed steels
– Vacuum degassing to eliminate O2, N, H
– Vacuum is soft, 0.1 - 0.2 mmHg
– the surface area of the droplets is larger than their volume
• Continuous casting
– conceived in the 1860s
– major improvements in efficiency and productivity and significant
reductions in cost
– the molten metal in the ladle is cleaned and equalized in
temperature by blowing nitrogen gas through it for 5 to 10 min.
The metal is then poured into a refractory lined intermediate
pouring vessel (tundish) where impurities are skimmed off. The
molten metal travels through water cooled copper molds and
begins to solidify as it travels downward along a path supported by
rollers (pinch rolls)
• Cast structures
– depend on
• the composition of the particular alloy
• the rate of heat transfer
• the flow of the liquid metal
Melting Practice and Furnaces
– Furnaces are charged with melting stock consisting of liquid and/or
solid metal, alloying elements, and various other materials such as
flux and slag forming constituents.
– Fluxes have several functions, e.g. for aluminum alloys:
• cover fluxes
• cleaning fluxes
• drossing fluxes
• refining fluxes
• wall cleaning fluxes
– To protect the surface of the molten metal against atmospheric
reaction and contamination the pour must be insulated either by
covering the surface of mixing the melt with compounds that form
a slag.
• Melting Furnaces
• electric arc
• induction
• crucible
• cupolas
– Electric arc Furnaces: high rate of melting, much less pollution,
and the ability to hold the molten metal for any length of time for
alloying purposes.
– Induction furnaces: used in smaller foundries, produce
composition controlled smaller melts.
• the coreless induction furnace (a crucible completely
surrounded with a water cooled copper coil, high frequency
current, a strong magnetic stirring action during induction
heating)
• a core or channel furnace (low frequency - 60 Hz, used in
nonferrous foundries, suitable for superheating, holding, and
duplexing)
– Crucible furnaces: heated with commercial gases, fuel oil, fossil
fuel, electricity. They may be stationary, tilting, or movable. Used
for ferrous and nonferrous metals.
– Cupolas: are basically refractory lines vertical steel vessels that
are charge with alternating layers of metal, coke, and flux. They
operate continuously, have high melting rates, and produce large
amounts of molten metal.
– Levitating melting: magnetic suspension of the molten metal. An
induction coil simultaneously heats a solid billit and stirs and
confines the metal.
• Foundries and foundry automation:
– the casting operations are usually carried out in foundries
– foundry operations initially involve two separate activities:
• pattern and mold making (CAD, CAM, and RP)
• melting the metals while controlling their composition and
impurities
– the rest of operations, such as pouring into molds carried along
conveyors, shakeout, cleaning, heat treatment, and inspection, are
also automated
• a die casting facility can afford automation
• a jobbing foundry producing short production runs may not be
automated
– The properties of the cast metal may be improved after casting:
• high temperature isostatic pressing (HIP) - argon is used to
pressurize the casting (P = 200 MPa, T = 2000C)
• applied for superalloy and Ti casting
• eliminates porosity and improves toughness and fatigue
strength
• steel and iron castings may be quenched and tempered
• Al and Ti castings - subjected to solid solution or precipitation
hardening treatments
• annealing - for homogenization of the micro and
macrosegregation
• stress relief - heat treatment
– It is necessary to consider
• the fluidity of the metal
• pressure and velocity distribution in the casting system
• heat extraction
• the propagation of the solidification front
• use of advanced computer programs
– Fluidity - the ability to fill the various details of the mold cavity
• it is affected by the modes of the solidification front
• by surface tension
• oxide films
• the thermal permeability of the mold material
• it improves by the temperature of the molten metal and the
mold (slower cooling, coarser grains)
• dendrites clog the channels
– Heat transfer
• from pouring to solidification and cooling to room temperature
• it depends on many factors related to the casting material and
the mold and process parameters
• Shrinkage
– Metals shink (contract) during solidification and cooling.
Shrinkage, which causes dimensional changes - and sometimes
cracking - is the result of:
• contraction of the molten metal as it cools prior to its
solidification;
• contraction of the metal during phase change from liquid to
solid (latent heat of fusion);
• contraction of the solidified metal (the casting) as its
temperature drops to ambient temperature
– The largest amount of shrinkage occurs during cooling of the
casting. The amount of contraction for various metals during
solidification is shown in Table 5.1. Not that gray cast iron
expands. The reason is that graphite has a relatively high specific
volume, and when it precipitate as graphite flakes during
solidification,k it causes a net expansion of the metal. Silicon has
the same effect in aluminum alloys.
• Basic requirements of casting processes
– mold cavity
• single use molds
• multiple use molds
– melting process
– pouring technique
– solidification process
– mold removal
– cleaning, finishing, and inspection
• Casting terminology
– construction of a pattern
– construction of a core
• the mold cavity
– riser - provides a reservoir of material that can flow into the mold
cavity to compensate for any shrinkage
– vents may be included to provide an escape of the gases
– gating system - to deliver the molten metal to the mold cavity
• Defects
– Depending on casting design and method, several defects can
develop in castings. Because different names have been used to
describe the same defect, the International Committee of Foundry
Technical Associations has developed standardized nomenclature
consisting of seven basic categories of casting defects:
• metallic projections, consisting of fins, flash, or massive
projections such as swells and rough surfaces
• cavities, consisting of rounded or rough internal or exposed
cavities, including blowholes, pinholes, and shrinkage cavities
• discontinuities such as cracks, cold or hot tearing, and cold
shuts. If the solidifying metal is constrained form shrinking
freely, cracking and tearing can occur. Although many factors
are involved in tearing, coarse grain size and the presence of
low melting segregates along the grain boundaries increase the
tendency for hot tearing. Incomplete castings result from the
molten metal being at too low a temperature or pouring the
metal too slowly. Cold shut is an interface in a casting that
lack complete fusion because of the meeting of two streams of
partially solidified metal.
– defective surface, such as surface folds, laps, scars, adhering sand
layers, and oxide scale
– incomplete casting, such as misruns (due to premature
solidification), insufficient volume of metal poured, and runout
(due to loss of metal from mold after pouring)
– incorrect dimensions or shape, owing to factors such as improper
shrinkage allowance, pattern mounting error, irregular contraction,
deformed pattern, or warped casting
– inclusions, which form during melting, solidification, and molding.
Generally nonmetallic, they are regarded as harmful because they
act like stress raisers and reduce the strength of the casting
• Porosity
– caused by shrinkage or trapped gases, or both
– porosity is detrimental to the ductility of a casting and its surface
finish
– porosity caused by shrinkage can be reduced or eliminated by
various means
• adequate liquid metal feeding
• external and internal chills
– The rate of heat dissipation affects the formation of shrinkage
cavities
– Hot tears are casting defects caused by tensile stresses as a result of
restraining a part of the casting.
– Cast metals are generally weaker in tension in comparison with
their compressive strengths
– casting process allows to distribute the masses of a section
– distribute masses in order to lower the magnitude of tensile stresses
in highly loaded areas of the cross section and to reduce material in
lightly loaded areas.
– It is recommended to make the small projection separate and attach
it to the large casting by an appropriate joining method.
– Machining should be performed only on areas where it is
absolutely necessary.
• The ribs should be as thin as possible
• Parabolic ribs are better than straight ribs in terms of economy
and uniformity of stress.
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Safety in foundries
– As in all other manufacturing operations, safety is an important
consideration, particularly because of the following factors:
• dust from sand and other compounds used in casting, thus
requiring proper ventilation and safety equipment for the
workers
• fumes from molten metals and lubricants, as well as splashing
of the molten metal during the transfer or pouring
• the presence of fuels for furnace, the control of their pressure,
the proper operation of valves, etc.
• the presence of water and moisture in crucibles, molds, and
other locations, since it rapidly converts to steam, creating
severe danger of explosion
• improper handling of fluxes, which are hygroscopic, thus
absorbing moisture and creating a danger
• inspection of crucibles, tools, and other equipment for wear,
cracks, etc.