Metal Casting Processes

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Metal Casting Processes

• 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

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• 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

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• Solidification of metals

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• 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

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• 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

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• Cast iron
• gray, white, ductile (nodular), and malleable
  2.25 to 4.4C 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

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• 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

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• 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

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• 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 Cgt3 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 - 0.15C
• 0.15 - 0.3C semikilled steels
• gt0.3C - fully killed steels

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• 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

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• 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)

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• Cast structures
• depend on
• the composition of the particular alloy
• the rate of heat transfer
• the flow of the liquid metal

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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.

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• 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)

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• 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.

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• 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

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• 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

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• 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

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• 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

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• 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.

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• 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

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• 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

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• 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.

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• 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

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• 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

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• The rate of heat dissipation affects the
  formation of shrinkage cavities

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• Hot tears are casting defects caused by tensile
  stresses as a result of restraining a part of the
  casting.

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• 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.

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• It is recommended to make the small projection
  separate and attach it to the large casting by an
  appropriate joining method.

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• 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.

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