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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 4.4%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

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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. Dissolvedgases 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 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

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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 significantreductions 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. Themolten 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 inductionheating)

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 thatare 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 andconfines 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, arealso 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 precipitationhardening 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 themold (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 ironexpands. 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 surfacefinish

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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