10- Fundamentals Of Metal Casting

Front 1

casting

Back 1

process:
a. pouring molten metal into a mold patterned after the part to be manufactured
b. allowing it to solidify
c. removing the part from the mold

Front 2

considerations

Back 2

-flow of molten metal into mold
-solidification and cooling
-influence of type of mold material

Front 3

mold

Back 3

where molten metal is poured into

Front 4

skin

Back 4

shell of fine equiaxed grains, located at the mold walls where the metal coolest rapidly

Front 5

columnar grains

Back 5

grains that have favorable orientation

Front 6

homogeneous nucleation

Back 6

grains (crystals) grow upon themselves, starting at the mold wall

Front 7

columnar dendrites

Back 7

between the liquidus (TsubL) and solidus (TsubS) where the alloy is in a mushy state, columnar dendrites make up the mush

Front 8

mushy zone

Back 8

between liquidus and solidus

Front 9

freezing range

Back 9

= TsubL - TsubS

Front 10

short freezing range

Back 10

difference of less than 50 degrees

Front 11

long freezing range

Back 11

difference greater than 110 degrees

Front 12

what effect does slow cooling rate have on dentritic structures?

Back 12

coarse dendritic structures with large spacing between dendrite arms

Front 13

what effect do higher cooling rates have on dendritic structures?

Back 13

structure becomes finer with smaller dendrite arm spacing

Front 14

amorphous

Back 14

develops for very high cooling rates

Front 15

G/R

Back 15

thermal gradient / rate, typically 10^5

Front 16

cored dendrites

Back 16

forms under high cooling rates, cored dendrites have surface composition different from that at their centers

Front 17

concentration gradient

Back 17

difference between composition of dendrites at surface and at centers in cored dendrites

Front 18

microsegregation

Back 18

during fast cooling of dendrites, rejection of alloying elements from the core during solidification causes a higher concentration of alloying elements on the surface of the dendrite

Front 19

types of segregation

Back 19

microsegregation, macrosegregation, normal segregation, inverse segregation, gravity segregation

Front 20

macrosegregation

Back 20

differences in composition throughout the casting itself

Front 21

normal segregation

Back 21

lower melting-point constituents in the solidifying alloy are driven toward the center
-higher concentration of alloying elements at center

Front 22

inverse segregation

Back 22

center of casting has a lower concentration of alloying elements

Front 23

gravity segregation

Back 23

segregation due to gravity, higher density inclusions or compounds sink and lighter elements float

Front 24

inoculant

Back 24

nucleating agent

Front 25

heterogeneous nucleation

Back 25

nucleation of the grains throughout the liquid metal, induced by the inoculent

Front 26

dendrite multiplication

Back 26

increasing convection in the liquid metal causes the dendrite arms separate

Front 27

semisolid metal forming

Back 27

dendrite arms are not strong and can be broken by agitation in the early stages of solidification

Front 28

rheocasting

Back 28

=semisolid metal forming

Front 29

thixotropic

Back 29

viscosity decreases when the liquid metal is agitated, improves castability

Front 30

thixotropic casting

Back 30

solid billet is heated to the semisolid state and then injected into a die-casting mold

Front 31

gating system

Back 31

molten metal flows through the gating system during gravity-casting, consists of a sprue, runners, and gates

Front 32

sprue

Back 32

tapered vertical channel through which the molten metal flows downward in the mold

Front 33

runners

Back 33

channels that carry molten metal from sprue into mold cavity or connect sprue to gate

Front 34

gate

Back 34

potion of the runner through which the molten metal enters the mold cavity

Front 35

risers

Back 35

reservoirs of molten metal to supply any molten metal necessary to prevent porosity due to shrinkage during solidification

Front 36

feeders

Back 36

=risers

Front 37

function of gating system

Back 37

trap contaminants (oxides, inclusions)

Front 38

bernoulli's theorem (eq)

Back 38

h + p/(pg) + (v^2)/(2g) = constant, h=elevation, p=pressure, v=velocity, p=density,

Front 39

law of mass continuity

Back 39

Q= A1V1=A2V2, Q=volume rate flow, A=cross-sectional area, v=average velocity, 1 and 2 = different locations in the system

Front 40

aspiration

Back 40

process whereby air is sucked in or entrapped in the liquid, prevent using tapered sprue to prevent molten metal separation from the sprue wall (caused by free-falling liquid having a smaller Ac as it gain downward velocity)

Front 41

relationship between height and Ac of sprue (eq)

Back 41

A1/A2=sqrt(h2/h1)

Front 42

velocity of molten metal leaving gate (eq)

Back 42

v=c[sqrt(2gh)], c=friction factor always between 0 and 1

Front 43

velocity of molten metal leaving gate if liquid level height x

Back 43

v=c[sqrt(2g)*sqrt(h-x)]

Front 44

turbulence

Back 44

important consideration in gating systems, flow that is highly chaotic, causes aspiration in casting systems

Front 45

reynolds number (eq)

Back 45

Re=v(Dp)/n, D=diameter, p=density, n=viscosity

Front 46

dross

Back 46

scum that forms on surface of molten metal, caused by reaction between liquid metal and air during severe turbulence (for Re>20,000)

Front 47

fluidity

Back 47

capability of molten metal to fill mold cavities

Front 48

factors of fluidity

Back 48

1. characteristics of molten metal
2. casting parameters

Front 49

viscosity index

Back 49

sensitivity to temperature

Front 50

how do inclusions affect fluidity?

Back 50

they raise viscosity which decreases fluidity

Front 51

fluidity and freezing range

Back 51

inversely proportional,
shorter range = higher fluidity

Front 52

inversely proportional to fluidity

Back 52

viscosity, viscosity index, surface tension, freezing range, inclusions, thermal conductivity of mold, heating

Front 53

linearly proportional to fluidity

Back 53

rate of pouring

Front 54

solidification time (eq)

Back 54

= C (Volume/Surface Area)^n, C=constant, n=2

Front 55

chvorinov's rule

Back 55

solidification time

Front 56

solidification time rule of thumb

Back 56

the more compact the faster it cools,
solidification time of a cylinder>cube>sphere for same surface area

Front 57

3 sequential events that cause shrinkage

Back 57

1. contraction of molten metal as it cools prior to its solidification
2. contraction of the metal during phase change from liquid to solid (latent heat of fusion)
3. contraction of the solidified metal (that casting) as its temp drops to ambient temp

Front 58

7 categories of defects

Back 58

A- Metallic projections
B- Cavities
C- Discontinuities
D- Defective surface
E- Incomplete casting
F- Incorrect dimensions or shape
G- Inclusions

Front 59

causes of porosity

Back 59

shrinkage, entrained or dissolved gases

Front 60

Microporosity

Back 60

develops when liquid metal solidifies and shrinks between dendrites and dendrite branches

Front 61

ways to reduce porosity caused by shrinkage

Back 61

-adequate liquid metal is provided
-internal or external chills (increase rate of solidification in critical regions)
-steep temperature gradient in alloys
-subjecting casting to hot isostatic pressing

Front 62

gases

Back 62

accumulate in existing porous regions, cause microporosity, can be removed by flushing with an inert gas

Front 63

shrinkage cavity

Back 63

gross porosity caused by shrinkage

Front 64

how to tell whether microporosity is caused by shrinkage between dendrites or by gases

Back 64

-for gases, porosity is spherical with smooth walls like swiss cheese
-for shrinkage, walls are rough and angular