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Metal Casting > class 5 > Flashcards

class 5 Flashcards

1
Q

3 advantages of shape-casting processes

A

1) complex part geometries
2) large parts possible
3) applicable to any melt-able metal
4) some suited to mass production

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2
Q

T or F for shape casting processes:
no further manufacturing operations needed to accomplish final part shape for net shape processes

A

True (why?)

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3
Q

What’s a foundry?

A

factory performing casting operations

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4
Q

difference between open and closed mold

A

open: mold open to atmosphere at top, open container in desired shape, must be flat at top
closed: mold has cavity entirely enclosed by the mold with passageway (gating system) leading from outside to the cavity

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5
Q

What’s a gating system and what does it entail?

A

passage way, in enclosed mold area, leading from outside to the cavity
molten metal poured into this to fill the mold

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6
Q

Two basic mold types that distinguish casting processes

A

expendable molds
permanent molds

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7
Q

which casting process is most commercially important?

A

sand casting

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8
Q

why is sand casting the most commercially important mold process?

A

-offers blend of versatility, cost effectiveness, adaptability
-withstand high temperatures, manages metal flow, gas venting & thermal contraction to prevent defects and impurities during process

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9
Q

what does sand casting allow?

A

formation of intricate shapes difficult to achieve with other methods

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10
Q

difference between pattern and core in sand molding?

A

pattern: determines external casting part shape
core: determines casting parts internal geometry if cavity involved/contained within

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11
Q

what’s superheat?

A

temp difference above melting point at which molten metal is poured
// amount of heat removed from molten metal between pouring and solidification

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12
Q

what flow of molten metal is favoured

A

laminar

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13
Q

why should turbulent flow of molten metal into mold be avoided?

A

issues such as accelerating formation of oxides in solidified metal
causes mold erosion or gradual wearing away of mold, due to impact of molten metal

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14
Q

gradual wearing away of mold can be caused by (think of flow types)

A

impact of turbulent molten metal flow

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15
Q

what’s the continuity law?

A

//continuity eqn; indicates volumetric flow rate is constant throughout liquid flow

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16
Q

what are some factors affecting molten metal fluidity during pouring into mold cavity?

A

1) pouring temp above melting point
2) metal alloy composition
3) viscosity of liquid metal
4) heat transfer to surroundings

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17
Q

how does pouring molten metal at a temperature above its melting point impact fluidity

A

increased fluidity- keeps metal in liquid state for longer period, reducing viscosity of fluid, enhancing ability to flow into intricate mold cavities

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18
Q

why is it ideal to have molten metal at a temperature above its melting point?

A

-molten metal fills mold more completely, reducing likelihood of defects e.g incomplete filling or cold shuts
-elevated temps lower surface tension of molten metal, further aiding flow and filling efficiency
-metal remains liquid for longer, allowing more time to flow and fill mold before solidification

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19
Q

what does excessive high temperatures lead to (in terms of molten metal pouring)

A

increased oxidation
damages the mold

20
Q

how does increased oxidation and mold damage occur with very high temps in metal casting?

A

oxidation- metal more readily reacts with oxygen in air, forming oxides on surface, degrades quality of cast metal, causes defects
mold damage-thermal shock to mold, leads to cracks or erosion, impacts lifespan and defects in cast part

21
Q

example of common oxidation reaction

A

rusting

22
Q

what’s rusting?

A

oxidation of iron, oxygen and moisture react with iron, degrading the surface overtime

23
Q

Iron oxide is more {1} and {2} than iron. The resulting material lacks the {3} integrity of iron. The rust will {4} and fall off. This then {5} the iron ‘new’ surface to further oxidation, continuing the cycle of this {6}.

A

1: brittle, flaky
2: flaky, brittle
3: structural
4: crumble
5: exposes
6: degradation

24
Q

how does composition of metal alloy impact its fluidity?

A

impurities–e.g. oxides, obstruct flow of molten metal, reducing fluidity
alloying elements–certain elements enhance/reduce fluidity
freezing range–alloys with short freezing range generally have higher fluidity

25
Q

why do alloys with short freezing range e.g. eutectic alloys have a higher fluidity?

A

uniform solidification–solidify at a single temperature, leading to uniform, rapid solidification (minimizes dendrites–tree like crystal structures that obstruct flow)
consistent flow–eutectic alloys quickly transition from liquid to solid, they maintain a consistent flow without interruptions caused by partial solidification

26
Q

example of alloying element that enhances fluidity, and one that reduces

A

adding silicon to aluminium increases, making it easier to cast complex shapes
magnesium reduces

27
Q

what’s a eutectic alloy?

A

two+ metals that solidify at single, sharp temperature (which is lower than the individual melting points of metals)

28
Q

At eutectic composition, the alloy forms a {1} mixture

A

1: uniform

29
Q

applications of eutectic alloys include

A

soldering (e.g. tin-lead solder)
brazing
casting (excellent fluidity and predictable melting behaviour)

30
Q

why are eutectic alloys ideal in casting?

A

excellent fluidity and predictable melting behaviour

31
Q

what does heat of fusion mean in casting?

A

amount of heat energy required to transform the metal from solid state to liquid state

32
Q

how does alloy solidification differ from that of pure metal?

A

most allows (exception of eutectic!!) start to solidify at the liquidus and complete solidification occurs at the solidus, where liquidus is a higher temp than solidus
pure metals–solidify at a single temperature equal to the melting point

33
Q

A eutectic alloy is a particular {1} in an alloy system, where solidus and liquidus temps are {2}. The temp is called the {3} temperature. Hence, solidification occurs at a {4} temperature, as opposed to over a temperature range.

A

1: composition
2: equal
3: eutectic
4: single

34
Q

Chvorino’s rule;

T_TS=C_m(V/A)^2

what does each component represent?

A

T_Ts: total solidification time
C_m: mold constant
V: casting volume
A: casting surface area

35
Q

identify 3 sources of contraction in metal casting after pouring

A

1) liquid contraction- contraction of molten metal after pouring
2) solidification shrinkage during transformation of state from liquid to solid
3) thermal contraction in the solid state

36
Q

what is meant by contraction in a metal casting?

A

reduction in volume, that occurs as metal cools and solidifies

37
Q

liquid contraction

A

occurs as molten metal cools from the pouring temperature, down to its solidification point, not super significant for casting design, but impacts overall volume of metal required

38
Q

solidification contraction

A

liquid-to-solid shrinkage, when metal transition from liquid to solid state

39
Q

why is the solidification contraction stage critical?

A

can lead to voids or defects if not properly managed

40
Q

to counteract issues with solidification contraction…

A

(proper riser design and placement) by feeding additional molten metal into the mold

41
Q

solid contraction

A

solidified metal cools to room temp, ‘patternmakers shrinkage’

42
Q

solid contraction must be {1} for in the {2} of the mold or pattern, to ensure { 3 } meets the desired {4}

A

1: accounted
2: design
3: final casting
4: dimensions

43
Q

what’s a chill

A

heat sink

44
Q

what’s the purpose of a chill

A

to encourage rapid freezing in certain regions of casting

45
Q

what’s a riser/feeder and it’s purpose

A

provide a reservoir of molten metal that compensates for shrinkage as casting solidifies, designed to solidify last

Metal Casting (1 decks)

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