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MECE 364 > Metal Casting Processes > Flashcards

Metal Casting Processes Flashcards

1
Q

What are the two categories of metal casting processes?

A

Expendable mold casting (Sand casting, investment casting, expended polystyrene casting, etc.) and permanent mold casting (Die-casting, thixotropic casting, centrifugal casting, etc.)

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2
Q

What are the advantages of casting?

A

Can be used to create complex part geometry, produce large parts, compatible with any metal that can be heated to a liquid state, is well suited to mass production, and some process are capeable of producing parts to net-shape/ near net-shape (a finished product).

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3
Q

What are the disadvantages of casting?

A

Limitations on mechanical properties (dendritic grain structure, defents), poor dimensional accuracy and surface finish (for some processes), and safety hazards to humans/the environment.

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4
Q

Cope

A

Upper half of the mold.

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5
Q

Drag

A

Bottom half of the mold.

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6
Q

Parting line

A

The line that separates the two halves of the mold.

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7
Q

Flask

A

The box that contains the two parts of the mold.

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8
Q

Pattern

A

The form of the mold cavity which has the shape of the part to be cast and is made of wood, metal, plastic, or another material.

The pattern is usually made oversize to allow for the shrinkage of metal as it solidifies and cools.

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9
Q

Core

A

The pattern that determines the internal surfaces of a casted part

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10
Q

Gating system

A

The channel, or network of channels, by which molten metal flows into the cavity from outside the mold.

Typically consists of a sprue through which the metal enters a runner that leads into the main cavity.

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11
Q

Riser

A

A reservoir in the mold that serves as a source of liquid metal to compensate for shrinkage of the casting during solidification.

The riser must be designed to freeze after the main casting to allow for any movement of the cast metal and to signify when the piece has set.

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12
Q

What are the required properties for a sand mold?

A

Refractory (a coating used to enhance the surface finish of the casting and reduce casting defects), permeability, round, fine grains, cohesive strength, and good collapsibility (to allow the casting to shrink while cooling).

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13
Q

Green sand mold

A

Sand, clay, and water. The green in the name means moist.

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14
Q

Cold-box mold

A

organic and inorganic binders are blended into the sand instead of clay and water.

This removes the need to bake the mold at a high temperature before use.

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15
Q

Hot-box mold

A

Termosetting resins are used and the molds need to be hardened at an elevated temperature.

This is most commonly used to make cores with a higher strength.

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16
Q

No-bake mold

A

Syntheic liquid resins are mixed with sand and the mixture hardens at room temperature.

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17
Q

What are the five main problems with metal casting?

A

Dendritic grain structure, shrinkage, fluid flow issues, heat transfer issues, and defects.

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18
Q

Dendritic grain structure

A

Is a tree-like structure that is formed near the edges of a cast part working from the outside in through a mush zone (solid/liquid phase).

It casues non-uniformity, segregation, porosity, and poor strngth in the cast part.

To prevent this from happening a higher/more uniform cooling rate must be achieved. Increased convection, the use of a nucleating agent, and mechanical vibration can also help.

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19
Q

Shrinkage

A

Is caused by the contraction of molten metal before solidification, the phase change itself, and temperature drops.

In sand casting, shrinkage is typically 4%, but can be as high as 7% in aluminum.

Thixotropic and die casting can help control shrinkage and keep it to <1%.

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20
Q

Fluid flow issues

A

Casting requires enough fluidity to fill the mild before freezing. This can be improved by lamiar flow, the reduction of turbulence, and vents,

Some things to avoid to help flow and the sundden change of flow direction or geometry, a Reynolds number of >2000, and the entrapment of gas.

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21
Q

Reynolds number

A

Reynolds number is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces.

Is 𝜌𝑣𝑑/𝜇. 𝜌 is the density,v is the velocity, 𝜇 is the viscosity of molten metal. 𝑑 is the diameter of the channel.

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22
Q

Heat transfer issues

A

This includes heat transfer between the metal, mold, and air.

The uniformity of cooling is important!

The total solidification time must be longer than the time it takes to fill the mold.

The solidiication should progress from the most distant point towards the riser.

The riser must be the last to cool. Use Chvorinov’s rule.

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23
Q

Chvorinov’s rule

A

Formula to determine solidification time.

𝑻=𝑪𝒎(𝑽/𝑨)^𝒏, usually, 𝒏=𝟐

T is the solidification time, V is the volume of the casting, A is the surface area of the casting, n is a constant, and Cm is the mold constant.

Cm depends on the properties of the metal such as density, heat cpacity, heat of fusion, and the mold.

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24
Q

What causes defects in casting?

A

Differential cooling rates and shrinkage, inclusions, gases, and decarburization.

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25
Q

Inclusions

A

Impurities from the metal or interaction of the metal with the mold surfaces or the outside atmosphere.

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26
Q

What defects are caused by differential cooling rates and shrinkage?

A

Macro-porosity, cracks, and hot tears.

Chills (an object used to promote solidification in a specific portion of a metal casting mold), exothermic padding, and pre-heated molds can help minimize differential cooling.

Risers can help with shrinkage and porosity.

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27
Q

What defects are caused by inclusions?

A

These defects reduce fluidity and act as stress raisers which reduces the strength of the final cast product.

Filters can be used to catch oxides and other impurites. Flux can be floated on top of molten metal to react with impurities to form slag which can then be removed.

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28
Q

What defects are caused by gases?

A

Porosity and voids.

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29
Q

What are surface defects?

A

Defects found on the surface of a cast part. They can include blows, scars, scabs, drops, buckles, etc.

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30
Q

Blow

A

A cavity produced by gases that displace molten metal

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31
Q

Scar

A

A shallow blow due to improper venting or permeability

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32
Q

Scab

A

Occurs when a portion of a face of a mold lifts or breaks down

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33
Q

Drop

A

Irregularily shaped projection on the cope (top) surface of a casting

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34
Q

Buckle

A

A long, shallow, broad v depression that occurs in the surface of flat castings

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35
Q

What are internal defects?

A

May not be visible from the surface of a part. Include blow holes and pin holes.

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36
Q

Blow holes

A

Well rounded cavities with a clean and smooth surface. They are created wehn excessive created gas is not able to flow through the mold.

37
Q

Pin holes

A

Small gas holes either at the surface or just below

38
Q

Wash

A

A projection on the drag (bottom) face of a csting that extends along the surface.

39
Q

Rat tail

A

A long, shallow, angular depression in the surface of a flat rating (?).

40
Q

Hot tear

A

Hot cracks that appear in the form of irregular crevices with a dark, oxidized fracture surface.

41
Q

Shrinkage cavity

A

A depression or internal void in a casting that results from the volume contraction that occurs during solidification.

42
Q

Swell

A

A slight, smooth bulge usually found on vertical faces of castings, resulting from liquid metal pressure

43
Q

Shift

A

A defect caused by a sidewise displacement of the mold cope relative to the drag.

44
Q

Misrun

A

Incomplete cavity filling.

45
Q

Design considerations when casting

A

Location and orientation of the parting line, geometric simplicity, corners and cross-sections, allowances for shrinkage and machining, avoid large flat areas (can be broken up with ribs), draft angles, and stress relief.

46
Q

Importance of the location and orientation of the parting line

A

Can affect the number of cores, method of supporting the cores, use of gating, weight of the final casting, dimensional accuracy, and ease of molding.

47
Q

Importance of corners and cross sections

A

Sharp corners, angles, and fillets with extreme radii should be avoided.

Uniform cross-sections and wall thickness will improve ease of casting. if not possible, ensure smooth cross-section changes.

48
Q

Draft angles

A

For the removal of parts from the mold.

~1% for sand molds
~2-3% for permanent die molds

49
Q

Shell molding

A

Mold is made up of a thin shell (~9mm) made of sand and held together by a thermosetting resing binder.

50
Q

Advantages of shell molding

A
  1. Surface of the shell mold cavity is smoother than a conventional green sand mold (2.5μm)
  2. Good dimensional accuracy is possible on small to medium sized parts (+/- 0.25mm)
  3. Collapsability of the mold is sufficeint to avoid tearing and cracking
51
Q

Disadvantages of shell molding

A

Requires a more expensive metal pattern, resin is expensive, possiblity for high porosity.

52
Q

Expended polystyrene casting

A

Uses a mold of sand packed aorund a polystyrene foam pattern that vaporizes when molten metal is poured into the mold.

53
Q

Advantages of expended polystyrene casting

A
  1. Do not have to remove the pattern before casting
  2. Is suitable for automated mass production (e.g. automobile engines)
54
Q

Disadvantages of expended polystyrene casting

A

A new pattern is required for each casting, high chance of porosity, foam vapors.

55
Q

Investment casting (Lost wax casting)

A

Wax patterns are produced and coated in a refractory material. The full mold is formed by continually coating it in the refractory material until it is rigid. The mold is then inverted and heated to melt out the wax. After the wax is gone, the mold is pre-heated at temperature to ensure contaminants are elimated and to help with flow. After the metal has set, the mold is broken away.

56
Q

Advantages of investment casting

A
  1. Very intricate parts can be cast
  2. Good dimensional control (+/- 0.075mm)
  3. Good surface finish can be achieved
  4. Wax can be re-used
  5. Additional machining is not often required (Net shape process)
57
Q

Disadvantages of investment casting

A
  1. More expensive
  2. Have to re-make the molds
58
Q

Basic permanent mold casting

A

Uses a metal mold made of two machined sections for easy and precise opening and closing. The shape must allow for the removal of the casting

Cores can be used to from interior surfaces. If removal of a metal core would be impossible, sand cores can be used (becomes semi-permanent casting).

59
Q

What are molds for lower melting point alloys made from?

A

Steel or cast iron.

60
Q

What are molds for casting steel made from?

A

Refractory material due to the high pouring temperatures.

61
Q

Advantages of permanent mold casting

A
  1. Good dimensional control and surface finish
  2. Rapid solidification caused by metal molds results in a finer grain structure (stronger castings)
62
Q

Disadvantages of permanent mold casing

A
  1. Typically limited to metals with loewr melting points
  2. Requires simpler part geometries
  3. High mold cost
63
Q

What is commonly made with permanent molds?

A

The high mold cost makes this a good process for automated, high volume production.

This can include automotive pistons, pump bodies, and castings for aircrafts and missiles.

64
Q

What are the most common metals used in permanent molds?

A

Al, Mg, Cu alloys, and cast iron.

65
Q

Die casting

A

A permanent mold casting processs where molten metal is injected into a mold cavity under high pressure.

The pressure is maintained during solidification.

66
Q

Hot-chamber die casting

A

Metal is melted in a container and a piston injects liquid metal under high pressure into the die.

Has a very high production rate (around 500 parts and hour), but is limited to low-metling point metals (Zn, Sn, Pb, Mg) that do not chemically attack the mechanical components.

67
Q

Cold-chamber die casting

A

Molten metal is poured into an unheated chamber from an external melting container. A piston then injects the metal into the cavity under pressure.

Has a high production rate, but not as fast as hot-chamber machines.

Casting metals include Al, brass, and Mg alloys.

68
Q

What kind of molds are made for die casting?

A

Made of tool steel, mold steel, or maraging steel. These molds require ejector pins to remove the parts from the die once it opens and may need lubricants to prevent sticking.

W and Mo have good refractory qualities and are used to die cast steel.

69
Q

Advantages of die casting

A
  1. Economical for large production qunatities
  2. Good surface finish accuracy
  3. Can create thin sections (down to 0.5mm)
  4. Rapid cooling causes small grain sizes and good strength
  5. Minimal shrinkage
70
Q

Disadvantages of die casting

A
  1. Limited to metals with low melting points
  2. Part geometry must allow for the removal of the die
71
Q

Semi-solid metal casting

A

Is a group of net shape/ near net shape processes completed with alloys at temperatures between liquid and solid.

The alloy is a mixture of both during castingy (mushy state) and must have solid metal globules in a liquid.

Stirring must occur to prevent the formation of dendrites.

72
Q

Advantages of semi-solid metal casting

A
  1. Allows for complex part geometries
  2. Thin part walls are possible
  3. Close tolerances
  4. Zero-low porosity, resulting in high strength casting
73
Q

Thixotropic molding

A

A type of semi-solid metal casting that uses a combination of injection molding and die casting techniques.

Mg is the primary metal, although Al and metal-matrix composites can also be used.

74
Q

Thixotropic

A

Viscosity is reduced when agitated.

75
Q

Process of thixotropic molding

A

Small chips are heated so that only the exterior melts, creating a slurry that is then injected into the mold. While being injected, the viscosity of the material is higher.

Slurry follows lamiar flow patterns without splashing around, reducing the porosity. This ends up with better mechanical properties.

76
Q

Lamiar flow

A

Laminar flow is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing

77
Q

Advantages of thixotropic molding

A
  1. Slurry requires less heat and therefore thicker sections are possible (less heat transfer issues)
  2. Thinner sections are possible
  3. improved accuracy (5x)
  4. Requires less draft angle (~0.5)
  5. Higher strength and elongation (compared to die casting)
  6. Longer mold life due to reduced thermal shock
78
Q

Disadvantages of thixotropic molding

A
  1. Process control is critical.
  2. The technology is still being improved
  3. More expensive per unit
79
Q

Centrifugal casting

A

A group of casting processes in which the mold is rotated at high speed so centrifugal for distributes molten metal to the outer regions of the die cavity.

This includes true centrifugal casting, semi-centrifugal casting, and centrifige casting.

80
Q

True centrifugal casting

A

Molten metal is poured into a rotating mold to produce a tubular part (mold rotation can also start afer pouring).

The outside shape of the part can be round, octagonal, hexagonal, etc.. The inside of the shape will theoretically be perfectly round due to radially symmetric forces.

Can be used to create pipes, bushings, and rings.

81
Q

Semi-centrifugal casting

A

Centrifugal force is used to produce solid castings rather than tubular parts.

Molds use risers at the center to supply feed material.

The density of metal in the final casting is greater in the outer sections due to the rotation. Because of this, it is often used to make parts wehre the center of the casting is machined away. Eliminating the loewst quality portion of the part.

Can be used to create wheels and pulleys.

82
Q

Centrifuge casting

A

The mold is designed with part cavities away from the axis of rotation. the nmolten metal poured into the mold is then distributed to these cavities by centrifugal force.

Is used to make smaller parts.

Radial symmetry is not required.

83
Q

What are the important factors to consider when selecting Al and Al alloys for casting?

A

High strength to weight ratios, corrosion resistance, thermal and electrical conductivity, and ease of machining.

84
Q

What are the main uses of Al and Al alloys?

A

1.Containers and packing
2. Buildings and other construction
3. Transportation (Aircrafts, cars, trains)
4. Electrical conductors (wires)
5. Consumer durables (appliances, cooking utensils)

85
Q

What are some ways to strengthen Al alloys?

A

Non-heat treatable alloys (1000, 3000, 4000, 5000) can be strengthened by strain-hardening.

Heat treatable alloys (2000, 6000, 7000) can be hardened by precipitation hardening.

86
Q

Precipitation hardening

A

Solution heat treatment: Requires a solution heated up to 540°C which brings it to the κ phase (fcc and ductile) before quenching to cool.

Aging: Is the precipitation of θ phase (CuAl2) at room temperature (natural aging) or at 150°C (artificial aging).

87
Q

Zinc (Zn) and Zn alloys

A

Uses for zinc:
1. 40% galvanizing steel (serves as an anode and prevents the cathose (steel) from corrosion
2. 20% die-casting
3. 20% brass products
4. 10% wrought products
5. 10% other

Some new Zn alloys have been discovered with improved strength, hardness, and wear resistance

88
Q

Magnesium (Mg)

A

Is the lightest of commercially imported metals nad costs nearly twice as much as Al.

Often used as alloying ingredients in Al, Fe, and steels.

89
Q

Copper (Cu)

A

Has excellent electrical and thermal conductivity properties.

The strength and hardness of copper are relatively low so it is quite often alloyed to make brass (Cu + Zn) and bronze (Cu + Sn).,

Brass is strong and corrosion resistant. This makes it well suited for water and electrical fittings.

MECE 364 (10 decks)

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