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2.008 Metal Casting. Outline. Introduction Process Constraints Green Sand Casting Other Processes. Some Facts. First casting: 5000-3000 BC Bronze, iron age, light metal age? Versatility. • Many types of metals • Rapid production • Wide range of shapes and sizes

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Presentation Transcript
slide1

2.008

Metal Casting

slide2

Outline

  • Introduction
  • Process Constraints
  • Green Sand Casting
  • Other Processes
slide3

Some Facts

  • First casting: 5000-3000 BC
  • Bronze, iron age, light metal age?
  • Versatility

• Many types of metals

• Rapid production

• Wide range of shapes and sizes

• Complex parts as an integral unit

slide7

Casting Process Physics and

Constraints

  • Phase Change

• Density

• Solubility

• Diffusion rates

High melting temperature

• Chemical activity

• High latent heat

• Handling

slide8

Analysis of Casting Processes

  • Fluid mechanics for mold filling
  • Heat transfer for solidification
  • Thermodynamics, mass transfer and heat transfer for nucleation and growth
  • Materials behavior for structure-property
  • relationships
slide9

Mold Filling

  • Bernoulli’s equation
  • Reynold’s number

• Turbulence

• Injection Molding : Re ~ 10-4

slide10

Cooling for Sand Mold

TEMPERATUR

METAL - MOLD

INTERFACE

METAL - AIR

INTERFACE

DISTANCE

slide11

Conductivity / Diffusivity

  • Conductivity (W/mK)
  • Cu ~ 400, Al ~ 200
  • Sand ~ 0.5, PMMA
  • Sand Casting
  • αsand < αmetal
  • Injection Molding
  • αtool metal ~ αmetal
  • Die Casting
  • αtool metal > αpolymer
slide12

Solidification Time : Sand Casting

  • Transient 1-D heat transfer

Solution

  • Solidification time

Chvorinov’s rule

slide13

Solidification Time : Die Casting

  • Transient 1-D heat transfer

Solution

  • Solidification time
slide14

Comparison:

Sand Mold vs Metal Mold

Sand Mold

Sand casting

Metal Mold

Die casting

slide15

Microstructure Formation

Schematic illustration of three basic types of cast structures

(a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic

slide16

Formation of Dendrites

Liquid

Liquidus

Solid

Temperature

Solidus

Liquid

Solid

Mushy zone

Alloying element

Solid

Liquid

Dendrites

slide17

Constitutional Supercooling

SOLUTE ENRICHED

LAYER IN FRONT OF

LIQUID-SOLID

INTERFACE

Liquid

LIQUID COMPOSITION

Solid

Temperature

Temperature

CONSTITUTIONALLY

SUPERCOOLED

REGION

slide18

Green Sand Casting

Mechanical

drawing of part

Core halves

pasted together

Drag pattern plate

Cope pattern plate

Core boxes

Cope after ramming with

sand and removing pattern,

sprue, and risers

Drag ready

for sand

Drag after

removing

pattern

Cope ready

for sand

Casting as

removed from

mold; heat treated

Casting ready

for shippement

Drag with core

set in place

Cope and drag assembled

ready for pouring

slide19

Green Sand Mold

  • Dimensional, Thermal and Chemical stability at high T
  • Size and shape
  • Wettability by molten metal
  • Compatibility with binder system
  • Availability and consistency
slide20

Pattern Design Considerations

(DFM)

  • Shrinkage allowance
  • Machining allowance
  • Distortion allowance
  • Parting line
  • Draft angle
slide21

Typical Shrinkage Allowance

Metal or alloy Shrinkage allowances

mm / m

Aluminum alloy

Aluminum bronze

Yellow brass (thick sections)

Yellow brass (thin sections)

Gray cast iron (a)

White cast iron

Tin bronze

Gun metal

Lead

Magnesium

Magnesium alloys (25%)

Manganese bronze

Copper-nickel

Nickel

Phosphor bronze

Carbon steel

Chromium steel

Manganese steel

Tin

Zinc

slide22

Typical Pattern Machining

Allowance

Allowances,mm

Pattern size, mm

Bore Surface Cope side

For cast irons

For cast steels

For nonferrous alloys

slide23

Gating System:

Sprue, Runner, and Gate

  • Rapid mold filling
  • Minimizing turbulence
  • Avoiding erosion
  • Removing inclusions
  • Controlled flow and thermal conditions
  • Minimizing scrap and secondary
  • operations
slide24

Riser: Location and Size

  • Casting shrinkage
  • Directional solidification
  • Scrap and secondary operation
slide25

Progressive Solidification in

Riser

Progressive solidification

Intermediate

rate

Fast

rate

Slow

rate

Riser

Temperature gradient

rising toward riser

Directional

solidification

slide26

Draft in Pattern

Patterns

Mold

slide27

Investment Casting

Wax

pattern

Injection wax or

plastic patterns

Ejecting pattern

Pattern

assembly

(Tree)

Autoclaved

Heat

Heat

Heat

Heat

Pattern meltout

Stucco coating

Slurry coating

Completed

mold

slide28

Investment Casting (cont.)

Casting Pattern

Finished product

Shakeout

Pouring

Pouring

slide29

Advantages of Investment Casting

  • Intricate geometry
  • Close dimensional tolerance
  • Superior surface finish
  • High-melting point alloys
slide30

Advantages of Investment Casting

Platen Toggle clamp

Gas/oil accumulator

Piston

Shot sleeve

slide31

Advantages of Die Casting

  • High production rates
  • Closer dimensional tolerances
  • Superior surface finish
  • Improved mechanical properties
slide32

Lost Foam Casting

Pottern molding

Coating

Cluster Assemble

Compacted in Sand

Shakeout

Casting

slide33

Invest assembly in flask

with backlip medium

Receive raw polystyrene beads

Vibrate to compact medium

Expand beads

Pour

Mold component pattern,

including gating system

Shakeout castings

Join patters (if multipiece)

Clean castings assembly

Coat pattern assembly

Inspect castings

Dry assembly

Ship castings

Lost Foam Casting

slide34

Advantages of Lost Foam Casting

  • No parting line
  • No cores
  • One-piece flask
  • Freedom of design
  • Minimum handling of sand
  • Ease of cleaning and secondary operation
slide35

Semi-solid Casting

Punch

Die

Induction

furnace

slide37

Casting Process Comparison

Cost *

Produciotn rate (Pc/hr)

Equipment

Labor

Die

Process

Sand

Shell-mold

Plaster

Investment

Permancnt mold

Die

Centrifugal

slide38

Cost - Casting

  • Sand casting
  • ‧Tooling and equipment costs are low
  • ‧Direct labor costs are high
  • ‧Material utilization is low
  • ‧Finishing costs can be high
  • Investment casting
  • ‧Tooling costs are moderate depending on the complexity
  • ‧Equipment costs are low
  • ‧Direct labor costs are high
  • ‧Material costs are low
  • Die casting

‧Tooling and equipment costs are high

‧Direct labor costs are low to moderate

‧Material utilization is high

slide39

Quality - Casting

  • Sand casting

‧Tolerance (0.7~2 mm) and defects are affected by shrinkage

‧Material property is inherently poor

‧Generally have a rough grainy surface

  • Investment casting

‧Tolerance (0.08~0.2 mm)

‧Mechanical property and microstructure depends on the method

‧Good to excellent surface detail possible due to fine slurry

 Die casting

‧Tolerance (0.02~0.6 mm)

‧Good mechanical property and microstructure due to high

‧pressure

‧Excellent surface detail

slide40

Rate - Casting

  • Sand casting

‧Development time is 2~10 weeks

‧Production rate is depending on the cooling time : t~(V/A)2

  • Investment casting

‧Development time is 5~16 weeks depending on the

complexity

‧Production rate is depending on the cooling time :

t~(V/A)2

  • Die casting

‧Development time is 12~20 weeks

‧Production rate is depending on the cooling time :

t~(V/A)1

slide41

Flexibility - Casting

  • Sand casting

‧High degree of shape complexity (limited by pattern)

  • Investment casting

‧Ceramic and wax cores allow complex internal configuration

but costs increase significantly

  • Die casting

‧Low due to high die modification costs

slide42

New Developments in Casting

  • Computer-aided design
  • Rapid (free-form) pattern making