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Metal Casting

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2.008 Metal Casting 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 ... – PowerPoint PPT presentation

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Title: Metal Casting


1
2.008 Metal Casting
2
Outline
  • Introduction
  • Process Constraints
  • Green Sand Casting
  • Other Processes

3
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
4
Example Sand Casting
5
Example Die Casting
6
Example Investment Casting
7
Casting Process Physics and Constraints
  • Phase Change
  • Density
  • Solubility
  • Diffusion rates
  • High melting temperature
  • Chemical activity
  • High latent heat
  • Handling

8
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

9
Mold Filling
  • Bernoullis equation
  • Reynolds number

Turbulence Injection Molding Re 10-4
10
Cooling for Sand Mold
TEMPERATUR
METAL - MOLD INTERFACE
METAL - AIR INTERFACE
DISTANCE
11
Conductivity / Diffusivity
  • Conductivity (W/mK)
  • Cu 400, Al 200
  • Sand 0.5, PMMA
  • Sand Casting
  • asand lt ametal
  • Injection Molding
  • atool metal ametal
  • Die Casting
  • atool metal gt apolymer

12
Solidification Time Sand Casting
  • Transient 1-D heat transfer
  • Solution
  • Solidification time
  • Chvorinovs rule

13
Solidification Time Die Casting
  • Transient 1-D heat transfer
  • Solution
  • Solidification time

14
Comparison Sand Mold vs Metal Mold
Sand Mold Sand casting
Metal Mold Die casting
15
Microstructure Formation
Schematic illustration of three basic types of
cast structures (a) Columnar dendritic (b)
equiaxed dendritic (c) equiaxed nondendritic
16
Formation of Dendrites
Liquid
Liquidus
Solid
Temperature
Solidus
Liquid
Solid
Mushy zone
Alloying element
Liquid
Solid
Dendrites
17
Constitutional Supercooling
SOLUTE ENRICHED LAYER IN FRONT OF LIQUID-SOLID INT
ERFACE
Liquid
LIQUID COMPOSITION
Solid
CONSTITUTIONALLY SUPERCOOLED REGION
Temperature
Temperature
18
Green Sand Casting
Mechanical drawing of part
Core halves pasted together
Cope pattern plate
Drag 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
19
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

20
Pattern Design Considerations (DFM)
  • Shrinkage allowance
  • Machining allowance
  • Distortion allowance
  • Parting line
  • Draft angle

21
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 Phosph
or bronze Carbon steel Chromium steel Manganese
steel Tin Zinc
22
Typical Pattern Machining Allowance
Allowances,mm
Pattern size, mm
Bore Surface Cope side
For cast irons
For cast steels
For nonferrous alloys
23
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

24
Riser Location and Size
  • Casting shrinkage
  • Directional solidification
  • Scrap and secondary operation

25
Progressive Solidification in Riser
Progressive solidification
Intermediate rate
Fast rate
Slow rate
Riser
Temperature gradient rising toward riser
Directional solidification
26
Draft in Pattern
Patterns
Mold
27
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
28
Investment Casting (cont.)
Casting Pattern
Finished product
Shakeout
Pouring
Pouring
29
Advantages of Investment Casting
  • Intricate geometry
  • Close dimensional tolerance
  • Superior surface finish
  • High-melting point alloys

30
Advantages of Investment Casting
Platen Toggle clamp
Gas/oil accumulator
Piston
Shot sleeve
31
Advantages of Die Casting
  • High production rates
  • Closer dimensional tolerances
  • Superior surface finish
  • Improved mechanical properties

32
Lost Foam Casting
Pottern molding
Coating
Cluster Assemble
Compacted in Sand
Shakeout
Casting
33
Lost Foam Casting
34
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

35
Semi-solid Casting
Punch
Die
Induction furnace
36
Advantages of Semi-solid Casting
37
Casting Process Comparison
Cost
Produciotn rate (Pc/hr)
Equipment
Labor
Process
Die
Sand Shell-mold Plaster Investment Permancnt
mold Die Centrifugal
38
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

39
Quality - Casting
  • Sand casting
  • ?Tolerance (0.72 mm) and defects are affected
    by shrinkage
  • ?Material property is inherently poor
  • ?Generally have a rough grainy surface
  • Investment casting
  • ?Tolerance (0.080.2 mm)
  • ?Mechanical property and microstructure depends
    on the method
  • ?Good to excellent surface detail possible due
    to fine slurry
  • ? Die casting
  • ?Tolerance (0.020.6 mm)
  • ?Good mechanical property and
    microstructure due to high
  • ?pressure
  • ?Excellent surface detail

40
Rate - Casting
  • Sand casting
  • ?Development time is 210 weeks
  • ?Production rate is depending on the cooling
    time t(V/A)2
  • Investment casting
  • ?Development time is 516 weeks depending on
    the
  • complexity
  • ?Production rate is depending on the cooling
    time
  • t(V/A)2
  • Die casting
  • ?Development time is 1220 weeks
  • ?Production rate is depending on the cooling
    time
  • t(V/A)1

41
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

42
New Developments in Casting
  • Computer-aided design
  • Rapid (free-form) pattern making
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