casting applications1

8/22/2019 Casting Applications1

http://slidepdf.com/reader/full/casting-applications1 1/20

ME 550 – Lecture 1 © 2005 Brian G. Thomas, University of Illinois at Urbana-Champaign, All Rights Reserved.

Casting Processes

• Foundry (sand) Casting• Investment Casting

• Directional solidification and single crystal pulling

• Permanent mold casting (eg. ingots)

• Die Casting

• Squeeze casting

• Centrifugal casting

• Rapid solidification processes

(strip casting, spray for powder metallurgy, etc.)

• Continuous Casting


Foundry Casting




Foundry (sand) casting is:

• one of the oldest manufacturing processes

• 6th largest U.S. industry

• Advantages – Easy to produce complex shapes

– Inexpensive (especially for small quantities)

– Only way to produce brittle alloy parts

• Disadvantages – Energy intensive

– Pollution control problems

– Defects


Inclusion-related defects in foundry

casting filling

Bubble trail

micro-graphs

J. Campbell & M. Jolly,Univ. Birmingham, UK




Schematic of bubbles and bubble

trails during mold filling

Mold cavity

runner

Down-sprue

J. Campbell & M. Jolly, Univ. Birmingham, UK


Oxide-inclusion stringers coating

dendrites

SEM image of oxide stringer from a

collapsed bubble-trail tubeM. Jolly, Univ. Birmingham, UK




Oxide-inclusion stringers coating

dendrites“eggs under tissue paper”

M. Jolly, Univ. Birmingham, UK


Die Casting

ASM Casting Handbook, 2009




Die Casting

Counter-gravity low-

pressure die casting

High-pressure die casting

Heat transfer controlled by:

- Interface resistance (mainly)

-mold & casting too)

Advantages:

Disadvantages:Small size (porosity problems for larger parts)

High capital cost (for dies): large numbers only

Nonferous only (low temperature)

Fast, economic production

(>400 parts / hour)

Thin sections possible

More sound than foundry

Very low cost per part



Centrifugal Casting

Products: eg. roll, cast-iron pipe

Advantages:

Disadvantages:

Radially symmetric parts only

OK for ferrous alloys

Sound, fine grain size, low porosity

Macrosegregation

expensive





RadiationHeating

Investment Casting

1. Wax pattern

2. Dip in ceramic slurry

5. Shake out

Finished casting3. Heat

(sinter ceramic and melt wax)4. Pour metal

“lost wax” process


Investment Casting

• Often cast wax patterns for producing clusters of

several patterns at once)

• Applications: small, precision parts,

eg. Jewelry, dental work, air foils, etc.

• Advantages

– Intricate shapes, thin walls, no parting line

– Good dimensional tolerance, good surface finish

– Low startup cost

• Disadvantages – Small parts only (slow heat transfer)

– Small quantities (pattern used only once)




Single Crystal Turbine Blades:

enable more efficient jet engines

Conventional

(fine grain) columnar

grains

Single

crystal

• Insert ceramic cores for inner spaces• Laser drill holes

• Air flow through blade creates boundary layer so blade surface

stays cooler than the surrounding gases

Directional solidification


Why directional solidification?

• Engines operate more efficiently at high

temperature – materials limitation

• Low temperature TH < 0.3

– Grain boundaries tangle dislocations

– Increase strength with fine grains

• High temperature TH > 0.5

– Grain boundaries have higher creep

– Increase strength by avoiding grain boundaries

• Ceramics would be good blade material if low-

temperature fracture resistance was better

=()




Investment casting

& directional solidification

InductionHeatingCoils

Water Cooled Chill Block

Slow Withdrawal

FurnaceColdZone

FurnaceHot Zone

SuperalloyMelt

RadiationBaffleTurbine Blades

PouringBasin

GrainSelector

RadiationHeating

RadiationCooling

CeramicShell(Mold)

Directional solidification

• avoid grain boundaries to

achieve high-temperature creep

resistance and strength

• Must avoid nucleation of new

grains

• Avoid cold liquid

• Cool very slowly, maintain steep

1-D temperature Gradient

• across mushy zone

• Baffle keeps top zone hot and

bottom cool, and high GYu et al, 1990





Grain Defects in Single-Crystal

Castings (eg. turbine blades)

Secondary grain originated from grain selector

High / low angle grain boundar


Single-crystal defects




Microstructure of single-crystal Ni

superalloy (from turbine blade)

Top view Side view

Note: Grain defect:

Different angle of

primary dendrite

arms


Directional Solidification Research

Vertical zone-melting process

-to study directional

solidification (dendrite growth,

etc.)

Also called:

“floating zone” or

“Bridgeman” process




consisting of alternating

plates of 2 different types

G/R maps to predict solidification structure

R =


Kurz & Fischer Fig. 4.18




G/R maps to avoid crystal defects


Freckle Formation - study

Ma et al




Freckle

Formation map

(measured)

CMSX-4 nickel superalloy

Tliq = 1375 oC

20mm x 180mm rod samples

planar cellular Cellular/dendritic dendritic

equiaxed

Elements content, wt %Cr Co Mo Al Ti Ta Hf Re

Ni

6.5 9.0 0.6 5.6 1.0 6.5 0.1 3.0matrix


Single Crystal Growing

• Eg. Single crystal growing of Si and

GaAs for semi-conductors

• Zone refining (purification)

• 1-D directional solidification processes:

– Bridgman process

– Czochralski

process




Czochralski process

• Solidify onto an existing seed crystal

• “pull” very slowly out of melt to avoid:

– Dislocations (from thermal stress)

• Stabilize melt flow with electromagnetic force;

rotating crystal

• Product: Si or Ga-As semiconductors

Wikipedia, 2012


Melt spinning

Spinning disk

plasma

Extremely high cooling rates > 106 oC/s

Nonequilibrium conditions and

microstrucctures

Large undercooling; amorphous metal

Product: rapidly quenched powder

Also achcieved by:

“spray atomization”

Purpose:

Wide range of unique properties and new

alloys:Eg. high-Si dissolved in Fe for transformers

Avoids segregation

Fine

powder

Inert atmosphere (He or

Ar)

Squeeze and sinter powder to net shape in furnace under pressure:

Hot Isostatic Pressing (HIP) “powder metallurgy




Strip Casting:Rapid Solidification of Thin Metal Strip


Rapid Solidification Processes

Single-wheel

melt spinning

(strip casting)Strip thickness variations, microstructure

From P. Steen, Cornell




Other Rapid Solidification Processes

• Liquid metal droplet spraying(eg. OSPREY)

• 3-D Laser deposition(like rapid prototyping for metal)

• Rheocasting (slush casting)

• Welding(soldering, brazing, liquid-metal arc welding,

laser welding, etc)


Semi-solid casting processes• Rheocasting

• Thixocasting

• Thixomolding (Mg)

• Often a raw-material for squeeze casting

• Expensive

• Very sound, no macrosegregation

High-quality die-cast parts

made from semi-solid

Microstructure of part

formed from semi-solid





Continuous Casting Process

Steel


Steel continuous casting mold




Continuous casting sub-mold

Courtesy Kawasaki Steel


Defects in

Con. Casting

Surface defects from:

- Level fluctuations

- Meniscus freezing

- Poor flux infiltration

Internal defects from:

- Inclusions

- Flux entrainment

Cracks from:

- Thermal stress

- Bulging, etc.

- Metallurgical

embrittlement

Water

Spray

Longitudinal Section through Slab Caster

(not to scale)

Molten Steel Pool

Solidifying Steel Shell

Flux

Rim

Submerged Entry Nozzle

Support

Roll

Roll Contact

Ferrostatic

Pressure

Bulging

Roll

Nozzle

Nozzle

copper

mold

Liquid Flux

Air Gap

Flux Powder

jet

nozzle

portargon

bubbles

Inclusionparticles andbubbles

Resolidified

Flux

Contact

Resistances

Oscillation

Mark

entrainment

CL

B.G. Thomas



ME 550 – Lecture 1 © 2005 Brian G. Thomas, University of Illinois at Urbana-Champaign, All Rights Reserved.Courtesy of SMS

Thin slab caster


R. O’Malley (Armco, Inc), D. Creech and B.G. Thomas (Univ. of Illinois)

5.2” Thin Slab Caster

(Armco Mansfield)Physical Water Model and

Computer Simulation

Continuous Casting of Steel




Choose a casting process for each product.

• a) cast iron engine blocks for tractors

• b) 5 large aluminum ashtrays

• c) 500,000 aluminum ashtrays

• d) a gold wedding ring with intricate detail.

• e) a life-size bronze statue of the President

• f) a single crystal turbine blade (airfoil)

• g) many stainless steel sheets for stamping and

forming into car mufflers

• h) 5 steel crank shafts

• i) 500,000 steel crank shafts

• j) steel rails (for railroad tracks)

casting applications1

Documents