Casting Processes I

Dermot Brabazon

Ref: Kalpakjian, Serope, Manufacturing engineering and technology. - 3rd ed. Reading, Mass : Addison-Wesley, 1995. - 0201538466 MAIN LENDING 670.42/KAL

(Also: MAIN LENDING 670/KAL )

Blowmoulding

The Material Transformation Process P

roductsIngotcasting

MoltenMaterial

Powders

CastingShapes

RollingForging/

Press forming

Stamping

Pressing

Sheet metalforming

ContinuousCasting/Rolling

Injectionmoulding

Mac

hini

ng

Fin

ishi

ng

Ass

embl

y

Raw

Mat

eria

l

Special

Extruding

Single crystalpulling

Firing/Sintering

SLS

Increasing level of detail

= Casting Processes

Casting • The conversion of raw materials into useful

shapes using phase transformations• One of the first steps in converting raw

materials into useful products• Applicable to most materials

– Metals– Ceramics– Plastics– Glass

• Also includes mixtures– Ceramic slips and slurries

Casting – a phase change forming process

• Form the shape from a fluid material state in a mould or container. These materials include

• Molten metals• Monomer solutions• Slips• Slurries

• Change the liquid into a solid by• Removing heat• Removing suspending liquid• Initiating a reaction

– apply heat– inject reactants– irradiate with photons

Casting Fundamentals • Casting advantages

– High shape complexity with internal cavities– Large shape size and variety– Wide variety of materials– Ease of production– Variety of materials that can be cast– Close tolerances (some processes)– High surface finish (some processes)– Excellent mechanical properties (some

processes)– Economic (for some lot sizes)

Kalpakjian pp 262-263

Casting Fundamentals • Casting disadvantages

– High setup costs (some processes)– Low tolerances (some processes)– Low surface finish (some processes)– Porosity (some processes)– Inhomogeneities (some processes)– Poor mechanical properties (some processes)

Casting Fundamentals• Overall Process:

– Make mould– Pour in fluid to solidify– Cool/solidify– Remove shape from mould

Types of Casting

• Molten materials which solidify on cooling– Metals, ceramics, glasses

• Liquids which solidify by reactions with light, activators/hardeners or moisture– Plastics

• Slurries which solidify by the extraction of the suspending medium (usually water)– Ceramics

Casting Fundamentals for Molten Material

• Factors affecting solidification characteristics from the molten state:– Fluidity

• Flow of molten material into the cavity– Heat transfer effects

• During solidification and cooling– Solidification effects– Influence of the type of mould material

Kalpakjian p 265

Fluidity of Molten Metal

• Fluidity is dependent on:– Characteristics of the fluid– Casting parameters

Fluidity - Characteristics of the fluid

• Basically, Fluidity is the ability of the liquid to flow into the mould

Kalpakjian pp 274-275

If these increase:• Temperature sensitivity of

viscosity• Surface tension• Inclusions• Freezing range

Fluidity Reduces then

Theory of Fluid Flow

• Theory has three components:– Bernoulli's theorem– Continuity law– Laminar vs turbulent flow

Kalpakjian pp 272-275

Bernoulli's theorem

where h is the elevation above a reference plane, p is the pressure at that elevation, v is the velocity of the liquid at that elevation, is the density of the liquid and g is the gravitational constant

h + ---- + ---- = constantp

g

v2

2g

Continuity Law

For an incompressible liquid:

• Av = constant, called the flow rate

where A is the cross sectional area and v is the velocity

Laminar vs. Turbulent Flow

• Laminar flow is preferred– Reynolds number (Re) less than 2000

• Turbulent flow (Re >4,000) can cause air entrapment and dross (oxide) formation – results in defects

Re = VD/

where V and D are a fluid characteristic velocity and distance; is density and is viscosity.

Parts of casting mould to be included in fluid flow analysis

• Pouring basin– where the molten metal enters the mould

• Gating system – connects the pouring basin to the rest of the

mould through • Sprue (connects the pouring basin to the runners)• Runners (carry the molten metal to the mould)

• Risers – act as reservoirs to supply molten material as it

solidifies and shrinks

Kalpakjian p 275

Fluidity - Casting parameters

• Mould design• Mould material• Mould surface characteristics• Degree of superheat• Rate of pouring• Heat transfer

Casting Fundamentals for Molten Material

• Factors affecting solidification characteristics from the molten state– Fluidity

• Flow of molten material into the cavity– Heat transfer effects

• During solidification and cooling– Solidification effects– Influence of the type of mould material

Kalpakjian p 265

Heat Transfer

• Very complex phenomenon• Very simple process

– A cold mould extracts heat from the melt causing it to solidify

• Critical to design of mould• Can compute a relative time for solidification

Kalpakjian p 275

Heat Transfer - solidification time

• Solidification time is proportional to the square of the volume divided by the surface area– A sphere will have a much longer solidification

time than a complex shape of the same volume

Effect of Cooling Rate

– Rate of cooling is critical for the structure of the material and hence its properties

• Slow cooling (~0.1K/s) gives large grain sizes• Fast cooling (~10 K/s) gives small grain sizes• Very fast cooling rates (>10 K/s) produce amorphous

materials– Implications:-

• Should design artifact to be thin and not massive• Require "chills" to control cooling rate

Casting Fundamentals for Molten Material

• Factors affecting solidification characteristics from the molten state– Fluidity

• Flow of molten material into the cavity– Heat transfer effects

• During solidification and cooling– Solidification effects– Influence of the type of mould material

Kalpakjian p 265

Solidification Effects

• This is where the material becomes important– Plastics

• Not as critical as for metals– Semiconductors

• Specialty crystal growing• Single crystal so no microstructures

– Glass• No microstructure (amorphous)

Kalpakjian pp 263-277

Solidification Effects - Metals

• Molten metal solidification events depend on the type of material– Pure metals– Alloy

Solidification of Pure Metals

• Solidification occurs at one temperature• Solidification occurs from the mould walls to

the center in a plane front• Grains tend to be equiaxed in the centre of

the casting, but grow outward from the mould wall in a columnar structure

• Nucleation agents can cause a more equiaxed structure (more uniform grains and size distribution)

Solidification of Alloys

• Eutectics behave similarly to pure metals• Cast grain structure depends on phase

diagram

Kalpakjian p 120

Copper-Nickel Phase Diagram

Solidification of Alloys

• Alloys with liquidus and solidus temperatures have a physical “mushy zone”

• “Mushy zone” has solid particles and liquid co-existing• Solid particles tend to be dendritic (tree like) in nature

that grow from the mould wall• Microstructure highly dependent on cooling rate• Freezing range is the temperature difference between

the liquidus and solidus temperatures• Ferrous alloys tend to have small freezing ranges• Aluminium and magnesium alloys tend to have wide

freezing ranges

Kalpakjian p 267

Crystallization Phenomena

Why is solidification so important for metals?

• The solidification events determine the microstructure of the product: – Grain size– Grain distribution– Grain morphology– Grain boundaries– Grain composition– Porosity content and type

Influence of grain size and microporosity

If these decrease:• Grain size • Microporosity

Kalpakjian p 269

• Cracking tendency decreases

• Strength and Ductility increasethen

Structure-Property Relationships • Slow cooling- uniform composition• Normal cooling- micro and macro segregation• Microsegregation

– Segregation of alloying elements within the grains or dendrites

– Dendrite surface has higher concentration of alloying elements than core

• Macrosegregation – Segregation of alloying elements across the

casting itself

Structure-Property Relationships • Types of macrosegregration

– Normal• Constituents with lower melting temperature are driven

away from the mould wall to leave a higher concentration at casting center

– Inverse• Melt enters the cavities among the dendrites formed at the

surface– Gravity

• Heavy elements sink to the bottom

• Macrosegregation gives rise to inhomogeneous microstructures and therefore bad mechanical properties

Avoidance of Macrosegregation • Use

– Nucleation agents– Create more grains and better chemical

homogeneity by mechanical means• Rheocasting - stir the metal while it is in the mushy zone• Vibration• Electromagnetic stirring

Solidification Effects - Shrinkage

• The metal shrinks as it cools – in the melt – as it solidifies as a solid (largest)

Kalpakjian p 279

Aluminium 6.6%Carbon Steel 2.5-3%Copper 4.9%Gray iron -2.55

Volume Solid Contraction of some metals:

Impact of Shrinkage on mould Design

• Dimensions of mould• moulds must be constructed to be larger than

the final product because the metal shrinks as it cools– Patternmakers ruler

• Warpage due to differential shrinkage• Defects due to induced stresses• Porosity

Part Porosity • Caused by shrinkage or gases• Detrimental to the strength and ductility of the

metal, the surface finish and pressure integrity of the part

Shrinkage Porosity • Caused by differential cooling• Thin sections cool faster than thick sections

leading to too little material in the thick sections

• When the thick sections begin to solidify, porosity develops due to the lack of feed metal which is often cut off by already solidified thin sections

• mould designers avoid this by the use of chills and proper flow channels and riser placement

Gas Porosity • Liquid metals have greater solubility for gases

than solid metals• Any gas in the melt appears as spherical

cavities• Melt treatment must include various

degassification processes• Can also have gases arising from reactions

(melt - mould)

List of other defect classes • Projections

– fins, flash, swells (massive), rough surfaces• Cavities

– internal, exposed, blowholes, pinholes• Discontinuities

– cracks, cold and hot tearing, cold shuts• Defective surface

– folds, laps, scars, adhering sand, oxide scale• Incomplete casts

– insufficient metal, leaky moulds• Incorrect dimensions or shape

– improper shrinkage allowance, warping, etc• Porosity See Kalpakjian pp 279-281

Casting Fundamentals for Molten Material

• Factors affecting solidification characteristics from the molten state– Fluidity

• Flow of molten material into the cavity– Heat transfer effects

• During solidification and cooling– Solidification effects– Influence of the type of mould material

Kalpakjian p 265

Influence of the mould material • Mould material impacts on:

– the heat transfer rate– the surface finish– the number of and hence grain size of the

microstructure• Selection of the mould material is strongly

influenced by the process