L5-Fundamentals of Metal Casting

7/26/2019 L5-Fundamentals of Metal Casting

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MetalCasting


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Casting

Casting is a process in which molten metal fows bygravity or other orce into a mold where it solidies in theshape o the mold cavity

Casting is usually perormed ina oundryFoundry = actory equippedor making molds, melting and

handling molten metal,perorming the castingprocess, and cleaning thenished castingWorkers who perorm castingare called foundrymen


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teps in casting !

"attern and mold making

Melt the metal

"our it into the mold

#et it ree$e

removing it rom the mold

Cutting o% sprues and risers Cleaning the component

&


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'(amples o Cast "arts

Crank handleormed by casting)some areas were

machined andassembled atercasting

C*clamps ormed bycasting +let andmachining +right

engine blocks


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Capabilities and -dvantages oCasting

Can create comple( part geometries

Can create both e(ternal and internal shapes

ome casting processes are net shape +no urther manuacturing

operations are required) others are near net shape

Can be used with any metal that can be heated to its liquid phase

-ll varieties o metals can be cast * errous and nonerrous

ome casting methods are suited to mass production

Can produce a wide variety o si$ed parts! #arge parts +cast parts weighing over .// tons have been

made ! engine blocks, wood burning stoves, railway wheels,

pipes, church bells, statues, etc0

mall parts! dental crowns, 1ewelry, small statues, rying pans


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#imitations o Casting

2i%erent disadvantages or di%erent casting processes!

#imitations on mechanical properties

"oor dimensional accuracy and surace nish or someprocesses) e0g0, sand casting

aety ha$ards to workers due to hot molten metals

'nvironmental problems

Casting deects which takes place due to lack omaintaining casting design guidelines in case o both parteatures design as well as mold design0

3he cost o pattern making, mold design, die design etc0


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"ouring cup*Molten metal ispoured throughit

prue*verticalportion o thegatingsystem

throughwhich metalsenters4unner*hori$ontalchannels toleads metalsinto maincavity

4iser*additionalvoid in the moldthat providesadditional metalto compensate orshrinkage

Cope* tophal o thepattern

2rag*bottom halo thepattern

Mold cavity*combination o themold material and cores

Casting process


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5eating 6"ouring the Metal

5eating

5eating urnaces are used to heat the metal to moltentemperature su7cient or casting

3he heat required is the sum o!

5eat to raise temperature to melting point

5eat o usion to convert rom solid to liquid

5eat to raise molten metal to desired temperature orpouring

"ouring

8or this step to be successul, metal must fow into all regionso the mold, most importantly the main cavity, beoresolidiying8actors that determine success!

"ouring temperature"ouring rate

3urbulence


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8luidity o Molten MetalCharateristics of the molten metal

Fluidity! 3he capability o a molten metal to ll mold cavities Viscosity! 5igher viscosity decreases fuidity Surface tension! 2ecreases fuidity) oten caused by o(ide lm Inclusions! 9nsoluble particles can increase viscosity, reducing

fuidity Solidication pattern! 8luidity is inversely proportional to theree$ing temperature range

Casitng parameters

Mold design! 3he design and si$e o the sprue, runners, and risers

a%ect fuidity Mold material and surface! 3hermal conductivity and roughness

decrease fuidity Superheating! 3he temperature increment above the melting point

increases fuidity Pouring rate: #ower pouring rates decrease fuidity because o

aster cooling Heat transfer! -%ects the viscosity o the metal


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8luidity o Molten Metal


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olidication o Metals-ter the molten metal poured into the mold, a series o events take

place during the solidication o the casting and its cooling to

ambient temperature0 3hese events greatly infuence the si$e,

shape, uniormity, and chemical composition o the grains ormed

throughout the casting, which in turn infuence its overall properties0

8actors a%ecting these events!

3ype o metal

3hermal properties o both metal and mold

:eometric relationship between the volume and the surace area

o the casting

3he shape o the mold


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olidication di%ers depending on whether the metal is

- pure element or

-n alloy

9t occurs into two stages! ;ucleation! olid particles orm within the liquid0

Crystal growth ! 3he crystal grown at nucleation stagestarts to grow in crystal growth stage and thus entire liquidis turned into solid by e(tracting heat0

.;i?/>Cu


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Characteristic grainstructure in an alloy

casting, showingsegregation o alloyingcomponents in center ocasting0

olidication o -lloys


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8errous alloys cast irons! wear resistance hardness, and good

machinability

magnesium base alloys * good corrosionresistance and moderate strength

cast steels * high temperatures required up to.@/ degree C

cast stainless steels * have a long ree$ing rangeand high melting temperatures, high heat andcorrosion resistance

;onerrous alloys aluminum base alloys

copper base alloys

$inc base alloys

high temperature alloys

Casting -lloys


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Metal olidication! e%ect o coolingrate

2endrites3ree*like structures that orm during the solidication o alloys

low cooling rates produce dendrites with larger branchspacing) aster cooling rates produce ner spacing) very ast

cooling rates produce no dendrites or grains


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5eat 3ranser

-n important consideration in casting is the heat transerduring the complete cycle rom pouring to solidicationand cooling to room temperature

olidication time!

3he metal that solidies rst is at the wall o the mold) thissolid layer thickens as time passes

hrinkage!

hrinkage during cooling can change the part dimensionsand sometimes cause cracking) it is caused by themetalAs thermal e(pansion properties and the phasechange between liquid and solid0


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5eat 3ranser


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5eat 3ranser


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4emoval, Cleaning, 8inishing and9nspection

-ter the casting is removed rom the mould it isthoroughly cleaned and the e(cess material usuallyalong the parting line and the place where the moltenmetal was poured, is removed using a portable grinder0

White light inspection, pressure test, magnetic particleinspection, radiographic test, ultrasonic inspection etc0are used

L5-Fundamentals of Metal Casting

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