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LECTURE 1

LECTURE 1. SOLIDIFICATION PROCESS. DBT 302 – Heat Treatment Objectives • Study the principles of solidification as they apply to pure metals. • To discuss the process of solidification and differentiate between homogenous and heterogeneous nucleation

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LECTURE 1

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  1. LECTURE1 SOLIDIFICATIONPROCESS

  2. DBT302–HeatTreatment Objectives •Studytheprinciplesofsolidificationastheyapply topuremetals. •Todiscusstheprocessofsolidificationand differentiatebetweenhomogenousand heterogeneousnucleation •Examinethemechanismsbywhichsolidification occurs. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  3. DBT302–HeatTreatment SolidificationProcess UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  4. DBT302–HeatTreatment SolidificationProcess •Allmetallicmaterials–ceramics,inorganic glasses,andtermoplasticpolymerareliquidor moltenatsomepointduringprocessing. •Forexample:Waterfreezestoice,molten materialssolidifyastheycoolbelowfreezing temperature. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  5. DBT302–HeatTreatment SolidificationProcess •SolidificationofCrystallineMaterialsrequired2 steps. –1stStep–Ultra fine Crystalline,knownasthenucleiof thesolidphase,formfromtheliquid. Theformationofstablenucleiinthemelt(nucleation) –2ndStep–Ultra fine Solid Crystallinebegintogrowas atomfromtheliquidareattachedtothenucleiuntilno liquidremains. Thegrowthofnucleiintocrystalsandtheformationof grainstructure. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  6. DBT302–HeatTreatment Variousstagesinthesolidificationofapolycrystallinematerial;thesquaregridsdepictunitcells. (a)Smallcrystallitenuclei. (b)Growthofthecrystallites;theobstructionofsomegrainsthatareadjacenttooneanotherisalso shown. (c)Uponcompletionofsolidification,grainshavingirregularshapeshaveformed. (d)Thegrainstructureasitwouldappearunderthemicroscope;darklinesarethegrainboundaries. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  7. DBT302–HeatTreatment Severalstagesinthesolidificationofmetals. (a)formationofnuclei, (b)growthofnucleiintocrystals, (c)joiningtogetherofcrystalstoformgrainsandassociated grainboundaries. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  8. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure Nucleusgrowintocrystalsindifferentorientations. Crystalboundariesareformedwhencrystalsjoin togetheratcompletesolidification. Solidifiedmetalcontainingmanycrystals- polycrystalline Crystalsinsolidifiedmetalsarecalledgrains. Grainsareseparatedbygrainboundaries. Morethenumberofnucleationsitesavailable,morethe numberofgrainsformed. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  9. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure •Numberofnucleationsitesavailableforfreezingmetal willaffectthegrainstructureofthesolidmetalproduced. •Fewnucleationsites-acoarseorlargegrainstructurewill produced. •Ifmanynucleationsites-afinegrainstructureresult. •2typesofgrainstructureusuallyproducedwhencasta puremetals: •Equiaxedgrains •Columnargrains UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  10. DBT302–HeatTreatment GrowthofCrystal&Formationof GrainStructure EquiaxedGrains: Crystals,smallerinsize,growequallyinalldirections. Formedatthesitesofhighconcentrationofthenuclie. ColumnarGrains Mold Example:-Coldmoldwall ColumnarGrains: Longthinandcoarse. Growpredominantlyinone direction. Formedatthesitesofslow coolingandsteeptemperature gradient. Example:-Grainsthatare EquiaxedGrains Knowledge.Sincerity.Exellence awayfromthemoldwall. UniversitiMalaysiaPerlis

  11. DBT302–HeatTreatment TechnologicalSignificance •Primaryprocessing-Processesinvolvingcasting ofmoltenmetalsintoingotsorsemi-finished usefulshapessuchasslabs. •Secondaryprocessing-Processessuchasrolling, extrusion,etc.usedtoprocessingotsorslabsand othersemi-finishedshapes. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  12. DBT302–HeatTreatment Nucleation •Nucleation-Thephysicalprocessbywhichanewphase isproducedinamaterial. •Criticalradius(r*)-Theminimumsizethatmustbe formedbyatomsclusteringtogetherintheliquidbefore thesolidparticleisstableandbeginstogrow. •Undercooling-Thetemperaturetowhichtheliquidmetal mustcoolbelowtheequilibriumfreezingtemperature beforenucleationoccurs. •Homogeneousnucleation-Formationofacriticallysized solidfromtheliquidbytheclusteringtogetherofalarge numberofatomsatahighundercooling(withoutan externalinterface). •Heterogeneousnucleation-Formationofacriticallysized solidfromtheliquidonanimpuritysurface. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  13. DBT302–HeatTreatment Aninterfaceiscreatedwhenasolidformsfromtheliquid. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  14. DBT302–HeatTreatment ApplicationofControlledNucleation Grainrefinement-Theadditionofheterogeneousnuclei inacontrolledmannertoincreasethenumberofgrains inacasting. Dispersionstrengthening-Increaseinstrengthofa metallicmaterialbygeneratingresistancetodislocation motionbytheintroductionofsmallclustersofasecond material. Solid-statephasetransformation-Achangeinphasethat occursinthesolidstate. Rapidsolidificationprocessing-Producingunique materialstructuresbypromotingunusuallyhighcooling ratesduringsolidification. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  15. DBT302–HeatTreatment GrowthMechanism Specificheat-Theheatrequiredtochangethe temperatureofaunitweightofthematerialonedegree. Solidificationfront-Interfacebetweenasolidand liquid. Planargrowth-Thegrowthofasmoothsolid-liquid interfaceduringsolidification,whennoundercoolingof theliquidispresent. Dendrite-Thetreelikestructureofthesolidthatgrows whenanundercooledliquidsolidifies. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  16. DBT302–HeatTreatment Whenthetemperatureoftheliquidisabovethefreezing temperatureaprotuberanceonthesolid-liquidinterfacewill notgrow,leadingtomaintenanceofaplanerinterface.Latent heatisremovedfromtheinterfacethroughthesolid UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  17. DBT302–HeatTreatment Figure(a):Iftheliquidisundercooled,aprotuberanceonthesolid-liquid interfacecangrowrapidlyasadendrite.Thelatentheatoffusionis removedbyraisingthetemperatureoftheliquidbacktothefreezing temperature. Figure(b):Scanningelectronmicrographofdendritesinsteel(x15). UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  18. DBT302–HeatTreatment SolidificationTime&DendriteSize Chvorinov’srule-Thesolidificationtimeofa castingisdirectlyproportionaltothesquareof thevolume-to-surfacearearatioofthecasting. Moldconstant(B)-Acharacteristicconstantin Chvorinov’srule. Secondarydendritearmspacing(SDAS)-The distancebetweenthecentersoftwoadjacent secondarydendritearms. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  19. DBT302–HeatTreatment Figure(a):Thesecondarydendritearmspacing(SDAS). Figure(b):Dendritesinanaluminumalloy(x50).(From ASMHandbook,Vol.9,Metallographyand Microstructure(1985),ASMInternational,Materials Park,OH44073-0002.) UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

  20. DBT302–HeatTreatment SolidificationDefects Shrinkage-Contractionofacastingduring solidification. Microshrinkage-Small,frequentlyisolatedpores betweenthedendritearmsformedbytheshrinkagethat accompaniessolidification. Gasporosity-Bubblesofgastrappedwithinacasting duringsolidification,causedbythelowersolubilityof thegasinthesolidcomparedwiththatintheliquid. Sievert’slaw-Theamountofagasthatdissolvesina metalisproportionaltothepartialpressureofthegasin thesurroundings. UniversitiMalaysiaPerlis Knowledge.Sincerity.Exellence

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