Presentation on theme: "CASTING TECHNIQUES FOR SINGLE CRYSTAL GROWING (S.C.G.)"— Presentation transcript:
1 CASTING TECHNIQUES FOR SINGLE CRYSTAL GROWING (S.C.G.) POLYCRYSTALLINE- ANISOTROPYSINGLE CRYSTAL- PROPERTIES SAME IN ALL DIRECTIONSCASTING OF GAS TURBINE BLADES BY S.C.G.
2 CASTING TECHNIQUES FOR SINGLE CRYSTAL GROWING (S.C.G.) CONVENTIONAL USE OF CERAMIC MOULDGRAINS WITH THE ABSENCE OF THERMAL GRADIENTDIRECTIONAL SOLIDIFICATION PROCESSCERAMIC MOULD PREHEATED.MOULD SUPPORTED BY WATER COOLED CHILL PLATES.AFTER POURING, ASSEMBLY LOWEREDCRYSTALS GROW AT CHILL PLATE SURFACE UPWARD. COLUMNAR GRAINS FORM
3 CONVENTIONAL USE OF CERAMIC MOULD GRAINS- AS WITH THE ABSENCE OF THERMAL GRADIENTPRESENCE OF GRAIN BOUNDARIES- MAKES STRUCTURE SUSCEPTIBLE TO CREEP AND CRACKING ALONG BOUNDARIES
4 DIRECTIONAL SOLIDIFICATION PROCESS, (1960’s) CERAMIC MOULD PREHEATED.MOULD SUPPORTED BY WATER COOLED CHILL PLATES.AFTER POURING, ASSEMBLY LOWEREDCRYSTALS GROW AT CHILL PLATE SURFACE UPWARD. COLUMNAR GRAINS FORMBLADE DIRECTIONALLY SOLIDIFIED WITH LONGITUDINAL- NOT TRANSVERSE- GRAIN BOUNDARIES. THUS STRONGER
5 SINGLE CRYSTAL BLADES, (1967), MOULD HAS CONSTRICTION IN THE SHAPE OF CORK SCREWTHIS CROSS SECTION ALLOWS ONLY ONE CRYSTAL TO FIT THROUGHWITH THE LOWERING, SINGLE CRYSTAL GROWS UPWARD THROUGH CONSTRICTIONSTRICT CONTROL OF MOVEMENT NEEDEDTHERE IS LACK OF GRAIN BOUNDARIES, MAKES RESISTANT TO CREEP AND THERMAL SHOCK.--EXPENSIVE
6 SINGLE CRYSTAL GROWING (S.C.G.) FOR SEMICONDUCTOR INDUSTRYCRYSTAL PULLING METHOD-CZOCHRALSKI PROCESSSEED CRYSTAL DIPPED INTO THE MOLTEN METAL, PULLED SLOWLY, (AT 10 m/ s), WITH ROTATIONLIQUID METAL SOLIDIFIES ON THE SEED AND CRYSTAL STRUCTURE CONTINUED THROUGHOUT
7 FLOATING –ZONE METHODPOLYCRYSTALLINE ROD (SILICON)- ALLOWED TO REST ON A SINGLE CRYSTALINDUCTION COIL HEATS THE PIECESCOIL MOVED UPWARD SLOWLY (20 m/ s)SINGLE CRYSTAL GROWS UPWARD WITH ORIENTATION MAINTAINEDTHIN WAFERS CUT FROM ROD, CLEANED, POLISHEDUSE IN MICROELECTRONIC DEVICES
8 PLASTER MOULD CASTINGFor casting silver, gold, Al, Mg, Cu, and alloys of brass and bronze.Plaster of Paris (Gypsum) (CaSo4.nH2O) used for cope and drag mouldingA Slurry of 100 parts metal casting plaster and 160 parts water used.Plaster added to water and not water to plaster. To prevent cracks, 20-30% talc added to plaster. Lime and cement to control expansionStirred slowly to form cream Poured carefully over a match plate pattern (of metal)Mould vibrated to allow plaster to fill all cavities.Initial setting at room temperature(setting time reduced by either heating or by use of terra-alba/ magnesium oxide)Pattern removedCope and drag dried in ovens at C(about 20 hours)Mould sections assembled
9 + points - points Dimensional accuracy 0.008 t0 0.01 mm per mm Excellent surface finish as no sand used.. No further machining or grindingNon ferrous thin sectioned intricate castings made.- pointsLimited to non ferrous castings.(sulphur in gypsum reacts with ferrous metals at high temperatures)Very low permeability as metal moulds used. Moulds not permanent, destroyed when castings removed.
10 FROZEN MERCURY MOULDING (MERCAST PROCESS) Frozen Mercury used for producing precision castingsMetal mould prepared to the shape with gates and sprue holesPlaced in cold bath and filled with acetone (to act as lubricant)Mercury poured into it, freezes at –20 C, after a few minutes (10mins)Mercury Pattern removed and dipped in cold ceramic slurry bath.A shell of 3 mm is built up. Mercury is melted and removed at room temperature.Shell dried and heated at high temperature to form hard permeable shape.Shell placed in flask- surrounded by sand-, preheated and filled with metal.Solidified castings removed.
11 For both ferrous and non ferrous castings For both ferrous and non ferrous castings.(melting temperature upto 16500C)Very accurate details obtained in intricate shapesExcellent surface finish, machining and cleaning costs minimum.Accuracy of mm per mm obtained.But, casting process costly.Casting cost high.