solidifikasi
DESCRIPTION
PembekuanTRANSCRIPT
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Proses SOLIDIFIKASI
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Bagaimana material membeku?
Apabila material (logam atau bukan logam) dalam kondisi cair diturunkan temperaturnya, maka energi kinetik rata-ratanya turun dan akan menyebabkan membekunya material tersebut.
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Pembentukan nuklei (bibit) ! Molekul (atom) umumnya dapat bergerak
bebas, tapi kadang kala ada yang saling bersatu (lengket)
! Pada energi kinetik yang lebih rendah, molekul akan lebih banyak bersatu lagi (bertempelan) satu dengan lainnya
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SEBUAH INTERFACE TERBENTUK KETIKA PADATAN MEMBEKU DARI CAIRAN
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Artinya …..
Ò Besarnya perubahan energi bebas untuk sistem adalah jumlah dari 2 faktor, yaitu volume dan luas permukaan.
Ò ΔG = 4/3 π r 3ΔGv + 4π r 2σ Ò Energi bebas volume naik jika radius/jari
jari pangkat tiga bertambah Ò Energi bebas permukaan meningkat jika
jari jari kwadrat bertambah
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o Penghalusan Butir - Penambahan nukleus/bibit pembekuan secara heterogen dan terkendali dapat meningkatkan jumlah butir yang terbentuk dalam pembekuan pada proses pengecoran.
o Penyebaran Kekuatan – peningkatan kekuatan tarik dengan membuat dislokasi sulit bergerak, yaitu dengan memberikan cluster kecil dengan paduan elmen lain
o Transformasi Fasa dalam Kondisi Padat- perubahan fasa yang terjadi pada kondisi padat
o Proses Pembekuan Secara Cepat – menghasilkan struktur material yang unik dengan memberikan kecepatan pendinginan yang sangat tinggi selama pembekuan
MEKANISME PENGUATAN LOGAM
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o Specific heat – panas yang dibutuhkan untuk mengubah temperatur dari unit berat material satu derajat
o Solidification front - Interface antara solid dan liquid.
o Planar growth – pertumbuhan halus dari solid-liquid interface selama pembekuan
o Dendrite – struktur padatan seperti pohon cemara yang tumbuh ketika cairan membeku
ISTILAH DALAM PEMBEKUAN
Mengapa terjadi proses pembekuan?
• Energi dari phasa kristal (padat) < energi dari phasa cair
• Beda energi tersebut disebut energi volume bebas (ΔGv)
• Dengan berkembang tumbuhnya phasa padat tersebut, jumlah energi volume bebas akan bertambah …….
• Tetapi nilainya negatif 8
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Tetapi……
• Saat pembentukan phasa padat tersebut, terbentuk suatu “batas antar permukaan” (interface) antar kedua phasa tersebut
• Energi bebas permukaan, σ berhubungan dengan interface ini
• Ketika fasa padat tumbuh, jumlah energi bebas permukaan naik dan….
• Nilainya positive…. 9
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Jumlah energi bebas dari sistim solid-liquid berubah dengan ukuran padatan. Padatan menjadi embrio jika radius lebih kecil dari radius kritis, dan jadi bibit (nucleus) jika radiusnya lebih besar dari radius kritis.
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Apa artinya …….
" JIKA HANYA BEBERAPA MOLEKUL YANG SALING MELEKAT (JUMLAH INTI KURANG) MAKA MEREKA AKAN LARUT KEMBALI
" JIKA JUMLAH MOLEKUL YANG SALING MELEKAT CUKUP, EMBRIO AKAN TUMBUH
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Nukleasi Homogen v This process is called homogeneous
nucleation v Hanya terjadi jika material very pure v Ukuran jari-jari kritis dari inti: r* = 2 σ Tm / (ΔHf * ΔT)
v ΔT à undercooling v Undercooling material umumnya 50 - 500
degrees C.
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Nukleasi Heterogen • Nukleasi homogen umumnya terjadi di
lab.
• Impurities provide a “seed” for nucleation
• Solidification can start on a wall.
• It’s like cloud seeding, or water condensing on the side of a glass.
• Adding impurities on purpose is called inoculation
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Ukuran Butir
• Solidification caused by homogeneous nucleation occurs suddenly and only produces a few grains
• In heterogeneous nucleation, solidification occurs on many “seeds”, so the grains are smaller, and more uniform
• Which will be stronger?
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Pertumbuhan Butir & Solidifikasi • If a melt is cooled slowly, and the
temperature is the same throughout, solidification occurs with equal probability everywhere in the melt. However….
• Metals are usually cooled from the container walls – so solidification starts on the walls
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Panas Peleburan
Ò When the liquid solidifies, energy must be removed.
Ò In planar growth the energy is conducted into the solid and out through the walls of the container.
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Apa yang terjadi jika logam cor tidak diberi inokulasi?
• Solidification starts on the walls. • The surrounding liquid is supercooled,
so the solid quickly grows, but… • All that heat that is evolved is hard to
conduct away. • Some of it is absorbed by the
surrounding liquid…which then heats up.
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Pembentukan Dendrit
• All that heat raises the melt temperature near the wall above the freezing point.
• The solid grows out away from the wall, “looking” for cooler metal.
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Solid Liquid
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Dendrit • Dendrites form as the metal solidifies
out into the melt, leaving molten metal behind, that has been reheated from the heat evolved in the solidification process.
• Dendrite formation is common, even in inoculated melts, however the better a melt is inoculated, the fewer dendrites
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Casting: Solidification • Grains perpendicular to wall shut-off other grains, so columnar structure naturally develops perpendicular to mold wall. • Grain boundaries tend to be weak à columnar castings tend to be brittle (unless loaded parallel to the column direction, as in turbine blades). • Equiaxed structure usually preferred for strength, can be achieved with innoculating agents and/or fast cool.
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Segregasi
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
(a) Shrinkage can occur between the dendrite arms. (b) Small secondary dendrite arm spacings result in smaller, more evenly distributed shrinkage porosity. (c) Short primary arms can help avoid shrinkage. (d) Interdendritic shrinkage in an aluminum alloy is shown (x 80)
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(a) The secondary dendrite arm spacing (SDAS).
(b) (b) Dendrites in an aluminum alloy (x 50). (From ASM Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International, Materials Park, OH 44073-0002.)
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Secondary Dendrite Arm Spacing
• The faster cooled off the liquid, the shorter the spacing between the arms
• Small SDAS results in higher tensile strength and yield strength (Why?)
• Very fine SDAS can be produced by cooling the metal as droplets
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o Recalescence - The increase in temperature of an undercooled liquid metal as a result of the liberation of heat during nucleation.
o Thermal arrest - A plateau on the cooling curve during the solidification of a material caused by the evolution of the latent heat of fusion during solidification.
o Total solidification time - The time required for the casting to solidify completely after the casting has been poured.
o Local solidification time - The time required for a particular location in a casting to solidify once nucleation has begun.
Cooling Curves
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(a) Cooling curve for a pure metal that has not been well inoculated. Liquid cools as specific heat is removed (betweens points A and B). Undercooling is thus necessary (between points B and C). As the nucleation begins (point C), latent heat of fusion is released causing an increase in the temperature of the liquid. This process is known as recalescence (point C to point D). Metal continues to solidify at a constant temperature (T melting). At point E, solidification is complete. Solid casting continues to cool from the point. (b) Cooling curve for a well inoculated, but otherwise pure metal. No undercooling is needed. Recalescence is not observed. Solidification begins at the melting temperature
Kurva Pendinginan Logam Murni • A pure metal solidifies at a constant
temperature equal to its freezing point (same as melting point)
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Pembekuan Paduan • Most alloys freeze over a temperature range
rather than at a single temperature
(a) Phase diagram for a copper‑nickel alloy system and (b) associated cooling curve for a 50%Ni‑50%Cu composition during casting.
Three Cast Structures of Solidified Metals • FIGURE 5.8 Schematic illustration of three cast structures of metals solidified in a square mold: • (a) pure metals; • (b) solid-solution alloys; and • (c) the structure obtained by heterogeneous nucleation of grains, using nucleating agents.
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o Chill zone - A region of small, randomly oriented grains that forms at the surface of a casting as a result of heterogeneous nucleation.
o Columnar zone - A region of elongated grains having a preferred orientation that forms as a result of competitive growth during the solidification of a casting.
o Equiaxed zone - A region of randomly oriented grains in the center of a casting produced as a result of widespread nucleation.
Struktur Coran
Cacat Solidifikasi
Ò Shrinkage É Cavities É Pipes É Control with a riser
Ò Gas Porosity É Gas may be dissolved in the melt,
then trapped in the solid
Ò Interdendritic Shrinkage
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Several types of macroshrinkage can occur, including cavities and pipes. Risers can be used to help compensate for shrinkage
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Waktu Solidifikasi • SolidificaIon takes Ime • Total solidificaIon Ime TTS = Ime required for casIng to solidify aMer pouring
• TTS depends on size and shape of casIng by relaIonship known as Chvorinov's Rule
where TST = total solidificaIon Ime; V = volume of the casIng; A = surface area of casIng; n = exponent with typical value = 2; and Cm is mold constant.
n
m AVCTST ⎟⎠⎞
⎜⎝⎛=
Konstanta Cetakan dalam Chvorinov's Rule
• Konstanta Cetakan Cm tergantung pada: – Mold material – Thermal properties of casting metal – Pouring temperature relative to melting point
• Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different