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Fundamentals of solidification

Fundamentals of solidification. Metalls. > Metals have a crystalline structure in solid state body > A crystal is an anisotropic, homogeneous body. The atoms have a 3 dimensional periodic structure. > The smallest unit of this structure is a so called „Elementary cell“

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Fundamentals of solidification

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  1. Fundamentals of solidification

  2. Metalls > Metals have a crystalline structure in solid state body > A crystal is an anisotropic, homogeneous body. The atoms have a 3 dimensional periodic structure. > The smallest unit of this structureis a so called „Elementary cell“ > Solid state bodies without this structure are amorphous, i.g. glas Quartz crystal (trigonal) Amorphous glas

  3. Crystal systems > There are 7 crystal systems with different angels and distances inside the elementary cell > Metals are belonging to the cubic and hexagonal systems Hexagonal elementary cell(Magnesium) Body centered cubic elementary cell (α-Fe) Face centered cubic elementary cell (γ-Fe)

  4. Atomic structure in body centered and face centered cubic lattice Body centeredP = 68% Face centeredP = 74 %

  5. Iron changes the crystalic structure with temperature (Allotropy)

  6. > The metallic atoms have a closed-packed structure > But the closed-packed structure have no 100% filling > There are gaps or holes between the metallic atoms, in which other atoms can be located In body centered structures there are 12 tetraedric and 6 oktaedric gaps

  7. In face centered structures there are 4 octaedric and 8 tetraedric gabs This is the basic of crystalline solid solution (mixed crystals) and solid solution alloys

  8. 2-dimensional defects Symmetric and unsymmetric small angel grain boundaries ( 10°)(regular edge dislotations but one crystal)

  9. Crystals with an orientation difference > 10°  grain boundary

  10. Polycrystalline structure Schematic grain boundary

  11. Morphology of the different solidification structures > Solidification is the transition from liquid to solid state. The solidification is an exogenous reaction. > The transition starts at the liquids-temperatureand ends at solidus-temperature. > There are two types of solidification morphology:exogenous (nucleation at the moulding surface)endogenous (nucleation in the melt)

  12. Melt • Type I: Exogenous solidification (small solidification period) Melt Quelle: Brunhuber (1984) smooth bore ← solidification → rough bore

  13. > pulpy type > spongy type • Type II: Endogenous solidification (wide solidification period) i.e. Ni-bronze, ductile iron, Cu-alloys Quelle: Brunhuber (1984)

  14. Microporosity caused by an endogenous spongy solidification morphology

  15. In technical alloys there are mixed types of solidification morphology (i.g. copper alloys) Melt

  16. Solidification morphology of Ferrous-alloys • Steel: exogenous-rough bore • Cast iron (dendritic solidification): endogenous-pulpy or spongy • Grey iron (eutectic solidification). Endogenous – shell-shaped • Ductile iron (eutectic solidification): endogenous - pulpy • White cast iron: exogenous – rough bore

  17. Sand casting Chill casting GJL Schematic solidification morphology of cast iron (influenced by heat flow). GJV GJS

  18. Typical macrostructure of a thickwalled casting • Globulitic, finely crystalline shell zone (high local undercooling caused by heat flow) • 2) Orientated radial crystallization inverse to heat flow • 3) Coarse crystalline centre zone (endogenous solidification) Quelle: S. Engler (1981)

  19. Homogenious and hertogenious nucleation > Technical melts normally solidifies with heterogeneous nucleation (wall surface, innoculants, oxidic particals etc.)

  20. Coarse crystalline finely crystalline > Nucleus formation and nucleus growth running parallel in the melt > Nucleus formation rate v and growth rate w are influenced by the undercooling of the melt > The undercooling of the melt is influenced by the cooling rate dT/dt and the chemistry of the melt (nucleus formation conditions) Velocity v, w Undercooling

  21. Dendrites directional solidificationnon-directional solidification center distance, l1 dendrite axis dendritearm equiaxed crystal (= crystalline grain) cut cut dendrite arms DAS, l2 dendrite arm spacing (DAS, l2) columnar crystal (= crystalline grain) (following Prof. S. Engler, Foundry Institute of the RWTH Aachen, Germany)

  22. Dendrite arm spacing (DAS) - quantitative image analysis freezing range Ts-l alloy X = 270 mm m = 10 DAS = 30 mm tf = local solidification time = local freezing rate (following BDG-Richtlinie / VDG Merkblatt P220, July 2011, Germany)

  23. A phase diagram shows us the thermodynamic state of metals and alloys in the thermodynamic equillibrium > It is a quantitative representation of the alloy as a function of temperature, chemical composition (and pressure) > Phase diagrams shows us the transition temperatures, the chemical composition of the phases and the metallurgical structure of phases > The phases diagrams are calculated for the thermodynamic equillibrium, real cooling or heating rates influence the transition temperatures and the solubility (composition) of the phases

  24. Phase diagrams Holding point at phase transition Cooling / heating curve of pure iron

  25. Phase diagrams Cooling curves Phase diagram Melt (Solid solution,mixed crystal) Time B (mass percentage) Development of a phase diagram Gießerei-Lexikon, 1997

  26. Liquid state Unlimited or limited solubility in solid state Different types of binary phase diagrams [Gießerei-Lexikon, 1997]

  27. Phase diagrams Monophase Binary phase Phase boundary

  28. The Fe-C-phase diagram Cast iron Steel • Stabiles System • Metastabiles System

  29. Solidification of cast iron Fe-C-phase diagram with 2,4 % Si

  30. Solidification of primary austenite At the liquids temperature the solidification starts with the nucleation of austenite dendrites in the melt

  31. Thermodynamic non equilibrium: Shell-type chemical composition of the dendrites The local chemical composition of the austenite dendrites are influenced by the solidification temperature and the solubility of carbon in the austenite.

  32. Micrographs of grey iron: original primary austenite dendrites between the Fe-C-eutectic phase

  33. Solidification of the eutectic phase At the eutectic temperature (1160° C – 1130° C) the residual melt solidifies in an eutectic phase between the primary dendrites.

  34. Solid-solid-transformation (eutectoide transformation) > At the eutectoid temperature (820° C – 770° C) the austenite transformed to pearlite. > The solid-solid transformation of the austenite to a lamellar ferrite-cementite-eutectoide starts at the grain boundary.

  35. Solid-solid transformation caused by diffusion of carbon > Directly after the eutectorid transformation exists 100 % pearlite > Caused by low cooling rates their is enough time for a diffusion of the carbon to the graphite phase. During the cooling period we have a formation of ferrite around the graphite phase and growth of the graphite phase. The diffusion of carbon is influenced by temperature, alloying elements like Cu, Mn, Sn and the distance to the next carbon particle.

  36. Binary phase diagram of Al-Si-alloys Melt Melt+Al Melt+Si Si-content in mass %

  37. Solidification of an AlSi7 – alloya) cooling down the melt C 1 S +  S +    +  Time A Masse-% B

  38. Solidification of a hypoeutectic AlSi7-alloyb) primary solidification C 1 primary  - dendrite Melt S +  S +    +  Time SEM-micrograph of an Al-dendrite A Masse-% B

  39. Solidification of a hypoeutectic AlSi7-alloyc) start of the eutectic solidification on the surface of the dendrites C 1 S +  S +   Melt   +    - primary A Masse-% B

  40. Solidification of a hypoeutectic AlSi7-alloyd) eutectic solidification of the retained melt C 1  - primary Eut (+) S +  S +    +  Time A Masse-% B

  41. Solidification of a hypoeutectic AlSi7-alloye) segregation of -phase (Silicon) out of the eutectic phase decreasing solubility of Silicon in  C 1 S +  S +    +   - primary Eut (+) Time -Segregation A Masse-% B

  42. Solidification of an eutectic AlSi12,5 – alloya) cooling down the melt to the eutectic temperature b) liquid-solid transformation (eutectic solidification at 577° C) C 2 S +  S +    +  Eut (+) A Masse-% B

  43. Solidification of an eutectic AlSi 12,5 – alloyb) segregation of -phase (Silicon) out of the eutectic phase (decreasing solubility of Silicon in ) C 2 S +  S +    +  Eut (+) -Segregation A Masse-% B

  44. Solidification of a hypereutectic AlSi17 – alloya) primary solidification of -phase (Silicon)b) eutectic solidification of the residual meltc) segregation of -phase ß - primary Melt C 3 S +  S +    +  ß - primary Eut (+) A Masse-% B -Segregation

  45. Microstructures of AlSi - alloys EutecticAlSi12 HypoeutecticALSi9 HypereutecticAlSi17

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