Metallurgy

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Metallurgy

• Dr. Waseem Bahjat Mushtaha
• Specialized in prosthodontics

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1) Metal

• Metal is the pure state are used much more in
  dentistry than in most other arts or industries.
  The pure metals that are commonly used in
  dentistry are gold and platinum, silver and
  copper titanium.

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Properties of metals

• 1) Metals are elements that ionize positively in
  solutions.
• 2) They are solids at room temperature (except Hg
  and gallium which are liquids and H2 which is a
  gaseous metal).
• Luster due to reflection of light waves by the
  free electrons and most of them are silvery in
  color (except that, copper is red and gold is
  yellow)

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• 4) All metals conduct heat and electricity
  because they have free electrons.
• 5) All metals have high strength, high hardness,
  and high melting temperature due to the metallic
  bonding.
• 6) They are malleable (can be hammered into
  sheets) and ductile (can be drawn into wires).
• 7) Give a metallic ring when they are struck.
• 8) All metals have high density which is related
  to the atomic weight and to the type of lattice
  structure that determines how closely the atoms
  are packed.

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Shaping of metals

• 1) Casting cast metal this is performed by
  melting the metal and shaping it in a mould. In
  dentistry a molten metal is poured into a mould
  made from a wax pattern embedded in an investment
  material

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• 2) Cold working (wrought metals)
• Metal can be hammered into sheets or pulled
  through dies to form wires at room temp. most
  dental appliances are cast structures, however
  orthodontic wires and clasps of partial dentures
  are wrought metals.

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• 3) Sintering (powder metallurgy)
• A metal powder can be pressed to produce an
  object. The product of this method is weak as
  there is little adhesion between the particles.
  The strength of the formed object can be improved
  by pressing and heating it in a non oxidizing
  atmosphere below the melting point of the metal
  to agglomerate the particles and improve
  adhesion. Amalgam tablets are made by sintering

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• 4) Electroforming
• The process of electrolysis is used to plate a
  metal on a conducting surface e.g. silver and
  copper plated dies.

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Cooling of molten metal

• A
  
• C
• D
• Temp B
• 
  F
• Time

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• If a metal is melted and then allowed to cool,
  and if its temperature during cooling, and if its
  temperature during cooling is plotted as a
  function of the time, the following figure
  results. As can be noted in the figure the
  temperature decreases regularly from A to B. An
  increase in temperature then occurs to C at that
  time the temperature becomes constant until the
  time indicated by D (C-D is the horizontal or
  plateau portion of the curve). After D the
  temperature decrease to room temperature at E.

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• The temperature T, as indicated by the horizontal
  or plateau portion of the curve at C-D, is the
  freezing or melting point.
• N.B. 1) During this time C-D the metal is
  solidifying and there is evolution of latent heat
  of fusion which compensates for the heat loss.
• 2) The initial cooling to B is called super
  cooling which is due to solidification and the
  release of the latent heat of fusion.

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Mechanism of crystallization

• Solidification starts at special centers called
  nuclei of crystallization. Some of these nuclei
  may be impurities which exist even in a pure
  metal. Growth of crystals form nuclei occurs in
  three dimensions (up down, anteroposteriorly
  and right to left) in the form of dendrites or
  branched structures (treelike branches). Growth
  continues until contact is made with adjacent
  growing crystals. Each nucleus gives rise to one
  crystal or grain. The grater the number of nuclei
  present the faster the solidification will be,
  and the smaller the size of each grain will be
  the tightly packed crystals are called grains
  and their boundaries are called grain boundaries
  

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The grain structure of the solidified material

• Each crystal of a metal is termed a grain. Each
  grain is grown from a nucleus. Within each grain,
  the orientation of the crystal lattice is
  uniform. Adjacent grains have different
  orientations, because the initial nuclei acted
  independently from each other. In other words,
  each grain starts from a different nucleus of
  crystallization and each grain, therefore, has an
  orientation different from that of its neighbor.
  The crystals do not join at their meeting points
  because their space lattices do not match space
  to space or row to row. If they did match exactly
  as they approached each other, they would
  probably join to form a larger grain, or crystal.

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Examination of the grain structure

• The grains can be seen with a microscope and
  photomicrograph can be made provided that the
  metal surface is properly prepared. The surface
  of the metal is flattened, polished, and then
  etched i.e. treated with chemical agents, which
  attack the grain boundaries of the metal more
  than the grains themselves. This is because atoms
  at the grain boundaries are more reactive, since
  they are not surrounded symmetrically by other
  atoms, as are the ones in the center of grain.

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Grain size

• There is an inverse relation between grain size
  and strength i.e. the smaller the grains are the
  stronger and the harder the cast structure is.
  The size of the grains depends upon the number of
  nuclei at the time of solidification. If the
  nuclei are equally spaced, grains will be
  approximately equal in size.

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• The solidification proceeds from the nuclei in
  all directions at the same time in the form of
  sphere. When these spheres meet, they are
  flattened along various surfaces. However, the
  tendency for each grain to remain spherical still
  exists, and the grain tends to have the same
  diameter in all dimensions. Such a grain is said
  to be equiaxed (not elongation). Dental castings
  generally tend to exhibit an equiaxed grain
  structure.

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Factors affecting grain size

• 1) Rapid cooling produces more nuclei of
  crystallization, thus more grains in a given
  volume, and therefore each grain is smaller.
• 2) Impurities or additives act as nucleating
  agents hence, refining (decreasing) the grain
  size .

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Factors affecting the grain size and shape

• 1) Rate of cooling
• Sloe cooling results in the formation of a coarse
  grain structure, whereas rapid cooling gives a
  fine grain structure because it produces more
  nuclei of crystallization. Rapid cooling of a
  molten metal is obtained in the following cases
  (a) when a mould of high thermal conductivity is
  used, (b) if the casting is small, and (c) if
  metal is heated just above its melting
  temperature.

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• 2) Nucleating agents
• Either impurities or additives can act as
  nucleating agents, hence refining the grain
  structure.
• 3) Cold working
• Drawing a cast metal into a wire transforms the
  grain structure into a fibrous structure, with
  high strength, high hardness but less ductility
  (brittle), also internal stresses are induced in
  the structure.

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• 4) Stress relief anneal (recovery)
• The process of releasing internal stresses by
  heating is called annealing. It is a low
  temperature which has little effect on the
  fibrous structure. A relief of the internal
  stresses will only occur.
• 5) Recrystallization
• Further heating of a cold worked material can
  change its elongated fibrous structure, into fine
  grain structure of improved properties. The metal
  is said to have been recrystallized.

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• 6) Grain growth
• If a metal is over heated, or heated for a longer
  time during recrystallization, grain growth
  occurs with a very high ductility and very low
  strength and hardness. This must be avoided if
  high strength and hardness are desired.

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Crystal imperfection

• Real crystal structure usually contains a variety
  of defects. Defect (point, line or plane) in
  crystals have a considerable effect on the
  properties of the metal or alloy.

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• a) Point defects
• 1- impurities these can cause distortion of the
  crystal lattice. Impurities may interstitial or
  substitution.
• 2- vacancies these can allow atoms to move in
  the crystal (solid state diffusion).

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• b) Line defects (dislocation)
• Dislocation is the movement of a row of atoms
  along each other in the lattice. This dislocation
  moves across the crystal, as show in A deforming
  it in a series of single steps, and the
  dislocation finally moves out of the crystal.

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• All the techniques used for improving the
  strength of metal depend on the stop of the
  motion of dislocations. Treatment, which will be
  discussed later, including alloying,
  precipitation hardening, grain refining, and cold
  working, can stop dislocation movement. For
  example, metal are grain refined to produce finer
  grain sizes. When a dislocation moves through a
  grain-refined metal it will encounter more grain
  boundaries than with a material with coarse
  grains. Dislocations become stuck on grain
  boundaries, thereby preventing further
  dislocation motion and strengthening the metal
  occurs. C) plane defect as
  grain boundaries.

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Deformation of metals

• At stresses below the proportional limit, the
  atoms in the crystal lattice are displaced in
  amount yet, when the stress is relieved, they can
  return to their original positions (stretching of
  the bonds). However, once the proportional limit
  is exceeded, a permanent deformation takes place
  and the structure does not return to its original
  dimensions when the load is released
  (dislocation) eventually, this displacement
  becomes so great that the atoms are separated
  completely and a fracture results (loss of
  cohesion).

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Practical consideration

• 1) Cooling a molten metal should be done rapidly
  to get a fine grain structure, if strength and
  hardness are important.
• 2) Cold working increases hardness and strength.
  However, this reduces ductility, so the material
  becomes more brittle. It becomes liable to
  fracture if further cold working is carried out,
  because the potential for further slip is lost.
  3)cold worked structures should be
  annealed to relief stresses and thus increasing
  ductility.

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