Metallurgy

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

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.

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)

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.

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

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.

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

4 )Electroforming:

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

Cooling of molten metal

A

C D

Temp B F

Time

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 .

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.

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

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.

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.

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.

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 stucture.

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.

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.

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.

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.

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.

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.

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).

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.

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 .

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).

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 .