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THE
USE OF TITANIUM IN ADVANCED SYSTEMS APPLIED TO ORTHOPEDIC AND TRAUMATOLOGICAL
SURGERY.
1)
Physical characteristics
Titanium is a pure element, present in nature
in the form of oxides and widespread throughout the earth’s crust: it is
the ninth element in order of abundance and the third most frequently used metal
in mechanical constructions after Aluminium and Iron. It is included in Group
4, period 4 of the periodic table of elements. The atomic number is 22 and, in
spite of being a transitional element, its behaviour is predominantly metallic |
Owing
to Ti’s affinity with Oxygen, Nitrogen and Hydrogen, the production and refinement
processes are extremely complex and therefore very costly.
The
first stage of production involves the transformation of the mineral into Titanium
Tetrachloride. The tetrachloride is then reduced using Magnesium (Kroll process)
or Sodium (Hunter process). These processes are now being flanked by the electrolytic
process. The metal or product takes the form of “spongy” or “granulous”
agglomerates which are separated from the by-product using leaching or vacuum
distillation.
The
raw metal (sponge or crystal) is then compacted into bricks, which are used to
make the consumable electrodes to smelt the metal in VAR (Vacuum Arc Remelting)
furnaces. During this stage binders are added to produce Titanium alloys.
Highly
purified Ti is a relatively pliable material whose mechanical characteristics
are not always suitable for the construction of parts under strain. For this reason,
Titanium is used as the base element to form alloys offering improved performances. |
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| At
room temperature, the metal has a compact hexagonal structure called the Alpha
phase; it then undergoes a crystalline transformation at around 882°C becoming
a centred cubic structure known as the Beta phase which remains stable until the
melting point at 1660°C. From a metallurgical point of view, Titanium alloys
can therefore be divided into three classes: Alpha, Alpha + Beta, Beta. Alpha
alloys are weldable, pliable and resistant, but cannot be subject to heat treatments.
Alpha + Beta alloys can undergo heat treatments and are easier to process
than Alpha alloys; they also have optimal mechanical characteristics and are rustproof.
Beta alloys are generally subject to heat treatments and are generally very hard
and fragile. Alpha-genic
elements can be distinguished when forming alloys: they extend the fields of existence
of the respective phases.
Alpha-genic elements Solid
replacement solutions: Al
Interstitial compounds: C, O, N, B
- Beta-genic
elements
- Intermetallic compounds: Mn, Si, Fe, Cr, Co, W, Ni
They
stabilise the Beta phase up to room temperature: Mo, V, Nb, Zr, Ta.
One of
the most widely used alloys for mechanical applications is composed of 90% Ti,
6% Al, 4% V. By extending the field of existence of the Alpha phase, aluminium
allows the Alpha-Beta transformation temperature to rise, stabilising the alloy
at room temperature and increasing its forgeability. Vanadium allows the Beta
phase to remain at room temperature and makes it more pliable during machining
at high temperatures.
Forging in the Alpha + Beta field gives the material
the first work hardening; in industrial uses at moderate temperatures the final
structure is obtained using heat treatments: the most widely used is Hardening
+ Ageing (Hardening and Tempering).
During hardening all or part of
the Beta phase present at high temperature is kept in a metastable mode; an acicular-martensitic
type structure is obtained that looks like the final dispersion of Beta in Alpha
(fig. 5).
Ageing is achieved by slow cooling in the field in which
the stable phases can be heard at low temperatures, and it allows the decomposition
of the supersaturated Beta phase and increased mechanical strength.
Tempering,
Ageing and Annealing can be varied according to the subsequent technological requirements. |
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Mechanical
characteristics
Elastic modulus
The
Elastic Modulus (or Young’s modulus) of a material can be defined as the
ratio between the specific force applied and the percentage deformation produced
by this force. The epithet “Elastic” implies a return to primary conditions
once the state of force has ended: namely, the stress-deformation diagram for
the interval in question is deemed to be linear.
The E.M. is typical of the
material used, rather than being a characteristic of the form or dimensions of
the sample chosen to obtain it. Of all those available today, TiAlV64 is the metallic
alloy whose modulus of elasticity is the most similar to cortical bone: the value
of E (E = Young’s modulus) for Ti (E = 110 GN/m2) is approximately
half that of austenitic steels (E = 200 GN/m2).
Comparison
between the physical properties of Titanium and some metals
| |
Ti |
Fe |
Al |
ni |
Cu |
| Density
(gr/cm3) | 4,51 |
7,9 |
2,7 |
8,9 |
8,9 |
|
Fusion temp. (°C) | 1668 |
1530 |
660 |
1453 |
1083 |
| Thermal
conductivity (Wm – 1 °C – 1) |
19 |
63 |
205 |
92 |
38 |
| Thermal
conductivity (Wm – 1 °C – 1) | 8,6 |
12 |
23 |
15 |
17 |
|
Electrical resistivity (10-8 W m) |
42 |
10 |
2,7 |
9 |
1,7 |
| Elastic
modulus (Kg/mm2) | 10000 |
21000 |
7000 |
21000 |
10500 |
| | Thermal
conductivity = 19 < other metals. Cu electrodes represent a preferential method
for the dissipation of heat generated by an electrical impulse at 250/300 milliseconds. |
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