Materials Selection for Total Hip Replacement

Titanium

Although the aerospace industry still provides the major market, titanium and titanium alloys are finding widespread use in other industries due to their many desirable properties.

Notable among these is their low densities, which fall between those of aluminium and iron and give very attractive strength to weight ratios.

In addition, titanium and titanium alloys really form stable protective surface layers which give them excellent corrosion resistance in many environments, including oxidizing acids and chlorides, and good elevated temperature properties up to about 550 C (1022 F) in some cases. 

Titanium metal is abundant in the earth's crust and is extracted commercially from the ore minerals rutile (titanium dioxide) and ilmenite (iron-titanium oxide).The commercial extraction process involves treatment of the ore with chlorine gas to produce titanium tetrachloride, which is purified and reduced to metallic titanium sponge by reaction with magnesium or sodium. The sponge blended with alloying elements as desired, is then vacuum melted. Several meltings may be necessary to achieve a homogeneous ingot which is ready for processing into useful shapes, typically by forging followed by rolling. For many applications the cost of titanium alloys can be justified on the basis of desirable properties. Pure titanium, like iron, is allotropic. At ambient temperature it has a hexagonal close packed (hcp) crystal structure which is stable during heating up to 883 C ( 1621 F) where it transforms to the body centred cubic (bcc) crystal structure. It remains bcc at higher temperatures until it melts at 1668 C (3034 F). On cooling, the transformation from bcc to hcp in pure titanium can not be suppressed by rapid cooling, the transformation occurring by martensitic type reaction. This is not, however, the case with titanium alloys, in which the transformation can be suppressed or modified. Thus the microstructure of titanium alloys frequently contain particles of the bcc phase of ambient temperature.

The spectrum of titanium-based materials can be divided into four classes depending their constituent phases; this in turn depends on their relative contents of alpha-stabilizing and beta-stabilizing alloying elements. 

The four basic classes are:

unalloyed or commercially pure titanium;

alpha and near alpha alloys;

alpha- plus -beta alloys;

beta-alloys

Products

Titanium mill products vary by forms: titanium bars, titanium ingots, titanium billets, extrusions, titanium strip, titanium sheets, titanium plates, titanium wire, titanium pipes (titanium tubes). All these products available for unalloyed titanium as well as from titanium alloys. Many grades of titanium and its alloys are also available as castings and forgings. Some beta alloys have superior forgeability; sheets can be cold-formed in the solution-treated condition. Ti 6 Al/ 4V is relatively difficult to cold form but is readily hot formed or even super plastically formed. 

Weldability

In general, weldability of titanium and its alloys can be readily performed but it is necessary to exclude reactive gases, including oxygen and nitrogen from the air, and to maintain clearness. Thus weld properties are heavily influenced by welding procedures, especially by the adequacy of inert gas shielding. Electron beam welding, gas metal arc welding, friction welding, laser welding, resistance welding are all used in some cases. Both alloys composition and microstructure are important in determining weldability, with the presence of beta phase having a deleterious effect. Thus unalloyed titanium and alpha alloys are generally weldable and welded joints generally have acceptable strength and ductility; postweld stress relief annealing of weldments is recommended. Some alpha-beta alloys, especially Ti 6 Al/4 V, are weldable in the annealed condition as well as in the solution treated and partially aged condition ( aging can be completed during the post-weld heat treatment. Strongly stabilized alpha-beta alloys can be embrittled by welding, the result of phase transformation occurring in the weld metal or the heat affected zone. Some beta alloys are weldable in the annealed or the solution treated conditions. 

Corrosion Resistance

Unalloyed titanium, the most corrosion-resistant of the titanium based materials, is resistant to nitric acid and many different chloride-bearing environments, including hot chloride solutions. It is also resistant to sulphides. The Pd-bearing unalloyed grades have improved resistance to corrosion in reducing media, so that it can be applied in hydrochloric, phosphoric, and sulphuric acid solutions. Since the corrosion resistance is based on the formation of a stable adherent protective surface oxide film, corrosion susceptibility can arise if the environment is such that the film can not regenerate itself when damaged; such a situation can arise for example in the case of crevice corrosion, where oxygen depletion and acidic conditions can occur in confident space. 

Creep and oxidation resistance

Unalloyed titanium has good creep resistance below 315 C (599 F).

Alpha alloys are generally stable for periods of 1000 hours up to 540 C (1004 F),

alpha-beta alloys up to about 370 C(698 F ) in the mill annealed conditions and as high as 425 C (797 F ) after heat treatment. 

APPLICATION OF TITANIUM AND TITANIUM ALLOYS

Aircraft Industry

remains the first and the major titanium consumer

Material requirements for aircraft building:
1) small weight
2) high specific strength
3) heat resistance
4) fatigue load resistance
5) crack resistance
6) corrosion resistance

Titanium and its alloys meet this requirement.

Three major trends of titanium application for aircraft building:
1. Fabrication of items of complex space configuration:
 - hatch and door edging where moisture is likely to be accumulated (high corrosion resistance of titanium is used)
 - skins which are affected by engine combustion product flow, flame preventing fire safety-proof membranes (high temperature of melting and chemical inactivity of titanium is used)
 - thin-walled lead pipes of air system (minimum thermal titanium extension ratio compared to all other metals is used)
 - floor decking of the cargo cabin (high strength and hardness is used)
2. Fabrication of designated high-loaded assemblies and units
 - landing gear 
 - fastening elements (brackets) of the wing
 - hydro cylinders 
3. Engine part manufacture (see next section).

The following is manufactured from titanium alloys for aircraft applications:
ailerons, panel and swivel wing assemblies, spar walls, panels, brackets, steering wheels,  wedge meshes, air intake ducts, lead pipes, frames, leading edge flaps and flaps, hydraulic systems, fasteners and a number of other parts.

In aircraft industry the most widely used titanium alloys are GOST VT1-0, VT22, VT6, VT3-1, VT8, VT9, VT25, ASTM Grade-2, Grade-3, Grade-4, Grade-5, Ti6Al-4V, Ti6Al2Sn4Zr2Mo, Ti6Al2Sn4Zr6Mo, IMI318  IMI550, IMI685, IMI829, IMI834.

Shipbuilding Industry

Titanium metal has many characteristics for use in ships, such as excellent corrosion resistance against sea-water, light weight and high tensile strength, good recycle ability, etc. For the future Ti usage, some basic data on Ti should be studied for Ti ship-building. Titanium has a big possibility for ship-material, specially, in some special purpose.

Automotive industry

In the automotive industry, uses for titanium in the automotive/motorcycle aftermarket and racing market. Engine parts such as connecting rods, wrist pins, valves, valve retainers and springs, rocker arms and camshafts, to name a few, lend themselves to fabrication from titanium, because it is durable, strong, lightweight and resists heat and corrosion. While titanium initially may be more expensive for these applications, designs that exploit its unique characteristics yield parts that more than pay for themselves with better performance and a longer life.

Architecture

Titanium has come of age as a competitive building material. New, more effective production techniques, combined with an abundance of raw and refined ore, have improved availability. Titanium's corrosion immunity, strength and physical properties combine to allow reduced wall thickness, lowering its installed unit cost. Well-researched designs that capitalize on its unique attributes and long-term savings from durability and low maintenance make titanium one of today's most cost effective building materials on a lifecycle basis. There are architectural titanium as coil, sheet, composite panel and tube. Sheet is the most commonly used in constructing of outer walls, roofs, shields.

Sports

The sporting goods industry uses the metal in the manufacture of tennis rackets, golf clubs, lacrosse stick shafts; cricket, hockey and football helmet grills, bicycle frames and components. Titanium alloys are also used in spectacle frames. This results in a rather expensive, but highly durable and long lasting frame which is light in weight and causes no skin allergies. The golf industry has found that lightweight titanium club heads can be bigger than those made of steel, enlarging the "sweet spot" of the club and thus increasing distance and accuracy.

The application of titanium in bicycle production started approximately 25-30 years ago and it was the first time titanium had been applied in sports. The most commonly used titanium alloy for bicycle frames is Тi 3Аl-2.5V (ASTM Grade 9) / PT3V.

Ti6Al-4V alloy is used in making knives for diving, but this alloy doesn’t provide the proper durability of blade edge in compare to other alloys. That’s why some manufacturers prefer to use GOST VT23 alloy.

Titanium is widely used in tourism and mountaineering – almost for all articles tourists and alpinists have in their rucksacks: bottles, cups, other food ware  are mostly made from CP titanium ASTM Grade 1 и Grade 2 (GOST VT1-0).

Medicine

Because of its corrosion resistance, titanium and its alloys are used extensively in prosthetic devices such as artificial heart pumps, pacemaker cases, heart-valve parts and load bearing bone or hip-joint replacements or bone splints. Titanium is completely inert to human body fluids, making it ideal for medical replacement structures such as hip and knee implants. Titanium actually allows bone growth to adhere to the implants, so they last longer than those made of other materials. Reconstructive titanium plates and mesh that support broken bones are also commonly used today.

High strength-to-weight ratio and superior ballistic properties make titanium well suited for armor applications. Used as protective armor on personnel carriers and tanks, it makes the vehicles much lighter, increasing mobility of the force. Personal armor vests and helmets for police made from titanium are far lighter and more comfortable than those made from competing materials.

For medical application following titanium grades are common: CP (commercially pure) GOST VT1-0, VT1-00, ASTM B 348 Grade 1, Grade 2 Grade 3, Grade 4, alloys VT6 / VT6S, ASTM B 348 Grade 5, Grade 23, Ti 6Al-4V ASTM F 1472, ASTM, Ti 6Al-4V ELI ASTM F 136.

Computer industry

Since titanium does not become magnetized, it is used in the structural parts surrounding computer components such as disk drives and microchips, which can be ruined by stray magnetism. In the computer industry, titanium is a promising substrate for hard disk drives. . Its non-magnetic properties prevent interference with the data storage process; its ability to withstand heat allows higher temperatures during the coating process, which improves manufacturing rates; and the purity of titanium permits closer read/write head tolerances, increasing disk capacity.

Food industry

Yet there is a very important sector that still needs to be analyzed, that is the food industry field. The absolute biocompatibility and non-toxicity of titanium, its resistance to the aggression of organic substances, to corrosion and erosion explain why this material plays a primary role in all food industry plants.

Titanium has been increasingly used in the manufacture of baking ovens for hams and sausages that is environment with the salinity of 10-14% and operation temperatures around 100 ⁰C, high humidity and continuous operation modes.      

All machinery, even if made of the beat stainless steel, has a useful life not   above 2 years, provided that it is subjected to regular servicing. There is a great demand for titanium for mincing machines that cut and actually mince the meat and in the field of sausage production.

Another field where titanium has started to be recently applied, which is still at an experimental level, not in technical terms but in terms of global efficiency, is the one related to pressurized boilers that is in coffee-making machines distillers.

More update research an studies indicate that there are food liquid, generally wine products, but also alcoholic beverages where titanium can be considered an excellent remedy, since it does not release the elements contains in steels with several titles.

The success of titanium originates from the excellent behavior of this material with high temperatures, its lightness, biocompatibility and resistance. 

Jewelry

Traditionally reserved for industrial uses, titanium has only recently been included as a jewelry material and is increasingly popular. Titanium is a good material for inlay work as it can be joined to different metals to make multi-tone pieces. Titanium is available in pure or alloyed form, but there is no reason to increase its strength with alloys for the purposes of jewelry, therefore the purest grade of titanium is the most desirable for this application.

The variety of titanium jewelry ware includes: rings, bracelets, chains, necklaces, pendants, earrings etc.
Titanium jewelry is lightweight and feels warm to the touch; it is easily adopted by the wearer, and its sober coloring is most sophisticated. It is far more durable and longer lasting than gold or platinum.