Titanium (pronounced /taɪˈteɪniəm/) is a chemical element with the symbol Ti and atomic number 22. It is a light, strong, lustrous, corrosion-resistant (including to sea water and chlorine) transition metal with a grayish color. Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other elements, to produce strong lightweight alloys for aerospace (jet engines, missiles, and spacecraft), military, industrial process (chemicals and petro-chemicals, desalination plants, pulp, and paper), automotive, agri-food, medical prostheses, orthopaedic implants, dental endodontic instruments and files, dental implants, sporting goods, jewelry, mobile phones, and other applications. Titanium was discovered in England by William Gregor in 1791 and named by Martin Heinrich Klaproth for the Titans of Greek mythology.

The element occurs within a number of mineral deposits, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere, and it is found in almost all living things, rocks, water bodies, and soils. The metal is extracted from its principal mineral ores via the Kroll process or the Hunter process. Its most common compound, titanium dioxide, is used in the manufacture of white pigments Other compounds include titanium tetrachloride (TiCl₄) (used in smoke screens/skywriting and as a catalyst) and titanium trichloride (TiCl₃) (used as a catalyst in the production of polypropylene).

The two most useful properties of the metal form are corrosion resistance, and the highest strength-to-weight ratio of any metal. In its unalloyed condition, titanium is as strong as some steels, but 45% lighter. There are two allotropic forms and five naturally occurring isotopes of this element; ⁴⁶Ti through ⁵⁰Ti with ⁴⁸Ti being the most abundant (73.8%). Titanium's properties are chemically and physically similar to zirconium.

History

Titanium was discovered included in a mineral in Cornwall, England, in 1791 by amateur geologist and pastor William Gregor, the then vicar of Creed parish. He recognized the presence of a new element in ilmenite when he found black sand by a stream in the nearby parish of Manaccan and noticed the sand was attracted by a magnet which was then named after him, Gregorite. Analysis of the sand determined the presence of two metal oxides; iron oxide (explaining the attraction to the magnet) and 45.25% of a white metallic oxide he could not identify. Gregor, realizing that the unidentified oxide contained a metal that did not match the properties of any known element, reported his findings to the Royal Geological Society of Cornwall and in the German science journal Crell's Annalen.

Around the same time, Franz Joseph Muller also produced a similar substance, but could not identify it. The oxide was independently rediscovered in 1795 by German chemist Martin Heinrich Klaproth in rutile from Hungary. Klaproth found that it contained a new element and named it for the Titans of Greek mythology. After hearing about Gregor's earlier discovery, he obtained a sample of manaccanite and confirmed it contained titanium.

The processes required to extract titanium from its various ores are laborious and costly; it is not possible to reduce in the normal manner, by heating in the presence of carbon, because that produces titanium carbide. Pure metallic titanium (99.9%) was first prepared in 1910 by Matthew A. Hunter by heating TiCl₄ with sodium in a steel bomb at 700–800 °C in the Hunter process. Titanium metal was not used outside the titanium bar laboratory until 1946 when William Justin Kroll proved that it could be commercially produced by reducing titanium tetrachloride with magnesium in what came to be known as the Kroll process. Although research continues into more efficient and cheaper processes (e.g., FFC Cambridge), the Kroll process is still used for commercial production.

Titanium of very high purity was made in small quantities when Anton Eduard van Arkel and Jan Hendrik de Boer discovered the iodide, or crystal bar, process in 1925, by reacting with iodine and decomposing the formed vapors over a hot filament to pure metal.

In the 1950s and 1960s the Soviet Union pioneered the use of titanium in military and submarine applications (Alfa Class and Mike Class) as part of programs related to the Cold War titanium bar. Starting in the early 1950s, Titanium

Physical

A metallic element, titanium is recognized for its high strength-to-weight ratio. It is a light, strong metal with low density that, when pure, is quite ductile (especially in an oxygen-free environment), lustrous, and metallic-white in color titanium bar. The relatively high melting point (over 1,649 °C or 3,000 °F) makes it useful as a refractory metal.

Commercial (99.2% pure) grades of titanium have ultimate titanium bar tensile strength of about 63,000 psi (434 MPa), equal to that of some steel alloys, but are 45% lighter. Titanium is 60% heavier than aluminium, but more than twice as strong as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys (e.g., Beta C) achieve tensile strengths of over 200,000 psi (1380 MPa). However, titanium loses strength when heated titanium bar above 430 °C (800 °F).

It is fairly hard (although not as hard as some grades of heat-treated steel), non-magnetic and a poor conductor of heat. Machining requires precautions, as the material will soften and gall if sharp tools and proper cooling methods are not used. titanium bar Like those made from steel, titanium structures have a fatigue limit which guarantees longevity in some applications.

The metal is a dimorphic allotrope with the hexagonal alpha form changing into the body-centered cubic (lattice) beta form at 882 °C (1,619 °F). The specific heat of the alpha form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the beta form regardless of temperature. Similar to zirconium and hafnium, an additional omega phase exists, which is titanium bar thermodynamically stable at high pressures, but which may exist metastably at ambient pressures. This phase is usually hexagonal (ideal) or trigonal (distorted) and can be viewed as being due to a soft longitudinal acoustic phonon of the beta phase causing collapse of (111) planes of atoms.

Chemical

The most noted chemical property of titanium is its excellent resistance to corrosion; it is almost as resistant as platinum, capable of titanium bar withstanding attack by acids, moist chlorine gas, and by common salt solutions. Pure titanium is not soluble in water but is soluble in concentrated acids.

While the following pourbaix diagram shows that titanium is thermodynamically a very reactive metal, it is slow to react with water and air.

This metal forms a passive and protective oxide coating (leading to increased corrosion-resistance) when exposed to titanium bar elevated temperatures in air, but at room titanium bar temperatures it resists tarnishing. When it first forms, this protective layer is only 1–2 nm thick but continues to slowly grow; reaching a thickness of 25 nm in four years.

Titanium burns in air when heated to 2,200 °F and in pure oxygen when heated to 610 °C (1,130 °F) or higher, forming titanium dioxide. It is also one of the few elements that burns in pure nitrogen gas (it burns at 800 °C or 1,472 °F and forms titanium nitride, which causes embrittlement). Titanium is resistant to dilute sulfuric and hydrochloric acid, along with chlorine gas, chloride solutions, and most organic acids. It is paramagnetic (weakly attracted to magnets) and has fairly low electrical and thermal conductivity.

Experiments have shown that natural titanium becomes radioactive after it is bombarded with deuterons, emitting mainly positrons and hard gamma rays. When it is red hot the metal titanium bar combines with oxygen, and when it reaches 550 °C (1,022 °F) it combines with chlorine. It also reacts with the other halogens and absorbs hydrogen titanium bar.

Titanium alloys

are metallic materials which contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures), light weight, extraordinary corrosion resistance, and ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, connecting rods on the Acura NSX and some premium sports equipment and consumer electronics. Auto manufacturers Porsche and Ferrari have also followed Acura's lead to use titanium alloys in engine components due to its durable properties in these high stress engine environments.

Although "commercially pure" titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications titanium is alloyed with small amounts of aluminium and vanadium, typically 6% and 4% respectively, by weight. This mixture has a solid solubility which varies dramatically with temperature, allowing it to undergo precipitation strengthening. This heat treatment process is carried out after the alloy has been worked into its final shape but before it is put to use, allowing much easier fabrication of a high-strength product.

Some alloying elements raise the alpha-to-beta transition temperature (i.e. alpha stabilizers) while others lower the transition temperature (i.e. beta stabilizers). Aluminium, gallium, germanium, carbon, oxygen and nitrogen are alpha stabilizers. Molybdenum, vanadium, tantalum, niobium, manganese, iron, chromium, cobalt, nickel, copper and silicon are beta stabilizers. Titanium alloys are usually classified as alpha alloys, near alpha alloys, alpha + beta alloy or beta alloys depending on the type and amount of alloying elements.

Generally, alpha-phase Titanium is stronger yet less ductile and beta-phase Titanium is more ductile. Alpha-beta-phase Titanium has a mechanical property which is in between both.

Titanium dioxide dissolves in the metal at high temperatures, and its formation is very energetic. These two factors mean that all titanium except the most carefully purified has a significant amount of dissolved oxygen, and so may be considered a Ti-O alloy. Oxide precipitates offer some strength (as discussed above), but are not very responsive to heat treatment and can substantially decrease the alloy's toughness.

Aside from titanium-based alloys, the term may refer to "binary" alloys which consist of a nearly even mix, atom-by-atom, of titanium and another element. Nitinol, a shape memory alloy, is a mixture of titanium and nickel, while niobium-titanium alloys are used as wires for superconducting magnets.

Many alloys also contain titanium as a minor additive, but since alloys are usually categorized according to which element forms the majority of the material, these are not usually considered to be "titanium alloys" as such. See the sub-article on titanium applications.

Titanium is a strong, light metal. It is as strong as steel and twice as strong as aluminum, but is 45% lighter than steel and only 60% heavier than aluminum. Titanium is not easily corroded by sea water and is used in propeller shafts, rigging and other parts of boats that are exposed to sea water. Titanium and titanium alloys are used in airplanes, missiles and rockets where strength, low weight and resistance to high temperatures are important. Since titanium does not react within the human body, it is used to create artificial hips, pins for setting bones and for other biological implants. Unfortunately, the high cost of titanium has limited its widespread use. Titanium is the ninth most abundant element in the earth's crust and is primarily found in the minerals Rutile (TiO2), Ilmenite (FeTiO3) and Sphene (CaTiSiO5). Titanium makes up about 0.57% of the earth's crust. The word titanium comes from the Greek word Titans the mythological "first sons of the earth". The pure elemental metal was not made until 1910 by Matthew A. Hunter, who heated TiCl4 together with sodium in a steel bomb at 700-800°C.

Grades

The ASTM defines a number of alloy standards with a numbering scheme for easy reference.

Grade 1-4 are unalloyed and considered commercially pure or "CP". Generally the tensile and yield strength goes up with grade number for these "pure" grades. The difference in their physical properties is primarily due to the quantity of interstitial elements. They are used for corrosion resistance applications where cost and ease of fabrication and welding are important.
Grade 5 is the most commonly used alloy. It has a chemical composition of 6% Aluminum, 4% Vanadium, remainder titanium, and is commonly known as Ti6Al4V, Ti-6AL-4V or simply Ti 6-4. Grade 5 is used extensively in Aerospace, Medical, Marine, and Chemical Processing.
Grade 6 contains 5% Aluminum and 2.5% Tin. It is also know as Ti-5Al-2.5Sn. This alloy is used in airframes and jet engines due to its good weldability, stability and strength at elevated temperatures.
Grade 7 contains 0.12 to 0.25% Palladium. This grade is similar to Grade 2. The small quantity of Palladium added gives it enhanced crevice corrosion resistance at low temperatures and high pH.
Grade 7H contains 0.12 to 0.25% Palladium. This grade has enhanced corrosion resistance.
Grade 9 contains 3.0% Aluminum and 2.5% Vanadium. This grade is a compromise between the ease of welding and manufacturing of the "pure" grades and the high strength of Grade 5. It is commonly used in aircraft tubing for hydraulics and in athletic equipment.
Grade 11 contains 0.12 to 0.25% Palladium. This grade has enhanced corrosion resistance.
Grade 12 contains 0.3% Molybdenum and 0.8% Nickel.
Grades 13, 14, and 15 all contain 0.5% Nickel and 0.05% Ruthenium.
Grade 16 contains 0.04 to 0.08% Palladium. This grade has enhanced corrosion resistance.
Grade 16H contains 0.04 to 0.08% Palladium.
Grade 17 contains 0.04 to 0.08% Palladium. This grade has enhanced corrosion resistance.
Grade 18 contains 3% Aluminum, 2.5% Vanadium and 0.04 to 0.08% Palladium. This grade is identical to Grade 9 in terms of mechanical characteristics. The added Palladium gives it increased corrosion resistance.
Grade 19 contains 3% Aluminum, 8% Vanadium, 6% Chromium, 4% Zirconium, and 4% Molybdenum.
Grade 20 contains 3% Aluminum, 8% Vanadium, 6% Chromium, 4% Zirconium, 4% Molybdenum and 0.04% to 0.08% Palladium.
Grade 21 contains 15% Molybdenum, 3% Aluminum, 2.7% Niobium, and 0.25% Silicon.
Grade 23 contains 6% Aluminum, 4% Vanadium.
Grade 24 contains 6% Aluminum, 4% Vanadium and 0.04% to 0.08% Palladium.
Grade 25 contains 6% Aluminum, 4% Vanadium and 0.3% to 0.8% Nickel and 0.04% to 0.08% Palladium.
Grades 26, 26H, and 27 all contain 0.08 to 0.14% Ruthenium.
Grade 28 contains 3% Aluminum, 2.5% Vanadium and 0.08 to 0.14% Ruthenium.
Grade 29 contains 6% Aluminum, 4% Vanadium and 0.08 to 0.14% Ruthenium.
Grades 30 and 31 contain 0.3% Cobalt and 0.05% Palladium.
Grade 32 contains 5% Aluminum, 1% Tin, 1% Zirconium, 1% Vanadium, and 0.8% Molybdenum.
Grades 33 and 34 contain 0.4% Nickel, 0.015% Palladium, 0.025% Ruthenium, and 0.15% Chromium .
Grade 35 contains 4.5% Aluminum, 2% Molybdenum, 1.6% Vanadium, 0.5% Iron, and 0.3% Silicon.
Grade 36 contains 45% Niobium.
Grade 37 contains 1.5% Aluminum.
Grade 38 contains 4% Aluminum, 2.5% Vanadium, and 1.5% Iron. This grade was developed in the '90 for use as an armor plating. The iron reduces the amount of Vanadium needed for corrosion resistance. It's mechanical properties are very similar to Grade 5.

Materials Selection for Total Hip Replacement

A schematic diagram of human body
The anatomy of the human body has been studied for hundreds of years. One area which has been the focus of much clinical attention in recent years is the ball and socket joint of the hip. It links the femur, the longest bone of the human body, and the pelvis. The spherical head of the femur inserts into a cup shaped recess known as the acetabulum.

An osteoarthritic femoral head
Many people suffer from hip joint pain, caused by the wear and tear produced by walking millions of steps a year. After many years this can lead to osteoarthritis which is the commonest form of degenerative disease of the hip. Often its progress is speeded up by osteoporosis, which causes loss of bone density. Accidents may also lead to irreparable damage of the hip. The picture shows an osteoarthritic femoral head of an 80 year old woman. The cartilage has been worn away, exposing the nerve ends in the underlying bone and leading to a grinding, painful gait. The only solution normally is a total hip replacement operation.

Main components of hip replacement
The picture shows a schematic diagram of the main components of a total hip replacement. The procedure involves sectioning the femoral neck, reaming a cavity into the femoral shaft and cementing or press-fitting a femoral stem component into the bone. The acetabulum is ground away. A plastic cup is then cemented or pressed into the machined hole. There are over 50,000 such hip replacement operations performed in the UK each year.

X-ray of a normal femoral component
X-ray of a migrated femoral component
After the implantation the joint replacement can be viewed using X-rays. The left figure shows a collared femoral component which is centrally located in the femoral canal. The acetabular cup has a metal back enabling the cup to be screwed into the pelvic bone. Although the operation is highly successful in approximately 90% of cases after 10 years, the number of failures is still unacceptably high. In the X-ray on the right we can see a typical form of failure. The implant has moved in the femoral canal. This is as a result of bone loss around the metal stem. The metal stem is very stiff compared to bone, and as a consequence carries most of the body weight. This causes bone to think it is not required, resulting in its loss.

A worn acetabular cup
Bone may also disappear in response to the wear debris produced from the articulation of the femoral head against the acetabular cup. The particles are pumped into the interface between the implant and the bone. Here you can see a worn acetabular cup. Millions of wear particles have been released into the body due to the abrasive action of the femoral head. The head may appear shiny, but if you looked at it under a microscope the surface would appear like a mountain range with lots of small peaks or asperities. These small peaks scratch away the softer plastic cup, releasing debris which causes cell response and eventually bone loss. The metal backing has also come away from the plastic cup.

Failed metallic femoral stems
Another, less common, source of failure is the fracture of the metallic femoral stem. This may occur as a result of the implant moving within the bone. You can see scratches on the surface of the implant, these lead to uneven loading and eventually stem fracture. However, this form of failure is less common as a result of using improved metal alloys.

Mechanical properties of implant materials
There are three main factors which will influence the performance of biomaterials in the human body: biocompatability, mechanical properties and degradation. The table shows the stiffness (Young’s modulus), strength and fracture resistance of a number of materials used for implants. At present we do not have any materials that can mimic perfectly the mechanical property of bone. Metals (steel, Co-Cr-Mo, Ti-6Al-4V) have sufficient strength and fracture toughness but have relatively high stiffness, which can lead to weight shielding problems. Ceramics (alumina, hydroxyapatite-HA) are generally very hard materials, they are strong in compression but exhibit low fracture resistance. polymer (polymethylmethacylate-PMMA and polyethlene-PE) have low stiffness values, reasonable fracture toughness but poor strength.

A Charnley stainless steel implant
Stainless steel is the most commonly used metal for femoral stems in hip replacements. It is an alloy of iron, chromium, nickel and molybdenum. It has extremely high resistance to corrosion, and thus does not degrade in the body. It can be shaped easily which is an important consideration for implant manufacturers who want to minimise production costs. However, problems may arise because of its relatively high stiffness, and the fact that some people may develop an allergic reaction to the nickel content. The picture shows a “Charnley” stainless steel hip implant. It is named after Sir John Charnley who pioneered in the 1960’s many of the techniques and design concepts still used today.

A Freeman cobalt-chromium implant
An alternative to stainless steel is cobalt chromium alloy (27-30% Cr, 5-7% Mo. rest Co). It has good wear properties and is more resistant to scratching. The fact that it contains no nickel means that it can be used in patients who have nickel sensitivity. The cobalt chromium implant shown here is a Freeman prosthesis designed to be used without cement and hence we can see holes for bone in-growth. The top section of the prosthesis is roughened to increase friction and hence stability. The bottom surface has been polished to prevent the stem from rubbing against the inside of the bone canal, which may lead to wear debris.

Johnson and Johnson titanium implants
Developed for the aerospace industry, titanium and its alloys have high strength in relation to their relatively low weight. A titanium implant has a stiffness of less than half that of stainless steel or cobalt chrome, which therefore reduces the effects of weight shielding. Its constituents give it excellent corrosion resistance, but it does suffer from a relatively low fracture toughness and poor wear properties. The two components shown are Johnson and Johnson prostheses manufactured from titanium alloy (Ti-6Al-4V). Although they are similar in geometry, the one implant would be cemented in position and the other uncemented. It can also be seen from the picture that these are modular femoral components - made up of two parts, the femoral stem and the femoral head. This allows surgeons to use finely polished wear resistant metal heads, which do not have to be the same material as the stem - an advantage considering the relatively poor wear resistance of titanium alloy.

PMMA bone cement
The second category of materials used for hip replacement are polymers. There are two main uses for them in total hip replacement. The first is as a grouting material in the form of poly(methylmethacylate) (PMMA) bone cement. PMMA bone cement polymerises in situ. Here you can see a surgeon injecting the doughy material into the femoral canal. It is mixed in surgery from a polymer powder and liquid monomer, and forms a hardened material in 10-15 minutes. The main problem with PMMA bone cement is that considerable heat is released to the surrounding bone during the curing process and this causes cell death. The resulting material has poor resistance to fracture. Other problems also include the shrinkage of the cement and the release of toxic monomer into the blood stream. The other major polymer used is polyethylene. Its main advantage is its wear resistance when used as a concave acetabular cup in a total hip replacement.

A ceramic femoral head
One means of reducing wear is to use extremely hard, polished materials that will be highly resistant to scratching and wear. Ceramics such as alumina and zirconia can be polished to produce a fine, hard surface finish. Therefore they can be used as femoral heads as shown in the picture. Another ceramic used in total hip replacements is hydroxyapatite. It consists of calcium phosphate, a mineral that forms one of the prime constituents of bone. Although it is a relatively weak and brittle material it does have good bioactivity. Therefore it can be used to coat implants, in the absence of bone cement, and achieve excellent fixation.

A hybrid implant
It is possible to combine the best mechanical properties of all the materials described and good engineering design in order to produce an implant with the optimum chance of long term clinical survival. Here is an example of such a 'hybrid' implant. It is a cobalt chromium Freeman, with a ceramic femoral head, hydroxyapatite coating and a nitrided surface finish, which hardens the surface of the stem and helps prevent scratching and the release of metal wear debris. However, there is one more parameter, which plays an important role in implant design, that is cost. Material scientists are constantly faced with the challenge of producing optimum material properties at minimum cost. Conclusions

The total hip replacement is just one of many examples that illustrates how materials can improve the quality of people's lives. Despite the great advances that have been made, there are still a number of problems that need to be solved if hip replacements are to be 100% successful and last the remaining lifetime of patients. Ultimately we would like to produce a material with identical properties to bone. British research groups and companies are leaders in the research and development of new biomaterials. They not only can contribute to improving the quality life of people but also the economic prosperity of the country.

The leading centre for biomedical materials science research in the UK is the Interdisciplinary Research Center based at Queen Mary University of London. Why not find much more about our undergraduate courses in Biomedical Materials Science and Engineering.

Dental Porous Titanium Foam Implants: An Idea with Bite

Prototype of porous Titanium attachment systems for dental implant

More than two out of three Canadians currently have a missing tooth. It's a gap that you'd think would be perfectly filled by high-tech solid titanium dental implants. Currently, however, only about one in a hundred gets a titanium implant. Most of us get by with dentures or bridges because of the current expense, discomfort and technical difficulty associated with dental implants.

But new porous titanium foam dental implants being developed at the NRC Industrial Materials Institute (NRC-IMI) in Longueuil, Quebec, have the potential to transform the way we replace lost or damaged teeth, and in the process take a major bite out of the billion-dollar dental implant market.

The team working on getting this technology from lab to the dentist's office recently won one of NRC's Business Case Challenge 2005 prizes.

"Our porous titanium foam implants will be more effective than any dental implants commercially available today," says Dr. Louis-Phillipe Lefebvre, a materials scientist and the project's leader. The team is currently in discussions with several potential partners for the commercialization of the technology.

Dental implants are pieces of the metal titanium that are surgically inserted into the jaw bone to take the place of the tooth's root, and then capped with a ceramic tooth. Titanium is the metal of choice for this purpose because it's biocompatible — it doesn't trigger the body's immune response system.

However, while 95 per cent of these implants are presently successful, there are some major shortfalls.

X-Ray of skull showing teeth and jaw

Patients must wait up to four months before bone has grown around the implant, anchoring it to the jaw and enabling the ceramic tooth to be fitted onto it. Also, current implants can only be inserted into the front of the jaw, where there's more bone available to anchor the implant.

Enter the porous titanium implant, which is created by a unique NRC-developed metallic foam creation process. Unlike current solid titanium implants, the NRC-IMI material is porous, much the same as a sponge, and more importantly in this case, bone. This provides a site for bone cells to grow into the implant and more solidly anchor it. This new porous yet durable material facilitates the creation of smaller implants that can be used in the back of the jaw. In addition, in some cases its use will avoid the need for the patient to have a bone graft, making the surgery simpler, faster and cheaper.

"This opens a totally new area for dentistry and represents an important new niche market for these porous implants," says NRC-IMI Business Development Officer Blaise Labrecque. Labrecque believes that once dentists see how effective the porous titanium implants are in the back of the mouth — including quicker healing — they'll also use them for replacing front teeth as well.

This summer and fall the NRC-IMI team working to commercialize the porous titanium implants is conducting tests to conclusively demonstrate how well bone grows into a titanium foam implant.

The results of these tests will be an important advance in getting the implants into the mouths of Canadians. Implants are the fastest growing area of dentistry. The worldwide dental implant market it estimated at $1.2 billion (U.S.) and growing at 15 per cent annually.

The NRC-IMI team members say that what's most exciting about their titanium foam technology is that it's a platform technology with numerous applications. The group is already collaborating with a major orthopaedic (hip joints, knees, etc.) implant manufacturer. For Lefebvre, getting into dentists' office will be an important and high profile first step in creating positive word-of-mouth about the innovative material.

"People readily imagine new applications when they see the material," says Dr. Lefebvre. "This means they can participate in the innovation and the dream."

Machining Titanium Implants

MEDICAL DEVICE MANUFACTURERS FACE TOUGH challenges. Their customers are demanding ever smaller, more complex parts produced with extraordinary accuracies from difficult to machine materials, such as titanium. On top of this, they must operate under the close scrutiny of regulatory agencies that require extensive and costly compliance documentation.

Orthopedic devices are designed to conform to the complex shape of bones and joints, so the machining of these parts is also complex. Devices machined from bar stock require a lot of material to be removed, resulting in an expensive process because of the low machinability rating of many of the materials involved. As a result, some parts are cast to near net shape, and that often requires fixturing that is complex and expensive. Another issue that adds to the complexity of machining is the tight tolerances required—0.002 in., or less—for most devices.

These pressures have given rise to new technologies to help shops that manufacture medical parts to cope and compete. Agile 12-axis turning machines tools, new insert grades and innovative thread-whirling machines are capable of producing complex parts to extreme tolerances, while innovations in EDM result in the production of high-quality parts at faster rates by eliminating the problems that were inherent in earlier technology.

TROUBLES WITH TITAMIUM
Stainless steels and titanium are the materials most used for medical implants. Stainless steels typically are used for devices that will not stay in the body permanently. Titanium typically is preferred for medical implants because of its light weight, high strength and biocompatibility. Also, titanium implants are compatible with magnetic resonance imaging and computed tomography imaging procedures, so they do not interfere with those procedures if the patient needs them after the implant is made.

Titanium 6AL-4V ELI is the standard material used for the manufacture of hip joints, bone screws, knee joints, bone plates, dental implants, and surgical devices. However, cobalt/chromium alloys are coming into use more often because they are stiffer, tighter grained and cleaner than titanium.

Machining titanium alloys requires cutting forces only slightly higher than those needed to machine steels, but titanium alloys have metallurgical characteristics that make them more difficult to machine than steels of equivalent hardness.

Titanium has a work-hardening characteristic that eliminates the stationary mass of metal (built-up edge) ahead of the cutting tool. That makes for a high shear angle in machining that causes a thin chip to contact a relatively small area on the cutting-tool face. Because of this work-hardening characteristic, feeds should not be stopped while tools and workpieces are in moving contact. The high bearing forces produced by machining in this way, combine with the friction developed by the chip as it rushes over the bearing area to result in a great increase in heat on a localized portion of the cutting tool. Heat generated by cutting titanium does not dissipate quickly because it is a poor conductor. Therefore, most of the heat is concentrated on the cutting edge and the tool face.

The combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.

To make matters worse, titanium alloys have a strong tendency to alloy with or to react chemically with the materials in cutting tools at tool-operating temperatures, and they have a tendency to gall as chips weld to the cutting edges of tools.

These difficulties multiply as tools start to wear, so tools used to machine titanium and its alloys should be watched carefully to make sure they are sharp, and they should be replaced before they dull. The rule-of-thumb in machining titanium and its alloys is that if you see any change in the machining process, you should change the tool immediately because it is likely that it is becoming dull

Another reason to keep tools sharp is that titanium can catch fire when cutting with worn or broken tools. The metal generates oxygen when it burns, so the fire can become self-sustaining. Therefore, many shops that machine titanium do not run "lights out," and they equip machines with fire-suppression systems.

With its relatively low modulus of elasticity, titanium has more "springiness" than steel, so work tends to move away from cutting tools unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing and tolerance problems. Consequently, rigidity of the entire system is very important, as is the use of sharp, properly shaped cutting tools.

12-AXES CONTROL MEANS LESS WORKPIECE HANDLING
The need to reduct the cost of producting complex parts is especially keen in the medical industry. This has given rise to advanced machine tools with up to 12 axes of motion that allow for total positioning capability in any spatial envelope, while increasing the number of operations that can be performed on a workpiece with a single setup and without repositioning or handling.

For example, 12-axis turning centers produced by Tornos Technologies US Corp. produce complex parts in a single setup with all 12 axes operating simultaneously. In addition to machining complex parts, the machines provide close tolerance work and fine surface finishes. The control in Tornos' 2000 series turning centers is coupled with software that runs on a Windows PC for complex part programs. The software is written so that the central clock in the control is used as an electronic cam, making the machine mimic the action of a conventional, camoperated machine.

Axis paths are calculated by data processing and stored by the control as data tables. Each axis has its own control chip—analogous to an "electronic cam"—that stores only its toolpath as a step table, a sequence of moves in one or two axes. A clock signal generator reads and executes steps every eight milliseconds. One data table is used for the toolpath axes, another for spindle speed, rotation and stops, and a third for machine functions. Data tables are programmed offline for each part.

The central clock synchronizes the reading of the multiple, individual toolpaths. The cycle in a camoperated machine is limited to 360o. In the Tornos machines, replacing the cams with stored programs eliminates physical, angular limits on the machine. The control reads data in parallel— not one at a time —so the machine can have four tools cutting simultaneously.

INSERTS FOR TURNING
To increase productivity and reduce tooling costs, Sandvik Coromant recently introduced a line of round inserts for turning hip joints. When used for internal turning of the spherical cup in a ball and socket hip joint, the company says these inserts provide a balance of security and productivity, while optimizing the roughing process when machining direct from castings.

In roughing applications, the inserts' round shape imparts a strong cutting edge and resistance to excessive notch wear resulting in fewer tool changes. Because lower temperatures are generated with these inserts, operators can increase feeds and speeds to maximize production. The company also offers toolholders with positive, D-style inserts for finishing and spherical turning. These also can be used for internal turning in applications where accessibility is limited.

Sandvik Coromant's round inserts are compatible with the company's CoroTurn 107 boring bars and its EasyFix method of achieving the correct cutting-edge center height.

The company says the round inserts and D-style inserts are good for machining titanium and cobalt chromium implants. When machining with round inserts and cobalt chromium, the company recommends grade GC1030. Grade GC1105 is the best grade for D-style inserts and cobalt chromium. Grade H13A achieves the best result while machining titanium for both round and D-style inserts.

WHIRLING THREADS
MEDICAL COMPONENTS continue to get more complex to allow for easier use by surgeons. However, this complexity requires new attachments for special turning and milling operations to generate the complex shapes required. For instance, Tornos has designed a new thread whirling unit to whirl the high helix angles used on some bone screws.

Unlike thread cutting and tapping, thread whirling produces clean contours without burrs. Thread whirling can be done for external thread cutting and internal tapping. The process is carried out on an automatic lathe and requires a high-frequency spindle turning at speeds to 30,000 rpm.

During internal tapping, the spindle axis must run parallel with the part being machined. The internal whirling process is 60 percent faster than conventional tapping. Also, the tools used have a longer useful life, and Tornos says that more than 2,500 titanium parts can be tapped without tool breakage. Cutting speeds are high, so machining time is shorter. There are no burrs or residual chips and the thread cutting depth can be more than three times the diameter of the thread. It is even possible to machine to the bottom of a blind hole.

For external threads, a device fitted to the end of the lathe rotates and inclines in relation to the thread pitch angle being produced. Machining is done by a bell-shaped tool comprising three cutters of the same section as the thread being machined. The spindle driving the whirlthreading tool revolves at high speed — up to 12,000 rpm — while the part simultaneously turns in the opposite direction at slow speed. The feed rate is synchronized with the two rotational speeds and the process continues until the required threading length is achieved. The hard metal tool must have the same shape as the thread being produced.

The surface of the threads produced is perfect because the tools rotate at high speed in the opposite direction to that of the part, thus avoiding the undesirable face lands that sometimes are found with conventional threading done with milling processes.

The whirling process also eliminates the long withdrawal of the bar from the guide channel, so it helps to avoid seizure due to an excessively long projection.

A BETTER WIRE EDM PROCESS
Wire electrical discharge machining (EDM) is a popular process for producing intricate, highly accurate medical devices because the process is not affected by workpiece hardness and can be used to machine hard, difficult-to-cut alloys. However, the process affects titanium in two ways: First, it changes the color of the natural highly corrosion-resistant oxide coating on the surface of titanium parts from gray to a bluish tinge. Secondly, and more troublesome, during the EDM process tiny droplets of copper and zinc from the wire are redeposited on the surface of the part being machined. That backplating requires an expensive cleaning process because copper is not biocompatible.

Charmilles Technologies has developed a new EDM generator — called CleanCut — that the company says cuts faster than previous technology and makes sharper shapes. The generator also reduces the undesirable surface effects of EDM machining. By generating alternately positive and negative discharges, the CleanCut generator eliminates the bluing of the surface of titanium parts. Also, Charmilles says the generator significantly reduces copper pollution of titanium surfaces reducing postmachining cleaning costs.

MICROMOLDING
The drive to make medical parts smaller and stronger has led Phillips Plastics Corp. to develop a process for micro molding titanium medical parts as small as 0.0001 cu. in. While the process follows the same guidelines as most other injection molded metals, titanium generally produces a rougher surface finish and thinner wall sections. Advantages of the process include high material usage and low waste, high cavity-to-cavity repeatability and tight tolerances of +60.001 in.

SATISFYING THE FEDS
The Food and Drug Administration's stringent Quality Systems Requirements govern the practices of medical device manufacturers. These quality standards require manufacturers making medical devices to document every action that is taken on a part while the manufacturer has custodial possession of it. All material — bar stock and castings — is serialized and all documentation must match. If at any time the serial number on the paperwork does not match the material, the entire lot must be scrapped.

To comply with FDA regulations and maximize machine tool productivity, Odyssey Medical in Memphis, Tenn. goes to great lengths to validate and improve process quality. Odyssey has implemented stringent controls for process prove-out and an in-process technique called Precontrol for monitoring part tolerance and process stability. Precontrol augments standard practices of control charts by triggering the operator to make adjustments to machine offsets well in advance of trouble. Tim Gooch, director of technical services at Odyssey, explains, "Part specifications control the acceptability of the part, but Precontrol controls the actions of the operator." The technique divides the tolerance band into green, yellow and red areas with the middle 50 percent of the tolerance band being the operator's target operating zone. Any two consecutive parts that exceed this tighter tolerance trigger the operator to adjust the machine offsets. Consecutive parts that are off on opposite sides of the tolerance indicate that the process is unstable requiring adjustments to the process. Parts outside the tolerance zone are, of course, scrapped.

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