Titanium has played a major part in the history of the world from the mid 20th century up until today. It was discovered in the 1700s, produced in small quantities until the late 1800s, and finally went into commercial production once the Kroll process was devised and the militaries of different countries started to understand its importance.
Titanium was discovered in 1791 by the clergyman and amateur geologist William Gregor as an inclusion of a mineral in Cornwall, Great Britain. Gregor recognized the presence of a new element in ilmenite when he found black sand by a stream and noticed the sand was attracted by a magnet. Analyzing the sand, he 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. Realizing that the unidentified oxide contained a metal that did not match any known element, Gregor 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 Müller von Reichenstein produced a similar substance, but could not identify it. The oxide was independently rediscovered in 1795 by Prussian chemist Martin Heinrich Klaproth in rutile from Boinik (the German name of Bajmócska), a village in Hungary (now Bojničky in Slovakia). 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 that it contained titanium.
The currently known processes for extracting titanium from its various ores are laborious and costly; it is not possible to reduce the ore by heating with carbon (as in iron smelting) because titanium combines with the carbon to produce titanium carbide. Pure metallic titanium (99.9%) was first prepared in 1910 by Matthew A. Hunter at Rensselaer Polytechnic Institute by heating TiCl4 with sodium at 700–800 °C under great pressure in a batch process known as the Hunter process. Titanium metal was not used outside the laboratory until 1932 when William Justin Kroll produced it by reducing titanium tetrachloride (TiCl4) with calcium. Eight years later he refined this process with magnesium and with sodium in what became known as the Kroll process. Although research continues to seek cheaper and more efficient routes, such as the FFC Cambridge process, the Kroll process is still predominantly used for commercial production.
Titanium sponge, made by the Kroll process.
Titanium of very high purity was made in small quantities when Anton Eduard van Arkel and Jan Hendrik de Boer discovered the iodide process in 1925, by reacting with iodine and decomposing the formed vapors over a hot filament to pure metal.[1]
Matthew A. Hunter, working for Rensselaer Polytechnic Institute in cooperation with the General Electric Company, suspected that the high melting point of titanium would make it a great candidate for their new incandescent lamp filaments. However, their calculations on the melting point were a bit off, and the project was abandoned, but because of their efforts, there was now a viable way to extract the metal from the ore [2].
Hunter’s process involved mixing TiO2 with coke and chlorine. By applying heat to the titanium dioxide, it would produce TiCl4, which could be reduced with sodium to create an alloying agent that would, for another few decades, mostly be used as an alloying agent in steel. At this point, it also became clear that the metal was a really good white pigment, and it became the choice for white paints and other products.
However, while this process did work, it was not very useful or effective for larger scale manufacturing. It wasn’t until 1938, when metallurgist William Kroll developed his method, that widespread use became a real possibility. His process, called the Kroll Process, used magnesium as the reducing agent instead of sodium and this is still the most widely used method today [3][4].
Even though larger production was a real possibility, by the end of 1947 only two tons of titanium were being produced in the United States. It took some encouragement from the government to really get the industry up and moving.
In 1948 the U.S. government began funding endeavors with titanium because of the realization of how important the metal could be for aircraft, missiles, spacecraft, and other military purposes. This was the first time a structural material received so much financial, scientific, and political attention. The Department of Defense provided a range of incentives to develop the metal, and by 1953 the annual production of titanium reached two million pounds [5].
By the early 60s, titanium had made its way into the commercial market and was used regularly in aircraft manufacturing. Still, it remained the metal of choice in the military for its unique characteristics, and for the next few decades with would still be considered an “aerospace metal.” In fact, by 2006, 73% of titanium metal in the U.S. was still used for aerospace construction [6].
Even so, more uses were being developed especially in the medical and dental industries [7]. In the 50s, Swedish doctor Per-Ingvar Brånemark conducted research on the way bone reacted to titanium. It was already known that titanium was non-toxic in the human body, but Brånemark saw how closely bone would adhere to titanium and, in 1965, placed the first titanium dental implant into a human volunteer. Since then it has been used in many prosthetics and other medical instruments [8].
Now, millions of tons of titanium are produced every year, although most of it (as much as 95%)[2] is still used in its dioxide form as a white pigment in food, paint, and cosmetics. As the market continues to grow, though, and mining and processing methods are improved, new applications in different industries continue to emerge [5].
1791 – Discovery of the metal by William Gregor
1793 – Confirmation and naming of the discovery by M.H. Klaproth
1910 – Matthew A. Hunter develops a method to extract the metal from the ore
1938 – William Kroll develops his method for extracting titanium
1947 – Two tons of titanium produced
1948 – The U.S. Government starts providing incentives to develop titanium manufacturing
1953 – Annual production reaches 2 million pounds
1965 – Per-Ingvar Brånemark places the first titanium dental implant
2010 – Over 6 million tons being produced annually
Titanium is the 22nd element on the periodic table and the ninth most abundant element in the earth’s crust (and the forth most abundant metallic element). The minerals in which it is found occur in alluvial and volcanic formations, and deposits usually contain between 2 and 12% heavy minerals – such as ilmenite, rutile, leucoxene, zircon, and others. Its unique properties make it an ideal metal for a wide range of applications [5].
Properties
The weight to strength ratio is one of the most attractive properties of titanium. This metal is as strong as steel and 45% lighter, but while it’s twice as strong as aluminum, it is 60% heavier. This ratio is what makes it so ideal for aerospace and other applications, but there are many other properties that make it such an important resource [2].
Tensile Strength – The tensile strength of titanium and its alloys range from 20,000 psi to more 200,000 psi, but most commercial grade titanium averages around 63,000 psi.
Fatigue Strength– A titanium alloy can have a very high-cycle fatigue strength. The actual strength can be determined by the surface finish, which is why care must be taken to avoid stress concentrators.
Density – Titanium is around 56% as dense as steel at 4.54 grams per cubic centimeter. It is somewhere between aluminum and iron.
Melting Point – The melting point is 3,034°F or 1,668°C.
Alloys – The properties of titanium make it easy to alloy with aluminum, iron, manganese, molybdenum, and other metals.
Coefficient of Thermal Expansion – The thermal expansion properties of titanium are lower than steel, copper and aluminum. The size changes very little under extreme temperature changes.
Electrical Conductivity – Titanium is not a good conductor of electricity. As a comparison, if copper were to have 100% conductivity, titanium would be around 3%.
Corrosion Resistance – A chemically inert oxide film forms on the surface of the metal, creating a high level of corrosion resistance to most mineral acids and chlorides.
Toxicity – This metal is non-toxic in the human body and biologically compatible with human tissue and bone.
Magnetics – Commercially pure titanium and all its associated alloys are non-magnetic.
Even though titanium ore is a relatively abundant element, because of the high reactivity of the metal with oxygen, nitrogen, and hydrogen in the air the mining, milling, and fabrication process is still very complex and costly. This, in turn, is what drives the price of titanium up, despite it being a relatively common metal [4].
Though the combined properties of titanium are coveted by many other metals, these properties still can be found. Due to the cost of titanium, the metal competes on a regular basis with other applicable metals. Titanium competes with metals like aluminum for strength, nickel for corrosion, and calcium carbonate for white pigment. There are several other metals that obtain excellent ratings in these applications, but none have been found with all the same combined efforts.
Mining Titanium
The expenses tied to mining titanium also contribute to the overall price of the materials. It is a fairly labor-intensive process that comes under the prevue of various regulatory requirements. Titanium is most commonly strip mined (open pit mining), and then it is sent, soil and all, to a processing center where it will undergo extraction through by means of the Kroll Process [10].
The Kroll Process was first introduced in 1938 and (combined with vacuum distillation) is still the most common method used to extract titanium metal from its ore. This process does not produce titanium at the rates in which it is possible to extract and refine steel or aluminum, but it remains the best option in both cases. (It is important to note that it s precisely this difficulty in extraction that contributes to titanium’s price, rather than the rarity of the metal.) The Kroll Process has developed a little over the year, but still follows the same basic steps:
Kroll Process Overview
Ore to Sponge
The first steps in the extraction and refining process do not actually produce a commercially usable material, but titanium sponge. The titanium oxide ore is combined with chlorine in a bed of petroleum coke, to form titanium chloride. The titanium reacts with the chlorine to create the gaseous titanium tetrachloride (TiCl4).
All the fine particles left over from the coke and ore must be removed from the tetrachloride, which is then liquefied and sent through a distillation to eliminate even more of the volatile impurities that will dissolve at the same relative boiling point as TiCl4. It will still take one more distillation process to bring the tetrachloride to 99.9% purity [2].
The titanium chloride is then reduced using magnesium or sodium as the reducing agent. The result of this reduction is the titanium sponge (a very porous version of the metal). This reduction takes place inside steel reactor that must be welded shut and heated to 1200C. The seal is a critical step because any moisture or oxygen in the mix will create a very brittle product.
Once it’s out of the reactor, the salt or magnesium must be cleaned out of the pores before the sponge is crushed and distilled again to reclaim some of the reagents and other materials.
Sponge to Ingot
Titanium sponge is not a usable product. It must be processed further into ingot. This is accomplished by putting the materials in a vacuum or argon environment where it can be melted down.
Vacuum arc re-melting is the most common process used to create titanium ingots as well as some high-performance alloys. Cold-hearth melting is also an option that involves an argon or vacuum chamber and can separate high-density contaminates.
This process allows the industry to extract and refine titanium at a commercially sustainable level, but it is still far from the effective process used for other metals. Metals like iron can be refined with a continuous flow through a blast furnace, but titanium must be processed in batched. The ore and sponge can be sealed in the reactors for days, and the total extraction and processing time can take weeks. Currently, the largest reactors can produce about a tone of titanium in a day, compared to the largest iron blast furnaces that produce around 10,000 tons (“Titanium: A Technical Guide” Second Edition, Matthew J. Donachie, Jr. Published: 2000).
Ingot to General Mill Products
After the titanium is made into ingots, it is ready to be processed further to produce general milled products such as billet, bar, extrusions, plate, sheet, tube, and wire. Extreme care has to be taken at this step because the product is easily influenced by the conditions around it.
A billet reduction process is the most common way to get started refining the titanium and producing a more refined grain. If the billet is intended for even more forging (secondary fabrications) it may go through a refining process several times. Roll cogging and hot roll finishing is then used to produce plate, sheet, strip, bar and other products.
Machining Titanium
Titanium has a number of characteristics that must be addressed in order to achieve effective production rates and a good, clean surface finish. It’s important to understand those properties in order to properly cut/drill/mill/polish titanium products. Some of the reasons milling must be done with extreme care include:
Titanium can react with the cutting tools, which could lead to seizing, galling, and cause other unwanted blemishes to the surface.
The low thermal conductivity of titanium leads to an unusual chip-forming tendency. This, in turn, can cause an excessive buildup of heat on the cutting tools. Normally, the high temperatures would dissipate in the chip (like it does with steel or aluminum). With titanium, all that heat is absorbed by the cutting tool.
Titanium is a low elastic modulus, which means that it is harder to cut, and may cause deflections of the work pieces. This also means that greater clearances of cutting tools may also be necessary.
These special characteristics make milling titanium a very time-intensive process. This is also why 40% to 50% of titanium’s costs are directly related to machining. While there are some other processes in development that could, in theory, increase production speeds, at the moment, it’s all about using the right tools in the right way [12].
Producers must find a way to balance cutting speed, tool wear, and profitable sustainability. This is not always an easy task, but there are some techniques and guidelines that make it easier to manage. These include:
Maintain the tools – If a cutting tool loses its sharp edge, it can cause very poor results, slow the process even further, and cause tearing and a bad surface finish. Dull machining tools can also lead to more heat buildup, which will contribute to further wear. These instruments must be kept sharp and in peak condition to extend their lifespan and produce consistent results.
Maintain high feed rates (and never stop feeding) – While cutting speed can be detrimental to titanium milling, feed rate is not. While there are limits, of course, the manufacturer needs to keep the highest rate of feed that will still deliver the expected results. Also, once a blade starts cutting into the titanium, it is important to keep the metal moving. If a tool stays in moving contact with titanium, it can cause hardening, smearing, or seizing.
Keep cutting speeds low – The cutting speed has a direct impact on the heat buildup in the tool. Too much heat results in damaged tools and poor products. Slightly higher speeds are possible with commercially pure titanium, but alloyed titanium requires lower speeds. Even a small increase in speed can lead to problem with the tools, though, so this should be carefully maintained.
Use plenty of cooling fluid – In order to offset the amount of heat accumulating in the tool and on the sample, generous amounts of cooling liquid need to be used. This liquid dissipates the heat and carries away the chips. This will help protect the edge of the tool and ensure a longer usable lifespan. Some tools use a through-spindle coolant that delivers the liquid right to the cutting edge while others require a high-pressure coolant pump that will stop chips from welding to the cutting edge.
The chemical element, titanium (Ti), has the atomic number 22 and an atomic weight of 47.90. It belongs to the first transition group and has a number of similarities with silica and zirconium. In lower oxidation states it has some similarities with chrome and vanadium as well.
When it is exposed to air at high temperatures, titanium metal and its alloys will oxidize immediately, forming a passive, but protective oxide coating. The same effect can be accomplished with nitrogen, which creates a coating of titanium nitride.
This protective oxide coating then serves to protect the metal from further oxidation, meaning that the first coat will appear quite readily, but anything that goes deeper will take a longer time.
Titanium is a thermodynamically reactive metal. It actually burns before the melting point is reached, which is why melting must be done in a vacuum or other inert atmosphere. By eliminating the oxygen, it is possible to heat it to the melting point without turning the metal to powder [12].
Titanium Alloys
Titanium alloys are grouped into four distinct types: commercially pure, alpha, beta, and alpha-beta. Each of these forms of the metal provides specific benefits, which means that different alloys will be used for different projects. Alloying elements are used to stabilize either the alpha or beta phase, which can create different characteristics to address different needs and to create titanium alloys that can be strengthened by heat treatment [13].
Commercially Pure Titanium – When titanium is not alloyed with any other metal, it is very ductile but has a lower strength. This makes it the preferred choice for applications in which corrosion resistance is a greater importance.
Alpha Alloys – These alloys are easy to weld and provide reliable strength at elevated temperatures. They are created with neutral alloying elements and alpha stabilizers, such as aluminum, but the results are not heat treatable.
Beta Alloys – These are the most common choices for projects that require higher tensile strength. A beta alloy is also heat treatable and contains sufficient beta stabilizers to maintain their beta phase even when quenched. Beta alloys use stabilizers like molybdenum or silicon, to create the desired characteristics.
Alpha-Beta Alloys – These are the most commonly used alloys because they combine the best characteristics of the other two alloys, creating a balance between strength, weight, and corrosion resistance. These alloys are heat treatable and are made with both alpha and beta stabilizers.
Unalloyed (Commercially Pure) Titanium Grades
CP titanium is graded on its corrosion resistance, formability, and strength. Unalloyed grades include 1 through 4, as wells as 7, 11, and 12. The higher the grade number, the higher the strength, however, the highest corrosion resistance and ductility is often found in the lower grade
Grade 1 (Ti 35A) – This grade has the highest formability and corrosion resistance but also has the lowest tensile strength
because of the low levels of iron and oxygen [14]. The most common forms for this grade are plate and tubing, and it is often used for chemical processing and marine applications.
Other jobs that will require a high resistance to corrosion, including food processing and pharmaceutical industries, bleaching or washing paper and pulp, medical tools and devices, anode and cathode cell components, desalinating various liquids, and consumer products utilize this grade on a regular basis.
Grade 2 (Ti 50A) – This is one of the most common grades of commercially pure titanium. It provides a good balance of strength, formability, and weldability. It has the same level of corrosion resistance as grade 1 titanium, but it is somewhat stronger [15]. It is commonly used in aerospace, marine, chemical, medical, and architectural applications such as:
Grade 3 (Ti 65A) – While this grade has a very high tensile strength and corrosion resistance, it is one of the least used grades of pure titanium. It still has some uses in a number of major industries, such as the marine and chemical industries. It may also be used in power plant cooling systems, but other alloys seem to be a more popular choice [16].
Grade 4 (Ti 80A) – This is a very strong grade of titanium that possesses high levels of formability and weldability [13]. This is the grade for most medical titanium and is used for much of the surgical hardware in use. It also acts as a good heat exchanger and can be found in cryogenic vessels. Some of the more common applications include airframe and aircraft engine components, chemical processing machinery, heat exchangers, desalinization plants, and corrosive waste disposal equipment.
Alloyed Titanium Grades
Grade 5 (Ti 6Al-4V) – This is the most common grade of all the alloys and accounts for about half of the titanium used in the world. It can be heat treated to increase its strength, can be used at extremely high temperatures (up to 600F), and it has a good ductility. Titanium grade 5 is alloyed with 6% aluminum and 4% vanadium and is commonly used in aircrafts, automobiles, recreational equipment, and many other major industries.
This is simply the mostly widely used grade, and therefore it can be found in almost any industry that requires structural components with a high strength-to-weight ratio and corrosion resistance [19].
Grade 7 (Ti-0.15Pd) – This grade is commonly found in the chemical industry because of its high corrosion resistance. IT is mechanically and physically very similar to grade 2 titanium, but because palladium is present as an interstitial element, it receives other beneficial effects [18]. It has an excellent weldability rating, and offers a good level of strength and ductility. This makes it a valid choice for pollution control equipment, desalination equipment, hydrometallurgical extraction, paper and pulp bleaching and washing, chemical processing equipment, and anode and cathode cell components.
Grade 12 (Ti0.3Mo0.8Ni) – This alloy is very weldable and can be hot or cold formed with press brake forming, drop hammer, or stretch forming. It is often used in heat exchangers or in high temperature chemical applications, as well as marine and aerospace products because of the high level of heat resistance [20].
Other uses include anything where superior corrosion resistance in extreme temperatures is a concern, and it is used to make valves, pumps, pipes, fittings, and a range of other products.
Grade 23(Ti 6Al-4V ELI) – This grade is alloyed with the same metals as grade 5, but it is more pure and has a reduction of oxygen content. It has a good damage tolerance, and is often used in coils, wires, strands, but it is the choice material for various medical and dental applications. ELI grade titanium can resist damage better than other alloys (with high fracture toughness) and has better performance at cryogenic temperatures.
ELI titanium is used for medical implants because of its biocompatibility, osseointegration, good fatigue strength, light weight, and, most particularly, for its corrosion resistance. This is a result of the stable, continuous oxide film that immediately forms on the metal when it is exposed to oxygen. The human body is full of fluids that can corrode normal metals, making this characteristic extremely important [21].
While this is the choice metal for surgical implants and equipment, it is still often used in aircraft components, salt water equipment, cryogenic vessels, and other structural components [19].
Titanium Oxides
Titanium is normally coated with an oxide layer that usually renders it inactive and protects it from corrosive elements. When it first forms, it is only 1-2 nm thick, but it will continue to grow slowly. Titanium dioxide is actually the most commonly used titanium compound. It is used as a white pigment in paint, makeup, sunscreen, and a range of other products [4].
Applications across Multiple Industries
Aerospace – The aerospace industry has long understood the value of titanium. The industry is always looking for new ways to produce more fuel-efficient planes, and the strong, light-weight metal is an ideal choice. It brings the necessary structural reliability without the excess weight. Some estimates say that 72% of the titanium metal in the United States is used in aerospace construction [5].
Currently, titanium is used in the air frame and wing structures as well as smaller parts like compressor blades, rotors, stator blades, and other components of a turbine engine.
Titanium has several characteristics that make it particularly valuable in the aerospace market. First, the weight-to-strength ratio means that designers can create aircraft that are much more fuel efficient and a structural design that requires less interior space. Second, the natural corrosion resistance is particularly important under the galleys, which can be a very corrosive environment. Finally, titanium has very reliable thermal expansion rates, which is important when it is likely to be subjected to extreme variation on a single journey.
Automotive – Compared to aerospace, the automotive industry has not been as fast to adopt titanium as a major structural component. Even though the same beneficial characteristics (corrosion resistance, light weight, strong) apply, the consumer market (which is much more influential for automobiles than for aircraft) is very cost conscious, which limits the amount of titanium that is used in most vehicles.
Currently, most titanium is most commonly found in race cars or other specialty vehicles where weight and performance are critical, and the actual price is an afterthought.
However, titanium offers enough potential benefits that this is starting to change. The corrosion and heat resistance means that engine parts can last much longer, and the light weight materials mean it won’t have to work as hard. Now, as processing costs are starting to decline, there are more engines that use titanium for connecting rods, valves, springs, pins, and other components [2].
Marine – The corrosion resistance of titanium makes it a choice material for a wide variety of marine applications. Titanium can easily stand up to the corrosiveness of salt water, and research continues to determine who well it can handle steam, oil, and stack gasses, too [3].
Currently, titanium is used in water-jet inducers, sweater valve balls for submarines, fittings for yachts, propeller shafts, fire pumps, heat exchangers, and a range of piping and exhaust systems. If the components come into contact with sea water, opting for titanium is a good choice.
Medical – Since titanium does not react adversely to the human body, it is the choice metal for artificial hips, pins for setting bones, and dental or other biological implants. Every year, millions of pounds of titanium are implanted in patients because its properties lead to high osseointegration (the way bone integrates with the metal). Due to its non-corrosive characteristics, it will last for a long time in the body. Doctors won’t have to go in and perform a replacement a few years later [21].
Beyond bone and joint replacements, titanium is used for dental implants, cardiovascular devices (pacemakers), and surgical instruments. Medical grade titanium alloys also provide a better strength to weight ratio than other metals, such as stainless steel.
Recreational – The strength-to-weight ratio of titanium makes it a quality material for many different recreational products. Currently it is used in everything from bicycles to golf clubs. Due to the costs associated with milling and fabricating the products, they can be more expensive than similar products made of aluminum or steel, but many consumers are willing to pay the increased prices for higher quality.
As the industry continues to develop, new trends and processes will affect how titanium is mined, processed and milled. A lot of the current work is focused on bringing the costs of production more into line with the other abundant metals that are used in so many applications. While the Kroll Process has remained the most commonly used method to produce usable titanium, there is a lot of interest in developing something that can surpass its abilities to produce titanium.
Some new alternatives have already appeared and are starting to gain more attention. The Armstrong process (22) and Fray-Farthing-Chen (FFC) Cambridge process [2] have been under development for some time now and are starting to get closer to industrial implementation [23].
Both of these processes use electrolysis to process the titanium. Each one uses different mediums to inject a little electricity into the mix – the Armstrong processes uses a flow of excess sodium, the FFC process uses a bath of calcium chloride – which makes it possible to extract oxygen from the titanium dioxide.
Previously, electrolysis wasn’t an option because the input oxide and output metal are not in liquid form, the way it is when metals like aluminum are processes. Titanium’s melting point is just too high to make the traditional electrolysis process feasible. These new processes are using titanium in powdered form, rather than ingots, to create an environment in which the electrical current can have the desired effect [24].
The demand for titanium is also expected to grow. While there was a definite downturn in the market in 2009 and another slowdown in 2012, the trends are pointing to a lot of growth in the industry. Some reports [25] expect the titanium industry to grow by 4 to 5% by 2018.
Aerospace remains the main market for titanium, accounting for 60 to 75% of the titanium currently being used (graphs). Even though a lot of the new generation of passenger aircraft is using a lot of carbon fiber reinforced polymers, titanium will still be in very high demand because these fibers are compatible with titanium but not with aluminum. In other words, the titanium will continue to be part of high-value-added parts.
1- https://en.wikipedia.org/wiki/Titanium
2 – http://www.berkeleypoint.com/learning/titanium.html
3 – http://www.mineralseducationcoalition.org/minerals/titanium
4 – http://minerals.usgs.gov/minerals/pubs/commodity/titanium/mcs-2013-titan.pdf
5- http://www.hardassetsinvestor.com/features/5051-abundant-titaniums-import-to-aerospace-well-known-but-other-industries-could-bring-new-demand.html?showall=&fullart=1&start=13
6 – http://www.asminternational.org/eNews/Titaniumaerospace.pdf
7 – http://metals.about.com/od/properties/a/Metal-Profile-Titanium.htm
8 – http://education.jlab.org/itselemental/ele022.html
9 – http://www.miningweekly.com/article/us-company-begins-tests-on-worlds-largest-titanium-deposit-2012-03-16
10 – Titanium: A Technical Guide” Second Edition, Matthew J. Donachie, Jr. Published: 2000
11 – http://www.lenntech.com/periodic/elements/ti.htm
12 – https://www.titaniumprocessingcenter.com/milling-titanium
13 – http://cartech.ides.com/datasheet.aspx?i=101&E=268
14 – http://www.azom.com/article.aspx?ArticleID=9412
15 – http://www.azom.com/article.aspx?ArticleID=9413
16 – http://www.azom.com/article.aspx?ArticleID=9415
17 – http://cartech.ides.com/datasheet.aspx?i=101&E=267
18 – http://www.azom.com/article.aspx?ArticleID=9399
19 – http://cartech.ides.com/datasheet.aspx?i=101&E=269
20 – http://www.azom.com/article.aspx?ArticleID=9345
21 – https://www.eao.org/
22 – https://www.youtube.com/watch?v=73HLzYuIfx0
23 – http://web.ornl.gov/sci/propulsionmaterials/pdfs/Emerging_Titanium.pdf
24 – http://www.metal-powder.net/view/4029/metalysis-leads-charge-for-change-in-titanium-production
25 – http://www.roskill.com/reports/minor-and-light-metals/titanium-metal