Metals - Titanium

Titanium has truly become the wonder metal for the bike industry. You wouldn't know it but, titanium is the fourth most abundant metal, after aluminum, iron, and magnesium. Its high cost of production and the complexity of the production process has historically placed some limits on the application of titanium. Titanium is a silver-grey colored metal that is 60 % heavier than aluminum.

Titanium Extraction and Refining

The element titanium was discovered in 1763 by an English cleric, William Gregor who was an amateur chemist with an inquiring mind. It was in the black sands of Cornwall that he discovered the new element that had up to that time, attracted little scientific interest. A few years later, an Austrian, Klaproth, extracted the same element from an ore widely known as "rutile", which is a mineral consisting of titanium dioxide (one titanium atom, two oxygen atoms), that is a reddish-brown substance with a slight metallic luster. While rutile is the highest grade ore containing titanium dioxide. Another ore called illmenite, is also a good source but is a lower grade ore containing a large percentage of iron. Illmenite is found largely in the Ural mountains and is the source of southern Russia's supply of titanium, which have become plentiful as the need for hard currency in the former USSR has become more acute. The magnesium reduction process used for the production of titanium was developed many years ago by a Dr. William Kroll of Luxembourg. Though he had patented the process the patent was taken over by the United States during World War II and further experiments were carried out by the US bureau of Mines. By 1949 the National Lead Company started on an experimental basis titanium production with a capacity of about 100 pounds per day. The first step in extracting the titanium from the ore, is to chlorinate the ore with gaseous chlorine in the presence of a carbon mixture or carbonaceous material, usually in the form of petroleum coke. This step produces titanium tetrachloride (one titanium atom, four chlorine atoms). What we now need to do is free the titanium from the chlorine. This is accomplished through the use of magnesium. Titanium in its melted form is highly susceptible to contamination with impurities introduced during the manufacturing process. These are generally fatal to titanium's physical properties. No good method has been found to remove contaminants acquired during the processing. Titanium, when molten, also combines with oxygen and nitrogen in the air, with such speed that most of the extraction and reduction processes must be carried out either in a vacuum, or in an inert atmosphere. The second step is to take the titanium tetrachloride along with a precise amount of pure magnesium (or sodium) metal and seal them in a heat resistant steel vessel at red-heat, in a vacuum or under an inert gas blanket to prevent oxidation. Within the heated vessel, the magnesium (or sodium) melts and a reaction takes place creating titanium metal and magnesium chloride (one atom magnesium, two atoms chlorine). During the process the excess magnesium chloride is removed from the reaction vessel several times, leaving titanium that still has some magnesium or magnesium chloride imbedded in the metal which must be removed. Several processes are used to remove these remaining components. Frequently the material is double melted or "distilled", to remove additional amounts of the magnesium and magnesium chloride, in a furnace, using a vacuum or inert argon gas for atmosphere. The final amounts of the magnesium chloride or sodium chloride are leached out of the metal with acid. The remaining material is known as "sponge" because in its less pure state, the metal had a porous appearance. The sponge is crushed for remelting into ingots. The melting process used by Titanium Metals Corporation, and widely used by others is a derivative of the von Bolton process of arc melting called "consumable electrode vacuum arc melting" using electrodes of tungsten, carbon, or consumable electrodes of titanium. The tungsten and carbon electrodes present difficulties in that they a subject to "spalling" (chipping or breaking off) and melting, particularly if they are spattered with titanium. Using this vacuum arc melting, the titanium can be alloyed, and is cast individually into ingots, through the use of a water-chilled copper crucible. The ingots are later converted into the common primary mill products of the metals industry, including billets, bar, plate, and strips. Secondary fabrication from the billet form can be performed including die forging and extrusion, with titanium tubing being chief among the extruded products.

Titanium Alloys and their Crystal Structure

Based on their crystal structure at room temperature, titanium alloys are divided into alpha, alpha-beta, and beta alloys based on their crystalline microstructures. The alpha alloys group is usually sub-divided into "commercially pure" titanium alloys known as "CP", and alpha or near-alpha alloys, which have slightly larger amounts of alloying elements. In the alpha-beta alloys, the alpha-beta ratio is important because it influences metallurgic grain size, metal workability, toughness and its ability to be joined through welding. Titanium like carbon has a different crystal structure depending on what elements, and the percentages of each, it's alloyed with. Carbon has a certain crystal structure when it exists perfectly in the form of a diamond, another when it is imperfectly graphite and is amorphous (having no regular form or being noncrystalline) in the form of anthracite or charcoal. This quality of carbon is known as "allotropy", and titanium is also allotropic. In pure titanium, the crystals have a structure or a "unit cell type" that is known as "close-packed hexagonal" or "CPH". The CPH crystal shape is its original or "alpha" phase. A "phase" is any structure that is physically or crystallographically distinct and usually visible under a metallurgical microscope. As the chemical composition of a metal or alloy is altered, or its temperature changed, new phases may form while existing ones may disappear. When the temperature of titanium is raised above 882¡ centigrade (1620¡ F) yet below 1727¡C (3140¡F), the titanium crystal changes by allotropic transformation into to a "body centered cubic" or "BCC" crystal structure or unit cell type. The BCC unit cell structure is referred to as its "beta" phase. This change to a BCC crystal structure remains only while the temperature is in this range, and reverts to the CPH type when it's back outside of this range. The allotropic transformation temperature also known as the "beta transus" is affected by the amount and type of impurities in the titanium or by the alloying elements. Adding aluminum, as an alloy element, to titanium stabilizes the alpha phase and raises the allotropic transformation temperature. Other elements are known to stabilize the beta phase, and lower the allotropic transformation temperature, they include chromium, molybdenum, and vanadium. With the addition of large amounts of the beta stabilizers, the beta phase can be made stable at or below room temperature.

Commercially Pure Titanium Alloys

As we've said, commercially pure (CP) titanium alloys consist of alpha alloys with extremely low amounts of what are called "interstitial" elements used as alloy elements. "Interstitial elements" are alloy elements (solute) whose atom is small enough to fit in the spaces ("interstices") between the primary or "solvent" metal's atoms. The interstitial atoms must be very small to fit in these spaces, and are generally non-metallic, in the case of alpha titanium alloys they are nitrogen, oxygen, and carbon. The primary difference between the various grades of commercially pure titanium is the content or amount of this interstitial element in the alloy. Alloys that have a higher purity, (less solute or interstitial element alloyed in the titanium), have lower strength, lower hardness and a lower alpha-beta (beta transus) transformation temperature. Commercially pure titanium alpha alloys are referred to by their American Society for Testing and Materials (ASTM) "grade" designation number. The commonly used CP alloys have oxygen as their primary interstitial element. They are known as "grade 1" which has 18/100ths of 1% oxygen as an interstitial alloy element, "grade 2" which has 25/100ths of 1% oxygen as an interstitial alloy element, "grade 3" which has 15/100ths of 1% oxygen as an interstitial alloy element, and "grade 4" which has 40/100ths of 1% oxygen as an interstitial alloy element. Although the strength of the alpha CP alloy increases as the interstitial element content rises, commercially pure titanium alloys have low to intermediate strength compared with the other alpha-beta and beta type titanium alloys. The CP alloys are commonly used in pipe and tubing form because they have a high ductility (can bend without breaking). You may remember a time when the relatively inexpensive Sakae Ringyo (SR) Powerbulge Titanium ATB was popular because it was made of titanium, it is made from CP tubing.

Complex Alpha or Near-Alpha Titanium Alloys

Alpha and near-alpha titanium alloys have principally alpha stabilizing agents as the alloy agents, therefore these alloys contain a high percentage of the alpha phase PCH crystal structure present in the metal at room temperature. Alpha alloys rely usually on aluminum and tin for the stabilizing (solute) element, have a single-phase structure that is weldable and good ductility. The alpha alloys are created through solid solution treatment (described in the aluminum part of this article) and have a significant hardening effect on the finished metal. For example, for each one percent of aluminum that's added to the alloy, its tensile strength increases by 8000 PSI, and for tin by 4000 PSI. Two important alpha alloys are the Ti-Pd and the Ti-5Al-2.5Sn. The Ti-Pd alloys which are grade 7 (UNS R52400) and grade 11 (UNS R52250) are alloyed from commercially pure titanium with approximately 15/100ths of 1% Palladium. The palladium is added to enhance corrosion resistance. The difference between the two grades is that grade 11 has a lower interstitial element concentration, which makes it softer, more ductile and formable than grade 7. The alpha alloy Ti-5Al-2.5Sn which is known as grade 6 (UNS R54250) is made from pure titanium with 5% aluminum and 2.5% tin and has a completely alpha phase grain structure. What we have learned so far is that the alpha phase structure of pure titanium leads to a weaker, less hard, more ductile metal, and that some alloy elements can be added to the pure titanium that retains the alpha structure, while increasing somewhat its hardness.

Alpha-Beta Titanium Alloys

Alpha-beta alloys contain percentages of beta stabilizing elements high enough in percentage to cause the beta phase to be present in the metal at room temperature. Alpha-beta alloys can be strengthened by solution treatment and aging (both processes described in the aluminum section of this article). The most common and widely used alpha-beta titanium alloy is Ti-6Al-4V, which is grade 5 (UNS R56400). The composition of Ti-6Al-4V is 90% Titanium with 6% Aluminum to increase strength in the alpha phase, and 4% Vanadium to partially stabilize an beta phase that remains present, (after alloying) at room temperature. A number of versions of grade 5 are produced with the difference between them being the amount of interstitial atoms (alloy or solute element) fit into the spaces (interstices) between the titanium (solvent element) atoms in the compound. The extra low interstitial grades of Ti-6Al-4V have a lower strength and higher ductility than standard grades of Ti-6Al-4V. It is Ti-6Al-4V titanium alloy that most after market titanium replacement bicycle bolts, spindles and axles are made from. Another alpha-beta titanium alloy is Ti-3Al-2.5V, this is the titanium alloy that Sandvik Special Metals has made so famous, in tubing form, in the bicycle industry. Sandvik is a maker of seamless tubing from ingot of this alloy. They will sell the tubing to small parts makers to be re-manufactured into bicycle components. More commonly, Sandvik is contracted to re-manufacture the titanium tubing on behalf of the bicycle parts maker to the maker's specifications, for a contracted price. Sandvik's tool set and titanium fabrication experience is probably un-parallelled, and our examination of their work has shown flawless quality in the miter cuts and joining welds. An example of the Sandvik workmanship is seen in some of the Mc Mahon Racing frames. As a two-phase, alpha-plus-beta alloy the Sandvik Ti-3Al-2.5V, (which they call "Ti-3-2.5" and is orally referred to in the bike industry as "Ti 325" (Ti three two five) or "325" (three two five)) there is very little of the beta phase present in the metal at room temperature, and the beta phase adds very little to the strength to the metal. The beta phase forms as a precipitation reaction within the grains of the alpha or single phase and at the grain boundaries. The alloy, is not responsive to heat treating and exhibits very little aging effect. The Ti-3Al-2.5V alloy is strengthened through the added 3% of aluminum, which also stabilizes the alpha phase and raises the alpha allotropic transformation temperature (beta transus). The alloy is further strengthened by cold working (described in the aluminum section of this article). The 2.5% Vanadium content in the alloy stabilizes the beta phase of the alloy at room temperature, and lowers the beta phase allotropic transformation temperature so the beta transus is approximately 935 degrees centigrade or 1715 degrees fahrenheit. Sandvik sells and uses Ti-3Al-2.5V tubing that has been cold worked to increase the materials strength. To reduce the residual stress left after cold working, the tubing is then "stress relieved" which is a process of heating the metal to a suitable temperature, below melting or "recrystalization", and holding this temperature long enough to reduce the residual stress of the cold work, then cooling it slowly enough to minimize the development of new residual stresses.

Beta Titanium Alloys

Beta alloys of titanium aren't commonly used in the bike industry, because the metal becomes so hard, and so brittle, that it looses the ductile properties you would need in a seat post, frame, or stem, but it has occaisionally used in B/B spindles.