1. The properties of titanium:
Titanium is a chemical element with the symbol Ti and the atomic number 22. It is one of the transition metals and sits in group 4 in the periodic table. Its appearance is similar to steel, and is an easily forgeable light metal. Its strength properties are similar to those of tempered steel, and it retains these properties up to around 635°C. Depending on how it is alloyed, it has a tensile strength of between 750 und 1150 N/mm2. However, with a specific weight of 4.51g/cm3, titanium is nearly 50% lighter than steel. Its melting point (1660°C) is higher than steel’s and it is exceptionally corrosion-resistant. It can withstand dilute hydrochloric acid and dilute sulfuric acid, and particularly resistant to chloride solutions, seawater and organic acids.
Titanium is antimagnetic. One of the consequences of this is that it is also almost fully insensitive to eddy currents, even in close proximity, where other common materials would heat up to unacceptable levels. Its high effectiveness in EMC-shielding makes titanium ideal for numerous applications where other materials fail or can only deliver insufficient absorption, allowing of course for weight and thickness. One other, intriguing characteristic is that titanium is the only element that can burn in pure nitrogen.
Titanium is also used as a micro-alloy component in steel. In a concentration of just 0.01-0.10 per cent by mass, it gives steel extra toughness, strength and ductility. In stainless-steel, it inhibits intergranular corrosion. In alloys with iron, aluminium, molybdenum or manganese, titanium is used to produce extremely strong, light and heat-resistant materials for aeroplane, ship and nuclear reactor construction. It is also used for surgical bone pins, and nowadays makes up a third of the engine of a modern airliner. Components of chemical equipment exposed to corrosive substances are in many cases made of pure titanium.
2. Occurrence in nature:
Titanium is one of the ten most common elements in the earth’s crust, but it only appears chemically-bound as a part of certain minerals, mainly limonite (FeTiO3), rutile (TiO2), and perovskite (CaTiO3). And although it is very widely distributed – typical farmland contains around 0.5% titanium – it only seldom occurs in substantial deposits. The most important of these are in Australia, Scandinavia, North America, the Urals and Malaysia. In 2010 deposits were discovered in Paraguay, but extraction there is currently still only at the planning stage.
The production process used for producing pure titanium is complex and demanding, which is why the metal is so expensive. Titanium production is 35 times more expensive than for common steel alloys, and 200 times more expensive than crude steel.
Because titanium reacts with conventional reducing agents, production using the reduction of its oxide is not an option. It was only with the discovery of the Kroll process that large-scale production became possible, and this system remains in use, largely unchanged, to this day. In the Kroll process, enriched titanium oxide (generally limonite or rutile) is heated with coke and chlorine to produce titanium tetrachloride and carbon monoxide; liquid magnesium is then used to reduce the titanium tetrachloride and produce so-called titanium sponge. The titanium sponge must be remelted in a vacuum arc furnace to produce alloys for making tubes, plate, sheet or bars.
ORE (ILMENITE/RUTILE) –> TITANIUM TETRACHLORIDE –> TITANIUM SPONGE –> TITANIUM ALLOYS –> TITANIUM RAW MATERIALS
4. Classification of titanium alloys: alpha, alpha-beta, beta
In terms of its internal structure, below 882°C, titanium exists as hexagonal alpha-titanium; above this temperature it switches to body-centred cubic beta-titanium. This transition point from alpha to beta is called the beta transus (βtr). And it is precisely this transition point that defines which class a titanium alloy belongs to. One attribute of the various categories of titanium alloy is that, depending on their type and quantity, these alloying elements stabilise either the alpha or beta phase of the alloy. Alpha elements raise the βtr and beta elements lower the βtr. Thus the alloying elements in a titanium alloy have a direct influence on the mechanical properties of the material. The aforementioned hexagonal crystal structure makes titanium relatively difficult to work, with significant impact on costs. For example, rolling titanium ingots into sheet titanium makes up around 50% of the cost of the end product.
– Alpha alloys
Alpha alloys of titanium have a high aluminium content, and reach high mechanical strength and corrosion resistance at temperatures between 300 and 500°C. Alpha alloys cannot be tempered, but they can be easily welded. The tube sets we use are made from alpha alloy.
– Alpha-Beta alloys
Alloying components like chrome, copper, iron, manganese, molybdenum, tantalum and columbium reach beta structure at normal ambient temperatures. Tempering is possible, which brings high mechanical strength. The disadvantage of this is the concomitant increase in brittleness, which makes the metal less workable. This alloy still has adequate ductility (breaking elongation is around 20%), therefore we use these for parts CNC-machined from solid blocks.
– Beta alloys
Pure beta alloys have a higher proportion of beta-stabilising alloying elements. Through heat-treatment (tempering), they achieve extremely high mechanical strength, and they have an excellent resistance to corrosion. These two attributes make beta alloys of titanium ideal for use in surgical implants.
– Division into grades
Titanium elements are often categorised according to the ASTM (American Society for Testing and Materials) standard, which has grades from 1 to 35. Grades 1 to 4 denote various degrees of pure titanium. These represent the weakest titanium alloys and are particularly easy to work at low temperatures. They belong to the class of alpha alloys.
6. We use two grades for our frame-building: Grade 5 (Ti6Al4V, EN 3.7165/3.7164) and Grade 9 (TiAl3V2.5, EN 3.7195)
Grade 5: The Ti6Al4V alloy is the commonest of the Alpha-Beta class titanium alloys, and the most common titanium alloy overall. It is still reasonably ductile (breaking elongation approx. 20%), so we use it for parts that are CNC-cut from solid blocks.
Grade 9: TiAl3V2.5 is however an Alpha alloy. It has good ductility and is exceptionally strong. It is extensively used in tubing production, from aircraft hydraulics to the tube sets for our bike frames. However, because high aluminium content leads to stress-cracking corrosion, the aluminium content must be limited to around 3%.
7. How do the specific properties of titanium alloys apply to our frames? And how do we work with titanium in our production processes?
- Oxidation is strongly accelerated at temperatures 550-600°C and above, necessitating elaborate and costly oxygen-free welding techniques: we weld our frames using a method known as argon shielding.
- Often only workable at high temperatures: our frames are prepared using meticulous adherence to a fixed welding sequence. Retrospective aligning of frames is very difficult, because the frame will drift back to its original form.
- Low modulus of elasticity (this can also be an advantage): exceptional resistance to denting, whether in accidents or general knocks.
- Cutting processes are very difficult: nevertheless all critical elements of our bikes – e.g. dropouts, braze-ons and postmount attachments – are CNC-cut from very hard Grade 5 titanium.
- High level of isotropy: multi-dimensional forces acting on the frame are exceptionally well absorbed by the metal. This stands in absolute opposition to carbon-fibre frames, which are thoroughly anisotropic.
- Extremely expensive, both in terms of the processing of the raw materials and the subsequent working using CNC-cutting and distortion-free tacking and welding of the frames.
- High specific strength: this allows the production of frames that are simultaneously very light and very stiff, with a long service life.
- Limited strength-loss at high temperatures: this means that durability is preserved at weld areas.
- High ductility, including at low temperatures: this means our frames and titanium components have enormous safety reserves – an unexpected failure or breakage of safety-significant parts like handlebars or forks can be more or less totally excluded. Titanium can deform by more than 30% before it breaks. This, incidentally, is quite the opposite of carbon fibre, that can only deform by between 0 and a maximum of 5% before breakage occurs.
- High resistance to fatigue: this means that we can make good use of its positive shock-absorption properties, without any penalty in terms of durability.
- High corrosion resistance, especially against salt water and sweat (protected by a resilient, quick-forming layer of TiO2): this makes our frames ideal for all-year use – road salt cannot damage the surface of the metal, because it is covered in a very resilient oxide protection layer (the passivation layer).
8. The key reasons why we use titanium for bike frame construction:
• Minimalist – timeless – beautiful
Our titanium frames are about more than just extreme durability. Its minimalist appearance gives a titanium frame a timeless quality that sets it above passing trends. It retains its value much better than any other frame material. We refrain from any use of paint or decals. The amount of labour required at both the building and finishing stages is immense, but once completed, the unadorned, hand-crafted frame is a joy to behold. The surface of titanium is very resilient to scratches and other environmental influences.
• Comfort through inherent shock absorption
A titanium-framed bike is comfortable to ride, and when high-quality tube sets are used, characterised by inherently stable handling: titanium frames demonstrate an exceptional shock-absorption value. For the absorption of impact energy from the surface being ridden on, inherent shock-absorption pays a key role. Introduced energy is absorbed through internal friction. It is therefore possible to build frames out of titanium that quickly abate vibration.
• Durability through resilience – minimal metal fatigue
Titanium frames stand out for their extreme robustness and very long service life, compared to frames built from other materials. Their resilience in a crash situation is without compare. Titanium’s outer surface is resistant to deformation and tearing. Titanium alloys share the high strength of steel, but with double the elasticity (= low modulus of elasticity). Similarly impressive is the chemical resistance of titanium: road salt and sweat cannot damage the pre-oxidised surface – in contrast to carbon fibre.