The Truth About Titanium

December 6, 1997 on 2:49 pm | In Product Tech. | No Comments

Background
As an element, titanium was discovered two centuries ago, but only until 1910 was it produced in metallic form. Titanium stayed in the lab until around 1940 when commercial processes were developed to produce useful quantities. Titanium is an abundant metal, about one percent of the earth’s crust is titanium. Titanium exists for the most part in common rocks as the mineral rutile (TiO2). Titanium is very reactive and easily forms compounds with other elements. The commercial reduction of mined titanium ore into titanium metal requires chemical treatments with chlorine and molten magnesium. The resulting titanium metal must be further purified and alloyed under vacuum in electric arc or the newer plasma furnaces. Titanium is one of the newer structural engineering materials available to the public.

Stronger than Steel

In recent years titanium alloys that were first developed for military aircraft applications have become popular for advanced bicycle frames and components. The major attraction to these alloys for bicycles is their low weight and high strength. For military aircraft, the attraction to these alloys is because they maintain an exceptional strength to weight ratio at temperatures from 420 °F to 1000 °F. Most titanium alloys match or exceed the tensile strength of structural steels but are 40% lighter. Titanium alloys usually have a lower modulus of elasticity (stiffness) than steel, but much higher than aluminum or magnesium. In comparison to aluminum and steel alloys, titanium alloys have a 30% or greater strength to weight ratio. Most titanium alloys are softer than steel and have poor wear properties. Thus, they do not make good rubbing, sliding or bearing surfaces. Using titanium for mechanical parts gives an unmatched weight savings without compromise in strength.

Careful Preparation

Titanium is not as easily manufactured and processed as steel or aluminum. High temperature processes such as casting, forming, and especially welding must be carefully set up and controlled to prevent embrittlement that leads to rapid failure of the part in service. Additionally, design rules for titanium must be obeyed. Most titanium alloys are notch sensitive. Notch sensitivity means that tensile stresses applied along a small inside radius of curvature will easily produce a crack. When designing titanium parts, it is a rule of the material to not have notches where high stress levels will cause cracks. Although titanium has a high strength to weight ratio in comparison to alloy steel, it still has about the same value of fracture toughness as 4340 steel.

Since common titanium alloys are notch sensitive, surface finish plays an important part in the fatigue life of a titanium part. A good surface finish is helpful to prevent crack formation. Highly polished titanium parts can have up to an order of magnitude increase in fatigue life over parts with a machined finish. Polishing titanium alloy parts takes time in the form of skilled labor. This coupled with a high material cost leaves us looking at a higher price tag for titanium parts as well as the answer to why the world of mechanical structures is dominated by steels.

Metallurgy

Pure titanium occurs in two solid forms. At room temperature a pure piece of titanium is in a form where all of its atoms are arranged in a hexagonal prism. Materials engineers call this pattern of arrangement a “hexagonal close packed” (HCP) crystal structure. When titanium is in the HCP form, materials engineers call it the “alpha phase” of titanium. If this alpha phase titanium is heated to 1620 °F (882 °C) the atoms rearrange from the hexagonal prism into a cubic pattern with an atom in the center of the cube, this pattern is called “body centered cubic crystal structure” (BCC) to materials engineers. This “phase” of titanium has its own name, “beta phase”. These two phases, alpha and beta, have different mechanical properties, the beta phase has higher strength than the alpha phase but it is brittle. Materials engineers can add other metals (alloying metals) in very small quantities to change the temperature at which the phases transform. That is, at what temperature alpha transforms into beta. The added metals that change the transformation temperatures or change the regions of stability for the phases are called phase stabilizers. Alpha stabilizers, such as aluminum, tin, and nickel all raise the alpha to beta transformation temperature. Beta stabilizers, such as tantalum, molybdenum, and vanadium, all lower the transformation temperature. Enough of the right beta stabilizer can lower the beta phase transformation temperature to room temperature (also known as your bicycle’s temperature).

In the right amounts and combinations, materials engineers have made titanium alloys that have both alpha and beta phases present at room temperature (alpha-beta alloys). These alloys exhibit a mixture of the mechanical properties of each pure phase. In other words, they are not as brittle as the pure beta phase but not as soft as the pure alpha phase. They are stronger than the alpha phase but not quite as strong as the pure beta phase. Cold working and heat treating processes will help mix the two phases finely and evenly through the bulk of the material.

Carbon, oxygen, nitrogen, and hydrogen can cause strengthening, and embrittlement. Oxygen will stabilize the alpha phase but also embrittle it. This can be a troublesome effect in some alpha-beta alloys, because oxygen not only embrittles the alpha phase on the surface exposed to air under high temperatures but grows more embrittled alpha phase from the beta phase that is exposed to air and high temperature. This occurs when titanium is processed under most high temperature operations such as electrical discharge machining (EDM), flame cutting, plasma cutting, and laser cutting. The brittle alpha phase formed on the surface is commonly called “alpha case”, similar to case hardening. This embrittled alpha phase acts as a crack initiator, and since cracks propagate easily through most titanium alloys, the embrittled alpha causes rapid mechanical failure in service. Any time significant heating (>500 °C) occurs with an environment that contains oxygen, the alpha case that forms will be embrittled and must be removed. During welding of titanium alloys the alpha phase that is formed will embrittle with oxygen from the air. To prevent this embrittlement, a shielding gas such as argon is used. TIG (Tungsten Inert Gas) welding has this inert gas feature but extra measures must be taken to make sure the weld and surrounding area is completely shielded as it cools down. Keeping parts clean and free of fingerprints, oils, and dirt, before welding will keep carbon from embrittling the alpha phase.

Alloys

In demanding the high specific strength of titanium some very important guides must be followed to obtain a part that will have lasting service. Alloy selection is a good first step. Not too many alloys are commercially available or affordable. The most common and widely produced titanium alloy is Ti-6Al-4V, sometimes referred to as Ti-64 or Ti-6-4 for short. The numbers in these designations are the weight percent of each of the elemental sub-constituents in the alloy. So a 100 pound block of Ti-6Al-4V alloy has 90 pounds of titanium, 6 pounds of aluminum, and 4 pounds of vanadium mixed all together. Ti-6Al-4V is what most all non-welded titanium bicycle parts are made from. Other more expensive alloys that are available may have higher strength or higher modulus but are more difficult to machine. The Ti-6Al-4V alloy does not weld well. There is usually a detrimental effect from welding on the flexural strength, modulus of elasticity, or most often the endurance limit of the welded joint. The endurance limit is the highest stress that can be cyclically applied to a component without failure ever occurring. For example; if the stress applied to a component is say ten percent higher than the endurance limit, then the component will fail in one million cycles. This is instead of never failing if the stress applied was lower than the endurance limit. So if we weld a Ti-6Al-4V alloy part it will fail sooner than if we use good bolts.

U.S. Tax Dollars

Military aircraft jet engines have miles of tubing around and in them to carry gasses, cooling fluids, oil, and fuel. Titanium tubing replaced stainless steel tubing because of its weight savings and high temperature corrosion resistance. Motivated by military applications, millions of dollars worth of research went into alloy development for titanium. Before titanium tubing could be used, a weldable alloy was needed since titanium fittings needed to be welded to the ends of the titanium tubes. The alloy that was developed for weldable tubing applications was Ti-3Al-2.5V. Titanium bicycle frames are manufactured from this alloy for a good reason, weldability. The bend radius test is one easy way to determine how well an alloy welds. Two square plates are welded together, edge to edge, with one weld pass. The welded plates are then put in a machine that determines the minimum radius that they can be bent around the weld. Brittle welds will not bend much, if at all, since they easily break. The more ductile the weld is the smaller the radius that the plates can be bent into. The Ti-3Al-2.5V alloy bends almost twice as small a radius than the Ti-6Al-4V alloy, making Ti-3Al-2.5V the choice for bicycle frame tubing.

Manufacturers and Quality

There are numerous manufactures of titanium alloys. Two of the largest producers are Teledyne and TIMET, both are American producers. There are also foreign producers of titanium. Russia has had/has an extremely large titanium processing capability and Russia is also known for outstanding metallurgy and quality for most metals. Interestingly the titanium for the larger parts of the U.S. Blackbird Spy Plane came from Russia (very indirectly) because at the time the United States did not have the production capability and quality that Russia did.

Because titanium alloys are very sensitive to processing variables its microstructure must be carefully inspected before parts made from it go into service. It is very easy to make bad titanium, and the result has been known to be disastrous. Material quality is extremely important in the aerospace and biomedical device manufacture. To ensure a safe product, the quality of each bar of titanium must be approved by metallurgical inspection. All titanium manufacturers make good titanium most of the time but inspection is needed to find the occasional bad batch. To have microstructural characterization done by commercial metallurgical centers, or to hire a materials engineer for this is expensive. This expense as well as the all the other finer details mentioned add to the personality of titanium. To the consumers of light and strong titanium bicycle parts and frames, this personality is expensive.-

by Robert Turner, PhD