Titanium and Titanium Alloy

Benefits of Titanium

Resistant to corrosion
One of the main reasons as to why titanium is so popular is due to its resistance to rust and corrosion. In many settings, metals that are exposed to a level of moisture will corrode over time, losing their natural shine and weakening them somewhat. This is why titanium is preferred, as it’s able to maintain its good as new look, upholding a professional feel.

Easily recycled
When living in an increasingly environmentally conscious society, you’ll be pleased to know that titanium can easily be recycled and used again in another form. The fact that titanium is a 100% recyclable eco metal is partly because it will not rust. This means that once you’ve finished using your titanium metal, you can pass it on elsewhere and reuse it with no defects.

Low density
Titanium has a supremely low density rating, making it very light yet also durable. This is why you will regularly spot titanium metal used in machinery that is found airborne, such as missiles, aircraft, and spacecraft. Not only is this metal easier to use during the manufacturing process, but it is also crucial in making these machines more streamlined and aerodynamic.

High melting point
With a very high melting point, titanium can withstand extreme temperatures without deformation or losing its properties. This makes it perfect for applications in jet engines, missile components, and power generation.

Benefits of Titanium Alloy

Types of Titanium

Types of Titanium Alloy

 

Alpha alloys

Alpha alloys are titanium alloys that are only purposely alloyed with oxygen. While other components such as carbon and iron can be found in small quantities, they only exist as impurities. As an interstitial alloying element, oxygen significantly boosts strength while decreasing ductility. The chemical and engineering industries are the primary users of alpha alloys.

Here, great corrosion behavior and deformability are more important than high (specific) strength. The main difference between commercially pure (cp) titanium grades is their oxygen concentration.

 

Near-alpha alloys

Near-alpha alloys of titanium are the most common high-temperature alloys. This alloy class is appropriate for high temperatures because it combines the superior creep behavior of alpha alloys with the high strength of alpha + beta alloys. However, their maximum working temperature is now limited to 500 to 550 ºC.

 

Beta and near-beta alloys

Beta alloys are another type of titanium material. Manufacturers create all titanium alloys by adding enough beta-stabilizing elements to titanium. These materials have been available for many years but have only lately gained popularity. They are more easily cold workable than alpha-beta alloys, heat treatable to high strengths, and some have better corrosion resistance than commercially pure grades.

 

Alpha and beta alloys

These are typically medium to high strength materials with tensile strengths ranging from 620 to 1250 MPa and creep resistance ranging from 350 to 400°C. In addition to tensile properties, they also have low and high cycle fatigue and fracture toughness characteristics.

As a result, people developed thermomechanical and heat treatment procedures to ensure that the alloys provide an optimal balance of mechanical properties for various applications.

Process of Titanium

Titanium is produced using the Kroll process. The steps involved include extraction, purification, sponge production, alloy creation, and forming and shaping. In the United States, many manufacturers specialize in different phases of this production. For example, there are manufacturers that just make the sponge, others that only melt and create the alloy, and still others that produce the final products. Currently, no single manufacturer completes all of these steps.

Extraction
At the start of production, the manufacturer receives titanium concentrates from mines. While rutile can be used in its natural form, ilmenite is processed to remove the iron so that it contains at least 85% titanium dioxide. These materials are put in a fluidized-bed reactor along with chlorine gas and carbon. The material is heated to 1,652°F (900°C) and the subsequent chemical reaction results in the creation of impure titanium tetrachloride (TiCl4) and carbon monoxide. Impurities are a result of the fact that pure titanium dioxide is not used at the start. Therefore the various unwanted metal chlorides that are produced must be removed.

Purification
The reacted metal is put into large distillation tanks and heated. During this step, the impurities are separated using fractional distillation and precipitation. This action removes metal chlorides including those of iron, vanadium, zirconium, silicon, and magnesium.

Production of the sponge
Next, the purified titanium tetrachloride is transferred as a liquid to a stainless steel reactor vessel. Magnesium is then added and the container is heated to about 2,012°F (1,100°C). Argon is pumped into the container so that air will be removed and contamination with oxygen or nitrogen is prevented. The magnesium reacts with the chlorine producing liquid magnesium chloride. This leaves pure titanium solid since the melting point of titanium is higher than that of the reaction.

The titanium solid is removed from the reactor by boring and then treated with water and hydrochloric acid to remove excess magnesium and magnesium chloride. The resulting solid is a porous metal called a sponge.

Alloy creation
The pure titanium sponge can then be converted into a usable alloy via a consumable-electrode arc furnace. At this point, the sponge is mixed with the various alloy additions and scrap metal. The exact proportion of sponge to alloy material is formulated in a lab prior to production. This mass is then pressed into compacts and welded together, forming a sponge electrode.

The sponge electrode is then placed in a vacuum arc furnace for melting. In this water-cooled, copper container, an electric arc is used to melt the sponge electrode to form an ingot. All of the air in the container is either removed (forming a vacuum) or the atmosphere is filled with argon to prevent contamination. Typically, the ingot is remelted one or two more times to produce a commercially acceptable ingot. In the United States, most ingots produced by this method weigh about 9,000 lb (4,082 kg) and are 30 in (76.2 cm) in diameter.

After an ingot is made, it is removed from the furnace and inspected for defects. The surface can be conditioned as required for the customer. The ingot can then be shipped to a finished goods manufacturer where it can be milled and fabricated into various products.

History

The use of titanium metal, in any form, only really developed after World War II. In fact, titanium was not isolated as a metal until the American chemist Matthew Hunter produced it by reducing titanium tetrachloride (TiCl4) with sodium in 1910; a method now known as the Hunter process.

Commercial production, however, did not come until after William Justin Kroll showed that titanium could also be reduced from chloride using magnesium in the 1930s. The Kroll process remains the most used commercial production method to this day.

After a cost-effective production method was developed, titanium's first major use was in military aircraft. Both Soviet and American military aircraft and submarines designed in the 1950s and 1960s began making use of titanium alloys. By the early 1960s, titanium alloys started to be used by commercial aircraft manufacturers as well.

The medical field, particularly dental implants and prosthetics, awoke to titanium's usefulness after Swedish doctor Per-Ingvar Branemark's studies dating back to the 1950s showed that titanium triggers no negative immune response in humans, allowing the metal to integrate into our bodies in a process he termed osseointegration.

Titanium Extraction

The overall process
Titanium is extracted from its ore, rutile - TiO2. It is first converted into titanium(IV) chloride, which is then reduced to titanium using either magnesium or sodium.

Conversion of TiO2 into TiCl4

The ore rutile (impure titanium(IV) oxide) is heated with chlorine and coke at a temperature of about 1000°C.

Note: There are other reactions occurring as well in which the other product is either carbon dioxide or carbonyl chloride, COCl2. AQA give the one producing carbon monoxide as their first choice, but will also accept one producing carbon dioxide.

Other metal chlorides are formed as well because of other metal compounds in the ore. Very pure liquid titanium(IV) chloride can be separated from the other chlorides by fractional distillation under an argon or nitrogen atmosphere, and is stored in totally dry tanks.

Note: Titanium(IV) chloride is a typical covalent chloride. It is a colourless liquid which fumes in moist air due to reaction with water to give titanium(IV) oxide and fumes of hydrogen chloride. Everything has to be kept very dry to prevent this happening.

Reduction of the titanium(IV) chloride
TiCl4 can be reduced using either magnesium or sodium. AQA will accept either, and so I am just giving the magnesium one.

Titanium(IV) chloride vapour is passed into a reaction vessel containing molten magnesium in an argon atmosphere, and the temperature is increased to about 1000°C. The reduction process is very slow, taking about 2 days, followed by several more days of cooling.

When it is cool, the reaction mixture is crushed, and dilute hydrochloric acid is added to react with any excess magnesium to form more magnesium chloride. All the magnesium chloride dissolves in the water present, and the remaining titanium is processed further to purify it.