History of titanium
  Titanium was first discovered in 1791 by British chemist Reverend William Gregor, and was originally named Gregorite after its founder. In 1793 it was once again discovered —independently—by German chemist M.H. Klaproth.

 He named it Titanium after the Titans of Greek Mythology: "the incarnation of natural strength". The element was successfully isolated in 1910. Titanium is the 22nd element on the periodic table. It is primarily found in the minerals Rutile and Limonite, which make up to 24% of the earth's crust—making Titanium the 9th most abundant element on Earth. It only occurs in nature in chemical combinations; the most common of which are oxygen and iron.

 The atomic of Titanium is 47.867amu. It is a low-density element (4510 kg/m3); approximately 60% less dense than steel (7850 kg/m3). Titanium contains no iron, making it a non-ferrous, non-magnetic substance. Titanium transfers heat well, with a higher melting point than steel. (Titanium melting point: 1993 K (3020 degrees F and 1660 °C). Steel melting point: 1923 K (3000 degrees F and 1650 °C). Titanium has the ability to passivate; therefore, it exhibits high levels of corrosion resistance to most mineral acids and chlorides.

 Once primarily used by the aerospace industry, Titanium and Titanium alloys are being used increasingly in medical and other industries due to their coveted properties: non-magnetic, low-density, non-corrosive and very attractive strength to weight ratios. Titanium is also nontoxic and biologically compatible with human tissue and bone, making it useful for artificial hip and knee replacement, heart valves, spinal and Maxillofacial and other implants.

  Titanium and titanium alloy is named as "future metal" for its excellent properties. It's lightweight, strong and high corrosion resistant, and become a great promising material. Titanium and titanium alloy is not just widely used in plane and aerospace industry. Now it's also widely used in chemical processing, petroleum, light industry, metallurgy, power generation and other industries.

  One of titanium's excellent properties is high corrosion resistance. Because of its special ability to oxygen, it can generate a tight dioxide film on the surface. And this will protect titanium from corrosion. This also works as it is in most kinds of liquid. So titanium has good stability in acid and other mediums. This property is much better than stainless steel and other nonferrous metals have, and even as good as platinum. But in certain mediums, if the film can be dissolved, titanium will corrode. For example, titanium in DRIE, hot hydrochloride, it will corrode. But if some metal ion or antioxidant is put in the liquid, the film will be protected, titanium will be steady.

  Chemical Industry

  Because of this steady property, as a corrosion resistant material, titanium is used more and more widely in chemical industry. For example, in chlorine industry, titanium is used as condenser. And this application is considered as a revolution in chlorine industry.

  Petroleum Industry

  Except for the five organic acids, when titanium is in organic compounds, it has good stability. So in petroleum industry, titanium used as excellent structure material, such as heat exchanger, reactor, hyperbaric container etc.

  Titanium Machining With High Efficiency

  Titanium alloy is a high strength metal with heavy viscosity, it's easy to generate and gather heat on cutting section during cutting. What's worse, there is a risk to cause fire when milling for a long time because of titanium's poor thermal conductivity. Since high speed cutting is not good for machining titanium alloy materials, it's a difficult problem for many companies engaged in producing top grade parts to make work efficient to process the titanium alloy. There are some methods to do the work better to a certain extent as below:

  1. Get a new high speed steel instead of traditional hard alloy. In most cases, hard alloys are the first choice for metal machining because of its good wearing resistance and red hardness and resonable machining costs. But people always forget their frangibility which is the the most important matter for machining titanium alloys. The abrasion of the knife edge is not the reason to make cutter disabled but the blade's broken. Furthermore, the high temperature caused by machining the titanium alloy material stop giving play advantages of high speed chipping. So large quantities of cooling fluid is required during high speed chipping. This cold and hot change causes an intensive impact between the cutter and the aparts, which can break the edge of blades of the fragile hard alloys. Then the expensive cobalt high speed steel and powder metallurgy high speed steel are the first choices for gang milling of titanium alloys. And this is the necessary condition to use high speed steel instead of hard alloys in traditional machining.

  2. Hard alloy cutter can reach a marvelous high speed to adopt the small radial direction cutting method to machine the titanium alloy parts with traditional machining. The small radial direction cutting method means to cut from the radial direction cutting depth which is much smaller than the cutter radius when machining the parts. This method can reduce the cutting time of each blade and extend the blades' no-cutting time, which means increasing the cooling time and getting a well control for cutting temperature, so the cutting speed can be raised substantially. Usually it can raise the cutting speed by 50% when the cutting depth is smaller than 25% of the diameter and by 100% when the depth is less than 10% of the diameter.

  Welding Technique of Titanium Alloy
  Titanium and its alloy are widely used in aerospace, chemical industry, shipbuilding etc, because of its low density, high strength, and corrosion resistance. At present, high quality plane and tank are being made of titanium alloy parts which are being used more widely in oil chemical industry. In the application of titanium in plane and its engine and chemical industry, welding technique is necessary. Welding technique is very important to the developing of titanium alloy application. This article is to point out some of the problems in the process of welding.

  1.Welding capability of titanium alloy

  Most titanium alloy can be processed in arcogen welding, and all of them can be processed in solid-state welding, such as TIG, MIG, low-current plasma arc welding, laser and electron beam welding. In actual fact, the occurrence tendency of flaw in titanium alloy welding is much smaller than ferrous materials (such as iron alloy and nickel alloy). Although titanium has such superior capability in welding, some engineers still consider it difficult in welding titanium alloy. Because of its especially demand of gas protection in welding, only those very professional can satisfy this requirement. In actual fact, many techniques could be applied in welding titanium alloy. As N2 O2 and carbon material are involved, the welding point become more fragile. So the welding area must be cleaned and protected by inert gas. Welding material are chosen as per the capabilities of welding alloy.

  2.Gas cavity in titanium alloy

  The occurrence of gas cavity in welding point is mainly due to the clean of the alloy and the length of welding time. Also O2 N2 CO2 and inert gas could result in the occurrence of gas cavity.

  The same as aluminum oxide, titanium oxide is moisture absorbing.

  Measures of reducing gas cavity in titanium alloy welding:
  1. welding material should be dry.
  2. prepared welding material should be used within 48 hours.
  3. Titanium and welding material should be cleaned.
  4. Welding should be protected by high purity nontoxic gas and helium gas.

  Process of Titanium Surface
  In dental industry, titanium is used as a superior material because of its good capabilities. But there is something important to be process during the manufacturing.

  In high temperature titanium is apt to react with elements of O, H and N in the atmosphere and with elements of Si, Al in the raw material. This reaction will bring a dirty surface on the casting which increases its hardness and brittleness, decreases its flexibility and plasticity.

  Because of its low density, fluid titanium has weak inertia as it flows. And this brings low flowing in the casting. The difference of temperature between casting and mould is a bit great, the temperature drops fast, so casting must be done under protective situation. These factors would result in gas cavities and other flaws on the surface and inside of the titanium castings.

  The flaws affect the quality of castings greatly. So it is much more important to process titanium casting surface than other dental alloy. Because of the special capabilities of titanium, such as low conducting of heat and electricity, hardness of surface, oxidizing etc, it is difficult to process titanium surface. General methods of surface processing can't bring good effect, so special methods must be applied.

  The purpose of titanium surface processing is not just making it smooth. The process can reduce conglutination of food and fungus, maintain the balance of nature in the mouth. The most important this improves the capabilities in corrosion, friction and applicability.

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