Titanium; An Overview of
Titanium (Ti) and titanium alloys have made significant progress in finding industrial use in the previous century due to some key advantages over other common industrial metals. Key benefits of titanium alloys over iron based and aluminum based alloys are its strength to weight ratio (Ti is roughly 60% of regular steel weight), outstanding combination of mechanical properties (from room temperature to elevated temperatures) and remarkable corrosion resistance.
The major challenge of Ti manufacturing is its high reactivity with gases present in atmosphere specially at elevated temperature causing immature failure and difficulty in fabrication. Consequently, manufacturing cost of Titanium ; starting from raw material production to the final shape is relatively high. However, eventual operating benefits outweigh its high manufacturing cost.
Applications
Majority of titanium applications are in aircraft industry; mostly owing to its high specific strength, optimum combination of mechanical properties and extraordinary corrosion resistance. Titanium maintains its optimum combination of strength, ductility, fatigue and creep properties from room temperature to moderately elevated temp. Hence, Ti alloys are used to manufacture both aircraft engine parts and structural parts. Extremely demanding rotating and non-rotating jet engine components (operating below 550 C /1022 F), highly stressed aero-structure components are routinely made by various Ti alloys. Alpha Ti alloys and pure Ti parts are used for applications where high corrosion resistance are the primary requirements – like marine applications, energy sectors, chemical and petrochemical processing plants etc. Additionally, Ti alloys are very popular in bio-medical applications because of its bio-compatibility (resistance to corrosion in body fluid). It has several cryogenic applications also.
Extraction and Manufacturing
Economically viable Titanium production started around 1960 after breakthrough extraction process invention by Wilhelm J. Kroll around 1940. Ti extraction involves chlorination of Ti ore (Rutile) to titanium tetrachloride (TiCl4). Then TiCl4 is reduced by sodium or magnesium under inert atmosphere. Current Ti production involves double or triple melting to ensure homogeneity. Electric-arc furnace under vacuum is used for melting. Vacuum ensures reduction in gas content, mainly hydrogen.
Ingots converted into mill products like billets, bars, plates etc by size reduction or cogging. Mill products go through secondary processing like forging, rolling, extrusion etc to form the final shape. Titanium parts are also produced by casting and powder metallurgy. Generally, parts get heat treated before and after secondary processing. There are three major types of heat treatments applied to Titanium: stress relieving, annealing, solutionizing & aging. Solutionizing & aging can be used to control mechanical properties by controlling microstructure while stress relieving is used to reduce stresses generated in previous operations. Annealing serves both the purposes. Actual anneal cycle is selected based on final mechanical property requirements.
Precautions should be taken during high temperature process so that Ti should not pick up gases like oxygen, nitrogen and hydrogen. Oxygen causes formation of α case on the surface. This is a very hard and brittle case and causes premature cracks and machine tool damage. It is removed either by machining or by chemical milling. Hydrogen content more than 100 – 200 ppm causes hydrogen embrittlement. High temperature furnace atmosphere and acids in the chemical milling are the main source of hydrogen pick up. Parts should be checked for final hydrogen content before placing in the service.
Metallurgy
Titanium has two elemental crystal structure: hexagonal closed packed (HCP) and body centered cubic (BCC). High temperature HCP structure is called β phase and low temperature BCC structure is called α phase. Based on the room temperature microstructure, Ti alloys are broadly divided into three types – α alloys, α – β alloys and β alloys. Apart from these alloys commercially pure Titanium has various applications also. Alloying elements and impurities controls the stability of the two phases. Hence, these are divided into two groups: α stabilizers (Al, O2, N2, C etc.) and β stabilizers (V, Fe, Mo etc.).
Final microstructure and resulting mechanical properties depend on both manufacturing operations and on heat treatments. Heat treatment cycle parameters, manufacturing operations (casting, forging, rolled etc.) control the final microstructure like grain size, phase constituents, phase distribution, morphology etc. Generally, final microstructure after heat treatment should consists of equiaxed primary α and transformed β. Transformed β is a mixture of acicular α in β matrix. Primary α is responsible for strength, ductility and low cycle fatigue while transformed β provides better creep and fracture toughness. Percentage and morphology of these phases determine the final properties. Wrong process parameters like cooling rate, heating temperature can result in undesirable microstructures like “Basket-weave” or Widmanstätten microstructure, grain boundary alpha etc. Care should be taken for selecting right process parameters based on section thickness, final property requirements etc.
β Transus is the temperature above which complete β is present and below which microstructure consists of a mixture of α and β. This is a very critical temperature for wrought Ti part production and heat treatment. Generally, mills certify this temperature for every heat because this temperature varies with variation in composition.
New Developments
Application of Ti has skyrocketed because of its abundant presence in the Earth’s Crust and its benefits over other common materials. New developments are continuing to innovate more cost effective and efficient processing and applications. Superplastic forming and concurrent diffusion bonding is already in production. Other areas of development are powder metallurgy, titanium based inter-metallic compounds, novel melting practices etc. Novel alloys with better properties are made by mechanical alloying in powder metallurgy which is not possible in conventional raw material processing.
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Fundamentals of Titanium Metallurgy |
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Metallurgy of Titanium Manufacturing |
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Metallurgy of Titanium Heat Treatment |
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Advanced Titanium Metallurgy |
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