Titanium and titanium alloy tube Unlike steels or aluminium alloys, titanium alloys are generally free of defects such as inclusions or porosity because of the double and triple vacuum arc melting procedures employed in producing them.
The resulting material structure provides good fatigue properties and almost no crack initiation due to inclusions or porosity.
However,fatigue behavior in titanium alloys is very sensitive to surface preparation, which can be more important than microstructural effects.
In contrast to working with ferrous metals, inducing compressive stresses in the surface actually reduces fatigue life rather than increasing it.
Similarly, the titanium surface finish is more sensitive to surface machining effects than that of ferrous metals, requiring care in producing the final finish.
In unalloyed titanium, fatigue life is also influenced by grain size, interstitial contents, and degree of cold work.
Decreasing grain size will increase fatigue life.
In alpha alloys fatigue life depends on grain size, degree of age-hardening, and oxygen content of the alloy.
Age-hardening in alpha alloys makes the crack grow faster, and, hence, reduces fatigue life.
The grain size effect is the same as unalloyed titanium, the finer grain size will give longer fatigue life.
For near-beta and beta alloys the microstructure plays a significant role in the fatigue limit as does the shape and size of the grains.
Endurance limits for most titanium alloys are observed to be 107 cycles or more.
The presence of a thin, tough oxide surface film provides excellent resistance to atmospheric and sea environments as well as a wide range of chemicals, including chlorine and organics containing chlorides.
What makes this possible is a mechanism similar to what occurs with stainless steel, the formation of a stable, self healing surface oxide.
Commercially pure titanium is the most commonly used titanium alloy for corrosion applications, especially when high strength is not a requirement, because it is relatively inexpensive.
Titanium is near the cathodic end of the galvanic series, allowing it to perform the function of a noble metal, but it may react pyrophorically in certain media.
Its corrosion-resistance is commonly improved by the application of an anodizing finish, surface coatings or alloying.
Explosive reactions can occur with fuming nitric acid containing less than 2% water or more than 6% nitrogen dioxide and on impact with liquid oxygen.
Increasing the water content above 2% removes the concern.
Pyrophoric reactions also can occur in anhydrous liquid or gaseous chlorine, liquid bromine, hot gaseous fluorine, and oxygen-enriched atmospheres.
Machined tube and Machinability The machining characteristics of titanium vary greatly and depend upon the alloy composition, heat treatment employed, and resulting hardness.
Instrumentation tube and machined tubes used for actuation systems are heavy walled tube and are made in both titanium and stainless steel.
Generally, titanium is more difficult to machine than carbon steels due to its reactive nature.
This property can result in poor cutting characteristics if inappropriate speeds and feeds are used during processing, reducing the cutting effectiveness by welding itself to the tool and alternately creating a hardened layer due to heat generated during the chip formation.
Low cutting speeds combined with high feed rates limit temperature extremes, while effectively getting below the hardened surface layer.
Pure titanium and alpha alloys require lower contact pressures than beta alloys but are still more difficult to machine than plain carbon steels.
Rigid setups are required to limit deflection because of the low modulus of titanium.
Titanium reacts rapidly at high temperatures with oxygen, nitrogen, and constituents in cutting tools.
The high strength of the alloy requires high contact pressures, which produces high tool-tip temperatures.
This combination of chemical activity and heat contributes to seizing, galling, and abrasion and to pyrophoric behaviour of small particles of titanium.
In addition, titanium has relatively poor thermal conductivity, exacerbating the temperature effects at the tool-tip.
The net effect is that machining of titanium requires careful selection of tools, speed, coolant, and atmosphere to get the desired results.
Titanium tends to oxidize rapidly when heated in air above 1200°F (650°C).
At elevated temperatures, it has the property of dissolving discrete amounts of its own oxide into solution.
For these reasons, the welding of titanium requires the use of a protective shielding, such as an inert gas atmosphere, to prevent contamination and embrittlement from oxygen and nitrogen.
Titanium's relatively low coefficient of thermal expansion and conductivity minimize the possibility of distortion due to welding.
Fabrication Processes Fabricating titanium is relatively difficult because of its susceptibility to hydrogen, oxygen, and nitrogen impurities, which cause embrittlement.
Elevated temperature processing, including welding, must be performed under special conditions that avoid diffusion of gases into the metal.
Forming is more difficult with titanium than with aluminum or iron-based materials.
Heat is usually required in most forming operations to reduce the "springback" of the material, improving the accuracy of forming.
Casting can be performed, but requires molds made from something other than sand, which is used with ferrous metals, because of the reactive nature of titanium.
Special moulds using sand combined with organic or graphite binders are typically used.
Apart from this consideration, conventional casting methods and mould design principles can be applied.
Superplastic forming can also be applied to those titaniums that exhibit a high strain rate sensitivity.
Ti-6-4 with a beta volume of 20% exhibits this characteristic at 870°C.