Machine Design


Depending on the predominant phase or phases in their microstructure, titanium alloys are categorized as alpha, alpha-beta, and beta. This natural grouping not only reflects basic titanium production metallurgy, but it also indicates general properties peculiar to each type.

The alpha phase in pure titanium is characterized by a hexagonal close-packed crystalline structure that remains stable from room temperature to approximately 1,620°F. The beta phase in pure titanium has a body-centered cubic structure, and is stable from approximately 1,620°F to the melting point of about 3,040°F.

Adding alloying elements to titanium provides a wide range of physical and mechanical properties. Certain alloying additions, notably aluminum, tend to stabilize the alpha phase; that is, they raise the temperature at which the alloy will be transformed completely to the beta phase. This temperature is known as the beta-transus temperature.

Alloying additions such as chromium, columbium, copper, iron, manganese, molybdenum, tantalum, and vanadium stabilize the beta phase by lowering the temperature of transformation from alpha to beta. Some elements, notably tin and zirconium, behave as neutral solutes in titanium and have little effect on the transformation temperature, acting instead as strengtheners of the alpha phase.

Alpha alloys: The single-phase and near-single-phase alpha alloys of titanium have good weldability. The generally high aluminum content of this group of alloys ensures good strength characteristics and oxidation resistance at elevated temperatures (in the range of 600 to 1,100°F). Alpha alloys cannot be heat treated to develop higher mechanical properties because they are single-phase alloys.

Alpha-beta alloys: The addition of controlled amounts of beta-stabilizing alloying elements causes the beta phase to persist below the beta transus temperature, down to room temperature, resulting in a two-phase system. These two-phase titanium alloys can be strengthened significantly by heat treatment consisting of a quench from some temperature in the alpha-beta range, followed by an aging cycle at a somewhat lower temperature. Beta-phase transformation, which would normally occur on slow cooling, is suppressed by the quenching. The aging cycle causes the precipitation of some fine alpha particles from the metastable beta, imparting a structure that is stronger than the annealed alpha-beta structure. Although heat-treated alpha-beta alloys are stronger than the alpha alloys, their ductility is proportionally lower.

Beta alloys: The high percentage of beta-stabilizing elements in these alloys results in a microstructure that is substantially beta. The metastable beta can be strengthened considerably by heat treatment.

Titanium is used in corrosive environments or in applications that require light weight, high strength-to-weight ratio, and nonmagnetic properties. While commercially available in many alloys, most requirements can be met by a grade of commercially pure titanium, titanium-0.2% palladium alloy, or by the high-strength Ti-Al-V-Cr (beta type) alloys. These grades, which are available in most common wrought mill forms, are covered by ASTM-AMS specifications and, in most cases, by a similar ASME specification.

Beta-21S is a new beta alloy developed as an oxidation-resistant aerospace material and as a matrix for metal-matrix composites. Composition is Ti-15Mo-2.7Nb-3Al-0.2Si, with molybdenum and niobium working synergistically to raise corrosion resistance to very high levels. It also offers one of the lowest hydrogen uptake efficiency levels of any titanium alloy. The combination of high strength and high corrosion resistance make it an ideal candidate for orthopedic implants, deep sour oil wells, and geothermal brine wells.

Like stainless steel, titanium sheet and plate work harden significantly during forming. Minimum bend-radius rules are nearly the same for both, although springback is greater for titanium. Commercially pure grades of heavy plate are cold formed or, for more severe shapes, warm formed at temperatures to about 800°F. Alloy grades can be formed at temperatures as high as 1,400°F in inert-gas atmospheres. Tube can be cold bent to radii three times the tube OD, provided that both inside and outside surfaces of the bend are in tension at the point of bending. In some cases, tighter bends can be made.

Despite their high strength, some alloys of titanium have superplastic characteristics in the range of 1,500 to 1,700°F. The alloy used for most superplastically formed parts is the standard Ti-6Al-4V alloy. Several aircraft manufacturers are producing components formed by this method. Some applications involve assembly by diffusion bonding.

Titanium plates or sheets can be sheared, punched, or perforated on standard equipment. Titanium and Ti-Pd alloy plates can be sheared subject to equipment limitations similar to those for stainless steel. The harder alloys are more difficult to shear, so thickness limitations are generally about two-third those for stainless steel.

Titanium and its alloys can be machined and abrasive ground; however, sharp tools and continuous feed are required to prevent work hardening. Tapping is difficult because the metal galls. Coarse threads should be used where possible.

Titanium castings can be produced by investment or graphite-mold methods. Casting must be done in a vacuum furnace, however, because of the highly reactive nature of titanium in the presence of oxygen. Typical applications for titanium castings are surgical implants and hardware for marine and chemical equipment such as compressors and valve bodies.

Generally, titanium is welded by gas-tungsten arc (GTA) or plasma-arc techniques. Metal inert-gas processes can be used under special conditions. Thorough cleaning and shielding are essential because molten titanium reacts with nitrogen, oxygen, and hydrogen, and will dissolve large quantities of these gases, which embrittles the metal. In all other respects, GTA welding of titanium is similar to that of stainless steel. Normally, a sound weld appears bright silver with no discoloration on the surface or along the heat-affected zone.

TAGS: Materials
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