Ti6Al4V, or Grade 5 according to the ASTM classification, is the most used titanium alloy covering around 50% of the entire production of this metal; the aerospace industry uses over 80%, the prosthetic sector about 3%, the rest is divided between the automotive industry and the production of sports equipment. The low element content version, Ti6Al4V ELI, is also used in applications with cryogenic temperatures.
The Lockheed SR-71 reconnaissance aircraft, better known as “blackbird”, with 93% of its weight made up of three different titanium alloys including Ti6Al4V, has broken a number of records as the highest speed ever achieved by a piloted aircraft, 3.530 km/h, and a maximum height of almost 26.000 m.
The fan of the turbofan aircraft engine (Turbofan) Trent 800, manufactured by the British company Rolls-Royce plc for the civil aviation sector, was built using twenty-six Ti6Al4V alloy blades.
Ti6Al4V belongs to the alpha-beta alloy category: the α-stabilising element present in the alloy is aluminium, with a 6% percentage, which gives the alloy excellent properties at high temperatures; the β-stabilising element is, on the other hand, vanadium, with a 4% percentage and boasts the primary function of stabilising the β phase, making it possible to improve the mechanical characteristics with the solubilisation heat treatment followed by a low temperature aging. This process makes it possible to improve the mechanical characteristics of the alloy by over 35% compared to those of the annealed material.
The best combination of resistance and ductility characteristics is achieved by performing the solubilisation process at about 900°C for the sheet metal and at about 950°C for the round and mass workpieces. The treatment is usually performed in a protective atmosphere: noble gases such as argon are used to minimise the formation of undesired surface oxides and the so-called “alpha case”.
The crucial parameter for the correct performance of the solubilisation treatment is the passage time from the heating furnace to the refrigerating unit, which must be as short as possible: in order to prevent deterioration of the mechanical characteristics, it is imperative that the 6-10 seconds are not exceeded, depending on the size of the component.
The cooling speed is also of fundamental importance: the correct performance of the treatment requires a gradient of over 800°C/min.
The solubilisation treatment performed by TAG meets the quality requirements of the main user groups such as the aerospace, biomedical and racing industries.
The cooling speed is maximised by the systems equipped with temperature-controlled water tanks, an inverter controlled stirring system and integrated management of the entire system.
In particular, it takes just 4.6 seconds to immerse the entire load in the cooling fluid. The aging temperature normally goes from 425°C to 650°C with processing times ranging from 4 to 8 hours.
Aging is a precipitation hardening process and is therefore performed according to a specific time and temperature. The maximum tensile strength and yield strength mechanical characteristics, at the expense of elongation, are achieved at temperatures of around 500°C.