Titanium is a relatively light metal (density = 4.54 g/cm3) but has high strength (96 ksi for 99.0% Ti), and so titanium and its alloys can compete favorably with aluminum alloys for some aerospace applications even though titanium costs much more ($3.85/lb for Ti versus $0.67/lb for Al in 2001. Titanium is also used for applications where it has superior corrosion resistance to many chemical environments such as solutions of chlorine and inorganic chloride solutions.
Titanium metal is expensive because it is difficult to extract in the pure state from its compounds. At high temperatures titanium combines with oxygen, nitrogen, hydrogen, carbon, and iron, and so special techniques must be used to cast and work the metal.
Titanium has the HCP crystal structure (alpha) at room temperature, which transforms to the BCC (beta) structure at 883°C. Elements such as aluminum and oxygen stabilize the a phase and increase the temperature at which the aphase transforms to the β phase. Other elements such as vanadium and molybdenum stabilize the beta phase and lower the temperature at which the βphase is stable.
Still other elements such as chromium and iron reduce the transformation temperature at which the β phase is stable by causing a eutectoid reaction that produces a two-phase structure at room temperature.
Why Use Titanium?
Titanium alloys have excellent corrosion and erosion resistance as well as very high tensile strength and toughness, even at extreme temperatures. Titanium’s benefits can be summarized as:
Titanium alloys are classified in four main groups based on the types and amounts of alloying elements they contain:
Alpha (a) – cannot be strengthened by heat treatment; low to medium strength, good notch toughness, formable, weldable, good creep resistance (superior to beta alloys) at somewhat elevated temperatures.
Near alpha – medium strength and good creep resistance.
Alpha-beta (a-b) – strengthened by heat treatment; medium to high strength, high formability, good creep resistance (but less than most alpha alloys), alloys with beta content less than 20% are weldable. The most familiar alloy in this category is Ti-6Al-4V.
Beta (b) – strengthened by heat treatment; high strength, fair creep resistance, prone to a ductile-to-brittle transition temperature.
Some alloying elements (Al, Ga, Ge, C, O, N) raise the alpha-to-beta transition temperature (alpha stabilizers) while others (Mo, V, Ta, Nb, Mn, Fe, Cr, Co, Ni, Cu, Si) lower the beta transition temperature (beta stabilizers).
While pure titanium is soft and relatively weak, heat treating can significantly enhance its properties. Titanium and titanium alloys are heat treated in order to:
Reduce residual stresses from fabrication (stress relieving)
Produce an optimum combination of ductility, machinability, and dimensional and structural stability (annealing)
Increase strength (solution treating and aging)
Optimize specific properties such as fracture toughness, fatigue strength, and high-temperature creep strength
Create specific material conditions
Standard heat treatments include:
Annealing – increases fracture toughness and ductility (at room temperature) and improves dimensional stability and creep resistance. Annealing may be necessary following severe cold work and to enhance fabrication and machining.
Homogenizing – improves chemical homogeneity in castings.
Solution treating and age hardening (aging) – a process of heating into the beta or high in the alpha-beta phase region, quenching and then reheating again to the alpha-beta region. A wide range of strength levels is possible. Fatigue strength increases while ductility, fracture toughness and creep resistance are enhanced.
Stress relief – used to reduce residual stresses during fabrication or following severe forming or welding to avoid cracking or distortion and to improve fatigue resistance. Strength and ductility will not be adversely affected and cooling rate is not critical.
Tempering – When titanium is quenched from an elevated temperature, reheated to a temperature below the beta-transus, held for a length of time and again quenched, it is said to have been tempered. Three variables exist in tempering: the phases present, the time held and the tempering temperature.
Custom heat treatments include:
Beta vacuum annealing and vacuum aging – improves fatigue and yield strength as well as elongation in alloys such as Ti-5553 (Ti-5Al-5V-5Mo-3Cr).
Brazing – induction, resistance and furnace brazing in an argon atmosphere or in vacuum. Cleanliness is important to avoid contamination.
Creep forming – takes advantage of the fact that titanium moves and takes a set well at temperature.
Degassing – involves removing of entrapped gases such as hydrogen (to under 50 ppm) to avoid embrittlement.
Diffusion bonding – powder metallurgy where individual particles fuse from intimate surface contact.
Hydriding – the deliberate addition of hydrogen to embrittle the material.
Isothermal transformation – involves quenching an alloy from the all-beta region into the alpha-beta field, holding and then continuing to quench to room temperature. This causes precipitation of the alpha phase from the beta.
Sintering – typically involving hot isostatic pressing and laser sintering of powder particles to form near-net-shape components.
Practical Considerations – What’s Important?
Heat-treat-furnace capacity is an important consideration since many titanium parts are volume-limited rather than weight-limited. Load support is a critical issue in many applications to prevent creep or other dimensional changes, especially for intricate or complex part geometry.
Temperature measurement and control must be exact – usually ±10 °F (5.5°C) or better throughout the entire working zone of the furnace. Work thermocouples are required because part temperature must be known. Caution: when heating parts over 1730°F (943°C), titanium cannot be in contact with a nickel alloy or stainless steels since eutectic melting will occur.
Vacuum pumping systems must be capable of reaching high vacuum levels - 1 x 10-5 torr or lower before starting to heat. This vacuum level must be maintained while heating (requiring very slow ramp rates) and at temperature. Diffusion pumping systems must be properly maintained for maximum efficiency and to avoid back streaming.
Since titanium is a strong “getter” material, vacuum-furnace interiors must be pristine. Ideally, all-metal hot zones and dedicated furnaces are desired, but graphite-lined furnaces, also used for other processes, are typical throughout the industry as a practical necessity. Thus fixtures and furnaces must be “baked out” (cleaned) before use typically at 2400°F (1315°C).
Finally, the reason why vacuum is used to process titanium is:
Superior buy-to-fly ratio (no wasted material)
Minimization of alpha case – avoidance of an oxygen contamination layer that is very brittle and must be removed by mechanical or chemical means (allows closer near-net-shape parts).
°F to °C Deduct 32, then multiply by 5, then divide by 9
°C to °F Multiply by 9, then divide by 5, then add 32