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• Explain how titanium is refined from ore.
• Explain how the strength of titanium is maximized while retaining
ductility.
• Identify the differences in hot-rolling, forging, extruding, casting, and
powder processing titanium compared to steel.
• Describe diffusion bonding and how it affects brazing titanium.
• Explain how chemical milling assists with the finish processing of
titanium.
Learning Objectives
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• Titanium is commonly used in golf clubs, bicycles, and eyeglass
frames.
• Less-visible titanium components include airplane parts, prosthetic
implants, and chemical reaction chambers.
• As a chemical, titanium dioxide has completely replaced lead oxide
for making white paint.
• Titanium alloys are lightweight and can withstand high temperatures
and corrosion.
Introduction
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• Titanium is refined from rutile, a naturally occurring form of TiO2
mined mostly in Australia and South Africa.
• Metallic titanium is produced using the Kroll process.
• Rutile crystals are extracted from ore.
• The crystals are heated with coke and chlorine gas to produce
titanium tetrachloride (TiCl4).
• The TiCl4 is reacted with magnesium metal in a sealed container.
• This reduces TiCl4 to porous titanium metal called “sponge titanium.”
Refining and Processing Titanium
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Sponge Titanium
Bjoern Wylezich/Shutterstock.com
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• The Kroll process was invented by William Kroll in the 1930s and
perfected in the 1940s.
• Method to commercially extract titanium and zirconium
• Kroll moved from Luxembourg to US in 1940 to escape Nazi
Germany.
• His metallurgical knowledge and process aided the Allied war effort.
Kroll Process
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• A new process to refine titanium recently came online.
• FFC Cambridge process produces metal powder directly from
titanium oxide by electrolysis.
• After this or Kroll process, impurities must be removed.
• Gaseous elements oxygen, nitrogen, and hydrogen
• Any remaining magnesium from Kroll process
FFC Cambridge Process
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• In the VAR process, sponge metal is formed into a rough electrode
and remelted in a vacuum remelt furnace.
• An electric arc is struck between titanium electrode and ingot pool.
• This melts tip of electrode and forms a remelt ingot.
• Most volatile elements are released and removed by vacuum pumps.
Vacuum Arc Remelting (VAR) Process
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• Any alloy additions are added and melted
into the ingot.
• Some critical applications require double
and triple remelting.
• Reduces volatile impurities and assures
uniformity.
Vacuum Arc Remelting (VAR)
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• Melting titanium in a vacuum is necessary to remove oxygen.
• Oxygen dissolves in titanium, forming a Ti-O alloy.
• TiO2 particles precipitate during cooling, sharply reducing ductility.
• Dissolved oxygen stabilizes hcp (alpha) phase of titanium, which is
undesirable.
• Alpha phase is harder and lower in ductility.
• It can produce internal cracks during hot work.
Remove Oxygen to Prevent Alpha Phase
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• Nitrogen also dissolves in titanium and has property effects very
similar to oxygen.
• Nitrogen must also be
removed by vacuum remelting.
• Iron is also undesirable in
titanium.
Impurities Affect Strength
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• Ingots as large as 36″ (91 cm) in diameter and up to 15,000 pounds
(6800 kg) are produced by vacuum melting.
• Obtaining sound, homogeneous ingots requires operators’ constant
attention.
• Final ingot can be processed by standard bulk deformation methods
(rolling, forging, or extruding).
• Producing billet, bar, plate, sheet, strip, or tube
Titanium Ingots
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• Titanium pieces must be protected from air during hot work.
• Above 1100°F (600°C), titanium absorbs oxygen and nitrogen, forming
a Ti-O-N alloy layer on the surface, called alpha case.
• Alpha case (hcp structure) has low ductility and will not undergo
phase transition during any heat treatment.
• When cooled, TiO2 and Ti2N particles form in alpha case, making it
brittle and likely to crack during use.
• Alpha case is removed by acid etching or mechanical grinding.
Manufacture of Titanium Products
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• As titanium alloys cool from high-temperature beta to lower-
temperature alpha phases, various ratios of phases develop.
• Some alloys remain entirely beta phase, and some entirely alpha.
• Different processing conditions produce different microstructures
and properties.
• This depends on thermal cycle and hot-working used.
Alpha and Beta Phases in Titanium
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• Above 1400°F (760°C), titanium ingots are easily
rolled.
• Hot-rolling titanium at temperatures just below
beta-alpha transition at 1620°F (882°C) produces
grain refinement.
• Where controlled atmosphere furnaces are
available, plates and bars can be heat-treated to
high strength.
Hot-Rolling Mill Products—Sheet and
Plate
mironov/Shutterstock.com
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• Process procedures strongly affect strength of titanium alloys.
• Small changes in procedures may affect impact and fatigue strength
of components.
• Some titanium alloys are designed to have both alpha and beta
phases at room temperature.
• UNS R56400 (Ti-6Al-4V) is an example, and it requires careful control
of deformation and temperature.
Alpha-Beta Titanium Alloys
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• During titanium forging, oxygen and nitrogen in air react to develop
alpha case.
• A major purpose of forging parts is to obtain metal flow patterns that
increase load capacity.
• Impact strength of titanium landing gear struts is determined by
forging temperature and degree of work in closed die.
• Preheating dies helps maintain workpiece temperature while forging.
Forging
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• Extruding titanium produces long lengths
of seamless pipe, as well as products with
complex cross sections.
• Titanium is extruded at temperatures
where high-temperature beta phase
exists.
• Ensures dynamic recrystallization and
maximizes plasticity
• Billet is heated then covered with molten
glass to extrude it.
Extrusion
Reprinted with Permission from Plymouth Tube Co. (www.plymouth.com)
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• When cold formed, titanium has high
level of springback.
• It must be “overformed” to achieve
design angles.
• Titanium has more springback for
two reasons.
• It has a lower modulus of elasticity.
• It has high yield strength.
Forming Titanium
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• Sometimes titanium is “warm formed” at temperatures below alpha-
beta transformation temperature.
• This reduces springback.
• Titanium’s greater springback is useful for eyeglass frames.
Titanium Warm Forming
CHARMANT USA Inc.
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• When formed slowly at 1562°F (850°C), titanium elongates 200%
(superplasticity).
• Eight times more than typical fully annealed metals
• Occurs because titanium recrystallizes dynamically
• Dislocation tangles never develop; titanium does not work harden.
Superplastic Forming
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• Example:
• Weight is placed on a titanium
sheet.
• This is placed on a die with cavities.
• Entire fixture is placed in vacuum or
inert gas furnace.
• Sheet deforms under load and
forms into die shape in few hours.
Superplastic Forming Example
Solar Atmospheres; Beckwood Press Company
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• Titanium castings may possess the
tensile and creep-rupture strength of
wrought titanium.
• Mold materials are typically graphite.
• Machined from blocks or compacted
from graphite powder
Casting Titanium
Radomir Rezny/Shutterstock.com
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• Cast titanium usually has small shrinkage voids and porosity.
• Cast aerospace parts are routinely hot isostatic pressed (HIP).
• Closes pores and restores fatigue strength
• Castings are placed into HIP chamber for two hours at 1650°F
(900°C).
• Argon atmosphere at 15 ksi (105 MPa) is used.
• About 1000 atmospheres of pressure
Hot Isostatic Pressing
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• Titanium powder can be compacted and sintered to form dense,
near net-shaped parts.
• Thermal processing must be done in vacuum or inert gas
atmosphere.
• Prevents oxygen absorption
• HIP processing achieves full density.
• Results in maximum ductility, yield strength, and impact strength
Titanium Powder Processing
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• Titanium metal powder is a fire hazard.
• It must be stored in flameproof containers.
• Only class D fire extinguisher puts out titanium powder fire.
• Burning titanium removes oxygen from water.
• This releases hydrogen that burns.
• Operations that process titanium powder must have class D fire
extinguishers in all working areas.
Titanium Powder Fire
Safety Note
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• Commercial-purity titanium can be stress relieved or annealed.
• Stress relieving is done at 900°F–1100°F (480°C–595°C).
• Annealing is done at 1200°F–1400°F (650°C–760°C).
• Time depends on how long it takes to heat workpiece’s center.
• Annealing is usually done by air cooling.
• Thin alpha case layer develops and may need to be removed later.
Heat Treatment of Titanium
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Titanium Going in Furnace for Heat
Treatment
mironov/Shutterstock.com
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• A duplex anneal purposely develops a specific ratio of alpha-to-beta
phases in microstructures.
• Improves creep resistance and fracture toughness
• Instructions are different for each alloy and production sequence
and must be followed exactly.
• Vacuum and inert gas furnaces are preferred for high-temperature
thermal cycles of titanium.
• For critical dimensions, hard vacuum is required.
Duplex Anneal (Alpha-Beta Anneal)
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• Ti-6Al-4V is most frequently used titanium alloy.
• It is solutionized, quenched, and aged to improve strength,
toughness, fatigue strength, and impact strength.
• Heated to 1750°F–1775°F (955°C–970°C) for one hour
• Water quenched quickly after removal from furnace
• Aged 4–8 hours at 900°F–1100°F (480°C–595°C)
• Produces microstructure of fine beta particles and alpha matrix
• Typical yield strength is 155 ksi (1069 MPa) with 16.5% elongation.
Precipitation Hardening Titanium Alloys
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• For aerospace parts, furnaces must be
calibrated frequently to meet standards.
• Aging must be higher temperature than
application.
• Compressor blades in jet engines run at or
above 900°F (405°C).
• Blades must be age hardened near upper end
of temperature range.
• Process instructions may be affected by
conditions of application.
Temperature Control of Aging
Chesky/Shutterstock.com; Jonathan Weiss/Shutterstock.com
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• Furnaces must be maintained to achieve uniform temperature within
25°F (14°C) across entire furnace.
• But 25°F (14°C) is entire permitted temperature range for heat-treating
Ti-6Al-4V titanium.
• Technicians and operators must carefully monitor processes.
• Make sure furnace calibrations are current and settings match
planned processing for parts.
Furnace Control
Practical Metallurgy
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• Titanium parts can be joined like many other metals.
• Arc welding, brazing, adhesive bonding, mechanical fastening
• Complex components can also be joined by diffusion bonding.
• Solid-state technique used to join similar and dissimilar metals
• Atoms of two metals diffuse together over time.
Joining Titanium
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• Experienced welders can join titanium using GMAW or GTAW.
• Workpieces must be cleaned thoroughly before welding.
• Inert cover gas of argon or helium is required.
• Supplementary inert cover gas (“trailer”) may be used to protect cooler
areas near welds.
Arc Welding Titanium
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• Welds must be absolutely protected from oxygen and nitrogen.
• Hot metal is embrittled when it absorbs either of these.
• Welding may be done inside controlled atmosphere chambers.
• Electron beam and laser welding are done in a hard vacuum.
Arc Welding Titanium (cont.)
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• Titanium has many useful applications in
chemical process reactors, pressure vessels,
and piping.
• Can withstand high temperatures and
pressures and remains chemically inert
• Titanium is preferred for papermaking using
wet chlorine gas.
• Desalination plants rely heavily on titanium.
Process Equipment
ImagineStock/Shutterstock.com
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• Brazing must minimize air exposure.
• Vacuum brazing is most common method.
• Some braze filler alloys are liquid below beta transformation
temperature for unalloyed titanium.
• Brazing in a vacuum chamber uses filler alloy foil between titanium
pieces at 1290°F (700°C).
• Workpieces keep original microstructure.
• Filler alloy melts and flows into joints.
Brazing Titanium
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• Higher-temperature braze filler alloys
are used where designs require
strength at high temperature.
• Entire assembly is heat-treated to
strengthen it after brazing.
• Achieves maximum strength
High-Temperature Brazing
Seven Cycle, Inc.
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• Braze joints made, then titanium parts
held at 1700°F (925°C)
• Aluminum in joint diffuses into
titanium at that temperature.
• Finished joint is alloyed titanium with
strength equal to parent.
Diffusion Bonding
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• Diffusion bonding and superplastic forming operations can be
combined into one furnace cycle.
• Multiple parts can be joined to form complex shapes.
Diffusion Bonding (cont.)
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• Brazing, diffusion bonding, and superplastic forming of titanium all
require careful monitoring.
• If vacuum chamber develops small leak, oxygen forms brittle alpha
layer on titanium, leading to surface cracks and reduced fatigue life.
• Close temperature control is necessary during long thermal
processing.
• Swings of 35°F (19°C) during superplastic forming change forming
rate, resulting in incomplete or distorted parts.
Maintaining Close Control during Brazing
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• Titanium machining conditions are
similar to stainless steel.
• Titanium can be polished and anodized.
Machining and Finishing
MarinaGrigorivna/Shutterstock.com
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• Chemical milling uses temperature-controlled baths of etching
chemicals consisting of nitric acid and hydrofluoric acid.
• Can remove selected portions of components, reducing weight without
producing scratches or notches
• Can also remove alpha case on mill products and forgings
Chemical Milling
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• Operators must be extremely careful working around hydrofluoric
acid.
• If acid contacts skin, it continues to react until reaching bone.
• Even a small wound may take months to heal.
• Safety kits with HF-neutralizing solutions must be close to any
operation involving this acid.
Hydrofluoric Acid
Safety Note
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• Titanium has excellent corrosion resistance
• Resists seawater, body fluids, fruit juices,
certain acids and bases
• Titanium is biocompatible with living things,
making it useful for many medical
applications.
• Hip and knee implants
• Pacemaker cases
• Dental implants
Corrosion Resistance and
Biocompatibility
Monstar Studio/Shutterstock.com
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• Most common titanium alloy is UNS
R56400 (Ti-6Al-4V).
• Contains 6% aluminum (Al) and 4%
vanadium (V)
• Yield strength is controlled through
heat treatment.
• Prior processing controls grain size.
• Table compares unalloyed grade 2
titanium (UNS R50400) with annealed
and heat-treated Ti-6Al-4V.
Summary of Mechanical Properties
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• Titanium can be recycled to reduce costs and environmental impact.
• Titanium is too costly to use for most consumer products.
• All manufacturers that use titanium recycle scrap.
• Aerospace industry is biggest user of titanium and biggest driver of
titanium recycling.
Recycling Titanium
Sustainable Metallurgy
