Melting and Casting of Titanium Aluminides...
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Henrik FRANZ, Head of R & D Metallurgy Division May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy Melting and Casting of Titanium Aluminides Challenges and Opportunities Henrik Franz ALD Vacuum Technologies GmbH, Hanau, Germany
Transcript of Melting and Casting of Titanium Aluminides...
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Melting and Casting of Titanium Aluminides
Challenges and Opportunities
Henrik Franz
ALD Vacuum Technologies GmbH, Hanau, Germany
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides
1st Generation Ti x Al (1970-1980)
2nd Generation Ti-48Al-2Nb-2Cr (1990)
3rd Generation Ti-45Al-8Nb-0.2C-0.2B (2000)
4th Generation Ti-46Al-8Ta (2009)
o α2-Ti3Al (hexagonal) + γ-TiAl (L10-cubic face centered)
o Intermetallic comp.→ high temp. strength
lightweight (3.8-4.5 g/cm3)
oxidation resistance (700-800 °C)
low RT ductility (< 1%)
o Al-content & alloying elements → micro structure
→ creep strength, oxidation resistance, α & β− transition temp.
o Impurities (e.g. Oxygen) → RT ductility↓
o Processing: casting, hot extrusion, forging, powder metallurgy
o Heat treatment processes → microstructure adjustment
UTILIZATION of TiAl – POTENTIALS -> TIGHT ALLOY SPECIFICATION & LOW IMPURITY CONTENT
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Alloy Production
o Reliable alloy production
o Feedstock selection, preparation and flexibility
o Homogenization
o Suppression and compensation of loss of volatile
elements
o Minimization of local superheating
(Heat of Mixing)
o Excellent leak tightness
o Cold wall techniques
o Process monitoring
2 MAIN PRODUCTION ROUTES ⇒ PLASMA
⇒ VAR
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Hearth
Torch
Feeding
Withdrawal
Titanium Aluminides - Alloy Production – Plasma Melting (PAM)
o Continuous Production Method (Feeding & Withdrawal)
o Melting under Helium or Argon (up to 1 atm.)
o Minimization of evaporation loss
o Flexible feedstock handling
o Excellent homogeneity
o Scrap recycling up to 100%
o Ingot diameter → productivity
→ risk of brittleness
o Processing under inert gas →online control of process gas
→ gas recycling
o Limited refining capabilities
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Power
Supply
Vacuum
System
Ingot
Electrode
Titanium Aluminides - Alloy Production – Vacuum Arc Remelting(VAR)
o Batch Production Method (1 Electrode = 1 Ingot)
o Melting under vacuum
o Compensation of evaporation loss by additional material
o Limited feedstock flexibility
o Electrode preparation by pressing & welding
o 2-3 times re-melting for proper homogenization
o Lowest oxygen, nitrogen and hydrogen contents
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
MELTING & CASTING TECHNIQUES
VAR SKULL MELTER
COLD WALL INDUCTION CRUCIBLE
CERAMIC CRUCIBLE
Gravity Casting
Centrifugal Casting
Tilt Casting
Counter Gravity Casting
TiAl –FEEDSTOCK
(VAR or PLASMA)
POWDER PRODUCTION
GAS ATOMIZATION
Gravity Casting
Centrifugal Casting
Gravity Casting
Centrifugal Casting
PLASMA PROCESSES
MIM
AM
Titanium Aluminides – Parts Production Routes
Forging
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Critical flow characteristic of TiAl-alloys
o Reduction of temperature loss while pouring
o Superheat is important
o Casting in hot mold
o Stable ceramic shell system
o Turbulence minimization
o Avoidance of inert gas entrapment
Special Mold Design
o Heating and cooling of the mold
o Heat expansion of TiAl-alloys
o Ductile → Brittle transition during cooling
Precise temperature management required
o TiAl-alloys tend to crack during cooling
Titanium Aluminides – General Requirements for Casting Processes
turbulent
tranquil
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
-
+
DC PowerSupply
CrucibleTilting Device
Electrode
Water cooled
Cu Crucible
with Melt
VacuumSystem
CentrifugeMold
Titanium Aluminides – Melting Techniques– VAR Skull Melter
Process Characteristics
o Low cycle time →high productivity
o Easy to operate and to maintain
o Easy process control
o High reliability/repeatability
o Electrode for multiple pours
o Low production costs
o Up to 1t pouring weight
o Limited scrap recycling
o Turbulent mould filling
o Small batch weights difficult
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
melt withmeniscusshape
cruciblesegment
inductor
bottom
slit
skull
current
(water cooled)
(water cooled)
(water cooled)
electromagnetic field
Second current
Titanium Aluminides – Melting Techniques– Cold Wall Induction Crucible (CIC)
Process Characteristics
o Excellent homogenization (thermal & chemical)
o Recharging and alloying
o 100% Scrap recycling
o No risk of impurity pick up
→ Unlimited holding time
o Multi-chamber design possible
o Vacuum or inert gas
o High energy consumption
o High cooling water demand
o 0.5 – 140 kg pouring weight
o Limited superheat
o Turbulent pouring process
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
o Oxygen solubility in binary Titanium Aluminum
alloys depend on :
Al-Content
Temperature
o γ-TiAl: Al-content < 50 at.%
→ oxygen equilibrium content > 1 %
⇒ If oxygen source available no fail system to limit oxygen pick up exists
o Stability expressed by standard Gibbs Free Energy
o Al2O3 → CaO → Y2O3 →REO
o Oxide mixtures ⇒ oxide activity ↓
o ln [(axMe ay
O)/a MeO ] <<0 ⇒ MeXOY → x Me + y O
Titanium Aluminides – Melting Techniques– Ceramic Crucible
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
o Yttrium contents around 0.5 wt.%
→ Oxygen < 1000 ppm wt.
o Strong temperature dependence
o Risk of formation of Yttria-inclusions during
solidification
→ Impact on properties
o Calcium contents around 0.2 wt.%
→ Oxygen < 1000 ppm wt.
o Calcium evaporates from melt
→ Oxygen enrichment
o CaO-Crucibles difficult to handle (hygroscopic)
Titanium Aluminides – Melting Techniques– Ceramic Crucible
o Both ceramic systems provide temporary
protection only
depending on → Temperature
→ Contact time
o Oxygen pick up will occur
→ Feedstock has to be low in oxygen
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
EU-Project
“IMPRESS”
2000
1995
TiAl-Valve
Casting
CIC + Permanent
Mould
TiAl-Valve Production
Proto-Type
“Wheelcaster”
2014
Investment
Casting
TiAl-Turbocharger
Wheels & Turbine
Blades
TiAl-Powder
Production by
EIGA/VIGA
Titanium Aluminides – History ALD
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Processing Routes – Automotive Valve Casting
o Continuous production
process
o Cold Wall Induction Crucible +
Centrifugal Casting
o Low cost approach
o Vertical mould spinning
o 50 Valves / cycle
o Excellent mould filling
Melting
Heating
Mould
Pre-
Heating
Pouring
Product
Take out
of
Product
o Permanent mould (niobium alloy)
o Cycle time < 25 min
o Up time > 80 % → 500,000 valves/year
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Processing Routes – Automotive Valve Casting
o Excellent mechanical properties (as cast & HIP)
o Chemical uniformity down to micro scale
o Oxygen content below 500-700 ppm wt.
o Plastic elongation >0.5%
o Successful tested
Ready from production
Ready for application
↓Cost target missed
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Processing Routes – Turbocharger Wheel Casting
o Cold Wall Induction Crucible + Gravity Casting
o Investment casting process
o Significant impact of mould preheating on casting yield
o Shell system to withstand high preheating temperatures
o Casting yield strongly depends on position of mould
o Yield reduction by small part/feeding system ratio
Misruns
Mould Temperature:
< 700 °C
Mould Temperature:
> 800 °C
o Low investment costs
o Low yield
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Processing Routes – Tilt Casting
o Adopted for TiAl by IRC Birmingham for large turbine blades
(IMPRESS Project)
o Cold Wall Induction Crucible
o Investment casting process
o Designed pouring sequence (incl. dwell & acceleration phases)
o Mould preheating
o 40 cm IGT-Blades successfully casted
o Long contact time mould/melt
→ stable shell system required
o Process adoptable at industrial scale
o Single piece casting process
→ limited productivity
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Processing Routes CIC + Counter Gravity Casting
Ceramic Crucible + Centrifugal Casting
o Acceleration starts from zero ⇒ turbulences↓
o Ceramic crucible ⇒ short melting time necessary
⇒ small batch weights
o Up scaling by additional casting units only
⇒ need for automation
o No mould preheating
o Low oxygen feedstock
o Hitchiner Process adopted by DAIDO (Levicast)
o Melting under vacuum → backfill with argon
o Mould immersion → mould evacuation → mould filling
o Smooth filling
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Ceramic CrucibleCold Wall Induction
Crucible
SuperheatLimited by ceramic
systemLimited
Feedstock Customized Flexible
Impurity pick
up
• Ceramic system
• Superheat
• Holding time
None
Batch Weight < 10 kg 2 - >100 kg
Holding Time Minimal Unlimited
Pouring
Process
Adjustable (laminar
to turbulent)Turbulent
Alloying No Limited
AtmosphereProtective gas
necessary
Vacuum or
protective gas
possible
ΣPreferable for
Pouring
Preferable for
Melting
Titanium Aluminides – Processing Routes – CIC + Ceramic Crucible
Combination of both techniques can offer
significant yield improvement at large
batch weights
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Melting and Casting Techniques - Summary
o State of the art TiAl-alloy production by Plasma or VAR
o Customized feedstock for casting processes (suitable size and chemical composition)
o Melt superheat and turbulences during pouring ⇒ Casting yield
o Various casting methods available ⇒ No general judgment
o Casting yield dominates CoO (high material costs)
o HIP and heat treatment of cast parts necessary
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Titanium Aluminides – Melting and Casting Techniques - Summary
oInvestment casting/Shell system
→ Gating and feeding system most important (design by simulations)
→ Stress impact on cast parts during solidification and cooling
→ Sufficient chemical resistance during mould filling
o Mould preheating favorable
o Melting in ceramic crucibles → Short cycle time → Small batch weights
o Combination CIC and ceramic crucible → Batch size ↑→ Casting yield↑ → Costs ↓
o Recycling technology necessary
Henrik FRANZ, Head of R & D Metallurgy Division
May 19-21, 2014 • Hilton Sorrento Palace, Sorrento, Italy
Thank You
for
Your Patience
