STRUCTURAL OPTIMIZATION OF TITANIUM ALLOYS BY ELECTRIC UPSET FORGING

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STRUCTURAL OPTIMIZATION OF TITANIUM ALLOYS BY
ELECTRIC UPSET FORGING

• Lembit Kommel
• Tallinn University of Technology
• Department of Materials Engineering

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ABSTRACT

• As result of Ti-alloys allotropically transitions
  the structure, texture and phases condition are
  very sensitive to electric conduction heating
  velocity, initial deformation temperature and
  rate of severe plastic deformation.
• The paper gives an overview of the state in
  structural optimization as art in manufacturing
  of preform from Ti-alloys with required
  microstructure and properties by electric upset
  forging (EUF) process use.
• A summary of electric upset forging parameters
  influences on changes of thermodynamics
  properties and their effect on structural
  optimization of Ti-alloys is given.
• Structural forming modes and their relationship
  to different Ti-alloys physical and mechanical
  properties are discussed in view of EUF process
  parameters.
• In addition, actual test results on materials
  texture and microstructure are shown.
• On base of these experimental results by natural
  bars upsetting the optimal EUF processing
  parameter for structural optimization of
  Ti-alloys were received.

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Introduction

• The material thermophysics properties are for
  example the heat capacity, coefficients of
  thermal expansion and heat conductivity, heat
  convection, heat distribution and others...
• For part of bar heating and for thermal balance
  calculation needed the required capacity of heat
  addition, which depends on coefficient of
  electrical resistance of metal and density of
  high strength heat-proof titanium based alloys
  for the turbo-jet steam turbines for fossil and
  nuclear power plants of structural parts such as
  blades are actual use.
• Components such as blades are highly loaded under
  the pressure of combined stresses from
  centrifugal stress during rotor rotation, bending
  stress and torsional stress from a compressed air
  or gas flow under service temperatures from -60
  up to 650C.

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Process

• The electric upset forging (EUF) process combines
  with rapid heating and forming functions in one
  operation, speeding both perform and finished
  forging production.
• During heating process with conduction electric
  current the free electrons and hydrogen atoms
  moving in metal with increasing of temperature
  and in that time takes place the regulation of
  metal electron structure.
• The regulation of electron structure in ones
  turns to increase the internuclear interaction of
  metal.

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Aims of investigation

• The aims of this structural optimization of
  heat-proof Ti-alloys are structure forming
  mechanism investigation at (a) electric
  conduction current density or velocity of rapid
  heating and at (b) deformation stress or
  deformation starting temperature during upsetting
  of bar in condition of severe plastic
  deformation.

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Materials

• The Ti-alloys in this study (in wt. ) were
• VT3-1 a??-titanium alloy TiAl6Mo2Cr2Fe0.5Si
• VT18U - pseudo a-titanium alloy
• TiAl7.6 Zr11MoNbSnSi
• VT25U - a??-titanium alloy TiAl5.8W1.1Mo7.4Zr6.5Sn
  3.8Si0.5

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Testing methods

• The natural cylindrical bars with diameter (d)
  from 15 to 50 mm were upsetted with testing of
  parameters on electric upsetting installation
  Hasenclever HG-125/560.
• Bars were heated by electric current density (i)
  from 10 to 26 A/m2 by heating velocity (??) from
  12 to 250C/s
• Severe deformed by deformation stress (?s) from
  30 to 500 MPa.
• By these parameters use the temperature of
  deformation starting was in interval of
  720-1300C and was increased during deformation
  in the central part of heated and deformed bar.
• For texture and microstructure investigation the
  specimens were cut off from bars in three parts
  initial, heated and deformed. These specimens
  (fabricated in three projections) were polished
  and etched for microstructure study.
• The texture and microstructure forming were
  studied with optical (Nikon CX) and scanning
  electron (Gemini LEO Supra-35) microscopes.
• For mechanical and physical properties measure
  the microindentation method with universal
  hardness tester Zwick Z2.5/TS1S was use. The
  universal hardness was measured by load of 100 N,
  creep and relaxation by load of 35 N during 5
  minutes.
• The operational properties of compressor blades
  were tested on electrodynamics vibration stand
  by frequency f1,0 1010-1080 Hz of
  self-fluctuation. During each test step (up to 6)
  of fatigue strength testing the 2?107 cycles were
  made.

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Mass heat capacity

• The thermophysics properties were determined by
  electric conduction heating method with heating
  velocity of 50C/s to temperature 1100-1200C.
• The coefficient of mass heat capacity (cp) was
  non-linear increased over two times.
• From temperatures at 600C the heat capacity
  rapidly increase and also, the heat addition want
  to increase for thermal balance of EUF process
  calculation.
• Ti-alloys have maximal mass heat capacity in
  region of phases transformation temperature.

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Coefficient of electrical resistance

• The coefficient of electric resistance has
  maximal value at temperatures 400-600C and to
  take decrease to minimal value by temperatures at
  800 C for (a??)-titanium alloys and at 930C for
  pseudo-a titanium alloy.
• The minimal value of electric conduction
  coefficient for all Ti-alloys was measured in
  region of phases transition.
• The coarse-grained ß-structure Ti-alloy has a
  lower coefficient of electrical resistance.

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Coefficient of temperature conductivity

• Up to 700C the coefficient of temperature
  conductivity was increased approximately
  linearly, then decrease by phases transitions
  and after phases transitions for ß-structure
  rapidly increases up to maximal value.

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Physical properties of Ti-alloys

• As was shown these physical properties of
  Ti-alloys have non-linear character by
  temperature increase.
• Near region of phase transition the physical
  properties change significantly.
• For calculation of optimal EUF process parameters
  the numerical values of physical properties dont
  use.

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Microstructural investigation

• Microstructure of pseudo-a Ti-alloy is shown in
  initial state.

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Microstructure formed During EUF

• Microtexture of pseudo-a Ti-alloy formed by EUF
  is shown.

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Microstructure forming mechanism by minimal
electric current density

• The initial coarse-grained microstructure was
  transformed by minimal electric current density
  of i 13 A/m2.
• By this the heating velocity was only 12C/s by
  minimal deformation rate 0.2 0.6 ? 10-3 m/s and
  maximal temperature 970C by optimal short time
  deformation stress ?s 140 MPa.
• The structure has view of deformed laminates of
  a- and ?-phases.
• Material was deformed by sliding on grain
  boundaries in condition of high velocity
  superplasticity.

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Microstructure forming mechanism by maximal
electric current density

• By electric current density increase up to 25
  A/mm2 the heating velocity was increased up to
  250 C/s.
• From internal stresses in the subgrains the
  microstructure in view of fine laminates was
  formed.
• By this the high heating velocity influences on
  incubation time which was increased and as result
  the temperature of polymorphous phases transition
  was increased too, from 980 up to 1130C.
• The strength characteristics were increased by
  mean plasticity.

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Microstructure forming mechanism by minimal
deformation stress

• The influence of deformation stress (by optimal
  electric current density i 17 A/mm2) on
  microstructure forming is illustrated.
• The subgrains size was increased significantly by
  laminates thickening and was identical to cast
  Ti-alloy microstructure.
• This material was heated by velocity of ?t
  102C/s and severe deformed by stress of ?s 50
  MPa at maximal temperature interval td
  1170-1280C and has by high strength a low
  ductility properties.
• Maximal temperature in heated part of bar was
  increased up to temperature of 1170C and as
  result the coarse laminates microstructure was
  formed.
• The material with this microstructure for blades
  manufacturing dont use.

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Microstructure forming mechanism by maximal
deformation stress

• Microstructure of Ti-alloy after EUF by maximal
  deformation stress is shown.
• By deformation stress increase up to 300-500 MPa
  the deformation mechanism was changed.
• Large subgrains were crushed at rapid heating
  and during severe deformation the ultrafine
  microstructure with mean grain size of 600 nm was
  formed.
• This microstructure was formed by deformation
  stress at 500 MPa and deformation temperature af
  800-850C.
• This Ti-alloy with ultra-fine grained
  microstructure has by relative high strength
  high-cycles fatigue stress and good plastic
  properties.
• The compressor blades with ultra-fine grained
  microstructure have high life extension 1. It
  was increased up to 2-3 times.

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Influence of electric current density on heating
velocity, EUF process duration and temperature of
deformation starting for Ti-alloy VT18U

• For each curves the electric current density is
  shown.

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Influence of deformation stress on temperature
of deformation starting and the maximal
temperature in the upsetted bar from VT18U

• Depending on deformation stress the mechanism of
  microstructure forming change
• By minimal stress (50-125 MPa) the temperature
  and temperature interval increase up to maximal
  and large grains of ß-phase can be formed.
• By mean stress (130-140 MPa) the deformation took
  place at phase transition temperatures without
  phase transitions.
• By maximal stress (150-450 MPa) the temperature
  of deformation decrease and grain size decrease
  also at shear stress by SPD.

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Effect of EUF on Ultimate Strength of titanium
alloy VT25U
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Effect of Processing on Fracture Toughness of
Pseudo-a Ti-alloy Depending on Direction of
Loading to Slip Lines
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Effect of Processing on Elongation and Reduction
of Area Depending on Direction of Loading to Slip
Lines
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Effetc of Synchronous Operation of the
Deformation Stress and Electric Current Density
on Temperature of Deformation Starting of the
Ti-alloy VT3-1
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Effect of electric current density and
deformation stress on heating speed and structure
forming mechanisms of titanium alloy VT3-1
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Ti-alloys optimal parameters for EUF installation
Hasenclever XG-125/560

• Optimal values of electric current density (i,
  102 x MA/m2 curve 1) and deformation stress
  (??, MPa curve 2) on heating velocity (??,
  C/s) on the surface (curve 3) and in the central
  part of bar (curve 4) depending on bar
  cross-section area (S 10-4 x m2) or for bar
  diameter from 16 to 50 mm. (Area, 10-4 x m2, S 10
  D 35.7 mm)

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Optimal Hydraulic Pressure in Upsetting Cylinder
for Hasenclever HG-125/560 During Processing of
Ti-alloys VT25U and VT3-1 Depending on Bar
Diameter and Degree of Deformation
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Conclusions

• Grateful to the physical properties of Ti-alloys,
  their low coefficients of heat capacity, low
  densities, and high electrical resistance the
  electric upset forging is very effective method
  for this metallic materials manufacturing.
• During rapid electric conduction heating the
  defects of structure, such as cracks, porous and
  chemical inclusions can be (in result of
  solid-to-solid diffusion) welded, large subgrains
  with ?-phase coarse laminates crushed, and new
  dislocations formed.
• Grateful to the rapid electric conduction heating
  the temperature of phases transitions increase
  with heating velocity increase.
• Deformation stress influences on temperature, and
  also on texture and microstructure forming
  mechanism.
• The different optimized microstructures, formed
  during EUF process with optimal parameters, can
  be receiving the needed mechanical and
  in-services properties of Ti-alloys.

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Thank You for the Attention!