Magnesium and titanium

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Magnesium and titanium

• EF420 lecture 12
• J S Taylor

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Reactive metal group

• Aluminium, magnesium, titanium
• Zirconium, hafnium, beryllium and uranium
• Left side / bottom corner of periodic table
• Readily react with air and water etc
• Oxide may protect metal and give good corrosion
  resistance
• Oxidation during welding a problem
• Finely divided metal may be pyrophoric
• Specific material hazards with some of group

3
Periodic table
Reactive metals Refractory metals Precious
metals
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Magnesium alloys
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Metallurgical properties

• Close packed hexagonal structure with high c/a
  ratio
• Slip only in basal plane
• No solid phase change
• High work hardening rate
• Cold working difficult
• Low ductility, notch sensitive
• High chemical affinity for oxygen
• Corrosion resistance is poor

6
Physical properties

• Lightest of common structural metals - density
  1.74Mg/m3
• Low elastic modulus (45GPa)
• Greater displacement under similar loads than Al
  or steel
• Absorbs vibrational and shock loading
• Melts at 649C, low thermal and electrical
  conductivity

7
Product forms

• Wrought products
• Extruded bars and shapes, wire
• Rolled sheet and plate
• Forgings, impact extrusions
• Sand and permanent mould cast alloys
• Die cast alloys
• Powder metallurgy not used because of high
  affinity for oxygen

8
Structural applications

• Alloys can show a weight saving over Al or steel
• Up to 350MPa tensile strength
• Transport, particularly aircraft and aerospace.
• High speed machinery - minimise inertia forces
• Poor wear, impact, creep and fatigue performance
• Not for elevated temperatures

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Other applications

• As an alloying ingredient in Al, Zn, Cu, and
  other metals
• Corrosion protection anodes
• Deoxidation agent and oxygen scavenger
• Reducing agent for producing Ti, Zr, Ha, U and Be
• Constituent in chemicals
• Photoengraving

10
Fabrication

• Excellent machineability, better than aluminium
  or steel
• Work hardens rapidly. Cold work limited
• Hot works at 230 - 370C (typically)
• No protective atmosphere necessary
• Good weldability with inert gas shielded
  processes
• Adhesive bonds and rivets used
• Cast-in inserts of steel, brass, bronze, etc

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Pure magnesium

• Has little structural use
• Soft, poor corrosion and oxidation resistance,
  difficult to work, expensive
• Getters in vacuum tubes, pyrotechnics
• Deoxidant, desulphuriser in manufacturer of Ni,
  steel, cast iron
• Reducing agent for Ti, etc

12
Magnesium alloys

• Up to 10 Al Zn, Mn
• Alloys containing zirconium, rare earths, thorium
  or silver are age hardenable
• Rare earths added as mischmetal or didymium

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Magnesium alloy designations
AZ91C-T6
2 letters Principal 2 alloy elements A
aluminium Z zinc
2 digits Quantity of principal alloys 9 9 Al 1
1 Zn
Temper designation -T6 solution treated and
artificially aged
1 letter Distinguishing code C Third alloy of
this type
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ASTM Designation codes

• F - As fabricated
• O - Annealed
• H10 and H11 - slightly strain hard
• H23, H24, H26 - strain hardened and partially
  annealed
• T4 - artificially aged
• T6 - Solution treat, aged
• T8 - Solution treat, cold work, aged

• A - Aluminium
• E - Rare earths
• H - Thorium
• K - Zirconium
• M - Manganese
• Q - Silver
• T - Tin
• Z - Zinc

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Typical sand cast alloys

• AZ91C-T6 9Al-0.7Zn-0.13Mn, moderate strength (91
  MPa yield, 275 MPa tensile), good ductility
• K1A 1Zr High damping capacity
• QH21A 2.5Ag-1Th-0.7Zr aircraft components for
  up to 250C (207 MPa yield, 275 MPa tensile)

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Typical die cast alloys

• AZ91B-F Most used die casting alloy
• AM60A-F Automotive wheels, good toughness,
  reasonable strength
• AS41A-F Good creep resistance up to 175C

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Wrought alloys

• AZ31B-F Extruded bars and shapes for battery
  components, moderate mechanical properties when
  used for structure.
• HK31A-H24 Sheet and plate 205 MPa yield and 260
  MPa tensile, excellent weldability

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Welding of magnesium

• Spot welding is relatively easy
• GTAW using AC or DCEP preferred for cleaning
  action
• GMAW using pulsed or spray techniques
• Gas shielding argon or argon-helium
• Preheat of none up to 260C to avoid cracking
• Stress relieve gt1.5 Al alloys to avoid SCC

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Fire precautions

• Ignition of alloys gt0.25mm thick during welding
  is remote
• Inert gas must be used
• Avoid accumulations of dust or chips
• Keep recommended powders for extinguishing
  magnesium fires handy
• Avoid accumulation of dust in water baths, as
  hydrogen gas froth is created.
• Some cleaning solvents are toxic

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Corrosion of magnesium

• Better than iron and equal to aluminium in dry
  clean environments
• About as good as steel in damp atmospheres
• Severe corrosion occurs in salt solutions eg
  marine environments
• Rapid attack by most acids, passivated by strong
  hydrofluoric and chromic acids
• Enamel or lacquer paints often used

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Corrosion of magnesium

• Anodic to all structural metals, including zinc
• Protective anodes
• Galvanic corrosion likely
• Stress corrosion of Mg-Al-Zn alloys is possible
• Welds are stress relieved at 150 to 260C
  depending on alloy

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Titanium and its alloys
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Extraction of titanium

• Abundant metal, 4th after Al, Fe and Mg
• Occurs as rutile and ilmenite sand
• Ore treated with chlorine gives tetrachloride
• Reduction of tetrachloride with Mg or Na
• All refractories attacked by molten Ti
• Vacuum melting in water-cooled copper hearths.
  Several remelts required.
• High cost material 10? the energy of steel
• Currently lowering of cost by Russia

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Properties of titanium

• Light weight 4.5 Mg/m3
• High strength
• 410 MPa yield for commercially pure metal to 1300
  MPa by alloying and heat treatment
• Excellent corrosion resistance in aqueous salt or
  oxidising acids
• Tight, microscopic oxide film on exposure to air
• Exposure to temperatures over 480C causes oxide
  to dissolve in titanium, causing embrittlement

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Applications

• High temperature strength (up to 480C) and high
  strength-to-weight ratio
• Aircraft aerospace, Soviet submarines
• Excellent corrosion resistance
• Chemical seawater piping and vessels
• Prosthetic devices
• Wets glass some ceramics
• Bonding applications

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Crystal structure

• Allotropic
• Up to 882C (Beta transus) cph a
• Over 882C bcc b - excellent hot workability
• Melts at 1670C
• c/a ratio for alpha Ti is less than ideal, slip
  is predominantly in pyramidal and prismatic
  planes
• Ductility is better than other cph metals, eg Mg
  and Zn

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Alloy element classes

• Interstitials - potent strengtheners
• Substitutional elements
• Alpha stabilising elements Al, O, N, C
• Isomorphous beta stabilisers are Mo, V, Nb and Ta
• Eutectoid beta stabilisers, Cu and Si (active),
  Cr and Fe (sluggish)

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Titanium alloy types
Neutral
Alpha stabilisers
T C
T C
bbcc
b
882
882
a
acph
Ti
Sn, Zr
Ti
Al, O, N, C
Isomorphous b-stabilisers
Eutectoid b-stabilisers
T C
T C
bbcc
bbcc
b-transus
882
882
a-transus
a b
b g
acph
acph
a b
a b g
Ti
Ti
Mo, V, Ta, Nb
Fe, Mn, Cr, Co, Ni, Cu, Si, H
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Titanium alloys

• Approximately 25 alloy grades in 4 groups
  commonly available
• Commercially pure grades
• Alpha and near-alpha alloys
• Alpha-beta alloys
• Metastable beta alloys

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Titanium alloy heat treatment

• Heating to beta region causes grain coarsening,
  unlike steels where austenitisation causes grain
  refinement
• Alpha-beta alloys and metastable beta alloys are
  age hardenable
• Beta can be retained on quenching, or transform
  to a by nucleation and growth, or can undergo a
  martensitic transformation

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Commercially pure alloys

• Contain controlled levels of oxygen, nitrogen,
  iron and carbon.
• O, N and C are interstitial elements
• Rapidly increase strength
• Rapidly reduce ductility
• Cause strain-aging phenomena
• Interaction between dislocations and impurities
• Yield strength levels 240 to 550MPa

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Commercial purity grades

• ASTM Grade 1 to Grade 12
• Increasing oxygen levels 0.18 to 0.25 max
• Grades 7 and 11 contain palladium, which
  increases corrosion resistance
• Grade 12 contains Ni and Mo for improved
  corrosion resistance
• Best for corrosion resistance
• Chemical plant and corrosive environments

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Alpha and near alpha alloys

• a-alloys cannot be heat treated to increase
  strength
• Contain Al, Sn, Mo, V, Zr, Mo
• Small amounts of b-stabilisers in near a alloys
  (Zr, V)
• Al is a strong solid solution hardener, increases
  recrystallisation temperature, alpha stabiliser
• Al content is limited (6 - 8) by the tendency to
  form a2
• Embrittlement, stress corrosion
• Sn increases strength, but has little adverse
  effect on ductility

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Properties of alpha alloys

• Yield strengths 320 to 830MPa
• Good creep strength and oxidation resistance
• Good properties at cryogenic temperatures

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Alpha-beta alloys

• Contain a-stabilisers b-stabilisers
• b-phase (bcc) is more ductile than a-phase (cph)
• Hot worked in high temperature range (b) at which
  they are more ductile
• Hot work in a b region followed by heat
  treatment is used to control mechanical
  properties
• Example Ti-6Al-4V(most popular alloy, 50 of all
  TI)
• Yield strengths 590 to 1210MPa

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Transformation in a b alloys

• Heating above the b-transus causes grain
  coarsening
• Transformation of b to a depends on cooling rate
• At slow cooling rates, a precipitates at the b
  grain boundaries to give low ductility
• Intermediate cooling rates give fine acicular a
  and good ductility
• Water quenching promotes martensite
  transformation
• cph a' in lean alloys
• Orthorhombic a" in higher alloys

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Heat treating a b alloys

• Annealing is at 650 to 850C in a b field,
  followed by air or furnace cooling
• Follows work in a b region
• Results in low yield stress and good ductility
• Eg Ti-6Al-4V Ry 900 MPa, A5 15
• Solution treatment is just below the beta
  transus, followed by quenching
• At top of a b range
• This increases the b content
• Subsequent aging allows fine a to precipitate,
• Eg Ti-6Al-4V Ry 1050 MPa, A5 12

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Metastable beta alloys

• 100 b retained on air cooling gives exceptional
  formability
• Age hardening at 480 to 590C for up to 24 h give
  yield strengths of 850 to 1430MPa by
  precipitation of coherent a-phase
• Cr and Si give fine precipitates, which improves
  high temperature strength
• Ductility and fracture toughness inferior to
  other grades
• Example Ti-15V-3Al-3Sn-3Cr

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Machineability

• Machineability is about same as 316 stainless
• Galls easily, low strength over 430C
• Low cutting speed
• High feed rates
• Sharp tools
• Cutting fluids required
• Low elastic modulus (100GPa)
• Slender sections chatter
• Dry or oil-contaminated turnings are a fire hazard

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Formability

• Cold forming of alpha alloys similar to steel
• Higher spring-back (low elastic modulus)
• Lower ductility (more generous radiuses)
• High strength alloys 'hot' worked at 200C to
  400C

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Fusion weld structure

• HAZ transforms to b and grains grow to large size
  close to fusion line
• Solidification of weld metal is by epitaxial
  growth from HAZ
• Large grain size in weld metal
• Grains meeting at centre of weld may cause
  weakness
• In thin materials grains may span thickness
  giving poor ductility

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Weldability of a alloys

• Applies to commercial purity, alpha and near
  alpha grades
• Good to excellent weldability
• High sensitivity to contamination by O2, N2, H2
  and C at over 430C
• Interstitials reduce number of slip planes in cph
  a from 3 (basal, prism, pyramidal) to 1 (prism or
  pyramidal)
• Causes contamination cracking
• Delayed hydrogen cracking is possible
• Weld properties are not greatly influenced by
  weld heating cycle

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Weldability of a b alloys

• Less sensitive to embrittlement by interstitials
• b is retained in weld metal and HAZ on cooling,
  martensite may form
• Weldability varies from fair to unweldable
• Alloys with a low level of b-stabilisers have
  fair weldability
• The a' martensite formed is tough and less hard
• Lower hardenability
• Eg Ti-6Al-4V
• Alloys with high level of b-stabilisers crack
  during welding
• Eg Ti-6Al-2Sn-6Mo
• PWHT is required to ensure ductility of a b
  alloys

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Welding metastable b-alloys

• b is retained in the weld metal and HAZ on
  cooling.
• Post weld aging increases strength
• Welds will be weaker than base material

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Welding processes

• GTAW, GMAW used for arc welding
• EBW, RW, Friction welding also used

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GTAW of titanium

• Autogenous is very successful for a-alloys
• Cleanliness has to be impeccable
• Clean environment, vacuum clean floor, walls and
  rafters
• Do not allow Ti to touch carbon steel
• High quality inert gas shielding is essential

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Gas shielding

• High purity argon
• Shield until weld has cooled to lt250C
• Weld in an argon environment
• Glove box, plastic bag, etc
• Open welding requires care
• Large gas shroud (20mm minimum)
• Trailing shoes or clamping bars with gas ports
• Back purging with clean argon essential
• Use gas sniffer to test O2 level

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Cleaning weld preparations

• No tap water, only deionised water
• Alcohol or ketone solvents
• Chlorinated solvents will cause SCC
• Grind preparation with tungsten burr to remove
  oxide skin and weld immediately

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Other processes

• Electron beam in vacuum chamber
• Laser welding for thin materials
• Resistance welding does not need inert gas
  shielding
• Friction welding can be done in air
• Brazing is less common

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Laser weld in Ti
Courtesy Cochlear Pulsed laser 3msec pulse 50msec
interval
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Inspection

• Weld colour and hardness are indicators of
  contamination.
• Colour should be silver,
• Hardness of weld not more than 50 HV10 above base
  material
• Weld surface will have large grain facets

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Corrosion resistance of welds

• Using proper procedures, corrosion resistance of
  welds in commercial purity alloys will match base
  material.
• Fe, Cr or Ni over 0.05 will reduce corrosion
  resistance of weld metal in nitric acid solutions
• Cleanliness is essential
• Heat effects of welding can reduce corrosion
  resistance of a b or metastable b alloys
• Correct procedures, proper filler, maintaining
  gas cover, and right PWHT are essential

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References

• ASM Handbooks
• AWS Handbooks
• www.alleghenytechnologies.com/titanium/