Titanium

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Titanium
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Resistance Welding

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• Lesson Objectives
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Keywords
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Donachie, Titanium, ASM, 1988
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Donachie, Titanium, ASM, 1988
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• Unique Properties
• Light Weight
• Excellent Corrosion Resistance
• Strong tightly adherent oxide
• High strength-to-weight ratio

• Applications
• Chemical Industry
• Aerospace Industry
• Marine Fields
• Medical Applications

• Phases in Pure Titanium
• Alpha hcp lt 1620F (880C)
• Beta bcc gt 1620F

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• Classification of Titanium Alloys
• Alpha near Alpha Alloys
• Alpha-Beta Alloys
• Metastable Beta Alloys

AWS Welding Handbook
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Can Lead to Brittleness Especially in welds (see
below)
AWS Welding Handbook
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AWS Welding Handbook
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• Commercially Pure Titanium
• Yield Strengths 35-80 ksi (interstitial O, N, C)

• Alpha Titanium Alloys
• Not heat treated to increase strength
• Used for moderate elevated-temp strength and
  creep resistance

• Near-alpha Alloys
• Contain small amount of Beta stabilizers
• Marginally heat treatable
• Most contain Si for improved high temp properties

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• Alpha-Beta Alloys (e.g. Ti-6Al-4V)
• Mixture of Alpha Beta
• Annealed or Solution heat treated and aged
• Outstanding strength to density ratio in heat
  treated condition
• Excellent fracture toughness when annealed

• Metastable Titanium Alloys
• Contain high percentage of Beta stabilizing
  elements
• Slow transformation - air cooling gives 100
  Beta
• Aging precipitates of fine Alpha can occur
• Aging give strength but ductility and fracture
  toughness sacrificed
• Frequently used for fasteners and springs

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Applications Using Titanium Alloys
For more information about applications
see http//www.titanium.org/GIsec2a.htm
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General Weld Problems

• Oxygen Contamination Cracking
• Oxygen gt 3000ppm WM HAZ Cracking
• Alpha (hcp) particularly susceptible
• Removal of surface oxide film recommended
• Gas shielding generally recommended (especially
  for seam welds)

• Degree of Oxidation
• Silver (mild oxidation)
• Straw
• Blue
• White (severe oxidation - weld problems likely)

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General Weld Problems

• Iron Contamination Cracking
• Iron particle dissolve into surface causing loss
  of
• Corrosion
• Embrittlement (when sufficient iron)
• Prevention
• Avoid steel fabrication near titanium
  fabrication
• Avoid airborne dust
• Avoid tools used for both Fe and Ti
• Scratch Brush just prior to welding
• Use clean gloves for handling

Http//www.twi.co.uk/bestprac/jobknol/jk24.html
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General Weld Problems

• Hydrogen Embrittlement
• Precipitation of titanium hydrides

• Hydrogen gt 200ppm can cause problems
• Use low moisture shielding gas

AWS Welding Handbook
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Example of Hydride Formation in Resistance Seam
Weld
25 ppm Hydrogen
2056 ppm hydrogen
H
450 days-Full around
2 days no hydrides
227 days-1/2 around
Toosky, R. Evaluation of Titanium Hydride
Formation in Resistance Seam Welded Dissimilar
Titanium Joints, Boeing Co., 1998
43 days-at toe
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Seam Weld Start, Nugget Pull-Out
Toosky, R. Evaluation of Titanium Hydride
Formation in Resistance Seam Welded Dissimilar
Titanium Joints, Boeing Co., 1998
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General Weld Problems

• Porosity
• H O partitioned between solidifying dendrites
• Micropores form when diatomic gas overcomes head
  pressure

• Mircopores coalesce to form macropores
• To keep O H out of weld pool, clean degrease

AWS Welding Handbook
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General Weld Problems

• Ductility Dip
• Alpha-beta alloys containing Nb rather than V
  more susceptible
• Alpha films at prior beta boundaries more
  susceptible

Niobium Containing

• Material in HAZ with these peak Temp can
  experience Dip
• Cracks are intergranular
• Believed to be caused by volumetric difference
  during transformation

AWS Welding Handbook
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Resistance Spot Welding

• Surface Cleaning - Mechanical or Chemical
  HF-HNO3
• To lower surface resistance to below 50 microhms
• To keep Titanium Oxide out of weld metal -
  embrittlement
• Handle with gloves
• Store in low-humidity less than 48 hours
• Higher resistivity for Alloys than steel lower
  current

Same Schedule as SS for Pure Titanium, But Not
For alloys

• Pure Ti can use SS Schedule
• Class 2 Electrodes with 3 in radius spherical
  face
• Gas shielding not required for spot welds

AWS Welding Handbook
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Schedule for Two Typical Alloys
AWS Welding Handbook
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Equiaxed Grains
Columnar Grains due to Effect of Alloys
AWS Welding Handbook
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Resistance Seam Welding

• Same Cleaning procedures as spot welding
• Class 2 Electrode Wheels with radius face
  wheel radius
• Inert shielding gas recommended

• Electrode Force and Welding Current slightly
  higher than for spot welds in the same material

AWS Welding Handbook
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Flash Welding Titanium

• Use machine capacities similar to that for steel
• Gas Shielding not absolutely necessary but
  recommended (Titanium Oxidizes rapidly)
• Use fast flashing rates shorter flashing times
  than steel (minimize weld contamination)
• Lower upset force than for steel
• Flashing and upset distances slightly less than
  for steel

Flash Welding Has Been Done on Following

• Commercially Pure Ti
• Ti-6Al-4V
• Ti-8Al-1Mo-1V
• Ti-5Al-2.5Sn

• Ti-6Al-2Sn-4Zr-2Mo
• Ti-6Al-2Sn-4Zr-6Mo

Info from AWS and Titanium Industries Inc.
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Some Titanium Welding Applications
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Froes, FH, et al, Non-Aerospace Applications of
Titanium Feb 1998, TMS
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SAAB 340 Aero Engine Shroud constructed from
commercially pure titanium using resistance, spot
and TIG welding. http//www.swantec.com/industry.h
tm
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Welding Battery Connectors for Implantable
Medical Device
Shield Gas (Ar) Tube
Weld Conditions 1000 amps 40 msec 3.5 kgrams
Electrodes
Battery Case Top
Feed Through Contacting Pin (glass sealed in case
top)
Weld Combinations (inch) 0.018 Ti tab to 0.035 Nb
pin 0.018 Ti tab to 0.020 Mo pin 0.018 Ti tab to
0.020 Ta pin 0.004 Ni tab to 0.020 Mo pin
Battery Case
Battery Tab
Berkowitz, F, et al, Implantable Medical Device
With High Reliability Electrical Connection using
Reactive Metals US Patent 5,712,462 Jan 27,
1998
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Resistance Weld Ti Alloy Honeycomb Structures for
High Speed Civil Transport (Requirement 72,000
hours _at_ 350F) (Al-Li max use temp 225F)
Weld
Weld between Ti 6242S face sheet and Beta 21S
honeycomb low heat treatment Low mechanical
Boundary Present
Heat Treat 1675-1825 F 15 min 4 hours
El-Soudani, S, Process for Enhancing the Bond
Strength of Resistance Welded Joints Between
Titanium Alloy Articles US Patent 5,830,289
Nov 3, 1998
Weld with heat treatment good properties