Reactive, Refractory, and Precious Metals

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Reactive, Refractory, and Precious Metals Alloys
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Resistance Welding

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Keywords
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AWS Welding Handbook
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Komuro, Welding of Zirconium Alloys, Welding
International Vol 8, 1994
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• Zirconium
• Similar to Ti but 50 higher density
• Rt-1600F hcp Alpha gt1600F bcc Beta
• Visible oxide at 400F loose scale at gt 800F
• Pure Zr UTS 60ksi YS 40ksi Elong 18
• Corrosion resistance in mineral and organic
  acids, sea water

• Uses
• Petrochemical
• Food Processing
• Nuclear Industry (Lower neutron absorption than
  SS, higher than other refractory alloys)

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Zirconium Alloys
Alpha Stabilizers Aluminum, Beryllium, Cadmium,
Hafnium (usually present), Carbon, Oxygen,
Nitrogen, Tin
Beta Stabilizers Cobalt, Niobium, Copper,
Hydrogen, Iron, Manganese, Molybdenum, Nickel,
Silver, Tantalum, Titanium, Tungsten, Vanadium
Low Solubility, Intermetallic Compounds Carbon,
Silicon, Phosphorous
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• Zirconium Alloys have been
• Spot Welded
• Seam Welded
• Flash Welded
• Upset Butt Welded
• Electromagnetic Force Butt Weld
• High Frequency Welded

Surface Cleaning Recommended

• Surface Cleaning - Mechanical or Chemical
  HF-HNO3
• To lower surface resistance to below 50 microhms
• To keep Zirconium 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

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Electromagnetic Force Butt Weld
Nuclear Fuel Rod Assembly
Komuro, Welding of Zirconium Alloys, Welding
International Vol 8, 1994
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The GTA Welds Have Been Replaced by Resistance
Butt Weld
Weld Current 20-24 kA Weld Time 1/60 sec
Komuro, Welding of Zirconium Alloys, Welding
International Vol 8, 1994
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Tube
End Plug
Post Weld Flash Removal is done
Komuro, Welding of Zirconium Alloys, Welding
International Vol 8, 1994
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Dissimilar Metals
Weld Zr Alloys only to itself or other reactive
refractory metals (Titanium, Niobium, Tantalum,
Hafnium) When welded to Iron or Copper,
extremely brittle intermetallics are formed.
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Cu
Fe
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• Hafnium
• Bright, ductile metal similar to zirconium
• RT-3200F hcp gt3200F bcc
• Density 2 times zirconium
• Better corrosion resistance than zirconium in
  water, steam, molten alkali metals
• Reacts slowly in air above 750F to form oxides
• Reacts above 1650F to form nitrides
• Reacts rapidly above 1290F to form hydrides

• Uses
• Nuclear applications making use of its strong
  neutron absorption
• control rods
• nuclear containers

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Hafnium is subject to severe embrittlement by
relatively small amounts of contamination by
nitrogen, oxygen, carbon, or hydrogen. Welding
usually done in a vacuum. Generally not suited
for resistance welding.
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• Beryllium
• hcp limited ductility at RT
• Density 70 that of Aluminum
• Cast or Powder metallurgy
• Inherent refractory oxide films

• Used
• Aerospace Structures
• Instrument platforms
• Nuclear - low neutron cross section

Beryllium and its compounds in the form of dust,
fumes and vapors are toxic and a serious health
hazard. Because of the chance of expulsion,
resistance welding processes are not considered
for this material.
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AWS Welding Handbook
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• Tantalum
• bcc to MP, good ductility at all temps
• Heavy, twice density of steel
• Good corrosion resistance for most chemicals
• Oxidizes gt 570F
• Attacked by hydrofluoric, phosphoric, sulfuric
  acid gt300F
• Reacts with chlorine and fluorine gases and
  C,H,N at elevated temps
• UTS30-50 ksi YS24-32ksi Elong 20-30

• Used
• Chemical handling
• Electrical capacitors
• High Temperature Furnace Components

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Tantalum Material Production

• Power Metallurgy
• Resistance Welding Not Recommended because
  porosity in weld will be excessive

• Vacuum Arc Melting Electron Beam Melting
• Suitable for Resistance Welding

Welding Handbook, AWS
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Tantalum Spot Seam Welding

• Alloying with Copper Electrodes Electrode
  Sticking is a Problem
• Weld Under Liquid (exclusion of Air, Cooling)
• a. Water
• b. Carbon Tetrachloride (old practice - Health
  Problems)
• Weld Times not to exceed 10 cycles
• Rapid Oxidation is a Problem
• Weld using inert gas or hydrogen

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• Niobium
• bcc with no allotropic transformation
• Tensile Strength gt 25 ksi YS gt 15 ksi
• Interstitials (O,N,H,C) effect mechanical
  properties
• Oxidizes rapidly at gt 750F
• Absorbs Hydrogen 500-1750F
• Reacts with carbon, sulfur and halogens at
  elevated Temp
• Excellent Corrosion Resistance in Aqueous
  solutions because of tenacious oxide formation

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• Niobium Alloys
• Alloy with Tantalum, Tungsten, Molybdenum,
  Hafnium, Titanium, Zirconium
• Produced into sheets, plates etc.

Welding Handbook, AWS
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Niobium Spot Projection Welding Problems

• Electrode Sticking
• AWS recommends using Projection or diffusion
  welding process
• Cooling with liquid nitrogen might help

• Contamination
• Inert shielding from atmosphere

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Molybdenum Tungsten

• bcc Structure
• Ductile to brittle transition temp near or above
  RT (welds will have little or no ductility)
• low solubility for O, N,C
• Grain Boundary Films of oxides, nitrides,
  carbides
• Welding performed in high purity inert
  atmosphere or vacuum
• Sensitive to stress concentrations and rate of
  loading
• Additions of Rhenium greatly improves ductility
  Mo 40-50Re, W 20-30Re
• W-25Re commercially available but tendency to
  solidification crack because of sigma phase

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Molybdenum Tungsten Resistance Weld

• Not generally resistance welded
• Exception is thermocouple wires (generally
  capacitor discharge)
• Exception is Tungsten for lamp filaments

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Tungsten Powder Compact Filament Welded to Nickel
Coated Steel Wire (Resistance Butt or Resistance
Spot)
Butt Welded
Nickel Coated Steel Wire
W Power Compact W BaTiO3 TiO2
Spot Welded
Mehrotra, V, et al Multiple Layer composite
Electrodes for discharge Lamps US Patent
5,847,498 Dec 8, 1998 see also Patents
6037714, Mar 14, 2000 5847497Dec 8, 1998
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Resistance Weld on Metal-Halide Lamp
Niobium Pin With lip (19) Thermal Expansion
Similar to Ceramic
Halide Resistant Mo or W Wire
W Electrode With Coil Wire
(W-Nb W-W) Resistance Welds
Huettinger, R Juengst, S, Metal-Halide Lamp
with Specific Lead Through Structure, US Patent
6,075,314 Jun 13, 2000
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Rhenium

• hcp crystal structure - different from other
  refractory alloys
• Highest modulus of all metals
• No ductile to brittle transition
• Low thermal conductivity (1/2 of Mo 1/3 of W)
• High resistivity (3-4 times Mo W)
• Superior tensile and creep properties
• Resistant to surface oxidation oxides that form
  have good conductivity
• However, embrittled by GB penetration of
  liquid-phase oxides
• Does not form a carbide (I.e. low intra-granular
  embrittlement)
• Available in sheet, strip, wire, tubing

AWS Indicates that Rhenium can be resistance
welded but no procedural data could be found
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Electro - Brazing of Reactive and Refractory
Alloys
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General Comment For Refractory Alloys
Electro-alloy


Ni Sheet
Caution (see below)
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Braze Weld w/o Carbon Block
Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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General Comments

• For Tungsten 1.) avoid brazing alloys with
  excessive nickel to prevent recrystalization
  in base metal due to high brazing temperatures
  2.) avoid
  contact with graphite to prevent carbide
  formation
• For Molybdenum 1.) prevent oxidation by using
  protective coatings
  2.) prevent
  contamination by interstitials 3.) prevent
  recrystalization by careful alloy
  selection 4.) use barrier layer (e.g.
  chromium) to avoid diffusion-induced
  embrittlement by intermetallic compound
  formation
• For Tantalum 1.) remove all reactive gases
  (O2, CO etc) Columbium 2.) electroplate with
  copper or nickel to prevent oxidation

Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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Messler, RW, Joining of Advanced
MaterialsButterworth-Heinemann, 1993
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Gold

• Bright yellow, malleable fcc
• Resistance to oxidation
• Good Electrical and Thermal Conductivity
• Alloyed with Copper silver for jewelry
• Alloyed with zinc, nickel, palladium, platinum
  for commercial

Resistance welding usually done on a small bench
type spot welder
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Silver

• Malleable fcc
• Resistance to oxidation
• Good Electrical and Thermal Conductivity
• Passive chemically (except S which tarnish)
• Dissolved Oxygen in liquid causes porosity
• Alloyed with Copper

• It is difficult to weld because of low
  electrical resistance
• Projection welds for contacts to relays
• Projections on silver to increase current
  density
• Welded to Phosphor-bronze, copper-nickel,
  beryllium-copper, brass

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Resistance Welding of Precious Metal Contact Tips
Application
Supply Wire (platinum, gold, iridium, osmium,
palladium, rhodium, rhenium, ruthenium, tungsten)
Resistance Weld (34 pounds, 8ms upslope, 1100
amps-16ms weld, 8 ms down slope)
Cut
2nd Weld and Coin (1410 amps, 400 pounds)
Tribble, D, Application of Precious Metal to
Spark Plug Electrode US Patent 6,132,277 Oct.
17, 2000
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Cochlear Electrode Array
Shape Memory Rod
Silicon Rubber Body
Electrodes
Resistance Weld Wires
Platinum Electrode Strips Spot Welded to Steel
Acid Etch Steel Plate Away
Kuzma, J Cochlear Electrode Array With
Positioning Stylet US Patent 6,119,044 Sept
12, 2000