Reactive, Refractory, and Precious Metals & Alloys.

Reactive, Refractory, and Precious Metals & Alloys

Resistance Welding

Lesson ObjectivesWhen you finish this lesson you will understand:•

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line workbook

Keywords

AWS Welding Handbook

Property Zirconium Titanium 18-8 StainlessMelting Point C 1852 1675 1400-1427Grain Structure 862C…HCP

>862C..BCC882C.. HCP>882C..BCC

FCC

Density g/cm3 6.5 4.5 8.0Electrical Resistance MicroOhm cm 40 45 72Thermal Cond. Cal/C cm sec 0.040 0.041 0.031Young's Mod. 9.9x103 10.85x103 20.3x103

Thermal Exp Cm/cm/C 4.9x106 9.0x106 16.5x106

Specific Heat cal/gC 0.066 0.1642 0.12

Komuro, Welding of Zirconium Alloys,Welding International Vol 8, 1994

Zirconium• Similar to Ti but 50% higher density• Rt-1600F {hcp} Alpha; >1600F {bcc} Beta• Visible oxide at 400F; loose scale at > 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)

Alpha Stabilizers: Aluminum, Beryllium, Cadmium, Hafnium (usually present), Carbon, Oxygen, Nitrogen, Tin

Zirconium Alloys

Beta Stabilizers:Cobalt, Niobium, Copper, Hydrogen, Iron, Manganese, Molybdenum, Nickel, Silver, Tantalum, Titanium, Tungsten, Vanadium

Low Solubility, Intermetallic CompoundsCarbon, Silicon, Phosphorous

Zirconium Alloys have been:• Spot Welded• Seam Welded• Flash Welded• Upset Butt Welded• Electromagnetic Force Butt Weld• High Frequency Welded

• 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

Surface Cleaning Recommended

Nuclear Fuel RodAssembly

ElectromagneticForce Butt Weld

Komuro, Welding of Zirconium Alloys,Welding International Vol 8, 1994

The GTA Welds Have Been Replaced by Resistance Butt Weld

Weld Current = 20-24 kAWeld Time = 1/60 sec

Komuro, Welding of Zirconium Alloys,Welding International Vol 8, 1994

Tube

EndPlug

Komuro, Welding of Zirconium Alloys,Welding International Vol 8, 1994

Post Weld Flash Removal is done

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.

Cu Fe

Hafnium• Bright, ductile metal similar to zirconium• RT-3200F {hcp}; >3200F {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

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.

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.

AWS Welding Handbook

Tantalum• {bcc} to MP, good ductility at all temps• Heavy, twice density of steel• Good corrosion resistance for most chemicals• Oxidizes > 570F• Attacked by hydrofluoric, phosphoric, sulfuric acid >300F• Reacts with chlorine and fluorine gases and C,H,N at elevated temps• UTS=30-50 ksi; YS=24-32ksi; Elong= 20-30%

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

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

Tantalum Spot & Seam WeldingAlloying with Copper Electrodes & Electrode Sticking is a Problem• Weld Under Liquid (exclusion of Air, Cooling)

a. Waterb. Carbon Tetrachloride (old practice - Health Problems)

• Weld Times not to exceed 10 cycles

Rapid Oxidation is a Problem• Weld using inert gas or hydrogen

Niobium• {bcc} with no allotropic transformation• Tensile Strength > 25 ksi; YS > 15 ksi• Interstitials (O,N,H,C) effect mechanical properties• Oxidizes rapidly at > 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

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

Welding Handbook, AWS

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

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 40-50%Re, W 20-30%Re• W-25Re commercially available but tendency to

solidification crack because of sigma phase

Molybdenum & Tungsten Resistance Weld

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

Tungsten Powder Compact Filament Welded to Nickel Coated Steel Wire (Resistance Butt or Resistance Spot)

Butt Welded

Spot Welded

Nickel Coated Steel Wire

W Power CompactW + BaTiO3 + TiO2

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

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

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

Electro - Brazing of Reactive and Refractory

Alloys

Ni Sheet

General Comment For Refractory Alloys Electro-alloy *

*

* Caution (see below)

Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

Braze Weld w/o Carbon Block

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 interstitials3.) prevent recrystalization by careful alloy selection4.) 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 oxidationMessler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

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

• 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

Silver

• 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

Resistance Welding of

Precious Metal Contact

Tips

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

ApplicationApplication

Cochlear Electrode Array

Shape Memory Rod

Silicon Rubber Body

Electrodes

Platinum Electrode Strips Spot Welded to Steel

Resistance Weld Wires Acid Etch Steel

Plate Away

Kuzma, J “Cochlear Electrode Array With Positioning Stylet” US Patent 6,119,044 Sept 12, 2000