Understanding Zirconium and Other Precious Metals in Resistance Welding

1. Reactive, Refractory, and Precious Metals & Alloys

2. Resistance Welding • Learning Activities • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • Lesson Objectives • When you finish this lesson you will understand: Keywords

4. AWS Welding Handbook

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

7. 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)

8. 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

10. 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

13. Electromagnetic Force Butt Weld Nuclear Fuel Rod Assembly Komuro, Welding of Zirconium Alloys, Welding International Vol 8, 1994

14. The GTA Welds Have Been Replaced by Resistance Butt Weld Weld Current = 20-24 kA Weld Time = 1/60 sec Komuro, Welding of Zirconium Alloys, Welding International Vol 8, 1994

15. Tube End Plug Post Weld Flash Removal is done Komuro, Welding of Zirconium Alloys, Welding International Vol 8, 1994

16. 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.

17. Cu Fe

19. 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

20. 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.

22. 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.

25. AWS Welding Handbook

27. 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

28. 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

29. 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

31. 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

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

33. 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

35. 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

36. Molybdenum & Tungsten Resistance Weld • Not generally resistance welded • Exception is thermocouple wires (generally capacitor discharge) • Exception is Tungsten for lamp filaments

37. 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

38. 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

40. 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

41. Electro - Brazing of Reactive and Refractory Alloys

42. General Comment For Refractory Alloys Electro-alloy * * Ni Sheet * Caution (see below)

43. Braze Weld w/o Carbon Block Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

44. 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 Materials”Butterworth-Heinemann, 1993

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

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

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

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