1. Reactive, Refractory, and Precious Metals & Alloys
The reactive, refractory and precious metals and alloys are taken together in one module as these metals tend to be found together in nature and have many aspects in common.

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

3. This chart is a slice of the periodic table and shows the break down of these various metals into their respective categories. We will look at the reactive metals first, each one at a time and then move to the refractory alloys and finally to the precious metals. The reactive metals are zirconium, hafnium, beryllium and uranium.

4. The physical properties of the reactive alloys are represented by those metals in the top portion of this table. These alloys tend to have higher melting points than the alloys we have examined before with melting points from 2000 to 4000 F.
AWS Welding Handbook

5. We will look at each of these reactive alloys, starting first with zirconium.

6. Komuro, Welding of Zirconium Alloys, Welding International Vol 8, 1994
This table lists some of the physical properties of zirconium together with a comparison with titanium and 18-8 stainless steel. The melting point of zirconium is a little higher than the Ti or SS. It is similar in grain structure to Ti having a BCC at high temperatures and a HCP at lower temperatures. Its density is between that of the Ti and SS. The electrical resistance of zirconium is the lowest listed with comparable thermal conductivity to Ti. The thermal expansion is quite a bit less than that of either Ti or SS.
Komuro, Welding of Zirconium Alloys,
Welding International Vol 8, 1994

7. Similar to Ti but 50% higher density
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)
So the zirconium is quite similar to Ti with slight higher density. The lower temperature phase is called Alpha and the higher temperature phase is called Beta. It forms a visible oxide at 400F and this oxide turns to a loose scaly oxide above 800F. The alloys have some corrosion resistance in mineral and organic acids and sea water. It also has lower neutron absorption than SS. It finds uses in petrochemical and food processing industries and in nuclear reactors.

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
Alloying elements which tend to stabilize the lower temperature phase alpha and those which tend to stabilize the higher temperature phase beta are listed here. Note that Hafnium is an alpha stabilizer and tends to be found in all the zirconium alloys because it is not fully separated in the refining process of zirconium. Several elements which have low solubility and tend thus to form intermetallic compounds are carbon, silicon and phosphorous.

9. Listed here are a few alloys of zirconium.

10. Surface Cleaning Recommended
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
According to the American Welding Society, zirconium alloys have been resistance welded with a variety of processes as listed here. Surface cleaning is recommended, remember the high temperature oxide is not extremely tenacious, it is kind of flaky so surface cleaning can easily be done either chemically or mechanically. The same etch used for Titanium seems to work pretty well. Like titanium, the cleaned parts should be handled with gloves, because the alloys are very reactive, to avoid further contamination and stored in low humidity chambers. These alloys have higher resistivity than the steel alloys so lower currents are required.

11. Again from the American Welding Society this table list spot welding procedures pure unalloyed zirconium sheet.

12. This table is a table of seam welding procedures for unalloyed zirconium sheets.

13. Komuro, Welding of Zirconium Alloys, Welding International Vol 8, 1994
Electromagnetic
Force Butt Weld
This is an example of nuclear fuel rods assembly which are electomagnetically force butt welded. The Zircaolly tubes contain the uranium oxide fuel elements and the tubes closed by butt welding. Each fuel tube is inserted into the cluster alternately with water cooling channels for the reactor operation.
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
The traditional processing of the fuel tubes was to place the uranium oxide into the tube, hold it in place with the spring and close the tube with gas tungsten arc welding. This took too much time so these welds have been replaced by resistance butt welds which take only a fraction of a second. The process is a capacitive discharge process, where magnetic force is used to pull the hammer containing the plug down onto the tube consummating the weld.
Weld Current = kA
Weld Time = 1/60 sec
Komuro, Welding of Zirconium Alloys,
Welding International Vol 8, 1994

15. End Plug Post Weld Flash Removal is done
Tube
End
Plug
As seen in this micrograph good leak free welds are made.
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.
The AWS indicates that the zirconium alloys can be welded to themselves and other refractory metals, but when welded to iron or copper, extremely brittle intermetallic compounds are formed.

17. Here are presented the Cu-Zr and Fe-Zr phase diagrams with the brittle intermetallics highlighted.

19. Bright, ductile metal similar to zirconium
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
Hafnium is that other element that finds itself associated with zirconium quite often. It is bright and ductile and also undergoes an allotropic phase transformation. From room temperature to 3200F it is hexagonal Close Packed and above 3200 it is body centered cubic. It is quite heavy with a density twice that of zirconium and it actually has better corrosion resistance than zirconium in many applications. At various temperatures it will form oxides, nitrides, and hydrides. It is a strong neutron absorber and is used in nuclear applications.
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.
The oxides, nitrides and hydrides are quite brittle and when ever welding is done it is recommended that it be done in a vacuum to avoid the formation of these compounds, thus is it generally not suited to conventional resistance welding techniques.

22. {hcp} limited ductility at RT Density 70% that of Aluminum
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.
Beryllium and its alloys are generally not resistance welded. It is hcp and has limited ductility. It is very light, manufactured in the cast form or by powder metallurgy and has an inherent refractory oxide. All of these factors make it extremely difficult to resistance weld. It has some applications in aerospace instrument and nuclear fields, but beryllium and its compounds are very toxic and as such are not generally considered resistance weldable.

23. Like beryllium, uranium is a very reactive element, and no references to resistance welding of this material could be found.

24. So let us now look at the group of alloys listed as refractory alloys as seen above.

25. This table lists the refractory alloys and their physical and mechanical properties. These alloys are refractory because of their extremely high melting points ranging form around 4000F to excess of 6000F. Because their melting points are so very high, they become very difficult to resistance weld.
AWS Welding Handbook

27. {bcc} to MP, good ductility at all temps Heavy, twice density of steel
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%
Some of the properties of tantalum are presented here. Note that it is quite heavy and has good corrosion resistance to most chemicals but it is susceptible to hot hydrofluoric, phosphoric and sulfuric acids. It is often used in the chemical processing industry and for electrical capacitors and high temperature furnace components.
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
Tantalum products are manufactured by powder metallurgy methods and it is also vacuum arc or electron beam melting and re-melting. List here are a few of the commercially available tantalum alloys. Resistance welding of the powder metallurgy parts is not recommended because any voids or porosity would cause problems.
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
Tantalum wrought product is both spot and seam welded, but because severe electrode alloying and sticking occurs, they are usually welded submerged under liquid.

31. {bcc} with no allotropic transformation
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 F
Reacts with carbon, sulfur and halogens at elevated Temp
Excellent Corrosion Resistance in Aqueous solutions because of tenacious oxide formation
Properties of niobium are listed here. Interstitials of O,N,H,C strengthen the niobium but make it brittle. It reacts with Carbon sulfur and halogens at elevated temperature.

32. Produced into sheets, plates etc.
Niobium Alloys
Alloy with Tantalum, Tungsten, Molybdenum, Hafnium, Titanium, Zirconium
Produced into sheets, plates etc.
Niobium is alloys with others of the reactive and refractory alloys as listed here, and it can be worked into wire, sheet and plate products.
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
Niobium can be resistance welded but there is considerable electrode sticking. AWS recommends projection welding to minimize this and suggest that perhaps shielding with liquid nitrogen for cooling might help.

35. Grain Boundary Films of oxides, nitrides, carbides
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 and Tungsten are taken together. Here are listed properties of these. Note that these metal tend to be brittle but the addition of rhenium greatly improves ductility. These are often used in wire form as thermocouples and are cross wire welded.

36. Molybdenum & Tungsten Resistance Weld
Not generally resistance welded
Exception is thermocouple wires (generally capacitor discharge)
Exception is Tungsten for lamp filaments
Capacitive discharge resistance welding or short cycle conventional resistance welding are used for thermocouple and lamp filament joining.

37. Nickel Coated Steel Wire
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
Composite sintered electrodes for gas discharge lamps with improved properties that make them suitable for use in a variety of lamp types are provided which comprise a refractory metal and a substantial amount of a refectory oxide emitter, either single layer or multiple layer the composites having been subjected to sintering at a high temperature and then resistance welded to a nickel coated steel contact wire
Spot Welded
Mehrotra, V, et al “Multiple Layer composite Electrodes for discharge Lamps” US Patent 5,847,498 Dec 8, 1998 – see also Patents , Mar 14, 2000 & Dec 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
The halides in a halide lamp are very corrosive to most metals and they perform at high temperatures. Selection of the wrong metallic current carriers will result in short life because of halide attack or cracking of the lamp ceramic member by differential thermal expansion. The niobium wire is used to nearly match thermal expansion and the molybdenum or tungsten wires and electrodes are selected because of their resistance to halide attack. Resistance spot welding solves the problems of joining the parts.
Huettinger, R & Juengst, S, “Metal-Halide Lamp with Specific Lead Through Structure, US Patent 6,075,314 Jun 13, 2000

40. {hcp} crystal structure - different from other refractory alloys
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
Rhenium is hcp and is therefore different in some respects from the other refractory alloys. Properties are presented here. Note that it has superior tensile and creep properties but it is seriously embrittled by liquid phase oxide grain boundary penetration, but it does not form carbides which cause a form of embrittlement in other reactive and refractory alloys. AWS Indicates that Rhenium can be resistance welded but no procedural data could be found
AWS Indicates that Rhenium can be resistance welded but no procedural data could be found

41. Electro - Brazing of Reactive and Refractory Alloys
Before leaving the reactive and refractory alloys completely, it should be noted that the electro-brazing process is often used for joining of these alloys.

42. General Comment For Refractory Alloys Electro-alloy *
Ni Sheet
In this process the sheets or parts to be welded are placed between electrodes and a braze alloy (often nickel sheet) is inserted between the sheets. The resistance heating melts the braze sheet and it flow to make the joint upon subsequent solidification. Note that when electrodes alone conduct too much heat away, they may be replaced by high resistance materials like carbon block. Note for some alloys the carbon can cause embrittlement and for some alloys, the use of nickel sheet can also cause embrittlement.
* Caution (see below)

43. Braze Weld w/o Carbon Block
This depicts a braze weld without the use of carbon blocks.
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 ) avoid contact with graphite to prevent carbide formation
For Molybdenum 1.) prevent oxidation by using protective coatings ) prevent contamination by interstitials 3.) prevent recrystalization by careful alloy selection ) 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
Above are listed general precautionary measures to be followed when electrobrazing various refractory alloys.
Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

45. Listed here and in the next slide are various braze alloys used with tungsten and molybdenum.
Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

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

47. List here are various braze alloys used with tantalum and niobium
Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

48. And finally are listed some additional braze alloys for these materials.
Messler, RW, “Joining of Advanced Materials”Butterworth-Heinemann, 1993

49. Let us just now take a quick look at the precious metals.

50. Here is a list of the precious metals
Here is a list of the precious metals. The ones we are most interested in are the gold, silver and perhaps platinum. The other metals get use so infrequently that they probably have little mass commercial interest.

51. Bright yellow, malleable {fcc} Resistance to oxidation
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
Here are some properties of gold. Note that it has good electrical properties so beside being used for jewelry, it finds use in electrical contact circuitry. It is usually welded with small bench type resistance welders of in electrical “clean room” conditions.

52. Resistance to oxidation Good Electrical and Thermal Conductivity
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
Some properties of silver are listed here. Like gold, it has good electrical and thermal conductivity and it is resistant to corrosion except for sulfur which causes tarnishing. Because of its low resistance it is often difficult to weld, never the less, it too finds use in welded or electrobrazed electrical contacts.

53. 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
A method of affixing a precious metal to an electrode of a spark plug is described in this patent. The process includes placing a length edge of a generally cylindrical precious metal wire on an electrode surface and resistance welding the wire to the electrode surface. This is followed by a second weld and coining operation.
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

54. 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
An implantable electrode array to improve hearing is made by spot welding platinum electrode strips onto a steel foil, and then resistance welding the wires to the strip electrodes. The steel foil is then acid etched away leaving the electrode array which is molded into a silicon rubber body. This body has a hole the full length of the body through which a cooled shape memory rod can be placed. When it warms to body temperature, it forces the body up against the ear for service.
Kuzma, J “Cochlear Electrode Array With Positioning Stylet” US Patent 6,119,044 Sept 12, 2000