Reactive metals have a strong affinity for oxygen and nitrogen at elevated temperatures and when combined form very stable compounds. At lower temperatures they are highly resistant to corrosion. Refractory metals have extremely high melting points. They may also exhibit some of the same characteristics of reactive metals.
The refractory metals are:
- Tungsten
- Molybdenum
- Tantalum
- Columbium (Niobium)
- Zirconium
- Titanium
- Beryllium
The reactive metals are becoming increasingly important because of their use in nuclear and space technology. They are considered in the difficult-to-weld category. These metals have a high affinity for oxygen and other gases at elevated temperatures and for this reason cannot be welded with any process that utilizes fluxes, or where heated metal is exposed to the atmosphere. Minor amounts of impurities cause these metals to become brittle.
Most of these metals have the characteristic known as the ductile-brittle transition. This refers to a temperature at which the metal breaks in a brittle manner rather than in a ductile fashion. The recrystalization of the metal during welding can raise the transition temperature. Contamination during the high temperature period and impurities can raise the transition temperature so that the material is brittle at room temperatures. If contamination occurs so that transition temperature is raised sufficiently it will make the weldment worthless. Gas contamination can occur at temperatures below the melting point of the metal. These temperatures range from 371°C up to 538°C.
At room temperature the reactive metals have an impervious oxide coating that resists further reaction with air. The oxide coatings melt at temperatures considerably higher than the melting point of the base metal and create problems. The oxidized coating may enter molten weld metal and create discontinuities which greatly reduce the strength and ductility of the weld. Of the three reactive metals, titanium is the most popular and is routinely welded with special precautions.
All of the refractory metals incur internal contamination or surface errosion when exposed to the air at elevated temperatures. Molybdenum has an extremely high rate of oxidation at high temperatures above 816°C. Tungsten is much the same. Tantalum and columbium form pentoxides that are not volatile below -3.9°C, but these provide little protection because they are nonadherent. Molybdenum and tungsten both become embrittled when a minute amount of oxygen or nitrogen is absorbed. Columbium and tantalum can withstand larger amounts of oxygen and nitrogen.
Titanium can withstand much more oxygen or nitrogen before becoming embrittled; however, small amounts of hydrogen will cause embrittlement. Zirconium can withstand about as much oxygen but much less nitrogen or hydrogen. Beryllium is similar to zirconium in this regard.
Welding Refractory Metals
It is obvious that these metals must be perfectly clean prior to welding and that they must be welded in such a manner that air does not come into contact with the heated material. Cleaning is usually done with chemicals. A water rinse is necessary to remove all traces of chemicals from the surface. After the parts are cleaned they must be protected from reoxidation. This is best done by storing in an inert gas chamber or in a vacuum chamber.Molybdenum is welded by the gas tungsten arc welding process and the electron beam process. The gas metal arc process can be used but sufficient thickness of molybdenum is rarely available to justify this process. Molybdenum has been welded by other arc processes but results are not too satisfactory. Welding with the gas shielding processes is accomplished in an inert gas chamber or dry box. This is a chamber that can be evacuated and purged with inert gas until all active gases are removed. Welding is done in the pure inert atmosphere with normally good results. The filler metal compositions should be the same as the base metal. The base metal in the heat-affected zone becomes embrittled by grain growth and recrystallization as a result of the welding temperatures. Recrystallization raises the transition temperature so that molybdenum welds tend to be brittle. Molybdenum is highly notch-sensitive, craters and notch effects such as undercutting must be avoided. Molybdenum can also be welded with the resistance welding processes and by diffusion welding.
Tungsten is welded in the same manner as molybdenum and has the same problems, only more intensely so. It has greater susceptibility to cracking because the ductile-to-brittle transition temperatures are higher. The preparation of tungsten for welding is more difficult. The gas tungsten arc welding process is used with direct current electrode (negative). Welding should be done slowly to avoid cracking. Preheating may assist in reducing cracking but must be done in the inert gas atmosphere.
Commercially pure tantalum is soft and ductile and does not seem to have a ductile-brittle transition. There are several alloys of tantalum commercially available. Even though the material is easier to weld, it should be well cleaned and for best results should be welded in the inert gas chamber. The gas tungsten arc welding process is recommended. Some tantalum products are produced by powder metallurgy technology and this may result in porosity in the weld. The arc cast product does not have porosity. Filler wire is normally not used when welding tantalum and for best results direct current electrode negative is used. High frequency should be used for initiating the arc. Helium is recommended for welding tantalum to provide for maximum penetration since joints are designed to avoid using filler metal.
There are several different alloys of columbium (niobium) available. Some are ductile and others brittle since the transition temperature is near room temperature. The gas tungsten arc welding process is used for the pure columbium and for the lower strength commercial alloys. In certain alloys the welding can be done outside of an inert gas chamber but special precautions should be taken to provide extremely good inert gas shielding coverage. For some of the alloys preheating is recommended to provide for a crack-free weld. Electron beam welding is used and columbium can also be resistance welded.
Welding Reactive Metals
Beryllium has been welded with the gas tungsten arc welding process and with the gas metal arc welding process, and it is also joined by brazing. Beryllium should not be welded without expert technical assistance. Beryllium is a toxic metal and extra special precautions should be provided for proper ventilation and handling.Zirconium and zirconium-tin alloys are ductile metals and can be prepared by conventional processes. Cleaning is extremely important and chemical cleaning is preferred over mechanical cleaning. Both the gas tungsten arc welding and the gas metal arc welding processes are used for joining zirconium. The inert gas chamber should be employed to maintain an efficient gas shield. Argon or argon-helium mixtures are used.
The zircalloys are alloys of zirconium which contain small amounts of tin, iron, and chromium. These alloys can be welded in the open in much the same manner as titanium. The electron beam process and the resistance welding processes have been used for joining zirconium.
The secret to the successful welding of titanium is cleanliness. Small amounts of contamination can render a titanium weld completely brittle. Contamination from grease, oils, paint, fingerprints, or dirt, etc., can have the same effect. If the material is cleaned thoroughly before welding and well protected during welding there is little difficulty in the welding of titanium.
The gas tungsten-arc and gas metal-arc welding processes can be used for welding titanium. Special procedures must be employed when using the gas-shielded welding processes. These special procedures include the use of large gas nozzles and trailing shields to shield the face of the weld from air. Backing bars that provide inert gas to shield the back of the welds from air are also used. Not only the molten weld metal, but the material heated above 1000°F (approx. 540°C) by the weld must be adequately shielded in order to prevent embrittlement.
When using the GTAW process a thoriated tungsten electrode should be used. The electrode size should be the smallest diameter that will carry the welding current. The electrode should be ground to a point. The electrode may extend 1-1/2 times its diameter beyond the end of the nozzle. Welding is done with direct current, electrode negative (straight polarity).
Selection of the filler metal will depend upon the titanium alloys being joined. When welding pure titanium, a pure titanium wire should be used. When welding a titanium alloy, the next lowest strength alloy should be employed as a filler wire. Due to the dilution which will take place during welding, the weld deposit will pick up the required strength. The same considerations are true when GMAW welding of titanium.
Argon is normally used with the gas-shielded process. For thicker metal use helium or a mixture of argon and helium. The purity of welding grade gases is satisfactory. They should have a dew point of minus 65°F (minus 54°C). Welding grade shielding gases are generally free from contamination; however, tests can be made before welding. A simple test is to make a bead on a piece of scrap, clean titanium, and notice its color. The bead should be shiny. Any discoloration of the surface indicates a contamination.
Extra gas shielding provides protection for the heated solid metal next to the weld metal. This shielding is provided by special trailing gas nozzles or by chill bars laid immediately next to the weld. Backup gas shielding should be provided to protect the underside of the weld joint. Protection of the backside of the joint can also be provided by placing chill bars in intimate contact with the backing strips. If the contact is close enough, backup shielding gas is not required. For critical applications use an inert gas welding chamber. These can either be flexible, rigid, or vacuum-purge chambers.
To guarantee that embrittlement of the weld will not occur, proper cleaning steps must be taken. Solvents containing chlorine should not be used. Recommended solvents would be tri-alcohol or acetone. Titanium can be ground with discs of aluminum oxide or silicon carbide. Wet grinding is preferred, however, if wet grinding cannot be used the grinding should be done slowly to avoid overheating the surface of the titanium.
