Welding of Dissimilar Metals

Abstract:

There are many applications in which weldments are made from metals of different compositions. The same is true of a mechanical wear problem, a high-temperature situation, or other conditions in which different properties are required from different parts of the same weldment.

This brings about the need for joining dissimilar metals. A successful weld between dissimilar metals is one that is as strong as the weaker of the two metals being joined, i.e., possessing sufficient tensile strength and ductility so that the joint will not fail in the weld. Such joints can be accomplished in a variety of different metals and by a number of the welding processes.

The procedures mostly described for obtaining quality welds in the literature have been between identical metals or at least of metals of similar composition and properties. There are many applications, however, in which weldments are made from metals of different compositions. The same is true of a mechanical wear problem, a high-temperature situation, or other conditions in which different properties are required from different parts of the same weldment.

This brings about the need for joining dissimilar metals. Welding dissimilar metals is being done more often; therefore, this article is devoted to providing information for successfully joining some of the more common metal combinations.

A successful weld between dissimilar metals is one that is as strong as the weaker of the two metals being joined, i.e., possessing sufficient tensile strength and ductility so that the joint will not fail in the weld. Such joints can be accomplished in a variety of different metals and by a number of the welding processes.

The problem of making welds between dissimilar metals relates to the transition zone between the metals and the intermetallic compounds formed in this transition zone. For the fusion type welding processes it is important to investigate the phase diagram of the two metals involved. If there is mutual solubility of the two metals the dissimilar joints can be made successfully. If there is little or no solubility between the two metals to be joined the weld joint will not be successful.

The intermetallic compounds that are formed, between the dissimilar metals, must be investigated to determine their crack sensitivity, ductility, susceptibility to corrosion, etc. The microstructure of this intermetallic compound is extremely important. In some cases, it is necessary to use a third metal that is soluble with each metal in order to produce a successful joint.

Another factor involved in predicting a successful service life for a dissimilar metals joint relates to the coefficient of thermal expansion of both materials. If these are widely different, there will be internal stresses set up in the intermetallic zone during any temperature change of the weldment. If the intermetallic zone is extremely brittle service failure may soon occur.

The difference in melting temperatures of the two metals that are to be joined must also be considered. This is of primary interest when a welding process utilizing heat is involved since one metal will be molten long before the other when subjected to the same heat source. When metals of different melting temperatures and thermal expansion rates are to be joined the welding process with a high heat input that will make the weld quickly has an advantage.

The difference of the metals on the electrochemical scale is an indication of their susceptibility to corrosion at the intermetallic zone. If they are far apart on the scale, corrosion can be a serious problem.

In certain situations, the only way to make a successful joint is to use a transition material between the two dissimilar metals. An example of this is the attempt to weld copper to steel. The two metals are not mutually soluble, but nickel is soluble with both of them. Therefore, by using nickel as an intermediary metal the joint can be made. Two methods are used:

  • use a piece of nickel, or
  • deposit several layers of nickel alloy on the steel, i.e., buttering or surfacing the steel with a nickel weld metal deposit.
  • The nickel or nickel deposit can be welded to the copper alloy using a nickel filler metal. Such a joint will provide satisfactory properties and will be successful.

    Another method of joining dissimilar metals is the use of a composite insert between the two metals at the weld joint. The composite insert consists of a transition joint between dissimilar metals made by a welding process that does not involve heating. By selecting the proper materials for the composite insert like metals can be welded to like metals in making the fusion weld joint.

    Welding Processes for Composite Inserts

    The following is a brief description of some of the welding processes that can be used for composite inserts that include transition joints and that do not employ filler metals.

    Explosion welding is used to join many so-called incompatible metals. In explosion welding the joint properties will be equal to those of the weaker of the two base materials. Since minimum heat is introduced there is no melting, no heat-affected zone, and no thermal compounds are formed.

    The characteristic sine wave pattern of the interface greatly increases the interface area. This process is used for cladding, but is also used to make composite transition inserts used for the fusion welding of dissimilar metals. Composites containing a transition joint are commercially available between aluminum and steel, aluminum and stainless steel, aluminum and copper, and other materials.

    Cold welding is used for making dissimilar metal transition joints. This process does not use heat, thus avoids the heat-affected zone and the intermetallic fusion alloy. Little or no mixing of the base metals takes place. It is commonly used to join aluminum to copper.

    Ultrasonic welding is also used for welding dissimilar metals since very little heat is developed at the weld joint. Ultrasonic welding can be used only for very thin materials or small parts.

    Friction welding is also used for joining dissimilar metals and for making composite transition inserts. Various dissimilar combinations of metal have been welded, including steel to copper base alloys, steel to aluminum, stainless to nickel base alloys, etc. In friction welding only a very small amount of the base metal is heated and that which is melted is thrown from the joint, therefore, the intermetallic material is kept to a very minimum. The heat-affected zone is also minimal.

    The high-frequency resistance process is also widely used for dissimilar metal welding. Here the heat is con¬centrated on the very surface of the parts being joined and pressure applied is sufficient to make welds of many dissimilar materials. It can be used for joining copper to steel at very high speeds.

    Diffusion welding is widely used for aerospace applications of dissimilar metals welding.

    Percussion welding is also used but this process is restricted to wires or small parts. The laser beam welding process has also been used but it is restricted, at this time, to very thin materials.

    The electron beam welding process has had wide application for joining dissimilar metals. Electron beam uses high-density energy and fast welding speed. It seems to overcome the difference of thermal conductivity when welding metals together having wide variation of thermal conductivity. In addition, the weld zone is extremely small and filler metal is not introduced. Since there is such a small amount of intermetallic compound formed electron beam does offer an advantage for many dissimilar combinations.

    The flash butt welding process will make high-quality welds between copper and aluminum. With proper controls all or most of the molten metal is forced out of the joint and the weld is complete as a solid state process. Flash butt welds are made in rods, wires, bars, and tubes.

    Arc Welding. There are many welding requirements to join dissimilar metals in which the above processes cannot be used. In these cases, the three popular arc welding processes are most often utilized. These are the shielded metal arc welding process, the gas tungsten arc welding process, and the gas metal arc welding process.

    Welding Aluminum to Various Metals

    There is a wide difference of melting temperatures, for aluminum, approximately 650°C, and for steel, approximately 1538°C. The aluminum will melt and flow away well before the steel has melted.

    The aluminum iron phase diagram shows that a number of complex brittle intermetallics are formed. It is found that iron aluminum alloys containing more than 12% iron have little or no ductility. In addition, there is wide difference in the coefficient of linear expansion, in thermal conductivity, and in specific heats of aluminum and steel. This will introduce thermal stresses of considerable magnitude.

    The most successful method is to use an aluminum-steel transition insert with each metal welded to its own base metal using any of the three arc welding processes.

    The other way is to coat the steel surface with a metal compatible with aluminum. The success of this type of joint depends on metal used to coat the steel, the thickness of the coating, and the bond between the coating and the steel surface. A coating of zinc on steel can be used and the aluminum welded to it by the gas tungsten arc welding process. A high-silicon aluminum filler wire should be used. Direct the arc toward the aluminum; pulsing will assist the welder.

    For welding aluminum to stainless steel transition inserts are available. It is also possible to use the coating technique. A coating for the stainless steel is pure aluminum coating, which can be applied by dipping clean stainless steel into molten aluminum. Another way to obtain a compatible coating is by tinning the stainless steel with a high-silicon aluminum alloy. The aluminum surface can then be gas tungsten arc welded to the aluminum. The arc should be directed toward the aluminum; pulsing will assist the welder.

    The welding of aluminum to copper is accomplished by using a copper-aluminum transition insert piece.

    Welding Copper to Various Metals

    Copper and copper-base alloys can be welded to mild and low-alloy steels and to stainless steels. For thinner sections, in the gauge metal thickness, the gas tungsten arc welding process can be used with a high-copper-alloy filler rod. The pulsed mode makes it easier to obtain a quality weld. The arc should be directed to the copper section to minimize pickup of iron.

    In the heavier thicknesses first overlay or butter the steel with the same filler metal and then weld the overlayed surface to the copper. It is important to avoid excessive penetration into the steel portion of the joint since iron pickup in copper alloys creates a brittle material. The copper must be preheated.

    Another method is to overlay the copper with a nickel-base electrode. A second overlay or layer is recommended on thicker materials. When making the overlay welds on thick copper, the copper should be preheated to ~540°C (1000°F).

    The overlay or buttered surface of the copper part should be smoothed to provide a uniform joint preparation. Effort should be made to minimize dilution of the copper with the nickel electrode. The shielded metal arc process, the gas tungsten arc or gas metal arc welding processes can all be used. The selection will depend on equipment available and the thickness of the material being joined.

    Copper can also be joined to stainless steel, and brass can be joined to mild and low-alloy steels.

    Welding Nickel-Base Alloys to Steels

    Nickel-base alloys such as Monel and Inconel can be successfully welded to low-alloy steel by using the Monel analysis of filler material when using any of the arc welding processes. In the case of Inconel to mild or low-alloy steel the Inconel base electrode would be used. The same situation applies also to the welding of Inconel or Monel to stainless steels. Here the Inconel or Monel type electrode is used.

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