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Metals Knowledge:Welding of Dissimilar Metals

Published: 19 Oct 2009 21:35:48 PST

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, ahigh-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 beforethe 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 weldcoppertosteel.The two metals are not mutually

soluble, butnickelis 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 nickelalloyon the steel, i.e., buttering or surfacing the steel with a nickel weld metal

deposit.

The nickel or nickel deposit can be welded to thecopper alloyusing 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 InsertsThe 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 weldingis 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 nomelting, no heat-affected zone, and no thermal compounds are formed.

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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 betweenaluminumand steel,

aluminum andstainless steel,aluminum and copper, and other materials.

Cold weldingis 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 iscommonly used to join aluminum to copper.

Ultrasonic weldingis 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 weldingis 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 processis also widely used for dissimilar metal welding. Here the heat is

concentrated 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 weldingis widely used for aerospace applications of dissimilar metals welding.

Percussion weldingis 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 weldingprocess has had wide application for joining dissimilar metals. Electron beam uses

high-densityenergyand 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 processescannot 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 MetalsThere is a wide difference of melting temperatures, for aluminum, approximately 650C, and for steel,

approximately 1538C. 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. Thesuccessof this type of joint

depends on metal used to coat the steel, the thickness of the coating, and the bondbetween the coating and the

steel surface. A coating ofzincon 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-siliconaluminum 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

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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 ironpickup 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 ~540C (1000F).

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 SteelsNickel-base alloys such as Monel and Inconel can be successfully welded to low-alloy steel by using the

Monelanalysisof filler material when using any of the arc welding processes. In the caseof 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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