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THINGS TO CONSIDER WHEN WELDING DISSIMILAR METALS
It is necessary in many
applications to weld metals of different compositions. A
successful weld between dissimilar metals is one that is as
strong as the weaker of the two metals being joined. It must
possess sufficient tensile strength and ductility so that the
joint will not fail. Such joints can be accomplished in a
variety of different metals and by many 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 zone.
MUTUAL SOLUBILITY
For fusion 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. 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.
THERMAL EXPANSION
The coefficient of thermal
expansion of both materials is another important factor. 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.
MELTING TEMPERATURES
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.
TRANSITION MATERIALS
In certain situations, the only way to make a successful joint
is to use a transition material between 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: (1) Use a
piece of nickel or (2) deposit several layers of nickel alloy on
the steel forming a nickel surface on the steel. The nickel can
then be welded to the copper alloy using a nickel filler metal.
Such a joint will provide satisfactory properties and will be
successful.
COMPOSITE INSERTS
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. A few welding processes that can be used for
making composite inserts include explosion welding, cold
welding, ultrasonic welding, friction welding, high-frequency
resistance welding, diffusion welding, electron beam and laser
welding, and flash butt welding.
ARC
WELDING
Three popular arc welding processes that are most often used for
welding dissimilar metals include shielded metal, gas tungsten,
and gas metal arc welding. The most popular combinations of
dissimilar metals that are joined include: aluminum to mild or
low alloy steel; aluminum to stainless steel; aluminum to
copper; copper to mild and low-alloy, or stainless steel; brass
to mild and low alloy steel; aluminum bronze to low alloy steel;
Inconel to mild and low alloy, or stainless steel; Monel to mild
and low-alloy, or stainless steel; ferritic or austenitic
stainless steel to mild and low alloy steel; or quenched and
tempered steel to mild and low alloy stainless steel. All of
these combinations can be successfully welded using the correct
procedures.
REFERENCE
Modern Welding Technology
by Howard B. Cary. 5th edition. 2002. Available
from the Publications Department, Hobart Institute, 400 Trade
Square East, Troy, OH 45373. Phone: 1-800-332-9448, ext. 5433
or 937-332-5433. Or purchase online at
http://www.welding.org
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