Preparation of useful bimetallic weld joints requires that the constituents of the joint do not form undesirable products while molten or during cooling. However, the usefulness of any weldment is based upon the required properties of the joint; therefore, some normally unacceptable bimetallic combinations may be usable, depending upon the joint requirements.
Generally, the strength of a bimetallic joint depends on the characteristics of the combined metals. Many of these combinations are ductile while others are extremely brittle and will fracture under application of very small stresses. Combined materials which are ductile, typically produce very desirable weldments; combinations which are devoid of ductility produce weldments with undesirable characteristics.
The following chart is provided as an aid in predicting the characteristics of various bimetallic combinations. It may also be useful in finding a solution to the joining of two incompatible metals, by finding an interposing metal which is compatible with each of the first two.
In the following chart, bimetallic combinations are divided into five groups in the order of decreasing desirability.
The SOLID SOLUBILITY group generally has high ductility, and produces excellent weldments; the strength is generally equal to or greater than that of the weaker of the two materials.
The COMPLEX STRUCTURES group can have almost any ductility. Use of joints in this group would require individual evaluations, but should be useful for many engineering applications.
The INSUFFICIENT DATA group has not yet been fully tested. The lack of complete data for these systems makes them suspect, and investigation of their properties is suggested.
The NO DATA AVAILABLE group will require careful study before use.
The INTERMETALLIC COMPOUNDS group is generally very brittle. These should be used only where the weld will not be subjected to stresses, or when no mechanical strength is desired in the joint. These joints may crack during cooling from fusion temperatures as a result of cooling stresses.