April 10, 2007
Certain metal combinations cannot be joined successfully with fusion welding. Deciding how to join them requires examining options, including potential material substitutions and process possibilities. This article discusses these options and offers a blueprint for the best solution.
|Photo courtesy of High Energy Metals, Inc.|
Weldabilityrefers to the ease (or to the difficulty) of welding a given material with a certain process and a specified procedure to obtain acceptable welds. If the procedure is simple, the material is considered easily weldable. If special precautions, such as preheating, specified heat input, controlled cooling, and postheating, are required, the material generally is considered not so weldable.
The American Welding Society (AWS) defines weldability as "the capacity of a material to be welded under the imposed fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service."
What happens when incompatible different materials are to be welded together?
In a few cases, as illustrated in the following, just modifying a simple procedure is enough to obtain acceptable joints. However, some material combinations will not stick together, no matter which procedure is used. When confronted with this situation, stop and consider the problem.
Two elements combine to create the joining difficulties:
If the material combination cannot be modified, then you may have to look for a more favorable process. If the process cannot be changed, then you should explore the possibility of changing at least one of the materials. Both issues must be examined to find the solution.
If the problem could be overcome by substituting one of the two materials with another more favorable to the process, you should seriously consider making that change.
Otherwise, consider if placing a third material—compatible with both the required materials—between them will achieve a more favorable combination. The transition can be accomplished by buttering (covering) the surface of one of the materials to be welded with a special electrode that then is welded to the other material.
For example, welding mild steel to stainless steel can be done by buttering the mild steel with a suitable stainless that will not cause dilution problems. The covered surface then can be readily welded to the austenitic stainless steel.
Most problems with welding incompatible materials are generated by fusion welding, because the combined metallurgy of each of the original materials prevents the production of sound joints.
Unsound joints result when there is a great difference between the melting temperatures of the two materials; when there is no appreciable solubility of either metal in the other in the solid state; and when brittle intermetallic compounds are likely to form.
Stresses can develop in the weld when there are big differences between the thermal expansion coefficients, thermal conductivity, and specific heat of the two materials.
Some examples of material combinations that cannot be fusion welded successfully are aluminum and steel (carbon or stainless steel), aluminum and copper, and titanium and steel. Nothing can be done to alter their metallurgical properties.
That leaves changing your process. To join materials that cannot be fusion welded, consider adopting another process for the material combination that will produce an acceptable joint with the required properties. Some possible joining processes that can produce adequate mechanical properties and are suitable for service conditions are:
Sometimes a mixed solution is preferred. For example, if a copper tube has to be welded to an aluminum tube, a bimetallic transition part—half of which is 100 percent copper and the other half, 100 percent aluminum—can be friction welded together. This transition part then is arc welded twice, so that the copper side faces the copper tube and the aluminum side faces the aluminum tube. The same principle applies to joining aluminum to steel or stainless steel.
If applicable, solid-state processes should be the preferred choice, taking into account any special joint requirements (such as overlap) for each of the processes listed above.
So what is the best solution for joining incompatible materials? It's the one that is readily available, permits the work to be performed in an acceptable timeframe, meets all the structural and service requirements, and has the minimum total cost.