While many similar-metal FRW joints are produced because of economic considerations, many dissimilar-metal FRW joints are produced because there are no alternative welding methods that can be used. Examples of these types of joints include dissimilar-metal combinations with widely different melting points and dissimilar-metal combinations that form incompatible phases when fusion welded.
Low-Carbon Steels to Medium-Carbon Steels:
- In general, low- and medium-carbon steels are joined to each other under a wide range of conditions, and high-carbon steels are readily joined to alloy steels using friction welding. High-speed tool steels are welded to alloy steel shanks for numerous machine-tool applications.
- Steel with carbon contents as high as 1.0%, such as 52100 steel, can be joined to lower-carbon alloys.
- Preweld heat treating may be required in some cases to better match the properties at the interface, and postweld heat treatment may be required in some cases to temper the interface region of the high-carbon steel grades.
Stainless Steels to Other Selected Metals:
Stainless steel alloys are comparatively easy to friction weld to other metals. For example, austenitic stainless steel to low-alloy steel, titanium and copper to stainless steel, and 1100 aluminum to stainless steel are examples of transition joints that are made by FRW.
Problems Common to Welding of Dissimilar Materials:
- In general, the same problems encountered when welding similar materials must be addressed when welding dissimilar materials.
- However, some problems are associated only with the welding of dissimilar materials or are greatly magnified during the welding of dissimilar materials.
- These factors include joint interfaces, low-melting phases, brittle phases, and different thermal expansions.
Joint Interfaces:
- While most similar-material welds are made with little concern for surface preparation, highly dissimilar-metal combinations are more sensitive.
- This happens for various reasons. In stainless steel to aluminum alloy welds, the oxide surface that forms on the aluminum picks up contaminants such as water and hydrocarbons, forming extremely tenacious surface layers.
- If this layer is not removed prior to welding, poor structural welds may occur. In stainless steel to refractory metal alloy welds, the oxide on the faying surfaces again contains contaminants such as water and hydrocarbons. The contaminants in this case are likely to alloy into the finished weldment.
- This alloying causes a reduction of structural integrity through the formation of low-melting or brittle phases at the weld interface.
Low-Melting Phase Formation:
- Some material combinations have very low melting point phases associated with mixing of constituents at the weld interface.
- The formation of these phases during the welding cycle is deleterious to the finished weld properties. Examples of combinations that fall into this category include iron-base alloys to titanium alloys and aluminum alloys to magnesium alloys.
- Low melting point eutectics are found in both of these metallurgical systems, and great care must be exercised during parameter development to prevent the formation of liquid phases during the completion of successful welds.
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