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This invention relates to electrodes for use in resistance spot welding, in particular to resistance spot welding of aluminum and aluminum alloys, and, in further particular, to composite electrodes having improved useful life and providing improved nugget formation when used to weld aluminum and alloys thereof.

BACKGROUND OF THE INVENTION

Resistance spot welding (RSW) is characterized by placing two workpieces of base metal, for example, low-, medium-, and high-carbon steels, alloy steels, stainless steels, nickel and nickel-based alloys, copper and copper alloys, aluminum, magnesium, titanium, and other alloys, including dissimilar metals or similar metals with the same of different sheet thicknesses. adjacent to one another, forcing the tip of at least one electrode against at least one of the workpieces, and passing a finite number of current cycles via the at least one electrode through the two workpieces. Metals with higher electrical resistivity and lower thermal conductivity are considered to be more amenable to RSW since it is possible to use a more-desirable lower welding current. When the base metals exhibit high thermal expansion, warping and buckling of the welded assembly can be a problem. In addition, hardness is a factor. Soft metals will be marked easily by the electrodes unless low electrode forces are used. Conversely, hard, strong metals require greater force to ensure adequate contact between the electrode and the workpiece. Finally, other factors such as oxide formation and plastic range can have significant affects on RSW.

In operation, resistance to the current melts the base metal at the interface between the two workpieces (the faying surface), thereby creating a lenticular-shaped zone of initially molten base metal which, when fused, forms a nugget which secures the two workpieces together. The current is typically short-time-pulsed, low-voltage, and high-amperage.

The electrodes used in RSW must exhibit the ability to conduct electricity to the workpiece efficiently, effectively transmit the necessary pressure to the workpiece, and rapidly transfer heat away from the interface between the electrode and the workpiece. Therefore, the most desirable electrodes will have high electrical and thermal conductivities, high hardness at elevated temperatures, and sufficient structural strength and stiffness to withstand the rigors of the weld process.

RSW is the most widely used joining method for thin sheet metals, particularly in the automotive industry. There is, particularly in the automotive industry, growing interest in the use of aluminum and aluminum alloys in automobile structures. (It is to be understood herein that any reference to aluminum, unless otherwise indicated, refers also to aluminum alloys.) It is recognized, further, that RSW is a key technology in the volume production of aluminum sheet structures. While conventional RSW is quite satisfactory for joining, for example, steels, other metals, particularly aluminum, present unique problems. First, aluminum has a high chemical affinity for oxygen and, therefore, forms a film of oxide when exposed to air. This oxide film not only presents a barrier of high electrical resistance which must be overcome to supply current to the workpiece, it also exhibits high heat transfer which conducts heat away from the workpiece so quickly that a nugget may not form