Abstract
The microstructures of the compound electrodes with tungsten insert were characterized using scanning electron microscopy. Resistance spot welding compound electrodes used in this study were prepared from precipitation-strengthened and cold-formed domed flat electrodes consisting of Cu-0.7Cr-0.2Zr alloy and tungsten insert. Detailed study of the microstructure of pure compound electrode, compound electrode after 800 cycles of mechanical compression load at room temperature and after 800 welds of the same sheets with protective coating were investigated using scanning electron microscope.
At room temperature, the cracks in the tungsten insert were observed. It was concluded that they originated from the technology of tungsten rods production. After welding, the microstructural results confirmed that Zn did not dissolve in tungsten. Unfortunately, the aluminium was found on the surface of the tungsten insert increasing the possibility to stamp the electrode to the welded sheet after 200 welds. The presence of aluminium is mostly in the form of aluminium oxides. However, the presence of Al-tungsten phases (WAl₄ and WAl₅) was not fully excluded. An important result is that it is necessary to address the problem of alumina, which, as was shown, affects the life of the studied compound electrode with tungsten insert.

Key words
resistance spot welding, compound electrode, tungsten insert, microstructure, life cycle

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