Correlating Electrode Degradation with Weldability of Galvanized BH 220 Steel during the Electrode Failure Process of Resistance Spot Welding

Electrode degradation in the continuous resistance spot-welding process of baked hardening (BH) 220 steel was evaluated by an electrode life test, and weldability tests were conducted by geometry feature measurement, mechanical property analysis, and electrode diameter measurement with 88 or 176 weld intervals. The analysis of weld geometry shows that the heat-affected zone (HAZ) width, nugget diameter, and nugget area tend to decrease rapidly, while the nugget height tends to increase with the weld repetitions until the welding heat input becomes too small to form an effective nugget. The maximum displacement and failure energy of the welded joints show a decreasing trend during the welding electrode failure process, while the peak load increases slightly until the 88th weld and then decreases. The cavities and pores in the nugget mainly appear after the 176th spot weld. The electrode diameter increases during welding. The reason for the increase in electrode diameter may be that the contact area between the electrode and the BH 220 steel sheets becomes smaller in the welding process, which causes the continuous sticking phenomenon between the electrode and the BH 220 steel sheets. In the absence of alloying, the edge of the electrode is geometrically deformed, while Cu–Zn–Fe alloying occurs in the area in contact with the BH 220 steel sheet.