In this study, the growth behavior of nugget and button diameter according to pre-welding conditions was investigated using dynamic resistance behavior in resistance spot welding. In the single welding condition, the nugget was generated when the welding current was stronger than 15 kA. When the current was increased to more than 35 kA, the shape of the nugget was unbalanced because the Peltier effect in resistance welding was too strong. With multi-welding using the pre-welding condition, the nugget diameter was reduced and melting was concentrated in the center, compared to the single-welding condition. This occurs because it plays a role in the lowering of surface resistance and the extension of the conduction area at the faying surface by breaking-down the oxide layer.
In this study, the effects of electrode surface design on the resistance spot weldability and degradation of the electrode following resistance spot welding (RSW) of aluminum 6014-T4 alloy were investigated. A new patterning method that can be produced through repetitive pressurization was applied to the electrode, producing a lattice-like pattern shape on the resulting patterned electrode. When RSW was performed using the lattice patterned electrode, the contact resistance decreased because of the effective removal of the oxide film from the surface of the aluminum alloy. As a result, heat generated by resistance on the E/S interface was reduced. Moreover, the growth rate of the weld nuggets formed with the patterned electrode in the thickness direction was lower than that of the nuggets formed with the as-received electrode, and there was comparatively less Cu-Al alloying of the patterned electrode. In addition, a continuous RSW process was performed on the alloy to observe the effect of the electrode surface design on electrode sticking. The results indicated that electrode surface shape can significantly influence resistance heat generation and electrode cooling effects, as well as produce welds with different weld morphology and microstructure. Finally, it was proved that the patterned electrode suffered less electrode degradation through EPMA on the electrode surface after the continuous RSW was completed.
Recently, lightweight vehicle bodies are in increasing demand to satisfy exhaust gas and environmental regulations around the world. In particular, aluminum alloys are widely used to manufacture lightweight parts, because of their excellent properties including corrosion resistance and mechanical properties. After the forming process, the welding process is important for manufacturing aluminum alloy parts. Resistance welding of aluminum alloys has several problems, due to internal weld defects such as cracks, shrinkage cavity, or porosity, which can result from the Al2O3 oxide film on the surface of the aluminum alloy. This study investigated electrode-force type controls to improve the weldability of the aluminum alloy. It was found that a high electrode-force on squeeze time can collapse the Al2O3 oxide film on the surface. It can reduce defects in the nugget by about 42%, by reducing heat input energy, compared to the continuous electrode-force 4 kN (reference value). Also, with high electrode-force during the hold time, defects were reduced by about 80%, by increasing the cooling rate. The weld quality has a great influence on the electrode-force type control, and internal defects in the nugget are greatly affected by the electrode-force on hold time.
This study entailed resistance spot welding conducted using 1.8 GPa-grade hot stamping boron steel heat-treated under varied conditions. The relationship between weldability and melting behavior of the intermetallic layer during resistance spot welding was examined. As regards the four heat treatment conditions beyond the austenitic temperature, it was found that the intermetallic layer thickened with increased time and temperature. Furthermore, the contact resistance also increased with increased time and temperature of the heat treatment. This was mainly because of the expansion of the area of Al-Fe-based intermetallic phases (e.g., FeAl2 and Fe2Al5) within the intermetallic layer. Contact resistance induces the thickening of the intermetallic layer and results, even with a low current, in the occurrence of expulsion due to the high heat generation in the faying surface. Subsequently, the weldable current range became narrow, and satisfying the property requirements was challenging. The welding using the heat treatment condition of 900 °C and 5 min showed sufficient current path area. This can be attributed to the phenomenon that the intermetallic layer was forced out to the rim of the corona bond area at an early stage of the welding. However, the welding using other conditions showed that the intermetallic layer remained on the border of the nugget and corona bond areas it was observed and verified via electron probe micro analysis. Consequently, the nugget was insufficiently formed and the fracture mode was a partial interfacial fracture. On the contrary, it was found that the adoption of the pre-pulse could enhance the weldability of all conditions by obtaining a larger contact area.
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