Analyzing Quality Issues of Medium Frequency Spot Welding Joints

In medium frequency spot welding, applying pressure is a key factor in heat generation during the welding process. Pressure application involves exerting mechanical force on the welding spot, which reduces contact resistance and balances resistance power.

This helps prevent localized heating during spot welding, ensuring even distribution of heat when power is applied. The duration of power application is also a crucial factor in heat generation. Heat generated during power application is dissipated through conduction. Even with a consistent total heat input, varying durations of power application lead to different maximum temperatures at the welding spot, resulting in varied welding outcomes.

Quality issues with spot welding joints primarily relate to their strength. This depends on factors such as nugget size (diameter and penetration), metal microstructure around the nugget and any defects present. For most metal materials, the strength of spot welding joints depends solely on the nugget size. However, materials sensitive to thermal cycling, such as quenchable steels, experience a drastic decrease in strength and ductility if the welding process is improper. In such cases, even with a sufficiently large nugget size, the joint cannot be used.

Materials prone to thermal hardening or cracking require post-heating with a lower working current after the main working current passes through to achieve annealing. Contact resistance is a critical factor directly related to heating at the contact point. When pressure application is consistent, contact resistance determines the surface condition of the welding material. Once the material is determined, contact resistance depends on the fine irregularities and oxide film on the metal surface.

While fine irregularities facilitate the desired heating range for contact resistance, the presence of an oxide film increases resistance, leading to localized heating. Therefore, it’s essential to remove any oxide film.

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