When the surface of the spot welding workpiece of capacitor discharge spot welder has oil, water, paint, oxide film and other dirt, the surface contact resistance increases sharply and fluctuates in a wide range, which directly affects the stability of the welding quality. To ensure stable joint quality, the surface of the workpiece must be cleaned before spot welding (including projection welding).
Cleaning methods are divided into mechanical cleaning and chemical cleaning. The former has sandblasting, shot blasting, brushing, polishing, buffing, etc. The latter is commonly used in acid cleaning or other chemical substances. It mainly dissolves and strips the rust, oil, oxide film and dirt from the metal surface. These two cleaning methods are generally selected according to the weld material, supply state, structure shape and size, production scale, production conditions and welding quality requirements.
Mild and low-alloy steels have weak corrosion resistance in the atmosphere and are often protected by corrosion-resistant oils during transport, storage and processing. If the paint surface is not contaminated by dirt or other undesirable conductive materials, the oil film is easily extruded under electrode pressure and does not affect the quality of the joint. For unpickled hot rolled steel plates, the oxide must be removed by sandblasting, shot blasting or chemical etching before welding. Except for a few steel plates, special cleaning is generally not required to weld. Aluminum plates need to be cleaned by wire brushing or chemical etching.
When spot welding stainless steel and high temperature alloys, the surface of the welded parts should be kept highly clean. If oil, dust and paint are present, they may increase the brittleness of sulfur and need to be cleaned by polishing, shot blasting or chemical etching. Important weldments are sometimes polished by electrolytic polishing, but the process is complex and productivity is low.
