Galling

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Galling can occur when metal parts, such as the threads of nuts and bolts, are forced together and rubbing generates friction among the asperities (high spots) on the surface. The friction causes heat, which is mainly isolated to these asperities. The asperities weld together but further displacement causes these tiny welds to break, which makes the surface even rougher, creating more opportunity for friction. Galling should not be confused with cold welding.

In metalworking that involves cutting (primariliy turning and milling) "galling" is used to describe a phenomena which often occur when cutting soft metal: workpiece material sticking to the cutter. It often occurs with aluminium and is a common cause of tool breakdown. When a soft material sticks to the cutting edges the effective surface area of the cutter increases. To achieve the pressure needed to cut the workpiece, a greater force is needed often resulting in extensive wear or breakage of the cutting tool.