Galling

Galling is a process that causes friction when metal parts like nuts and bolts are rubbed together among the high spots on the surface. The friction in turn generates heat and causes the asperities (i.e., high points) to weld together. It is not the same as cold welding, which is common in metals like steel.

Galling can occur even if the parts are brought together slowly, and it is prevented by the presence of grease or surface coatings, even if the surface coatings increase friction. It does not occur when joining dissimilar materials (e.g., threading 18-8 stainless steel into 17-4 stainless steel) even though both of those materials are susceptible to galling.

In metalworking that involves cutting (primarily turning and milling), galling is used to describe a phenomenon which often occurs when cutting soft metal: the worked material sticking to the cutter. It often occurs with aluminum 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. Therefore, to achieve the pressure needed to cut the workpiece, a greater force is needed resulting in more wear or breakage of the cutting tool.