Galling - Incidence and Location

Incidence and Location

Galling or adhesive wear is often found between metallic surfaces where direct contact and relative motion have occurred. Sheet metal forming, thread manufacturing and other industrial operations may include made parts of stainless steel, aluminium and titanium that are particularly susceptible to galling.

In metalworking that involves cutting (primarily turning and milling), galling is often used to describe a wear phenomenon which occurs when cutting soft metal. The work material is transferred to the cutter and develops a "lump". The developed lump changes the contact behavior between the two surfaces, which usually increases adhesion and resistance to further advancement and, due to created vibrations, can be heard as a distinct sound. An example of a change in material behavior can be seen in figure 4.

Galling often occurs with aluminium compounds and is a common cause of tool breakdown. Aluminium is a ductile metal, which means it possesses the ability for plastic flow with relative ease, which presupposes a relatively consistent and large plastic zone. In comparison, brittle fractures exhibit a momentary and unstable plastic zone around the cutter, which gives a discontinuous fracture mechanism that deters the accumulation of heat.

High ductility and flowing material can be considered a general prerequisite for excessive material transfer and galling build-up because frictional heating is closely linked to the constitution (physique) of plastic zones around penetrating objects and, as mentioned, brittle fractures seldom generate a great amount of heat.

Galling can occur even at relatively low loads and velocities because it is the real local pressure or energy density in the system that induces a phase transition, which often leads to an increase in material transfer and higher friction.