galling

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Galling is "a form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (i.e., lumps) above the original surface". In other words galling is material transfer from one metallic surface to another caused by movement and plastic deformation.^[1]

Galling usually refers to adhesive wear and transfer of material between metallic surfaces during sheet metal forming and other industrial applications.

Mechanism

The friction or plastic deformation of one surface generates heat and increase adhesion between the asperities (i.e., high points) found on the mating surfaces, this is the initial interaction between the mating points for galling interaction. This process can be compared to cold welding, because cold welding is not cold and exhibits an increase in temperature and energy content derived from applied pressure and plastic deformation in the contact zone. In other terms, an applied load and plastic deformation increase the temperature and the chance for chemical or metallic bonding between the two surfaces.

Note: In dynamic contact and sliding friction, is increased compressive pressure equal to a raise in potential energy and temperature into the system. Because initially there is only limited amounts of transferred energy and thermal conductivity away from the contact zone through the small surface area on the system boundary and new energy is continuously forced into the system as the sliding progresses allowing a constant increase in energy content and temperature in the contact zone. If the right condition is achieved and the energy transfer away from the contact zone is less than the added energy will the accumulated energy cause a clear change in the sheet materials contact and plastic behaviour, causing an increase in adhesion and friction, see Figure 3).

In metalworking that involves cutting (primarily turning and milling), galling is used to describe a phenomenon which often occurs when cutting soft metal: the work material is transferred to the cutter and develops a "lump". The developed "lump" changes the contact behaviour between the two surfaces which usually increase the friction and resistance for further advancement. The change in contact behaviour can be described as friction welding or cold welding which sometimes but not always significantly increases the transfer of material to the cutter, see Figure 3).

"Friction welding" demands phase transition in both the mating materials which creates a weld after cooling.

"Cold welding" demands phase transition in at least one of the mating materials which creates a minimum amount of chemical and metallic bonding between the surfaces to create a solder.

A "solder" exhibits phase transition in one of the mating materials and chemical or metallic bonding in a much smaller volume between the mating surfaces, compared to friction welding.

Galling can occur even at relatively low loads and velocities because it's the real pressure or energy content in the system that inflict phase transition.

Galling often occurs with aluminium compounds and is a common cause of tool breakdown. Aluminium is a ductile metallic compound and possesses the ability to plastically deform which is needed to develop a plastic zone around the cutter. The ability to deform plastically can be considered as a general prescription for excessive material transfer and galling build-up because frictional heating is closely linked to the constitution (physique) of plastic zones around penetrating objects. In comparison, brittle fracture seldom generates heat.

Galling should not be confused with attraction between surfaces without involving plastic deformation, this type of attraction should only be compared with adhesive surface energy theories. Different energy potentials at the surfaces can develop adhesive bonds or cohesive forces that holds the two surfaces together even though they are separated by a distance. However, surface energy and the cohesive force phenomenon is not the same as galling, because galling involve plastic deformation of at least one surface.

However, the present research is not clear on this point and surface energy and cohesive forces are possibly involved in the initial material transfer, see Figure 1) where only surface-oxide asperities are in contact, but hard to distinguish from more severe attraction caused by increased pressure and plastic deformation of the sheet material. Oxides are brittle and it is probable that most energy in the fracture mechanism is consumed in brittle fracture which creates wear debris that can be transferred to the opposing surface. But this means that the transferred oxide material will instantly act as a penetrating body and the concentration of energy, pressure and frictional heating is immediate. The formation and constitution (physique) of plastic zones around penetrating objects are arguably the main factor for excessive material transfer, lump growth and galling build-up even in the initial contact process, see Figure 1).

Prevention

Galling is prevented by the presence of grease or surface coatings, even if the surface coatings increase friction. It usually 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.

See also

• Tribology
• Rheology
• Surface engineering
• Pin on disc tribometer

Notes

References