Galling

For the botanical concept, see bark-galling.

Galling

Galling, according to ASTM standard G40 (2006), 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.

Galling usually refers to transfer of material between metallic surfaces during sheet metal forming (SMF) and other industrial applications.

The galling 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 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 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 conditions is achieved and the energy transfer away from the contact zone is less than the surplus energy will the sheet material exhibit a clear change in it´s contact and plastic behaviour, causing a 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.

• 
  Figure [1]
  The characteristic pattern show no breakthrough of the oxide surface layer, only small amounts of adhesive material transfer and a flattening damage of the sheets surface. This is the first stage of material transfer and galling build-up.
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  Figure [2]
  The characteristic pattern illustrates continues lines or stripes indicating a breakthrough of the oxide surface layer.
  This type of contact can, in different proportions, be found simultaneously with
  Figure 3)
• 
  Figure [3]
  The characteristic pattern illustrates an "uneven surface", a change in the sheet materials plastic behaviour and a larger deformed volume compared to flattening of surface oxides, Figure 1).
  This type of contact can, in different proportions, be found simultaneously with
  Figure 2)

Galling 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. 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.

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. Perhaps the surface energy is involved in the initial contact process, see Figure 1), but hard to distinguish from more severe attraction caused by increased pressure and plastic deformation.

What prevent galling

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.

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[Category:Tribology, Friction, Adhesive wear, Welding]