Friction can be one of the most frustrating factors to control for in metalworking. It can cause excessive heat, worn edges and tools, and seized-up parts. It can also cause metal galling. In order to prevent metal galling, it is important to understand what it is, what causes it, what materials are most susceptible, and methods that can be used to mitigate or eliminate its effects.
What is Metal Galling?
Metal galling is a type of wear that can cause a metal to unintentionally adhere to another metal that it is in contact with. This can result in the metal separating from the component it was originally attached to. Metal galling typically happens very rapidly, rather than after a large number of cycles, although additional cycles once galling has taken place can damage the material further.
Sometimes galling can occur at the microscopic level, while other times galling may cause large, easily visible metal wear on components such as bolts or pistons. Microscopic galling may not completely eliminate the usefulness of the metal component but for operations with tight tolerances, even microscopic galling can be enough to cause performance failures.
Galling is also different from other forms of wear because of the adhesion it causes. Most other forms of wear involve erosion of one metal as a result of contact with another material, however, galling causes the metal to adhere to the other material it is in contact with.
What Causes Metal Galling?
Galling happens as a result of friction forces being great enough to cause adhesive wear. Typically, this occurs when two or more metals are in contact with one another under very high loads. Metal galling also requires that a material be ductile and have a crystal structure that promotes cohesive attraction. Many metals have these two characteristics which is why galling frequently occurs on metal materials.
Another critical requirement for metal galling to occur is a large amount of friction. Friction provides the energy necessary for adhesion to occur.
When all three components of these components are present – friction, ductility, and cohesive attraction – metal galling can occur. This means that galling can frequently impact softer metals in contact under high loads.
What Metals are Most Susceptible to Metal Galling?
Metals such as aluminium and austenitic stainless steels are two of the most prominent metals that encounter galling issues. Hardened materials like tool steels and martensitic stainless steels are less likely to have metal galling occur.
When Does Metal Galling Occur?
Applications with ductile metals placed under high amounts of friction with one another are often the most susceptible to galling. Stainless steel and aluminium fasteners are among the most frequent instances of metal galling. This is due to the friction force that the fasteners undergo as they are torqued. Any type of metal machinery that becomes subjected to excess friction is also at risk for metal galling.
How to Prevent Metal Galling?
There are a number of excellent ways to prevent metal galling:
- Select a metal that is not very prone to it, such as a hardened tool steel.
- Reduce the amount of friction by using a lubricant.
- Reduce the amount of friction in threaded fastener applications by selecting a coarser thread.
- Ensure the contacting metal surfaces are clean, as debris can also increase friction which can lead to galling.
- A helicoil insert may also be effective at reducing fastener galling in metals such as aluminium.
- Reduction of the load placed on the contacting metallic surfaces can also inhibit metal galling.
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