An important aspect of any part’s design and maintenance is the component’s resistance to wear. Adhesive wear is the product of two metal surfaces being rubbed together. Under a sufficient load, this will penetrate the oxide film on the surface of metals like aluminum and stainless steel. When the force exceeds the metal’s strength, adhesion or cold welding occurs.
At lower pressures, a slow wearing between the mating surfaces is the result. With higher force applied, cold welding occurs over a larger area with greater frequency and the resulting effect is referred to as galling.
What is galling?
Galling is a kind of wear on materials that is the result of friction between sliding surfaces. It causes one surface to be pulled away from the other due to the adhesive effect of cold welding. It is especially prevalent when there is a large amount of force that compresses the two surfaces against each other.
To explain it in its simplest terms, galling is due to friction and adhesion between two surfaces, which causes the crystal structure beneath the surface to slip and tear. A galled surface will appear gouged with pockets of missing material interspersed with lumps of the new material stuck on the surface.
Galling will be most widespread in metal surfaces with a lack of proper lubrication that allows them to slide without creating friction. Certain metals are more prone to galling because their crystalline structure. Aluminum and stainless steel are susceptible to galling, whereas carbon steels that have been fully hardened are not. Other examples of metals that are resistant to galling include brass and bronze.
What kinds of applications and operations are susceptible to galling?
Manufacturers need to select metals with specific properties for applications that involve frequently sliding surfaces. For instance, ball bearings of the wrong material are subject to galling. It should be noted it doesn’t matter if the two metals are of different or similar grades.
Galling is a frequent phenomenon with the thread flights of screws and bolts when they are fastened with excessive force. The metals most susceptible to such galling are those that have a natural passivation layer. This includes both stainless steel and aluminum, as well as titanium and other metals with natural corrosion resistance.
Aluminum is more likely to be affected by galling because it is a ductile metal. Galling is a common cause of tool breakdown with aluminum components. Because aluminum has high plastic flow, it can lead to the excessive material transfer and friction that is a signature of galling.
Manufacturers need to be particularly alert to the fact that galling can happen even at lower stress loads. The actual determinant of galling is the real energy density of an operating system, as well as material transfer and higher friction.
How can galling be prevented?
One of the best ways to prevent galling is to use an adequate amount of lubricant. However, this alone may not be enough because a certain kind of galling is possible even in the presence of lubricant. The main goal is to retain an adequate thickness in the lubricant layer, reducing the possibility of plastic deformation. When plastic deformation occurs, it creates heat, which can change the viscosity of the lubricant being used.
Galling can be reduced or avoided all together by making sure that threaded parts like bolts and screws are clean and intact. If the threads have been damaged already, then the likelihood of galling increases.
Another good rule of thumb if galling is a concern is to slow down the installation speed. As noted earlier, velocity alone does not prevent galling, but more careful installation can allow for straighter alignments and more flush insertions. With stainless steel or aluminum parts that are prone to galling, it may be necessary to avoid using power tools during the installation process.
Another tactic to minimize galling is to try and decrease the surface roughness of the mating components in the subject application. Decreasing the contact load can also mitigate the onset of galling, though this may be the factor least subject to manipulation. Alloy selection is one of the best ways to delay the onset of intense adhesive wear, but there is always a balance to be struck between different material properties such as corrosion resistance, hardness and wear resistance.
Your Technical Services Professional
Stainless steel and aluminum, because of their enhanced corrosion resistance, are more susceptible to galling than other materials. This is a particular weakness for these metals, and so it’s especially important that manufacturers take galling into account when selecting the grade and alloy for a given application.
At Clinton Aluminum, our team of technical professionals has a wealth of experience. Our staff is committed to working with our clients through every step of their procurement process. Contact us today to learn more.