Skip to content 
An Overview of Aluminum Galling
Galling refers to a severe form of adhesive material wear that occurs when two opposing surfaces of a metal, such as aluminum, in relative motion stick to each other through molecular forces, developing accidental cold welding. Metal galling or cold welding is typical in applications such as metal forming, engine pistons, hydraulic cylinders, bearings, etc.
This adhesive wear usually happens in high-speed applications with little loads, and when aluminum is exposed to slow sliding speeds and high loads—the effects of galling increase on any metal machinery or surfaces with poor lubrication and high temperature.
Aluminum alloys are typical metals that are more prone to galling because they undergo high friction force when torqued. While hardened steels like martensitic stainless steels exhibit good resistance to galling, softer metals generally gall easily. Plasticity and ductility are principal properties that influence the ability of a material to gall. Galling can occur regardless of whether the metallic material in contact is the same or different.
Alloys of aluminum and some other metals, such as titanium and stainless steel, develop corrosion resistance as a passive oxide layer, making them susceptible to galling when high contact force damages this oxide layer and exposes the bare reactive material. Consequently, galling is a major issue in components such as aluminum bolts because it freezes the bolt threads, and an increase in tightening force may strip the threads or shear off the bolt’s head.
In metalworking operations such as milling, turning, punching, and bending, galling involves the transfer of the work material onto the cutter, forming a lump. This lump changes the interaction between the blank and the cutting tool, causing greater adhesion and resistance to further cutting.
How Does Galling Occur?
The surfaces of metal parts usually appear smooth to the naked eye but have irregularities, such as voids on a smaller scale. The microscopic high points/ surface irregularities (asperities) of the contact area between two or more metals generate heat and friction when they rub against each other under high pressure or load. Local stress happens in the contact area, and lumps grow continuously while penetrating the protective oxide layer.
The high energy density in the contact area creates metal atom bonding and causes electron transfer between the two metal surfaces’ microscopic fibers. It fuses down the contact zone, developing cold welding from the natural plastic behavior of the metal. Galling typically entails excessive material transfer since it is adhesively pulled from one surface to another, creating a raised lump called a gall.
Galling doesn’t form progressively like other forms of wear, such as fatigue and abrasive wear. It develops swiftly and spreads rapidly as the elevated lump causes more galling. Although galling can happen at the microscopic scale, it sometimes causes vast, easily visible aluminum wear.
Therefore, it would help to consistently check machine parts susceptible to adhesive wear to ensure optimal overall performance. Detecting this problem late often has high-cost implications and unwanted production delays.
Factors Contributing to Galling in Aluminum
Below are various factors that trigger galling in aluminum:
- High Stresses: Direct contact and fast movement of the aluminum material while under extreme pressure significantly increase levels of stress. These stresses contribute to the effect of aluminum plasticity, making it more susceptible to galling. However, it is essential to understand that galling sometimes occurs at low stress since the real energy density of an operating system, material transfer, and higher friction are actual determinants of galling.
- Exposed Surfaces: Freshly cut aluminum materials bond easily when pressed against each other due to the absence of a protective oxide layer on their surface.
- Similar Metals: Microscopic electron transfer is more effective between two metals with close metallurgical properties. For example, an aluminum fastener will easily bind with aluminum materials with the same properties.
- Debris: Tiny particles or dirt trapped between the surfaces of aluminum material act as an abrasive as the surfaces undergo sliding contact, making them more susceptible to galling. Likewise, the area of contact deforms gradually, making localized deformation, which will gradually deform as the sliding motion continues.
Helpful Ways of Preventing Galling in Aluminum
Unchecked galling can pose complications, from major mechanical or equipment failures to increased maintenance costs. Here are various ways to prevent galling in aluminum metals:
Material Selection
Aluminum, austenitic stainless steel, and titanium are some of the most galling-prone materials. However, they are often chosen for specific applications due to their desirable properties, including machinability, strength, and corrosion resistance.
Meanwhile, some materials are more prone to galling than others. Harder materials are generally more resistant to galling than softer ones. As such, it would be best to combine aluminum with a harder material like stainless steel to reduce adhesion issues.
Additionally, dissimilar grades work well against mechanical abrasion since they possess varying atomic structures. Materials such as brass, bronze, and cobalt are other alternatives that are less susceptible to galling. So, using different metals for the nut and bolt is best. Various grades of aluminum have varying hardness levels and ensure unequal damage to the oxide layers under high-contact force.
Lubrication
Lubrication is an effective strategy in preventing galling in sliding aluminum components. Adequate application of lubricants helps minimize friction and contact temperature and functions as a protective layer between two surfaces. Specialty anti-galling lubricants, anti-seize lubricants, grease, and oil are common for preventing galling. Furthermore, a thick lubricant layer must be maintained to minimize the possibility of plastic deformation in the sliding surfaces.
Maintaining Sufficient Clearance in Part Design
Increasing the contact area between sliding surfaces of sliding aluminum components can reduce surface contact pressure and minimize galling. AT machining experts advise maximizing the contact surface area when designing these parts. The design should support adequate lubrication of the sliding components- adding holes or cavities in the part’s design can help retain lubricant in the contact zone. Specifying tolerances when designing sliding parts is essential, as sufficient clearance minimizes friction and allows proper functioning.
Surface Roughness Treatments
Rough surfaces can intensify galling because the asperities of each surface can get interlocked. Conversely, very smooth surfaces can cause galling when the material cannot capture and keep lubricant, resulting in a lack of texture on the surface. The ideal surface roughness to combating galling is more than 0.25 mm Ra and below 1.5 mm Ra.
Surface finishing treatments like buffing, grinding, and polishing are viable options for reducing the roughness of materials with a surface roughness above 1.5 mm Ra. Finishing treatments such as abrasive blasting are suitable for increasing surface roughness and masking surface defects.
Anti-galling Coatings
Aluminum parts susceptible to galling can be prevented with anti-galling coatings formulated to minimize friction. Tungsten disulfide is an effective coating type that acts as a dry film lubricant, reducing friction between two surfaces. Other options include diamond-like carbon (DLC) coatings and plasma electrolytic oxidation (PEO). Unlike wet lubricants that attract dirt and debris, they are applied and re-applied before assembly and usage.
Chromium plating, hard coat anodizing, and electroless nickel are suitable alternatives for reducing galling in aluminum parts. They also offer corrosive resistance benefits. Also, diamond-like carbon is another popular anti-galling coating that can increase the hardness of a metal part’s surface.
Use Clean and Undamaged Materials
Contaminants on the metallic surfaces often promote galling, which exacerbates cold welding. Experts recommend keeping the material free from debris through proper storage, shipment, and handling since parts can rub against each other during transport. Also, keep threaded components like bolts and screw clean and intact because damaged thread or debris can increase unexpected adhesion.
Conclusion
Galling in aluminum is a prevalent challenge in metal-to-metal contact applications, and it can occur for several reasons. However, you can manage this problem through proper preventive measures such as selecting the right materials, surface treatment, reducing installation speed, optimizing the part’s design, and adequate lubrication.
AT-Machining is a renowned and trusted CNC machine shop that provides quality metal parts and prototypes using processes such as CNC machining, additive manufacturing, sheet metal fabrication, metal casting, and aluminum extrusion. Our skilled experts can provide professional assistance in developing the perfect galling-resistant metal parts. We offer various material options and surface finishing treatments, including anodizing, plating, and bead blasting. Request an instant quote on your custom-made sliding aluminum parts now!
