How to Choose Abrasives for Aluminum Surface Treatment

Source: The Fabricator
Authors: Jim Sucharski and Neil Otis

This article was first published in the July 2023 issue of Canadian Fabricating & Welding.

When working with any type of aluminum material, metal fabricators aim to prevent abrasives from becoming gummy. Abrasives specifically designed to reduce heat load can help avoid this issue.

Aluminum is a non-ferrous metal, meaning it contains no iron. It belongs to the same category as brass or bronze. In terms of hardness, it is very soft. This softness causes it to stick to consumables during processes like preparing for welding or painting.

Therefore, the goal is to find a product—whether a cutting wheel, grinding wheel, coated abrasive product, or even a file—that creates conditions under which aluminum does not stick to the abrasive.

Aluminum comes in many different grades, ranging from very soft and ductile to the very hard 7075 series. However, due to its softness, the material's properties need to be carefully considered.

Aluminum is an excellent conductor of both heat and cold. It transmits both quickly, which is why it is used for cookware. Aluminum-core stainless steel frying pans, for instance, distribute heat more quickly and evenly. This also makes aluminum products lighter. Aluminum is used in everything from cutlery to shipbuilding, where weight is a factor.

From a safety perspective, working with abrasives on aluminum generates aluminum dust, which can be highly explosive. It’s important to keep safety in mind by using downdraft tables and wearing proper personal protective equipment whenever possible.

When choosing tools for working with aluminum parts—from pre-weld cleaning to polishing—you must consider these fundamental material properties and address them effectively.

Power Tool Concerns

When evaluating a process, there are three main questions to answer:

1、What material am I working with?

2、What type of tool am I using?

3、What is my ultimate goal (cleaning, blending, polishing)?
The second question is at the heart of the discussion. Are you using an electric or pneumatic tool? If electric, is it variable speed, straight-shaft, or right-angle?

For pneumatic tools, you must answer similar questions.

What RPM are you using? What type of mounting system are you using? Is it a threaded spindle? All these factors affect which product is suitable for a particular tool. Adjusting the airflow of a pneumatic tool will affect torque but not speed. If the tool’s rating isn’t correct, it may stall when pressure is applied.

Carbide Inserts Remove Chips without Generating Dust

Most pneumatic tools operate at a pressure between 90 and 95 PSI, with speeds ranging from 18,000 to 25,000 RPM. It’s essential to understand your tools to use them properly.

Preparing to Handle Sheet Metal

When cutting materials for use, it is necessary to remove sharp burrs from the sheet to avoid injury. Typically, workshops use a 50mm flap disc for edge deburring. A file can also be used for this task without requiring electricity or pneumatics. This method is low-cost and doesn’t produce airborne particles.

Cleaning Aluminum Sheets

When most people think of oxidation, they think of rust on carbon steel. However, every material has its own way of rusting. Aluminum tends to turn whitish-gray. The oxide layer needs to be removed to reach the substrate.

A brush is the ideal tool for this job. It must be made of stainless steel. If a carbon steel brush is used on aluminum, it may embed material and affect the surface finish and any weld seams. A brass brush can also be used, but ensure it is made of brass wire, not brass-plated wire. Brass-plated wire is cheaper, but once the coating wears off, it will contaminate the base material.

You have two types of wire brushes to choose from: crimped wire brushes and knotted wire brushes. For cleaning and preparing round or square materials from sheet metal, crimped wire brushes, whether cup-shaped or bevel-shaped, are ideal. The wires on the brush lift and release contaminants. On the other hand, knotted wire brushes absorb and retain contaminants, which is undesirable.

If you are working with larger surfaces, bevel brushes may be particularly useful. For very large surfaces, such as 1.75-inch thick steel plates used for building aircraft carriers, larger cup brushes are ideal. The key is to maintain a surface size similar to when you started.

Aluminum Welding Preparation

If you are preparing for butt welding, beveling is necessary. Traditionally, the first choice is to use grinding wheels to grind these bevels. The thickness of the sheet determines what kind of angle and how deep that angle needs to be. Since this is a method that depends on the operator applying force, the angle may vary.

Another option is to use flexible grinding discs. This requires using coated abrasives suitable for aluminum. Ceramic abrasive discs are popular in this regard because they have more fracture points in their design, so they wear down slower than standard aluminum oxide discs. However, regardless of the type of disc used, it is important that it has a lubricant or top coat to reduce friction and, in turn, reduce heat generation.

Heat is always the enemy, as it affects not only the base metal but also the consumables. With the proper coating, grinding discs run at a lower temperature, preventing material from quickly sticking to the abrasive.

Sometimes, the right tool for the job may be as simple as a file, which doesn’t require electricity or pneumatics. The advantage of this method when handling aluminum is that it doesn’t produce airborne particles.

A chamfer planer is a third option. They come in both 30-degree and 45-degree angles, and depending on the chamfer design, they can create more consistent bevel shapes. Some models are now designed like planers, with a top guide to help you guide along the material’s edge, improving precision. Chamfer planers designed for aluminum typically have deeper grooves to easily pull soft material off the surface and discharge it from the tool more efficiently.

Another option is to use carbide inserts mounted on a disc and installed on an angle grinder. Using carbide inserts means no dust is produced; instead, chips and chunks are removed. From a health and safety perspective, this is a positive approach. There is also no odor during use.

Achieving the Final Finish

At the start of the surface treatment process, the last of the three previously mentioned issues comes into play.

Where are your pain points? What type of surface finish do you want to achieve? The answers to these questions determine what type of abrasive to use.

If you want a vertical or horizontal scratch pattern, a linear polishing tool can provide a good vertical or horizontal scratch pattern. If you want a non-directional finish, non-woven material, typically with a grit rating of 180, can provide a good non-directional polish. Using a specialized tool can achieve a turned finish, and using a non-woven pad with a scratch rate of 1800 grit or equivalent can achieve a mirror polish. If you switch to a felt pad and use polishing paste, you can get an even brighter finish.

The key to working with aluminum is to ensure that all consumables are rated for the base material you are preparing. It is important to ensure that the material is free of contaminants, that heat is dissipated, and that consumables do not load up quickly.