Finishes of Aluminum Extrusions

Aluminum extrusion can be accompanied by secondary services such as deburring to smoothen piercing edges and surface treatment to give the extruded pieces the desired finishing and boost corrosion resistance. Surface treatment doesn't do much for the quality of the aluminum.

That depends on the alloys and the extrusion technique. That said, surface treatments have a crucial role in improving the performance and physical appearance of extruded pieces.

The aluminum extrusion sector is getting competitive as different players establish shops to take advantage of the super-duper profit margins. Providing secondary services such as surface treatment is a sure way to deliver pieces that customers will love and make our business stand out.

What is aluminum finishing?

Aluminum finishing is the process of treating the surface of aluminum to improve its properties. It’s usually done as a secondary procedure following extrusion and it can spell the difference between a “good” and a “magnificent” product. Indeed, some of the benefits of surface treatment include:

Improving corrosion resistance

Aluminum is naturally resistant to corrosion thanks to the thick layer of oxide that forms on its surface. But the oxide layer isn’t durable. It can scratch off easily, exposing the metal to alkalis, acids, and other harsh elements. Finishing techniques help overlay the metal with a corrosion resistant surface, improving its performance in harsh environments.

Thermal insulation

Aluminum has an excellent strength to weight ratio that makes it advantageous for use in aerospace components, industrial equipment, pipes, and ductwork. However, some of these applications involve high temperatures, and aluminum being a superheat conductor is a big problem. Surface treatment can be done to give the metal a reflective or highly emissive exterior, reducing heat transfer as desired.

Improving hardness

Aluminum is suitable for making aircraft landing gear, medical devices, and different machine tools. However, it’s a relatively soft metal, which is advantageous when it comes to forming it into different structures, but it can dent, scratch, and melt easily when exposed to friction. Different treatments can be done to give the metal an abrasion resistant surface, improving performance in high friction environments.

Improving lubricity

When we talk of lubricity, we mean the stickiness of aluminum with heat exposure. This can be a problem when you are keen to reduce friction between parts. Sticky surfaces multiply friction, putting parts at risk of wear and tear. Surface treatment can help make the surface of extruded pieces more slippery, thus reducing friction.

Improving UV resistant

Treatment can also be done to make aluminum surfaces resistant to UV radiation. This is especially important in extruded pieces intended for use in outdoor furniture and automotive parts. Not only can UV cause fading, but solar heat can also accelerate aging. Protecting from UV will help keep your furniture looking like new for longer.

Improving antibacterial properties

Some extruded pieces are used to make medical devices and food processing equipment. Such tools are typically required to meet certain sanitary standards and specifications, including the ability to ward off bacteria.

This can be achieved by making surfaces smooth or with nano-porous topographical patterns to reduce adhesion and contamination and also make them easy to clean. Additionally, the surface can be enhanced with copper or silver ions, which naturally ward off bacteria from equipment.

Improving appearance

Surface treatments can also help improve the overall appearance of extruded pieces. It often starts with deburring, which removes sharp edges, resulting in a smooth and clean surface ready for surface treatment and decorative finishing to improve the metal's properties and colors.

Common aluminum finishing options

There are many different types of finishing techniques that can be done on aluminum. Some finishing techniques are relatively inexpensive, which will help keep production costs low, while others are extremely pricey. Overall, it’s vital to consider your budget and the application of the extruded pieces.

Mechanical finishes

Grinding is a common aluminum surface treatment technique. It involves the use of a spinning abrasive wheel to remove sharp spots from the flat surface of extruded aluminum pieces. It improves the surface quality and may be accompanied by painting, anodizing, or other secondary protective and decorative finishing options to achieve desired results.

The alternative is polishing, where an abrasive material is used to eliminate scratches from the surface of extruded aluminum, resulting in a smooth, lustrous, and shiny finish often referred to as mirror-bright.

Anodizing

Following mechanical finishing, anodizing can be done to improve appearance even further and also boost corrosion and wear resistance. It’s an electrochemical procedure that thickens the natural oxide layer on the metal’s surface into a super film of anodic oxide.

Anodizing is usually performed in a tank and the operating conditions depend on the applications of the extruded aluminum pieces. Basically, we can do hard anodizing for parts intended for use to make aircraft landing gear, medical devices and machine tools. This is where we reduce the drum temperature to below 5 degrees Celsius while keeping the sulfuric acid concentration around 10-15%, the voltage between 15-25V and the current density around 1.5-3 A/dm² to create a harder oxide.

On the other hand, soft anodizing is done on pieces intended for use in less stressful applications, and it involves higher drum temperature (64-72 °F) and acid concentration (15-20%) plus lower voltage (10-15 V) and current density (1-1.5 A/dm²)

Generally, anodized aluminum is incredibly resistant to corrosion and wear and tear. It’s also resistant to UV damage, making it suitable for use to make automotive parts and outdoor furniture. Additionally, it’s super easy to dye an anodized surface because it comes out rougher and really receptive to dyes and paints. Above all, anodized pieces are super easy to melt and recycle without pretreatment, such as paint stripping.

Powder coating

Powder coating is a popular alternative to other finishing techniques for giving aluminum pieces durable and protective finishing. It involves the use of spray guns or systems to negatively charge finely grounded polyester, acrylic, or resin particles and spray them onto the aluminum’s surface. The aluminum being treated must be grounded to allow the native particles to stick to the surface.

Following applications, the piece is cured in an oven at temperatures ranging from 150 to 450 degrees Celsius, depending on the powder material. This forces the powder to melt and fuse into a uniform protective surface.

Powder coating is a good alternative to painting which involves the use of solvents that can end up in the environment as volatile organic compounds, resulting in great harm.  Also, when painting, you might lose a large portion of your solvents via evaporation. On the other hand, powder coating doesn’t suffer from that problem, plus any spilled powder can be collected and reused.

Compared to anodizing, powder coating is cheaper. The equipment used in powder coating is expensive and sophisticated. It requires the use of tanks, chillers, and rectifiers, plus phosphoric or sulfuric acid.

Some cleaning, rinsing, etching, anodizing, and sealing is also necessary, and these require time and labor. On the other hand, powder coating is a simple three-step process involving pre-treatment, application, and curing and it only requires dry powder. There’s no limit to the color and finish options.

Painting

Painting is another surface treatment procedure that can be done to aluminum pieces to give them a protective and decorative sheen. It involves pre-treatment, such as cleaning, sanding, and priming, followed by painting and coating with an enamel sealer. The paint is usually a liquid solvent that can be sprayed or applied using a brush and then sealed to protect it from fading, chipping, and scratching.

Using solvents is less expensive than dry powder coating, and painted surfaces are typically easier to refinish. That said, the resulting coating may not be as durable as powder coating. Plus, the use of liquid solvents often results in spillages and volatile organic compounds. Spillages create extra cleaning work, while VOCs can cause harm to the environment.

Electroplating

Electroplating involves immersing pieces of extruded aluminum into an electrolyte solution containing dissolved metal ions of another metal and then introducing a current so the dissolved ions can migrate and bond onto the aluminum surface for a durable, protective, and decorative finish.

First, the aluminum workpiece must be pretreated to remove grease, stains, rust, and inorganic contaminants. Following greasing, pickling, and rinsing the workpiece, the experts prepare the electrolyte solution. Popular metal ions include tin, nickel, silver, and gold.

Tin improves electrical conductivity, while nickel boosts corrosion resistance without interfering with the metals strength to weight ratio. Silver improves lubricity, solderability, surface conductivity, and antibacterial properties, while gold is commonly used on pieces intended for medical implants.

Electroless nickel plating

Alternatively, you can choose electroless plating treatment where no electric current is introduced. Rather, the metal ions are left to deposit via an autocatalytic process. This procedure is often done using nickel alloys, particularly nickel-phosphorus. The phosphorus content boosts the coating's protective properties.

Electroless nickel plating starts just like other plating procedures.

First, the aluminum workpiece must be pre-treated to remove grease, rust, and inorganic impurities. Next, the workpiece is introduced into a solution of nickel ions, and sodium hypophosphite is introduced as a reducing agent.

Depending on your preferences, you can leave the coating as is or apply other metal coating such that the ENP becomes an undercoat that holds and strengthens subsequent metal plating. Generally, ENP increases corrosion resistance without affecting aluminum’s excellent strength-to-weight ratio. It also increases hardness, but only if the phosphorus content is kept low, so feel free to play with the phosphorus content as the application demands.

Ceramic coating

Ceramic coating is where you choose to cover aluminum workpieces with thin layers of permanent or semi-permanent ceramic material. Ceramic paints are typically available in liquid form and contain some amount of silicon dioxide, which helps the chemicals bond onto aluminum’s surface for a highly emissive and hydrophobic effect.

One common application of ceramic coating is insulating aluminum for use in aerospace components, roofs, and medical and industrial equipment. Ceramic coatings reduce heat absorption, helping achieve desired thermal insulation.

Ceramic coating can also be done on top of painted aluminum surfaces to protect paints from extreme heat, UV oxidation, scratches, and other activities that can cause damage. It acts as a strong protective surface that will keep aluminum pieces looking like new for years to come.

Nitriding

Nitriding involves transferring nitrogen onto the surface of an extruded aluminum part to create a case-hardened layer. The technique is over a century old, and it involves baking aluminum pieces in ammonia. It’s usually done in nitrogen-rich furnaces at extremely high temperatures.

First, the aluminum workpieces must be degreased, pickled, and rinsed to eliminate impurities, then loaded in baskets and transferred into the furnace. The furnace must be filled with nitrogen to displace off any oxygen that would otherwise combust with ammonia explosively.

The furnace is heated to nitriding temperatures, then ammonia is introduced, and the door is closed for safety and odor prevention. The process gives aluminum a hard and protective coating.

Sublimation

Sublimation, otherwise known as metal imprint, involves the transfer of dyes into pieces of pre-treated aluminum. It's a simple procedure for creating a surface with decorative patterns and improved durability. Basically, the pre-treated aluminum pieces are wrapped in film and heated to allow the dyes and patterns to transfer from to the pieces.

If you want even more durable surface coating, you can perform powder coating prior to sublimation.

This way, the aluminum workpieces are coated with a clear coat before the sublimation procedure.

Combining powder coating and sublimation eliminates the need for UV coating and other special finishes following the procedure since the powder coating acts as a protective finish to keep your aluminum safe from harsh elements and scratches. Once the dye has been transferred, the workpieces will be ready for use.

Alright, folks, that is from us at Weiye Aluminium. Surface treatment is a sure way to improve the look and performance of extruded aluminum.

We provide over 300 finishing treatments so our customers can get aluminum products with the surface quality they want. If you’ve already made up your mind on what finishing technique to do, please let us know, and we’ll be happy to oblige.