Aluminum alloy surface treatment processes

Thread Content

Surface treatment: It involves using mechanical and chemical methods to create a protective layer on the surface of the product, which serves to protect it. In nature, it enables a stable state, enhances the resistance of the organism and the aesthetic appeal of the product, thereby increasing its value. The selection of surface treatment types should first take into account the operating environment, service life, and aesthetic considerations; of course, economic value is also a key factor to be considered. The surface treatment process includes pretreatment, film formation, and post-film treatment. Packaging, storage. Processes such as shipment, where pretreatment includes mechanical treatment and chemical treatment. Mechanical treatment includes processes such as sandblasting, shot blasting, grinding, polishing, and waxing, with the aim of removing irregularities from the product’s surface and correcting other defects in its appearance. Chemical treatment removes oil and rust from the product’s surface, and it forms a layer that enables the coating material to bond better or to transform into an active metallic structure, ensuring that the coating remains stable and enhancing the adhesion of the protective layer, thereby providing protection for the substrate. Common surface treatment methods for aluminum include chemical treatments such as chromating, painting, electroplating, anodizing, and electrophoresis. Mechanical treatments include processes such as drawing, polishing, sandblasting, and grinding. Aluminum alloy sheets can be divided into two main categories based on their surface treatment: unpainted products and painted products. 1) Uncoated products (1) These can be divided into hammered aluminum sheets (with irregular patterns), embossed sheets (with regular patterns), and pre-passivated or anodized aluminum sheets with surface treatments. (2) Such products are not painted on the surface of the sheet material; they have low requirements regarding the appearance of the surface, and their price is also low. 2) Painted products (1) Classification: Based on the coating process, they can be divided into spray-coated sheet products and pre-rolled coated sheets ； Based on the type of paint, they can be classified into polyester, polyurethane, polyamide, modified silicon, epoxy resin, fluorocarbon, etc. (2) Among the various coatings, the main difference in performance lies in their resistance to ultraviolet rays from sunlight; among those used on the front side, fluorocarbon paint (PVDF) is the most common, as it has strong resistance to ultraviolet rays ； A polyester or epoxy resin coating can be chosen for the back side as a protective finish. Classification of common surface treatment processes 1 Anodizing: Primarily refers to the anodizing of aluminum; it utilizes electrochemical principles to form an Al2O3 alumina layer on the surface of aluminum and aluminum alloys. This oxide layer possesses special properties such as protection, decorative value, insulation, and wear resistance. Process flow: Single color, gradient color: Polishing/sandblasting/brushing → Degreasing → Anodizing → Neutralization → Dyeing → Sealing → Drying. Two-color: ① Polishing/sandblasting/brushing → Degreasing → Masking → Anodizing 1 → Anodizing 2 → Sealing → Drying; ② Polishing/sandblasting/brushing → Degreasing → Anodizing 1 → Laser engraving → Anodizing 2 → Sealing → Drying. Technical features: 1. Enhanced strength ； 2. Enable any color except white ； 3. Achieve nickel-free sealing to meet the requirements for nickel-free materials in Europe, the United States, and other regions. Technical challenges and key points for improvement: The yield rate of anodization affects the cost of the final product. To improve this yield rate, it is important to use the appropriate amount of oxidizing agent, as well as the right temperature and current density. This requires manufacturers of structural components to keep exploring ways to make improvements during the production process. 2. Electrophoresis is used for stainless steel, aluminum alloys, etc.; it enables the products to take on various colors while maintaining their metallic luster, enhances their surface properties, and provides good corrosion resistance. Process flow: Pretreatment → Electrophoresis → Drying. Technical features: Advantages: 1. Rich range of colors ； 2. No metallic texture; can be combined with sandblasting, polishing, embossing, etc ； 3. Processing in a liquid environment enables surface treatment of complex structures ； 4. The technology is mature and suitable for mass production. Disadvantages: Its ability to cover defects is average, and electrophoretic coating of die-cast parts requires high standards for pre-treatment. 3 Micro-arc oxidation is a process in which a high voltage is applied in an electrolyte solution (usually a weakly alkaline solution) to generate a ceramicized surface film layer; this process is the result of the combined action of physical discharge and electrochemical oxidation. Process flow: Pretreatment → Hot water washing → MAO → Drying. Technical features: Advantages: 1. Ceramic texture, matte appearance without any glossy areas; smooth to the touch and fingerprint-resistant ； 2. Wide range of substrates: Al, Ti, Zn, Zr, Mg, Nb and their alloys, etc ； 3. The pre-treatment is simple; the product boasts excellent corrosion and weather resistance, as well as good heat dissipation properties. Disadvantages: Currently, the colors are limited; only colors such as black and gray are available, as more vibrant colors are not yet feasible to achieve ； Costs are primarily affected by high power consumption, making it one of the most costly aspects in surface treatment. 4 PVD Vacuum Coating: Physical vapor deposition (PVD) is a manufacturing process used in industry, involving the use of physical methods to deposit thin films. Process flow: Cleaning before PVD → Vacuuming in the furnace → Target cleaning and ion cleaning → Coating → End of coating process, cooling and removal from the furnace → Post-treatment (polishing, AFP). Technical features: PVD (Physical Vapor Deposition) enables the application of high-hardness, highly wear-resistant cerametallic decorative coatings on metal surfaces. 5 Electroplating is a process that uses electrolysis to apply a layer of metal film to the surface of a metal, thereby preventing corrosion and enhancing properties such as wear resistance, electrical conductivity, reflectivity, and aesthetic appearance. Process flow: Pretreatment → Cyanide-free copper alloy → Cyanide-free brass-tin alloy → Chromium plating. Technical features: Advantages: 1. The coating has a high gloss, giving it a high-quality metallic appearance ； 2. The substrate materials include SUS, Al, Zn, Mg, etc., and the cost is relatively lower than that of PVD. Disadvantages: Poor environmental protection and a high risk of environmental pollution. 6 Powder spraying: A powder coating device (electrostatic spray gun) is used to apply the powder coating onto the surface of the workpiece. Under the action of electrostatic forces, the powder adheres evenly to the surface of the workpiece, forming a powdered coating layer ； The powder coating is leveled and cured through high-temperature baking, resulting in a final coating with various effects (depending on the type of powder coating). Process flow: Loading → Electrostatic dust removal → Spraying → Low-temperature leveling → Baking. Technical features: 1. A wide range of colors available, with options for glossy and matte finishes ； 2. Lower cost, suitable for building furniture products and the casings of heat sinks, etc ； 3. High utilization rate – 100% utilization, environmentally friendly ； 4. Strong ability to mask defects ； 5. It can mimic wood grain effects. 7 Brushing is a surface treatment method that creates linear patterns on the surface of a workpiece through grinding, serving a decorative purpose. Based on the patterns after drawing, it can be divided into: straight-line drawing, random-pattern drawing, wavy patterns, and spiral patterns. Technical features: The brushing treatment enables the metal surface to acquire a non-mirror-like metallic luster; it also helps to eliminate minor imperfections on the metal surface. 8 Sandblasting is a process that uses compressed air as power to generate a high-speed jet stream, which is used to shoot particles at high speed onto the surface of the workpiece to be treated. This causes changes in the appearance or shape of the outer surface of the workpiece, resulting in a certain level of cleanliness and varying degrees of roughness. Technical features: 1. Enables different reflective or matte effects. 2. It can remove tiny burrs from the surface of the workpiece, making the surface smoother; it eliminates the hazards caused by burrs and improves the quality of the workpiece. 3. Remove the residues left over from the pre-treatment process, improving the surface finish of the workpiece; this allows the natural metal color of the workpiece to be revealed, making its appearance more attractive and pleasing to the eye. 9 Polishing: A process of modifying the surface of a workpiece using flexible polishing tools, abrasive particles, or other polishing media. For different polishing processes – rough polishing (the basic polishing step), medium polishing (the finishing step), and fine polishing (the polishing for a glossy finish) – using the appropriate polishing wheels can achieve the best polishing results while also improving polishing efficiency. It improves the dimensional accuracy or geometric precision of the workpiece, yields a smooth surface or a mirror-like finish, and can also eliminate gloss. 10 Etching: What is generally referred to as etching, also known as photochemical etching, involves exposing a pattern, developing it, removing the protective film from the area to be etched, and then exposing it to a chemical solution; this process causes dissolution and corrosion, resulting in a textured or hollowed-out shape. Process flow: Exposure method: The process starts with determining the material dimensions based on the design – material preparation – material cleaning – drying → film application or coating – drying → exposure – development – drying – etching – film removal – completion. Screen printing method: Cutting the material – cleaning the sheet metal (stainless steel or other metal materials) – screen printing – etching – film removal – completion. Technical features: Advantages: 1. Enables fine processing of metal surfaces ； 2. Imparting special effects to the metal surface. Disadvantage: The corrosive liquids (acids, bases, etc.) used in etching are mostly harmful to the environment.