What are the characteristics of surface treatment processes for titanium alloy materials?

Titanium alloys are various alloy metals made from titanium and other metals. They are relatively scarce and expensive. Titanium alloys are characterized by high strength, light weight, corrosion resistance, and high temperature resistance. They can be used in various fine chemicals and are mainly applied in the automotive, aerospace, nuclear, chemical and petrochemical, dental, and medical repair industries.

Surface treatment methods for titanium alloys are not only to enhance their aesthetics, but more importantly, to improve their surface properties and adaptability through these surface treatment and modification processes. Titanium alloy materials mainly undergo the following surface treatment processes:

I. Electroplating
Electroplating on titanium alloy surfaces primarily involves nickel, gold, and copper plating. The purpose is to improve the adhesion, conductivity, and brazing properties of the titanium alloy. However, electroplating also presents some problems, namely poor adhesion between the plating layer and the substrate. Therefore, pretreatment is necessary before plating:

① Sandblasting: This removes impurities from the titanium alloy surface, including oxide scale, oil, rust, and machining marks, making the surface rougher. This results in a stronger adhesion of the subsequent surface plating, making it less prone to peeling.

② Pickling: This removes oxide scale, oil, and impurities generated during high-temperature rolling, making the surface smooth and improving its properties, preventing corrosion, oxidation, and wear. However, if pickling is not performed correctly, subsequent processing will be significantly affected. Therefore, the pickling process must be strictly followed according to process requirements.

II. Micro-arc Oxidation

This process involves placing titanium parts in a specialized alkaline electrolyte under high voltage (200-800V). Numerous tiny electric sparks, known as micro-arcs, are generated on the workpiece surface. These arcs instantly produce temperatures of 3000-5000℃, melting and oxidizing the titanium substrate. A layer of titanium dioxide ceramic film grows in situ, not as a spray coating, but as an integral part of the substrate. The hardness of this micro-arc oxidation film is significantly higher than that of acid-washed or sandblasted bare titanium, and it also improves the corrosion resistance of the substrate surface, making it highly durable in seawater, disinfectants, and saline environments. The non-conductive nature of ceramics also makes it resistant to oxidation at high temperatures.

III. Coloring
Coloring improves the surface properties of titanium alloy materials and allows for different colors to be achieved according to customer requirements, enhancing the decorative appearance of components. Titanium alloy surface coloring processes include: atmospheric oxidation, anodizing, chemical treatment, and micro-arc oxidation. Currently, anodizing is the most commonly used coloring method for titanium alloys.

IV. Blackening
Blackening is a common chemical surface treatment method. The principle is to create an oxide film on the metal surface, completely isolating the metal from the air, thus achieving rust prevention. Blackening also increases the surface gloss of the metal, making it more aesthetically pleasing, and can reduce some of the deformation caused by internal or external forces.

V. Polishing
Polishing removes surface burrs and achieves a bright surface. It involves grinding, polishing wheels, and polishing fluids to make the metal surface smooth and shiny. A smooth surface is easier to process in subsequent surface treatments. Common polishing methods are divided into two types: mechanical polishing and electrolytic polishing. Mechanical polishing uses sandpaper, polishing wheels, and polishing paste to grind layer by layer, achieving a high mirror finish. Electrolytic polishing, also known as chemical polishing, uses a polishing solution for immersion, resulting in no mechanical grinding marks. It is suitable for small workpieces, and thin-walled titanium foil is less prone to deformation.
VI. Thermal Spraying

Thermal spraying involves melting metal or ceramic powder at high temperatures using flames or electric arcs, then atomizing it into fine droplets using a high-pressure gas stream. This atomized droplets are sprayed onto the surface of the substrate, and after cooling, an additional surface layer is formed. This process differs from micro-arc oxidation and is simple and easy to learn.

For titanium alloy surface treatment, we need to select the appropriate surface treatment method based on the usage environment and product application requirements to better reflect the product's characteristics.