Micro-Arc Oxidation Technology for Aluminum and Aluminum Alloys

1. Technical content and technology key

(1) Content and process flow of micro-arc oxidation technology

The technology of micro-arc oxidation of aluminum and aluminum alloy materials mainly includes pretreatment of aluminum-based materials; micro-arc oxidation; and post-treatment three parts. The process flow is as follows: aluminum-based workpieces → chemical degreasing → cleaning → micro-arc oxidation → cleaning → post-processing → finished product inspection.

(2) Microarc oxidation electrolyte composition and process conditions

Example 1. Electrolyte composition: K2SiO3 5~10g/L, Na2O2 4~6g/L, NaF 0.5~1g/L, CH3COONa 2~3g/L, Na3VO3 1~3g/L; solution pH 11~13; temperature It is 20-50°C; the cathode material is stainless steel plate; the electrolytic method is to first rapidly increase the voltage to 300V and hold it for 5 to 10 seconds, then increase the voltage of anodization to 450V and electrolysis for 5 to 10 minutes. Example 2 Two-step electrolytic method, relying on the previous step: oxidize the aluminum-based workpiece in an aqueous solution of 200g/L K2O · nSiO2 (potassium water glass) with an anode current of 1A/dm2 for 5 min; Step 2: Rely on the microsteps The arc-oxidized aluminum-based workpiece was washed with water and then oxidized in an aqueous solution of 70 g/L Na3P2O7 with an anode current of 1 A/dm2 for 15 min. The cathode material is a stainless steel plate; the solution temperature is 20 to 20°C and 20 to 50°C.

(3) Influencing factors

1 Influence of alloy material and surface state: Microarc oxidation technology does not require high alloy composition of aluminum-based workpieces. Microalloyed materials that are difficult to handle in common anodizing, such as copper-containing and high-silicon cast aluminum alloys, can be micro- Arc oxidation treatment. The surface condition of the workpiece is also not required to be high, and generally no surface polishing treatment is required. For workpieces with high roughness, the surface after micro-arc oxidation treatment is repaired to become more even and smooth; and for workpieces with low roughness, the surface roughness is improved after micro-arc oxidation.

2 Influence of electrolyte solution and its components: The micro-arc oxidation electrolyte is the key to obtaining the qualified membrane layer. Different electrolyte compositions and oxidation process parameters result in different film properties. Microarc oxidation electrolytes often contain a certain metal or non-metal oxide alkaline salt solution (such as silicates, phosphates, borate, etc.), and its presence in the solution is preferably in a colloidal state. The pH range of the solution is generally between 9-13. Depending on the nature of the film, some organic or inorganic salts may be added as auxiliary additives. Under the same micro-arc electrolysis voltage, the higher the electrolyte concentration, the faster the film formation rate and the slower the solution temperature rise. Conversely, the film formation rate is slower and the solution temperature rises faster.

3 Effects of Oxidation Voltage and Current Density: The control of microarc oxidation voltage and current density is also critical to obtaining an acceptable film layer. Different aluminum-based materials and different oxidizing electrolytes have different micro-arc discharge breakdown voltages ( breakdown voltage: the electrolytic voltage of the workpiece surface just after the micro-arc discharge is generated), and the micro-arc oxidation voltage is generally controlled to be larger than the breakdown voltage Ten to hundreds of volts are performed. Oxidation voltage is different, the formed ceramic membrane performance, surface state and film thickness are different. According to the requirements of the film performance and different process conditions, the voltage of micro-arc oxidation can be changed in the range of 200-600V. Microarc oxidation can be controlled by voltage or controlled current method. When the voltage is controlled by micro-arc oxidation, the voltage value is generally controlled in sections, that is, a certain thickness of insulating oxide film is formed on the surface of the aluminum substrate at a certain anode voltage. Then increase the voltage to a certain value for micro-arc oxidation. When the micro-arc oxidation voltage has just reached the control value, the oxidation current passing through is generally large, reaching about 10A/dm2. With the extension of the oxidation time, the ceramic oxide film is continuously formed and improved, and the oxidation current is gradually reduced. Less than 1A/dm2. The waveform of the oxidation voltage has a certain influence on the performance of the film layer and can use voltage waveforms such as DC, sawtooth or square waves. The control current method is more convenient than the control voltage method, and the current density of the control current method is generally 2-8 A/dm2. When the control current is oxidized, the oxidation voltage begins to rise quickly. When the micro-arc discharge is reached, the voltage rises slowly. With the formation of the film, the oxidation voltage rises quickly and is maintained at a higher electrolytic voltage later.

4 Influence of temperature and agitation: Unlike conventional aluminum anodization, the temperature of the micro-arc oxidation electrolyte allows a wide range of temperatures and can be performed at a temperature of 10 to 90°C. The higher the temperature, the more water vaporizes the interface between the workpiece and the solution, and the faster the formation of the membrane, but the roughness increases. At the same time, the higher the temperature, the faster the electrolyte evaporates, so the temperature of the micro-arc oxidation electrolyte is generally controlled in the range of 20 to 60°C. Since most of the energy of micro-arc oxidation is released as heat energy, the temperature of the oxidation liquid increases faster than that of conventional aluminum anode oxidation. Therefore, the micro-arc oxidation process must be equipped with a large-capacity heat exchange refrigeration system to control the bath liquid temperature. Although a large amount of gas precipitates on the surface of the workpiece during the micro-arc oxidation process, the electrolyte has a certain degree of agitation. However, in order to ensure uniform oxidation temperature and system components, mechanical devices or compressed air is generally used to stir the electrolyte.

5 Effect of micro-arc oxidation time: The micro-arc oxidation time is generally controlled at 10 to 60 minutes. The longer the oxidation time, the better the film's densification, but its roughness also increases.

6 Cathode Material: The micro-arc oxidation cathode material uses insoluble metal materials. Since the micro-arc oxidation electrolyte is mostly an alkaline liquid, the cathode material may be carbon steel, stainless steel or nickel. In this way, a cathode or an electrolytic cell made of the above material may be used.

(7) Post-treatment of the film: After micro-arc oxidation, the aluminum-based workpiece can be directly used without post-treatment, and the oxidized film can also be sealed, electrophoretically painted, mechanically polished, etc. to further improve the performance of the film.

(4) Equipment for micro-arc oxidation

1 Micro-arc oxidation power supply equipment is a kind of special power supply equipment with high voltage and high current output. The output voltage range is generally 0-600V. The output current capacity depends on the surface area of ​​the workpiece to be machined. General requirements are 6-10 A/dm2. The power supply shall be provided with constant voltage and constant current control devices. The output waveform may be DC, square wave, sawtooth wave, etc. depending on the process conditions.

2 heat exchange and refrigeration equipment. Since the surface of the workpiece has a high oxidation voltage during the micro-arc oxidation process and passes a large electrolytic current, most of the generated heat is concentrated at the interface of the film layer and affects the quality of the formed film layer. Therefore, micro-arc oxidation must be used. Supporting heat exchange refrigeration equipment, so that the electrolyte cooling in time to ensure that the micro arc oxidation within the set temperature range. The electrolyte can be cooled by circulating convection, which can not only control the temperature of the solution, but also achieve the purpose of stirring the electrolyte.

(5) Quality Inspection of Film

There is no special standard for the quality inspection of micro-arc oxidation ceramic film, and the detection standard for the performance of aluminum conventional anodic oxide film can be used.

2. Advantages and disadvantages and scope of use

The use of micro-arc oxidation technology to surface-enhanced aluminum and its alloy materials, has a simple process, small footprint, strong processing capacity, high production efficiency, suitable for large industrial production and other advantages. The micro-arc oxidation electrolyte does not contain toxic substances and heavy metal elements. The electrolyte has strong anti-pollution ability and high rate of reusable recycling. Therefore, it has little environmental pollution, meets the needs of high-quality clean production, and also meets the needs of China's sustainable development strategy. The micro-arc oxidation treatment of aluminum-based ceramic surface layer has high hardness (HV>1200), strong corrosion resistance (CASS salt spray test> 480h), good insulation (film resistance >100MΩ), film layer and base metal bonding Strong, and has good wear and heat shock resistance and other properties. Micro-arc oxidation technology has strong processing ability, and can obtain oxide film layers with different characteristics by changing process parameters to meet the needs of different purposes; also, by changing or adjusting the composition of the electrolyte, the film layer has certain characteristics or presents different colors. It is also possible to use different electrolytes to perform multiple micro-arc oxidation treatment on the same workpiece to obtain a ceramic oxide film layer with different layers and different properties.

Because of its advantages and characteristics, the micro-arc oxidation technology has an extremely wide application prospect in the industrial fields such as machinery, automobiles, defense, electronics, aerospace and civil construction. Mainly used for surface strengthening treatment of aluminum-based components that have special requirements for wear resistance, corrosion resistance, thermal shock resistance, high insulation properties, etc.; also can be used in construction and civil industries. High requirements for decorative and wear-resistant corrosion resistance The surface treatment of the aluminum substrate can also be used for surface strengthening treatment of special aluminum-based alloy materials that cannot be treated by conventional anodizing. For example, aluminum-based pistons, piston seats, cylinders, and other aluminum-based components of automobiles and other vehicles; various aluminum-based molds in the mechanical and chemical industries; the inner walls of various aluminum cans; and various aluminum-based zeros in aircraft manufacturing. Components such as warehouse floors, roller bars, rails, etc.; as well as various aluminum-based hardware products in the civilian industry, fitness equipment.

The micro-arc oxidation technology still has some shortcomings, such as the need to further improve the research of process parameters and supporting equipment; the oxidation voltage is much higher than the conventional aluminum anodization voltage; the safety protection measures should be done during operation; and the temperature of the electrolyte should rise. Faster, need to have a larger capacity of refrigeration and heat exchange equipment.