The increasingly prominent problems of energy crisis and environmental pollution have made magnesium alloys that meet the development direction of “excellent performance, near-final processing and recyclable” materials stand out and become the most popular application material of this century. High-efficiency, energy-saving magnesium alloy die-casting parts will remain the main application of magnesium alloys for a long period of time now and in the future. Due to the high electronegativity of magnesium (-2.36 V SCE), the corrosion resistance in the atmosphere is extremely poor. Therefore, the magnesium alloy die casting must be properly surface treated according to specific requirements before use. In the production cost of magnesium alloy die-casting parts, the surface treatment accounts for about 40%, so surface treatment is crucial for the production and application of magnesium alloy die-casting parts.

At present, the surface treatment research of magnesium alloy die-casting parts is not the same, not as mature and standardized as aluminum alloy surface treatment, which restricts the application of magnesium alloy die-casting parts to some extent. This paper intends to apply the existing magnesium alloy die-casting parts. Surface treatment technology is briefly summarized and analyzed for key technical issues and development directions.

1. Surface treatment technology of magnesium alloy die castings

The surface of magnesium alloy die-casting parts generally needs to be pretreated (cleaned, degreased, pickled, etc.), coated, painted (painted, sprayed, metallized, etc.). The surface of the magnesium alloy die castings is commonly referred to. Processing refers to the process of coating, the main function of which is to form a protective film with corrosion resistance on the surface of the die-casting part with good adhesion to paint, plastic or metal. At present, the main use in the field of magnesium alloy die-casting is wet. Surface treatment method. That is, the surface treatment method using the treatment solution. The existing surface treatment techniques are different. We classify the surface treatment technology of magnesium alloy die casting into chemical film formation and anodization according to the film formation conditions. Two major types of film formation are introduced below.

1. Chemical film formation

The chemical film-forming technology mainly refers to a type of surface treatment technology in which a magnesium alloy die-casting part forms a protective film layer on the surface of the workpiece by a chemical reaction between the workpiece and the chemical treatment liquid for a certain period of time in the chemical solution. It mainly includes treatment technologies such as chromization, phosphating, zinc substitution and chemical corrosion.

1) Chromating treatment

The chromizing treatment is a surface treatment technique in which a workpiece is immersed in a treatment solution mainly composed of chromic acid or dichromate to produce a protective layer of magnesium oxide, chromium oxide or chromate on the surface. Typical processing specifications are shown in Table 1. Generally, the chromium ion-containing treatment liquid has a good film forming property, and the surface quality of the workpiece is not high, and is suitable for both thick and thin workpieces. The chrome-treated film layer is uniform and has a thickness of usually 0.5 to 3 μm. The corrosion resistance of the workpiece can be increased by several tens of times. The research on chrome treatment of magnesium alloy has been carried out more and more mature. However, due to the fact that chromium is far from the side effects on the human body and the environment. So its application has been gradually reduced.

      2) Phosphating treatment

Phosphating treatment is a surface treatment technique in which a workpiece is immersed in a solution mainly composed of phosphoric acid or phosphate or sprayed with a spray gun to produce a complete phosphate protective film layer on the surface. Typical processing specifications are shown in Table 2. The film forming property of the phosphating treatment liquid is not as good as that of the chromizing treatment liquid, and the surface quality of the workpiece is high, and it is generally not suitable for the surface treatment of the thin-walled die-casting (the wall thickness is less than 2 mm) having a poor surface quality. The thickness of the phosphating film layer is large, and as the bottom layer of the paint, the adhesion, moisture resistance and corrosion resistance of the film can be increased by several tens to hundreds of times. There are few studies on magnesium alloy phosphating treatment, and the current application is very limited.

3) Zinc replacement treatment

Zinc displacement treatment is the impregnation of the workpiece in a solution based on a zinc salt (usually adding pyrophosphate). Surface treatment techniques that allow the surface to be zinc replaced to produce a complete protective film layer. Typical processing specifications are shown in Table 3. The zinc replacement film not only has good corrosion resistance, but also can significantly improve the adhesion property of the metal. After the zinc replacement treatment, it can be subjected to copper plating, nickel plating, chrome plating, etc., so the zinc replacement treatment is mainly used for metal plating. Surface treatment of the former die casting.

4) Chemical corrosion treatment

The chemical corrosion treatment of magnesium alloy die-casting parts is carried out in two steps. The first step is to use chromic acid and nitric acid solution for corrosion; the second step is treated in hydrofluoric acid solution to remove the chromic acid formed during the first step of treatment. The film simultaneously forms a magnesium fluoride film layer on the surface of the workpiece. Typical treatment solutions are shown in Table 4. Since the surface of the workpiece is uneven due to chemical corrosion, it is difficult to obtain a smooth and bright film layer. However, since the adhesion of the film layer is improved, the chemically etched surface can be directly subjected to nickel plating, chrome plating or the like.

2. Anodized film

Anodizing into a film mainly refers to the magnesium alloy die casting as an anode, under the action of an applied voltage, by a certain temperature in a solution for a period of time, using the electrochemical reaction between the workpiece and the treatment liquid to form a protection on the surface of the workpiece. A type of surface treatment technology for oxide film layers. The cathode is typically a trough or stainless steel plate. Anodizing into a film mainly includes conventional anodizing and plasma micro-arc anodizing techniques.

1) Conventional anodizing treatment

The conventional anodizing treatment is also an anodizing treatment technique in which a workpiece is subjected to a low voltage. Typical processing specifications are shown in Table 5. Usually, after conventional anodizing treatment, a relatively dense oxide film layer is formed on the surface of the workpiece, and the film layer is mainly composed of magnesium oxide and hexagonal magnesium hydroxide, and is well bonded to the substrate. Compared with the chemical film forming process, the conventional anodized film layer has good corrosion resistance, wear resistance, high mechanical strength, and the dimensional accuracy of the workpiece hardly affects, and the coating process can be omitted in some cases of use. It can be directly used as the final treatment, so the conventional anodizing treatment technology has been widely used.

2) Plasma micro-arc anodic oxidation treatment

The voltage of the conventional anodizing treatment is raised to a certain value, and a plasma arc discharge is generated in the anode region. The micro-arc diameter is generally between several micrometers and several tens of micrometers, and the residence time on the surface of the workpiece is about several tens of milliseconds, and the corresponding temperature can be as high as several thousand degrees, so that the surrounding liquid can be vaporized. Forming a high temperature and high pressure zone in which a large amount of electrons and positive and negative ions can be generated under the action of an electric field, thereby generating a special physicochemical action, so that the formed oxide film becomes an ordered structure of ceramics (mainly by a cubic structure) The magnesium oxide composition), this special magnesium alloy die casting surface treatment technology is a plasma micro-arc anodizing treatment technology. This is a new treatment method that has just been explored in recent years. The typical processing specifications are shown in Table 6. Since the micro-arc discharge causes the temperature of the treatment liquid to continuously rise, in order to ensure a constant temperature of the treatment liquid, a circulating cooling system is also required for surface treatment. Since the obtained ceramic film layer is in-situ growth of the substrate, it is complete, compact, and has good adhesion to the substrate and the paint, and has excellent corrosion resistance, wear resistance and electrical insulation. At present, plasma micro-arc anodic oxidation treatment technology can be used to prepare decorative, protective and functional ceramic surfaces according to specific requirements. It has been applied in actual production, and further research is in constant exploration.

3) AC plasma micro-arc oxidation treatment technology

In the past six months, the author has studied the surface treatment of magnesium alloy die-casting parts, and developed the AC plasma micro-arc oxidation treatment technology and the corresponding treatment solution. The technology uses the power frequency alternating current to perform the micro-arc oxidation treatment on the magnesium alloy die-casting parts. Compared with the anodizing treatment described above, the technology does not need a DC power supply, the equipment is simple, and the surface treatment time of the workpiece is greatly shortened. The processing efficiency is more than 20 times higher than other methods. When processing magnesium alloy die-cast mobile phone casings, all the requirements of surface treatment can be achieved within a few minutes of processing time. The preliminary application in actual production has proved that the AC isolating micro-arc oxidation treatment technology is a very practical and promising magnesium alloy die-casting surface treatment technology. At present, the application of this technology in other aspects is under study, and I believe that it will achieve gratifying results in the near future.

Second, the key technical issues of surface treatment

The purpose of the surface treatment of magnesium alloy die-casting parts is to obtain a high-quality film layer that meets the requirements according to specific requirements and using appropriate techniques. Since various existing surface treatment technologies still have their own problems, it is necessary to understand the key issues in different processing technologies to achieve this goal.

1. Key technologies in chemical film formation

Since the chemical film formation is completely dependent on the chemical reaction between the treatment liquid and the workpiece, unlike the anodization film, there is an applied voltage, so in order to form a high quality film layer, in addition to strictly complying with the operation of each link In addition to the procedures, the uniformity of the surface composition of the die-casting parts should be given full attention. Only a uniform surface can form a uniform treatment layer, so it should be ensured that the composition of the magnesium alloy liquid is uniform, the composition of the die-casting part is uniform, and the surface of the die-casting part is sufficiently cleaned, especially the chromium-free treatment liquid with low film forming properties. When the thin-walled (less than 2mm) die-casting parts are processed, the alloy liquid composition design, the injection speed, the mold condition, etc., which affect the uniformity of the surface composition of the die-casting part, are strictly controlled to ensure uniform and stable surface composition of the die-casting part.

2. Key technologies in anodizing film formation

Since the anodization film formation is carried out under the action of an applied voltage, the surface quality requirements of the die casting are not as harsh as chemical film formation, and die castings of a certain surface quality (sometimes even without pretreatment). It is always possible to find a suitable processing voltage and processing time to obtain a high quality film layer. For anodizing to form a film, the most critical issue is the effectiveness of the treatment fluid. The reduction of the surface quality requirements of the die castings gives the release agent an opportunity to enter the treatment liquid. If the selection of the release agent and the treatment liquid does not match, the rapid failure of the treatment liquid will result, and in the plasma micro-arc oxidation film formation. The temperature of the treatment liquid will rise rapidly, and this temperature rise will cause the failure of some treatment liquids, affecting the surface treatment effect. Therefore, the reasonable selection of the release agent and the treatment liquid and the constant temperature of the treatment liquid are the key technical problems of the anodization film formation.

Third, the future development direction

At present, the surface treatment technologies of magnesium alloy die-casting parts have their own advantages, and the chemical film-forming equipment is simple and the operation pollution is small. It has great advantages in combination with coating treatment; anodizing film formation including AC plasma micro-arc oxidation treatment technology can achieve a variety of different processing purposes according to specific requirements. However, these technologies still have their own problems, and they need to be gradually improved and developed in the future.

From the perspective of improving the quality of surface treatment and meeting the needs of various products, it is a future development direction to develop short-term, multi-functional and high-quality film-forming and final processing. Plasma micro-arc oxidation Film formation technology will be rapidly developed.

Standing at the long-term development level, from the perspective of environmental protection, the realization of “green surface treatment” will be the fundamental principle for future development. The development of environmentally-friendly and non-hazardous treatment fluids and corresponding treatment technologies will be the future. The main development direction, the current development of treatment liquid to non-chromium, has taken a gratifying step.

At present, there is still a certain contradiction between protecting the environment and meeting the needs of various products. The development of surface treatment technology for magnesium alloy die-casting parts depends on the organic combination of the two and the close cooperation of production, learning and research.

Keyword: magnesium alloy die casting

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