Published in： 2021-01-22 08:12:00, published via： cyanbat | clicks: 0 | Information Sources：
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Its machining and corrosion and mechanical properties have many characteristics: fast heat dissipation, light weight, good rigidity, certain corrosion resistance and dimensional stability, impact resistance, wear resistance, good attenuation performance and easy recycling; in addition, there are high Features of thermal and electrical conductivity, non-magnetic, good shielding and non-toxic.
Application scope: Magnesium alloy is widely used in portable equipment and automobile industry to achieve the purpose of lightweight.
machining and corrosion and mechanical properties have many characteristics: fast heat dissipation, light weight, good rigidity, certain corrosion resistance and dimensional stability, impact resistance, wear resistance, good attenuation performance and easy recycling; in addition, there is high thermal conductivity And electrical conductivity, non-magnetic, good shielding and non-toxic characteristics.
Application scope: Magnesium alloy is widely used in portable equipment and automobile industry to achieve the purpose of lightweight.
Although the specific gravity of magnesium alloy is heavier than plastic, it has higher strength and elastic modulus per unit weight than plastic. Therefore, with the same strength parts, magnesium alloy parts can be made thinner and lighter than plastic. In addition, because the specific strength of magnesium alloy is higher than that of aluminum alloy and iron, the weight of aluminum or iron parts can be reduced without reducing the strength of parts.
Magnesium alloy has the highest relative strength (ratio of strength to mass). The specific stiffness (ratio of stiffness to mass) is close to aluminum alloy and steel, much higher than engineering plastics.
In the elastic range, when magnesium alloy is subjected to an impact load, the energy absorbed is half larger than that of aluminum alloy parts, so magnesium alloy has good shock resistance and noise reduction performance.
The melting point of magnesium alloy is lower than that of aluminum alloy, and its die-casting performance is good. The tensile strength of magnesium alloy castings is equivalent to that of aluminum alloy castings, generally up to 250MPA, up to more than 600Mpa. Yield strength and elongation are not much different from aluminum alloys.
Magnesium alloy also has good corrosion resistance, electromagnetic shielding performance, and radiation protection performance, which can be 100% recycled.
Magnesium alloy parts have high stability. The casting line of die casting parts has high dimensional accuracy and can be processed with high precision.
Magnesium alloy has good die-casting forming performance, and the minimum wall thickness of die-casting parts can reach 0.5mm. It is suitable for manufacturing various die-casting parts of automobiles.
However, the coefficient of linear expansion of magnesium alloy is very large, reaching 25～26 μm/m℃, while that of aluminum alloy is 23 μm/m℃, brass is about 20 μm/m℃, structural steel is 12 μm/m℃, and cast iron is about 10 μm/m. m℃, rocks (granite, marble, etc.) are only 5-9 μm/m℃, and glass 5-11 μm/m℃.
Magnesium alloys are alloys based on magnesium and other elements. Its characteristics are: low density, high specific strength, large elastic modulus, good shock absorption, greater impact load capacity than aluminum alloy, and good corrosion resistance. The main alloying elements are aluminum, zinc, manganese, cerium, thorium, and a small amount of zirconium or cadmium. Currently the most widely used is magnesium aluminum alloy, followed by magnesium manganese alloy and magnesium zinc zirconium alloy.
The specific gravity of magnesium alloy is the lightest among all structural alloys. Therefore, without reducing the strength of the parts, the weight of aluminum or iron parts can be reduced. The specific strength of magnesium alloy is significantly higher than that of aluminum alloy and steel, and the specific stiffness is equivalent to that of aluminum alloy and steel. In the elastic range, when magnesium alloy is subjected to impact load, it absorbs more energy than aluminum alloy parts, so magnesium alloy has good shock resistance and noise reduction performance. Under the same load, the vibration damping is 100 times that of aluminum and 300 to 500 times that of titanium alloy. The electromagnetic shielding performance is good. The shells of 3C products (mobile phones and computers) must be able to provide superior anti-electromagnetic protection, and the magnesium alloy shell can completely absorb electromagnetic interference with frequencies exceeding 100db. Good texture, the appearance and touch texture of the magnesium alloy are excellent, making the product more luxurious and less prone to corrosion in the air.
Magnesium alloy has an absolute advantage in heat dissipation relative to alloys. For radiators of the same volume and shape of magnesium alloy and aluminum alloy, the heat (temperature) produced by a certain heat source is easier to transfer from the root of the heat sink than aluminum alloy. With the speed to the top, the top is easier to reach high temperature. That is, the temperature difference between the root and top of the aluminum alloy radiator is smaller than that of the magnesium alloy radiator. This means that the temperature difference between the air temperature at the root of the heat sink made of magnesium alloy and the air temperature at the top is larger than that of the heat sink made of aluminum alloy material, thus accelerating the diffusion and convection of the air inside the radiator and improving the heat dissipation efficiency. Therefore, at the same temperature, the heat dissipation time of magnesium alloy is not half that of aluminum alloy.
Therefore, magnesium alloy is an ideal material for LED and other lighting, automotive application parts, and other parts that require high quality, high strength, and high toughness.
Chemical treatment
The chemical conversion coating of magnesium alloy can be divided into chromate series, organic acid series, phosphate series, KMnO4 series, rare earth element series and stannate series according to the solution.
The traditional chromate film has a dense structure with Cr as the framework, and the Cr containing structured water has a good self-repair function and strong corrosion resistance. However, Cr is highly toxic and the cost of wastewater treatment is high. Therefore, the development of chromium-free conversion treatment is imperative. The magnesium alloy is treated in KMnO4 solution to obtain a chemical conversion coating of amorphous structure, which is equivalent to chromate film in corrosion resistance. The chemical conversion treatment of alkaline stannate can be used as a pretreatment for electroless nickel plating of magnesium alloys, replacing traditional processes containing harmful ions such as Cr, F or CN. The porous structure of the chemical conversion coating shows good adsorption during the activation before plating, and can change the bonding force and corrosion resistance of the nickel plating layer.
The conversion coating obtained by organic acid treatment can simultaneously possess comprehensive properties such as corrosion protection, optics, and electronics, and occupies a very important position in the new development of chemical conversion treatment.
The chemical conversion film is thin, soft, and weak in protection, and is generally only used as an intermediate layer for decoration or protection.
Anodizing
Anodizing can obtain a better wear-resistant and corrosion-resistant coating base coating than chemical conversion, and has good bonding force, electrical insulation and thermal shock resistance. It is one of the commonly used surface treatment technologies for magnesium alloys. .
The electrolyte of traditional magnesium alloy anodic oxidation generally contains chromium, fluorine, phosphorus and other elements, which not only pollutes the environment, but also harms human health. In recent years, the corrosion resistance of the oxide film obtained by the environmentally-friendly process researched and developed is greatly improved compared to the classic process Dow17 and HAE. The excellent corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodization, so that the formed oxide film has good density and integrity.
It is generally believed that the pores in the oxide film are the main factors affecting the corrosion resistance of magnesium alloys. Studies have found that adding an appropriate amount of silicon-aluminum sol to the anodizing solution can improve the thickness and density of the oxide film to a certain extent, and reduce the porosity. In addition, the sol component will cause the film formation rate to increase rapidly and slowly in stages, but basically does not affect the X-ray diffraction phase structure of the film.
However, the anodic oxide film is brittle and porous, and it is difficult to obtain a uniform oxide film layer on complex workpieces.
Metal coating
Magnesium and magnesium alloys are the most difficult metals to plate. The reasons are as follows:
(1) Magnesium oxide, which is easily formed on the surface of magnesium alloy, is not easy to clean up, which seriously affects the adhesion of the coating;
(2) The electrochemical activity of magnesium is too high, and all acidic plating solutions will cause rapid corrosion of the magnesium matrix, or strong substitution reaction with other metal ions, and the coating bonding after substitution is very loose;
(3) The second phase (such as rare earth phase, γ equal) has different electrochemical characteristics, which may cause uneven deposition;
(4) The standard potential of the coating is much higher than that of the magnesium alloy substrate. Any through hole will increase the corrosion current and cause serious electrochemical corrosion. The electrode potential of magnesium is very negative. It is difficult to avoid hydrogen evolution caused by pinholes during plating. ；
(5) The compactness of magnesium alloy castings is not very high, and there are impurities on the surface, which may become the source of pores in the coating.
Therefore, the chemical conversion coating method is generally used to dip zinc or manganese, etc., then plate copper, and then perform other electroplating or electroless plating treatments to increase the bonding force of the coating. Magnesium alloy electroplating layer has coatings such as Zn, Ni, Cu-Ni-Cr, Zn-Ni, etc. The electroless coating is mainly Ni-P, Ni-W-P and other coatings.
A single electroless nickel layer is sometimes insufficient to protect magnesium alloys well. It has been studied that by combining the electroless Ni layer and the alkaline electroplating Zn-Ni coating, the about 35μm thick coating can withstand 800-1000h neutral salt spray corrosion after passivation. Some people also use electroless nickel plating as the bottom layer, and then use DC electroplating nickel to obtain a microcrystalline nickel coating. The average crystal grain size is 40nm. Due to the refinement of the crystal grains, the porosity of the coating is greatly reduced and the structure is more compact.
Electroplating or electroless plating is a surface treatment method that simultaneously obtains superior corrosion resistance and electrical, electromagnetic and decorative properties. The disadvantage is that the Cr, F and the plating solution in the pretreatment are serious to the environment; most of the plating layer contains heavy metal elements, which increases the difficulty and cost of recovery. Due to the characteristics of the magnesium matrix, the binding force needs to be improved.
Laser machining
Laser treatment mainly includes laser surface heat treatment and laser surface alloying.
Laser surface heat treatment is also called laser annealing, which is actually a rapid surface solidification treatment method. The laser surface alloying is a new technology based on laser surface heat treatment. Laser surface alloying can obtain alloy layers of different hardness, with metallurgical bonding interface. Single-layer and multi-layer alloying layers can also be prepared on high-purity magnesium alloy by using the cladding action of laser radiation source.
When a broadband laser is used to prepare a Cu-Zr-Al alloy cladding coating on the surface of a magnesium alloy, the alloy coating has high hardness, elastic modulus, and wear resistance due to the reinforcement of various intermetallic compounds formed in the coating Resistance and corrosion resistance. However, due to the presence of the rare earth element Nd, the laser multi-layer coating obtained after the laser rapid melting treatment can significantly refine the crystal grains, which can improve the compactness and integrity of the cladding layer.
Laser machining can process surfaces with complex geometries, but magnesium alloys are prone to oxidation, evaporation, vaporization, pores, and thermal stress during laser machining. It is very important to design the correct machining technology
Other machining
Ion implantation is a method in which accelerated high-energy ions (Al, Cr, Cu, etc.) impact the surface to be processed at high speed under the action of an electrostatic field with a voltage of ten to hundreds of KV in a high vacuum state and inject into the inside of the sample. The injected ions are neutralized and left in the vacancies or gaps of the sample solid solution, forming an unbalanced surface layer.
Some studies believe that the improvement in corrosion resistance is due to the densification of natural oxides, the radiation of implanted ions, and the formation of magnesium nitrides. The performance of the modified layer obtained is related to the amount of implanted ions and the thickness of the modified layer, and the MgO on the surface of the substrate also has a certain promoting effect on the improvement of the corrosion resistance of the modified layer.
Vapor deposition is vapor deposition of coatings. There are physical vapor deposition (PVD) and chemical vapor deposition (CVD). It is used to greatly reduce the content of impurities such as Fe, Mo, Ni in the magnesium alloy, and at the same time, the coating is used to cover various defects of the substrate to avoid the formation of local corrosion cells, thereby achieving the purpose of improving the corrosion resistance.
Compared with other surface treatment technologies of magnesium alloys, organic coating protection technology has the advantages of diverse varieties and colors, wide adaptability, low cost, and simple process. At present, solvent-based organic coatings are mainly used widely. Powder-based organic coatings are solvent-free, low pollution, uniform thickness, and better corrosion resistance. In recent years, they have been more popular in magnesium alloy parts such as automobiles and computer housings.