Development and processing of magnesium alloy matters needing attention

1. Introduction Since the beginning of the 1990s, the application trend of major metal materials in the world has changed significantly. The application of traditional materials such as steel, copper, lead and zinc has grown slowly, while the light metal materials represented by magnesium alloy have sprung up. The rate of 20% per year continues to grow. Magnesium alloys can be classified into cast magnesium alloys and wrought magnesium alloys. Magnesium alloys are mainly classified into Mg-AI-Zn-Mn, Mg-AI-Mn and Mg-AI-Si-Mn, Mg-AI-RE, Mg-Zn-Zr and Mg- depending on the alloy composition. Zn-RE system. Density (20 ° C): 1.738 g / cm 3; Melting point: 650 ° C; Boiling point: 1107 ° C; Heat of fusion: 8.71 kJ / mol; Heat of vaporization: 134 kJ / mol; Specific hot melt (20 ° C): 102.5 J / kg. Linear expansion coefficient: 25.2×10-6/K; thermal conductivity: 155.5 W/mK; resistivity: 44.5 nΩ·m; conductivity: 38.6% IACS 2. Magnesium alloy performance characteristics and application status Magnesium alloy has the following characteristics: (1) Light weight: magnesium alloy has a specific gravity of about 1.7, which is 1/4 of zinc, 1/5 of steel, and even more than aluminum alloy. The proportion of 2.7) is also 1/3 lighter.

(2) The «high strength, light weight» characteristics of magnesium alloys make it possible to replace these materials in the traditional applications of steel, cast iron, zinc alloys and even aluminum alloys.

(3) Excellent thermal conductivity, excellent shock absorption with respect to engineering plastics, better mechanical strength, impact resistance and wear resistance.

(4) Anti-EMI electromagnetic wave: Magnesium alloy is a non-magnetic metal with excellent electromagnetic shielding performance.

(5) High dimensional stability: not easily changed due to changes in ambient temperature and time.

(6) Recyclable: Magnesium alloy has 100% complete recovery characteristics, which is more in line with today’s environmental protection requirements.

(7) Machining characteristics: If the power required for magnesium cutting is 1, aluminum is 1.8, brass is 2.3, and cast iron is 3.5; and the specific gravity is light, the cutting inertia is small, and high-speed cutting is possible. The main use of magnesium alloys is in weight reduction. At present, the application industry of magnesium alloy die-casting products is mainly automotive components, accounting for more than 80%, followed by the 3C industry. Others such as bicycles, equipment tools, sporting goods and aerospace defense are also within its application range. Application Industry – Application Products Automotive Parts – Seat Brackets, Dashboards and Brackets, Motorized Window Motor Housings, Lifters and Axle Armatures, Accelerator Pedal, Acoustic Housing, Rear View Frame Bicycle Parts – Avoid Shocker parts, frame, crank, flower shell, three/five parts, rims, brakes, hand-held electronic communication – notebook computer case, MD case, mobile phone case, projector case, space defense – aviation communicator And radar casings, aircraft landing gear wheel housing sporting goods – tennis racket, snowboard holder, bat, mid-shot bow and handle equipment tools – portable electric saw casing, fish fishing automatic take-up 匣, control valve, camera 77% of the chassis, camera shell information, and communication products: 39% of notebook computers, 19% of digital cameras, 14% of mobile phones, 5% of digital cameras, 6% of projectors, and 17% of other electronic products. 18% of steam and locomotive parts: 88% of auto parts and 12% of locomotive parts. 5% of agricultural and forestry machinery: Among them: 41% of agricultural and forestry machinery, 27% of electric tools, 8% of sports goods, and 24% of others. The US government and the three major automobile companies (Ford), General Motors (GM), and Chrysler (Prysler) proposed the PNGV (Partnership for a New Generation of Vehicles) program in 1993, hoping to develop a 6-seat fuel-efficient vehicle in 2004, for every 100 The target of 3 liters of fuel consumption is mainly due to the lightweight design and development of the car body structure and power system. Suspension system assembly – part name: wheel, spare tire, control arm (2, rear), control arm (2, front), engine frame, rear bracket inner assembly – part name: instrument panel, beam , instrument panel bracket, seat back seat, airbag part steering wheel assembly – steering wheel parts body assembly – part name: bumper reinforcement beam, casting door lining, casting A/B column, windshield switch reinforcement Material, luggage rack, side mirror brake system assembly – part name: ABS parts, clutch / brake pedal bracket, pedal parts electrical machinery parts – AC box, audio / EEC parts, wiper motor, AC / AC bracket power System assembly – transmission (valve body, box, side cover, starter), gearbox (15% of total weight 12kg), engine block, support bracket, cover (oil / water pump, locomotive motor), cylinder head , suction manifold, engine bracket, oil cover, front cover.
3. Magnesium alloy processing problems Based on the above excellent characteristics, magnesium alloy has a strong advantage in the future development, more in line with contemporary requirements for environmental protection and sustainable development, and is the best choice to replace steel and aluminum. Due to the high chemical activity of magnesium metal, it brings a series of problems to the processing of magnesium alloy parts, which hinders the promotion and use of magnesium alloys, mainly reflected in:

(1) It is highly prone to galvanic corrosion. Special attention must be paid to smelting and manufacturing, and it is also more difficult to prevent corrosion than other metals. Therefore, in order to make the application of magnesium alloy more extensive, more processing procedures are needed for the corrosion mechanism, anti-corrosion mechanism, surface treatment technology and workpiece anti-corrosion design of magnesium alloy.

(2) Low ignition point. The influence of temperature must be taken into consideration during the cutting process to prevent the burning of the chips, and corresponding measures and conditions must be taken in the process to truly prevent the accident from happening.

(3) The problem of workpiece deformation. The linear expansion coefficient of magnesium alloy is larger than that of steel and cast iron. The cutting heat and temperature difference will directly affect the precision of magnesium alloy parts. It is necessary to select the machining allowance, tool geometry parameters, cutting amount and the design and testing method of fixtures. There are good measures in other areas.

4. Cutting technology points

4.1 Anti-rust measures during processing should be neatly discharged in the designated location, not allowed to touch the ground; moisture-proof measures should be taken in the place where the parts are stored; the surface of the parts coated with anti-rust oil should be wiped with cloth and gasoline, and dried. Processing; all chemical treatment processes, ie unsealing, oxidation and painting processes shall be recorded on the process card; after the parts are unsealed, the injection processing shall not exceed 15 days and nights; after the parts are crossed, the oxide film will be destroyed, so the marking shall be It should be carried out before the final oxidation; it should not be cooled by lubricating oil and coolant with dry cutting. It can be lubricated and cooled with oil when machining the thread; all machining processes should be carried out before the final oxidation. In special cases, the last oxidation is allowed. Individual machining. All sharp corners of the part should be rounded for best protection; oxidation is processed every 10 days and nights starting from the mechanical removal of the oxide film.

4.2 Sub-surface anti-rust treatment (1) Surface pre-cleaning (pre-treatment): Magnesium alloy products are easily mixed with oxides, lubricants, greases and other contaminants through die casting or forging, so the purpose of pretreatment is Clean the surface of the magnesium alloy for subsequent processing. Mainly used in two ways to clean: 1 mechanical cleaning, with different grinding, rough polishing, dry and wet abrasive spraying to achieve the required surface roughness; 2 chemical cleaning, solvent cleaning, alkaline washing, pickling, causing them different Surface state. (2) Passivation treatment: Passivation treatment is to use an electroless chemical reaction between a metal surface and a solution to produce an insoluble, inorganic salt surface film. The purpose of the passivation film is to improve the corrosion resistance, and the passivation film can also be used as a coating. The base to increase the adhesion of the coating. (3) Anodizing treatment: Anodizing treatment can produce anodized film, which greatly improves the corrosion resistance; produces metallic luster texture, has beautiful decorative effect, and its oxide film also serves as a subsequent coating base to increase the adhesion of the coating. If the subsequent sealing treatment is performed well, the resulting dense oxide layer can improve the surface hardness and corrosion resistance. (4) Electroplating treatment: The surface color and appearance of the magnesium alloy material can be changed by electroplating technology to achieve the desired functional and decorative purposes. Generally, when considering the improvement of the adhesion of the plating layer, several pre-treatments such as zinc replacement, copper replacement, and electroless nickel plating are performed before the plating treatment. (5) Metal coating: The metal coating can further prevent corrosion and increase the mechanical properties and hardness of the surface, so as to achieve the ultimate selection of lightweight magnesium alloy under special environmental conditions.

4.3 Measures to prevent combustion during magnesium alloy processing (1) Measures in daily management process Magnesium alloy parts are processed at the specified site, and the work site needs to be clean, bright and ventilated. It is necessary to clean up the magnesium chips at the work site in time, and it is not allowed to stack the chips next to the machine tool. Instead, special chips should be placed outside the factory to place the chips. Dry powder fire extinguishers or dry sand for fire fighting should be installed next to each processing machine. (2) Technical specification measures Select the appropriate cutting amount and tool geometry parameters, try to avoid the cutting depth of less than 0.05mm, keep the sharp and intermittent cutting state of the tool, and avoid the formation of thin strip-shaped chips. (3) Equipment environmental measures Dry high-speed cutting is used as much as possible. Without using coolant, the air is naturally cooled and equipped with flushing and chip evacuation equipment. Since the magnesium alloy powder that is in contact with water is easy to produce, mineral oil should be used as much as possible during production (acidic cutting agents are strictly prohibited), and the oil-resistant cutting fluid machine must have a leak-proof design. In addition, when removing magnesium chips, do not touch the water as much as possible.

4.4 Measures to reduce the deformation of parts during processing The magnesium alloy has small cutting force, low cutting energy consumption, less heat generation during cutting, easy chip breaking, small tool wear and prolonged service life. Therefore, magnesium alloy can be processed for high speed and large cutting. This partly offsets the weakness of the higher price of magnesium alloys. In principle, any tool material, including ordinary carbon steel, can be used to process magnesium alloys. In high-volume machining, carbide tools are generally used. Diamond tools are mainly used in the case of strict surface quality requirements. Use diamond tools. However, the magnesium alloy has a low ignition point (650 ° C), and must be cooled strongly with mineral oil during processing, and the magnesium chips are quickly removed from the processing area. In addition, in high-speed cutting, the magnesium alloy has a built-up edge on the surface of the bonding tool. The measures to reduce the deformation mainly include the following aspects: (1) Technical specification: Separate roughing from finishing, and remove most of the margin in the roughing stage. For frame-type magnesium alloy parts, especially thin-walled parts, The method of attaching the damping material reduces the deformation of the cutting vibration during machining. Consider the damping process during machining to reduce the cutting allowance and temperature difference. Artificial or natural aging and corresponding tempering heat treatment are arranged between the roughing and finishing processes to release the internal stress of the part as much as possible and reduce the deformation after processing. (2) Reasonable tool geometry parameters: adopt large rake angle, generally γ=20°～30°, rough processing takes smaller value, α=10°～15°, λs=60°～75° during finishing. Large blade inclination increases cutting force. At the same time, the influence of the wear of the flank face on the cutting force should be considered. Therefore, controlling the wear of the flank face to maintain the sharpness of the tool is also an effective way to reduce the cutting force. The lead angle has a direct influence on the cutting force. Decreasing the lead angle and increasing the ratio of the cutting depth to the amount of the cutter have a good effect on the heat dissipation in the cutting zone. Generally, the lead angle Hγ=30° to 45°. (3) Choosing the right cutting amount: It is processed by high-speed milling technology, and the cutting heat is basically taken away by the chip. It is also possible to appropriately select the large cutting amount and the large cutting depth.

5. Conclusion Magnesium alloy is developing from general structural castings to high-performance light-alloy structural frames and sophisticated application conditions. It is being promoted from military to civilian use, while processing technology is developing toward numerical control, high-speed, and automation technologies. The development of manufacturing and the simultaneous formation of components; the manufacturing technology is developing in the direction of informationization, digitization and design-manufacturing. China’s magnesium alloy manufacturing technology is in a new era of change, it will provide more advanced materials and manufacturing technology for the development of a new generation of industrial and civilian equipment.