Current situation and Prospect of the hottest magn

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The current situation and Prospect of magnesium alloy forming technology

in recent years, the demand for lightweight materials is increasing. As a structural material, magnesium alloy is widely used in automotive, electronic, communication and other industries because of its small specific gravity, high specific strength and specific stiffness, good thermal conductivity and conductivity, good machinability, excellent damping and electromagnetic shielding, and easy processing, forming and recycling, Known as "Green engineering materials for the 21st century".

according to different forming processes, magnesium alloy materials are mainly divided into two categories: cast magnesium alloy and deformed magnesium alloy. The former mainly obtains magnesium alloy products through casting, including sand casting, permanent mold casting, investment casting, lost foam casting, die casting, etc. among them, die casting is the most mature and widely used technology. The latter produces plates, rods, tubes, and molds with various sizes through deformation Materials and forgings. And through the control of material structure and the application of heat treatment process, higher strength, better ductility and better mechanical properties can be obtained, so as to meet the needs of more structural parts. In addition, semi-solid forming of magnesium alloys as a new casting technology has also been widely studied and applied

1 cast magnesium alloy

casting is the main forming method of magnesium alloy. A variety of casting methods, including sand casting, metal mold casting, investment casting, lost foam casting and die casting, can be used for magnesium alloy forming. At present, more than 90% of magnesium alloy products are die cast

1.1 die casting

die casting is the most important and widely used forming process of magnesium alloys. Magnesium alloy has excellent die-casting process performance: magnesium alloy liquid has low viscosity, good fluidity, and is easy to fill complex cavities. Die castings with a wall thickness of 1.0mm ~ 2.0mm can be easily produced with magnesium alloy, and now the minimum wall thickness can reach 0.6mm. The casting slope of magnesium die castings is 1.5, while that of aluminum alloys is 2 ~ 3 degrees. The dimensional accuracy of magnesium die castings is 50% higher than that of aluminum die castings. The melting point and crystallization latent heat of magnesium alloy will certainly want power battery enterprises to reduce the cost of power battery lower than that of aluminum alloy. The erosion of mold in the process of die-casting is smaller than that of aluminum alloy, and it is not easy to stick. Its mold life can be 2-4 times longer than that of aluminum alloy parts. The die-casting cycle of magnesium alloy parts is shorter than that of aluminum parts, so the production efficiency can be increased by 25% than that of aluminum alloy. The machinability of magnesium alloy castings is better than that of aluminum alloy castings. The cutting speed of magnesium alloy castings can be 50% higher than that of aluminum alloy castings, and the processing energy consumption is 50% lower than that of aluminum alloy castings. Production experience shows that due to high production efficiency, the total cost of small magnesium alloy parts cast in hot chamber is lower than that of aluminum alloy parts cast in cold chamber

die cast magnesium alloys can be divided into four series according to their composition: AZ (mg Al Zn) series (AZ91), am (mg Al Mn) series (AM60, AM50), as (mg A1 Si series (AS41, AS21), AE (mg Al re) series (aea2)

The AZ series alloy AZ91 has good casting performance and the highest yield strength. Its die castings are widely used in various forms of components such as car seats, gearbox housings, etc. Am series alloys AM50 and AM60 have high elongation and toughness, and are used in occasions with high impact load resistance and safety, such as wheels, doors, etc. As series magnesium alloys AS41, AS21 and AE series afa2 are heat-resistant die-casting magnesium alloys developed in the 1970s

Cold and hot chamber die casting methods are widely used in magnesium alloy die casting. Generally, hot chamber die-casting machine is used for thin-wall castings, and cold chamber die-casting machine is used for thick wall castings. Magnesium alloy hot chamber die casting machine is the most widely used special equipment for magnesium alloy die casting abroad at present. It has the characteristics of high production efficiency, low pouring temperature, long injection mold life, easy to realize melt protection and so on. The main disadvantage is that the equipment cost and maintenance cost are high

during magnesium alloy die casting, the high-speed turbulent movement of the alloy liquid during filling and molding makes the gas in the cavity unable to be discharged, which will lead to loose tissue and even bulge or deformation on the surface of the casting. Die casting process parameters such as pressure, speed, melt temperature, mold temperature have a significant impact on the performance of castings. Many new die-casting methods, including vacuum die-casting, oxygen filled die-casting and squeeze casting, overcome the above shortcomings to a certain extent, reduce the defects such as porosity and porosity of castings, and improve the density of castings. Zak, an Ohio precision molding company, introduced the metal compression molding technology (MCF) of magnesium alloy, which is a molding method of pressurizing the whole casting surface. It solidifies under pressure, improves the microstructure, reduces the grain size and porosity, and the castings are dense and uniform. It can be used for castings with high performance requirements and complex shapes

1.2 investment casting is necessary to protect human health. Investment casting is one of the more advanced casting technologies in the world at present. In principle, investment casting is suitable for preparing small volume and high-precision castings. At present, it has been used to produce aluminum alloys and even nickel based superalloys. In the development process of magnesium alloy castings, investment casting can be used to produce some workpieces with complex structure, thin wall thickness in some parts, and strict requirements on surface roughness and dimensional tolerance

when using investment casting to produce castings, it has the characteristics of no mold taking, no core and no parting surface. Therefore, Xu Shengming, a professor of Tsinghua University and vice president of Jiangxi University of science and technology, said previously that the dimensional accuracy and surface roughness of its castings are close to those of investment castings. In addition, investment casting provides sufficient freedom for the structural design of castings. The original components assembled by multiple parts can be cast as a whole by gluing them together after sectioning, so many complex parts can be produced economically. However, the equipment investment and unit casting cost of investment casting are high, and the workpiece size is limited. In addition, there is a high activity reaction between magnesium and oxide ceramics for investment casting materials and bonding materials, which greatly limits its application. When producing magnesium alloy thin-walled parts, the mold needs to be preheated in order to fill the thin-walled parts. However, too high preheating temperature and pouring temperature will promote the reaction between magnesium alloy and mold. Some studies have shown that ZrO2 is a promising mold material when using low mold preheating temperature

1.3 lost foam casting

lost foam casting is a new casting technology with nearly no surplus and precise molding. It has many advantages, such as molding sand does not need binder, casting sand falling and sand treatment system is very simple, and it is easy to achieve cleaner production; The casting has no parting surface and ejection inclination, which can make the structure of the casting high; The processing and assembly time can be reduced, and the casting cost can be reduced by 10% - 30%

the preliminary experimental research shows that the characteristics of magnesium alloy are very suitable for the lost foam casting process, because in addition to the above characteristics, the lost foam casting of magnesium alloy also has the following unique advantages: ① at the pouring temperature, the decomposition products of foam shape of magnesium alloy are mainly hydrocarbons, benzene, styrene and other aerosols, which have a natural protective effect on the liquid magnesium alloy that is very easy to oxidize during hedge molding; ② The use of dry sand negative pressure molding avoids the contact between magnesium alloy liquid and water in molding sand and the resulting casting defects; ③ Compared with the currently widely used magnesium alloy die-casting process, its investment cost is greatly reduced, and the dry sand has a good concession to reduce the hot cracking tendency of magnesium alloy during solidification shrinkage; The slow and stable filling speed of molten metal avoids the involvement of gas, so that the mechanical properties of castings can be further improved by heat treatment. Therefore, the lost foam casting of magnesium alloy has a great application prospect

the solidification and chemical properties of magnesium alloys cause many problems in lost foam casting, especially insufficient pouring and oxidation combustion. Due to the low density and specific heat capacity of magnesium alloy, the heat required to vaporize the foam pattern comes from the latent heat of high-temperature liquid magnesium alloy, which hinders the filling. Moreover, the crystallization temperature range of magnesium alloy is wide, so the indenter effect of molten metal during EPC filling is small, and it is easy to stop flowing too early, resulting in insufficient pouring defects. The chemical reaction of magnesium alloy may be controlled by using flame retardant applied in magnesium alloy sand casting industry and auxiliary use of pattern coating with high porosity, and controllable atmosphere can also be used to prevent oxidation and combustion during pouring. In addition, the high-density foam pattern absorbs more heat and produces more liquid and gaseous products, which reduces the filling property of magnesium alloy. However, the reducing atmosphere produced by the foam pattern in the pouring process reduces or even prevents the oxidation and combustion of magnesium alloy, which ensures the safety of magnesium alloy in the processing and molding process, and is also conducive to ensuring the cleanness and high quality of magnesium alloy melt

2 wrought magnesium alloy

wrought magnesium alloy is different from the liquid forming of cast magnesium alloy, but is formed by extrusion, rolling and forging in the temperature range of 300 ℃ - 500 ℃. Compared with cast magnesium alloys, wrought magnesium alloys have higher strength, better ductility, better mechanical properties, and lower production costs due to the elimination of casting structural defects and refinement of grains by deformation processing

2.1 plastic deformation

in wrought magnesium alloys, the commonly used alloy systems are mg Al Zn system and mg Zn Zr system. Mg Al Zn series deformation alloys are generally medium strength and high plasticity deformation materials. The content of aluminum in magnesium is 0-8%. Typical alloys are AZ31, AZ61 and AZ81 alloys. Because mg Al alloys have good comprehensive properties of strength and plasticity and low price, they are the most commonly used alloy systems. Mg Zn Zr alloys are generally high-strength materials, and their deformation ability is not as good as mg Al alloys. They are generally produced by extrusion process. The typical alloy is ZK60 alloy. Mg Mn system is also a high-strength wrought magnesium alloy. Its main advantage is excellent corrosion resistance and weldability, but it has poor casting performance, large shrinkage, hot cracking tendency and less application. In addition, the addition of elements such as nd, th, Yb, SC and Mn can significantly improve the corrosion resistance of wrought magnesium alloys

at present, the plastic forming process of magnesium alloys is mainly forging and extrusion, and a small amount is rolling forming, and all of them need hot working. Therefore, deformation temperature is an important parameter, and deformation rate and stress state are also important considerations

1) forging forming: the forging performance of magnesium alloy depends on three factors: solidification temperature, deformation rate and grain size of the alloy. In order to ensure good processability, AZ and

zk magnesium alloy billets or billets with malleability must be used. The satisfactory grain size of these two series alloys can be obtained by adding grain refiners and alloying elements. However, the grain size of the casting structure generally does not meet the forging requirements. The ingot must be extruded to obtain the required grain size for forging, and then forged at a high speed change rate. Magnesium alloy is forged within 55 ℃ below its solidus temperature, and cracks may be formed when the forging temperature is too low. Hydraulic press and low-speed mechanical press are common equipment for die forging

2) extrusion forming: magnesium alloy can be extruded into various pipes, bars and profiles. Including profiles with concave corners and dark grooves, thin-walled tubes with large diameter and variable cross-section thickness and other difficult to machine products. Extrusion materials are also AZ and ZK magnesium alloys, and the temperature is generally controlled between 300 ℃ - 460 ℃. The specific temperature selection is also related to the specific alloy brand and extrusion shape. Because magnesium will produce a lot of heat during deformation, it must be fully cooled during extrusion, otherwise the alloy temperature may exceed the solidus temperature and cause cracking

3) rolling forming: magnesium cast into a flat shape with round edges

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