Die Casting
Application of magnesium alloy precision die casting in aerospace field
1. Lightweight is one of the important development directions of aerospace component materials. Magnesium alloy is currently the lightest metal structural material in practical application. The density of pure magnesium is 1.74 g/cm3, which is about 2/3 of aluminum alloy, 1/3 of zinc alloy, 1/4 of steel, and titanium alloy. 2/5 of that of most engineering plastics. The application of magnesium alloys can bring huge benefits of weight reduction and a significant improvement in the tactical performance of aircraft. The fuel cost savings brought by the same weight reduction of commercial aircraft and automobiles is nearly 100 times that of the latter, while the fuel cost savings of fighter jets is nearly 10 times that of commercial aircraft, and more importantly, its maneuverability can be greatly improved. Improve its combat effectiveness and survivability. In the aerospace field, magnesium alloys are widely used in the manufacture of important components on aircraft, missiles, spacecraft, and satellites to reduce the quality of parts, improve the maneuverability of aircraft, and reduce the launch cost of spacecraft.
Magnesium alloys have the advantages of high specific strength and specific stiffness, high damping, electromagnetic shielding, good dimensional stability, thermal conductivity, excellent casting, machining performance and easy recycling, etc., and are known as "green engineering in the 21st century". Material". However, magnesium alloys have the following disadvantages: poor corrosion resistance, low ignition point; low material strength, especially poor high temperature strength and creep resistance; magnesium alloy castings are prone to shrinkage porosity and hot cracks, low yield, and plasticity of magnesium alloy deformed parts Processing conditions are difficult to control, resulting in unstable microstructure and mechanical properties. These shortcomings limit its application in the aerospace field.
This paper introduces the research and application status of magnesium alloys in the aerospace field. Combined with the current development trend of high-performance magnesium alloys and the research status of forming technology, the application of magnesium alloys in the aerospace field is prospected.
2. Research status of high-performance magnesium alloys
2.1 High-strength cast magnesium alloy
Casting magnesium alloy has excellent casting performance and machining performance, and is often used in casing and shell parts of aero-engines, helicopter transmission systems, etc., and can well meet the performance requirements of parts for materials. Such magnesium alloys are mainly cast by different liquid forming methods, including Mg-Al series, Mg-Zn series and Mg-RE series.
The earliest application is the Mg-Al alloy. At present, magnesium alloyed with aluminum alloy accounts for about 43% of the total application of magnesium alloys, and aluminum is still the most widely used alloying element in magnesium alloys. Most of the current research is on magnesium alloys based on AZ91. The new high-strength magnesium alloys developed by adding Ca, Y, Sc, Mn, etc., have achieved good application results in the aerospace field. For example, Zhu et al. found that the addition of trace Mn elements in Mg-9Al-2Sn alloy can form Al8 (Mn, Fe) 5 phase, and play a role in grain refinement and aging strengthening. Mg-9Al-2Sn-0.1 After Mn alloy T6 treatment, the tensile strength, yield strength and elongation can reach 292 MPa, 154 MPa and 5%, respectively.
In the Mg-Zn system, Zn mainly plays the role of solid solution strengthening, and the yield limit of the alloy can be increased after heat treatment. In addition, Zn can also eliminate the adverse effects of impurity elements such as iron and nickel in magnesium alloys on corrosion performance. Feng Kai conducted a systematic study on the microstructure and properties of Mg-(5%-20%)Zn-(0-6%)Al alloys by adjusting the ratio of Zn/Al, in order to obtain the alloys with high strength and significant strengthening effect of heat treatment. Magnesium alloy, and the alloy is suitable for semi-solid forming. The study found that ZA54, ZA56, ZA72 and ZA74 alloys (metal type) all have excellent mechanical properties, and the tensile strength of ZA74 can reach 352 MPa after semi-solid thixotropic die casting. Wang et al. added a small amount of Cu element on the basis of the existing Mg-Zn-Al-Mn alloy, the purpose is to make the alloy solid solution at a higher temperature, promote more Zn to dissolve into the magnesium matrix, and increase the subsequent aging strengthening Effect. The study found that the yield strength of the Mg-8.0Zn-1.0Al-0.5Cu-0.5Mn alloy can reach 228 MPa, the tensile strength can reach 372 MPa, and the elongation is 16%. Generally, zirconium with a content of more than 0.5% is added to Mg-Zn alloys, and its purpose is to refine the grains and form Mg-Zn-Zr alloys with higher performance. However, such alloys are sensitive to micro-shrinkage and thermal cracking tendencies. By adding rare earth elements, the casting properties and creep resistance of the alloy can be significantly improved. The ZE41 developed based on this has a high strength at 200℃, and the service temperature of EZ33 can reach 250℃.
Rare earth elements have solid solution and precipitation strengthening effects on magnesium alloys. Adding rare earth elements to magnesium alloys can improve the room temperature and high temperature strength of the alloy, improve the high temperature creep resistance, improve the casting performance, and at the same time help to improve the corrosion resistance, so that the Mg-RE alloy has high high temperature strength, excellent Creep resistance, good heat and corrosion resistance. At present, Y, Nd, Gd, and Sm are the main elements added to Mg-RE alloys. Fu Penghuai optimized the composition of the Mg-Nd-Zn-Zr alloy and found that the addition of trace Zn elements to the Mg-Nd alloy changed the plastic deformation mechanism of the alloy, and the β' or β" phase was precipitated during aging, thereby significantly improving the Mg-Nd alloy. Mechanical properties of Nd alloys. Among them, Mg-3Nd-0.2Zn-Zr (NZ30K) has excellent mechanical properties (see Table 1). Zhang Zhenyan replaced Y and S of WE series with Gd and Sm with higher limit solid solubility. Nd element, Mg-( 6-10 ) Gd-( 2-6 ) Sm-Zr alloy was systematically studied. The results show that the alloy has good solid solution strengthening and aging strengthening effect. For cast alloys, peak aging GS62K alloy The room temperature tensile strength of GS102K alloy is the highest (360 MPa), but the yield strength (204 MPa) is lower; the comprehensive room temperature mechanical properties of GS102K alloy are the best, the yield strength, tensile strength and elongation are: 237 MPa, 347 MPa and 3 respectively. . 2%. He Shangming systematically studied high-strength and heat-resistant Mg-( 6-12) Gd-( 1-3) Y-Zr alloys and found that the yield strength, tensile strength and elongation of GW103K and GW123K were combined: 241 MPa -370 MPa-4.1% and 248 MPa-328 MPa-1.1%, the aging strengthening phase is mainly β' phase.
Table 1 High performance casting and wrought magnesium alloys | ||||
Alloy Composition/wt% | Forming Process | Tensile Properties | ||
Tensile Strength/MPa | Yield Strength/MPa | Elongation/% | ||
Mg9AI-2Sn-0.1Mn | Metal mold casting | 292 | 154 | 5 |
Mg-7Zn4AI | Die casting | 338 | - | 12.1 |
Mg8.0Zn-1. 0AI-0.5Cu-0.5 Mn | Metal mold casting | 372 | 228 | 16 |
Mg-3Nd-0.2Zn-Zr | Die casting extrusion | 300 | 140 | 11 |
Mg-6Gd-2Sm-Zr | Die casting extrusion | 360 | 204 | 8.1 |
Mg-10Gd-2Sm-Zr | Metal mold casting | 347 | 237 | 3.2 |
Mg-10Gd-3Y-Zr | Die casting extrusion | 370 | 241 | 4.1 |
Mg-12Gd-3Y-Zr | Metal mold casting | 328 | 248 | 1.1 |
Mg-5AI-0.3Mn-1.5Ce | Metal mold casting | 203 | 88 | 20 |
Mg-11AI-3Zn-0.4 Mn | Hot rolled | 353 | - | 9.5 |
Mg-9Zn-0.6Zr-0.6Er | Extrusion | 372 | 342 | 18 |
Mg-14.4Zn-3.3Y | Extrusion | 380 | 365 | 8 |
Mg-10Er-2Cu | Extrusion | 380 | 320 | 15 |
Mg-10Er-2Cu-V | Extrusion | 430 | 370 | 11 |
Mg-2.7Nd-0.2Zn-0.4Zr | Extrusion | 417 | 394 | 2.6 |
Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr | Extrusion + Hot Rolling | 473 | 372 | 10.2 |
Mg-3Y-1.5Zn | Equal channel corner extrusion | 473 | 445 | 6.2 |
2.2 High-strength wrought magnesium alloy
The strength and elongation of wrought magnesium alloys are generally better than those of cast magnesium alloys, because after hot deformation of magnesium alloys, the structure is refined, the composition is more uniform, and the interior is denser. Therefore, a large number of wrought magnesium alloys are used in aerospace vehicles, especially missiles, satellites and space shuttles. This kind of alloy composition also includes Mg-Al system, Mg-Zn system and Mg-RE system.
In recent years, domestic and foreign scholars have optimized the composition of existing magnesium alloys and developed different high-strength wrought magnesium alloys. Wang et al. added a small amount of Ce into the Mg-5Al-0.3Mn alloy, and after aging, the Al11 Ce3 phase was precipitated and distributed along the grain boundaries. After hot rolling of Mg-5Al-0.3Mn-1.5Ce alloy, the grains were refined and the size was 15 μm. The yield strength and tensile strength were increased from 88 MPa and 203 MPa to 225 MPa and 318 MPa, respectively. From 20% to 9%. Zhang Jinlong et al. found that after extrusion of AZ113 magnesium alloy, the grain size was reduced from 120 μm to 30 μm, the performance was greatly improved, the tensile strength could reach 353 MPa, and the elongation was 9.5%; after T5 treatment, The tensile strength of the alloy reaches 420 MPa.
Zhang added a trace amount of rare earth Er on the basis of Mg-Zn alloy, and developed a Mg-9Zn-0.6Zr-0.6Er alloy with high strength, high toughness and low cost. Due to the addition of Er in the alloy, a thermally stable phase containing Er and Zn is formed, which improves the deformation ability of the alloy and improves the structure of the alloy. After extrusion, the particle size of the second phase MgZn2 is reduced, and the mechanical properties are improved. Alok Singh et al. studied the quasicrystal reinforced Mg-Zn-Y alloy and found that the yield strength, tensile strength and elongation of Mg-14.4Zn-3.3Y after solution, extrusion and aging treatment were 365 MPa, 380 MPa and 380 MPa, respectively. 8%.
Du et al. studied the effect of V on Mg-Er-Cu alloys containing LPSO structure, and found that the addition of V changed the LPSO structure in the alloy from a coarse block to a fine layer. The combined yield strength, tensile strength and elongation of Mg-10Er-2Cu and Mg-10Er-2Cu-V alloys after extrusion were 320 MPa-380 MPa-15% and 370 MPa-430 MPa-11%, respectively. Zhang et al. extruded the Mg-2.7Nd-0.2Zn-0.4Zr solid solution alloy, and its yield strength, tensile strength and elongation could reach 394 MPa, 417 MPa and 2.6%, respectively. Xu et al studied different extrusion processes, large deformation hot rolling and aging treatment of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy sheets, and found that the mechanical properties of alloys formed by different processes were different. It has high mechanical properties during aging. The yield strength, tensile strength and elongation can reach 372 MPa-473 MPa-10.2%. The improvement of mechanical properties is mainly due to the β' phase in the grain, the LPSO structure and the grain boundary. Precipitation of dispersed cubic Mg-Gd-Y phase. He Shangming's research found that Mg-12Gd-3Y-Zr wrought magnesium alloy was subjected to strain aging treatment after hot extrusion, and its room temperature mechanical properties could reach Rm = 491 MPa, Rp 0.2 = 436MPa, A = 3% ~ 6%. Chen Bin prepared Mg95.5Y3Zn1.5 and Mg97Y2Zn1 by ordinary casting process combined with equal channel angular extrusion. The highest yield strength and tensile strength of Mg95.5Y3Zn1.5 alloy reached 444.6 MPa and 472.7 MPa, respectively. Mg97Y2Zn1 alloy The highest yield strength and tensile strength reached 406.2 MPa and 455.2 MPa respectively. Zhang Zhenyan found that the GS102K alloy extruded at 400℃ + peak aging at 200℃ had the best comprehensive room temperature mechanical properties, yield strength, tensile strength and elongation were 315 MPa, 410 MPa and 4.9%, respectively.
2.3 High damping magnesium alloy
The development and application of high damping magnesium alloy is one of the effective measures to prevent vibration and noise. The damping mechanism of pure magnesium and its alloys belongs to the dislocation damping of defect damping, and its internal friction can be divided into two types: damping resonance type and static hysteresis type. The high damping applied in engineering mainly uses the static hysteresis type which is related to amplitude and independent of frequency. At present, high-strength damping magnesium alloys are mainly developed through alloying, deformation process modification, and modification of existing high-strength magnesium alloys.
Adding Zr, Ni, Mn, Cu, Si, Ca, La, Nd and other elements to magnesium alloys can form different damping magnesium alloys, among which the more typical ones are Mg-Zr series, Mg-Ni series and Mg-Cu-Mn series magnesium alloy. Recently, Fu Penghuai et al. studied the damping performance of high-strength magnesium alloys such as AZ91D, ZK51, WE43, NZ30K, GW103K, GW83K at room temperature, and found that the specific damping performance of the above-mentioned magnesium alloys at 10% yield strength P0. . 71% to 8. 55%, belonging to medium damping performance materials, the order of damping performance is WE43 < ZK51 < GW103K < AZ91D < GW83K < NZ30K. Peak aging heat treatment after solution treatment usually reduces the damping properties of high-strength magnesium alloys, which is associated with finely dispersed precipitates, such as β phase in AZ91D alloy, β' (cbco) metastable phase in WE43, and β in GW103K. 'Metastable phase, β″ (DO19) metastable phase in NZ30K, it is difficult for dislocations to get rid of the strong pinning effect of these precipitation phases, the smaller the area swept by the dislocation movement, the smaller the kinetic energy consumption of material vibration is. , the damping performance of the material is worse. The high temperature overaging treatment may improve the damping performance of the alloy, which is mainly due to the increase of the kinetic energy of vibration consumption of the material due to the coarsening of the precipitates and the reduction of the density of the precipitates.
In recent years, many studies have shown that the LPSO structure of magnesium alloys can improve the damping properties of the alloys while increasing the strength. Tang et al. showed that the LPSO structure can improve the damping properties of Mg-Zn-Y-Zr alloys and explained it with the Shockley incomplete dislocation movement model. Qing et al. found that with the increase of the LPSO structure in the Mg-Ni-Y alloy, the critical strain amplitude of the alloy gradually decreased, resulting in a weakening of the dislocation pinning effect of the alloy and an increase in the damping performance of the alloy. Lu et al. found that in Mg-Zn-Y alloys, the damping performance of alloys containing rod-like LPSO structures was higher than that of alloys containing bulk and layered LPSO structures. Wang et al. also found that in Mg-Cu-Mn-Zn-Y alloy, LPSO can improve the yield strength and damping performance of the alloy. In addition, Xu et al. also found that the stacking fault in Mg-4Er-4Gd-1Zn makes the alloy also have high damping performance. The mechanism by which the LPSO structure improves the damping properties of the alloy cannot be described by the G-L dislocation damping theory and needs to be further studied.
2.4 High temperature magnesium alloy
At present, traditional high-temperature casting magnesium alloys mainly develop Mg-Al-Zn-Ca, Mg-Al-Si, Mg-AL-RE, Mg-Zn-Cu alloys and rare earth magnesium alloys. Good creep performance. In addition, Zhu et al. compared the microstructure, tensile strength and creep properties of different die-casting high-temperature magnesium alloys and found that MRI230D (Mg-6.5Al-2Ca-1Sn-0.3Sr), AXJ530 (Mg-5Al-3Ca-0.2Sr) and The yield strength of AM-HP2+ (Mg-3.5RE-0.4Zn) is better than that of A380 aluminum alloy, but the room temperature toughness is poor. In contrast, AS31 (Mg-3Al-1Si) and AE (Mg-Al-RE) series alloys have good room temperature toughness and lower yield strength than A380 aluminum alloy. The creep resistance of MRI230D, AXJ530 (Mg-5Al-3Ca-0.2Sr), AE44 (Mg-4Al-4RE) and AM-HP2+ at 150 °C and 175 °C is comparable to that of aluminum alloys.
As mentioned above, Mg-RE alloys have high high temperature strength, excellent creep resistance, good heat resistance and corrosion resistance. Among them, WE54 and WE43 rare earth magnesium alloys containing Nd and Y have excellent comprehensive mechanical properties, and the operating temperature is as high as 250 ℃, which is widely used in the aerospace industry. Mordike B L et al. studied several typical creep-resistant rare earth magnesium alloys such as Mg-Y-Zn-Zr, Mg-Zn-Y, Mg-Y-Zr, and found that Y element has a better strengthening effect, and pointed out that WE alloys It is an ideal base alloy system for creep-resistant magnesium alloys; Sc and Mn are added to Mg-Y alloys to develop Mg-4YSc-1Mn alloys with creep resistance better than WE43 alloys. Gao Yan's research found that the tensile strength and yield strength of the as-cast Mg-10Y-5Gd-0.5Zr alloy after T6 treatment were significantly higher than those of the WE54 alloy in the same state. The tensile strength at 300 °C reaches 326 MPa and 261 MPa, respectively. Zhang Zhenyan's research found that the tensile strength of the peak-aged Mg-Gd-Sm-Zr extrusion alloy decreases with the increase of the test temperature (T). The tensile strength is above 250 MPa, and the mechanical properties drop significantly when the temperature is higher than 200 ℃, and the drop exceeds that of the peak-aged cast alloy. Except for GS62K alloy, the tensile strength of other Mg-Gd-Sm-Zr alloys from room temperature to 250℃ is 30-85 MPa higher than that of WE54 commercial forged heat-resistant magnesium alloy. Extruded alloys are suitable for room temperature to 200°C, and cast alloys are suitable for room temperature to 250°C. Yin Dongdi's research found that Mg-11Y-5Gd-2Zn-0.5Zr (WGZ1152) T6 alloy in the range of 25 ~ 400 ℃, the tensile strength and yield strength are significantly better than commercial heat-resistant magnesium alloy WE54-T6 and piston alloys Heat-resistant aluminum alloy AC8A-T6. Its tensile and yield strength at 300 °C are higher than 250 MPa and 225 MPa, respectively. In addition, under the same stress condition at 300 °C, its minimum creep rate is two orders of magnitude lower than that of WE54-T6, more than one order of magnitude lower than that of AC8A-T6, and comparable to HZ32-T5 heat-resistant magnesium alloy with the highest application temperature. He Shangming's research shows that the high temperature stability of Mg-Gd-Y-Zr alloy system is very good, and it still has good mechanical properties between 200 and 250 ℃. The creep resistance and corrosion resistance of this alloy system are also very good. The instantaneous high temperature tensile strength of GW123K and GW102K is higher than that of 2618 heat-resistant aluminum alloy and WE54 commercial heat-resistant magnesium alloy.
3、Magnesium alloy forming process
Magnesium alloy forming process can be mainly divided into liquid forming process and solid state forming process. Among them, liquid forming mainly includes gravity casting, low pressure casting, die casting, semi-solid casting, squeeze casting, etc. Solid-state forming, also known as plastic forming, includes extrusion, forging, rolling, stamping, deep drawing, etc. In addition, there are some new forming technologies, such as rapid solidification/powder metallurgy, spray deposition, etc.
3.1 Liquid forming process
3.1.1 Gravity casting
Magnesium alloys can be produced by different gravity casting methods, such as sand casting, investment casting, metal casting, semi-metal casting, shell casting, etc. Among them, the sand casting of magnesium alloys has gone through the development stage of ordinary clay sand, water glass sand and no-bake resin sand. The use of no-bake resin sand molding core-making process can improve the quality of castings, simplify the process procedures, facilitate the realization of mechanized and automated production and improve the service life of molds, and reduce harmful gases. It is the direction of large and complex castings toward precision. Investment casting, also known as the lost wax method, usually coats several layers of refractory material on the surface of the wax mold. After it is hardened and dried, the wax mold in it is melted to form a shell, which is then fired and then poured. Get castings. Because the obtained casting has high dimensional accuracy and surface finish, it is also called "investment precision casting".
3.1.2 Low pressure casting
Magnesium alloys generally have small heat capacity and large solidification interval, and are prone to casting defects such as cracks, uneven filling, segregation and coarse structure, and it is difficult to produce large, thin-walled or complex castings. High-quality magnesium alloy castings can be produced by utilizing the smooth filling and sequential solidification characteristics of low-pressure casting. Li Xinlei [34] et al. developed a special mixed protective gas system for low-pressure casting of magnesium alloys, which realizes the mixing of compressed air, SF6 and CO2 in proportion, and provides three-way gas supply branches for atmospheric pressure smelting, low-pressure process protection and leakage protection. road. The gas mixing system can achieve effective flame retardant protection for the whole process of low pressure casting of magnesium alloys. In addition, they also proposed a dual-station structure design, through the movement and lifting of the trolley, to realize the switching of two magnesium alloy low-pressure casting equipment between different stations, to ensure the continuous production of magnesium alloy castings. Low pressure casting continuous production technology. Ding Wenjiang et al. [2] combined the coating transfer core technology, the crucible liquid metal sealing technology and the low pressure casting technology, and developed the precision low pressure casting molding process for large magnesium alloy castings, and adopted the double furnace furnace and pressure converter method to ensure High purity of magnesium liquid. This process has many advantages, such as precision molding, high dimensional accuracy, dense pressure solidification structure, and smooth coating transfer surface. At present, it also has the ability to develop and trial-produce magnesium alloy castings with a mass of 100 kg in small batches.
3.1.3 Die casting
Magnesium alloy has a low melting point (about 650 ℃ for pure magnesium), small latent heat of solidification, fast solidification speed, low viscosity of alloy liquid and good fluidity, which is especially suitable for die casting production. However, conventional die-casting parts cannot be heat treated and cannot be strengthened by aging. The vacuum die-casting, oxygen-filled die-casting and semi-solid die-casting developed in recent decades can solve this problem. Vacuum die casting first removes the gas in the cavity during the die casting process, thereby reducing or even eliminating the pores and dissolved gases in the die casting, and improving the mechanical properties and surface quality of the casting. Oxygen-filled die casting is to fill the cavity with oxygen or other reactive gases to replace the air in the mold before the melt is filled. During the filling process, the reactive gas reacts with the molten metal to form dispersed metal oxides, so as to eliminate the gas and pores in the die casting. Semi-solid die casting can be divided into rheological die casting and thixotropic die casting. Rheological die-casting is to transfer the prepared semi-solid slurry directly to the pressure chamber for die-casting, while thixotropic die-casting is to reheat the prefabricated semi-solid ingot with fine structure to the semi-solid interval for die casting. Whether it is rheological die casting or thixotropic die casting, the molten metal fills smoothly in the semi-solid range, avoiding the turbulent filling method, so it can significantly reduce the loose shrinkage of the die casting.
In addition, Harbin Institute of Technology and Hong Kong Jiarui Group have cooperated in recent years to develop a new process - casting and forging dual-control forming technology. The process can continuously complete both die casting and forging processes in the same forming process, while achieving precise control over the shape, size and performance of the part. Casting and forging dual-control molding can not only control the shape and size of parts like die casting, but also can make the parts plastically deform, and the strength can be greatly improved, and the strength can also be improved by heat treatment.
3.1.4 Semi-solid casting
Semi-solid forming is a new and advanced process method. Compared with traditional liquid forming, it has the advantages of low forming temperature, long die life, improved production conditions and environment, refined grains, reduced pores, loosened shrinkage cavities, and improved microstructure. Density, improve casting quality and other advantages.
Semi-solid forming can be divided into rheological forming and thixo forming. At present, in the preparation of semi-solid slurry, scholars at home and abroad have invented different preparation processes, all of which aim to obtain an ideal semi-solid structure in which small, round primary particles are uniformly suspended in the liquid phase, and then combine different processes Forming is carried out to give full play to the advantages of semi-solid forming. Among them, semi-solid thixotropic injection molding is the most mature, with the advantages of simple process and high degree of automation, and has been widely used in the production of magnesium alloys.
3.2 Solid state forming process
3.2.1 Extrusion
Magnesium alloys have poor plasticity and are suitable for extrusion, generally warm extrusion and hot extrusion, and the extrusion temperature is usually 300 to 450 °C. Magnesium alloy extrusion has the following advantages: grain refinement, increased strength by retaining the extruded fiber texture, excellent surface quality and good dimensional accuracy. At present, magnesium alloy pipes, bars, profiles, strips and other products are mainly formed by extrusion. However, magnesium alloy extrusion also has disadvantages such as slow extrusion speed, large deformation resistance, and anisotropy of material mechanical properties due to the formation of texture after extrusion processing.
3.2.2 Forging
Magnesium alloy forging generally has two methods: free forging and die forging. Commonly used magnesium alloys for forging are ZK series and AZ series magnesium alloys. The mechanical properties of magnesium alloy forgings usually depend on the degree of strain hardening produced during the forging process. The lower the forging temperature is, the more significant the strain hardening effect is. However, when the temperature is too low, the forging is easy to crack, and when the temperature is too high, the oxidation is serious. A new technology developed in the traditional forging process, precision forging, can form high-precision, complex-shaped forgings with streamlines distributed along the geometrical shape of the forgings, and at the same time can improve the bearing capacity of the forgings. In addition, the magnesium alloy has a narrow forging temperature range, the thermal conductivity is twice that of steel, and the contact mold cools down quickly, which easily leads to a decrease in plasticity, an increase in deformation resistance, and a decrease in filling performance, so it is suitable for isothermal forging. Precision stamping and forging technology is a new process that combines stamping and forging, and it is a promising process for magnesium alloy forming. Mainly, the heated magnesium alloy billet is punched and forged in a heating die, and an ordinary mechanical forging machine can be used. The key technology lies in the design of the forming die and forming process, as well as the control of parameters such as die temperature, deformation rate and deformation speed. . Compared with die casting and semi-solid forming processes, this technology has the advantages of high production efficiency, high yield and low cost.
3.2.3 Rolling
Strips and sheets of magnesium alloys are generally produced by rolling. The rolling process can refine the grains, improve the structure of magnesium alloys, and significantly improve the mechanical properties of magnesium alloys. The rolling temperature is a key parameter in the rolling process of magnesium alloys. When the rolling temperature is too low, high stress concentration can lead to twin nucleation and shear fracture; when the rolling temperature is too high, the grains tend to grow and the hot brittleness of the sheet increases.
3.2.4 Magnesium plate
Different temperature drawing process magnesium alloy has poor plastic deformation ability at room temperature, and it is almost impossible for magnesium alloy sheet to be stamped at room temperature. Even under high temperature isothermal stamping, its formability is still poor. Fan Likun et al. developed a magnesium plate differential temperature deep drawing process and related sheet metal forming experimental machine. Through computer program control, different parts of the magnesium plate can be differentially heated according to the degree of deformation during deep drawing, and the dynamic control can be accurately controlled. The blank holder force comprehensively utilizes the deformation ability of magnesium alloy at high temperature and the work hardening ability at low temperature, so as to achieve a higher ultimate drawing ratio at a lower forming temperature.
4. Research and application of magnesium alloys in the aerospace field
The application of magnesium alloys in the aerospace field is of great significance. For every 1 kg reduction in the load mass, the take-off mass of the entire launch vehicle can be reduced by 50 kg, and the structural mass of the ground equipment can be reduced by 100 kg; if the mass of the fighter is reduced by 15%, the rolling distance of the aircraft can be shortened by 15%, and the range can be increased by 20%. %, increasing the payload by 30%; the jet engine structure can be reduced by 1 kg, the aircraft structure can be reduced by 4 kg, and the ceiling height can be increased by 10 m. Magnesium alloys began to be used in aviation in the 1920s. Table 2 shows the application of magnesium alloys in the aerospace field in the 20th century. It can be seen from the table that magnesium alloys are mainly used in the manufacture of aircraft, bombers, missiles and other military equipment. During the war, the weapons and equipment suffered a lot of battle damage, the service life was short, and the shortcomings of poor corrosion resistance of magnesium alloys were concealed. In the 1970s and 1980s, due to the discovery of poor corrosion resistance of magnesium alloys, especially poor electrochemical corrosion, fatigue and creep resistance, and the rapid development of aluminum alloys, the amount of magnesium alloys was greatly reduced. In the 1990s, under the promotion of the development of the automobile industry, some existing problems of magnesium and its alloys were solved, which brought the second climax of research and application of magnesium alloys, and also caused the aerospace industry again. Interest in using magnesium materials.
Table 2 The application of magnesium alloy in the 20th century in the aerospace industry | |
Era | Application |
20's | Aircraft propeller |
30's | Engine crankcase, engine parts. Balloon basket. Passenger aircraft seats, landing wheels |
40's | JU88 Landing Gear Support Frame, FW190 Windshield Bracket, Landing Gear Hell&Jum90 Parts. BMW801 Engine Parts, Machine Gun Bracket Ring, Radio Equipment Base, Directional Gauge, Nylon Wheel, B:36 Bomber Parts |
50's | RR Dat engine components. S55 helicopter engine base. Rocket and missile components. Helicopter gearboxes, wheel and engine components, main landing wheels. C421 and C124 transport aircraft floor beams |
60's | B-47 and B-52 main landing wheels, satellite parts. HC-helicopter floor, aircraft cockpit roof frame, Appllo vibration monitoring equipment. S64B landing gear gearbox |
70's | F20 reduction gear and cockpit roof frame.CH53E helicopter transfer box |
80's | Helicopter drive train, PW 100 submerged wheel engine parts, Garrett TPE331 and other turbine engine parts. Constant speed transmission. Auxiliary power equipment, intake pipe, jet engine transmission gearbox |
4.1 Research and application status of foreign aerospace field
4.1.1 Research progress on magnesium alloy investment
Entering the 21st century, magnesium and magnesium alloys are re-entering the aerospace stage. Since 1996, the European Union has carried out a series of projects in the fields of FP4, FP5 and FP6 to study magnesium alloys. Among them, there are two main projects that play a guiding role in the future aerospace field. The details are as follows:
From March 2005 to December 2008, the European Union implemented a research project called "Aeronautical application of wrought magnesium (AEROMAG)" within the framework of FP6, with a total of 20 units participating. Among them, there are 3 aerospace manufacturing companies, namely Airbus, Eurocopter and Italian Alenia, 2 Russian research institutes, namely St. Petersburg Institute of Light Metals and Moscow Institute of Aeronautical Materials, and With the participation of 7 universities and 8 magnesium production enterprises, all-round research on the smelting, forming process, combustion performance, surface treatment, connection technology and structural performance of magnesium alloys has been carried out, and a series of achievements have been obtained.
The magnesium alloys used in this project are mainly Mg-Al-Zn, Mg-Zn-Zr-Re and Mg-Y-Re alloys. Figure 1 shows the mechanical properties of commercial alloys and newly developed alloys. A series of research results show that magnesium alloys can replace medium-strength 5xxx aluminum alloys. For a certain performance, magnesium alloys can reach or even exceed high-strength 2xxx alloys, but comprehensively consider strength, fatigue, processing, corrosion resistance, temperature resistance and other properties. , there is no magnesium alloy equivalent to 2xxx aluminum alloys.
This shows that in a long period of time, it is impossible for magnesium alloys to partially replace aluminum alloys at the structural manufacturing level, but they are widely used in the aviation industry. From August 2006 to September 2009, the EU implemented a sister project of the AEROMAG project within the framework of FP6, the Development of New Magnesium Forming Technologies for the Aeronautics Industry (MAGFORMING). A total of 12 units participated in the project. The project utilizes different forming techniques such as forging, superplastic forming, roll bending, bladder hydroforming, deep drawing and creep forming to manufacture different aerospace parts. The MAGFORMING project has developed a number of viable commercial forming methods for conventional and novel magnesium alloys. Based on the results of these two projects, Ostrovsky I et al. predicted that during 2015-2020, the use of 10% to 15% magnesium alloy components in civil aircraft is a realistic target.
In addition, Germany has also increased its investment in magnesium alloy research since 2006. In February 2015, the German seat manufacturer ZIMFLUGSITZ GmbH has successfully used the Elektron® 43 magnesium alloy provided by MagnesiumEletron in the United States to produce an aviation seat (see Figure 2), which maintains strength and toughness on the premise. , replacing the existing aluminum alloy seats, reducing the weight by 25%. This is a huge progress for wrought magnesium alloys to replace aluminum alloys in aviation. If it can pass the strict test of the US Federal Aviation Administration (FAA), the requirement that magnesium alloys cannot be used in the SAE standard AS8049 will be cancelled, and a new SAE standard AS8049c will be released in 2015. Elektron® 43 magnesium alloy can be predicted to bring lightweighting benefits to commercial aircraft designers and operators in the near future. It is certain that the significant advantages of magnesium and magnesium alloys, the future aerospace industry will increasingly rely on the huge potential of magnesium alloys in terms of weight reduction.
4.1.2 Application of magnesium alloys in aerospace vehicles
Magnesium alloy composite materials have been used in structural parts such as brackets, bushings, and beams on the US Navy satellite, and their comprehensive performance is better than that of aluminum matrix composite materials. The starter rocket "Hercules" of the "Dreenay" spacecraft
600 kg of wrought magnesium alloys were used; 675 kg of wrought magnesium alloys were used in the "Tiscaviller" satellite; the "Verger" rocket casing with a diameter of about 1 m was made of magnesium alloy extruded tubes of. The British Bloodhound MK-2 (projectile diameter 546 mm, maximum speed Mach 2.7) surface-to-air missile pods are made of new heat-resistant magnesium alloy castings and forgings. Taking advantage of the low density of magnesium alloys, Japan has developed a structural optimization design method aimed at improving the characteristics of supersonic aircraft with magnesium alloy wings, and has successfully produced ultra-small artificial satellites with a mass of only 1 kg using magnesium alloys.
The kinetic energy penetration and damage missile KEPD-350 (Figure 3), developed in cooperation with German company Taurus Systems and Swedish company Bofors, was delivered in 2010. The full bomb is 5 m long, has a range of 350 km, and has a flight Mach number of 0.9 to 0.95 M. In the missile structure, more than 30 kinds of parts such as reinforcing frame, wall plate, rudder surface and partition plate are made of high-performance magnesium alloys such as GW83 and ZK61 of about 100 kg respectively. The total mass of the whole bomb is controlled within 1400 kg, which ensures the damage efficiency of the warhead, and can penetrate 4 layers of steel reinforced concrete layers and then explode. In 2007, the AGM-154C developed by the American Raytheon Company for the U.S. Navy (Figure 4) is equipped with the British BAE Systems Broch Penetration Warhead (227 kg), the projectile length is 406 cm, and the gliding distance of low-altitude throwing It can reach 22 km, the gliding distance of high-altitude throwing can reach 120 km, and the hit accuracy can reach within 3 km. It can effectively attack solid tactical targets such as industrial facilities and logistics systems. Among them, the charge warhead can penetrate 125 mm thick armor plate, and each fragment weighs about 30 g, which can damage light vehicles within 15 meters and aircraft within 75 meters. In order to ensure the effective damage capability of the warhead, in addition to a large number of aluminum alloys, a certain amount of high-strength and tough cast magnesium alloys are also used, such as connecting cabins, tail cabins, wing skeletons, equipment boxes, etc. The modified AZ91E and AZ91D can meet the overall requirements of penetrating bombs in terms of mechanical properties and corrosion resistance. The mass of the full bomb is controlled within 483 kg.
4.2 Research and application in the domestic aerospace field
The magnesium alloys used in the aerospace industry in my country mainly include cast rare earth magnesium alloys ZM2, ZM3, ZM4, ZM5, ZM6, ZM9 and deformed rare earth magnesium alloys MB25 and MB26. Among them, ZM2 is used in the front casing, rear casing and main casing of turbojet-7 and turbojet-13 engines. ZM3 is used to manufacture the front cabin castings of the turbojet-6 engine of the J-6 aircraft and the centrifugal casing of the turbojet-11 engine; ZM4 is used to manufacture the casing of the aircraft hydraulic constant speed device. The accessory drive case and reducer case of a turboprop engine are made of ZM5 (Fig. 5), the accessory case of the Kunlun engine developed in my country is made of ZM5 magnesium alloy, and the accessory drive rear case of a gas turbine starter (Fig. 6) ) ZM6 magnesium alloy is selected. A certain type of helicopter main reducer main box adopts ZM6 magnesium alloy, which can be used in marine environment. MB25 can manufacture aircraft fuselage truss and operating system bolt arms, supports and other stress components. my country's fighters, bombers, helicopters, transport aircraft, airborne radars, surface-to-air missiles, launch vehicles, satellites and spaceships all use rare earth magnesium alloy components.
According to relevant information, nearly 400 magnesium alloy structural parts were selected for a certain type of aircraft. The body of the Hongqi-9B missile developed and produced by our army is made of high-strength magnesium alloy materials, so the total mass of the missile body is controlled to 1200 kg, the volume is also greatly reduced, and the maximum speed is increased to Mach 6.
At present, my country's universities, research institutes and aerospace enterprises have also done relevant research on the application of magnesium alloys in the aerospace field, and have achieved a series of results. Lu Yan from Huazhong University of Science and Technology studied the forming process of complex-shaped magnesium alloys, and successfully formed the complex magnesium alloy aircraft upper receiver by isothermal precision forging process.
Using anti-gravity vacuum low pressure lost foam casting method, various complex Mg alloy castings such as aircraft guide wheels and exhaust pipes were cast and formed. Beijing Satellite Manufacturing Plant has carried out a lot of technical research and equipment transformation in recent years, breaking through the anti-corrosion treatment, machining and welding technology of large-scale magnesium alloy surfaces, and realizing the application of large-scale magnesium alloy structural parts in many spacecraft. Carrying out technical research on magnesium alloy surface coating, micro-arc oxidation, high-emissivity surface anodizing treatment, etc., breaking through the comprehensive surface treatment technology of magnesium alloy surface anti-corrosion, electrical conductivity and high-emissivity thermal control requirements, realizing magnesium alloy in The application in the chassis of spacecraft electronic products achieves the goal of lightweight products. The G04 magnesium alloy developed by the Institute of Metal Research, Chinese Academy of Sciences was successfully applied to the electric control box of the Shenzhou spacecraft (SZ-6), reducing its weight by about 13 kg. The part is specifically used in the space environment from the earth's surface to the orbit of a low-Earth orbit spacecraft. Subsequently, the alloy has successfully produced other spacecraft parts such as Tiangong-1. Combining advanced magnesium alloy materials with new molding processes, Shanghai Jiaotong University has successfully produced a certain type of light missile cabin (Figure 7), the engine casing, a certain type of light missile wing, and a seamless tube with a diameter of 145 mm for a certain type of missile. Preparation of light missile casing (Figure 8), a certain type of helicopter tail reduction casing and a certain type of missile casing (Figure 9), a certain type of radar components (Figure 10).
4.3 Comparison of research and application status at home and abroad
The demand for magnesium alloys in the aerospace field at home and abroad is basically the same. However, the research and development and application of magnesium alloys abroad are relatively early. In the 20th century, magnesium alloys with different compositions such as AZ91E, EZ33, ZE41, QE22, EQ21, WE43, and WE54 have been developed for the aerospace field. As mentioned in 4.1, the United States, Germany and other developed countries in Europe have carried out a number of research projects on magnesium alloys, and have achieved many beneficial results, and their industrial base is also relatively strong. The research and application of magnesium alloys in my country is relatively late. During the 10th Five-Year Plan, my country has successively started the development and industrialization of related magnesium alloy materials in the key research plan and the 863 plan, which effectively promoted the application of magnesium alloys in my country.
At present, in the development of high-performance magnesium alloys, the newly developed magnesium alloys in my country and foreign commercial alloys have slight differences in alloy composition. For example, commercial Elektron 21 and NZ30K magnesium alloys have been applied in the aerospace field, and Elektron 21 is an alloy formed by adding a small amount of Gd element to NZ30K. This shows that on the existing high-strength magnesium alloys in my country, the composition can be adjusted by micro-alloying to suit different occasions, so as to commercialize the alloy and standardize the product. In terms of the production and application of high-performance magnesium alloy products, domestic products are currently limited to some simple and small-volume non-stressed/secondary load-bearing structural parts used in low temperature environments, while foreign countries have some large, complex and load-bearing structural parts. application. In addition, there is still a certain gap between my country's corrosion protection technology and foreign countries, and the casting yield is also low. The main reason is that my country's research on advanced magnesium alloy materials is relatively short, and the industrial base is also relatively weak.
5. Outlook
With the rapid development of my country's aerospace industry, lightweight is bound to become the mainstream of the aerospace manufacturing industry, and the application of new lightweight and high-strength magnesium alloy materials in the aerospace field will become more and more extensive. my country is a country with large magnesium alloy resources. It is undoubtedly the best choice at present to accelerate the development of magnesium alloy technology, improve the technical level of my country's magnesium industry, and transform my country's magnesium industry from resource advantages to economic advantages. The promotion and application of new magnesium alloy materials in the aerospace field requires mutual cooperation between relevant universities, research institutes and aerospace enterprises, continuous innovation in technology, standardization of aerospace magnesium alloy products, and expansion of the application scope of magnesium alloys in the aerospace field.