Research Status and Future Development Trend of Die-casting Aluminum Alloy (Part 1)
Aluminum is the most abundant metal element on the earth. After being made into aluminum alloy according to functional needs, it still has low density, high specific strength, good corrosion resistance, good thermal stability, good mechanical processing performance, stable recycling performance, and recycling efficiency. High, low cost and other advantages. In March 1994, Audi launched the first luxury sedan Audi A8 with the world's unique Audi AFS (Aluminum Space Frame, AFS) at the Geneva Motor Show. The weight of the vehicle is reduced by 50% and the strength performance is increased by 60. %. In 2013, the American Ford Centennial Ford GT vehicle adopted an all-aluminum structure, and most of the outer body panels were manufactured using aluminum alloy superplastic forming technology, achieving a 64% reduction in vehicle weight. At present, as far as the global automobile industry is concerned, the average consumption of aluminum alloy per car exceeds 120Kg, which accounts for about 10% of the total vehicle weight. The trend of replacing steel with aluminum is increasing.
The current advanced manufacturing processes mainly include: laser welding, hydroforming, squeeze casting and high vacuum die casting processes, etc. The widely used lightweight materials include high-strength steel, aluminum alloy, magnesium alloy, reinforced plastics and composite materials, etc., and the structure is optimized The design includes topology optimization, size optimization, shape optimization, and multidisciplinary comprehensive design optimization. Compared with other manufacturing processes, the die-casting forming process has high production efficiency, high dimensional accuracy, excellent mechanical properties, high material utilization, and better economic benefits for mass production. Among aluminum alloys for automobiles, die-cast aluminum alloys and other cast aluminum alloys account for about 80%, and processed aluminum materials (plates, strips, foils, tubes, rods, shapes, wires, forgings, powders, pastes, etc.) account for only about 20% , The consumption of die-casting parts accounts for about 70% of the total consumption of casting products, so die-casting aluminum alloy products account for about 54% to 70% of automotive aluminum. Since 2009, my country's automobile sales have occupied the world's first place for eight consecutive years, increasing from 13 million in 2009 to over 27 million in 2019.
In recent years, with the increasingly fierce competition in the automotive market, various manufacturers are developing in the direction of high quality, high reliability, light weight, energy saving, environmental protection, and low cost. With the stricter international environmental protection policies year by year, new energy vehicles have become In the future, the key development direction of the rail transit field, high-strength, high-quality new energy vehicle structural components (automobile body, column, chassis, shock tower, etc.) will also increasingly need to be manufactured by die-casting process. All these show that aluminum alloy die-casting occupies a pivotal position in the die-casting industry and is the mainstream of the die-casting industry.
01. Die-cast aluminum alloy classification
In addition to better die-casting process performance and better mechanical properties, die-cast aluminum alloy also needs to have the following process properties:
(1) Good thermoplastic rheological properties. It should have good thermoplastic rheological properties when the superheat is not high and near the liquid and solidus temperature to achieve the filling of complex cavities, form a good casting surface, and avoid shrinkage defects. The production.
(2) The smaller linear shrinkage rate prevents cracks and deformation during the die-casting process and improves the dimensional accuracy of the product.
(3) The small solidification temperature range facilitates rapid simultaneous solidification and reduces the number of defects such as internal shrinkage holes.
(4) Good high-temperature thermal strength, to avoid thermal cracking or serious deformation during mold opening.
(5) Good casting/mold interface performance, no chemical reaction with the die-casting mold, low affinity, and avoid sticking and alloying reaction at the casting/mold interface.
(6) Good physical and chemical properties, it is not easy to inhale and oxidize in the high-temperature molten state, so as to meet the needs of long-term heat preservation during the die-casting process.
According to the alloy composition, die-cast aluminum alloy can be divided into four series: Al-Si (Al-Si-Cu, Al-Si-Mg), Al-Cu, Al-Mg and Al-Zn. The crystallization temperature interval is small, the silicon phase solidification crystallization latent heat and specific heat capacity are large, the linear shrinkage rate is small, and it has good fluidity, filling performance and small thermal cracking and loose tendency, so it is the most widely used. Although Al-Cu die-casting alloy has high mechanical properties, the addition of Cu element reduces the corrosion resistance of the material, and the service life of die-casting products is greatly reduced, and the die-casting process is prone to segregation and cracking, so the application range is relatively wide. Small. Compared with Al-Si series die-casting alloys, Al-Mg series die-casting alloys have poor casting properties, large fluctuations in mechanical properties and large wall thickness effects, easy cracking during die-casting, and a greater tendency to stress corrosion; Al-Zn series die-casting alloys have a natural Aging can obtain better mechanical properties, but its corrosion resistance is poor, and it is prone to thermal cracking and stress corrosion, so it is less used. Table 1 shows the grades, chemical composition and mechanical properties of commonly used die-casting aluminum alloys. Among them, Al-SI-Cu alloys are the most widely used, such as AlSi9Cu3 (A380) and AlSi11Cu3 (ADC12). As shown in Table 1, die-cast aluminum alloys generally add Si, Cu, Mg, Mn, Fe, Ni and rare earth elements.
Since the solidification latent heat of Si element is much greater than that of Al element, adding an appropriate amount of Si can improve the process flow properties of the alloy, reduce the tendency of hot cracking, and improve the air tightness, corrosion resistance and thermal conductivity of the material. When adding 25% Si element , The shrinkage rate of the solidified body of the die-casting alloy can be reduced to 0, which can be used to manufacture pistons and other die-cast products that require extremely high dimensional stability. Fe element can improve the demoulding performance. Generally, the addition amount needs to be greater than 0.6%. Excess Fe element is easy to form needle or flake β-AlFeSi phase, which reduces the plasticity of the alloy. You can increase the content of Fe element while adding an appropriate amount of Mn element (≤ 0.5%) to improve the demoulding performance of the alloy. The addition of Cu can enhance the corrosion resistance, mechanical strength and thermal conductivity of the alloy, and improve the process fluidity, creep resistance, fatigue strength and machining performance of the alloy; when Cu is used as a strengthening phase, it is solid-soluble in the aluminum matrix or particles In the presence of crystalline compounds, the strength and hardness of the alloy can be significantly improved, but the elongation is slightly reduced; when Cu forms a network compound, the strength and elongation of the alloy are greatly reduced. The Mg element can improve the corrosion resistance and mechanical strength of the alloy, and the tendency of the mucous film will be reduced accordingly, which improves the machining performance of the alloy. Al-Si-Cu die-casting alloy materials usually add 0.5-1.0% Sm element to refine the size of the eutectic silicon phase, reduce its secondary dendrite arm spacing, and improve the strength and plasticity of the alloy; add a proper amount of Sr element (0.02～0.08%) While improving the morphology of eutectic silicon, it can achieve grain refinement and effectively improve the comprehensive mechanical properties of die-casting alloys. The addition of Ni element with a content of 1 to 1.5% can improve the strength, hardness and corrosion resistance of the alloy, and significantly reduce the damage of the alloy to the mold. At present, the most commonly used die-cast aluminum alloy A380 in China increases the content of Si, Mn and Mg, reduces the content of Fe and Cu, and adds 0.04% Sr and 0.05% Be elements, and the elongation can be increased to more than 5.0%.
02. Typical application of die-cast aluminum alloy
The development of the die-casting industry is closely related to the automobile industry. In recent years, people’s requirements for automobiles have gradually tended to indicators such as high performance, low pollution, and low energy consumption. The weight of automobiles plays a decisive role in fuel economy. Reduced by 0.7L/100Km, lightweight design has become the most critical indicator of current fuel-based and new energy vehicle design. In the 1940s, the Italian Fiat Automobile Company developed aluminum alloy cylinder heads and applied them to some models; in the 1950s, a German automobile company improved the low-pressure casting technology and produced aluminum alloy die castings with complex structures, and began to mass Production of aluminum alloy die-casting parts such as engine back cover and air-cooled cylinder head; since the 1960s, due to the development of high-pressure die-casting technology, the application of die-cast aluminum alloy in automobiles has increased significantly, gradually replacing cast iron, laying the foundation for the widespread use of aluminum alloy in the modern automobile industry The foundation of die casting.
At present, the application scope of aluminum alloy die castings in the automobile industry at home and abroad is classified according to the use function. They have been used in structural parts, stress parts, safety parts and decorative parts, mainly including the following aspects:
(1) Power system: cylinder block, cylinder head, cylinder head cover, crankcase, cylinder head cover, oil pan, piston, pump body, pump cover, intake pipe, generator housing, engine gear chamber, six Rocker arm seat, various engine supports, etc.
(2) Transmission system: transmission housing, transmission oil circuit board, clutch housing, shift fork, gearbox bracket, etc.
(3) Steering system: chain cover, rack housing and turbine housing.
(4): Chassis assembly: suspension bracket and beam.
(5) Body: wheel hub, frame and decorative products.
(6) Others: the lower end cover of the shock absorber, the compressor bracket, the clutch pedal and the brake pedal, etc.
03. Application research on microstructure and properties of die-cast aluminum alloy
The injection pressure commonly used in die-casting ranges from thousands to tens of thousands of KPa, up to 2×105KPa, and the injection molding speed is about 10-50m/S, sometimes even as high as 100m/S, so the filling time is very short. Generally within 0.01～0.2s. Compared with other casting processes, the process characteristics of die casting are mainly reflected in "high-speed filling and high-pressure solidification". Under high-speed filling, the melt can quickly fill the mold cavity and accurately replicate the size, shape and surface characteristics of the cavity; however, the liquid metal in the high-speed filling process is in a turbulent state, which is easy to form holes and defects, and the entrained gas is covered There is an oxide film (about 1μm thick) wrapped by the free surface, which significantly reduces the mechanical properties of the alloy material. Combined with the rapid cooling of the metal cavity under high pressure, the alloy melt can be rapidly solidified in a very short time to obtain a uniform structure and a solidified structure with fine grains. The surface of the die-cast product is usually a porous surface structure, and the outermost layer is fine. The dendritic α-Al matrix structure, from the outside to the inside, is the Al-Si eutectic structure with increasing volume fraction.
For die-cast aluminum alloys, the primary α-Al matrix phase and Si phase in the alloy can usually be refined by adding rare earth elements, and at the same time, the silicon phase modification of the eutectic Si phase can be realized, and the strength and ductility of the alloy material can be improved. There are three main forms of rare earth elements in die-cast aluminum alloys, which are solid solution in α-Al matrix, segregation in phase boundary, grain boundary and dendrite boundary, and solid solution in compound or in compound form. . After adding a proper content of rare earth elements, the rare earth elements are easy to be enriched at the front of the solid-liquid interface during solidification, which increases the concentration gradient of the front liquid phase, resulting in component subcooling. When the subcooling is greater than the critical subcooling required for nucleation At this time, the liquid phase nucleation rate of the alloy will be greatly increased, the secondary dendrite spacing of the alloy will be reduced, and the melt will be refined, so that the clusters will increase and decrease, and the grain growth time will be prolonged. The delay improves the flow properties of alloy materials.
After 1.0% Sm is added to ADC12 die-cast aluminum alloy, the eutectic Si phase changes from coarse needles to short rods and fine spherical structures. SDAS is reduced from 51μm to 15μm, and the average grain size is reduced from 90μm to 40μm, as shown in Figure 5. This is the microstructure test results of A380 die-cast aluminum alloy with different content of Sm. After adding trace La element and undergoing solution aging heat treatment, the coarse and massive polygonal primary silicon in the alloy structure disappeared, and the eutectic silicon morphology changed from long needles to short rods with dispersed distribution; the morphology of harmful compounds was improved and solid solution The higher the temperature, the more harmful intermetallic compounds are dissolved and the finer the crystal grains. When the addition amount of La element is 0～0.15%, the tensile strength, hardness and impact toughness of the alloy continuously increase with the increase of the addition amount, and the mechanical performance peak is achieved at the addition amount of 0.15%. When the addition amount of the La element exceeds 0.15 % When it is easy to form a needle-like rare earth-rich phase, which deteriorates the mechanical properties of the material. Adding a proper amount of Fe element (>0.6%) to die-casting aluminum alloy can significantly improve the mucosal phenomenon of die-casting molds, but when the content of Fe element is too high, it is easy to form a large number of Al3Fe, Al7Fe2, Al-Si-Fe flake structure phases , Split the α-Al matrix structure, deteriorate the plastic toughness of the alloy material; when the Fe element addition exceeds 1.0%, the excessive Fe element strengthens its tendency to segregate to the lower part of the aluminum liquid, and a large number of hard spots are easily formed on the surface of the die-casting alloy. Reduce the surface quality of die-cast products. Usually, a small amount (0.2-0.45%) of Ni is added to the die-cast aluminum alloy, and the die-cast product can be polished to obtain a smooth surface and improve the surface quality of the die-cast product. Adding ≤3.0% of Zn element to the die-cast aluminum alloy containing copper and silicon will help to improve the mechanical properties of the die-cast alloy material.
Cr element can form a large number of dispersed phases with a size of 0.01～0.1μm in the alloy, inhibit the recrystallization and grain growth of the alloy, play the role of grain refinement, and improve the toughness of the alloy; when the Cr element is added at 0 ～0.1% with the increase of the addition, the mechanical properties of the alloy material will decrease. When the Cr element is added in the range of 0.1～0.2%, with the increase of the addition, the mechanical properties of the alloy will continue to rise, and the addition content is 0.2% To obtain the best value. The added content of Mg element generally does not exceed 1.0%, which can generate an appropriate amount of Mg2Si strengthening phase in the alloy, and at the same time refine the eutectic Si phase without segregation, and improve the strength of the alloy. Adding a small amount of Sc element can effectively refine the grain structure and greatly extend the low-cycle fatigue life of the material. Usually, adding an appropriate amount of Cu to the die-cast aluminum alloy can passivate the ends of the coarse dendrites and turn them into fine flower-like grains. The morphology of the alloy grains changes and the micro-segregation of the alloy is reduced. The dispersion and precipitation of α-Al2Cu intermetallic compound in the early stage of solidification can hinder the migration of grain boundaries and reduce the growth rate of grains. Under fast cooling conditions, the dendritic structure is refined, the grain distribution is more uniform, and the alloy density is improved, and the Cu element It can significantly reduce the liquidus temperature of the alloy, promote crystal branching to form fine necks, prone to crystal proliferation, and significantly refine the grain structure.
In addition to the optimization of mechanical properties of die-cast aluminum alloy through composition design, heat treatment process parameters can also be adjusted to improve the mechanical strength and plastic toughness of die-cast aluminum alloy. At the same time, the combination of low-pressure speed and high-pressure die-casting process parameters can achieve the densification of the alloy structure, which is effective Improve the mechanical properties of die-cast products. For Al-Mg die-cast aluminum alloys, the process fluidity, linear shrinkage rate and thermal cracking sensitivity of the alloy are mainly improved by controlling the content of Fe element and adjusting the addition ratio of Si and Mg elements. The research team of Ding Wenjiang of Shanghai Jiaotong University solved the problem of the adhesion film of Al-Mg-Si-Mn-Cu-Nd-La die-casting alloy by adjusting the content of Mg and Mn elements and adding a small amount of RE elements, and realized the high toughness and freedom of alloy materials. strengthen. Research by Zhang Yinshuai of Xi'an University of Technology pointed out that adding 0.85% of Y element can significantly improve the elongation and impact toughness of ADC12 die-casting alloy. After T4 heat treatment at 520℃ for 8 hours, the tensile strength can reach 225MPa, and the impact toughness can reach 10.8J/cm2; When the Ce element addition amount is 1.07%, the casting fluidity of ADC12 die-casting alloy can be significantly improved. Die-casting aluminum alloy is easy to be involved in gas during the filling and injection process, and high-cooling speed die-casting forms a higher supersaturated solid solubility. The conventional heat treatment process is prone to deformation and bulging. Therefore, low-temperature aging heat treatment process is generally used. The low-temperature aging temperature of die-cast aluminum alloy is generally selected at 150℃～175℃. With the increase of aging time, the strength of the alloy material continues to rise, the plasticity continues to decrease, and the low-temperature aging time is general.No more than 12h. Luo Wenbo of University of Science and Technology Beijing studied the effect of heat treatment on the mechanical properties of rheological die-casting A356 aluminum alloy. When the artificial aging of T51 heat treatment is adopted for 3h, the alloy strength can reach 290MPa and the elongation can reach 8.5%; when the artificial aging of T6 heat treatment is adopted for 1h, the alloy resists The tensile strength and elongation increased to 310MPa and 16.5% respectively. With the extension of the artificial aging time, when the aging holding time was selected as 4.5h, the tensile strength reached a peak value of 335MPa and the elongation decreased to 10.5%.
Source: Proceedings of "The 14th China International Die Casting Conference & The 5th International Conference on Nonferrous Alloys and Special Casting Technology"
Author: Fan Zhenzhong (from China Aviation Development Beijing Institute of Aeronautical Materials), etc.
This article refers to the URL: https://mp.weixin.qq.com/s?src=11×tamp=1629080114&ver=3255&signature=SLSSVnMwRC7GnIq7Py--Doqrl5YjWufCIR5OSm-MRLJsJmx0O8ciwgnLKVxVZr2ZufqAwYZZ1GonewsNoq3GVXVZr2ZufqAwR5YGFY3G3GVXVZR2ZufqAwR5YP
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