SMM9: at the 2020 (second) China Industry Expo New Materials Forum-China Automotive New Materials Application Summit Forum and China (8th) Aluminum processing Industry chain supply and demand Trading Summit, co-sponsored by Shanghai Nonferrous Metals Industry Association, Shanghai Automotive Engineering Society, Suzhou Die casting Technology Association and Shanghai Nonferrous net. Li Jianxiang, chairman of Guangdong Hesheng Industrial Aluminum Co., Ltd., gives a speech on automotive aluminum alloy extruded profiles and application cases from the aspects of performance requirements of automotive aluminum alloy profiles, selection of automotive aluminum alloy materials, alloy secondary development and process development, introduction of automotive aluminum alloy materials and application cases.
Performance requirements of aluminum alloy profiles for vehicles
1. Have appropriate strength and stiffness
Li Jianxiang said that the main demand for using aluminum alloy in cars is to replace steel and achieve lightweight. The overall strength of the structure needs to meet the requirements of lightweight vehicles. According to the recent requirements of China's automobile lightweight circuit map (2016-2020), the strength target value of AHSS grade high strength steel 600MPa. Aluminum alloy is expressed as the development of 5 series, 6 series high formability alloy, the amount of 190 kg per bike, there is no quantitative index for the strength of aluminum alloy.
The difference between the mechanical properties of aluminum alloy and steel:
Due to the high yield-strength ratio and low elongation of aluminum alloy, the cold deformation of aluminum alloy is much more difficult than that of steel, and aluminum is easy to rebound.
Elastic modulus: the elastic modulus of aluminum is low, about 70GPA, and that of general alloy steel is about 200GPA, which is 3 times that of aluminum. Aluminum is less resistant to impact.
Second, it has good anti-fatigue performance.
When the automobile is used for a long time and long distance, the structural parts can not be replaced, so the fatigue resistance of aluminum alloy is very important. There are many methods to measure fatigue resistance, and usually the data found in the material manual are not accurate. The following provides a running test data commonly used in the vehicle industry, which is very practical for the selection of automotive alloy materials. Figure 2 is the fatigue test data of aluminum extruded materials for high-speed rail in China.
III. Corrosion resistance
Because of the use of surface anticorrosion technology, automotive parts are not afraid of corrosion in the external environment, but the stress corrosion and electrochemical corrosion formed by materials through multi-pass cold processing and connection have a great impact on the service life and safety of automotive products. Ruri cars, high-speed rail used to be the mainstream use of 7N01 materials, and then all changed to use 6N01, is caused by corrosion resistance.
From the point of view of heat source, the connection between materials can be divided into hot connection and cold connection. Cold connection, such as riveting, pays more attention to the requirements of structural design, while hot connection requires matrix material selection. Considering the welding quality requirements such as hot crack tendency and corrosion resistance, not all aluminum alloy materials are suitable to be used as welding base materials.
For the alloy which can not be strengthened by heat treatment, the decrease of post-welding strength is mainly due to the fact that the temperature of HAZ exceeds the recrystallization temperature, recrystallization and grain growth occur, and the strength decline coefficient depends on the welding peak temperature. For the heat treatment strengthened alloy, the main reason for the decrease of post-weld strength is that the grain in the HAZ is coarse, and the strengthening phase dissolves into the matrix due to the action of heat source, which weakens the dispersion strengthening effect. The strength decline coefficient mainly depends on the second phase properties and thermal cycle characteristics of the material.
5. Examples of special requirements
Bending performance (three-point bending): some requirements of automotive aluminum alloy profiles exceed the general standards of the aluminum processing industry, such as the performance requirements of anti-collision beams, and the car factory requires three-point bending tests of anti-collision beams. The parameters corresponding to the bending test results can not be found through the conventional performance characteristics of aluminum extruded materials, which is not positively related to the absolute value of material strength. Our study shows that bending cracking has a high correlation with the microstructure of the material. The bending property of the material with complete longitudinal fiber structure is the best. It is closely related to the cross-section design of the material, and the difference of wall thickness design has a great influence on the bending property, and the elongation of the material is positively related to the bending property.
Crushing performance (energy absorption effect): the energy absorption effect of materials cannot be determined by conventional thinking of materials. it is generally believed that materials with high elongation are not easy to crack during compression. Billiton aluminum researchers have done a large number of experiments show that the axial compression properties of 6xxx series alloys, it is best to use the average compression force to express that (mean crush force) MCF, increases linearly with the tensile strength, and the elongation is not a good predictor of fracture sensitivity in the crushing process. Among all kinds of alloys tested, AA6008 has the highest strength at zero crack. The same mechanical properties of materials, fiber tissue products show the best energy absorption effect.
Welding performance and buckling performance: Japanese car factory uses aluminum tube as shock absorber inner and outer casing. Years of research shows that two core indexes of shock absorber aluminum tube: welding performance and buckling performance. Hesheng designed a pipe expansion test method to evaluate the welding performance of metallurgical welds. The performance of flexion was evaluated by flexion test.
Material selection of aluminum alloy for automobile
Except for 6063 and 6061 commonly used alloys for automotive aluminum alloy extruded profiles, there are differences among American, Japanese and European systems.
American Alcoa, Alcan (Billiton) to develop and use mature alloys, mainly 6005A, 7003 and so on;
The Japanese series are mainly alloys developed by Showa Electric, Guhe Electric, Kobe Aluminum, Sumitomo and Nippon Light Industry, such as 6N01, 7N01, 7005 and so on.
The European system is mainly based on alloys developed by Hydro and Sapa Aluminium, mainly 6082 and 6013.
Generally speaking, most of the automotive materials choose medium strength or medium to high 6 series alloys, and some low grade 7 series alloys. Properly distinguish and select materials according to the use of the product.
Limitations of alloy selection: we can not always choose the best alloy according to the needs of aluminum for car body, because some materials may not be extruded to meet the conditions of industrial mass production. This is mainly based on two factors: alloy extrudability and quenching sensitivity.
Secondary development and process development of alloy
The international standard is a universal and broad alloy, and there is a big difference in the performance of the same brand. For example, the most commonly used A6063 alloy, Si0.2-0.6% and Mg0.45-0.9%, Hesheng experience shows that its most basic mechanical properties are Rm 180-260Mpa, and the difference lies in the difference in alloy content. our calculation shows that the Mg2Si content of A6063 main alloy is 0.6-1.2%. Therefore, if you really want to make good use of A6063, you need to do secondary development according to the actual use of the product, and select the appropriate micro-area within the standard brand range. Control the content of the main alloy and various parameters that affect the performance, such as the amount of excess Si in 6063, the allowable value of Fe phase, and the content of Cu in A6061, which is related to the solid solution strengthening effect of the product, quenching sensitivity, aging curve, surface yellowness after anodic treatment, etc., which is why we emphasize that aluminum extruded material users need suppliers with alloy development ability.
In addition to the reasonable composition of the alloy, there is also a very important index, that is, how the alloy works effectively. We sometimes find that the composition of the alloy is fine, but the properties do not meet the requirements. The main reason is that the alloy has not fully formed a strengthening phase, such as the strengthening phase of A6063 alloy Mg2Si, which has three basic morphologies: columnar structure (rare phase), rod-like structure (é phase) and needle-like structure (é phase). Only the Europa "phase has a good strengthening effect, and the thick columnar and rod-like structures basically have no strengthening effect, so, our process design should control the optimal conditions from the whole thermal process of aluminum alloy melting casting, ingot homogenization treatment, extrusion solid solution and artificial aging, so that the added alloy elements can maximize the formation of the only" phase in the T6 state, and our basic goal will be achieved.
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