Summary
Based on EN AW-6056 aluminum alloy wire, the necessary improvement and adjustment were carried out. The key manufacturing process and main mechanical properties of aluminum alloy bolts were studied, and aluminum alloy bolts meeting the requirements of lightweight coupling were developed. The developed aluminum alloy bolts passed the engine reliability bench test, indicating that its application on the engine is feasible.
Introduction
The lightweighting of automobiles has made aluminum parts more and more widely used, but the connection technology has been regarded as the most critical technology for the actual improvement of light weight. Among them, the screwing technology is the most important. The reason is that the technical bottleneck exposed by the use of traditional steel bolts or screws (hereinafter referred to as steel bolts) to join aluminum parts: electrochemical corrosion caused by steel and aluminum joints, and thermal expansion of steel bolts and aluminum joints Inconsistent coefficients, when temperature changes will lead to additional stress on the threaded coupling pair, increase the creep risk of the aluminum alloy, and weaken the ability to maintain the preload. Whether the above-mentioned joint reliability problem can be solved is a key point for whether aluminum parts can be effectively applied in light weight, and it is also a problem that the screw-connection technology of aluminum parts is constrained or urgently solved. The author has successfully developed aluminum alloy bolts based on the improved aluminum alloy wire based on EN AW-6056, and has been put into practical use on the engine.
1. Wire rod for aluminum alloy bolts
In order to comprehensively improve the performance of aluminum alloy bolts, based on EN AW-6056 aluminum alloy wire, the necessary improvements and adjustments have been made:
(1) Reduce the maximum content of Si element to reduce the sensitivity of intergranular corrosion;
(2) Reduce the Fe element content to ensure the mechanical properties of the aluminum alloy. It should be noted that when the content of Fe in the aluminum alloy is more than 0.3%, the mechanical properties of the aluminum alloy are degraded rapidly. In addition, excessive amounts of Fe can cause aluminum alloy waste to be unrecyclable;
(3) , Compressed Cu element limit range. The factors affecting the sensitivity of intergranular corrosion are mainly Cu elements (but at the same time can also increase the strength of aluminum alloys), followed by heat treatment processes;
(4) , Compressed Mn element limit range. Mn has a strengthening effect on the properties of the material, and the solid solution strengthening effect is good, and the aluminum alloy can be further strengthened in EN AW-6056. Its effect on aluminum alloys is mainly in the process of recrystallization. It should be noted that in an aluminum alloy in which the amount of Mn added is more than 0.1%, Mn increases the deformation resistance and increases the quenching sensitivity;
(5) , reduce the maximum content of Mg elements. It should be noted that the corrosion resistance of aluminum-magnesium alloy is very good, and it can strengthen aluminum (the tensile strength of aluminum alloy can increase by about 34 MPa for every 1% increase of magnesium), but Mg has hot brittleness and is prone to cracking;
(6) Reduce the maximum content of Cr element. Cr can improve toughness and improve the corrosion resistance of the alloy. The content of Cr is preferably 0.15% to 0.3%;
(7) Reduce the maximum content of rare earth element Ti. Adding 0.02% to 0.1% Ti and 0.01% to 0.2% Cr to EN AW-6056 can improve the forging properties of the alloy and refine the grain of the alloy. However, it should be noted that Ti will reduce the hardness when it is solid solution, and will increase the hardness when forming precipitates;
(8) , the range of compression Zn element limits. Zn has little effect on the mechanical properties and processing properties of the material. The influence of Zn content and distribution on aluminum alloy is mainly for aluminum materials that require anodizing and coloring. If the content is improperly controlled, the surface of the part will form a coarse crystal structure, and even a "galvanizing" defect will occur during the anodizing process;
(9) Add rare earth element Zr. EN AW- 6056 is not specified. Zr is corrosion resistant, insoluble in hydrofluoric acid and aqua regia, especially resistant to hydrochloric acid and sulfuric acid. At high temperatures, it can react with non-metallic elements and many metal elements to form solid solution compounds;
(10) , other elements. When the mass fraction of the rare earth element is 0.2%, the crystal grains can be refined and the corrosion resistance of the alloy can be improved.
The comparison of the chemical element composition of EN AW-6056 aluminum alloy wire is shown in Figure 1. Moreover, the average grain size in the longitudinal and transverse microsections of the bolt test piece is ≤ 150 μm.
Fig.1 Comparison of chemical composition of EN AW-6056 aluminum alloy wire before and after comparison
2 Main mechanical properties of aluminum alloy bolts
2.1 General mechanical properties
After the corresponding process, especially the heat treatment process, the aluminum alloy bolt can achieve the following mechanical properties: tensile strength Rm ≥ 400 MPa; specified non-proportional extension 0.2% stress Rp0.2 ≥ 350MPa; elongation A ≥7%; The bolts and screws are tested and the wedge load reaches the level of Figure 2. It should be noted that the minimum breaking torque is only applicable to bolts or screws that cannot be subjected to tensile testing. The conventional mechanical properties of the aluminum alloy bolts basically meet the requirements of 4.6 grade steel bolts.
This aluminum alloy bolt has a maximum ambient temperature of 150 °C. Under the condition of continuous service at 1000 °C for 1 000 h, the performance indexes of bolts are stable. During the temperature increase, each performance index is attenuated by ≤10% compared to the target value at the specified ambient temperature.
Figure 2 Parameters for wedge load test
2.2 Fatigue performance
The influence coefficient method is mainly used. The design idea is: Starting from the S-N curve of the material, and considering the influence of various influence factors, the S-N curve of the bolt is obtained, and the anti-fatigue design is performed accordingly. Limited life designs typically use the sloped portion of the S-N curve. The aluminum alloy bolt has no failure before the stress amplitude σa = 15 MPa and the number of oscillation cycles reaches 107. Fatigue performance has reached an ideal level.
2.3 Corrosion resistance
Intergranular corrosion (IGC) is a localized corrosion caused by electrochemical heterogeneity of the structure, which causes loss of bonding between the grains and complete loss of metal strength. Grain boundary corrosion mainly affects fatigue, strength and plasticity.
The study of intergranular corrosion of the aluminum alloy bolts can be carried out from the heat treatment process as well as the alloys and rare earth elements. The factors affecting corrosion sensitivity are mainly Cu elements (but at the same time can also increase the strength of aluminum alloys), followed by heat treatment.
When the aluminum alloy bolt is artificially aged, the slow cooling will increase the corrosion sensitivity of the grain boundary. When the hardness reaches the peak value, the grain boundary corrosion sensitivity is also the largest. The overaging will reduce or even eliminate the grain boundary corrosion sensitivity tendency, but it will lead to the point. eclipse.
There are three main types of corrosion associated with intercrystalline crystals: intergranular corrosion, exfoliation corrosion, and stress corrosion (cracking). Since the peeling corrosion is likely to occur in the 2000 series, the 5000 series, and the 7000 series copper-containing or copper-free (inter-crystalline corrosion-sensitive) aluminum alloy, the aluminum alloy bolt peeling corrosion may not be specially considered.
For the aluminum alloy bolts, the intergranular corrosion is mainly controlled. However, it should be noted that when increasing the strength of the aluminum alloy by increasing the content of Cu, the risk of stress corrosion is generally increased. Therefore, the increase in the content of Cu should not cause the stress corrosion cracking sensitivity of the bolt. The ratio of the area shrinkage of the bolt under ambient conditions æ–é¢ the ratio of the area shrinkage to the inertness under inert conditions should be greater than 95%.
3, the key manufacturing technology of aluminum alloy bolts
(1) The basic process route is: cold forming - cleaning - heat treatment (annealing + age hardening) - twisting thread - cleaning - lubrication - acceptance.
(2) The basic technical route of heat treatment is shown in Figure 3. Unlike steel hardening whose hardness and microstructure are controlled by non-diffusion martensitic transformation, the hardness of the aluminum hardening and the microstructure are controlled by the precipitation-strength strength growth stage.
Figure 3 Heat treatment technology route
The basic purpose of this aluminum alloy bolt heat treatment is:
1. Solution annealing is to obtain a homogeneous solid (to obtain a solution + recrystallized microstructure);
2. Quenching is to obtain a supersaturated solid solution;
3. Aging (time/temperature) is to control the precipitation of the strength growth stage.
Material properties are achieved through the corresponding molding and heat treatment processes. At temperatures above 160 °C, the bolts shall be subjected to at least one solution annealing and age hardening. The heat treatment process must be carried out in a continuous furnace and must be threaded after heat treatment.
The average grain size in the longitudinal microscopic section is ≤150 μm, and the average grain size in the transverse microscopic section is ≤150 μm. The key factors for heat treatment include the amount of deformation of the bolts, temperature control, and characteristics of the wire material. The highest strength is formed at the specific age hardening temperature and time of the alloy; the highest strength grade is formed at the specific solid solution annealing temperature and time of the alloy. The heat treatment process is shown in Figure 4.
Figure 4 Heat treatment process of aluminum alloy bolts
3.1 Surface treatment
The bolts are treated with oxidation and sealing. Based on the anodizing and micro-arc oxidation treatment, it has a good effect of improving the resistance to intergranular corrosion. The aluminum alloy bolts are surface oxidized. Since the surface corrosion resistance of the aluminum alloy wire is not strong, it is necessary to perform surface treatment by anodizing to increase the corrosion resistance, wear resistance and appearance of the aluminum bolt. The main process is:
(1) Surface pretreatment. The surface of the bolt is cleaned chemically or physically to expose a clean substrate to facilitate the acquisition of a complete, dense, artificial oxide film. It is also possible to obtain a specular or matt (matte) surface by mechanical means;
(2) Anodizing. After surface pretreatment of the bolt, under certain process conditions, the surface of the substrate is anodized to form a dense, porous, strong adsorption AL203 film layer;
(3) , sealing. The pores of the pores of the porous oxide film formed after the anodization are closed, so that the oxide film is prevented from being contaminated, and the corrosion resistance and abrasion resistance are enhanced. The oxide film is colorless and transparent. By using the strong adsorption property of the oxide film before sealing, some metal salts are adsorbed and deposited in the pores of the film, so that the appearance of the profile can show many colors other than the natural color (silver white), such as: black, bronze, Gold and stainless steel. Oxidation treatment and sealing treatment requirements: GB / T8013.1 grade V; salt spray corrosion resistance (CASS) is 72 h, grade ≥ 9; alkali resistance ≥ 125s; wear resistance f ≥ 300 g / μm.
3.2 Threading
Thread tanning is arranged after the heat treatment process to further optimize the mechanical properties of the bolt, minimize the sensitivity of intergranular corrosion, increase the fatigue strength (reduce the effect of thread bottom) and minimize the risk of thread damage.
Moreover, the material properties of aluminum dictate the necessity of cold forming of the entire stem including the bolt neck using a multi-station cold forming apparatus. It should be noted that the aluminum alloy bolts should be avoided in the manufacturing process of steel bolts and screws, tools (tools, tools, etc.).
3.3 Control of friction coefficient
The bolt is coated with a friction coefficient stabilizer by dipping, and as the last process of the whole manufacturing process, the effective control of the tightening speed is realized, and the variation of the friction coefficient value between the mass-produced fasteners is effectively reduced to stabilize the tightening torque and improve The locking force of the fastener under vibration and vibration.
It is a special dry lubricant using an aqueous colloidal dispersion friction coefficient stabilizer containing a high molecular polymer. It can be coated by dipping, and cured by drying to form a transparent lubricating film with good adhesion. And because of this special dry lubrication, because the film layer is extremely thin, the problem of influencing the tolerance is avoided. Parts pre-coated with lubricated water wax can be stored in reserve, ready for assembly and automated.
4. Application of aluminum alloy bolts on automobile engines
At present, aluminum alloy bolts are applied to a total of six parts of the engine. Specifically:
Oil and gas separator installation (Figure 5(a)),
Phase sensor installation (Figure 5(b)),
Speed ​​sensor installation (Figure 5(c)),
Pump assembly installation (Figure 5(d)),
Vacuum pump installation and thermostat housing mounting (Figure 5(e)).
Figure 5 Engine application aluminum screw parts
The above mounting points are assembled by the torque method. Through the 800 cycles, 400h alternating load working endurance test of the engine and the simultaneous detection of the residual torque of the six joints using the aluminum alloy bolts, the tightening torque of the aluminum alloy bolts and joints fully meets the design requirements.
The aluminum alloy bolts developed in this project have solved some key problems in the connection, and the improvement effect is obvious:
(1) , to ensure that the expansion coefficient of the bolt and the connected part is basically the same.
Since the bolt and the connected parts are made of aluminum alloy, the expansion coefficient is basically the same, which is beneficial to reduce or even eliminate the additional stress caused by the temperature change of the threaded coupling pair, thereby reducing the creep of the aluminum alloy and improving the pre-tightening ability. .
(2) Improve the clamping force retention capability of the coupling body. Since the modulus of elasticity of an aluminum alloy is about 1/3 of that of steel, the rigidity of an aluminum bolt of the same size is 1/3 of that of a steel bolt. The advantage is that the same plastic relaxation or load corresponding to the creep variable decreases, and the clamping force of the coupling body is better.
(3) , electrochemical corrosion is completely avoided. When aluminum alloy bolts are used to connect, the electrochemical corrosion of the aluminum alloy can be avoided or minimized, thereby solving the problem of loose joints from the source.
(4) Shorten the length of the threaded joint. In order to avoid the thread tripping, when the steel bolt is used to connect the aluminum alloy material, a larger screwing length is required, so that the structural size of the coupled member and the coupling body is significantly increased.
(5) The bolt itself is lighter. The aluminum alloy has a very high specific strength and its strength is equivalent to a low carbon steel bolt that is not heat treated, but its density is only 1/3 of that of steel, and the bolt itself has a light weight advantage.
Conclusion
Through the independent development of aluminum screws, torque development and verification, the aluminum screwing technology has been successfully applied on the engine. This achievement is very important for the effective advancement of aluminum components in lightweighting.
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