Aluminum alloy wheels are a good substitute for steel wheels, which have been widely used in cars and passenger cars. In 2000, the world's demand for aluminum alloy wheels has reached 110 million. Authoritative sources predict [1] that in the next ten years, the aluminization rate of China's car wheels will reach or approach 50%, calculated based on the aluminization rate of 5 wheels (1 spare wheel) per car and 50% of the wheels, and consider other vehicles As for aluminum wheels used in repair and retail, China's demand for aluminum wheels is expected to exceed 10 million in 2010 and 180 million in 2020. Therefore, the market potential of aluminum alloy wheels is huge.
    In order to improve the operation reliability, durability and appearance of aluminum alloy wheels, aluminum alloy wheels are generally painted after heat treatment. Before the coating process, pretreatment of three processes of oil removal, rust removal and phosphating should be carried out. In the later stage of pretreatment, drying treatment is required. The temperature is 210°C. After drying, painting is performed. According to the manufacturer's requirements, painting processes such as spraying, powder coating, and electroplating are applied. After painting and electroplating, baking is performed. At present, the common baking process is 160℃, 100℃ double baking. Therefore, the coating and baking process has a certain influence on the structure and mechanical properties of T6 treated aluminum alloy wheels. Studying the influence of the coating process on the mechanical properties of the alloy has certain practical significance for optimizing the T6 heat treatment process.
    1 Test process
    At present, the main alloy of the cast aluminum alloy wheel is A356 alloy, and its composition is shown in Table 1. After the wheel is formed, T6 heat treatment is performed. The solution temperature is 540 ℃, time 6h, aging temperature 180 ℃, time 4h. After aging, follow the parameters in Table 2 Three-level painting.
    Table 1A356 alloy wheel hub alloy element mass fraction%

 

    Tensile test is used to study the changes of alloy mechanical properties. The tensile test bar is sampled at the rim of the hub, and the tensile test bar is processed into a 5 times standard tensile test bar. The tensile test is carried out on a WDW-50 microcomputer-controlled electronic universal testing machine with a tensile rate of 5 mm/s.
    Conductivity and DSC tests are used to study the microstructure changes of the alloy. Conductivity measurement is carried out at room temperature, the sample is a rectangular sample of 20mm × 8mm × 4mm, to study the alloy dislocation, precipitation phase and solid solution changes; DSC test is carried out on a DSC differential thermal analyzer, the sample is A round sample with a diameter of 5 mm and a height of 1 mm was subjected to DSC analysis immediately after heat treatment to study the changes in precipitation transition temperature and peak value of the strengthening phase and the equilibrium phase.
    2 Test results and discussion
    Study the changes of hardness, tensile strength, yield strength and elongation of alloy under T6 and two-stage aging heat treatment process: T6 process is 540 ℃ 6h + 180 ℃ 4h; two-stage aging process is 540 ℃ 6h + 110 ℃ 2h + 180 ℃ 4h.
    2.1 Effect of coating process on mechanical properties of alloy
    The effect of coating on the strength properties of the alloy under the two heat treatment processes is shown in Figure 1, and the effect on elongation is shown in Table 3.
    Table 3 Elongation% of A356 alloy under two coating process systems

Craft category	Unpainted	First class painting	Secondary coating	Three-level painting
T6 process	9	9	9	10
Two-stage aging process	10	10	10.5	10

    As can be seen from Figure 1, the influence of the coating process on the properties of the alloys of the two heat treatment processes is consistent. After the first-level coating, the strength performance of the alloy decreases. After the second-level coating, the strength performance of the alloy increases, and after the third level Coating, the performance of the alloy has been reduced, but compared with the uncoated process, the performance of the alloy is increasing, especially for the two-stage aging process, the coating process has a more obvious impact on the strength. It can be seen from Table 3 that the effect of coating process on elongation is not obvious. Therefore, the coating process improves the strength properties of the alloy to a certain extent without changing the elongation of the alloy.
    After the first-level coating, the electrical conductivity of the alloy increased significantly. After the second-level coating, the electrical conductivity decreased. After the third-level coating, the electrical conductivity of the alloy rebounded. Compared with the first-level coating, the conductivity of the alloy The first-level coating and the third-level coating have little effect on the electrical conductivity of the alloy. Literature [2] pointed out that the influence of heat treatment on the conductivity of Cu-Mg-Cr alloys pointed out that due to the dissolution of chromium in the copper matrix, the probability of free electrons scattering during movement increased, resulting in a decrease in conductivity. For A356 alloy, after the first-level coating, on the one hand, due to the solid solution of silicon element in α-Al continues to dissolve from the matrix, on the other hand, the precipitated silicon element can be used as the nucleation core of Mg2Si [3-4] , Making Mg2Si aggregate and grow, reducing the uniformity and density of Mg2Si distribution in the matrix, thereby reducing the alloy's scattering rate of free electrons, resulting in an increase in electrical conductivity. Regarding the effect of alloy microstructure on electrical conductivity, secondary coating and tertiary coating have a certain effect on the electrical conductivity of the alloy, but the change is not large, indicating that the silicon dissolution and Mg2Si aggregation have basically ended at this time.
    2.3 DSC analysis
    Literature [5] pointed out that when studying the yield strength model of A356 alloy, the solid solubility of silicon in α-Al is between 0.5% and 1.2%. The yield strength increase due to the solid solution of silicon does not exceed 2N/mm2 to 3N/mm2 . Regarding the strengthening mechanism of Al-Mg-Si alloy, the literature [6-8] believes that the desolvation sequence of the alloy is supersaturated α solid solution-GP region-β″ phase-β′ phase-β phase. When the GP region is formed, The GP zone and the matrix produce elastic strains near the boundary, which hinders the movement of dislocations and improves the strength of the alloy; with the extension of the aging time, the CP zone quickly grows into a needle or rod shape, which is the β″ phase, and its elasticity in the C axis direction The strain field caused by coherent bonding is the largest, and its elastic stress is also the highest. When the β″ phase grows to a certain size, its stress field is spread throughout the matrix, and the strain zones are almost connected. At this time, the strength of the alloy is high; "On the basis of the phase, Mg and Si atoms are further enriched to form a local coherent β'transition phase. The elastic strain around the matrix reaches its maximum value and its strength decreases; when a stable β phase is formed, it loses its contact with the matrix. In the coherent relationship, the coherent strain disappears, and the strength decreases. Therefore, the change in alloy strength should be mainly attributed to the transition between precipitated phases.
    DSC analysis of A356 alloy in solution state, T6 process and three-level coating process is shown in Figure 3. The DSC curve of the solid solution state is analyzed, where point A is the precipitation peak in the GP area, point B is the precipitation peak of β″ phase, C1 and C2 are the precipitation peaks of β′, and D is the precipitation peak of β equilibrium phase. Compare the solution state and aging State DSC curve, β″ and β′ precipitation temperature is basically the same, but the peak of ageing state DSC curve β″ is significantly higher than the solution state DSC curve. Ageing state curve β phase precipitation temperature increases, because the ageing process is beneficial to strengthen the phase β″ and The precipitation of the β′ phase hinders the precipitation of the equilibrium phase β. The precipitation temperature of the β-phase of the DSC curve of the first-level coating decreases. Compared with the first-level coating, the temperature of the β-phase of the DSC curve of the second-level coating increases, while the precipitation temperature of the β-phase of the third-level coating decreases. The peaks of β-phase precipitation in coatings increase obviously, while the peaks of β″ and β′ phases weaken obviously. In A356 alloy, the strength increase of the alloy mainly comes from the precipitation strengthening of β″ and β′ phases, while the balance relative alloy strength has no contribution. After aging, the strength of the alloy increases. Due to the formation of a large number of dispersed β″ and β′ phases during the aging process, after the first coating, it is conducive to the transformation of α′ and β′ phases to the equilibrium phase β phase, and the number of strengthening phases is reduced. , And the strength of the alloy is reduced; after the secondary coating, the strength of the alloy may be increased because the secondary coating hinders the transition of β″ and β′ phases to the equilibrium phase β phase, and the release of vacancies and dislocations in the solid solution increases the strengthening phase The reason is that after the three-stage coating, the alloy equilibrium phase β phase is greatly increased, and the number of β″ and β′ phases is reduced, thereby reducing the strength of the alloy.