Aluminum alloy wheels are important parts of automobiles, not only responsible for the safety of the performance of the entire vehicle, but also have the effect of enhancing the appearance of the entire vehicle. In recent years, polished and electroplated aluminum alloy wheel products have quickly occupied the high-end market in the automotive field with their beautiful appearance. In order to meet the coating needs of polishing and electroplating products, cast aluminum alloy wheels
The manufacturing level also puts forward higher requirements.
A certain type of polished aluminum alloy wheel adopts low-pressure metal casting and T6 heat treatment process. The front surface of the spoke retains the casting surface and is sprayed with transparent paint after polishing. After the polishing process of this polished aluminum alloy wheel product, it is found that there are black markings on the front of the root of the wheel. After the spraying process, the black markings are more obvious in the sun. The concentrated black markings on the spoke roots of the wheel have seriously affected the appearance of the polished aluminum alloy wheel, which needs to be solved urgently.
1 test detection
1.1 Chemical composition detection
The chemical composition analysis of the aluminum alloy wheel material is shown in Table 1, and the test results meet the ASTME1251.94 (1999) standard. However, by comparing the composition of the black spot area and the normal area at the spoke position, it is found that the Si content in the black spot area is relatively high.

1.2 Metallographic inspection
Samples were taken from the dark spot area and the normal area without dark spots for metallographic analysis, and the microstructure of the two is obviously different. As shown in Figure 1, under an optical microscope, the Si distribution in the black spot area is uneven, the segregation phenomenon is serious, and the microscopic shrinkage is also serious.

1.3 Measurement of dendrite spacing
Measure the dendrite distance between the black spot and the normal area, calculate and compare the crystal solidification time of the two... As shown in Table 2, the dendrite spacing in the black spot area is larger than that in the normal area. By calculating and comparing the crystal solidification time of the two, the crystal solidification time in the black spot area is significantly longer.

1.4 Grain size detection
Samples are taken from the dark spots and normal areas without dark spots for grain size analysis, as shown in Figure 2. According to the GB/T3246.2-2000 standard, it is judged that the grain size in the black spot area is level 4, the grain size in the normal area is level 3, and the grain size in the black spot area is obviously larger.

2 casting simulation
2.1 The simulation software used for simulation pre-processing and parameter setting is MagmaSoft, and the modeling software is UG. In the simulation, the material of the aluminum alloy wheel hub is set to A356.2, the pouring temperature is 700¡ãC; the material of the mold is set to X40CrMo5V1, and the initial temperature is 480¡ãC. The heat transfer parameters between the casting and the mold are set to A1Si7Mg in the MagmaSoft database. Mold type, and set the cooling system according to the actual use of the mold [2].

In order to get better results, the simulation uses the same mold to repeat the casting 5 times, and the mold temperature after the previous simulation is used as the initial temperature of the next simulation. At the same time, the mold opening, casting out, and mold closing are considered between the two simulation analyses. When the mold is left in the air to cool down, after 5 calculations, save the 5th simulation results.

2.2 Simulation results
For the simulation results of the casting solidification temperature field, the solidification temperature of the casting blank at the front of the spoke root is relatively high, and this phenomenon only appears on the front of the casting blank, and the spoke side and back are not found. The high temperature position in the simulation result is consistent with the black spot on the actual casting blank.

3. Analysis and discussion
From the analysis of the metallographic and chemical composition detection results, it can be seen that the Si composition in the black spot area is segregated seriously and accompanied by obvious microscopic shrinkage. Through the detection of dendrite spacing and grain size, the grains in the black spot area are coarse, the dendrite spacing is large, and the solidification time is long. In the improvement test, 6 wheels with black spots on the front face were machined by 1 to 1.5mm, and it was observed that the black spots were significantly reduced or even disappeared as the machining amount increased. This is also the same as the higher temperature of the spoke roots in the casting simulation. The location only exists on the surface and the results are consistent.

In summary, the black spot phenomenon only exists on the surface of the casting. The excessively high temperature area is because the surface of the casting is not crusted in time, which causes the crystallization time to be too long, which disrupts the overall solidification sequence of the casting. Si segregation and microscopic shrinkage appear in the shrinking area, that is, the black spot phenomenon seen after the wheel hub is polished.

After analysis and discussion, it is preliminarily judged that the cause of the black spot formation is that the molten aluminum flows too fast, the local temperature of the mold is too high, and the local cooling strength of the mold is insufficient, causing the black spot area at the root of the spoke to be repeatedly scoured, and the molten aluminum fails to pass quickly. Crust; during the crystallization and solidification period of the casting, this area plays a role in feeding the area around it, so segregation and shrinkage appear at the root of the spoke to form black breaking.

4. Trial production verification and results
Based on the above analysis results, the idea to solve the black spot problem is to strengthen the local cooling strength of the mold to ensure the sequential solidification of the castings, so that the castings at the roots of the spokes can be crusted in time and the feeding channel is smooth. To this end, the following trial production plans are made: ¢ÙReduce the molten aluminum filling speed to reduce the impact of turbulence and overheating at the roots of the spokes; ¢ÚUnder the premise of ensuring good fluidity of the molten aluminum, minimize the temperature of the molten aluminum and shorten the crystallization time before the flow of the molten aluminum ¢ÛIncrease the temperature of the top mold of the mold to ensure smooth feeding channel; ¢ÜSet up a local forced cooling system in the bottom mold of the mold to make the castings in the dark spot area quickly encrustate and improve the surface quality of the castings.

On the basis of the above adjustments, after repeated adjustments to the on-site cooling parameters, the segregation and shrinkage defects at the roots of the spokes of the polished aluminum alloy wheels were finally solved, and the black spots were completely eradicated.

5 Conclusion
1) Black spots appearing on the surface of polished aluminum alloy wheels are essentially black spots formed by segregation and shrinkage caused by local overheating of the casting;
2) The idea to solve the black spot phenomenon is to locally strengthen the cooling effect, so that the surface of the casting can be quickly encrusted, and the casting can be solidified in sequence;
3) When designing aluminum alloy wheel polishing products, try to avoid wide-spoke polishing surfaces, or design the front surface of the spokes to be polished first, which can block the source of black spots;
4) The aluminum hub polishing product design should be as thick as possible, and the weight-reducing pockets should be as thick as possible to avoid segregation and shrinkage caused by the blockage of the feeding channel similar to the spoke root.