**4. Conclusions**

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powder coating cycles on the microstructure and mechanical properties of the 18-inch wheels.

Based on the results obtained in the present study, it can be drawn that the different cooling rate of the wheels, ejected from the die at high temperature, produces different amount of distortions. By increasing the water temperature, the amount of distortions linearly decreases. Water cooling at a temperature higher than 70°C produces similar distortion as air cooling.

Considering the T6 heat treatment applied to the wheel production, a solution heat treatment of 6 h at 540°C is sufficient to dissolve completely the Mg-rich phases and to achieve a homogeneous solid solution. This solution treatment causes spheroidization and coarsening of the eutectic Si particles, leading to substantial changes in the microstructure throughout the 18-inch wheel. Higher solution temperatures lead to incipient melting at grain boundary and in the interdendritic regions. On quenching this liquid, reprecipitation of silicon and other intermetallic particles occur, and the average size increases. Quenching also leads to a large amount of shrinkage porosity adjacent to melted regions, which can coalesce and lead to the complete fracture of the wheel.

Furthermore, quenching is usually carried out from solution temperature to room temperature to obtain a supersaturated solid solution of solute atoms and vacancies, in order to achieve an elevated strengthening subsequent ageing. Here, the wheel distortion progressively reduces by increasing the temperature of water quenching, and a temperature higher than 80°C is sufficient to avoid distortion, allowing to achieve at the same time the required mechanical properties.

Finally, the powder coating of the wheels influences the final mechanical properties by activating the diffusion mechanism of the solute atoms, such as Mg and Si. This leads to the precipitation of dissolved elements and the coarsening of existing precipitates. The result is an increase of the hardness of about 3% after each coating cycle. This means that the powder coating can be integrated into the whole T6 heat treatment cycle of wheels, with a great impact on productivity and manufacturing cost of wheels.
