**2.1 Alloy and casting parameters**

The cast wheels were produced with an AlSi7Mg alloy (EN AC-42100, equivalent to the US designation A356), whose composition is indicated in Table 1. The material was melted in a furnace set up at 730 ± 5°C. The melt was degassed with a rotary impeller by using nitrogen and modified with Sr-containing master alloy. AlTi5B1 rod type grain refiner was also added to the molten metal. The hydrogen level was evaluated before casting through a Reduced Pressure Test (RPT).


Table 1. Chemical composition of A356 alloy used in the present work (wt.%)

The die cavity is geometrically complex and is comprised of four sections: a bottom die, two side die sections, and a top die. These die sections are made by an AISI H13 tool steel. The temperature in the die, measured with thermocouples, was in the range of 450-520 ± 10°C.

The casting process is cyclic and begins with the pressurization of the furnace, which contains a reservoir of molten aluminium. The excess pressure in the holding furnace forces the molten aluminium to fill the die cavity in 60 ± 4 s with a final pressure of 0.4 ± 0.015 bar. An overpressure of 1.2 ± 0.03 bar, reached after 10 ± 2 s from the end of the filling, was then applied for 210 ± 5 s. During solidification, cooling rates are controlled by forcing air (2–3 bar) through internal channels in the top and bottom dies, at various times during casting

Optimizing the Heat Treatment Process of Cast Aluminium Alloys 201

solutionizing and water quenching (T6). This is a typical underageing treatment used in the manufacture of wheels. The rejected or sound wheels are finally moved to Stages E or F

After machining and cleaning operations, the wheels are generally powder coated and left inside an air electric furnace at 170 ± 5°C for 1 hour, including the heat-up time. Fig. 3. shows a typical thermal cycle of the wheels during powder coating. In the present work the

effect of coating cycles has been studied by varying the number of cycles from 1 to 3.

Fig. 3. Thermal cycle used for powder coating wheels; thermocouples are placed directly into the furnace chamber and embedded into the hub and the spoke region of the wheel

Detailed microstructural characterisation of the as-cast and T6 heat treated wheels was carried out using an optical microscope and a scanning electron microscopy (SEM) equipped with an energy-dispersive spectrometer (EDS). The quantitative analysis of various phases in the microstructure were characterised using an image analyser software. The samples, drawn from the hub, the spoke and the rim region of the wheels, were mechanically prepared to a 3-µm finish with diamond paste and, finally, polished with a commercial fine silica slurry. Average secondary dendrite arm spacing (SDAS) values were obtained using the linear intercept method. A series of at least 10 photographs of each specimen were taken and several measurements were done, in order to obtain reliable mean values. To quantify the microstructural changes during solution heat treatment, the image analysis was focused on the size and shape factor of the eutectic Si particles. Size is defined as the equivalent circle diameter (*d*); the shape factor (α) is the ratio of the maximum to the minimum Ferets. To obtain a statistical average of the distribution, a series of at least 15 photographs of each specimen were taken; each measurement included more than 700 particles. The secondary phases, such as the Mg-rich particles and the Fe-rich intermetallics, were excluded from the analysis. Further, the polished specimens were chemically etched in

Brinell hardness measurements were carried out throughout the casting, on well defined locations, by using a load of 250 kgf, according to the standard ASTM E92-82. An average over 15 measurements was taken to evaluate the hardness of each wheel. Target hardness

a Keller etchant (7.5 mL HNO3, 5 mL HCl, 2.5 mL HF and 35 mL H2O).

values after complete T6 heat treatment range between 90 and 95 HB.

respectively, as indicated in Fig. 2.

**2.3 Microstructural characterization** 

**2.4 Distortion and hardness testing** 

cycle. On the side dies, cooling can be ensured by air jets, aimed at various sections of the exterior face. After the complete solidification, the side dies open and the top die is raised vertically. The wheel remains fixed to the top die prior to be ejected onto a transfer tray rolled under the top die. The die is then closed and the cycle begins again. Typical cycle times are 5–6 min. The wheel was then automatically picked up by a robot and cooled. To obtain a set of different cooling rates, water in the temperature range of 30-90°C was adopted. Slow cooling rate in air was also used.
