**2.2 Heat treatment and powder coating cycle**

The wheels were T6 heat treated in an industrial plant, whose lay-out is shown in Fig. 2 (Manente, 2008). The lay-out consists of a one-way line, where the wheels, loaded in suitable steel frame (handling unit), follow and complete the whole heat treatment cycle.

Fig. 2. Lay-out of the T6 heat treatment plant used in the present work (Manente, 2008)

A robot provides for loading 30 wheels in a five plane basket (Fig. 2 – Stage A). The basket is then moved into an air circulating tunnel furnace, where it is driven forward in 30 consecutive steps (Fig. 2 – Stage B). In the first 6 steps, the wheels are heated up to the set up solution temperature, while in the further steps they are maintained at temperature. The wheels were solution treated at 540 ± 5°C for 4, 5, 6, 7 and 8 hours (including heat up time) and immediately quenched (Fig. 2 – Stage C). The quenched delay was measured to be 20 s. To obtain a set of different quench rates, water at different temperature was adopted as quenchant. The water temperature ranged from 50 to 95°C. Slow quenching in air was also used. Table 2 shows the targeted and achieved quench water temperatures.


Table 2. Targeted and achieved temperature of water quenching

The wheels are subsequently transferred to an air circulating tunnel furnace, where they artificial aged (Fig. 2 – Stage D). This stage consists of 20 steps, where in the first 4 steps, the wheels are heated up to the set up ageing temperature, while in the further steps they are maintained at temperature. The wheels were artificially aged at 145 ± 5°C for 4 hours after

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

The wheels were T6 heat treated in an industrial plant, whose lay-out is shown in Fig. 2 (Manente, 2008). The lay-out consists of a one-way line, where the wheels, loaded in suitable

steel frame (handling unit), follow and complete the whole heat treatment cycle.

Fig. 2. Lay-out of the T6 heat treatment plant used in the present work (Manente, 2008)

used. Table 2 shows the targeted and achieved quench water temperatures.

Table 2. Targeted and achieved temperature of water quenching

A robot provides for loading 30 wheels in a five plane basket (Fig. 2 – Stage A). The basket is then moved into an air circulating tunnel furnace, where it is driven forward in 30 consecutive steps (Fig. 2 – Stage B). In the first 6 steps, the wheels are heated up to the set up solution temperature, while in the further steps they are maintained at temperature. The wheels were solution treated at 540 ± 5°C for 4, 5, 6, 7 and 8 hours (including heat up time) and immediately quenched (Fig. 2 – Stage C). The quenched delay was measured to be 20 s. To obtain a set of different quench rates, water at different temperature was adopted as quenchant. The water temperature ranged from 50 to 95°C. Slow quenching in air was also

Targeted 50 60 70 75 80 85 90 95 Achieved 48 58 67 75 81 86 89 94

The wheels are subsequently transferred to an air circulating tunnel furnace, where they artificial aged (Fig. 2 – Stage D). This stage consists of 20 steps, where in the first 4 steps, the wheels are heated up to the set up ageing temperature, while in the further steps they are maintained at temperature. The wheels were artificially aged at 145 ± 5°C for 4 hours after

Water temperature (°C)

adopted. Slow cooling rate in air was also used.

**2.2 Heat treatment and powder coating cycle** 

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 respectively, as indicated in Fig. 2.

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

### **2.3 Microstructural characterization**

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

#### **2.4 Distortion and hardness testing**

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

Optimizing the Heat Treatment Process of Cast Aluminium Alloys 203

The microstructure of the modified A356 alloy consists of a primary phase, α-Al solid solution, and an eutectic mixture of aluminium and silicon. The α-Al precipitates from the liquid as the primary phase in the form of dendrites. The scale of microstructure in different zones of the wheel was characterized by means of SDAS measurements. The coarseness of the microstructure varied inversely with the casting thickness, i.e. the solidification rate. Typical microstructure of the as-cast wheel is shown in Fig. 5, referred to the hub, spoke and rim zones, corresponding to 55, 36 and 22 μm in SDAS respectively. Local solidification times (tf) were estimated by means of SDAS measurements through the following

( )( )

l 0 eut 0

⎝ ⎠

m1k C C ⎛ ⎞ ⎛ ⎞ ⎜ ⎟ Γ ⎜ ⎟ ⎝ ⎠ = − − −

<sup>C</sup> D ln

sl l

where Γsl is the Gibbs-Thomson coefficient, Dl the diffusion coefficient in liquid, ml the slope of the liquidus curve, k0 the partition coefficient, C0 and Ceut are the initial alloy concentration and the eutectic composition respectively. The solidification time was estimated to be 184 s in the hub, 52 s in the spoke and 12 s in the rim zone. The solidification sequence is approximately directional, starting at the outermost point of the wheel (rim) and

Fig. 5. Microstructure of as-cast wheel with reference to the different positions analysed

continuing toward the centre of the wheel (hub), where the ingate is located.

<sup>C</sup> SDAS 5.5 <sup>t</sup>

eut

0

1 3

(1)

f

**3.1.2 Microstructural observations of as-cast wheels** 

relationship (Dantzig & Rappaz, 2009):

The amount of distortions of the wheels was carried out after post-cast cooling (ε) and after quenching (εt), by using a circular gauge, which allows to calculate the maximum variation of the diameter of the wheel along the rim. Generally, the maximum accepted distortion of a wheel is 1.5 mm, while wheels with higher distortions are normally rejected. This is a typical standard used for wheel manufacturing (Manente, 2008).
