**3.2.2 Partial melting**

208 Recent Trends in Processing and Degradation of Aluminium Alloys

**0**

Fig. 10. (a) Linear regression analysis of eutectic Si equivalent radius with t1/3; the point zero in the time axis represents the as-cast condition; (b) frequency distribution of the shape

> **20 30 40 50 60 SDAS (µm)**

**Solution treatment 540°C for 6h**

**As-cast**

Fig. 11. Average diameter *d* of the eutectic Si particles as a function of SDAS; data refer to

changes in particle distribution are not observed by increasing the solution times within 4 hours, even if the distribution curves flatten with solution time and their peaks move to the

**Spoke**

**5**

**10**

**15**

**Frequency (%)**

**20**

**25**

**30**

 **0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Shape factor,** α

**Hub**

**As cast 30 min 60 min 120 min 240 min 480 min**

**02468 Time of the solution heat treatment,** *t* **1/3 (min)**

**0.4**

**0.8**

**1.2**

**1.6**

**Rim**

the different positions of the as-cast and solution heat treated wheels

**Equivalent diameter,** 

*d* **(µm)**

**2**

**2.4**

**2.8**

**Linear fit (R2=0.97)**

(a) (b)

factor *α* after solution treatment at 540°C for different times (Pedersen, 1999)

**0**

**0.4**

**0.8**

**1.2**

**Equivalent radius,** *R* **(µm)**

**1.6**

**2**

The increase of solution temperatures for the heat treatment of the wheels would be desirable since it increases the diffusion rate of Si atoms in the Al matrix, leading to rapid fragmentation and coarsening mechanism of eutectic Si particles, and, therefore, to shorten the total time of the T6 heat treatment cycle. It was demonstrated that for a given short solution treatment time of 9.5 minutes, increasing the temperature from 540 to 550°C the number fraction of Si particles with a diameter of greater than 1 μm increases by more than 10%. Similar changes in the distribution of the shape factor for Si particles are observed by increasing the solution temperature, that is the number fraction of the particles with a shape factor of greater than 0.5 increases by approximately 10% (Zhang et al., 2002). Earlier works (Shivkumar et al., 1990b) showed that extremely high coarsening occurred at temperatures greater than 540°C for A356.2 alloys. However, the major problem associated with higher heat treatment temperatures remains the liquid phase formation, which increases with temperature.

In the present work, the possibility to heat the wheels at higher solution temperature was evaluated. A Fourier thermal analysis was carried out to determine the evolution of the solid fraction during solidification of the A356 alloy used for wheel production. A detailed description of the equipment, the casting procedure, and the process parameters is given elsewhere (Piasentini et al., 2005). The relationship between fraction of solid (fs) and temperature of solidifying A356 alloy is shown in Fig. 12 for a cooling rate of 1°C/s. With increasing solution temperature above 540°C (final solidification point), the amount of liquid phase (100 fs) increases slowly at first and then rapidly near the Al-Si eutectic reaction of ~560°C, at which point the fraction of liquid (100-fs) is about 15%.

At relatively lower solution temperatures, melting starts at grain boundaries and interdendritic regions. In alloys with a dendritic structure, local melting starts generally at interdendritic channels, since these often contain high concentrations of alloying elements/impurities. At higher solution temperatures, local melting may also start at grain

Optimizing the Heat Treatment Process of Cast Aluminium Alloys 211

Therefore, regions of the wheel solidified at high cooling rate, such as the rim, show large amounts of liquid phase formation as compared to those solidified at lower cooling rate, such as the spoke, presumably due to greater segregation of solute elements at

Fig. 14. Fracture path developed by coalescence of shrinkage porosity due to quenching of

Fig. 15 shows the microstructure of artificial aged wheels, which were water quenched at 45 and 95°C. The size and shape of the eutectic Si particles were not influenced by the quenching condition used in the present work. The different quenching media influenced probably the Mg2Si and Si precipitates in the α-Al matrix obtained by subsequent artificial ageing. Detailed TEM investigations on A356 alloy, reported elsewhere (Zhang & Zheng, 1996), revealed that, at the peak-aged condition and with a water quench at 25°C, the α-Al matrix consists of a large number of needle-shaped and coherent β″-Mg2Si precipitates. The size of the precipitates is approximately 3 to 4 nm in diameter and 10 to 20 in length. With a

Fig. 15. Microstructure in the hub of the wheels; the micrographs refer to artificial aged A356

alloy solubilised at 540°C for 6 h and water quenched at (a) 45 and (b) 95°C

interdendritic regions and grain boundaries.

liquid phase

**3.3 Quenching** 

**3.3.1 Microstructural observations** 

Fig. 12. Solid fraction versus temperature of the A356 alloy used in the present work

boundaries. However, it is difficult to distinguish between interdendritic and grain boundary melting in the microstructure. Interdendritic and grain boundary melting is shown in Fig. 13. The Fe-rich intermetallics melt at solution temperatures above 550°C leading to formation of spherical liquid droplets within the dendrites/grains. At high solution temperatures the width of the grain boundary melted zone increases, and spherical interdendritic liquid droplets enlarge and coalesce to form a large network of interdendritic liquid. On quenching this liquid, reprecipitation of silicon and other intermetallic particles may 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 casting, as seen in Fig. 14. The amount of liquid phase formed with high temperature solution treatment depends greatly on the initial solidification rate.

Fig. 13. Interdendritic and grain boundary incipient melting

Therefore, regions of the wheel solidified at high cooling rate, such as the rim, show large amounts of liquid phase formation as compared to those solidified at lower cooling rate, such as the spoke, presumably due to greater segregation of solute elements at interdendritic regions and grain boundaries.

Fig. 14. Fracture path developed by coalescence of shrinkage porosity due to quenching of liquid phase
