**2. Material and experimental techniques**

198 Recent Trends in Processing and Degradation of Aluminium Alloys

changes the morphology of eutectic Si from polyhedral, or fibrous morphology in the modified alloys, to globular structure. Various efforts have been made to investigate the effects of solution temperature and time on microstructure and mechanical properties of AlSiMg foundry alloys (Zhang et al., 2002; Rometsch et al., 1999; Pedersen & Arnberg, 2001; Shivkumar et al., 1990a; Dwivedi et al., 2006; Taylor et al., 2000; Langsrud & Brusethaug,

*Quenching* is usually carried out to room temperature to obtain a supersaturated solid solution of solute atoms and vacancies, in order to achieve an elevated strengthening subsequent ageing (ASM Handbook, 1991; Liščič et al., 1992; Komarova et al., 1973; Totten et al., 1998; Totten & Mackenzie, 2000). The most rapid quench rate gives the best mechanical properties, but it can also cause unacceptable amounts of distortion or cracking in components (Auburtin & Morin, 2003). Thus, parts of complex shape, often with both thin and thick sections, are commonly quenched in a medium that provides a slower cooling. This quenchant can be hot water, an aqueous solution of polyalkylene glycol, or other fluid medium such as forced air or mist. In this way the heat transfer coefficient between the piece and the quenchant is reduced, the heat transfer from the surface is delayed and a more uniform temperature between the surface and the centre is obtained (Liščič et al., 2010; Totten et al., 1998; Totten & Mackenzie, 2000; Bates, 1987; Bates, 1993). Therefore, a balance between fast cooling and distortion minimization is required in quenched components. *Artificial ageing* consists of further heating the casting at relatively low temperatures (120- 210°C) and it is during this stage that the precipitation of dissolved elements occurs. These precipitates are responsible for the strengthening of the material. In AlSiMg alloys, the decomposition of the supersaturated solution begins with the clustering of Si atoms. This clustering leads to the formation of coherent spherical GP zones, consisting of an enrichment of Mg and Si atoms, that elongate along the cube matrix direction to develop into a needle shape coherent β″ phase. With prolonged ageing, the needle shaped GP zones grow to form rods of an intermediate phase, β′, which is semicoherent with the matrix. The final stable β-Mg2Si phase forms as an incoherent platelets on the α-Al matrix and has ordered facecentered-cubic structure. Several studies have been made to investigate the effect of artificial ageing temperature and time on strengthening mechanism of cast AlSiMg alloys. Ageing in the temperature range 170-210°C gives comparable peak yield strength (Rometsch & Schaffer, 2002; Alexopoulos & Pantelakis, 2004), and, with higher temperatures, the time to peak can be shortened. At ageing temperatures higher than 200°C, the β″ phase is

1998; Cáceres et al., 1995; Cáceres & Griffiths, 1996; Wang & Cáceres, 1998).

substituted by the β′, which contributes less to strengthening (Eskin, 2003).

manufacturing cost.

It is of vital importance to consider both the foundry process and the T6 heat treatment on the whole, in order to achieve the required performances and specific properties (Merlin et al., 2009). While the benefit of T6 heat treatment is accepted, the additional cost and production time associated with such a heat treatment are substantial. Considering the whole production cycle of a standard automotive aluminium alloy wheel made by a lowpressure die-casting process (LPDC), the casting process normally takes less than 6 min, while a typical T6 heat treatment cycle may take more than 10 h. This means that shortening the total time of the T6 heat treatment cycle has a great impact on productivity and

In the present work, some process variables, which play a key role in production cycle of wheels have been investigated and improved. An integrated methodology for developing and optimizing the production and the final quality of A356-T6 18-inch wheels, in terms of casting distortion and hardness, is proposed. This study focuses on examining both the An approach for optimizing wheel production has been applied on A356-T6 18-inch wheels, which are 5-spoke wheels in the T6 temper, with a diameter of 457 mm and a rim width of 203 mm. Fig. 1 shows a sketch of the analysed wheel, which is generally cast by LPDC. The casting has a weight of about 18 kg.

Fig. 1. Sketch of the low-pressure die-cast wheel analysed; the ingate is located in the hub region
