**Optimizing the Heat Treatment Process of Cast Aluminium Alloys**

Andrea Manente1 and Giulio Timelli2 *1Cestaro Fonderie Spa 2University of Padova, Department of Management and Engineering Italy* 

### **1. Introduction**

196 Recent Trends in Processing and Degradation of Aluminium Alloys

diffraction of the implanted pieces reveals the presence of AlN in the cp and hcp crystalline phases where the peak intensities increase along with the nitrogen content. The presence of the hexagonal phase has been detected when either pure nitrogen or a 50% mixture have been used, suggesting a correlation between the h phase and the enhanced microhardness. Raman spectroscopy has confirmed the signature peak of AlN and, in addition to XRD, shows that the general surface improvement is enhanced with the N proportion in the Ar/N

A compromise between high hardness and low roughness in pure nitrogen is observed due to a competition between sputtering and nitriding after, at least, 1 hour of treatment. In particular, maximal microhardness values were found in samples treated with the equal part mixture. The best roughness was achieved with this gas mixture in all cases, although increasing along with the implantation pulse width up to a 300 nm peak at 150 μs. Such a performance can be maintained in a pure nitrogen plasma, provided that longer 1 hour

The authors are grateful to the technical collaboration received from Israel Alejandro Rojas Olmedo, Hannalí Millán Flores, Everardo Efrén Granda Gutiérrez, María Teresa Torres

Anders André, editor. (2000), "Handbook of plasma inmersion implantation ion and

Conrad, J. R., (1987) Sheath thickness and potential profiles of ion-matrix sheaths for

Conrad J. R., Radtke J. L., Dodd R. A., Worzala Frank J. and Tran Ngoc C. (1987) Plasma

Herman D. A., Gallimore A. D. (2008) An ion thruster internal discharge chamber

Manova D., Mändl S. and Rauschenbach B. (2001), Oxygen behaviour during PIII-nitriding

Selvaduray G., Sheet L. (1993) Aluminium nitride: review of synthesis methods Source,

Wang J. A., Bokhimi X., Morales A., Novaro O., López T. and Gómez R. (1999). Aluminum

cylindrical and spherical electrodes, *Journal of Applied Physics*, Vol.: 62, No. 3, pp 777

source ion-implantation technique for surface modification of materials, *Journal of* 

electrostatic probe diagnostic technique using a high-speed probe positioning

of aluminium, *Nuclear Instruments and Methods in Physics Research Section B: Beam* 

Local Environment and Defects in the Crystalline Structure of Sol-Gel Alumina

deposition". Ed. John Wiley and Sons, ISBN 0-471-24698-0, USA

system, *Review Scientific Instruments*, Vol. 79, 013302, 10 pages.

*Materials Science and Technology*, Vol. 9, No. 6, pp 463-473

Catalyst, *J. Phys. Chem. B,* Vol. 103, pp*.* 299-303

*Interactions with Materials and Atoms*, Vol. 178, No. 1-4, pp 291-296.

mixture concentration.

**6. Acknowledgment** 

**7. References** 

– 779

implantation periods are performed.

Martínez, Pedro Angeles Espinoza and Isaías Contreras Villa,.

*Applied Physics*, Vol 62, No. 11, pp. 4591-4596

The unfailing increased use of light alloys in the automotive industry is, above all, due to the need of decreasing vehicle's weight. The same need has to be taken into account in order to face up also both energetic and environmental requirements (Valentini, 2002). In terms of application rates, Al and its alloys have an advantage over other light materials. The reduced prices, the recyclability, the development of new improved alloys and casting processes, the increased understanding of design criteria and life prediction for stressed components and an excellent compromise between mechanical performances and lightness are the key factors for the increasing demand of Al alloys. A consolidated example of aluminium alloy employment regards the production of wheels, which, together with an improved aesthetic appearance, guarantees an improvement of driving, like directed consequence of the inertia reduction. These critical safety components are somewhat unique as they must meet, or exceed, a combination of requirements, from high quality surface finish, as wheels are one of the prominent cosmetic features of cars, to impact and fatigue performance. Due to their excellent castability and good compromise between mechanical properties and lightness, AlSiMg alloys are the most important and widely used casting alloys in wheel production (Conserva et al., 2004). Further, the increasing application of these alloys has been driven by the possibility to improve the mechanical properties of cast components through the use of heat treatments. Various heat treatments, e.g. different combinations of temperatures and times, have been standardized by Aluminium Associations and they are used in Al foundry depending on the casting process, the alloy type and the casting requirements (ASM Handbook, 1990). Standard T6 heat treatment is generally applied in wheel production. This heat treatment provides two beneficial effects for cast aluminium alloy wheels: an improved ductility and fracture toughness through spheroidization of the eutectic silicon particles in the microstructure and a higher alloy yield strength through the formation of a large number of fine precipitates which strengthen the soft aluminium matrix (Zhang et al., 2002). The T6 heat treatment comprises three stages (ASM Handbook, 1991): solution heat-treating, quenching and artificial aging.

*Solution heat-treating* at relatively high temperature is required to activate diffusion mechanisms, first, to dissolve Mg-rich phases formed during solidification and, then, to homogenize the alloying elements, such as Mg and Si, so as to achieve an elevated yield stress subsequent ageing (ASM Handbook, 1991). Further, the solution heat treatment

Optimizing the Heat Treatment Process of Cast Aluminium Alloys 199

effect of cooling rate on wheel distortion and hardness during the post-cast and quenching steps, and the influence of the solutionizing temperature and time, and the powder coating

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

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

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

Alloy Al Si Fe Cu Mn Mg Zn Ti Sr A356 bal. 7.20 0.135 0.009 0.010 0.265 0.004 0.126 0.0279

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

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

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

**2. Material and experimental techniques** 

casting has a weight of about 18 kg.

**2.1 Alloy and casting parameters** 

Reduced Pressure Test (RPT).

region

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

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

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 manufacturing cost.

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 effect of cooling rate on wheel distortion and hardness during the post-cast and quenching steps, and the influence of the solutionizing temperature and time, and the powder coating cycles on the microstructure and mechanical properties of the 18-inch wheels.
