**4. Modelling of the heat treatment process**

66 Heat Treatment – Conventional and Novel Applications

β''.

ageing and artificial ageing time and temperature [64-66].

β'' phase. The β'' phase is therefore preferred, rather than the Q'' phase. It is however not clearly stated when the Q'' phase forms at the expense of the β'' phase in cast alloys. For wrought alloys it has been shown that the fraction of the Q'' phase increases with natural

**Figure 6.** Sequence of phases found during age hardening of Al-Mg-Si alloys [60-62]. Supersaturated solid solution (SSSS) decomposes as Mg and Si atoms are attracted first to themselves (cluster) then to each other to form precipitates GP(I), sometimes also called initial-β''. GP(I) zones either further evolve directly to a phase β'' and then to a number of other metastable phases labelled β', B', U1, U2 (another one, U3, has been postulated theoretically), or first form an intermediate phase called pre-

 **Figure 7.** TEM images of Al-Si-Mg alloy subjected to 2 different heat treatments. (a) solutionising and quenching, immediate aging at 180°C for 540 min, (b) solutionising and quenching, natural ageing for

The precipitation of metastable Mg-rich phases depends on the Mg-to-Si ratio. The excess of Si in solid solution can significantly alter the kinetics of precipitation and the phase composition. In other words, equilibrium phases are enriched in Mg and metastable phases are enriched in Si. Silicon precipitates are observed if stable phases are formed [67,68].

10,000 min at 20°C, aging at 180°C for 540 min [63].

Designing an alloy and a heat treatment process for a material that meets specified requirements for a certain component can be facilitated by the use of models. Development of models can also help in the search for new alloys as knowledge is gained about the influence of a specific part of the microstructure on the alloy properties. The first model where the yield strength is coupled to the evolution of the microstructure during artificial ageing was developed by Shercliff and Ashby in 1990 [69]. They defined their model as a mathematical relation between the process variables (e.g. alloy composition, heat treatment temperature and time), and the mechanical response of the alloy (e.g. yield strength, hardness), based on physical principles (e.g. thermodynamics, kinetics of precipitation, strengthening mechanisms etc.).

More refined models have been developed since then for prediction of yield strength [70,71] and elongation to fracture [72] after artificial ageing. To be able to model the tensile strength after heat treatment, the evolution of the microstructure has to be modelled from casting to artificial ageing. Empirical equations as the Hollomon's [73] and the Ludwigson's [74] and equations where the parameters are coupled to the microstructure as in the KM strain hardening theory can be used to describe the plastic deformation behavior. The KM strain hardening theory has already been successfully used to couple the plastic deformation behavior to the microstructure for heat treatable wrought alloys and Al-Si-Mg casting alloys.

#### 68 Heat Treatment – Conventional and Novel Applications

The Scheil equation is a simple model giving fair results for segregation profiles and fraction of particles formed during solidification for aluminum alloys. The Scheil equation assumes no diffusion in the solid and complete diffusion in the liquid [75]. The correctness of the predictions of the Scheil segregation model depends on the diffusivity of the alloying elements in the α-Al phase.

A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 69

Although many previous investigations into the thermal processing of Al-Si-Cu/Mg casting alloys have been carried out, most focus on a single aspect of the overall process and a comprehensive experimental study considering all heat treatment stages is still required. This review shows that it is of vital importance to take the whole heat treatment process into consideration in order to achieve the optimal mechanical properties of an alloy. It is not sufficient to consider only the solution treatment and the artificial ageing parameters. Furthermore, the development of process models for the prediction of microstructure and mechanical property changes in aluminum alloys has focused on wrought alloys, while casting alloys that contain more complex microstructures have been overlooked and the evolution of the solution treated microstructure and its influence on subsequent ageing

**5. Conculsions** 

**Author details** 

A.M.A. Mohamed

F.H. Samuel

**7. References** 

standards

**6. Acknowledgement** 

behaviour has not been incorporated into the models.

*Department of Metallurgical and Materials Engineering,* 

*Materials Technology Unit (MTU), Qatar University, Doha, Qatar* 

*Université du Québec à Chicoutimi, Chicoutimi, Québec, Canada* 

Technology Unit, MTU, at Qatar University for her help and support.

of Materials Science and Engineering A 2012; A543 22-34.

Aluminium Alloy. Int. J. Cast Metal. Res. 1999;12 67–73. [4] Stefanescu D. Casting, ASM International 1988, vol.15. [5] Pekguleryuz M.O. Aluminum physical metallurgy. 2007.

and Design 2009;30(10) 3943-3957.

*Faculty of Petroleum and Mining Engineering, Suez Canal University, Suez, Egypt,* 

The authors would like to thank Professor Mariam Al-Maadeed, Head of Materials

[1] Mohamed A.M.A, Samuel F.H., Alkahtani S. Influence of Mg and Solution Heat Treatment on the Occurrence of Incipient Melting in Al-Si-Cu-Mg Cast Alloys. Journal

[2] Mohamed A.M.A, Samuel A.M., Samuel F.H., Doty H.W. Influence of Additives on the Microstructure and Tensile Properties of Near-Eutectic Al-10.8%Si Alloys. Materials

[3] Djurdjevic M., Stockwell T., Sokolowski J., 1999. The Effect of Strontium on the Microstructure of the Aluminium-Silicon and Aluminium-Copper Eutectics in the 319

[6] ASTM. (2006). Aluminum and Magnesium Alloys Annual (Volume 02.02). ASTM

From the as-cast microstructure the time needed for dissolution and homogenization can be modelled. The model developed by Rometsch et al. [76], which handles solution treatment of Al-Si-Mg alloys, is an example of a simple, but efficient model. The evolution of the microstructure during artificial ageing involves nucleation, growth and coarsening. Two main approaches are used; precipitates having an average radius or precipitates having a size distribution. For the case of precipitates which a size distribution, coupled nucleation, growth and coarsening can be calculated, while for an average radius growth is sequentially followed by coarsening.

The strength of an alloy derives from the ability of obstacles, such as precipitates and atoms in solid solution, to hinder the motion of mobile dislocations. The strength contributions from atoms in solid solution and from shearable and non-shearable precipitates change during ageing, while contributions from lattice, dislocations and grain boundaries are constant. Small and not too hard precipitates are normally sheared by moving dislocations, see Figure 8.a. When the precipitates are larger and harder the moving dislocations pass the precipitates by bowing, leaving a dislocation ring around the precipitate, see Figure 8.b. The strength of the precipitates increases with size as long as it is sheared by dislocations. When dislocations pass the precipitates by looping, the alloy strength decreases with increasing radius of the precipitates. Figure 8.c shows the different strength contributions to the total yield strength for different ageing times.

**Figure 9.** Dislocations passing a precipitate by a) shearing and b) looping (Orowan mechanism) [77] c) Illustrates the different strength contributions to the total yield strength
