Morphology of intermetallic phases in Al-Si cast alloys and their fracture behaviour

Applications of Al-Si cast alloys in recent years have increased especially in the automotive industry (dynamic exposed cast, engine parts, cylinder heads, pistons and so on). Controlling the microstructure of secondary aluminium cast alloys is very important, because these alloys contain more additional elements that form various intermetallic phases in the structure. Therefore, the contribution is dealing with the valuation type of intermetallic phases and their identification with using optical and scanning microscopy. Some of the intermetallic phases could be identified on the basis of morphology but some of them must be identified according EDX analysis. The properties of aluminium alloy are affected by morphology of intermetallic phases and therefore it is necessary to study morphology and its fracture behaviour. The present work shows morphology and typical fracture behaviour as the most common intermetallic phases forming in Al-Si alloys.


Introduction
Aluminium alloys have been the most common materials in different industries, but especially in the areas of aerospace and automotive, on account of their high stiffness/weight ratio and strength/weight ratio, good formability, good corrosion resistance etc. ( Al-Si-Cu alloys usually contain Cu (2 -4 %), a certain amount of Fe, Mn, Mg and Zn that are pre-sent either unintentionally, or they are added deliberately to provide special material properties. These elements partly go into solid solution in the matrix and partly form intermetallic particles during solidification (RIOS C.T., 2003). The influence of intermetallic phases to mechanical and fatigue properties depends on size, volume and morphology of these phases (TILLOVÁ E., 2010). The formation of these phases should correspond to successive reactions during solidification with an increasing number of phases involved at a decreasing temperature. In practice, Bäckerud et al. (BÄCKERUD L., 1986; BÄCKERUD L., 1992) identified these reactions in Al-Si-Cu alloy: • 602°C: α-dendritic network. The exact temperature depends on the Si and Cu concentration in the alloy; • 590°C: Liq. → α-phase + Al 15 (Fe,Mn) 3 Si 2 ; • 575°C -507°C: Liq. → α-phase + Si + Al 5 FeSi; • 525°C -507°C: Liq. → α-phase + Al 2 Cu + Al 5 FeSi + Si. Reduction of temperature allows nucleation of Cu-enriched eutectic (Al + Al 2 Cu); • 507°C: Liq.→ α-phase + Al 2 Cu + Si + Al 5 Mg 8 Si 6 Cu 2 . • 483°C: End of the alloy solidification (solidus temperature). Suwanpinij et al. (SUWANPINIJ P., 2003) calculated crystallization temperatures of major phases by the Gulliver-Scheil model in the 380-type alloy (Fig. 1).

Fig.1 The cooling curve of 380-type alloy (SUWANPINIJ P., 2003)
The present study is a part of a larger research project, which was focused on a study of secondary Al-Si cast alloy. The purpose of the present article is to investigate the microstructure of recycled AlSi9Cu3 cast alloy with a combination different analytical techniques and valuations of fracture behaviour structure components.

Methodology of research
As an experimental material was used secondary AlSi9Cu3 alloy, that contains 9.4 % Si, 2.4 % Cu, 0.9 % Fe, 0.28 % Mg, 0.24 % Mn, 1.0 % Zn, 0.03 % Sn, 0.09 % Pb, 0.04 % Ti, 0.05 % Ni, 0.04 % Cr. The secondary alloy (prepared by recycling of aluminium scrap) was received in the form of 12.5 kg ingots. Experimental material was molten into the metallic mould (chill casting). The melting temperature was maintained at 760 °C ± 5 °C. Molten metal was purified with salt AlCu4B6 before casting and was not modified or grain refined. The microstructure of ex-perimental material was studied using light microscope Neophot 32 and SEM observation with EDX analysis using scanning electron microscope VEGA LMU II linked to the energy dispersive X-ray spectroscopy (EDX analyzer Brucker Quantax). The samples for metallographic observations (1.5 cm x 1.5 cm) were prepared by standard metallographic procedures (wet ground, polished with diamond pastes, finally polished with commercial fine silica slurry -STRUERS OP-U and etched by standard (Dix-Keller, 0.5 % HF) reagent. Some samples were also in order to reveal the threedimensional morphology of the silicon phase and intermetallic phases deep-etched for 30 s in HCl solution. The fracture surface (and fracture behaviour of structure components) was observed by using scanning electron microscope on Charpy impact specimens after impact bending test.
Al 5 FeSi phases precipitate in the interdendritic and intergranular regions as platelets (appearing as needles in the metallographic microscope). Long and brittle Al 5 FeSi platelets (more than 500 µm) can adversely affect mechanical properties, especially ductility, and also lead to the formation of excessive shrinkage porosity defects in castings (RIOS C.T., 2003; TAYLOR J. A., 2004). The β platelets appeared to be the main nucleation sites for the eutectic Si, eutectic Al 2 Cu and Cu-rich phase. Excess Mn may reduce Al 5 FeSi phase and promote formation Fe-rich phases Al 15 (FeMn) 3 Si 2 (known as alpha-or α-phase) in the form "skeleton like" or in the form "Chinese script". Phase The Cu-rich intermetallic phases observed in experimental material are important when material is heat treated. These phases led to formation Ѳ and Ѳ´ precipitates with body centered-tetragonal crystal structure with the stoichiometry Al 2 Cu in substructure and improve mechanical properties of materials (BISWAS A., 2014).

Fig. 3 The morphology of Cu-rich phases, etch. Dix-Keller
The fracture surface was influenced very significantly by structural components (α-phase, eutectic silicon, intermetallic phases) and their distribution in the cross section. The overall appearance of the fracture surface is not only a violation of the matrix (αphase), but also the shape and size of eutectic Si and intermetallic phases.
The matrix is characterized by high plasticity while the crystals of eutectic silicon and Fe-rich intermetallic phases have higher values of hardness and so almost zero values of plastic properties. Therefore, the fracture surface of AlSi9Cu3 cast alloy composed of matrix and Cu-rich intermetallic phases ductile fracture and cleavage fracture of hard and brittle structural components (eutectic Si and Fe-rich intermetallic phases).

Summary and conclusions
In the present study, the morphology and fracture behaviour of intermetallic phases in AlSi9Cu3 cast alloy was investigated. From the analysis of the results the following conclusions can be drawn: In AlSi9Cu3 cast alloy two Fe-rich phases were observed -Al 5 FeSi needles; skeleton-like Al 15 (FeMn) 3 Si 2 and two Cu-rich intermetallic phases -Al 2 Cu and Al-Al 2 Cu-Si.
Al 15 (FeMn) 3 Si 2 phase was dominant thanks to the presence of Mn. The morphology and size of iron phases are undesirable.
The morphology of structural components (eutectic silicon, intermetallic Fe-rich and Cu-rich phases) significantly affects the fracture surface of Al-alloy.
The fracture surface of as-cast state is forming of transcrystalline cleavage and ductile fracture. On the fracture surface, transcrystalline cleavage fracture is dominant. Transcrystalline cleavage fracture is related to the presence of large hexagonal plate-Si particles in the structure and also brittle iron intermetallic phases. The transcrystalline ductile fracture of Al matrix (αphase) and Cu rich intermetallic phases is observed in the smaller surface.