Improved microstructure and mechanical properties of secondary Aluminium-Silicon alloys

Primary aluminum alloys are widely used in the manufacturing of safety parts for the automotive industry. In this context, the aluminium silicon alloys are one of the most used groups of foundry alloys due to their excellent castability and good mechanical properties.

Secondary aluminum alloys are rich in impurities, in particular in iron and copper, resulting in worse mechanical properties and corrosion resistance compared to primary alloys. For these reasons, these alloys are not used when the cast part has to fulfill high mechanical property requirements. However, the use of recycled alloys brings potential energy saving and a competitive price.

The most deleterious impurity element in secondary alloys is Iron due to its tendency to form brittle intermetallic phases such as b-Al5FeSi. The plate/needle like morphology of the b-Al5FeSi phase impairs mechanical properties, especially, ductility and fracture toughness.

a)                                                                    b)

Figure 1. As-cast microstructure of the AlSi7Mg0.3Fe0.3 modified with CrV (alloy 5). a) Coarse Chinese script a- compound in the interdendritic spaces and b) fine a-iron compounds within the eutectic Al-Si.

The microaddition of some elements, such as Mn, Be, Cr, and Co, prevents the formation of β-Al5FeSi phases by changing the morphology of the of β-Al5FeSi phase into a less harmful one (Chinese script or globular). Therefore, these microadditions could improve the quality of secondary alloys with high iron content, however, the introduction of additional alloying elements will increase also the overall amount of intermetallic compounds.

a) Alloy 2                                                     b) Alloy 2

c) Alloy 3                                             d) Alloy 3

e) Alloy                                                 6 f) Alloy 6

Figure 2. SEM micrographs showing observed iron compounds and corresponding EDX spectra: a) and b) for alloy 2 with 0. 04 wt % Mn; c) and d) a- iron compound of alloy 3 with 0.16 wt % Mn; and e) and f) a- iron compound of alloy 6 with 0.26 wt % MnCrV.

IK4-AZTERLAN investigates the effect of individual and/or combined additions of Mn, Cr and V on the morphology of the intermetallic iron compounds and mechanical properties of a recycled AlSi7Mg0.3Fe0.3 alloy cast in sand moulds. Some conclusions of this ongoing research are exploited in the following lines:

1. Mechanical properties comparable to the ones of the corresponding primary alloy can be achieved by appropriate additions of alloying elements.

2. Independent of the alloying element addition, individual or combination of different elements, it is necessary to add about the same quantity of alloying elements as the Fe content (Mn+Cr+V:Fe = 1:1) in order to transform the b- compounds into Chinese script and/or fine a- iron compounds.

3. The quality index, Q = UTS + K • log E, was found to decrease strongly by the presence of intermetallic iron compounds, in particular b-Al5FeSi compounds, and to lesser extent by high quantities of a-iron compounds.

4. The highest quality index, Q equal or above 400 MPa, is achieved when no b-Al5FeSi compounds or only in a very low fraction (< 0.03 %) are present in the microstructure and the content of elements added to avoid the formation of b-Al5FeSi compounds is at optimum.

5. As the content of alloying elements is further increased the size and also the area fraction occupied by intermetallic compounds increases and hence, the quality index Q decreases.

Table 1: Mechanical properties of the alloys with different microadditons.