SMAC: E-C-T. Electromagnetic Casting Table

EFFECT OF ELECTROMAGNETIC FIELD ON HOMOGENITY OF CONTINUOUS CASTING

ALUMINIUM ALLOY  EN AW 6060

More than 80 percent of the aluminum used worldwide in extruded sections are aluminum alloys, and more than 80 percent of those alloys belong to the 6xxx series. Alloy EN AW 6060 is the most widely used of series 6xxx (Al-Mg-Si).

In these alloys, Mg and Si combine to form the chemical compound Mg2Si (Magnesium-Silicide) that induces the process of the primary hardening phase.

The production of billets for aluminum extrusion requires a certain degree of homogeneity.

But in the second phase there are some segregated particles, inside the cast structure of the billets, dendrites that must be homogenized by a heat treatment in order to minimize or eliminate such segregation of alloying elements and to obtain as much as possible a deformation behavior. Worldwide practice suggests a long homogenizing process at elevated temperatures in order to achieve such condition.

Meanwhile, the extrusion process is strongly influenced by the amount of Mg and Si in solid solutions and by the size, the distribution and the shapes of Mg2Si precipitated particles.

Facts are that grain boundaries have relatively open regions where the strain energy is minimum and that’s why there are some segregation of the un-dissolved particles both on micro and macro scale.

  

Most of the alloying elements added to aluminum alloys are less soluble in the solid phase than in the liquid phase, then a gap between the liquid and the solid will exist leading to the segregation of these alloying elements. This segregation is reduced by heating the ingots for an extended period close to the solid temperature in order to homogenize the structure.

The subject of this study is to compare the microstructure and the segregation in the grain boundaries for the billets that are

·       direct-chill (DC) cast with homogenizing heat treatment and

·       the DC casting in electromagnetic filed (ECT) without the homogenized process.

The first effects of improved metallurgical  properties with ECT casting has been done  with laboratory testing phase, followed by industrial application in a DC foundry on the existing casting tables. In this foundry DC cast (with ECT) alloy series 2xxx and 6xxx.

Billets produced from alloy 6060, 6012 and 6082 on this casting table modified with magnets are produced with an increased speed of the 20÷30% compared to the processed known at the present time; moreover billets are extruded without homogenizing heat treatment process.

The evidence is that billets cast with ECT have better surface quality, the structure is very fine, and an amazing homogeneity with small, globular grain without boundary segregation.

It is usual that ECT almost eliminates macro porosity and has good influence on grain refining.

The next figures show metallographic analyses of samples of billets produced from alloy 6060 with

·       DC cast in electromagnetic field without homogenization process (fig.1)

·       and traditional DC cast with homogenizing process (fig.2).

Figure 1 clearly show microstructure samples of billets cast with ECT with clearly marked fine, globularly Mg2Si particles and very small particles coming from AlFe3 phase. Moreover there is no segregation zone near to billet surface.

Figure 2 show a microstructure with un-dissolved, coarse Mg2Si particles which come from unfinished homogenizing process, so that the process of Mg2Si phase has not been transformed into solution.

Fig 3 and fig. 4 show, by a polarization light device, the grain size of both processes.

CONCLUSION: Using electromagnetic field for casting, instead of traditional DC cast with homogenizing process, we can achieve:

·       significant similar distribution and globular shape during the different phases

·       significant similar distribution of smaller parts in the Al3Fe phase

·       absence of surface segregation zones.

All these points indicate homogeneity within the structure and the absence of segregations, so extrusion without heat treatment is possible. These are facts with a real proof.