Characterization of recycle-like Al-Si alloys processed by severe plastic deformation

Abstract

Three aluminium alloys from the Al-Si-Fe system were selected as an object of the present study. The chemistry of the investigated materials, with relatively large additions of iron and silicon, was selected to obtain large volume fractions of iron and silicon containing second phases, which are commonly observed in secondary (recycled) aluminium. Thus, three recycled-like alloys with particle content changing in controlled manner were formed. The three recycle-like Al-Si-Fe alloys were successfully deformed at room temperature by Equal Channel Angular Pressing (ECAP) to various strain levels. The effect of particles on the microstructures and mechanical properties due to ECAP deformation and subsequent annealing was analyzed. Moreover, changes to the coarse second phase constituents during plastic deformation were inspected.

The results of this work have shown that large particles fracture early during the first ECAP pass, whereas smaller particles require larger plastic strains in the material before they fracture. A rapid particle size reduction during the first ECAP pass proceeds at a slower rate in subsequent passes. Elongated iron-containing phases fractured more readily than the more compact silicon phase. The general tendency was to reduce the aspect ratio of particles, which in turn lowered the loads transmitted to particles through particle-matrix interfaces upon deformation.

Analyses of particle rotation phenomena during plastic deformation were approached through statistical measurements and direct observations on the surface of split ECAP samples. The experimental results showed that the average rotation angle of all particles increased with increasing strain. Rotation of elongated particles was shown to be well approximated by a purely geometrical treatment and ,in case of ECAP, the rotations can be reasonably described by a simple model taking into account the initial orientations of particles with regard to the shear direction. Compact particles rotated in a more random fashion, what was most probably a consequence of short inter-particle distances leading to complex stress fields and slip patterns in the matrix.

Particles were observed to accelerate deformation induced microstructure refinement in their immediate surroundings. In consequence the measured grain size was finer in materials containing larger fractions of coarse particles. Lattice rotations close to separated particles were observed to follow a constant pattern and were assumed to be related to stresses arising from strain incompatibilities between stiff particles and the ductile matrix. Lattice rotations in particle clusters were more randomized what was most probably a consequence of overlapping stress fields from different particles constituting the cluster.

Deformation by ECAP led to considerable strengthening and the most significant increase occurred during the first ECAP pass. The increase in strength was accompanied by a reduction in ductility. Generally, a higher strength was obtained after deformation of materials containing larger fraction of coarse second phases.

Annealing of deformed materials at low temperatures (T<130°C) restored ductility to that of the non-ECAPed condition with little reduction in strength. Prolonged ageing did not improve ductility any further but gradual softening could be observed. At intermediate annealing temperatures (200°C - 250°C) the softening was more rapid and so was the strength reduction. Ductility was not improved above that of non-deformed materials until recrystallization occurred. Fully recrystallized materials were observed to have considerably improved plastic properties, indicating the beneficial effect of breakup of the cast microstructures.

Microstructures of heavily deformed materials were observed to coarsen upon annealing. The coarsening was relatively uniform during annealing in a low temperature regime. It was similar to the process commonly termed as ‘continuous recovery/ recrystallization’. Annealing at temperatures above ~250°C resulted in discontinuous growth of dislocation free grains nucleated predominantly in particle deformation zones (PDZs).

The effect of particles on crystallographic grain orientation distributions was analyzed in both deformed and annealed materials. Textures were shown to be weakened by the presence of coarse particles what suggests that there is a considerable spread of orientations in the PDZs. Textures weakened also upon annealing and the process was observed to be faster in alloys containing more second phase constituents.

Analysis of the strain distribution in the shear zone of the ECAP process was performed based on grids and based on microstructural features. The grid based strain measurements documented the large-scale strain variations whereas the microstructure based strain measurements allowed for finer scale observations of strain distribution in deformed state.