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Tensile stress occurs in the vicinity of upper surface of the specimen
in the severe plastic deformation zone, which increases the cracking and
fracture tendency of the specimen and impedes the further ECAP processing.
In this paper, the conventional ECAP die (@J=16^o and @F=90^o) was modified
to eliminate the tensile stress and enhance the compressive stress in the
severe plastic deformation zone, therefore reducing the cracking and
fracture tendency of the specimen. Finite element analysis demonstrated
that the stress state changes from tensile to strongly compressive when
using the modified die. A modified die was made and employed to extrude the
commercially pure aluminum to verify its effectiveness experimentally. The
billet was successfully extruded for 20 passes without obvious surface
defects with the modified die, compared to 13-14 passes at most for the
conventional die. Consequently, much more fine and uniform microstructure
was obtained with the average grain size of 200-300nm, while the average
grain size is ~500nm in the case of using the conventional die.
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