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The Influence of Die Bearing Geometry on Surface Recrystallisation of 6xxx

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The Influence of Die Bearing Geometry on Surface Recrystallisation of 6xxx

Nick Parson, Alexandre Maltais, Rio Tinto Alcan, Jonquiere, Quebec, Canada,
Chris Jowett, Consultant to Rio Tinto Alcan, Kingston, Ontario, Canada


ABSTRACT – The control of the grain structure of 6xxx extrusions has become a requirement for many
applications, particularly in the automotive sector. Mechanical properties, corrosion resistance, and
formability can all be impacted by this aspect of the microstructure. Often it is a requirement that the
product be unrecrystallized in the as extruded condition with a maximum surface recrystallized depth.
For some applications, the same degree of control is required after secondary forming and heat treatment
operations. The main challenge is to prevent coarse grain at the surface where the material experiences a
more severe thermo-mechanical history. Alloy composition and ingot microstructure are known to have
an effect on the formation of a surface recrystallized layer. Extrusion conditions also contribute to its
creation or otherwise, but often the only option to the extruder is to reduce extrusion speed to
unacceptable levels. The use of conic dies has been previously shown to have an impact on this problem.
Using data generated on the RTA experimental extrusion press, the current study describes how changes
in die bearing length and angle of choke on conventional die bearings can play a major role in controlling
grain structure.

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