Answer. Friction is a factor in direct extrusion because the work billet is squeezed against the walls
of the container so that friction resists the movement of the billet toward the die opening. In indirect
extrusion, the billet does not move relative to the container walls, and thus there is no friction.
19.22 What does the centerburst defect in extrusion have in common with the roll piercing process?
Answer. They are both examples of how compressive stresses applied to the outside surface of a
solid cylindrical cross section can create high tensile stresses in the interior of the cylinder.
19.23 What is wire drawing and bar drawing?
Answer. Wire and bar drawing are bulk deformation processes in which the cross section of a wire
or bar is reduced by pulling (drawing) it through a die opening.
19.24 Although the workpiece in a wire drawing operation is obviously subjected to tensile stresses, how
do compressive stresses also play a role in the process?
Answer. Compressive stresses are present in wire drawing because the starting metal is compressed
as it is forced through the approach of the die opening.
19.25 In a wire drawing operation, why must the drawing stress never exceed the yield strength of the
work metal?
Answer. Because if the drawing stress exceeded the yield strength, the metal on the exit side of the
draw die would stretch rather than force metal to be pulled through the die opening.