Defects occurring within the process window
As mentioned above, operating the extrusion process outside the process window
leads to various product defects. However, a number of defects can still arise when
the extrusion is operated within the process window. Die lines and surface pickups
are, among them, the most severe and relevant to the AA 6XXX series alloys. They
can virtually appear throughout the process window.
Extrudate temperature
Die lines are longitudinal depressions or protrusions formed on the product surface.
A good measure of the severity of the die line defect is the roughness of the
extruded surface. Die lines can be categorised into macro die lines and micro die
lines. Macro die lines are formed inside the bearing area (as shown in Fig. 1.2 (a))
and are closely related to the roughness of the die bearing surface. Micro die lines
are much less deeper and are attributed to linear alignment of cavitations
interspersed with the fractured iron phase precipitates [1]. Impressions of die lines
are shown below:
Extrusion speed
Hot cracking
Incipient
melting
Undissolved
Mg2Si
Insufficient
pressure
A
C B
Process
window