External extrusion defects
In addition to internal defects (such as centerburst) or metallurgical deficiencies (from variations in grain size and metallurgical structure, resulting from recystallization after inhomogeneous deformation), there are also external , friction-related defects in both hot and cold extrusion that, when severe, make a product unsuitable for use. Surface cracking, whether circumferential (fir-tree cracking), or at roughly 45o to circumferential (“crow’s feet” cracking), or even longitudinal (“splitting”), is a common problem in both extrusion and drawing. The fir-tree effect (speed effect) occurs in hot-short materials with friction on the die land as a contributory cause. A cold extrusion defect of similar appearance can also be attributed to the simultaneous occurrence of slip on the die face and sticking on the die land.Other external defects include:
1. Subsurface defects caused by intermittent stick-slip on the container wall may emerge at the surface in the form of laminations or flakes.
2. Pickup on the die or on the die land leads to deep scoring of the extruded surface. Pickup tends to be cumulative and may force a die change and cleanup, often after extrusion of each billet in hot extrusion.
3. In lubricated extrusion, the surface roughens in the presence of a thick lubricant film, a phenomenon often described as “orange peel”. An excessively thick, unstable film leads to bambooing and, in some less-ductile materials, even to shear cracks. If the billet has been turned in preparation for extrusion, the machining marks are stabilized by the lubricant.
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