Back-end Defects in Aluminium Extrusion: Causes and Prevention

Back-end defects are internal defects that may develop during the final stage of aluminium extrusion. They are associated with non-uniform metal flow, billet surface contamination and deformation of the remaining billet material.

Unlike many surface defects, back-end defects are not always visible on the extruded profile. They may remain inside the product, appear during subsequent processing or affect mechanical performance.

Two important types of back-end defects are back-end coring and funnel formation (piping). Although both occur near the end of the extrusion stroke, their formation mechanisms are different.

Metal Flow During Extrusion

During direct aluminium extrusion, friction between the billet and the container wall restricts the movement of the outer billet layers. Meanwhile, the central material flows towards the die more rapidly.

As extrusion progresses, some surface and subsurface material accumulates near the rear end of the billet, ahead of the advancing dummy block.

This material may contain oxide films, surface contaminants, inverse segregation and coarse intermetallic particles.

Under normal conditions, much of this material remains in the billet butt. However, if extrusion continues too far, contaminated metal may move towards the billet centre and eventually enter the extruded product.

Back-end Coring

Back-end coring occurs when material originating from the outer layers of the billet flows inwards and subsequently passes through the die.

The mechanism involves several stages:

1. Billet surface layers are restrained by friction against the container wall.
2. Surface and subsurface material accumulates near the rear corner of the billet.
3. The advancing dummy block promotes radial inward flow of this material.
4. During the final stage of extrusion, contaminated material may enter the product.

Valberg et al. (2020) demonstrated this mechanism using experimental observations and finite element modelling.

In extruded rods, contaminated material may form an irregular or approximately annular region between the central core and the outer surface.

Coring is therefore not necessarily a defect located on the centreline.

Funnel Formation (Piping)

Funnel formation, also known as piping, is a different type of back-end defect.

As the remaining billet becomes short, deformation of its rear surface may produce a funnel-shaped depression. If extrusion continues, this depression can extend towards the die and form a central cavity in the extruded product.

The essential difference is that piping involves the formation of a central void, whereas coring involves the inward movement of billet surface or subsurface material.

Both defects are associated with the final stage of extrusion, but they should not be treated as identical phenomena.

Billet Skin Contamination

The billet surface is particularly important because it may contain oxides, segregated alloying elements and other contaminants.

When this material enters the extrusion, it may create internal discontinuities or regions with different metallurgical characteristics.

Negozio et al. (2020) investigated billet skin contamination in industrial extrusion experiments involving AA6063 and AA6082 profiles.

In one AA6082 experiment, contamination was detected approximately 3.7 m before the stop mark associated with the billet change. The observed contaminated region reached approximately 15–20% of the profile cross-section.

These findings demonstrate that contaminated material may enter the product before the nominal end of the extrusion cycle.

However, the reported distances and percentages relate to particular experimental conditions and cannot be used as universal trimming recommendations.

Consequences for Extruded Products

The consequences of back-end coring depend on the alloy, extrusion geometry and intended application.

In small architectural profiles made from 6xxx alloys, contaminated material may be extensively deformed during extrusion. In some cases, it does not produce an obvious defect in the untreated profile.

However, it may cause streaking after anodising, particularly when contaminated material intersects the product surface or a transverse weld region.

For structural extrusions, the consequences may be more serious.

Parson et al. (1992) demonstrated brittle failure during tear testing of a 6082 extrusion containing a back-end defect.

This does not mean that every back-end defect causes brittle fracture. It shows, however, that internal contamination may significantly affect fracture behaviour even when surface appearance alone does not reveal the problem.

Butt Discard and Back-end Trimming

Two different sources of material loss must be considered when controlling back-end defects.

Butt discard is the remaining billet material left in the container and removed after extrusion.

Back-end trimming is the portion of the extruded product removed from its rear end because it may contain defective or contaminated material.

An insufficient butt discard may allow contaminated material to enter the extrusion. An unnecessarily large butt discard, on the other hand, reduces metal recovery.

Similarly, excessive back-end trimming increases scrap without necessarily improving quality.

The objective is not simply to maximise the butt discard or trimming length, but to establish suitable limits based on extrusion conditions and product quality requirements.

Identification of Back-end Defects

Back-end defects may be difficult to detect by visual inspection alone.

Depending on the defect and product geometry, examination methods may include metallographic analysis, macroetching and inspection of longitudinal or transverse sections.

Coring may appear as an irregular band or region containing oxide films, inclusions or other discontinuities.

Piping is characterised by a central cavity or related loss of continuity.

For investigations involving successive billets, the stop mark provides a useful reference for determining where contamination appears along the extruded length.

Table 1. Back-end Extrusion Defects: Identification and Prevention

Prevention of Back-end Defects

Effective prevention requires control of billet quality, metal flow and the final stage of extrusion.

Important measures include:

• Maintaining suitable billet surface quality and limiting surface segregation and contamination.
• Establishing an appropriate butt discard length.
• Controlling extrusion conditions and avoiding unnecessary continuation of the final stroke.
• Maintaining the dummy block and container liner in suitable condition.
• Avoiding excessive dummy block lubrication.
• Establishing back-end trimming requirements using inspection results rather than fixed assumptions.

Billet–container friction plays an important role in retaining surface layers near the container wall. However, the accumulated material may subsequently move inwards if extrusion continues too far.

Metal flow during billet upsetting may also contribute to the formation of back-end defects.

Therefore, prevention should combine suitable billet quality, proper press maintenance and extrusion practices with an appropriate butt discard length.

Conclusion

Back-end defects are closely related to metal flow during the final stage of aluminium extrusion.

Coring results from the inward movement of contaminated billet surface material, while piping involves the formation of a central cavity.

Both defects can reduce product quality, but their consequences depend on the alloy, extrusion geometry and application.

Understanding these mechanisms helps extrusion plants select suitable butt discard and trimming practices, reduce unnecessary scrap and improve product reliability.

References

1. Parson, N. C., Hankin, J. D., and Bryant, A. J. (1992). The Metallurgical Background to Problems Occurring During the Extrusion of 6XXX Alloys. Proceedings of the Fifth International Aluminum Extrusion Technology Seminar, Vol. 2.
2. Valberg, H. S., Lefstad, M., and Costa, A. L. M. (2020). On the Mechanism of Formation of Back-End Defects in the Extrusion Process. Procedia Manufacturing, 47, 245–252. https://doi.org/10.1016/j.promfg.2020.04.207
3. Negozio, M., et al. (2020). FEM Validation of Front End and Back End Defects Evolution in AA6063 and AA6082 Aluminum Alloys Profiles. Procedia Manufacturing, 47, 202–208. https://doi.org/10.1016/j.promfg.2020.04.178