Aluminium extrusion defects

Surface defects of aluminium profiles

We have tried to collect as much information as possible on surface defects of extruded aluminum profiles. These are profiles made of soft and medium-strength aluminium alloys such as 6060, 6063, 6005A, 6061 and 6082. The surface quality characteristics of profiles made of high-strength aluminium alloys, such as alloys of the 2xxx, 5xxx and 7xxx series, may also have their own defect types due to its high alloying and peculiarities of their manufacturing technology [1, 2].

The defects classification is mainly based on the classification of surface defects from the source [3] and also from the source [4]. Terms and definitions are brought in accordance with the document [5]. The descriptions and images of defects are supplemented and expanded according to the sources [1,2, 6-10].

All presented surface defects are classified into three sections for reasons of their occurrence:

  • Extrusion-press-related surface defects – Part 1, see below.
  • Metallurgical-related surface defects – Part 2, coming soon.
  • Mechanically determined surface defects – Part 3, coming soon.

Part 1: Extrusion-press-related surface defects

1.1 Pickups (pick-ups, pickups)

Small particles of aluminium and aluminium oxides that were torn from the surface of the aluminium profile and later settled on its surface again. They are in the form of a comma or comet, which are oriented in the direction of extrusion. May occurs in connection with the “die lines” defect [4].

Torn, comma-like spots on the surface of extruded products caused by a local material deposition on the surface of the die [5].

Picture 1 – Schematic view of the “pick-ups” defect [4]

Figure 2 – “Comet tail” appearance of the “pick-ups” defect [3]

Figure 3 – Model of the “pick-up” defect [7]

1.2 Die lines

Grooves running in the direction of extrusion, which often end at points where alumi-nium material particles adhere. May occurs in connection with “pick-ups” defect [4].

Continuous longitudinal line formed on an extruded product caused by minor irregularities and/or the build-up of aluminium or non-metallic inclusions on the bearing surfaces of the die [5].

Micro die lines. Even with an optimum die bearing length and extrusion temperature, together with highly polished die bearings, die lines are still observed to occur. These die lines are finer and shallower and are termed micro die lines [3].

Picture 4 – Schematic view of the “die lines” defect [4]

Picture 5 – Die lines and micro die lines [3]

1.3 Bearing streaks

Light or dark streaks running in the extrusion direction. The usual reason is a too abrupt change in the length of the die bearing [1, 4].

Longitudinal discoloration, typically lighter than the surrounding metal, which can occur as a result of uneven cooling, where there are large changes in wall thickness. Wide streaks are often termed “stripes” [5].

Figure 6 – Schematic view of the “die-related streaks” defect [4]

Figure 7 – Die-related streaks [3]

Figure 8 – Variations in bearing lengths at the junction points must be properly blended to prevent streaking.
Three different types of blending processes [1]

1.4 Chamber and web markings 

Streaks running in the extrusion direction in the area of changes in profile cross-section (webs, hollow chambers, etc.). They have the form of convex / concave bulges or through different reflections of the strand surface due to differences in structure [4].

Figure 9 – Schematic view of the “chamber and web markings” defect [4]

Figure 10 – Chamber and web markings [3]

1.5 Longitudinal extrusion seem streaks (weld lines, charge welds)  

Narrow streaks running in the direction of extrusion, depending on the lighting conditions, lighter or darker than the rest of the material. Longitudinal press seam marks are process-related and unavoidable, but their extent can be influenced [4].

Figure 11 – Schematic view of the “longitudinal extrusion seems streaks” defect [4]

Figure 12 – A longitudinal extrusion seem on 6063 profile [1]

1.6 Front-end defect (transverse-weld defect)

Parabolically shaped defect. It caused by oxides and lubricants from the billet end surface being trapped when two billets are welded during billet-to-billet extrusion resulting in poor welds [5].

Figure 13 – Schematic view of the “transverse weld” defect [4]

Figure 14 – A transverse weld [8]

1.7 Back-end defect

Cone-shaped defect resulting from surface contaminations and oxides being built up in front of the advancing extrusion pad due to container friction resulting in annular separation in the rear of the extrusion [5].

Figure 15 – Schematic view of the “back-end” defect [4]

Figure 16 – A back-end defect [8]

  1.7 Speed ​​cracks (speed tear)

Cracks in the surface of the strand mostly running transversely to the pressing direction, at the edges of the strand at an angle (45º) to the extrusion direction, in extreme cases cracks appearing like a fir tree [4].

Transverse surface cracks, preferentially in corner radii or extremities of a profile, caused by localized high temperature [5]

Figure 17 – Schematic view of the “speed cracks” defect [4]

Figure 18 – Speed cracks [3]

  1.8 Blisters

Bubbles of different sizes arranged in lines or irregularly in the extrusion direction [4].

Raised spot, inside hollow, on the surface of products caused by the penetration of air into a subsurface zone. A void resulting from blister that has ruptured is often termed “blow hole” [5].

Figure 19 – Schematic view of the “blisters” defect [4]

Figure 20 – A big blister and small blisters [3]

Figure 21 – A blow hole [3]

 1.9 Surface waviness

A surface waviness can appear along the entire length of the strand at short distances. The defect can appear on the entire strand surface but also on one or more partial surfaces. Tend to occur in alloys that can be easily extruded [4].

Figure 22 – Schematic view of the “surface waviness” defect [4]

Figure 23 – Surface waviness [4]

Figure 24 – Waving due to incorrect metal flow through the die [10]

1.10 Chatter marks

Marks on the surface of the strand are repeated with more or less equal distances. This defect is typical for low-alloy extrusion alloys which have very high extrudability [4].

Regularly spaced superficial marks, transverse to the extrusion direction, produced by vibration between the metal and the working surface during fabrication [5].

Figure 25 – Schematic view of the “chatter marks” defect [4]

1.11 Stop mark of upsetting

Circular ring at right angles to the pressing direction (in the area of the transverse press seam) for billet-to-billet extrusion [4].

Figure 26 – Schematic view of the “stop mark of upsetting” defect [4]

Figure 27 – A stop mark of upsetting [4]

1.12 Snap marks (stop marks)

The ring with different cut-outs transversely to the pressing direction can occur at any point on the strand [4].

The band-like pattern around the full perimeter of an extruded section and perpendicular to its length. It caused by an abrupt change of an extrusion parameter during the process. If the extrusion process is abruptly suspended, then the term “stop mark” is used. [5].

Figure 28 – Schematic view of the “snap (stop) marks” defect [4]

Figure 29 – A “stop marks” defect [6]

1.13 Broken die

The deviation from the desired cross-section due to the absence of a certain portion of the extrusion die used or displacement of parts of complex extrusion die [3].

Figure 30 – Schematic view of the “broken die” defect [4]:

left – а displacement of parts of the porthole extrusion die;
right – а crack in the solid extrusion die


Most likely, with all the care in developing this document, we did not achieve a complete classification of defects in aluminium profiles from 6xxx aluminum alloys. Some mistakes are inevitable as well. Therefore, we ask users of this work to send suggestions, additions and any criticism to the e-mail This will help to make the necessary improvements in a later new edition.


  1. P. K. Saha, Aluminum Extrusion Technology – ASM International, 2000.
  2. Extrusion of Aluminium Alloys / T. Sheppard – Springer Science+Business Media Dordrecht, 1999
  3. Extrusion Defects Fundamentals & Solutions for Optimum Finish / J. Fourmann – Rio Tinto – 2017
  4. Merkmale von Aluminium-Strangpreβprofilen – IWK, Aachen, 2001
  5. GAG Guidance Document 001, Terms and Definition – 2011.
  6. Product defects in aluminum extrusion and their impact on operation cost / A.F.M Arif et al, KFUPM, Saudi Arabia, 2002.
  7. Surface quality of aluminium extrusion products / X. Ma, PhD Thesis – University of Twente, The Netherlands, 2011
  8. Defect Analysis in Anodizing / Barry R. Ellard – ET 2000 – 2000
  9. The Identification and Prevention of Defects on Anodizing Aluminium Parts / T. Short – Metal Finishing Information Service Ltd – 2003.
  10. Care of Aluminium – – 2021

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