Effect of Process Variables on the Formation of Streak ... · prevent build-up of grease on the shear blade etc. 3 Effect of Extrusion Process on the Formation of Streak Defects 3.1

High Temp. Mater. Proc., Vol. 31 (2012), pp. 105–111 Copyright © 2012 De Gruyter. DOI 10.1515/htmp-2012-0024

Effect of Process Variables on the Formation of Streak Defectson Anodized Aluminum Extrusions: An Overview

Hanliang Zhu,1;� Malcolm J. Couper2 andArne K. Dahle3

1 Institute of Materials Engineering, Australian NuclearScience & Technology Organisation, Kirrawee DC, Syd-ney, Australia

2 ARC CoE for Design in Light Metals, Monash Univer-sity, Clayton, Australia

3 ARC CoE for Design in Light Metals, Materials Engi-neering, University of Queensland, Brisbane, Australia

Abstract. Streak defects are often present on anodized ex-trusions of 6xxx series aluminum alloys, increasing the fab-rication cost of these products. Moreover, streaking of-ten only becomes visible after etching and anodizing treat-ments, rather than in the as-extruded condition, making itdifficult to identify the original causes and influencing fac-tors of these defects. In this paper, various process vari-ables that influence the formation of streak defects on an-odized aluminium extrusions are reviewed on the basis ofa literature review, industrial practice and experimental re-sults. The influencing factors involved in various process-ing steps such as billet quality, extrusion process, die designand etching process are considered. Effective measures forpreventing the formation of streak defects in industrial ex-trusion products are discussed.

Keywords. Aluminum extrusion, microstructure, surfaceimperfection, streak defect.

PACS®(2010). 81.05.Bx, 81.20.Hy, 81.40.Lm, 81.65.Cf.

1 Introduction

Variation in surface gloss as well as surface defects such asstreaking is often present on anodized extrusions of 6xxxseries aluminum alloys, increasing the fabrication cost ofthese products. Surface gloss is considered to be the amountof incident light that is reflected at the specular reflectance

* Corresponding author: Hanliang Zhu, Institute of MaterialsEngineering, Australian Nuclear Science & TechnologyOrganisation, Locked Bag 2001, Kirrawee DC, Sydney, NSW 2232,Australia; E-mail: [email protected].

Received: October 22, 2011. Accepted: January 27, 2012.

angle of that surface, and streaking is typically charac-terised by narrow bands with a different surface gloss thanthe surrounding material [1–5]. The origin of streaking isa difference in intensity and diffuse nature of the reflectedlight. Hence, the formation of streak defects is dependenton surface topography of the final anodized products [2–4].

During anodising of an extrusion, an oxide layer isformed on top of the metal surface. Previous research indi-cates that the gloss after anodising correlates strongly withthe metal surface roughness and to a lesser degree withthe oxide surface roughness [2, 6]. The reason is that alu-minum oxide is transparent and the metal/oxide interfaceis therefore the prevailing reflecting surface whose surfacecharacteristics have a greater effect on the gloss of the an-odized part [2, 6]. Hence, any surface imperfections in themetal/oxide interface will increase the diffuse part of thereflected light and any inhomogeneous distribution of thesesurface imperfections results in the occurrence of streak de-fects [2].

Surface imperfections are created on the extrusion sur-faces during etching [2, 3]. It has been found that themost common surface imperfections are etching pits, grainboundary grooves and grain etching steps [2, 3]. Extrusionsurface defects such as die lines may also remain on theetched surface under certain conditions, forming surface-defect-remains [2]. Etching pits are created on the extru-sion surface during etching due to different reaction ratesbetween intermetallic particles or inclusions and the alu-minium matrix. In 6xxx series alloys, the major intermetal-lic phases are primary Fe-rich intermetallic particles andMg2Si precipitates. During etching, the detachment of theFe-rich intermetallic particles from the Al matrix results inthe formation of large etching pits with size up to 10 µmon the etched surface [2, 4]. Also, the detachments of in-clusions or contaminants produce very large etching pitswith irregular shape corresponding to the size and shapeof the inclusions or contaminants. Moreover, the preferen-tial dissolution of Mg2Si precipitates leads to the formationof small etching pits with size directly related to the origi-nal size of the precipitates [7]. Grain boundary grooves areformed due to preferred grain boundary attack during etch-ing. The severity of grain boundary grooves depends onwhether grain boundaries are attacked in preference to thematrix and on the kinetics of the attack [2]. Grain etchingsteps are created on the surface of polycrystalline materi-als due to pronounced chemical attack of particular crystalplanes [2,3]. After etching, the surfaces of pronouncedly at-

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106 H. Zhu, M. J. Couper and A. K. Dahle

3 mm

2 mm

Web Intersection

Normal region Normal region

(b)

(c) (d)

(e) (f)

(a)

Figure 1. Profiles of anodized aluminium extrusions with (a) 0.17 wt.% Fe and (b) 0.29 wt.% Fe, and SEM morphologyof anodized low-iron extrusion in (c) web intersection region and (d) normal region, and high-iron extrusion in (e) webintersection region and (f) normal region [10]. Arrows indicate grain boundary grooves.

tacked planes are lower than for other planes, forming grainetching steps [2, 3]. Surface-defect-remains may be presenton the etched surface due to insufficient etching or seriousextrusion surface defects etc. [2, 8].

Overall, surface imperfections are created on the extru-sion surface due to uneven chemical attack on the surfacemicrostructure during etching. Consequently, the intensityof streak defects depends on the surface microstructure aswell as its degree of homogeneity, which are influenced byvarious factors involved in the different processing stepsof the anodized extrusions [8]. In this paper, the influ-ence of various process variables involved in billet qual-ity, extrusion process, die design and etching process onthe formation of streak defects are reviewed. Moreover, thepreventive measures corresponding to different influencingfactors are discussed for industrial extrusion products aswell.

2 Effect of Billet Quality on the Formationof Streak Defects

2.1 Billet Composition

Certain elements such as Fe, Mg and Si in the billet alloyscan influence the amount, distribution and morphology ofMg2Si and Fe-rich intermetallic particles in the final extru-sion, and hence influence the formation of streak defects.In order to investigate the effect of Fe content as well as theprimary intermetallic particles on the formation of streakdefects, different amounts of iron were added to the billetalloy to obtain iron contents of 0.17 wt.% and 0.29 wt.%in the billets of low-iron and high-iron extrusions, respec-tively [9, 10]. Figure 1 presents the surface appearance andmorphology of the anodized samples. The anodized surfaceof the low-iron extrusion has a bright streak defect as shownin Figure 1 (a), whereas that of the high-iron extrusion ex-



Effect of Process Variables on the Formation of Streak Defects 107

hibits relatively uniform colour (Figure 1 (b)). The grainboundary grooves in the low-iron extrusion are deeper andwider in the normal region (Figure 1 (d)) than in the webintersection region (Figure 1 (c)). In contrast, there is nosignificant difference in the two regions of the high-iron ex-trusion (Figure 1 (e) and 1 (f)). Hence, lower severity ofgrain boundary grooves in the web intersection of the low-iron extrusion is responsible for the bright streak defect.The difference in severity of grain boundary grooves in thelow-iron extrusion is due to the difference in surface mi-crostructure caused by non-uniform metal flow during ex-trusion. An optimum alloy composition with balancing thesurface appearance and other properties are effective waysfor reducing the intensity of this streak defect.

2.2 Billet Microstructure

Billet microstructure defects such as surface segregationand surface chill zones can influence the occurrence and in-tensity of billet streaks on anodized aluminum extrusions.During extrusion, the difference in microstructure betweenthe surface and the bulk of billets with these defects resultsin the formation of inhomogeneous surface microstructureon the extrudate. After etching, a heterogeneous distribu-tion of surface imperfections is produced on the etched ex-trusions due to uneven response of the inhomogeneous sur-face microstructure, leading to the formation of streak de-fects on the final anodized products. Hence, the intensitiesof these streaks are influenced by the degree of segregationand the depth of the chill zone. Usually, the chill zone ex-tends much deeper into the billet than the surface segrega-tion zone. The segregation streak defect can be preventedby using good quality billet, while the chill zone streakcannot be avoided completely, but the intensity can be re-duced by controlling the DC casting process to minimizechill zone depth [11].

2.3 Billet Contaminants

Billet contaminations such as billet skin, oxidized metal,other inclusions and foreign materials, e.g. lubricant andcarbon from run-out table, are important factors that influ-ence the formation of contamination-type billet streaks andsome extrusion process streaks due to inherent process im-perfections which are discussed in the next Section. Thesecontaminations can be entrapped in the material during theextrusion process due to non-uniform metal flow. Figure 2presents a schematic diagram showing the mechanisms ofbillet skin entrapment during the extrusion process. Whena hot billet is forced to flow by compression through thedie orifices, friction occurs between the metal and the die.As a result, the surface skin of the billet material and othercontaminations on it are moving backwards and the mainpart of the billet is pressed forward through the die un-der normal friction and temperature conditions. Hence, the

(a)

(b)

Figure 2. Mechanisms of billet skin entrapment duringextrusion. (a) Flow of billet skin via back end and (b) flowof billet skin via front end.

billet skin and other contaminations are entrapped into thesection via backend due to non-uniform metal flow (Fig-ure 2 (a)). Also, the flow of billet skin and other contami-nations can be entrapped into the section via the front end(Figure 2 (b)). Since the billet skin is enriched in Mg2Si,Fe-rich intermetallic particles and other contaminations, ahigh density of etching pits is formed in the location con-taining large amounts of billet skin due to direct dissolutionof the Mg2Si precipitates and detachment of the Fe-rich par-ticles and other contaminations during etching.

Effective measures for preventing contamination-typebillet streaks involve avoiding the entrapment of billet skinand other contaminations. Some measures are to: check diedesign to ensure die cavity at least 20 mm away from con-tainer wall; check container alignment and liner condition(no excessive wear); clean container regularly to preventbuild-up of aluminum oxides; increase butt length; avoidusing excessive lubrication on billet or dummy block; re-place graphite lubricants with boron nitride; not use anycarbon blocks on run-out table, but Kevlar rollers instead;prevent build-up of grease on the shear blade etc.

3 Effect of Extrusion Process on the Formationof Streak Defects

3.1 Process Imperfections

The inherent process imperfections of the extrusion processcan result in the formation of extrusion process streaks suchas “longitudinal” or “transverse” weld and mixing of differ-ent alloys [8,12]. For example, during extrusion using morethan one billet, it is common practice to extrude billet on bil-let using a welding chamber or feeder ring to hold the backof the previous billet in the die and provide a surface forthe next billet to weld on to. The end of the previous billetmay contain billet skin and other contaminations on it, re-sulting in streaks. Figure 3 shows the streak defects formedat longitudinal extrusion welds. During extrusion of a hol-




Figure 3. Streaks due to longitudinal extrusion welds. Thewelds are formed due to splitting and rejoining of themetal flowing through the webs of the die. Arrows linkthe streaks with the corresponding webs of the die.

low profile, the metal flowing through the die has to splitup around the webs of the die and then rejoins, resulting inthe formation of longitudinal extrusion welds in the profile.The weld line has a different microstructure with the matrix,resulting in the occurrence of streak defects. In addition,some hollow dies have a big pre-chamber. When changingfrom one alloy to another, the weld area may have a mix-ture of two different alloys, forming different microstruc-ture, and hence resulting in streaks.

Some “longitudinal” or “transverse” weld streaks are dif-ficult to eliminate completely, but their intensity can be re-duced by optimising die design, which is discussed in Sec-tion 4. Also, as described above, “longitudinal” or “trans-verse” weld streaks are formed due to the entrapment of thebillet skin and other contaminations, so the preventive mea-sures described for the contamination-type billet streaks arealso applied to reduce the intensity of these streaks. For pre-venting the formation of streaks due to mixing of differentalloys, the measures include cleaning container adequatelybefore alloy change; not using pre-filled dies for anodisedprofiles if billet from different suppliers are used; not mix-ing billets from different suppliers even if they are of similaralloy type (e.g. 6063); rationalising alloy selection.

3.2 Process Parameters

Extrusion process parameters are important factors to influ-ence the metal flow through the die. Non-uniform metalflow can result in a heterogeneous deformation within theextrusion. Moreover, the inhomogeneous deformation canlead to an inhomogeneous distribution of microstructurewithin the extrudate, and hence the formation of streak de-fects after etching [2]. The most important process parame-ters that influence metal flow, deformation and microstruc-ture during the extrusion process are temperature and extru-sion speed [2].

The temperature achieved in the extrusion as it exits thedie is a result of the starting billet temperature i.e. preheat-ing temperature plus the net temperature rise. During ex-trusion, most of the work of deformation is transformedinto heat, causing a net temperature rise, which can be sev-eral hundred degrees [13]. If the temperature is increased,the flow stress is reduced and the deformation is thereforeeasier. Localization of deformation can lead to localizedheat generation as well as a locally high temperature, espe-cially when conduction of heat from the deformation zoneis poor [13]. The locally high temperature can further ac-celerate the local deformation, resulting in different grainsize, texture development and precipitation behavior of in-termetallic particles in the deformation zone compared toits surroundings. Hence, streak defects can be formed inthis location after etching.

The effects of preheating temperatures of billet and con-tainer on the formation of streak defects are complicatedbecause they have an effect on the whole surface of the ex-trusion. Different preheating temperatures may affect thedegree of temperature rise and have an effect on the bright-ness level of streaks. However, if the billet temperature ismuch lower than the container temperature, the flow patternof the billet skin can be changed and the billet surface mate-rial is entrapped into the section through the die, resulting inthe formation of streak defects [4]. On the contrary, if thebillet temperature is much higher than the container tem-perature, the temperature is much higher in the centrelinethan the surface of the billet, leading to faster flow of thecenterline material and forming streak defects [14]. It hasbeen suggested that the container should be at least 10 ıCless than the minimum billet temperature to prevent the oc-currence of streak defects [15].

The higher the ram speed, the less time available for heatconduction, and there will therefore be more localized heatgeneration due to the increase in ram speed. The temper-ature rise can be significant for plastic deformation at highstrain rates. Hence, the extrusion speed has a great influ-ence on the quality of the extrusion surface. Karhausen etal. [1] found that a linear dependency is observed when themeasured temperature in the bearing is plotted against thelogarithmic ram speed. The total heat generation appears tobe almost solely dependent on extrusion speed and die lay-out. After anodising, sections from a die with flow controlvia bearing length remain rather homogeneous, while sec-tions from a die with flow control via feed exhibit a matterappearance in the thick area. In addition, it is also reportedthat high speed worsens the streak defects for extrusionswith thin shapes [16]. Overall, higher extrusion speeds canmake the die streaks more pronounced.

3.3 Extrusion Surface Defects

Serious extrusion surface defects such as rough surfaces,die lines and pick-up can be created in the extrusion surface




(a) (b)

(c) (d)

10 mm

500μm 200μm

Figure 4. Streaks due to extrusion surface defects. (a) Profile of an anodized aluminium extrusion with streak defects,(b) surface morphology of the streaked region, and (c) and (d) remains of etching pits in the anodized surface.

due to a variety of causes. If these surface defects could notbe completely removed from the etched surface after a suf-ficient etching treatment, extrusion process streaks may beobserved in the final anodized extrusions. Figure 4 showsdefects due to remains of serious extrusion surface defects.The deep etching pits could not be flatted by the etchingtreatment, and remain in the anodized surface. The align-ment of many etching-pit-remains results in a streak re-gion in which has discontinuous and dull colour pit areas.The die-line-remains can form continuous streaks in the an-odized surface [2]. All these defects are formed due to re-mains of serious extrusion surface defects after etching, andare referred as extrusion process streaks in the present pa-per. These streaks can certainly be eliminated by prevent-ing the formation of serious surface defects on the extrusionsurface through optimizing the extrusion process etc.

4 Effect of Die Design on the Formationof Streak Defects

Good die design can make the metal flow through the dieorifices smoothly and uniformly, and thus decrease the fric-tion between the materials and die and the difference indeformation and microstructure in different locations, re-ducing the tendency for streaking. Hence, die design is animportant step to prevent the formation of die streaks andsome extrusion process streaks.

The typical areas where the die streaks might occur intypical extrusion profiles are illustrated in Figure 5. In theextrusion profile of Figure 5 (a), the streak occurs on thesurface opposite the intersection. This streak defect may becaused by too slow flow at the tips of the screw, which re-tards the flow of the entire screw area, or too fast flow at theintersection due to a low surface area-to-volume (S/V) ratioin this localised region. In order to remove the streak defectfrom this extrusion profile, reducing the bearing length atthe tips of the screw and increasing the bearing length forthe die bearing opposite the screw are effective measures.Also, all bearing changes should be smooth with properblending. Similar like the streak on the extrusion profileof Figure 5 (a), the streak defect on the extrusion profile ofFigure 5 (b) is also due to slower flow or faster flow of thevertical leg of the profile in comparison to that of the hori-zontal section. The remedial measure is to reduce the bear-ing length for the vertical leg with slow flow, or to increasethe bearing length or use a choked bearing for the verticalleg with fast flow. These designs can achieve a balancedflow in the area around the intersection, and hence elimi-nate the streak. The streak defect in Figure 5 (c) is mostlikely related to a slow flow in the weld area due to a smallweld chamber or excessive restriction of the metal feedinginto the weld chamber. Effective measures for eliminatingthis streak are modifying web and weld chamber designs,such as reducing web width/height and increasing the weldchamber width/height [3].




(b) (a) (c)

Figure 5. Examples of die streaks and typical areas wherethey might occur.

In addition, the intensity of streaks along the longitudi-nal welds of a hollow profile can be reduced by optimisingdie design. A general approach recommended is to designthe ports in such a way that the longitudinal welds are lo-cated in the regions of the profile that are not critical froma cosmetic perspective [11]. Figure 6 presents a schematicdiagram showing optimum die design for preventing longi-tudinal welds of hollow profiles in critical locations. Thechange in the position of ports can make the weld lineslocated at the corners of the profile. Moreover, Hauge etal. [4] modified the ratio between the height of the webs andthe height of the weld chamber. With this modification thetemperature difference between the seam weld and the restof the section wall could be reduced. This gave almost novariation in grain size between the seam weld and the restof the wall. The result was an equal etching response andalmost no visible longitudinal streaks after anodising [4].

From the above results, it can be concluded that althoughthe metal flow in the die is almost always non-uniform,causing localization of deformation, good die design cancorrect the die to allow a non-uniform shape to flow uni-formly from the die exit. Therefore, die design is a crit-ical factor to influence the uniformity of metal flow aswell as the homogeneity of deformation and microstructure.Hence, good die design is a key factor for preventing diestreaks.

5 Effect of Etching Process on the Formationof Streak Defects

The surface imperfections causing streaks are produced onthe extrusion surface during the etching treatment and thestreak defects are revealed after the etching treatment aswell. Hence, the etching process is an important step toinfluence the formation and severity of surface imperfec-tions as well as streak defects on aluminum extrusions [17].For example, if the etching treatment is insufficient, the dielines may not be completely removed from the etched sur-face and their remains become one part of the surface im-perfections on the etched surface. If the distribution of dieline remains is inhomogeneous, etching process streaks canoccur. These streaks are different from the extrusion pro-cess streaks due to extrusion surface defects. In that case,the surface defects are so serious that surface defects stillremain although sufficient etching treatment has been con-ducted. The etching process streaks can be eliminated by

(b)

(a)

Figure 6. Schematic diagram showing optimum die designfor preventing longitudinal welds of hollow profiles.(a) Old die design and (b) optimum die design.

sufficient etching treatments. Also, the severity of the sur-face imperfections is influenced by the degree of etching,which may also influence other types of streaks.

The most important etching parameters include etchingtime, etching temperature and composition of the etchingsolution. The results of etching trials indicate that withincreasing etching time at a low etching temperature, theseverity of die-line-remains decreased while grain boundarygrooves became more severe [17]. If the etching was insuf-ficient, a streak defect appeared on the etched surface dueto inhomogeneous distribution of die-line-remains. With in-creasing etching temperature, the surface imperfections be-came less serious. Figure 7 shows surface morphologiesof the samples after etching at 40 ıC, 70 ıC and 90 ıC for5 min. It is noted that die lines are still observed on thesample etched at 40 ıC, while they completely disappearedfrom the samples etched at 70 ıC and 90 ıC. However,grain boundary grooves are observed but are not signifi-cant on the 40 ıC etched surface (Figure 7 (a)), and becomemore serious on the 70 ıC etched surface (Figure 7 (b)),but completely absent from the 90 ıC etched surface (Fig-ure 7 (c)) [17]. These results indicate that increasing etchingtemperature greatly accelerates the dissolution of the sur-face material, flattening the extrusion surface defects veryquickly. Also, at the high etching temperature of 90 ıC,the dissolution of both grain interiors and grain boundaryregions are accelerated so much that any difference in dis-solution rate between the two regions is greatly decreased,preventing the formation of grain boundary grooves. Fur-thermore, there is no significant difference in the morphol-ogy of grain boundary grooves between the normal and webintersection regions after etching for 5 min at 70 ıC and90 ıC. Hence, sufficient etching treatment can effectivelyeliminate the occurrence of etching process streaks.

6 Summary

The influencing factors involved in various processing stepssuch as billet quality, extrusion process, die design and etch-ing process and the corresponding preventive measures forvarious streak defects on anodized aluminium extrusionsare summarized:




(a)

30μm

(c)

30μm

(b)

30μm

Figure 7. Surface morphology of web intersection region in extrusion samples etched for 5 min at (a) 40 ıC, (b) 70 ıCand (c) 90 ıC.

1. Billet quality: some chemical elements in the billet al-loys can influence the amount, distribution and morphol-ogy of intermetallic particles and hence affect the for-mation of streak defects. Surface segregation, surfacechill zones and contaminations in the billet can result inthe formation of billet streaks, which can be effectivelyeliminated by good control of the billet quality or re-moved by mechanical treatment of the section.

2. Extrusion process: the inherent process imperfections ofthe extrusion process can result in the formation of ex-trusion process streaks such as “longitudinal” or “trans-verse” weld and mixing of different alloys. Good diedesign can locate the longitudinal weld streak in the re-gions that are not critical from a cosmetic perspective.Carefully cleaning container and avoiding mixing of dif-ferent alloys can effectively prevent the streaks due tomixing of different alloys. Improper process parame-ters can promote the occurrence of die streaks and anoptimum process assists in eliminating or reducing theintensity of these streaks.

3. Die design: good die design can correct the die to allowa non-uniform shape to flow uniformly from the die exit,eliminating streak defects. Adjusting die bearing lengthsaccording to section thickness, avoiding abrupt changesand smooth bearing transition are integral elements ingood die design.

4. Etching process: sufficient etching treatment can effec-tively prevent the occurrence of etching process streaks.

References

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[3] X. Zhang, H. Zhu, M. J. Couper and A. K. Dahle, Proceed-ings of the 9th International Aluminum Extrusion Technol-ogy Seminar & Exposition, Florida, 2008, Vol. 2, 455–464.

[4] T. Hauge and K. F. Karhausen, Aluminum Extrusion 3(1)(1998), 32–37.

[5] P. Ratchev, R. Labie, B. Verlinden and R. V. D. Broeck,Z.Metallkd 91 (2000), 510–515.

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[10] H. Zhu, X. Zhang, M. J. Couper and A. K. Dahle, Metall.Mater. Trans. A 40 (2009), 3264–3275.

[11] N. C. Parson, J. D. Hankin and A. J. Bryant, Proceed-ings of the 5th International Extrusion Technology Seminar,Chicago, 1992, Vol.2, 13–23.

[12] A. J. Bryant, W. Dixon and R. A. P. Fielsing, Light MetalAge June (1999), 30–54.

[13] P. K. Saha, Wear 218 (1998), 179–190.[14] J. Xie and J. Liu, Theory and Technology of Extrusion of

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Effect of Process Variables on the Formation of Streak ... · prevent build-up of grease on the shear blade etc. 3 Effect of Extrusion Process on the Formation of Streak Defects 3.1