24. improving mechanical properties of chip-based aluminum extrudates by

7/25/2019 24. Improving Mechanical Properties of Chip-based Aluminum Extrudates By

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Materials Science and Engineering A 539 (2012) 194204

Contents lists available atSciVerse ScienceDirect

Materials Science and Engineering A

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e a

Improving mechanical properties of chip-based aluminum extrudates byintegrated extrusion and equal channel angular pressing (iECAP)

M. Haase a,, N. Ben Khalifa a, A.E. Tekkaya a, W.Z. Misiolek b

a TU Dortmund University, Institute of Forming Technology and Lightweight Construction, Baroper Strae 301, D-44227 Dortmund, Germanyb Lehigh University, Institute for Metal Forming, 5 East Packer Avenue, Bethlehem, PA 18015, USA

a r t i c l e i n f o

Article history:

Received 7 December 2011

Received in revised form 20 January 2012

Accepted 21 January 2012

Available online 28 January 2012

Keywords:

Chip extrusion

Equal channel angular pressing (ECAP)

Aluminum alloy recycling

Die design

Mechanical properties

Microstructure

a b s t r a c t

In order to improve the mechanical properties of profiles extruded from aluminum chips, a four turn

equal channel angular pressing tool was integrated into an extrusion die (iECAP die). AA6060 aluminum

alloy turning chips were cold pre-compacted to chip-based billets and hot extruded through the iECAP

die on a conventional forward extrusion press. Mechanical properties and microstructure of the chip-

based billets extruded through theiECAP diewere investigatedand compared to those extruded through

a conventional flat-face die and a porthole die. To evaluate the performance of the iECAP processed chip-

based profiles, conventional cast billets were extruded through the flat-face die as a reference material.

To investigate the influence of temperature on mechanical properties and microstructure of chip-based

profiles, the extrusion was performed at 450 C and 550 C.Tensile tests revealed superior mechanical properties of the chip-based billets extruded through the

iECAP die in comparison to chip-based billets extruded through the flat-face and the porthole die as well

as to cast billets extruded through the flat-face die.

2012 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Direct extrusion of aluminum chips

The direct recycling of aluminum chips using hot extrusion was

first proposed and patented by Stern in 1945 [1]. Gronostajski et al.

[2]have investigated the direct conversion of aluminum chips to

final products using a three step method: granulation of the chips

using a cutting device, cold pre-compaction and finally hot extru-

sion. The investigations were conducted consecutively with pure

aluminum, AlMg2 and AlCu4 alloys. Theyproducedspecimens with

a residual porosityof about 5% after hotextrusion, withan extrusion

ratio of 4:1. Hardness andtensileproperties of the chip-basedspec-

imen were lower compared to extruded cast billets. Gronostajski

et al.[3]proposed the following factors to contribute significantlyto the bonding quality of aluminum and aluminum alloy chips

with an introduced consolidating phase: (i) the amount, form

and size of the consolidating phase; (ii) the degree of fineness of

the aluminum and aluminum alloy chips; (iii) the cold pressing

Corresponding author. Tel.: +49 231 755 2654; fax: +49 231 755 2489.E-mail addresses:[email protected](M. Haase),

Nooman.Ben [email protected](N. Ben Khalifa),

[email protected](A.E. Tekkaya),[email protected]

(W.Z. Misiolek).

parameters; (iv) the shape of the extrusion dies; (v) the degree of

reduction; (vi)the lubrication method and the lubricants used; and

(vii) the temperature and rate of extrusion. The results of direct

recycling of machining chips consisting of different alloys with-

out previous granulation were published by Fogagnolo et al. [4].

Hot and cold pre-compaction processes before hot extrusion were

compared.They foundthat onlythe combination of hotcompaction

and hot extrusion led to a sufficient chip bonding for low extrusion

ratio (6.25:1), whereas for higher extrusion ratio (25:1) both pre-

compaction methods led to a sufficient bonding of the extruded

chips. For higher extrusion ratio, the difference in ultimate ten-

sile strength (UTS) between hot and cold pre-compacted billets

after extrusion was negligible. Therefore, Fogagnolo et al. [4]pro-

posed the combination of cold pre-compaction and hot extrusion

to be the most promising process in terms of cost to benefit ratio.Tekkaya et al.[5]have studied the recycling of AA6060 aluminum

alloy chips using cold pre-compaction andhot extrusion. The influ-

ence of different chip geometries, produced by milling and turning

operations, on tensile properties of the extruded chip-based billets

was investigated. During extrusion, a two-feeder porthole die with

an extrusion ratio of34:1 was used in order to break oxide lay-ers coveringthe chips andachieve good bonding of pure metal.The

mechanical propertiesshowed comparable results to extruded cast

billets, independent of chipgeometry and chip production method.

Generally, the introduced plastic strain during extrusion is

defined by the extrusion ratio. Increasing the extrusion ratio will

0921-5093/$ see front matter 2012 Elsevier B.V. All rights reserved.

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M. Haase et al. / Materials Science and Engineering A 539 (2012) 194204 195

result in an increase of the introduced plastic strain but also in an

increase of the extrusion force[6].However, the selection of the

die design for a given extrusion ratio can provide a different level

of strain and therefore different conditions for chip bonding[5].

1.2. Equal channel angular pressing as a consolidation tool

Severe plastic deformation (SPD) processes introduce ultra-

large plastic strain into bulk metals which results in the formation

of an ultra-fine grained microstructure withgrain sizesbelow 1m

and grain boundaries of high misorientation angles [6]. A well

known SPD process is equal channel angular pressing (ECAP), also

known as equalchannel angular extrusion(ECAE),whichwas intro-

duced and patented by Segal in 1977[7]. Through simple shear,

ECAP is able to introduce plastic strain into the processed mate-

rial without changing the cross-section of the workpiece[6]. An

overview of the principles of ECAP is given by Valiev and Langdon

[8].

Xiang et al. [9] have compared the microstructure and mechan-

ical properties of aluminum powders consolidated in the ECAP

process at elevated temperatures and in the process of hot extru-

sion. The hardness value of the specimens consolidated with ECAP

was higher compared to the extruded samples. Xiang et al. [9]

related this to the fine microstructure and a high density of dis-locations introduced into the workpiece. Xia et al. [10]compared

the effect of ECAP on microstructure and mechanical properties of

both pure aluminum powder previously consolidated with back

pressure ECAP (BP-ECAP) and cast aluminum billets. The consoli-

dated workpiece showed a finer grain structure and higher tensile

strength. Balog et al. [11]found that the application of back pres-

sure was inevitable when using ECAP as a consolidation tool for

Al particles in order to avoid surface cracks of the processed spec-

imen. Luo et al. [12] recycled titanium machining chips directly

by using BP-ECAP at elevated temperature. After two passes, full

density and good bonding were achieved. According to Luo et al.

[12], the oxidelayers graduallycrackedwith repeated deformation,

potentially leading to an additional dispersion strengthening.

ECAP has its own limitations, first, the length to diameter ratioof the workpiece is limited to a critical value to prevent bending.

Second, the ram of the press has a limited travel distance, which

limits the length of the workpiece. Third, due to inhomogeneous

microstructure and the appearance of cracks near both ends of the

workpiece, material has to be cut off in these regions. Fourth, the

process is labor intensive because theworkpiece mustbe reinserted

into the die after every pass, making industrial application difficult

[13].

Paydar et al. [14] have proposed the combined process of

forward extrusion and ECAP (FE-ECAP) for the consolidation of

commercial pure aluminum particles. The method was carried out

at 200 C with a ram speed of 0.2 mm/s and an extrusion ratio of7.1:1. Superior hardness, tensile strength and comparable ductil-

ity were achieved with the new method compared to extrudedingot samples and samples consolidated with forward extrusion.

Paydar et al. [14] related this to a finer average grain size. Pay-

dar et al.[15]also proposed the combined process of ECAP before

forward extrusion (ECAP-FE) for consolidation of commercial pure

aluminum particles. As an advantage to the previously presented

concept, the extrusion after ECAP automatically creates back pres-

sure. However, this concept led to a lower ductility at similar

strengthcompared to FE-ECAP.Ying et al. [16] investigated thesolid

state recycling of AZ91 Mg alloy machining chips through the pro-

cess of cold compaction, hot compaction, extrusion and single pass

ECAP. The average grain size was proven to be much smaller com-

pared to cast AZ91 alloy processed by extrusion and ECAP. Results

were attributed by Ying et al. [16]to the dispersion of oxide con-

taminants. Without single pass ECAP, the extruded cast workpiece

showed superior strength and ductility compared to the recycled

ones. After applying ECAP, the recycled workpiece showed higher

strength but lower ductility compared to the cast billet processed

under the same conditions.

Recently, Orlov et al.[17]have presented a two turn ECAP tool

integrated into a conventional extrusion tool, based on the princi-

ples of a patentby Estrin et al. [18]. They produced bars of ZK60 Mg

alloy, which showed improved strength and ductility compared to

the initial state at an extrusion ratio of

19:1.

2. Tool design

In this paper, the concept of integrated extrusion and ECAP

(iECAP) was modified in order to improve the mechanical proper-

ties of profiles extruded from aluminum chips. For this purpose,

the number of ECAP turns was increased to four and the angle

between the channels was reduced. Mechanical properties and

microstructure of chip-based profiles produced with the iECAP die

were investigated and compared to chip-based profiles produced

withconventional hot extrusion using boththe flat-face die and the

porthole die. Cast billets were also extruded through the flat-face

die to provide a reference material.

The die was designed for the use on a conventional hydraulicextrusion press. A schematic illustration of the die is presented in

Fig. 1.

The tool consists of five parts with part one including the con-

ventional extrusion die and parts two to five including the four

ECAP steps. The extrusion part was fabricated as a prechamber die

for a profile with a rectangular cross section of 20 mm20mm,having a corner radius ofrprofile = 3.1mm. The resulting extrusion

ratio (ER) for this profile is8.7:1, the resulting true strain afterthe extrusion step can be calculated to Extrusion =ln (ER)2.16[19].Four ECAP turns were implemented into the die, equivalent

to routes CAC in conventional ECAP, where C indicates a sam-

ple rotation of 180and A no rotation between consecutive passesaround extrusion direction axis [20]. Backpressure is automatically

applied to the processed material after every ECAP turn when thematerial flows to the die wall in the proposed tool with the excep-

tion of the last ECAP turn. The channel angle was fabricated with = 90 according to Nakashima et al. [21], whoshowed thatimpos-ing a very intense plastic strain is important in order to achieve

a fine grained microstructure. The die was designed without an

outer arc of curvature, which leads to the highest strain in every

ECAP turn fordies with = 90[22]. The channel displacement wasdesigned asK= wc = 20mm, wherewc is the channel width, refer-

ring to Raab[23],who proposedK= dcfor round specimens, where

dcis the channel diameter, in order to achieve strain homogeneity

in the cross section of specimens processed by ECAP with parallel

channels. A graphite based guiding tool was used behind the last

ECAP step and the die exit to maintain straight material flow.

3. Experimental materials and methods

3.1. Experimental material and processing

The material used in the following experiments was aluminum

alloy AA6060, provided by apt Hiller GmbH. The chemical com-

position of the material (seeTable 1)was analyzed by apt Hiller

GmbH by optical emission spectroscopy using a Thermo Scientific

ARL 3460 Metals Analyzer.

The aluminum was processed in three consecutive steps. First,

aluminumchips wereproduced by turningof thecast billet. Second,

thechips were cold compactedon a hydraulicpressintochip-based

billets. Third, the billets were forward extruded on a hydraulic

extrusion press at elevated temperature using the billet-to-billet


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196 M. Haase et al. / Materials Science and Engineering A 539 (2012) 194204

Fig. 1. Schematic illustration of the four turn iECAP die and the corresponding profile cross section. is the channel angle and K is the channel displacement. All given

dimensions are in mm.

Table 1

Chemical composition of the AA6060 aluminum alloy used in the study.

Si Fe Cu Mn Mg Cr Zn Ti Others Al

0.45 0.208 0.016


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Fig. 3. Tools used for the investigation of chip extrusion and the corresponding material extracted from the die after extrusion (a) conventional flat-face die, (b) four-feeder

porthole die and (c) iECAP die.

Table 3

Fabricated specimens and corresponding process parameters.

Specimen Die Billet temperature Material

Reference (Ref.) Flat-face 450 C CastFlat-face die, low temperature (FL) Flat-face 450 C ChipsFlat-face die, high temperature (FH) Flat-face 550 C ChipsPorthole die, low temperature (PL) Porthole 450 C ChipsPorthole die, high temperature (PH) Porthole 550 C ChipsiECAP die, low temperature (EL) iECAP 450 C ChipsiECAP die, high temperature (EH) iECAP 550 C Chips

To evaluate the performance of the chip-based billets extruded

through the designed iECAP die, chip-based billets were also

extruded through a conventional flat-face die and a four-feeder

porthole die. Both dies were used to manufacture solid profiles

with a rectangular cross section of 20mm20mm, resultingin theextrusion ratio of

8.6:1. The tools and the corresponding material

removed from the dies are shown in Fig. 3.

As reference specimen, conventional cast billets of the same

alloy as the chips, with a diameter ofdbillet =62mm and a length

oflbillet = 85 mm, were extruded through the flat-face die. All bil-

lets were homogenized in an electrical furnace for 6 h at 550 C. Inorder to investigate the influenceof billet temperature on mechani-

cal properties and microstructure of the chip-based extrudates, the

billets were extruded at 550 C and 450 C. The chip-based billetsextruded at 450 C and the cast billets were cooled in the furnaceafter homogenization and before extrusion. Ram speed was set to

1 mm/s, die and container temperature to 450 C. The specimennotation for the combinations of dies used, billet temperature and

material are shown inTable 3.

3.2. Mechanical characterization

In order to compare the mechanical properties of the extruded

profiles, tensile tests and hardness measurements were con-

ducted. Tensile tests were performed with an initial strain rate

of 2.5103 s1 at room temperature on a Zwick/Roell Z250 ten-sile test machine. Tensile test specimens were fabricated parallel

to extrusion direction (ED) by milling in accordance with EN ISO

6892-1:2009 standard and pulled to failure. All reported results

are an average of at least three tensile tests. The dimensions of the

tensile specimens are given inFig. 4.The first tensile specimen for

each condition was machined out of the extruded profile after at

least 1000 mm from the profiles front end.

Vickers hardness was measured with an applied loading force

of 1.961N (i.e. HV0.2) and a holding time of 10 s at room temper-

ature. The measurement was conducted on a Struers Duramin-1

with a Vickers diamond indenter in accordance with DIN EN ISO

6507-1:2005 standard. Specimens were taken from the EDTD

plane, mechanically ground using SiC paper (grit 500, 1000, 2400

and 4000 for 180 s each) and polished for 600 s with colloidal sil-

ica. The mean values of HV are an average of at least six hardness

measurements.

3.3. Microstructure

The microstructure was characterized for all extruded profiles

usinglight optical microscopy (LOM) under polarized light on Zeiss

Axio Imager.M1m with Zeiss AxioCam MRc. Specimens were taken

from the EDTD plane, mechanically ground and finally polished

with colloidal silica as it was done for the hardness measurements.

Fig. 4. Tensile test specimen (light gray) machined out of the extruded profile (dark gray).L0is the gage length for the strain sensor, all given dimensions are in mm.


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Fig. 5. Extrusion force vs. remaining ram displacement. Billets were extruded at a

billet temperature of 550 C and a ram speed of 1 mm/s through different dies.

The polished specimens were electrolytically etched using Barkers

reagent[24]with an applied voltage ofU=25V dc for 120s on a

Struers LectroPol-5.The average grain size wasmeasuredusingthe

linear intercept method in accordance with ASTM E112-96 (2004)

standard.

4. Results

4.1. Extrusion force

The extrusion force is one of the characteristic factors in the

extrusion process. Therefore, it is very advantageous to investigate

the extrusion force for new die designs and to compare them to

known die sets. Chip-based billets of the same length (117mm)were prepared and extruded at a billet temperature of 550 C anda ram speed of 1 mm/s through the iECAP die, the porthole die and

the flat-face die. For comparison, a cast billet of the same length

was extruded through the flat-face die. Cast billets were extruded

through all die sets to fill the dies before performing all the exper-

iments. The extrusion forces related to the different die sets are

shown inFig. 5.

A discard of30mm is left in the press containerafter theextru-sion of each billet. For easier comparison of the initial steps of

extrusion for different dies, the curves inFig. 5are aligned to the

end-point of the extrusion stroke and the extrusion force is shown

as a function of the remaining ram displacement. The maximum

extrusion force of the cast billet extruded through the flat-face die

was 0.73 MN at a remaining ram displacement of106mm. Thechip-based billets extruded through the flat-face and porthole die

showed a maximum extrusion force of

0.73 MN and

1.7MN at a

remainingram displacementof85mmand76 mm,respectively.The chip-based billet extruded through the iECAP die had a maxi-

mum extrusionforce of1.98 MN ata remainingram displacementof78mm.

4.2. Material flow in iECAP and imposed strain

In order to reveal the material flow within the iECAP die, the

material was taken outof the die and the containerof theextrusion

press using caustic soda, cut in the EDND plane, ground, polished,

etched with Barkers reagent and investigated under polarized

light.Fig. 6shows a collage of images of the aluminum processed

with the iECAP die revealing dead metal zone (DMZ) formation.

The presented figure (Fig. 6) is a collage of images, so the overall

dimensions are slightly incorrect due to cutting losses of1 mmfor each cut. Regions of different grain sizes can be observed.

4.3. Bonding quality of chips recycled with the iECAP die

During the extrusion process, the single chips were welded

together. No voids between the chips can be observed for the chip-

based specimen extruded through the iECAP die in Fig. 7(a). The

appearance is comparable to the cast billet extruded through the

iECAP die shown inFig. 7(c). The shape of the single chips can be

made visible within the extrudateusing Barkers reagent, as shown

inFig. 7(b). When treating the extruded cast billet with the same

reagent, no significant difference can be seen in the macroscopicappearance of the extruded cast billet (Fig. 7(d)).

Fig. 6. Collage of the microstructure images in the EDND plane of the iECAP die and magnifications.


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Fig. 7. Chip-based and cast specimens processed with the iECAP die and cut in the

EDTD plane. (a) and (c): after grinding and polishing. (b) and (d): after grinding,

polishingand etching.The presentedimagesare collagesof smallerlocalizedimages.

ED is upwards.

4.4. Microstructure analysis

The microstructures of the processed specimens, labeled

according to Table 3, are shown in Fig. 8 for the specimen extrudedat 450 C andFig. 9for the specimen extruded at 550 C.

The microstructure analysis of the reference specimen reveals

an elongated grain structure and a thick zone of peripheral coarse

grains (PCG). The FL specimenshows equiaxedand elongated grains

withthe PCGzone smaller comparedto thereference specimen. The

FH specimen shows a similar microstructure to the FL sample with

a bigger PCG zone. The PL specimen has a similar microstructure to

the PH specimen with equiaxed and elongated grains at the center

of the profile, corresponding to the seam weld line, and equiaxed

grains elsewhere. The PCG zone is smaller for the PL specimen. The

EL specimen has equiaxed grains without a PCG-zone while the EH

sample shows equiaxed grains anda PCG zone similar in size to the

PH sample.

The dimension of the PCG zone increases for every die set withincreasing temperature, however, no increase in average grain size

could be measured for all chip-based specimens when PCG was

not taken into account. The iECAP processed specimens had an

average grain size of 31.8m, similar to the chip-based specimens

processed with the flat-face or porthole die.

4.5. Hardness measurement

The results of the Vickers hardness measurement across the

specimen width within the EDTD plane for the specimens

extruded at 450 C and 550 C are shown in Fig. 10(a) and (b),respectively.

The reference specimen has the lowest andthe FL specimen has

the highest average hardness value of41HV and53HV, respec-tively. The PL specimen has an average hardness value of44HV,while the hardness value of the EL specimen is46HV. The FHspecimen has 49 HV, the highest average hardness value amongthe specimens extruded at 550 C. The PH and EH specimen havean average hardness value of48HV and 43 HV, respectively.

4.6. Tensile test

The true stress vs. true strain curves for the specimen extruded

at450 Cand550 C arepresented in Fig. 11(a) and (b), respectively.The reference specimen has a maximum true stress value of

167MPa at a true strain value of 0.15. The FL and PL specimenshave maximum true stress values of162MPa and168MPa at

correspondingtrue strain values of 0.14 and0.17, respectively. At a

maximum true strain value of 0.24, the EL sample has a maximum

true stress value of174MPa.The FH and PH specimens have maximum true stress values of

169MPaand178MPa at correspondingtrue strain values of 0.15and0.16, respectively. TheEH specimen has a maximum true stress

value of195 MPa at a corresponding true strain value of 0.24.

5. Discussion

5.1. Extrusion force

As it can be seen inFig. 5,the extrusion of the cast billet begins

earlier compared to the chip-based billets, although all billets have

the same initial length. The density of chip-based billets achieved

by cold compaction was80%. The chip-based billets are furthercompacted in the press container before the extrusion takes place.

A significant increase in extrusion force during upsetting can be

observed for the chip-based billet extruded through the flat-face

die. This can be related to the effectof hotcompaction beforeupset-

ting of the chip-based billet in the press container. While extruding

aluminum through the porthole or theiECAPdie,a small aluminum

shell is formed in the press container. Because of the formation of

the shell during filling the dies, the initial extrusion force does notstart at zero at the next extrusion cycle due to additional friction

within the press container.

The earlier initialization of upsetting for the chip-based billet

extruded through the flat-face die compared to the other die sets

is due to slightly higher initial billet density of the billet extruded

through the flat-face die.

The maximum extrusion forces of cast and chip-based billets

extruded through the flat-face die are similar. The theoretical

length of the chip-based billet at full compaction is20% shortercompared to the lengthof the cast billet due to its 80% density. This

indicates thata higher extrusionforce is necessaryfor the extrusion

of chip-based material under the same conditions.

The porthole die and the iECAP die show higher maximum

extrusion forces than the flat-face die. For the porthole die, this

can be related to two different mechanisms. The first is the force

needed to divide the aluminum into four single strings. The second

is the increase in friction force due to the increase in friction sur-

faces compared to the flat-face die. For the iECAP die, the increase

in extrusion force can be related to the four shear zones present

within the ECAP steps and to the increase in friction surfaces. For

this die, all friction surfaces after the prechamber can be seen as

onesingle bearing surface of complex geometry leading to the final

profile geometry.

5.2. Material flow in iECAP and imposed strain

It can be observed inFig. 6that four defined dead metal zones

appear in the ECAP steps of the iECAP die. Eivani and Karimi Taheri

[25]investigated the effect of DMZ formation on imposed strain

and pressing force during conventional ECAP using upper bound

analysis. It was proposed that the formation of the deformation

zone and therefore of the DMZ, depends on the friction condition

and the channel angle. They assumed the DMZ to act as a die wall

with an outer arc of curvature, leading to an angle of deformation

zone (). According to Eivaniand KarimiTaheri [25], thisanglecan

be calculated as:

= 2 cot1

1m1+m (1)

wherem is the friction factor andis the channel angle. The pre-

sentediECAPtool has a channel angleof = 90in every ECAP step.

Assumingthe frictionfactor to be m = 1, the deformationzone angle


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Fig. 8. Collages of themicrostructures of thespecimens extruded at 450C with corresponding magnifications. Specimensweretakenfrom theEDTDplane, ED is upwards.

can be calculated to be= 90in every ECAP step by using Eq.(1).An increasing angle of deformation zone is resulting in a decrease

of the total imposed strain[22].The introduced equivalent plastic

strain for a single pass was proposed by Eivani and Karimi Taheri

[25]as:

tot=1

3

2

1 m1+ m+

2 cot1

1 m1+ m

1+m

2

(2)

For m = 1, the introduced total plastic strain is tot 0.9 in one

ECAP step which leads to a total imposed plastic strain in the ECAP

part of the iECAP die ofECAP3.6. As observed from the revealedmicrostructure in Fig. 6(a), theactual deformation angleseems less

than= 90. However, a zone of ultra fine grains between coarsegrains occurs along the theoretical curvature related to an angle of

90, as it can be seen inFig. 6(b), so shear along this curvatureis assumed.

5.3. Bonding quality of chips recycled with the iECAP die

Referring to Gronostajski et al. [3], extrusion ratio, extrusion

temperature and die shape are significant process parameters


8/11


Fig. 9. Collages of the microstructures of the specimens extruded at 550 C and of the reference specimen with corresponding magnifications. Specimens were taken fromthe EDTD plane, ED is upwards.

controlling the quality of the chip bonding. Taking into account

the investigation of Fogagnolo et al. [4], the processing route of

cold compaction and hot extrusion through a flat-face die was not

leading to sufficient chip bonding for an extrusion ratio of6.3:1.The iECAP die used in the experiments had an extrusion ratio of8.7:1, the investigated flat-face die and porthole die of8.6:1.The usage of the flat-face die led in some cases to a peeled off sur-

face of the extruded profile as it is shown in Fig. 12, which confirms

the results of the referred investigation.

All chip-based billets extruded through the porthole die and

the iECAP die of comparable extrusion ratio showed sufficient chip

bonding without surface defects.

For all die sets used in the performed experiments, the bond-

ing quality increased with increasing temperature. These results

confirm the results of Ceretti et al. [26],who presented a new pro-

cedure for the identification of the extrusion welding criterion.

They simulated aluminum seam welding in porthole die extru-

sion by performing a flat rolling experiment of a sandwich of two


9/11


Fig. 10. Results of hardness measurement over the specimens width in the EDTD plane (a) specimens extruded at 450 C, (b) specimens processed at 550 C and referencespecimen.

Fig. 11. Tensile test results of the processed specimens. (a) Specimens extruded at 450 C and (b) Specimens extruded at 550 C and reference specimen.

rectangular AA6061 aluminum alloy specimens in order to bond

them. They varied the testing temperature and the rolling ratio in

order to investigate the effect of temperature and pressure on the

welding quality. In theexperiment of Ceretti et al. [26], higher tem-peratures led to a better solid state bonding, measured by optical

investigation and micro-hardness tests in the normal direction to

the bonding line.

Takinginto account thegeometryof the die, thewelding quality

increased when changing the die set from flat-face die to porthole

die and then to the iECAP die. This can be related to the longer

Fig. 12. Peeled off profile surface during extrusion through the conventional flat-

face die for chip extrusion at an extrusion ratio of8.6:1.

deformation zone. Referringagain to Ceretti et al. [26], itwasshown

that a better solid state bonding was achieved for a higher rolling

ratio, i.e. higher pressure and strain. By using the porthole die,

a higher amount of pressure affects the chip bonding due to thepressure occurringin the welding chamber anda higher amount of

strain is introduced into the specimen due to the additional shear

whenthe materialflow isdividedintothe four singlestrings. There-

fore, the bonding quality increases using a porthole die instead of a

flat-facedie. TheiECAP dieneededthe highest extrusionforce of the

investigated die sets which leads to a high amount of pressure on

the chips during extrusion. In addition, every ECAP turn within the

die leads to a back pressure for the previous turn and to additional

strain through shear. Referring to Xia et al.[27],when consolidat-

ing particles using ECAP, the application of back pressure leads to

a synthesis of fully dense bulk material. The combination of high

pressure, additional strain and back pressure explains the superior

bonding quality of the iECAP processed specimen.

5.4. Microstructure analysis

The microstructure of extruded chip-based billets differs from

the microstructure of extruded cast billets (reference specimen)

in terms of shape and size of the grains ( Figs. 8 and 9). Chip-

based billets extruded throughthe flat-face diehavea smaller grain

size compared to cast billets extruded through the same die. The

microstructure analysis revealed a mixture of equiaxed and elon-

gated grains for chip-based profiles produced withthe flat-face die.

When using the porthole or the iECAP die for chip extrusion, the

grains become equiaxed and the PCG zones become smaller com-

pared to chip-based or cast billets extruded through the flat-face

die.


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Fig.13. Preventedgraingrowthdue tochip boundary ina chip-based specimen,ED

is upwards.

A positive effect concerningthe extrusionof chips instead of cast

material is the prevention of grain growth, as chip boundaries act

as natural barriers between grains, which can be seen inFig. 13.

Although the single chips are not visible after extrusion, the

chip structure still exists and could be made visible through

metallographic techniques. It is assumed that only a partial break-

ing of the oxide layer between the single chips occurs, which

prevents the chips from perfect welding and leads to the shown

effect. Comparing the microstructure of the chip-based profiles

with the reference specimen in the EDTD plane, the average grainsize of chip-based profiles is68% smaller compared to cast bil-lets extruded through the flat-face die. This can be related to the

dispersion of oxide contaminants, as it was assumed by Ying et al.

[16],and to the prevention of grain growth due to chip boundaries

as proposed by the current authors.

5.5. Hardness measurement

Theoverall hardness distributionover theEDTD plane is rather

homogeneous for all specimens, the average standard deviation of

each specimen is lower than 2.5. When increasing the extrusion

temperature from 450 C to 550 C, the hardness value increasedfor the chip-based specimen extruded through the porthole die by

9.1%, while it decreased by7.5% and6.5% for the chip-basedspecimen extruded through the flat-face andthe iECAP die, respec-

tively. All chip-based specimens have a higher hardness compared

to the cast billet extruded through the flat-face die. This can be

related to the smaller average grainsize of the chip-based specimen

compared to the extruded cast billet. Additionally, Luo et al. [12]

suggested an additional dispersion strengthening due to gradual

fragmentation of the oxide layers between chips.

The hardness of an individual chip after turning and before

cold-compaction and hot extrusion was82HV. As a result ofhot extrusion, all extruded chip-based profiles exhibited a lower

average hardness compared to the chips after turning. The chip-

based billets extruded throughthe flat-face diewereexposed to the

lowest strainand therefore to the lowest temperature in the defor-

mation zone during extrusion for both extrusion temperatures of450 C and 550 C compared to the chip-based billets extrudedthrough the porthole die andiECAPdie. This resulted in the highest

hardness for the FL specimen and the FH specimen for the corre-

sponding extrusion temperatures.

5.6. Tensile test

The maximum true stress values and the corresponding true

strain values for the FL specimen and PL specimen are comparable

to the reference specimen. Having a comparable maximum true

stress value, the EL specimen shows a 60% higher corresponding

true strain value than the reference specimen. The high ductility of

the EL specimen benefits in case of further mechanical processing

of the extruded profiles in terms of their forming limit.

The FH specimen shows a comparable maximum true stress

value to the reference specimen at the same true strain value.

Slightlyhighertrue stress (6.6%)andtruestrain(6.7%) values canbe observed for the PH specimen. The EH sample shows superior

strengthand ductility, havinga 16.7% higher maximum truestressvalue at a true strain value 60% higher compared to the reference

specimen.

The tensile test results strongly depend on the extrusion tem-

perature.The maximum true stress value of the specimen extruded

at 550 C are always higher than of those extruded at 450 C withthesame dieset. This is most significant forthe iECAP dieprocessed

specimen with an increase in maximum true stress of12.1% com-pared to an increase of6% and4.3% for the porthole die andflat-face die processedspecimen, respectively, whenincreasing the

extrusion temperature from 450 C to 550 C. The maximum truestrain level increased by 0.1 for the flat-face die and decreased by

0.1 for the porthole die processed specimen when the temperature

was changed from 450 C to 550 C. No influence on the true strainvalue could be measured for the iECAP die processed specimens.

For the chip-based specimen extruded at 550 C, an increase in themaximum true stress value of5.3% and10% can be observedwhen changing the die set from the flat-face to the porthole and

from the porthole to the iECAP die, respectively. For the investi-

gated parameters, a change of the dieset leads to a more significantchange in strength and ductility than a change of temperature.

No correlation between thevalues of thehardnessmeasurement

(Fig. 10) and the tensile test results (Fig. 11) can be observed for the

chip-based specimens. The hardness values represent the material

response of an individual chip or eventually two chips and the chip

boundary, as thedimension of the indentation is smaller compared

to the dimension of a chip. The tensile properties are represent-

ing the material response as an average of the chips in the whole

deformation zone of the tensile test specimen. This includes the

mechanical properties and microstructure of the individual chips,

whichcorrelate to the hardness values, andin addition the bonding

between the single chips and the contribution of the PCG zone.

The microstructure of the chip-based specimen extruded

through the flat-face die at 450 C (Fig. 8,FL) appears to result insuperior tensile results compared to the cast reference specimen

(Fig. 8, Ref.). However the tensile properties are slightly inferior for

the chip-based specimen. This can be related to insufficient chip

bondingwhen extrudingchip-basedbilletsthrough theflat-facedie

at the low extrusion ratio of8.6:1. Tensile properties of the chip-based specimens increased when the die set was changed from

flat-face die to porthole die and then to iECAP die (Fig. 11).These

more complex extrusion dies provided deformation routes result-

ing in better chip bonding, more equiaxed grains and a decrease in

the size of the PCG zone. The inferior tensile results of the porthole

die compared to the iECAP die are due to the presence of the four

welding lines where thematerial strings of the four portholes were

welded together in the welding chamber.

6. Conclusion

A four turn equal channel angular pressing tool integrated

into an extrusion die (iECAP die) was designed to improve the

mechanical properties of profiles extruded from aluminum chips.

Microstructure and mechanical properties of chip-based billets

extruded through the iECAP die were compared to chip-based bil-

lets extruded through the flat-face die and the porthole die as well

as to cast billets extruded through the flat-face die. The following

conclusions can be drawn:

- At the low extrusion ratio of8.7:1, the deformation path of theflat-face die did not guarantee sound chip bonding. However, the

porthole and the iECAP die provided deformation conditions for


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the same extrusion ratio resulting in successful solid state recy-

cling of aluminumchips. The superior bondingqualityof thechips

extruded throughthe iECAP diecan be related to a high amount of

pressure affecting the chips, additional strain and back pressure

due to the ECAP turns.

- The analysis of the material flow in the iECAP die revealed

dead metal zone formation in the corners of the ECAP steps,

related to frictionbetween aluminum andthe diewalls. Thedead

metal zone is considered to act as an extension of the die wall,

leading to a deformation zone angle of90. Additional strainofECAP 3.6 was introduced into the material in the four ECAPparts of the iECAP die.

- For chip-based billets extruded through the iECAP die, no chip

boundaries could be observed after extrusion. After etching, the

chip boundaries could be made visible again. It is assumed that

chip welding is partially prevented due to remainingoxide layers

on the chips. Theremaining oxide layers areassumed to be obsta-

cles forthe graingrowth over the chip boundaries and potentially

lead to dispersion strengthening.

- The chip-based billets extruded through the iECAP die and the

porthole die, respectively, showed equiaxed small grains, while

the chip-based billets extruded throughthe flat-facedie showed a

combination of equiaxed and elongated grains with a bigger PCG

zone compared to the other die sets.- Using the iECAP die instead of the flat-face or the porthole die

as a tool for solid state recycling of aluminum machining chips,

under the presented conditions, leads to improved chip bonding

and superior strength and ductility.

- Compared to cast billets extruded through the flat-face die, hard-

ness measurement andtensile testsrevealedsuperiormechanical

properties of chip-based billets extruded through the iECAP die.

Acknowledgments

The support for W.Z. Misiolek as Mercator Visiting Professor

at TU Dortmund University has been provided by the German

Research Foundation (DFG) while he has been also supported by

the Loewy Family Foundation at Lehigh University in Bethlehem,PA, USA through Loewy Professorship. The corresponding author

acknowledgesthe financial support granted by theGraduateSchool

of Energy Efficient Production and Logistics in Dortmund.

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24. improving mechanical properties of chip-based aluminum extrudates by

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