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Materials Science and Engineering A 472 (2008) 251–257

A physical approach to the direct recycling of Mg-alloyscrap by the rheo-diecasting process

G. Liu ∗, Y. Wang, Z. FanBrunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge UB8 3PH, UK

Received 7 December 2006; received in revised form 8 March 2007; accepted 9 March 2007

bstract

Due to the rapidly growing market for magnesium diecastings in the automotive industry in the past decade, there is a steady increase inagnesium scrap sourced from both end-of-life vehicles (old scrap) and the manufacturing processes (new scrap). In addition, the energy required

o recycle magnesium is only 5% of that for extraction of primary magnesium from Mg-containing minerals. This makes magnesium recyclingxtremely beneficial for cost reduction, preservation of natural resources and protection of the environment. In this paper, a physical approach isresented to magnesium recycling using the rheo-diecasting (RDC) process. The RDC process was applied to process AZ91D alloy sourced fromoth primary alloy ingots and diecast scrap. The experimental results showed that the RDC process could be used to produce recycled AZ91D

lloy with fine and uniform microstructure and very low level of porosity. The intermetallic compounds containing the impurity elements were finend spherical particles distributed uniformly in the alloy matrix. No oxide particle clusters and oxide films were found in the RDC microstructure.he tensile properties of the recycled AZ91D alloy were comparable to those produced from the primary alloy ingots. 2007 Elsevier B.V. All rights reserved.

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eywords: Recycling; Rheo-casting; Mg-alloy; Microstructure; Mechanical pr

. Introduction

Due to the increasing concerns for environmental protec-ion and ever-tightening government regulations for sustainableconomic development, there is an increasing pressure onutomakers to improve fuel consumption to reduce greenhouseas emissions. The automakers have studied the relationshipetween vehicle mass and fuel economy for decades. The major-ty of the studies to date conclude that for every 10% reductionn vehicle weight there will be a corresponding 6–8% decreasen fuel consumption. Magnesium alloys, as the lightest of alltructural metallic materials, find increasing applications in theutomotive industry for vehicle weight reduction. Since the early990s we have seen a 15% average annual growth rate of diecastg-alloy components in the automotive industry [1]. It is pre-

icted that the usage of light alloys in cars will continue to rise

t an even faster pace in the next decade. This fast growth rateeans a fast increase in Mg-alloy scrap from both manufac-

uring source (new scrap) and end-of-life vehicles (old scrap).

∗ Corresponding author. Fax: +44 1895 269758.E-mail address: guojun.liu@brunel.ac.uk (G. Liu).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.03.026

magnesium inventory analysis has been conducted for Ger-any up to 2020 [2], which concluded that the major source

f magnesium scrap is end-of-life vehicles. In Germany alone,he stored automotive magnesium will reach 51 kT in 2020, rep-esenting an average annual growth rate of 80%. The generalicture of magnesium inventory is expected to be similar in theeveloped countries. In addition, the source for new scrap is alsorowing fast. In typical magnesium diecasting operations onlyround 50% of the material input ends up as finished products,nd the remaining 50% of magnesium alloys is accumulated ascrap [3]. Furthermore, unlike other materials for engineeringpplications, metals, such as aluminium and magnesium, cane recycled repeatedly without loss of their inherent properties.ecycled light alloys from a scrap source only consume 5%f the energy required to produce the same amount of primarylloys. The application of recycled light alloys in cars can lead tosubstantial reduction of greenhouse gas emissions. Therefore,

ecycling Mg-alloy scrap is becoming an important technicalnd economical challenge.

A major barrier to the recycling of Mg-alloys is the existencef substantial amount of inclusions and impurity elements in thecrap (both new and old), the former causes severe lose of ductil-ty and strength, and the latter reduces significantly the corrosion

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52 G. Liu et al. / Materials Science an

esistance. The major challenges in reprocessing light alloy scrapre dealing with the increased inclusions and impurity elements.onventional wisdom is to reduce the amount of such inclusionsnd impurities by a chemical approach [4–6], e.g., flux or flux-ess refining, and hydrometallurgy process. However, there aretill some problems in these recycling technologies, such as pro-ess complexity, low productivity and high energy consumption.herefore, there is a growing need for more effective recyclingrocesses for Mg-alloy scraps.

In recent years, the semisolid metal processing (SSM) haseen shown to possess a number of technological advantagesver the traditional casting processes. It is proved to be par-icularly suitable for light alloy components with improved

echanical performance [7,8]. Among these SSM processes, theheo-diecasting process (RDC), recently developed by BCASTt Brunel University, is a promising technology in terms of therocess flexibility, component integrity, microstructural homo-eneity and cost-effectiveness [9–11]. In the RDC process, thentensive shearing and turbulence are imposed to the solidifyingiquid by the high speed rotating twin screws, resulting in a finend uniform microstructure. The aim of the present study is tonvestigate the feasibility to use the RDC process as an enablingechnology for direct recycling of magnesium alloy scrap.

In this paper, we present the RDC process as a physicalpproach to direct recycling of Mg-alloy scrap. The experimen-al results presented in this paper include variation of chemicalompositions, microstructural evolution, and mechanical prop-rties of RDC recycled AZ91D alloy processed under differentonditions. The discussion will be focused on the effects of pro-essing on microstructure and mechanical properties of recycledg-alloys and the advantages of RDC as a physical approach to

irect recycling of Mg-alloy scrap.

. Experimental

The primary AZ91D alloy used in the present study was pro-ided by MEL (Magnesium Elektron Ltd., Manchester, UK),nd its chemical compositions are listed in Table 1 in com-arison with the ASTM specifications for the same alloy [12].he AZ91D alloy scraps used in the present study were fromiecasting and consisted of biscuits and gates (70 wt%), runners20 wt%), overflows (9 wt%), and dross (1 wt%). The primaryZ91D ingot with or without either 50 or 100 wt% scrap waselted in a steel crucible, using an electrical resistance furnace

t a holding temperature of 675 ◦C. A mixed gas of N2 con-aining 0.4 vol.% SF6 was used to protect the alloy melt from

xidising during the melting.

The alloy samples were prepared by the RDC process, devel-ped recently by BCAST at Brunel University. This novelechnology is an innovative one-step SSM processing technique

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able 1hemical compositions of the primary alloy used in this study in comparison with th

lement Zn Al Si Cu

rimary alloy 0.67 8.6 0.03 0.0lloy specifications [12] 0.45–0.9 8.5–9.5 0.05 0.0

gineering A 472 (2008) 251–257

or manufacturing near net shape components of high integrityirectly from liquid alloys. Detailed description of the RDC pro-ess can be found elsewhere [13]. The RDC equipment consistsf two basic functional units: a twin-screw slurry maker (TSSM)nd a standard cold chamber high-pressure diecasting (HPDC)achine. The TSSM has a pair of screws rotating inside a barrel.he fluid flow inside the slurry maker is characterised by highhear rate and high intensity of turbulence. In the present study,he rotation speed of the TSSM was fixed at 800 rpm, and thehearing time at 30 s. A 280-t standard HPDC machine was usedo produce the standard tensile test samples. For all the experi-ents in this investigation, the die temperature was kept constant

t 235 ◦C. The dimensions of the tensile test samples were 6 mmn gauge diameter, 60 mm in gauge length and 150 mm in totalength.

The as-cast samples were then subjected to various heat treat-ents with the optimised conditions identified previously [14].he samples were solution treated at 413 ◦C for 5 h and quenched

n water (T4), following by aging at 216 ◦C for 5.5 h (T6). T5reatment was carried out at 216 ◦C for 5 h. The evaluation ofechanical properties under as-cast and various heat treated con-

itions was carried out at room temperature on an Instron tensileest machine, with a strain rate of 6.7 × 10−4 s−1.

The microstructures of the RDC samples with and withouteat treatment were examined by optical microscopy (OM) withuantitative metallography. The specimens for OM were pre-ared by the standard technique of grinding with SiC abrasiveaper and polishing with an Al3O2 suspension solution, fol-owed by etching using a solution of 5 vol.% concentrated HNO3n 95 vol.% ethanol. A Zeiss AxioVision optical imaging sys-em was utilised for the OM observations and the quantitative

easurements of the microstructure features. Scanning electronicroscopy (SEM) was carried out using a Zeiss Supera 35achine with a field emission gun, equipped with an energy

ispersive spectroscopy (EDS) facility and operated at an accel-rating voltage of 15 kV. The chemical composition analysis wasonducted on a WAS Foundry Master with at least five burns onach of the three different areas of each sample.

. Results

.1. Microstructures

Fig. 1 shows the general microstructure of AZ91D Mg-alloyroduced by the RDC process from both primary alloy ingot and00% diecast scrap. It can be seen that both RDC samples exhibit

he typical microstructural features of AZ91D alloy produced byhe RDC process. There is no obvious difference in microstruc-ure between the two alloy samples. As shown in Fig. 1, there is aertain volume fraction of the spherical primary �-Mg particles

e ASTM standard specifications for AZ91D Mg-alloy (all in wt%)

Mn Fe Ni Be Others

1 0.22 0.002 0.0007 0.001 <0.0115 >0.17 0.004 0.001 – 0.01

G. Liu et al. / Materials Science and Engineering A 472 (2008) 251–257 253

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rseen that these particles have a spherical morphology and fineparticle size (1–3 �m), and are well dispersed in the remainingliquid matrix. EDS analysis of such particles (see Fig. 3) hasconfirmed that such inclusion particles contain mainly Al, Mn

ig. 1. Microstructure of RDC AZ91D Mg-alloy manufactured from (a) primarylloy ingot and (b) 100% scrap.

eing uniformly distributed in the entire section of the sample.he relatively large and spherical particles are formed inside theSSM under high shear rate and high intensity of turbulence.he average particle size and the volume fraction of the primary-Mg globules were measured to be 37.8 �m and 19.4% for therimary alloy. Quantitative metallography showed a very similaresult for the RDC recycled alloy, with the average particle sizend volume fraction of the primary �-Mg phase being 37.6 �mnd 19.5%, respectively. Both alloy samples show a very highasting quality and high integrity with the total porosity being.2–0.5 vol.%.

The remaining liquid of the semisolid slurry produced by thewin-screw slurry maker will undergo a further solidification inhe shoot sleeve and inside the die cavity of the HPDC machine,hich has been referred as to secondary solidification [15]. A

ew of �-Mg dendrite fragments can be found randomly in bothlloy samples, as shown in Fig. 1. Such dendrites were formedn the shot sleeve and then fragmented when they were passinghrough the narrow gate of the die, giving rise to the observedendrite fragments in the final microstructure. The remaining

iquid in the semisolid slurry finally solidified in the die cavitynder high cooling rate, and the resulting microstructure is veryne �-Mg particles (less than 10 �m) defined by the eutectic-Mg17Al12 phase, as shown in Fig. 2.

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ig. 2. SEM micrograph showing the detailed microstructure of RDC recycledZ91D alloy from 100% scrap.

Fig. 3 is a backscattered electron SEM image of the RDCecycled AZ91D alloy showing the inclusion particles. It can be

ig. 3. SEM backscattered electron image of RDC recycled AZ91D alloy show-ng the size and morphology of intermetallic particles (marked by the arrow).lso shown here are the EDS results from an intermetallic.

254 G. Liu et al. / Materials Science and Engineering A 472 (2008) 251–257

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ig. 4. Chemical compositions of AZ91D alloy processed under different condi-ions: (a) compositions of elements Al, Zn and Mn; (b) compositions of elementsi, Fe and Be.

nd O. It is more likely that such inclusion particles are Al8Mn5articles associated with O. Further quantitative EDS analysisevealed that some of the bright particles contained small amountf Fe.

Detailed microstructural analysis has confirmed that neitherxide films nor clusters of oxide particles existed in the RDCecycled AZ91D alloy samples.

.2. Chemical compositions

There are three main alloying elements (Al, Zn, and Mn)nd five minor detectable elements (Be, Si, Fe, Cu, and Ni)n the AZ91D Mg-alloy. Fig. 4 presents the chemical compo-itions of AZ91D alloy at different processing states and withifferent starting materials. It can be seen that for both alloyinglements and impurity elements (except Fe), the slight changesn compositions occurred after melting of the primary alloy, andhere were no obvious changes in compositions after RDC pro-essing with different starting materials. After melting of therimary alloy ingot Al content increased by 0.31 wt% and Znontent decreased by 0.04 wt%. It should be noted that all thesehanges in composition are well within the alloy specifications

see Table 1). The standard deviation of chemical analysis isabulated in Table 2. Therefore, it can be concluded that RDCrocessing of AZ91D diecast scrap (even with 100% scrap)roduces samples within the ASTM specifications. Ta

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ig. 5. Iron content of RDC AZ91D alloy processed from different startingaterials.

The iron concentration of the RDC recycled AZ91D alloy islightly higher than that in the primary alloy, as shown in Fig. 5.owever, from Fig. 5, it is important to point out that, the RDCrocess does not increase Fe content in the final RDC castingsompared with former melting stage. As shown in Table 2, forhe primary alloy, Fe content increased from 38 ppm in the ingoto 40 ppm in the molten alloy and 41 ppm in the RDC sam-le; while for the scrap alloy, Fe content had no change afterhe RDC process. In general, Fe is picked up mainly from these of iron equipment during melting. Table 2 also shows that,fter RDC of AZ91D Mg-alloy, there was no obvious changen chemical concentrations for other impurity elements com-ared to that after melting, although Ni has increased from0 ppm in the molten alloy to 45 ppm in the RDC recycledamples.

.3. Mechanical properties of RDC recycled AZ91D alloy

Table 3 summarises the tensile properties of the RDC recy-led AZ91D alloy under various processing conditions. Theechanical properties for the primary alloy prepared at the

ame conditions are also given for comparison. Fig. 6 shows

able 3ensile properties of the RDC AZ91D alloy processed under different conditions

rocessing conditions Yield stress(MPa)

UTS (MPa) Elongation (%)

DC as-castPrimary 139.8 ± 4.1 250.1 ± 7.6 6.86 ± 0.6850% Scrap 139.6 ± 2.3 249.3 ± 8.2 6.67 ± 0.92100% Scrap 139.6 ± 2.7 249.2 ± 6.6 6.39 ± 0.59

DC + T4Primary 96.9 ± 4.0 263.8 ± 10.7 11.61 ± 1.57100% Scrap 95.9 ± 2.1 260.9 ± 9.0 11.33 ± 1.32

DC + T5Primary 142.8 ± 1.9 242.4 ± 8.9 5.27 ± 0.51100% Scrap 144.9 ± 1.3 243.0 ± 7.3 4.54 ± 0.53

DC + T6Primary 134.0 ± 6.0 259.2 ± 8.5 5.59 ± 0.82100% Scrap 134.1 ± 6.4 260.6 ± 12.6 4.76 ± 0.88

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ig. 6. Mechanical properties of RDC AZ91D alloy processed from differenttarting materials.

direct comparison of the tensile properties of RDC AZ91Dlloy produced with primary alloy, 50% Scrap and 100%crap. It can be seen that both yield strength (YS) and ulti-ate tensile strength (UTS) for the RDC recycled Mg-alloy

re comparable with that of the primary AZ91D alloy. How-ver, recycling of the AZ91D scraps does cause decrease inuctility, but the reduction is comparably small, being 0.5%ompared with that of the primary alloy. The RDC alloy recy-led from 100% scraps has a considerably high elongation levelf 6.4% at as-cast state, which is much higher than 3.0–3.3%,chieved by the normal HPDC process using primary alloy ingot12,16].

Fig. 7 shows the strain–stress curves for the RDC AZ91Dlloy at different heat treatment conditions. It can be seen that6 treatment improves the UTS significantly, whilst T5 treat-ent results in the highest YS but with some sacrifice of the

longation. It is important to observe that the solution treatmentithout aging (T4) provides a superior combination of elonga-

ion and UTS. In the present study, the achieved maximum UTSnd elongation for the RDC AZ91D alloy were 264 MPa and

.1% under as-cast state, and 278 MPa UTS and 13.9% under4 treated condition.

ig. 7. Strain–stress curves for RDC AZ91D alloy obtained under differentrocessing conditions. Also shown here are the strength vs. elongation plot forome of the tested samples.

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. Discussion

.1. Microstructural evolution

Solidification of the AZ91D alloy during the RDC process haswo distinct stages, i.e., the primary solidification under intensiveorced convection inside the TSSM and the secondary solidifi-ation under high cooling rate inside the die cavity [15]. Ashown in Fig. 1, the relatively larger primary �-Mg particles,8 �m on an average, are formed inside the slurry maker duringhe primary solidification. The secondary solidification occurshen the semisolid slurry is transferred into the shot sleeve and

s responsible for the solidification of the remaining liquid in theemisolid slurry. The high cooling rate (about 103 K/s) inside theie cavity is obviously responsible for the fine primary �-Mgrains (less than 10 �m in diameter) and the fine inter-granularutectic �-Mg17Al12 phase, as shown in Fig. 2.

In addition to the fine and homogeneous microstructure, theecycled alloy exhibits high integrity. The total porosity in theDC alloy produced from 100% scrap was measured to be.2–0.5 vol.%, which is significantly less than the usual poros-ty levels observed for the conventional HPDC AZ91D alloy12]. In addition, no oxide clusters or oxide films were found inhe RDC recycled AZ91D alloy using the SEM/EDS analysis.part from the eutectic �-Mg17Al12 phase, it was found that then-containing intermetallic particles observed in the recycled

lloy are usually associated with oxygen and have an extremelyne size (1–3 �m in diameter). This is a significant advantagef the RDC technology compared to the conventional diecast-ng technologies, where the oxide particle clusters, oxide filmsnd intermetallic particles containing impurity elements wereound in the cast microstructure usually with a large size and aon-uniform distribution [17].

.2. Mechanical properties

The properties of magnesium alloy can be considerablyffected by the incorporation of impurities within the recycledlloys. These chemical impurities can lead to a severe reductionn ductility. The ductility decrease up to 50% has been foundn some magnesium alloy systems [4]. Therefore, the qualityf recycled magnesium alloys has to be controlled carefullyhrough chemical refining before diecasting. In this study, how-ver, there is no considerable reduction in ductility for the RDCecycled alloy in comparison with the primary RDC alloy. Thelongation can be as high as 6.4% for the 100% recycled alloy,hich is more than double of that achievable by HPDC processsing the same primary alloy [12]. It is also interesting to notehat the YS and UTS of the recycled alloy are essentially keptn the same level as that for the primary alloy. The good com-ination of strength and ductility of the RDC recycled AZ91Dlloy can be attributed to the following factors:

1) Much reduced porosity in the as cast microstructure. The

porosity level has been reduced from 2–5 vol.% in the HPDCsample to 0.2–0.5 vol.% in the RDC sample.

2) Oxide and intermetallic compounds in the RDC sampleshave a fine size, spherical morphology and uniform dis-

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gineering A 472 (2008) 251–257

tribution, and therefore, their harmful effect to mechanicalproperties has been effectively eliminated.

3) Fine and uniform microstructure throughout the entire RDCsample can promote uniform deformation, and hence elim-inating stress concentration.

.3. RDC process for recycling magnesium scrap

A major challenge in recycling Mg-alloy scrap is to deal withhe increased inclusions and impurity elements. In this work, weave developed a physical approach to recycle magnesium scrapsing the RDC process. Instead of eliminating the inclusionsnd impurity elements themselves in the conventional chemicalpproaches, here we eliminate the harmful effects of the inclu-ions and impurity elements. Alloy melt prepared from scrap ishysically treated under intensive forced convection to elim-nate the detrimental effects of both inclusions and impuritylements, so that higher grade light alloy products can be pro-uced from their scrap. Since the RDC recycled Mg-alloy hasetter mechanical properties than that of the primary alloy pro-essed by the conventional technologies (e.g., HPDC process),his new approach is termed as upcycling, which is in contrast tohe current concept of recycling where the scrap is converted inton existing grade of alloy often with inferior quality comparedith the corresponding primary alloy grade.Compared with the conventional chemical approach to recy-

ling, the physical approach developed here has the followingdvantages:

1) Through enhanced nucleation (known as effective nucle-ation [15]), the primary intermetallic compounds containingimpurity elements will have a much finer particle size and acompact morphology (Fig. 3), resulting in improved ductil-ity (Fig. 6). Therefore, intensive melt shearing can increasethe tolerance of light alloys to impurity elements.

2) Inclusions, such as oxide particles and oxide films, willbe pulverised (1–3 �m in size) and dispersed uniformlythroughout the alloy matrix, and become strengtheningphases, rather than detrimental factors, leading to a signifi-cant improvement of mechanical properties by eliminationof the stress concentration points.

3) Due to the chemical uniformity and fine and uniformmicrostructure provided by the RDC process, it is expectedthat the RDC recycled Mg-alloy will assist to improve cor-rosion resistance, due to the well dispersed intermetalliccompounds containing Fe, Ni, or Cu.

4) The RDC process can be used to produce high qualitycastings directly from Mg-alloy scrap. Compared with theconventional chemical approach, it is more efficient, lessenergy intensive, lower cost and less environmental impact.

Finally, it should be pointed out that, RDC process as a phys-cal approach to recycling of Mg-alloy scrap does have its own

imitations. It is more suitable for direct recycling of higher-rade scrap, such as diecast scrap and well sorted end-of-lifeehicle components. However, for lower grade scraps, such asurnace dross and old scrap mixed with other materials, the con-

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Proceedings of the 6th International Conference on Mg-alloys and their

G. Liu et al. / Materials Scienc

entional chemical approaches have to be used to purify the alloyo certain level before the RDC process can be used for directecycling.

. Conclusions

1) Rheo-diecasting (RDC) process can be used to recycle highgrade Mg-alloy scrap to produce high integrity Mg-alloycastings.

2) The RDC recycled Mg-alloy has fine and uniformmicrostructure and extremely low porosity.

3) The RDC recycled AZ91D alloy has chemical compositionswell within the ASTM specifications for the same alloy.

4) No oxide particle clusters and oxide films were found in theRDC recycled Mg-alloy.

5) Intermetallic compound particles containing the impurityelements have a fine size, spherical morphology and a uni-form distribution in the alloy matrix.

6) The RDC recycled AZ91D alloy exhibits superior mechan-ical properties, particular elongation, over the primaryAZ91D alloy processed by the conventional HPDC process.

cknowledgement

The financial support from EPSRC, UK is acknowledgedith gratitude.

[

Engineering A 472 (2008) 251–257 257

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