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Materials Science and Engineering A 447 (2007) 341–346

The correlation between wall thickness and propertiesof HPDC Magnesium alloys

E. Aghion a,∗, N. Moscovitch b, A. Arnon a

a Ben-Gurion University of the Negev, Department of Materials Engineering, Beer-Sheva 84105, Israelb Magnesium Research Institute, Beer-Sheva 84100, Israel

Received 21 May 2006; received in revised form 4 October 2006; accepted 21 October 2006

bstract

A systematic study was carried out to evaluate the correlation between high pressure die casting (HPDC) parameters and the mechani-al properties of Magnesium alloys. The tested alloys included two conventional alloys: AZ91D and AM50A and a newly developed alloy,RI153M.The nature of the above correlations was semi-empiric, based on computer simulation of the die casting process using MAGMA® simulation

oftware and actual measurements of mechanical properties. This was combined with microstructure analysis with the aim of understanding thenteraction between high pressure die casting and the obtained properties. Special attention was also given to the complex effect of the wall thicknessf die cast specimens and their subsequent porosity level. 2006 Elsevier B.V. All rights reserved.

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eywords: Magnesium alloy; Die casting; Solidification; Mechanical propertie

. Introduction

The development of magnesium applications for the auto-otive industry has received significant attention due to its lighteight and consequent potential to reduce both fuel consump-

ion and green house effect [1–3]. Magnesium applications werelso developed for the electronics industry, where weight reduc-ion and electro-magnetic shielding are of primary importance.n both cases, the dominant process for producing cost effectiveomponents is high pressure die casting (HPDC) [4,5]. How-ver, the increasing demand for HPDC magnesium componentsequires proper understanding of the correlation between HPDCarameters and properties of the end product [6]. This is partic-larly important for newly developed Magnesium alloys whereimited HPDC experience exists [7].

The aim of the present study is to evaluate the correlation

etween HPDC parameters and the properties of conventionalnd new Magnesium alloys for die casting applications. This isequired for part design as well as for optimizing the die cast-

∗ Corresponding author at: Department of Materials Engineering, Ben-Gurionniversity of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel.el.: +972 8 647 7498; fax: +972 8 647 7935.

E-mail address: egyon@bgu.ac.il (E. Aghion).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.10.065

ting simulation

ng process in order to obtain adequate properties in serviceonditions. The tested alloys in this study include the newlyeveloped MRI153M, and AZ91D and AM50A as referenceaterials. The chemical compositions of the selected alloysZ91D and AM50A met ASTM standards. The chemical com-osition of MRI153M is 4.7–7.3% Al–0.17–0.6% Mn–0–0.8%n–0.03% Si–0.003% Cu–0.001% Ni–0.004% Fe–1.8–3.2%a–0–0.5% Sr–balance Mg. The typical mechanical propertiesre shown in Table 1 [8]. It should be noted that the casta-ility of MRI153M is similar to that of AZ91D. However,t has a significant advantage in terms of creep resistance atigh temperature applications [9]. The scientific tools of thistudy include a computer simulation model which was combinedith measurements of mechanical properties and microstructure

ssessments.

. Experimental

Semi-empiric correlations between HPDC parameters androperties of the tested alloys: MRI153M, AZ91D, and AM50A

ere separately obtained by computer simulation modeling,echanical testing, and microstructure analysis. The computer

imulation was carried out using MAGMA® simulation softwareor evaluating the solidification features. The thermo-physical

342 E. Aghion et al. / Materials Science and Engineering A 447 (2007) 341–346

Table 1Typical mechanical properties of the tested die cast Mg alloys (separately diecast specimens)

Properties AZ91D AM50A MRI153M

Tensile yield strength [MPa]20 ◦C 160 125 170150 ◦C 100 75 135

Ultimate tensile strength [MPa]20 ◦C 240 230 250150 ◦C 120 110 190

Compression yield strength [MPa] 160 125 170Elongation in 60 mm [%] 5 15 6Impact strength [J] 4 15 8Young’s modulus [GPa] 45 45 –Hardness [Brinell] 70 60 45F

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The soundness of the specimens versus their wall thickness interms of porosity for AZ91D, AM50A, and MRI153M is shown

atigue strength [MPa] 95 9 120

roperties used for the purpose of this simulation are presentedn Table 2 [10]. For the purpose of the simulation, the parame-ers provided to the MAGMA software for the MRI153M alloyere those of the AZ91D alloy with the correct non-equilibrium

olidification range for MRI153M. Mechanical testing in termsf ultimate tensile strength, yield strength, impact, hardness, andlongation were carried out on rectangular die cast tensile spec-mens with a width of 12 mm and thicknesses between 1.5 and2 mm as shown in Fig. 1.

The tensile specimens were obtained using a Hydra OL-320old chamber die casting machine. According to the processptimization, the melt and die temperatures for MRI153M were60–670 and 200–250 ◦C, respectively. As for AZ91D, the meltnd die temperatures were 650–670 and 150–200 ◦C, respec-ively, while for AM50A the melt and die temperatures were80–695 and 150–200 ◦C, respectively. Tensile specimens wereested by X-ray radiography to ensure soundness. Only soundnd high-quality specimens with reasonable porosity levels weresed for the detailed study. In general, the porosity was measuredsing Archimedes’ principle only in the gauge area (the area toe studied).

The microstructure assessment, mainly in terms of grain size,as carried out on a cross-section of the rectangular die cast

pecimens. The technical method of measurement was based on

ounting the number of grains along arbitrary lines drawn on theetallographic images.

able 2hermo-physical properties used in MAGMA simulation [10,11]

roperty AZ91D AM50A MRI153M

ensity at 20 ◦C [g/cm3] 1.81 1.77 1.82inear thermal expansion coefficient[�m/mK]

26.0 26.0 25.9

hermal conductivity at 20 ◦C [W/Km] 51 65 64pecific heat [kJ/kg K] 1.02 1.02 1.09atent heat of fusion [kJ/kg] 370 370 –on-equilibrium solidification range[◦C]

598–434 620–434 601–506Ft

Fig. 1. Tensile specimen drawings and their associated dimensions in mm.

. Results and discussion

A schematic illustration of the rectangular die cast tensilepecimens showing the cross-section and X-direction used forhe following analysis is introduced in Fig. 2. The X-directionndicates the direction of heat transfer during solidificationhich affects the cross-sectional properties of the specimen. For

he purpose of this study, several assumptions were made:

1) One dimension geometry, including heat transfer solidifica-tion (only in the X-direction);

2) uniform die temperature in a thin layer near the die surface;3) parallel layers of similar microstructure;4) symmetry conditions are being applied.

ig. 2. Specimen transverse cross-section and the direction of heat flow fromhe cavity.

E. Aghion et al. / Materials Science and Engineering A 447 (2007) 341–346 343

Fig. 3. Porosity percentages according to specimen wall thickness.

Table 3Tensile properties of die cast AZ91D vs. specimen thickness

Specimen thickness[mm]

Yield strength[MPa]

Ultimate tensilestrength [MPa]

Elongation[%]

12 141 ± 4 210 ± 12 4.1 ± 0.89 148 ± 2 229 ± 9 5.3 ± 0.56 164 ± 1 241 ± 8 5.1 ± 0.5

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Table 4Tensile properties of die cast AM50A vs. specimen thickness

Specimenthickness [mm]

Yield strength[MPa]

Ultimate tensilestrength [MPa]

Elongation [%]

12 107 ± 1 200 ± 28 6.7 ± 2.49 112 ± 1 230 ± 7 10.6 ± 2.06 118 ± 0.5 236 ± 9 11.0 ± 2.03 124 ± 0.5 231 ± 11 11.0 ± 1.51.5 136 ± 5.8 230 ± 16 8.0 ± 2.6

Table 5Tensile properties of die cast MRI153M vs. specimen thickness

Specimen thickness[mm]

Yield strength[MPa]

Ultimate tensilestrength [MPa]

Elongation [%]

12 158 ± 2 209 ± 15 3 ± 0.79 163 ± 2 216 ± 9 3 ± 0.56 172 ± 2 221 ± 8 3 ± 0.5

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cmothe grain size at the mid-section (Fig. 6) due to the differences

3 163 ± 1 243 ± 9 4.2 ± 0.51.5 175 ± 6 236 ± 14 3.2 ± 0.6

n Fig. 3. In general, it was evident that increased wall thicknessesulted in reduced porosity. This can mainly be explained byhe fact that die casting of thin walled specimens encountersigh turbulent flow of the molten metal, during the first phasef die casting, inducing volume porosity. The volume porosityn this case can be identified as micro-cavity porosity that isbtained from the combination of gas porosity and shrinkage.n addition, the increased porosity in thin walled specimens cane attributed to the significantly lower effectiveness of the thirdhase (pressure phase) of the die casting process compared tohe effectiveness of this phase in thick specimens. It should beointed out that the aim of the third phase is to reduce the cavityorosity resulting from the metal shrinkage, as well as to reduceas porosity which is generated from the dissolved gas in theolten metal [12].The correlation between specimen thickness and mechan-

cal properties, namely yield strength, ultimate strength, andlongation, is shown in Tables 3–5 for AZ91D, AM50A,nd MRI153M, respectively. This has shown that increased

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Fig. 4. The correlations between TYS of M

3 172 ± 1 219 ± 8 2.7 ± 0.31.5 179 ± 6 218 ± 12 1.9 ± 0.4

all thickness results in decreased yield and ultimatetrength, while elongation was increased for all the selectedlloys.

The combined results of all the alloys in terms of YTS arehown in Fig. 4. This revealed relatively similar behavior ofZ91D and MRI153M compared to AM50A. The similarity

n yield strength behavior of AZ91D and MRI153M probablyriginates from their similarity in chemical composition.

In order to evaluate the mechanical strength of the threeelected alloys, hardness tests were performed. The resultsbtained for AZ91D, AM50A, and MRI153M according to thepecimen wall thickness are shown in Fig. 5. This clearly showshat in both AZ91D and MRI153M, similar mechanical strengthas achieved.The differences in mechanical properties of the surface,

ompared to the mid-section, can be explained throughicrostructure analysis of the relevant specimen. Grain size

f AM50 at the surface was significantly smaller compared to

n solidification rate between the surface and the mid-section.ence, the increased strength at the surface can be explained in

erms of the Hall–Patch equation. In general, it should be pointed

g alloys and their casting thickness.

344 E. Aghion et al. / Materials Science and Engineering A 447 (2007) 341–346

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Fig. 7. Average grain size (across the entire cross-section) for all specimens anda

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Fig. 5. Hardness results of AZ91D, AM50A, and MRI153M.

ut that there is a clear correlation between the grain diame-er and the thickness of the specimen. As the wall thickness ofhe die cast specimen increases, the grain size also increases,s shown in Fig. 7, and the solidification time lengthens ashown in Fig. 8. Fig. 9 illustrates the solidification time ver-us grain time and from the values of “n” it is evident that theres a strong dependence of the grain size on the solidificationime in AZ91D alloy, a very low dependence in AM50A and

medium dependence in MRI153M. This behavior is due tohe differences between the castability of the various alloys. Inact, AZ91D has the highest castability while AM50A has theowest one. In general, the castability is attributed to the forma-ion of MgO particles in the melt during the filling of the dieast cavity. Since there is a strong connection between the meltbility to form MgO particles and the Al content in the alloythe higher it is, the lower its ability to form oxides will be),

he content of MgO particles in the flowing path of AM50Aill be higher compared to that of AZ91D. The increased con-

ent of MgO particles generates a high level of nucleation sitesnd consequently a reduced grain size. For example, solidi-

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Fig. 6. Microstructure of die cast AM50 with 12 mm transverse cross-sec

lloys as acquired by optical microscopy.

cation time of 3 s will generate a grain size of 7.8 �m inM50A while the same solidification time will result in a0.1 �m average grain size in AZ91D. This characteristic canxplain the significant differences in the “n” values depicted inig. 9.

The semi-empiric correlations of the above properties witholidification rate were obtained using computer simulation inhe form of MAGMA’s® simulation software.

The relationships between the solidification rates in terms ofolidification time versus tensile yield strength are illustrated inig. 10. This indicates that reduced solidification rate result in

ncreased grain diameter and consequently reduced yield point.imilar results are shown in Figs. 11 and 12, which indicate that

ncreased grain diameter and longer solidification time resultsn reduced impact energy.

tion (T = distance from surface) with measured local grain diameter.

E. Aghion et al. / Materials Science and Engineering A 447 (2007) 341–346 345

Fig. 8. A typical cooling curve for AM50A alloy as received from MAGMA simulation software.

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ig. 9. Solidification time vs. average grain size simulated for AZ91D, AM50M, andime)n.

Fig. 10. The correlation between TYS of die cast M

MRI153M, and fitted according to the relationship: grain size = (solidification

g alloys and associated solidification time.

346 E. Aghion et al. / Materials Science and Engineering A 447 (2007) 341–346

Fig. 11. Impact vs. grain diameter of the selected alloys.

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[10] Hydro-Magnesium, Die cast Magnesium alloys, Data sheet, August 2002.

Fig. 12. Impact vs. solidific

. Conclusions

Semi-empiric correlations between high pressure die castingarameters and mechanical properties of AZ91D, AM50A, andRI153M were established. These correlations highlight the

lose relationship between the microstructure, properties, androcess parameters. In fact, it is evident that proper computerimulation of the casting process can be combined with experi-ental data to obtain sound and high quality die cast Magnesium

lloy components.

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