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Abstracteveral families of magnesium alloys have been developed to operate at the elevated temperatures experienced in automotive drive train applications.

These alloys are based on additions of certain elements, in particular silicon, calcium, rare earths and strontium. This paper surveys the different types of high temperature alloys comparing elevated temperature properties, castability and factors influencing cost. While high temperature properties have attracted the most attention, the ability to produce sound castings is equally important. Castability testing was conducted on several alloys including AE44, AS31, AJ62, MRI153, AM-HP2+ and AXJ530. Factors influencing alloy cost are also very important. During 2011 a rise in rare earth prices caused a significant shift in costs. Rare earth prices have declined greatly in recent months and the outlook for these alloys is increasingly positive with Western producers commencing production. When considering creep performance, cost and castability together the alloy AE44-2 (made using only two rare earth elements: cerium and lanthanum) stands out as the best performing alloy.

IntroductionThe most commonly used magnesium alloys are based on Mg-Al, for example AZ91, AM60 and AM50. These alloys have a good combination of castability and room temperature properties but are unsuited for use at elevated temperatures. At temperatures above 100-130°C they undergo excessive creep deformation even at low stress levels. The poor creep resistance of Mg-Al alloys has been considered to be associated with the formation of Mg17Al12 at elevated temperatures [1,2]. A number of special alloys have therefore been developed with improved high temperature performance [3-10]. These have additional elements that; 1) form high melting point compounds with aluminium to suppress the formation of Mg17Al12 (rare earths,

calcium and strontium), 2) form high melting point compounds with magnesium (rare earth, silicon and tin) and/or 3) form strengthening precipitates (calcium and neodymium).

Several different organisations have developed and tested high temperature alloys for commercial applications. A selection of these alloys is listed in Table 1 along with the elements employed to enhance creep resistance.

An alloy’s ability to withstand high temperatures is one factor in determining its suitability for commercial application. The other two key factors are price and castability. If the price of creep resistance enhancing elements is excessive then the alloy will be uneconomic. Similarly if non-standard procedures are

creep resistant magnesium alloys and their properties

S Alloy Si RE Ca Sr Sn

ae44 √

aS31 √

aJ62 √

Mri153a √ √

Mri153M √ √

Mri230D √ √ √

aM-hP2+ √

aXJ530 √ √

Table 1. Creep resistance enhancing elements used in selected high temperature magnesium alloys

When conSiDering creeP PerforMance, coSt anD caStability together the alloy ae44-2 (MaDe uSing only tWo rare earth eleMentS: ceriuM anD lanthanuM) StanDS out aS the beSt PerforMing alloy.

Suming Zhu, Mark Gibson and Mark Easton, CAST-CRC Zisheng Zhen and Trevor Abbott, Magontec Limited

required for production then costs may be an issue.With regards to castability, an alloy with excellent creep

resistance that is unable to form acceptable parts due to cracking or filling problems is also unsuitable for commercial application.

Creep resistanceCreep resistance of magnesium alloys has been studied extensively by the Australian research organisation: CAST. The creep behaviour of several alloys is shown in Figure 1. Alloys AM-HP2+, AXJ530 and AE44 have excellent creep resistance under a stress of 90 MPa at 150°C.

Tests conducted at a single temperature or stress do not give

a complete picture of alloy performance. A more complete overview of relative performance is shown in Figure 2 where several stress levels have been applied to each alloy. The graph shows the effect of stress on the strain after 100 hours at 150°C. The order of creep performance from best to worst is AM-HP2+ > AE44-4 > AE44-2 > (AE42, MRI153M) > (AS21, AJ62, MRI153A) > AS31 > (AM60, AZ91).

AE44 alloys AE44 is an Mg-4%Al-4%rare earth alloy. It is shown in the figures as two alloys: AE44-4 and AE44-2. During 2011 rare earth prices rose dramatically then fell back (Figure 3).

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Figure 1. Creep behaviour of various magnesium alloys at 150°C and 90MPa.

Figure 2. Creep performance of various magnesium alloys at 150°C for a range of different stress levels.

Figure 3. Chinese prices of four elements and two elements rare earth misch metals.

an alloy With eXcellent creeP reSiStance that iS unable to forM accePtable PartS Due to cracking or filling ProbleMS iS alSo unSuitable for coMMercial aPPlication.

Metal casting technologies June 2012 21

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Rare earths typically occur as a mixture of elements with cerium being the most abundant followed by lanthanum, neodymium and praseodymium. Traditionally the lowest cost rare earth metal has been that obtained by direct conversion of the ore to metal without separation of individual elements. However, in recent years the increased demand for neodymium has led to two elements misch metal, containing just cerium and lanthanum, becoming considerably cheaper than four elements misch metal.

In view of these developments AE44 is now provided in

two versions. AE44-4 contains Ce, La, Nd and Pr while AE44-2 contains just Ce and La.

While the creep performance of AE44-2 is reduced from AE44-4 it still exceeds the performance of all other alloys apart from AXJ530 and AM-HP2+. When die castability is considered the benefits of AE44-2 over other alloys is greatly enhanced.

Die castabilityIn some alloys, high temperature creep resistance comes at the expense of castability. If an alloy is prone to cracking or does not easily fill the die then high temperature creep resistance is of little use.

Quantification of die castability is not as straightforward as determining creep properties. Castability refers the several different aspects including tendency for cracking, level of porosity, fluidity and ease of filling, resistance to sticking and die soldering, surface finish and melt handling (including oxidation resistance and tendency for clogging problems in the melt transfer system).

In addition to different measures of castability, differences

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Figure 4. Castability die used by CAST. Top picture shows the plan view with the gate at the bottom. The bottom picture shows the side view of the central box section.

Figure 5. Castability ratings for cracking and filling. Rating were made on parts cast using the die shown in Figure 4. Higher values = better castability

traDitionally the loWeSt coSt rare earth Metal haS been that obtaineD by Direct converSion of the ore to Metal Without SeParation of inDiviDual eleMentS.

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Figure 6. Creep strength (stress for 0.1% strain 100h 175°C) vs castability (filling+cracking average).

also exist for a single measure and alloy when used in different casting sizes and geometries.

Despite these differences, attempts have been made to rank alloys according to castability, in particular tendency for cracking and ease of filling. Figure 4 shows a die developed by CAST to assess castability [11,12]. It was designed to contain difficult to cast features such as difficult to fill thin sections, decreasing then increasing section thickness in the flow direction, thick and thin sections adjacent to each other and stressed regions to induce cracking.

Each alloy was cast under four conditions: high and low die temperature and high and low second stage velocity. Ten castings from each condition were rated for each of the alloys. Ratings were made on a scale of 0-5 (0 worst, 5 best) according to the size and number of cracks and the completeness of filling.

The castability rating results are shown in Figure 5. Of the high temperature alloys AE44, AS31 and AM-HP2+ performed well. MRI153M, AXJ530 and AJ52 performed poorly.

In Figure 6 the creep performance is plotted against castability. The alloys with calcium are highlighted. Calcium can cause a number of issues related to castability. Cracking severity is often high in these alloys and this factor is included in the castability rating. Other effects are not shown in the rating including melt handling problems due to the formation of CaO films on the melt. This can lead to clogging issues in cold chamber machines without good cover gas protection in the transfer tube.

AE44 and AM-HP2+ stand out in Figure 6 as alloys with

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CAS9220-HALFPAGE-FA3.indd 1 23/05/12 1:17 PM

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both good creep properties and good castability. AS31 rates particularly well in terms of this castability test but has poor creep resistance.

CostThe cost of an alloy is as important as properties, especially in automotive applications. The recent rare earth price fluctuations mentioned above were of concern during 2011 but are now trending downwards (Figure 3). Additionally, non-Chinese rare earth

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Figure 7. Creep performance plotted against alloy ingredient prices. The data relates to prices in March 2012 and does not include contributions from processing costs or yield losses.

Dr Suming Zhu, CAST CRC Research Fellow, Monash UniversityDr Zhu received his PhD from City University of Hong Kong in 1997. Prior to joining Monash, he worked as lecturer at Dalian University of Technology, a research associate at City University of Hong Kong, and an industrial research scientist at Japan Ultra-High Temperature Materials Research Institute. Dr Zhu’s primary research focus is on microstructure and mechanical behaviour of structural metal alloys. His current research work is in magnesium alloys, including their mechanical properties and strengthening mechanisms, creep behaviour and deformation mechanisms, and micro-structural characterisation by electron microscopy.

Dr Mark Gibson, Research Group Leader Process Science & Engineering, CSIRODr Gibson gained his PhD from the University of Wollongong in 1989, joining CSIRO shortly after. In his current role, Dr Gibson is engaged in a number of research activities relating to the manipulation of alloy microstructure through careful control of composition and processing to optimise targeted functional properties. This has largely been associated with magnesium alloy development for light-weight automotive applications; however it is now extending more broadly into titanium alloys and non-equilibrium processing.

Dr Mark Easton, Program Manager Automotive Applications, CAST CRCDr Easton graduated with a PhD from The University of Queensland in 1999. Since then, he has worked in the light metals industry and in research organisations. He has been a research program manager with CAST CRC for the past seven years, a role which has included the oversight of magnesium alloy development activities. His research interests focus on metals processing, particularly casting, and how the developed microstructure affects alloy properties. Dr Easton won the prestigious GKSS International Magnesium Award in 2009.

Dr Zisheng Zhen, Technical Director, Magontec AsiaDr Zhen gained his PhD in materials science and engineering from University of Science and Technology Beijing, China, in 2003. He then conducted further research work on magnesium alloys at Oxford and Brunel Universities in England, and at the GKSS Magnesium Innovation Center in Germany. In 2009, Dr Zhen became the Manager of Research and Development at Magontec Xi’an. He took up his current role as Technical Director, Magontec Asia in 2011.

Dr Trevor Abbott, Magontec GmbH in GermanyDr Abbott leads the alloy development activities with Magontec GmbH in Germany. He joined Magontec’s predecessor organisation, Australian Magnesium Limited (AML) in 2005 as the Manager of Research and Development, and became AML’s Chief Technology Officer in 2009. Prior to this, Dr Abbott was the Sector Leader for magnesium applications in CAST CRC, and earlier, a senior researcher at BHP Steel. Dr Abbott has extensive experience in industrial research, and its application in commercial environments. Prior to his transfer to Germany, he led a CAST CRC/AML project to develop specialised magnesium coatings, and played a key role in commissioning the production facility of an alloy factory in Henan Province, China.

AUTHORS

producers are also coming on-line in US, Australia, Malaysia and elsewhere which should help stabilise the price.

In Figure 7 an approximate indication of alloy cost is shown. The prices shown are the cost of alloy ingredients relative to AZ91D ingredients (March 2012 prices). This cost does not include production cost or yield losses and so should be used as a rough guide only. AM-HP2+, AE44-4 (Ce-La-Nd-Pr) and MRI230D are all considerably more expensive than AZ91D. AE44-2 (Ce-La) is much lower cost than AE44-4 and represents the best all round combination of creep properties, castability and cost.

AXJ530, MRI153M, AJ62, MRI153A and AS31 all have low cost ingredients but are limited by poor castability and/or poor creep properties.

ConclusionsFor a high temperature magnesium alloy to be adopted in automotive applications it needs to satisfy several criteria. Arguably the most important are creep resistance, castability and cost. The magnesium-aluminium-rare earth alloy AE44-2 (rare earths = Ce, La) uniquely meets all three criteria with good castability, creep resistance and cost.

In situations where creep resistance is of greatest importance the alloy AM-HP2+ stands against other high pressure diecast alloys. It exhibits both good castability and unmatched creep resistance but at a significantly higher price than AE44-2. n

Metal casting technologies June 2012 25

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2. M.s. Dargusch, G.L. Dunlop, and K. Pettersen: in Magnesium Alloys and Applications, B.L. Mordike and K.U. Kainer, eds., Werkstoff-Informationsgesellschaft, frankfurt, Germany, 1998, pp. 277-82.

3. T.K. Aune and T.J. Ruden: sAe Trans., 1992, vol. 101, pp. 1-7.

4. M.O. Pekguleryuz and J. Renaud: in Magnesium Technology 2000, H.I. Kaplan, J. Hryn, and B. clow, eds., TMs, Warrendale, PA, 2000, pp. 279-84.

5. e. Baril, P. Labelle, and M.O. Pekguleryuz: JOM, 2003, vol. 55(11), pp. 34-39.

6. B.R. Powell, A.A. Luo, V. Rezhets, J.J. Bommarito, and B.L. Tiwari: sAe Trans., 2001, vol. 110, pp. 406-13.

7. P. Bakke and H. Westengen: in Magnesium Technology 2005, n.R. neelameggham, H.I. Kaplan, and B.R. Powell, eds., TMs, Warrendale, PA, 2005, pp. 291-95.

8. P. Lyon, J.f. King, and K. nuttall: in Procedinging of the 3rd International Magnesium conference, Institute of Materials, London, UK, 1996, pp. 99-108.

9. M.A. Gibson, c.J. Bettles, M.T. Murray, and G.L. Dunlop: in Magnesium Technology 2006, A.A. Luo, n.R. neelameggham, and R.s. Beals, eds., TMs, Warrendale, PA, (2006), p. 327-31.

10. M.A. Gibson, s.M. Zhu, and J.f. nie: in Proceedings of the Light Metals Technology conference 2007, saint-sauveur, Québec, canada, 2007, pp. 35-40.

11. M. Gibson, M.A. easton, V. Tyagi, M. Murray, and G. Dunlop, in Magnesium Technology 2008, M. Pekguleryuz, e. nyberg, R. s. Beals, and n. neelameggham, eds., TMs, Warrendale, PA, 2008, pp. 227-232.

12. strobel K, easton MA, Tyagi V, Murray MT, Gibson MA, savage G, Abbott T. Int. J. cast Met. Res. 2010;23:81.

Reprinted with permission from the Proceedings of the 69th Annual International World Magnesium conference, published by the International Magnesium Association.

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