Effects of Calcium Addition on Properties of Mg Alloys: A Review

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Effects of Calcium Addition on Properties of Mg Alloys: A Review

88ISBN: 978-1-4673-4948-2 2013 IEEE

Abstractthe Magnesium alloys attract the automobile,aerospace and electronic equipments manufacturing industries

with its superior specific strength, processability andelectromagnetic shielding. Aluminium alloys and polymer matrixcomposites are the strong contenders for magnesium alloys.Alloying is the way to improve the properties of the magnesiumto defend its position as a structural material. Calcium one of thelow cost alloying elements which improves the mechanicalproperties, provides thermal stability and enhances the formationof a thin and uniform oxide layer and thus improves theoxidation resistance and also increases the ignition temperature

with a decrease in elongation and loss of fluidity. This articlesummarizes the effects of Calcium addition on the properties ofmagnesium alloys.

Index TermsMg-Ca alloys, creep resistance, strengtheningmechanism, corrosion.

I. INTRODUCTIONA wide spectrum of materials is presently available for the

innovative product designer. Although the versatility of the

various engineering plastics especially fiber reinforced plastics

has made a strong impact on material utilization, the metals

have defended their strong position as materials for high

performance, loaded structures [1]. Magnesium, a density of

2/3 that of aluminium, and little higher than fiber reinforced

plastics, combined with excellent mechanical and physical

properties as well as processability and recyclability, make

magnesium alloys an obvious choice when designing for light

weight. Magnesium and its alloys are becoming widelyrecognized in automotive, aircraft, and also in electronic

consumer products with its good electromagnetic shielding

characteristics [1],[2].

Commercial cast magnesium alloys for automotive

applications are AZ and AM series alloys (AZ91D, AM50A,

and AM60B). These alloys offer an excellent combination of

mechanical properties, corrosion resistance, machinability and

die-castability. But, they have poor creep resistance above

125 , which makes them inadequate for major powertrain

applications. A new improved high performance magnesium

alloys, could well be used in the automatic transmission cases,

which can operate up to 175 , engine blocks where up to

200 , and engine pistons where even higher than 300 .

Creep resistance is a major requirement for use of magnesium

in automotive powertrain components that are currently made

up of aluminium or cast iron [2]. Present consumption of

aluminium and magnesium in passenger cars is 120-140 kg

and approximately 5 kg per vehicle respectively [3].

Magnesium in pure form is soft and can only be hardened

by deformation. For structural applications a variety of

magnesium alloys have been developed. The alloying

elements are added both to secure adequate processability of

the metal, and to obtain the performance required by theapplications [1]. Magnesium is classified as an alkaline earth

metal. It is found in Group three of the periodic table. It

possesses a similar electronic structure to Beryllium (Be),

Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra)

[4]. The alloy elements used for enhancing thermal stability of

magnesium alloy mainly consists of rare-earth elements, the

alkali soil elements (Ca, Sr, Ba) and the IV,V Race elements

such as silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi).

These elements can form high melting point compounds with

Mg, Al and other alloying element (for example, Zn, Mn, Zr)

to realize good thermal stability by dispersion strengthening of

alloys [5].

Alloying with Ca in the range from 0.01 to 3% is becomingmore common in the development of cheap creep resistant

magnesium alloys essentially to replace -Mg17Al12phase in

Mg-Al alloys [4]. Fig.1. shows the direction of new

magnesium alloys development based on the required

properties for different performance range. This grouping

clearly shows that the Ca containing magnesium alloys

Effects of Calcium Addition on Properties of

Mg Alloys: A Review1. S.Gunabalan, 2. Dr. R. Elansezhian

1. Ph.D. (Research Scholar), 2. Associate Professor, Dept. of Mechanical Engineering, Pondicherry

Engineering College, Puducherry.


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89ISBN: 978-1-4673-4948-2 2013 IEEE

participate in many groups such as creep resistance, die-

castability and wrought Magnesium alloys [6]. This is the

indication of importance of calcium in development of new

magnesium alloys.

Fig.1. Direction of Mg alloy development to improve the performance [6]

II. MICRO STRUCTURE AND MECHANICAL PROPERTIESThe mechanical properties of the pure magnesium can be

improved by the addition of alloying elements. Ca addition

from 0.5% to 1.5% (mass fraction) in Mg-5Zn-5Sn system

increases the yield strength and creep properties of the Ca-

containing Mg-5Zn-5Sn alloys. As-cast Mg-5Zn-5Sn alloys

are mainly composed of -Mg, CaMgSn, MgZn and Mg2Snphases. The CaMgSn intermetallics is more easily formed due

to the higher electronegative difference value between Ca and

Sn elements, while the Mg2Ca intermetallics is less easily

formed due to the lower electronegative difference value

between Mg and Ca elements. The CaMgSn compound which

has a high thermal stability can possibly restrict the growth of

-Mg primary phase and secondary solidification phasesduring solidification. Consequently, the Ca-containing Mg-

5Zn-5Sn alloys obtain finer microstructures than the Mg-5Zn-

5Sn alloy. This improves the tensile properties both at room

temperature and at 150 , which is beneficial to the creep

properties. Among the Ca-containing Mg-5Zn-5Sn alloys, the

alloy added 1.5% Ca exhibits the optimum yield strength and

creep properties, however, the maximum ultimate tensile

strength of 174.7MPa and elongation of 4.79% at room

temperature and ultimate tensile strength of 147.9MPa and

elongation of 14.21 % at 150 are achieved in the alloy added

with 0.5% Ca [7].

Mg-3Sn-1Ca alloy shows higher ultimate tensile strengthand elongation at room temperature (138MPa) and at 150

(120MPa) than Mg-3Sn-2Ca alloy with 127MPa at room

temperature and 116MPa at 150 . However, the yield

strength of the Mg-3Sn-1Ca alloy is 101MPa at room

temperature and 82MPa at 150 , which is lower than that of

Mg-3Sn-2Ca alloy having yield strength of 112MPa and

104MPa respectively [9].

The ultimate tensile strength and elongation of ZA104 +

0.3Ca alloy are superior to those of ZA104 + 0.6Ca alloy.

Tensile properties of ZA104 + 0.3Ca die-casting alloy are

comparable to those of AZ91D alloy. Ultimate tensile strength

and fracture strain of the two ZA experimental alloys decreasewhile calcium content increases [8].

III. CREEP PROPERTIESThe creep properties of magnesium alloys are mainly

related to the structure stability at high temperatures. The

creep properties of the Ca-containing Mg-5Zn-5Sn alloys are

possibly related to the formation of eutectic and/or primary

CaMgSn phase with high thermal stability. With the Ca

amount increasing from 0.5% to 1.5%, the amount and size of

the CaMgSn phase increase gradually. Ca addition to the Mg-

5Zn-5Sn alloy does not significantly change the fracture mode

of the alloy. The tensile rupture of the experimental alloysoccurs along inter-granular boundary, but the cracks seem to

extend easily along the interfaces between the coarse primary

CaMgSn particle and -Mg matrix which possibly results inthe relatively poor mechanical properties of the Mg-5Zn-5Sn-

1.0Ca and Mg-5Zn-5Sn-1.5Ca alloys [7]. Mg-3Sn-2Ca alloy

has more potential as elevated temperature magnesium alloy

due to its higher creep properties as compared to Mg-3Sn-1Ca

alloy [9]. The microstructure of the Mg-Ca alloys is

characterized by the discontinuously distributed primary

magnesium phase and Mg2Ca phases. Increase in calcium

content decreases the volume fraction of the primary

magnesium phase. Mg2Ca is the reason for the creep

resistance [10].In Mg-Al based alloys Mg17Al12 phase is the reason for the

poor creep property. Ca suppresses Mg17Al12 phase in AC515

alloy due to the suppression of its generation caused by a

strong combination with Ca and Al atoms [11]. The Ca added

to AZ91D + 3% Ca alloys forms insoluble Al2Ca which

decreases the amount of Mg17Al12 phase in the matrix. In

AZ91D + 5% Ca, Mg17Al12 is not detected, but Al2Ca

detected. In AZ91D + 1% CaO alloy Mg17Al12 is detected as

the minor phase. In the case of the AZ91D + 10% CaO alloy

only -Mg and Al2Ca are detected as the matrix phases [12].

IV. STRENGTHENING MECHANISMGrain refinement is the most desirable strengthening

mechanism. The strengthening mechanism of Ca-containing

Mg-5Sn-5Zn alloy is mainly attributed to the microstructure

refinement and/or the formation of the CaMgSn phase [7].

With the combined addition of 0.5% Ca and 1.0% Y to AZ91

alloy, the grain size in microstructure decreases. Further

increasing Ca content can cause the change of microstructure


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from dentritic to equiaxed. When 1.5% Ca and 1.0% Y are

added to AZ91 alloy, the grains are remarkably refined, and

the average grain size is 2030 m. the grain refinement maybe the result of the formation of new particles that can act as

potent nucleation sites for magnesium [13].Micro-hardness of Mg-Zn-Ca system increases with the

increase in Ca content and age hardening occurs after aging at

200 in the flakes probably due to the precipitation

strengthening of the fine precipitates Mg2Ca and

Ca2Mg6Zn3[14].

The combined addition of Y and Ca can refine the as-die-

cast microstructure, result in the formation of Al2Ca phase and

Al2Y phase, and inhibit the precipitation of Mg17Al12 phase.

The combined addition of Y and small amount of Ca has little

influence on the ambient temperature tensile properties, but

increasing the content of Ca can improve significantly the

tensile strength at both ambient and elevated temperatures. In

AZ91-1Y alloy, formation of coarse block and rod-like Al2Y

phase are seen. The increase of ambient ultimate tensile

strength of AZ91-1Y-1.5Ca alloy is due to microstructure

improvement, refinement of Al2Y phases and dispersion

strengthening with Al2Ca phases and Al2Y phases.

Unfortunately, the combined addition of Y and Ca can

decrease the elongation at 150 [13].

V. CORROSIONIn spite of the great advantages of magnesium alloys, their

application as engineering material is still restricted by their

high susceptibility to corrosion [15]. Magnesium is highly

resistant to alkalies, chromic and hydrofluoric acids. But it isless resistant to other acidic or salt-laden environments.

Magnesium is anodic to any other structural metal and will be

preferentially attacked in the presence of an electrolyte [16].

Adding Ca to AZ91 magnesium can refine the

microstructure, improve the elevated temperature properties

and the corrosion resistance of magnesium alloys [17]. As-

Cast AZ91 magnesium alloy containing 0.14 wt.% Ca consists

of -Mg17Al12, Mg17Al8Ca0.5 and Mg2Ca. When the alloys are

exposed to 3.5% NaCl solution no corrosion attack is observed

in Mg17Al8Ca0.5 and Mg2Ca phases [18]. On other hand

addition of calcium to the Mg6% Zn1% Mn5% Si alloy

decreases the corrosion resistance [19].

VI. DAMPING PROPERTIESThe structural materials required to withstand the intended

load and also required to absorb shock load occasionally. Pure

magnesium has the better damping properties among various

metallic materials. As-cast Mg1 wt%Ca binary alloy exhibitsgood damping capacities. The damping curve of Mg1 wt%Ca

alloy is close to high damping pure magnesium, while it is

much higher than AZ91D alloy, which indicates that Mg1wt%Ca alloy exhibits good damping capacity. Mg1 wt%Caalloy could be considered as a potential candidate for high

damping magnesium alloy. But further increase in Ca contentin MgCa binary alloy exhibits the lower damping capacities[20].

VII. CAST ABILITYMagnesium alloys, especially those with aluminium as a

major alloying element show a very good castability [6]. In

magnesium alloy casting, the viscosity of the molten metal is

increases clearly with increasing the amount of Ca addition

and firmer surface oxide film is formed by Ca addition. The

flow length is shortened by Ca addition on the effect of the

above factors [11]. Ca can be introduced in magnesium alloy

casting as Mg-Ca master alloy. Mg-Ca master alloys arecommercially available in 15, 20 and 30% Ca. Recommended

addition temperatures for these master alloys depend on the Ca

content and range from 680 720C. Dissolution or mixingtime of 1530 minutes after the addition is recommended.Recoveries are quite high around 8090% under fluxlessalloying conditions. As mentioned above, standard magnesium

fluxes react with calcium so flux free melt protection is the

preferred method for Ca addition [4].

Ca addition to the Mg-5Zn-5Sn alloy can result in the micro

structural refinement of the alloy. It is well known that the Ca

atom has a larger atomic radius than the Zn and Sn atoms (Ca:

0.197 nm; Zn: 0.131 nm; Sn: 0.141 nm), with the Ca addition

to the Mg-5Zn-5Sn alloy, the Ca element is mainly rich in thesolid-liquid interface during solidification [7].

More than about 1% Ca addition to AM50 alloy

significantly improves creep resistance. It is accompanied by a

tendency to hot cracking. By the addition of approximately

0.2%Sr, such casting cracks are significantly suppressed, and

besides the enhancement in creep resistance and mechanical

properties. The improvement of creep resistance by Sr

addition seems to be attributed to the suppression of grain

boundary sliding due to the creation of thermally stable Al-Sr

compound along the grain boundary and the suppression of

discontinuous precipitation of Mg17Al12 phase [4],[11].

VIII. OXIDATION AND IGNITION RESISTANCEA. Oxidation

The interaction of oxygen with base metal forms the oxide

layer. PillingBedworth principle of oxidation is the conditionof oxidation film on the surface of the oxidizing metal. The

PillingBedworth ratio () is the ratio of the oxide volume to


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the volume of the original metal which is consumed to form

the oxide. When > 1, oxide can expand to bring compressivestress, under the action of which the oxidation film becomes

tightly coherent. The value of magnesium is 0.81, so theMgO film is loose and there are a lot of small holes onoxidation film which allows oxygen infiltrating into the

molten alloy [21],[22],[23].

The surface changes, of pure magnesium during casting and

solidification under ambient atmosphere without any

protective gas, are seen such as severe oxidation begins and

tarnished surface appears in 10 seconds just after being

poured. Severe burning starts in 10 seconds and continues

even with white flame to the point when all melts [5].

Ca strongly improves oxidation performance. As little as

1% of Ca is enough to reduce the oxidation rate to extremely

low values, even for longer oxidation times. Rare earths

(added in the form of mischmetal) are not as effective as Ca,

and slightly higher contents are necessary to achieve similar

performance [15]. Elements with high affinity to oxygen such

as Ca, Sr or Zr enhance the formation of a thin and uniform

oxide layer and prevent growth of thick nodular oxide features

[24].

B. IgnitionThe lowest temperature associated with the observation of a

flame is called Ignition temperature. Ignition also defined as

the temperature at which an exothermic oxidation reaction

becomes self-sustaining at a rate which causes a significant

temperature increase. The ignition temperature measurement

is critical when the heat accumulates in the specimen [25],[26]. In the case of magnesium machining, most of the energy

put into cutting processes is converted into heat, this leads to a

risk of fire due to chip ignition [4].

Surface changes, of Mg alloy with 0.3wt% CaO during

casting and solidification under ambient atmosphere without

any protective gas, see no burning appears and shiny surfaces

are maintained even after the solidification is completely

stopped. The similar phenomena occurs in AZ31-

0.27wt%CaO, AM50-0.27wt%CaO, AM60-0.3wt%CaO, and

AZ91-0.27wt%CaO magnesium alloys [5].

The ignition point of AZ91 alloy increases linearly with the

increase of Ca content. The data of ignition points of Ca-

bearing AZ91 alloys can be fit as a linear equation:

(1)

Where is the ignition point in and x is the analyzedcontent of Ca (mass fraction, %) in AZ91 alloy. Ca-bearing

AZ91 alloys are in agreement with linear equation when the

content of Ca is lower than 1.3% (analyzed content). When the

content is higher than 1.3%, the ignition temperature is

unstable. The minimum ignition temperature is higher than

770 and some of the alloys do not burn up to 900 . The

testing results of ignition points of Ca-bearing AZ91 alloysimply that the proper addition of Ca is effective to improve the

ignition point of AZ91 alloy up to 770 , but at a higher

temperature the ignition of AZ91 cannot entirely be inhibited

by addition of Ca [27].

The ignition point of Mg-5Ca alloy is about 1,030 .The

average ignition point of Mg-5Ca powder in the size range of

150-250 m, is 685 , this average ignition temperatureincreases with the increasing particle size [11].

IX. DEVELOPMENT OF ECO MAGNESIUM ALLOYAs environmental benefits provided by lightweight, being

unlimited, and recyclable, magnesium alloys have thepotential to grow significantly in the future by Eco-Mg

(Environment Conscious Magnesium) approach. The simple and

plain approach of Eco-Mg alloy is to introduce CaO particlesin the range of 0.3wt% to 0.7wt% as an ingredient into

conventional cast and wrought magnesium alloys for (1) non-

SF6 process, (2) Be elimination, (3) improved melt

cleanliness, (4) ensured original process adjustability for

casting, forming, joining as well as surface treatment, (5)

improved mechanical property by grain refinement andinternal soundness, (6) ensured safety during manufacturing

and application by raising oxidation and ignition resistances of

machined chips and products, and (7) improved recyclability.

CaO of over 0.5~0.7wt% can be introduced for special

purposes to develop creep-resistant, fire-retardant or fire-proofmagnesium alloys [5]. It is confirmed that CaO is reduced to

Ca through phase analysis. Mg2Ca phase is formed even in

0.07% CaO added pure magnesium by reduction, while

Mg2Ca phase is formed over 1.35% in Ca added magnesium.

With respect to CaO content, the hardness of CaO added pure

magnesium increased by grain refinement [28].

X. RECYCLABILITYCa content in magnesium alloys disappears during recyclingdue to the reaction of Ca with the fluxes so flex free melt

protection is recommended for Ca addition[4]. In a fluxed

magnesium melting process, a suitable flux also can minimizethe reduction of Ca content during recycling for Ca added

magnesium alloys. KCl and NaCl are the suitable fluxes for

magnesium alloys with Mg2Ca or Al2Ca phases without Ca

content loss [5].

XI. SUMMARY AND CONCLUSIONSThe research in the area of Ca addition on Properties of


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magnesium alloy is summarized as follows:

1) In Mg-Al based alloys Mg17Al12 phase is the reason forthe poor creep property. Ca suppresses Mg17Al12 phase in

AC515 and AZ91D + 3% Ca alloys forms insoluble

Al2Ca which decreases the amount of Al12Mg17 phase inthe matrix.

2) Ca addition from 0.5% to 1.5% (mass fraction) increasesthe yield strength and creep properties of the Ca-

containing Mg-5Zn-5Sn alloys. The maximum ultimate

tensile strength of 174.7MPa and elongation of 4.79% at

room temperature and ultimate tensile strength of

147.9MPa and elongation of 14.21% at 150 are

achieved in the alloy added with 0.5% Ca.

3) Increasing Ca content in magnesium alloys enhances thegrain refinement and also change of microstructure from

dentritic to equiaxed. When 1.5% Ca and 1.0% Y are

added to AZ91 alloy, the grains are remarkably refined,

and the average grain size is 2030 m.4) Ca addition of more than about 1% to AM50 alloy

significantly improves creep resistance but increases the

cast cracking tendency. By the addition of approximately

0.2%Sr, casting cracks are significantly suppressed, and

besides increasing the creep resistance and mechanical

properties.

5) The fluidity of the molten metal decreases with increasingthe amount of Ca addition. 0.07% CaO added pure

magnesium is reduced to Ca forming Mg2Ca phase, while

Mg2Ca phase is formed over 1.35% in Ca added

magnesium. So Ca can be effectively added to

magnesium alloys in the form of CaO.

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Effects of Calcium Addition on Properties of Mg Alloys: A Review