Effect of Antigravity-Suction-Casting Parameters on Microstructureand Mechanical Properties of Mg­10Al­0.2Mn­1Ca Cast Alloy

Tomomi Ito1,+, Masafumi Noda2, Hisashi Mori3, Yoshio Gonda1,Yuta Fukuda1 and Satoshi Yanagihara1

1Magnesium Division, Gonda Metal Industry Co., Ltd., Sagamihara 252-0212, Japan2Department of Mechanical Science and Engineering, Chiba Institute of Technology, Narashino 275-0016, Japan3Materials Technology Division, Railway Technical Research Institute, Tokyo 185-8540, Japan

We investigated the effect of the cooling rate and molten metal temperature on the microstructure and mechanical properties of a sub-rapidly solidified magnesium alloy, Mg­10Al­0.2Mn­1Ca, prepared by antigravity suction casting using a water-cooled steel mold. Themicrostructure of the antigravity-suction-cast material without water cooling consisted of coarse grains (grain size: 780µm), with networks of anAl­Ca compound at the grain boundaries. The higher cooling rate of the water-cooled steel mold promoted the formation of the Al­Cacompound and voids in accordance with increases in the internal and external temperature gap and differences in the solidification rate of themold. The formation of voids and the shrinkage were suppressed, however, by adjusting the cooling rate and decreasing the molten metaltemperature. The particle size of the Mg phase was refined to 135µm and the grain-boundary compounds were finely dispersed in the Mg phase.The as-cast alloy showed an ultimate tensile strength of 166MPa and an elongation of 8%. The microstructure and mechanical properties of theas-cast alloy were dependent on the cooling rate and molten metal temperature, but they were not dependent on the casting speed.[doi:10.2320/matertrans.MC201401]

(Received February 17, 2014; Accepted April 30, 2014; Published June 13, 2014)

Keywords: magnesium alloy, microstructure, mechanical properties, cooling rate, antigravity suction cast

1. Introduction

Magnesium alloys are the lightest practical metals withgood relative strength, attenuation properties, and machina-bility. As Mg resources are also abundant,1) Mg alloys areincreasingly being used as casings and vehicle parts in orderto improve energy efficiency and to reduce weight.2) Onthe other hand, many vehicle components use Al alloys.2)

In order to replace Al alloys with Mg alloys, the rigidity,mechanical properties and noncombustibility of the Mgalloys need to be improved before they can serve as structuralparts, and the manufacturing costs must be reduced. Akiyamaet al. showed that adding more than 0.5mass% of Ca metalin Mg alloys can increase the ignition point to above 800°C,thereby improving noncombustibility.3) Kawamura et al.developed a noncombustible Mg alloy that does not combustabove 1100°C.4) Currently, attempts are being made towardincreasing noncombustibility and nonflammability of Mgalloys in the structural materials field. For example, somesurface-treated Mg­Al­Zn­Ca series alloys are characterizedas nonflammable materials. However, the mechanical proper-ties and processing conditions of Mg alloys are ofteninfluenced by additive elements, and combustion duringcasting leads to the formation of impurities and oxides.Hence, the melting and casting conditions are as importantfor fabricating noncombustible Mg alloys as they are forfabricating other Mg alloys.

A typical Ca-added Mg alloy is the flame-resistant Mgalloy, in which approximately 0.5­2mass% of Ca metal isadded to a Mg­Al­Zn series alloy.5­7) There have beenreports on the effects of the addition of Ca or Al and thecooling rate on the mechanical properties of the resultingalloy,8) where the intermetallic Al2Ca compound was present

as networks and as coarse agglomerations.9) In the productionof the cast material, an airtight environment is important foringot combustion to counter contamination in the melt duringthe melting process. In addition, a solution treatment aftercasting is necessary owing to the development of an Al­Caintermetallic material during gravity casting, in which thecasting speed is relatively low. Improving the workability isthe largest problem with the manufacture of Mg alloys,followed by the need to lower the production cost.

Twin-roll casting is a method of fabricating sheet materialwith low equipment cost;10) however, segregation occurs atthe center of the material under some casting conditionsand the sheet thickness is currently limited to about4mm.11,12)

An alternative casting method is antigravity suction castingmethod for producing cast parts, in which the melt is directlycast into a steel mold while cooling takes place as a result ofthe pressure difference.13,14) In this method, the equipmentused is simple, the casting speed is high, and semisolidcasting into the mold without reducing the temperature ispossible because there is no contact with the atmosphere asthe melt is injected into the mold. In addition, metal oxidesthat form and aggregate on the melt surface can be bypassedby extracting clean melt from below the surface. This castingmethod is expected to solve the problems of conventionalcasting methods because the production cost is reduced whilehigh-quality melt is produced. This method of antigravitysuction casting has not been applied to the fabrication ofneither Mg alloys nor thick plates, however. Therefore, in thisresearch, we prepared a cast, noncombustible Mg alloy usingthe method, and we investigated how the melting conditionsaffected the purity of the melt and the formation of defectsduring casting. We also investigated the effect of the coolingrate on the granulation of the microstructure and theimprovement in mechanical properties.+Corresponding author, E-mail: ito495@gondametal.co.jp

Materials Transactions, Vol. 55, No. 8 (2014) pp. 1184 to 1189Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, VI©2014 The Japan Institute of Metals and Materials

2. Experimental Procedure

The noncombustible Mg alloy fabricated in this researchwas a Mg­10Al­0.2Mn­1Ca (mass%) alloy (AMX1001).A Mg­Al­Mn mother alloy (AM60BMg), a Mg­30%Camother alloy, and a pure Al ingot (99.7%) were weighed tothe final stoichiometry, melted in a steel crucible, andantigravity suction casting was performed. The material washeated and melted in the crucible under either an airatmosphere or an inert (Ar) atmosphere. Bubbling of 0.2-MPa Ar gas was carried out for 20min when the melttemperature reached 680°C, while a lid with a glass windowwas placed on the crucible. In the event of combustion duringthe heating and melting process, the fire could beextinguished with a gas mixture of nitrogen gas and sulfurhexafluoride (SF6). The melt temperature was lowered to610°C during bubbling and melt settling, and the slug wasremoved immediately. Antigravity suction casting wasconducted by sinking the down sprue 300mm into the meltin a SS400 steel mold that had a width of 95mm, a thicknessof 15mm, and a length of 2m, ensuring that the melt didnot come into contact with the atmosphere during casting.Figures 1(a) and 1(b) show a schematic of the antigravity-suction-casting system. The pressure in the mold was set to0.044MPa lower pressure compared with atmosphericpressure by using vacuum pomp connected to the mold.The mold was submerged in the molten metal, and the castingwas carried out.

As this research focuses on ingot combustion duringheating, the purity of the melt, and the cooling rate duringcasting, we experimented with four sets of casting conditionswith different parameters, which are shown in Table 1. Moldand material were cooled over a distance of 1200mm,beginning at 600mm above the initial melt level. The moldand material in the casting process were cooled for 300 s fromjust before the start of casting, with the cooling-watertemperature set at 30°C or 90°C. The mechanical propertieswere tested, and the microstructure was observed in the

center of the cooling region. The mechanical properties of thecast material were investigated by tensile tests and hardnesstests. The specimens for the tensile tests were cut to a gaugediameter of 4mm and a gauge length of 25mm. Tensile testswere performed at an initial strain rate of 1.3 © 10¹3 s¹1 atroom temperature, and the tensile direction was set parallel tothe casting direction. The hardness was measured using aVickers hardness tester, applying a load of 300 g for 30 sin a cross section perpendicular to the casting direction.X-ray fluorescence analysis (XRF) was used to analyze thestoichiometry, and the microstructure of a cross sectionperpendicular to the casting direction was observed usingoptical microscopy (OM) and scanning electron microscopy(SEM).

3. Results and Discussion

3.1 Effect of the melting atmosphere on the castmaterial

Melting under an air atmosphere resulted in partial ignitionon the surface of the Mg­30Ca (mass%) mother alloy duringheating, as shown in Fig. 2(a). However, combustion did notoccur during heating and melting under an Ar atmosphere.When melting occurred under an air atmosphere, a protectivegas was supplied to the melt surface from the timecombustion or flames were confirmed in the crucible untiljust before the molten melt was injected into the mold.Figures 2(b) and 2(c) show the melt surface after melting inair and Ar atmospheres, respectively, followed by settling.Figure 2(b) shows that chunks of slag formed at the meltsurface; the amount of recovered slag was 1.6 kg against amelt of 45 kg. In contrast, a film was formed on the meltsurface when melting occurred under an Ar atmosphere, and400 g of slag was recovered. Here, the total amount of slag isthe average value of slag recovered from 10 antigravitysuction castings. The inclusion density in the cast materialfrom the melt obtained in air and under an Ar atmospherewas measured 20 times using a cross section measuring95mm © 15mm. The inclusion density was 0.04 inclusions/mm2 for melting in an air atmosphere, whereas the densitydecreased to 0.01 inclusions/mm2 for melting in an Aratmosphere. This implies that oxides (the oxide dimensionwas observed to be under 20 µm) produced by combustionthat could not be removed as slag became trapped in the castmaterial. Controlling the melt atmosphere prevented ignitionof the base metal, thereby reducing the amount of slag andthe inclusion density in the cast material.

Figures 3(a)­3(d) show OM and SEM micrographs of themicrostructure of cross sections of cast materials that were

crucible

thermocouple

initial melt levelh = 600 mm

h = 600-1800

heater

molten metal

(a)

15 m

m

center

95 mm

43.5 mm

side5 mm

(b)

vacuumpump

chiller

mm

sprue

Fig. 1 Schematic of the antigravity-suction-casting system: (a) overview;(b) position of the thermocouples.

Table 1 Casting conditions of antigravity-suction-casting process.

CaseMolten metaltemperature,

T/°C

Castingspeed,

V/mms¹1

Cooling-watertemperature,

T/°C

1 610 250 ®

2 610 750 ®

3 610 750 30

4 610 750 90

Effect of Antigravity-Suction-Casting Parameters on Microstructure and Mechanical Properties of Mg­10Al­0.2Mn­1Ca Cast Alloy 1185

melted in air and Ar atmospheres. There were voids in thecross sections of the cast materials, and the voids observedin the inner areas of the cast materials melted in air andAr atmospheres appeared as dots and lines at the grainboundaries. The area ratios of voids measured using imageanalysis (SigmaScan Pro5.0) are shown in Fig. 4. Most of thevoids were under 100 µm, and the void area ratios for Ar-atmosphere melting was reduced to 25% in comparison withair atmosphere. It has been reported that increasing thecooling rate causes the formation of voids at the boundarybetween Al­Ca compounds and the Mg phase, whichdevelop into cracks because of differences in the cooling orprecipitation rate between the two phases.15,16) The SEMmicrograph in Fig. 3(b) does not show precipitation of Al­Cacompounds near the voids, but it indicates that voids mainlyformed at grain boundaries of the Mg phase and were morefrequently found in cast material melted in air atmosphere. Asa result, void formation was found to be affected by the purityof the melt and the cooling rate.16) In general, the shape ofshrinkage cavities are known to be affected by the amount ofdissolved gas and the solidification process. Partial ignitionat the metal surface was observed during melting in air

atmosphere, and thus the formation of hydrogen gas bycontact with water is thought to have increased. Blowing ofgas and stirring are considered effective in removing gas.17)

In this study, we prevented combustion during heating andused Ar bubbling; hence, the molten metal is expected tocleaned shown in Table 2. Despite this fact, voids werefound, at grain boundaries for example, as shown in Fig. 3.It is possible that bubbling did not completely remove theimpurities, although we did not carry out gas analysis. Thedifference between the corrosion resistances of meltedmaterial from pure ingots and recycled material are known.18)

Prevention of ignition and partial combustion should improvethe melt quality, regardless of whether pure ingots or recycledmaterial is used.

3.2 Effect of cooling during casting on antigravity-suction-cast material

Antigravity suction casting carried out with melting in anAr atmosphere resulted in high melt purity. Four castingconditions (cases 1­4), which primarily differed in thecooling rate during casting, are shown in Table 1. Coolingimmediately after melt injection into the mold was achievedby circulating water around the mold, as shown in Fig. 1(a).When there was no cooling during casting, as in case 1 andcase 2, many surface defects of the cast material wereobserved upon removal from the mold, and some parts werewhite with no metallic luster (Fig. 5(a)). On the other hand,cooling during casting produced a melt that had a goodexternal appearance with a metallic luster (Fig. 5(b)). As nolubricant was coated on the mold, the surface defects of thecast material shown in Fig. 5(a) arose from shrinkage duringsolidification. The change in color was the result of contact ofmaterial in the mold with the atmosphere. Figures 6(a) and

(a) (b)

(c) (d)

Fig. 3 Optical and SEM micrographs of the microstructure of antigravity-suction-cast materials: (a), (b) in air atmosphere; (c), (d) in Ar atmosphere.The casting conditions of Case 4 were used.

0

50

100

150

200

250

300

350

400

450

500

Air atmosphere Ar atmosphere

Num

ber

of v

oid

in th

e cr

oss

sect

ion > 1000 μm2

600 - 1000 μm2

200 - 600 μm2

100 - 200 μm2

80 - 100 μm2

60 - 80 μm2

40 - 60 μm2

20 - 40 μm2

0 - μm20 2

Fig. 4 Number of voids in the cross section of the cast materials preparedusing the casting conditions of Case 4.

Table 2 Chemical composition (mass%) of antigravity-suction-cast materi-als.

Meltingatmosphere

Al Ca Mn Si Fe Mg

Air 10.3 0.97 0.16 0.03 0.001 bal.

Ar 10.3 1.06 0.12 0.02 0.001 bal.

(a)

(b) (c)

crucible

ingot

slag

Fig. 2 (a) Partial ignition from the ingot’s surface during heating in an airatmosphere. (b) Chunks of slag formed on the melt under an airatmosphere. (c) A film formed on the melt surface under an Aratmosphere.

T. Ito et al.1186

6(b) show the cross sections of the cast material fabricated bycases 3 and 4, respectively. There were continuous cracksat the center of the cast material when the cooling watertemperature was 30°C. In contrast, setting the coolingtemperature to 90°C prevented formation of continuouscracks, indicating that decreasing the melt cooling rate couldsuppress formation of cracks in the cast material. Thermo-couples were placed at the center and near the side walls ofthe mold, as shown in Fig. 1(b), to directly measure the melttemperature during antigravity suction casting with cooling.The thermocouples were placed 1200mm higher than theinitial melt surface; the casting conditions of case 4 wereused (the melt temperature was 610°C and the cooling-watertemperature was 90°C). The measured results are shown in

Fig. 7. The maximum temperature of the melt at the height ofthe thermocouples was 420°C, a value below the solidustemperature. In this casting method, solidification began aftercasting and the melt was sucked up when it was half-solidified and rapidly solidified in the cooling region.Therefore, the casting conditions of case 3 resulted incontinuous cracks, as shown in Fig. 6(a), because the moldend offered supercooling compared to the condition in case 4(Fig. 6(b)) and the melt was less likely to reach the center ofthe cast material.

Figures 8(a)­8(d) show the OM micrographs of castmaterials prepared under the casting conditions of cases1­4; the average grain sizes of the Mg phase are also shown.In cases 1 and 2, in which there was no cooling during

(a)

(b)

10 mm

Casting direction

Fig. 5 Surfaces of antigravity-suction-cast materials: (a) prepared withoutwater cooling and using the casting conditions of Case 2; (b) preparedwith water cooling and using the casting conditions of Case 4.

(a)

(b)

10 mm

Fig. 6 Cross sections of antigravity-suction-cast materials: (a) preparedwith a cooling-water temperature of 30°C and using the casting conditionsof Case 3; (b) prepared with a cooling-water temperature of 90°C andusing the casting conditions of Case 4.

200

300

400

500

0 10 20 30

Tem

pera

ture

, T

/ °C

Time, t / s

mold centermold side

Fig. 7 Measurement of the melting temperature during antigravity suctioncasting under the conditions of Case 4.

d = 780 μm

d = 775 μm

d = 130 μm

(d)

(c)

(a)

(b)

30 μm

d = 135 μm

200 μm

200 μm

30 μm

Fig. 8 OM micrographs of antigravity-suction-cast materials preparedusing the casting conditions of (a) Case 1, (b) Case 2, (c) Case 3, and(d) Case 4.

Effect of Antigravity-Suction-Casting Parameters on Microstructure and Mechanical Properties of Mg­10Al­0.2Mn­1Ca Cast Alloy 1187

casting, the average grain sizes of the Mg phase were 780 and775 µm, respectively, and thus the effect of casting speed oncrystal grain refinement was small. Without cooling, thecrystal morphology contained coarse grains and the inter-metallic compounds that formed around the grain boundariesdeveloped into networks. In contrast, the average grain sizeof the Mg phase formed with cooling during casting was135 µm, a decrease to approximately one-fourth the size ofthe cast material without cooling. In general, AMX1001alloys are known to form Al­Ca intermetallic compounds,19)

which decrease the performance of the rolling process aftercasting.19) The Al­Ca compounds in the cast material withoutcooling formed networks6,19) during casting according toFigs. 8(a) and 8(b). The cast material prepared with coolinghad more refined grain size and the network of intermetalliccompounds reduced in thickness as compared to the castmaterial prepared shown in Figs. 8(c) and 8(d).

The room temperature tensile properties and Vickershardness of the prepared cast material are given in Figs. 9(a)and 9(b). Compared to the cast material prepared withoutcooling, cooling improved the yield stress by approximately25%, the ultimate tensile strength by approximately 10%, andthe hardness by approximately 4%. The elongation in case 4was double that of case 2. Some inclusions and brittlefractures were observed at the fracture surface, therebyreducing the elongation in case 1. The reasons for thesuperior test results of case 4 are the refinement of crystalgrains and dispersion of intermetallic compounds, asmentioned earlier.

3.3 Measurement of cooling rate during solidificationThe cooling rate between the solid and liquid phases could

not be measured in antigravity suction casting. Therefore, asmall mold with a cooling range of 1000mm in whichcooling water could circulate was prepared, and the coolingrates at the side wall and center of the mold were measured.The mold had a board thickness of 35mm, a board width of

120mm, and a length of 1m shown in Fig. 10(a). Themeasurement results are given in Figs. 10(b)­10(d), whichshow that the cooling rate at the mold center was 2.8°C/swithout cooling and 12°C/s with cooling, regardless ofwhether the cooling-water temperature was 30°C or 90°C.The cooling rate at the mold’s side wall was 3.4°C/s withoutwater cooling, and 21.2 and 29.4°C/s with a cooling-watertemperature of 30 and 90°C, respectively. A difference of0.6°C/s in the cooling rate was found between the side andcenter of the cast material without water cooling, and thisdifference is thought to depend on the heat capacity of thesteel mold. When the cooling-water temperature was set to 30and 90°C, the cooling rates were 17.4 and 9.2°C/s faster,respectively, at the cast material side than the inside.Therefore, when the cooling-water temperature was 30°C,the material near the side edges of the mold becameovercooled and induced continuous cracks inside the castmaterial. In contrast, the difference in cooling rates was9.2°C/s when the cooling-water temperature was 90°C,which means that the cooling rates of the mold side andcenter were more balanced. However, it would be difficultto obtain a uniform internal microstructure in thickly castmaterial. One topic for future studies includes investigationson whether a uniform internal microstructure can be attainedin thickly cast material by controlling the cooling rate andmelt temperature.

4. Conclusions

This research focused on the antigravity suction casting ofMg­10Al­0.2Mn­1Ca, a noncombustible Mg alloy. Purifi-cation of the melt during processing and the effect of coolingrate on the mechanical properties of the microstructure wereinvestigated. Using an Ar-gas melting atmosphere preventedcombustion on the Mg­Ca mother alloy surface, and theamount of slag and oxide inclusions in the cast materialdecreased to one-fourth of the original amount. Casting while

(a)

50

60

70

80

1 2 3 4

Har

dnes

s (

HV

)

Case Numbers

(b)

0

2

4

6

8

10

0

50

100

150

200

Elo

ngat

ion,

δ(%

)

Nom

inal

str

ess,

σ/ M

Pa YS UTS

El

Fig. 9 (a) Tensile properties (average of n = 5) and (b) Vickers hardness ofantigravity-cast-materials prepared under various conditions (average ofn = 5).

450

500

550

600

650

0 10 20 30

Time, t / s

450

500

550

600

650

Tem

pera

ture

, T/ °

C

mold centermold side

450

500

550

600

650

0 10 20 30

Tem

pera

ture

, T

/ °C

chiller

mold

molten metal

thermocouple(a)(b)

(c) (d)

Fig. 10 Measurement of molten metal temperature in a small mold:(b) without water cooling; (c) with a cooling water temperature of 30°C;(d) with a cooling-water temperature of 90°C. The cooling rate wasmeasured for a molten metal temperature in the range of 500­590°C.

T. Ito et al.1188

the melt was cooled under a controlled atmosphere couldreduce the injecting temperature of the melt. The particle sizeof the Mg phase was refined to 135 µm, and the Al­Cacompounds were finely dispersed into the Mg phase. Theproof stress and ultimate tensile strength were 123 and166MPa, respectively, and the elongation was 8%.

Acknowledgment

This study was supported by the Innovative StructuralMaterials Project (Future Pioneer Project) from Ministry ofEconomy, Trade and Industry, Japan.

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Effect of Antigravity-Suction-Casting Parameters on Microstructure and Mechanical Properties of Mg­10Al­0.2Mn­1Ca Cast Alloy 1189