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Effect of Strontium, Magnesium and Iron Content on Mechanical Properties of Rheocast Al 7 mass%Si Mg Alloys

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Effect of Strontium, Magnesium and Iron Content on Mechanical Properties

of Rheocast Al-7 mass%Si-Mg Alloys

Satoru Sato

1

, Yasunori Harada

1;2

, Naoki Ishibashi

1

and Mitsuru Adachi

1;2

1

Technical Development Center, Ube Machinery Corp., Ltd., Ube 755-8633, Japan

2Aluminum Wheel Division, Ube Industries, Ltd., Ube 755-0057, Japan

Newly developed Rheocasting process makes semi-liquid slurry in metallic vessel without stirring process. Semi-liquid castings made by this process show good mechanical properties. The effects of strontium, magnesium and iron contents on the mechanical properties of Rheocast and squeeze cast Al-7 mass%Si-Mg alloys were investigated. Elongation of Al-7 mass%Si-Mg alloy Rheocastings was higher than those of squeeze castings when strontium was not added or iron content was up to 0.27 mass%. Both Rheocastings and squeeze castings increased in strength but decreased in elongation with increasing magnesium content in Al-7 mass%Si-xmass%Mg (x¼0:23{0:64) alloy.

[doi:10.2320/matertrans.F-MRA2008845]

(Received July 3, 2008; Accepted November 7, 2008; Published January 25, 2009)

Keywords: rheocasting, slurry, mechanical properties, strontium, magnesium, iron

1. Introduction

Semi-solid and semi-liquid casting processes have been developed as one of the high pressure casting processes to get sound castings without harmful shrinkage defects in fast

casting cycle. In 1970’s, M. C. Flemings et al.constructed

theoretical basis of these processes. According to their investigations, semi-liquid metal (described as ‘‘slurry’’ in this paper) having non-dendritic microstructure could be obtained by cooling melt with mechanical stirring and this slurry could be cast into complex shape because of its good rheological properties. They also advocated these casting

processes as ‘‘Rheocasting’’ and ‘‘Thixocasting’’.1,2) The

Rheocasting consists of slurry making and direct slurry casting. This process is also called semi-liquid casting. Thixocasting process is more complex because solid billets are made from slurry in prior to casting and the billets are reheated to the desired semi-solid temperature at the casting.

Almost 15 years later of the basic studies of Flemingset

al., Thixocasting process was applied to the first mass

production. However some disadvantages were revealed, for example, high material cost and poor castability compared with conventional casting processes. Therefore, there were strong demands for industrialization of the Rheocasting process to reduce material cost. But in the Rheocasting process it was difficult to supply slurries stably in synchron-ized timing with casting cycle.

In late 1990’s, some Rheocasting processes that overcame the problems of the Thixocasting process appeared. One of the first commercial Rheocasting processes was developed by

Shibataet al.3,4)by utilizing vertical shot unit with induction

coils in order to apply strong stirring force to the melt in the shot sleeve to make slurry one shot by one shot directly. Another Rheocasting process developed by authors is very simple. In this process non-dendritic, high quality slurry can be prepared at a rotating table near the casting machine

without stirring process.5) The process consists of pouring

low superheat melt into the metallic vessel and cooling the melt to desired semi-liquid temperature with uniform temperature distribution.

Characteristics of slurry and heat treatment effects on mechanical properties of AC4CH aluminum alloy have been

discussed,5,6) but effects of strontium as refiner of eutectic

silicon, magnesium as strengthen element and iron as harmful element have not been described yet. This paper aims to investigate effects of above mentioned three elements on mechanical properties of Rheocastings comparing with squeeze castings. Squeeze casting is another good process to get sound castings.

2. Experimental Procedures

Figure 1 shows a flow diagram of the newly developed Rheocasting process. Low superheat melt is gently poured into the metallic vessel. After pouring, top and bottom of the

(3)Induction heating (1)Pouring

(2)Air blow cooling Ceramics

Ceramics Air

(4)Ejecting Induction coil

Ladle

Vessel

Shot sleeve

[image:1.595.308.550.512.768.2]
(2)

vessel are covered with ceramics insulators to prevent over cooling at both portions. Then the slurry is cooled to desired temperature with slow cooling rate and induction heated at the last stage of cooling process to control temperature distribution. Then, the slurry is transferred with the metallic vessel to the shot sleeve of high pressure casting machine and ejected by turning upside-down.

Table 1 shows chemical compositions of three series of Al-7 mass%Si-Mg alloys analyzed by optical emission spectrometer. All alloys were made from mother ingots, such as 25 mass%Si, 10 mass%Mg, 5 mass%Ti, Al-10 mass%Sr and Al-Al-10 mass%Fe. The alloy was melted in the graphite crucible and degassed by argon bubbling device before the start of each alloy series. Experiments of each alloy series started from the lowest level of the remarked element. After conducted planned casting schedule, the additional mother ingot was added into the melt to increase amount of the remarked element. This procedure was repeated until the end of each alloy series.

The high pressure casting machine used in this study had vertical shot unit (shot force 660 kN) and horizontal die clamping unit (die clamping force 3.2MN). Table 2 and Fig. 2 indicate casting conditions and dimensions of stepped plate castings and tensile test specimens, respectively.

Tensile test pieces were cut from 15 mm thick portion of castings and machined to stick shape with 6 mm diameter and 35 mm length as shown in Fig. 2. All castings were heat treated with T6 condition: i.e. solutionizing at 803 K for 7.2 ks, water quenching and artificial aging at 433 K for 7.2 ks.

Tensile test was conducted by universal testing machine

with strain rate 5:6104. Number of specimens for each

condition was three. Fracture surfaces of tensile specimens were observed by scanning electron microscope, and micro-structures around fracture surfaces were observed by optical microscope. The compositions of unknown compounds were determined by electron probe microanalysis.

3. Results and Discussions

Figure 3 shows typical microstructures of Al-7 mass%Si-0.32 mass%Mg alloy Rheocastings and squeeze castings. Microstructures of squeeze castings consist of fine dendritic alpha phase and eutectic structure among dendrite arms. On the other hand, microstructures of Rheocastings are com-posed of globular primary alpha phase and eutectic structure among primary alpha phase. The size of globular alpha phase

[image:2.595.309.544.75.243.2]

is 100 to 150mmand each particle exists individually.

Table 1 Chemical compositions of Al-7 mass%Si-Mg alloys. (mass%)

Element Si Fe Mg Ti Sr

Series 1 7.1 0.10 0.32 0.12 0–0.105

Series 2 6.8 0.14 0.23–0.64 0.12 —

[image:2.595.47.290.84.138.2]

Series 3 6.6 0.11–0.51 0.34 0.13 —

Table 2 Casting conditions.

process Cast temp., Tcast/K

Shot velocity, Vs/ms1

Metal pressure, PM/MPa

Squeeze 993 0.05 103

Rheocast 853 0.10 123

biscuit gate

2

5

15 100

100

100

200

φ

6

0.02 15R

35 90

15R

Fig. 2 Schematic drawings of stepped plate castings and tensile specimens.

(a)

100

µ

m

(b)

[image:2.595.46.291.189.241.2] [image:2.595.128.469.291.459.2]
(3)

3.1 The effect of strontium content

Figure 4 shows mechanical properties of series 1 alloys with different strontium content from zero (no addition) to 0.105 mass%. Contents of other elements were constant. Both Rheocastings and squeeze castings show maximum elonga-tion at 0.009 mass% strontium. The elongaelonga-tion of Rheocast-ings was higher than those of squeeze castRheocast-ings at no

[image:3.595.55.283.72.243.2]

strontium addition and at 0.105 mass% strontium addition. Figure 5 shows microstructures of Rheocast and squeeze cast Al-7 mass%Si-Mg alloy without strontium addition and 0.009 mass% strontium addition. As shown in Fig. 5(c), there were large plate-like silicon particles in the micro-structure of squeeze cast Al-7 mass%Si-Mg alloy without strontium. In contrast to this, very fine eutectic silicon particles were observed in the microstructure of the same alloy fabricated by the Rheocasting process, as shown in Fig. 5(a). This alloy contains 2 ppm sodium but also 10 ppm phosphorous according to the additional chemical analysis. At this level of sodium and phosphorous content, an effect of eutectic silicon refinement by alkaline-earth elements de-creases with decreasing the cooling rate during

solidifica-tion.7)Since slurry has very small amount of latent heat for

solidification from semi-liquid state to completely solid because 30 to 50% of the alloy had already solidified, so cooling rate of remaining liquid phase in the slurry would be higher than those of squeeze castings. This is because finer eutectic structure and larger elongation could be obtained in Rheocastings without strontium addition. In the microstruc-ture of both squeeze castings and Rheocastings with 0.105 mass% strontium addition, some large particles formed. Figure 6 shows electron probe microanalysis results of the large particles. According to this analysis, these particles were Al-Si-Sr compound. It is well known that existence

of Al-Si-Sr compounds reduces elongation7) because these

Sr (mass%)

Squeeze Rheocast

0 10 20

Elongation (%)

250 300 350

UTS,

σ

B

/

MP

a

No addition 0.01 0.1 1

Fig. 4 Effect of strontium content on tensile strength and elongation of T6 heat-treated Rheocast and squeeze cast Al-7 mass%Si-0.32 mass%Mg alloys.

(a)

10

µ

m

(d)

(c)

(b)

[image:3.595.127.470.415.758.2]
(4)
[image:4.595.158.439.71.332.2]

compounds easily break under some stress as shown in Fig. 7 and are the start point of crack propagation. Therefore the decreasing elongation in both squeeze castings and Rheocastings would be due to these Al-Si-Sr compounds.

Ultimate tensile strength was almost the same for both processes, but it must be noted that squeeze casting shows nearly 10 MPa higher strength than Rheocasting. This result will be discussed in the next section.

3.2 The effect of magnesium content

Figure 8 shows effects of magnesium content on mechan-ical properties of Rheocastings and squeeze castings. Ultimate tensile strength increased and elongation decreased with increasing magnesium content for both casting proc-esses, but tensile strength of squeeze castings was about 10 MPa higher than Rheocastings for all magnesium content as shown in Fig. 8. Elongation of squeeze castings decreased remarkably with increasing magnesium content from 0.23 to 0.32 mass%. On the other hand, the elongation of

Rheocast-Al

Sr

Si

SEI

10

µ

m

Fig. 6 Composition analysis of strontium contained brittle compound in a Rheocast Al-7 mass%Si-0.32 mass%Mg alloy.

10

µ

m

Fig. 7 Microstructures of brittle compounds around fracture surfaces of 0.105 mass% strontium added Rheocast Al-7 mass%Si-0.32 mass%Mg alloy.

250 300 350

UTS,

σ

B

/MP

a Squeeze Rheocast

0 10 20

0

Mg(mass%)

Elongation(%)

0.8 0.6

0.4 0.2

[image:4.595.158.438.371.513.2] [image:4.595.312.540.566.758.2]
(5)

ings at 0.32 mass% magnesium was still high and reduced at 0.46 mass% magnesium.

As magnesium content increases, intermetallic compounds

like Mg2Si and Al-Si-Fe-Mg system would form and

increase their volume fraction. As shown in Fig. 9, the volume fraction of the intermetallic compounds was low at 0.32 mass% magnesium but it increased at 0.64 mass%

magnesium so much as the compounds spreading all over the eutectic structure area. And larger plate type compounds can be observed more in squeeze castings than Rheocastings. Figure 10 shows electron probe microanalysis results of T6 heat-treated Rheocastings that contains 0.64 mass%

magne-sium. In eutectic structure area, silicon, Mg2Si and

Al-Si-Fe-Mg system compound were observed.

(a)

10

µ

m

(d)

(c)

(b)

Fig. 9 Effect of magnesium content on microstructure of T6 heat-treated Rheocast (a), (c) and squeeze cast (b), (d) Al-7 mass%Si-Mg alloys. Magnesium content; 0.32 mass% (a), (b) and 0.64 mass% (c), (d).

(c) Mg

2

Si

(a) Si

(b) Al-Si-Fe-Mg

5

µ

m

(a)

(b)

(c)

Si

Al

Mg

Fe

Si

[image:5.595.124.474.71.422.2]

Si

Mg

[image:5.595.130.468.471.667.2]
(6)

Increasing of tensile strength with increasing of magne-sium content would be due to volume fraction of precipitated

Mg2Si. Thus squeeze castings seem to contain higher

magnesium in alpha phase than Rheocastings. According to additional electron probe microanalysis results of 0.64 mass% magnesium samples, magnesium content in primary alpha phase is 0.71 mass% for Rheocastings and

0.79 mass% for squeeze castings. This difference of magne-sium content seems to cause 10 MPa strength difference. The difference of magnesium content in the primary alpha phase would be due to the difference of solidification rate between two processes. In Rheocasting process, melt is cooled slowly to the semi-solid state. This results in effective rejection of solute elements from the primary alpha phase. This is the reason why magnesium content in primary alpha phase of Rheocastings is lower than those of squeeze castings.

Additionally to say, rejected solute elements are concen-trated to the liquid phase and they solidify in eutectic area as

the intermetallic compounds, Mg2Si or Al-Si-Fe-Mg system.

This would be due to the morphological and volume fraction difference of the intermetallic compounds between Rheo-castings and squeeze Rheo-castings. Therefore lower elongation of squeeze castings may be due to larger and more platelet shaped morphology of intermetallic compounds.

3.3 The effect of iron content

Figure 11 shows the effect of iron content on mechanical properties of Rheocast and squeeze cast Al-7 mass%Si-Mg alloys. The elongation of squeeze castings was decreasing from 12.7% to 9.1% with increasing iron content from 0.11 mass% to 0.27 mass%. On the other hand, the elongation of Rheocastings did not decrease like those of squeeze castings. It was 17.9% and 17.1% at iron content 0.11 mass% and 0.27 mass%, respectively. As shown in Fig. 12, the 0

10 20

0

Fe(mass%)

Elongation(%)

250 300 350

UTS,

σ

B

/

MP

a Squeeze

Rheocast

0.6 0.4

[image:6.595.56.283.72.262.2]

0.2

Fig. 11 Effect of iron content on tensile strength and elongation of T6 heat-treated Rheocast and squeeze cast Al-7 mass%Si-0.34 mass%Mg alloys.

10

µ

m

(a)

(b)

(d)

(c)

[image:6.595.128.469.421.758.2]

(e)

(f)

(7)

amounts of iron containing compounds were very small at 0.11 mass% iron for both casting processes. At 0.27 mass%

iron, the plate shaped compounds which with 10 to 20mmin

length appeared for both processes. This type of compounds generally reduces elongation. But it would not be true for Rheocastings because of very fine eutectic silicon. At 0.51 mass% iron, large compounds over 1 mm length came to appear and the elongation of castings for both processes decreased to around 7%.

4. Conclusions

The effects of strontium, magnesium and iron contents on mechanical properties of T6 heat-treated Rheocastings were investigated and compared with those of squeeze castings. (1) Rheocastings showed high elongation (17.9%) without strontium addition due to fine eutectic structure. It decreased to 13.6% at 0.105 mass% strontium addition because of brittle Al-Si-Sr compounds.

(2) Ultimate tensile strength in both processes increased with increasing magnesium content, while elongation decreased. There was about 10 MPa difference in the ultimate tensile strength between Rheocastings and squeeze castings. This is

because magnesium content in the primary alpha phase in Rheocastings, that depends on the precipitation strengthen-ing, was lower than squeeze castings.

(3) As increasing iron content, the amount of iron containing intermetallic compound increased. But the elongation of Rheocastings at 0.27 mass% iron was almost the same as those at 0.11 mass% iron. This would be due to fine eutectic structure in Rheocastings.

REFERENCES

1) M. C. Flemings, R. G. Riek and K. P. Young: Mater. Sci. Eng.25(1976) 103–117.

2) M. C. Flemings: Metall. Trans.22B(1991) 269–293.

3) R. Shibata, H. Yamane, T. Soda and T. Kaneuchi: J. JFS69(1997) 885– 891.

4) T. Kaneuchi, T. Imamura and R. Shibata: J. Jpn. Soc. Technol.41(2000) 1197–1200.

5) M. Adachi, S. Sato, H. Sasaki, Y. Harada, N. Ishibashi and T. Kawasaki: Collected Abstracts of the 1998 Autumn Meeting of the Japan Inst. of Light Metals (1998) pp. 69–70.

6) Y. Harada, S. Sato, T. Ueno, N. Ishibashi and M. Adachi: Collected Abstracts of the 1998 Autumn Meeting of the Japan Inst. of Light Metals (1998) pp. 71–72.

[doi:10.2320/matertrans.F-MRA2008845]

Figure

Fig. 1Schematic drawings of Rheocasting process using metallic vessel.
Table 1Chemical compositions of Al-7 mass%Si-Mg alloys. (mass%)
Fig. 5Effect of strontium content on microstructure of T6 heat-treated Rheocast (a), (b) and squeeze cast (c), (d) Al-7 mass%Si-0.32 mass%Mg alloys
Fig. 7Microstructures of brittle compounds around fracture surfaces of 0.105 mass% strontium added Rheocast Al-7 mass%Si-0.32 mass%Mg alloy.
+3

References

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