liquid interface during solidification

Scripta Materialia, Vol. 39, No. 7, pp. 969 –975, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00

Pergamon

PII S1359-6462(98)00242-5

PORE BEHAVIOR AT THE SOLID/LIQUID INTERFACE DURING SOLIDIFICATION J.M. Kim1, D.G. Kim2, H.W. Kwon3 and C.R. Loper, Jr. Materials Science & Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA 1 Now at the Pennsylvania State University, University Park, PA, USA. 2 Professor, Metallurgical Engineering, Dong-A University, Pusan, Korea. 3 Professor, Metallurgical Engineering, Yeungnam University, Kyongsan, Korea. (Received January 13, 1998) (Accepted in revised form May 28, 1998)

Introduction Porosity formation in Al-Si alloys is a complicated phenomenon that involves mushy solidification associated with the segregation of hydrogen and difficulties in feeding liquid metal through the mushy zone. Mechanisms of pore formation have been widely studied both theoretically and experimentally, however, only little attention has been directed to the growth mechanism of pores in relation to the solid/liquid interface movement. The stage at which pores are nucleated is an important factor that influences the morphology and distribution as well as the size of pores in the casting. [1] When a substantial amount of gas is present in the melt, pores may be formed at the beginning of freezing in castings. Once a pore is formed, it will be affected by the advancing solid/liquid interface. [2– 4] The pore may increase in size while being pushed by the advancing interface or may be entrapped by the interface. This pore push/engulfment phenomenon may affect significantly the final pore size and distribution in castings. In this study, a unidirectionally solidifying alloy was quenched to investigate the effect of the solid/liquid interface on pore formation. Upward and downward freezing conditions were established to study the effect of gravity. The pore behavior at the moving solid/liquid interface was discussed both theoretically and experimentally.

Experimental Procedure The schematic diagram of the experimental setup is shown in Figure 1. A 25.4 mm I.D. alumina tube, containing a quartz tube and a 0.25 mm D. K-type thermocouple, was placed in a tube furnace. The specimen to be studied was placed inside the 6 mm D. Quartz tube having a 1 mm thick wall. An A356 alloy was used as the charge material and its chemical composition is shown in the Table 1. After the alloy was melted, the quartz tube was moved up or down and held at a certain position, so that directional solidification started from either the top or bottom. The solidification velocity was 100 mm/s for the upward freezing condition and was 160 mm/s for the downward freezing condition, over a length 969

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Figure 1. Schematic diagram of the directional solidification experimental setup.

of about 40 mm. The effect of gravity on the pore formation was studied by comparing these two conditions. After a given period of time the tube was quenched into the ice-water mixture. A 12 cm length of sample was cut longitudinally for metallographic examination. The pore morphology and distribution with respect to the solid/liquid interface were analyzed by using an image analysis system. Results and Discussion Modeling for the Behavior of Pores at Advancing Solid/Liquid Interface Theories on particle pushing/engulfment phenomenon have been developed for its application to particle distribution in ceramic particulate-reinforced metal matrix composites. [5–7] This model was modified for the pore case. A pore formed in the melt will float due to the buoyancy force exerted on it. [8] The pore accelerates to a constant terminal velocity, Vf, at which point the upward buoyancy force is equal to the downward force due to the gravity and the drag force. Since the density of the pore is very small, the force due to gravity is negligible. A terminal floating velocity is obtained when the upward buoyancy force is equal to the downward force due to the drag force. This velocity of floating is expressed as: TABLE 1 Melt Chemistry of A356 Alloy. Si

Fe

Mg

Zn

Ti

Sr

Ni

Pb

Sn

Ca

Na

7.05

0.07

0.38

0.02

0.14

0.004

0.008

0.00

0.01

0.002

0.004

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TABLE 2 Data used for Pore Behavior Map [6 – 8]. ao(m) 210

2310

[5]

r(kg/m3)

h(Pazs)

a

Dso(N/m)

2371[5]

0.005[5]

0.0036[7,8]

0.047[7]

Vƒ 5

2 2 g R r 9 h

(1)

where R is the pore radius, g is the acceleration due to gravity, h is the melt viscosity, and r is the density of the melt (the density of the pore was assumed as zero). In the case when a solid/liquid interface is moving upward (freezing occurs from the bottom), the pore will float or experience the pushing/engulfment interaction with the interface, depending on the velocity of floating and velocity of the interface movement. It is obvious that when the interface is moving downward, pore escape from the interface by flotation is impossible. There will be a critical velocity of interface movement where pushing/engulfment transition exists. When the velocity of the moving interface is slower than critical, the pore will be pushed, and when the velocity is more rapid than critical, a pore will be entrapped by the interface. The critical velocity, Vc, can be obtained by considering three forces acting on the pore; gravitational force (Fg), drag force (Fd), and force due to the interfacial energy (Fi). The equilibrium velocity, Vc, must satisfy the following equation: Fg 2 Fd 1 Fi 5 0

for the upward freezing and

Fg 1 Fd 2 Fi 5 0

for the downward freezing

(2)

Thus (5), Vƒ 5

F S

a0 d Ds0 3 ha R a 0 1 d

D

h

6

2 Rrg 3a

G

(3)

For n 5 2, d 5 a0, the critical velocity is obtained as follows: Vc 5 Vc 5

a 0D s 0 2 a 0R r g 1 12 ha R 9 ha 2

a 0D s 0 2 a 0R r g 2 12 ha R 9 ha 2

for the upward freezing

(4)

for the downward freezing

where: a0 5 atomic distance, Ds0 5 interfacial energy difference (5ssp 2 slp 2 ssl), and a 5 kp /km; kp 5 thermal conductivity of pore, and km 5 thermal conductivity of melt. In the case of a concave interface, a . 1, consequently, a greater drag force results, while a convex interface (a , 1) results in a smaller drag force. A pore behavior map was developed from this analysis using equation 4 and the data of Table 2. The thermal conductivity of hydrogen at 900 K was used for the pore [8], and the thermal conductivity of aluminum used was 109.5 Wm21 [7]. In the case of a pore in the aluminum melt: Dso 5 (ssp 2 slp 2 ssl 5 0.98 2 0.84 2 0.093) 5 0.047 (N/m) $ 0 (nonwetting)

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Figure 2. Pore behavior map (a) upward freezing (b) downward freezing. E (Engulfment) P (Pushing) F (Floating) VF (floating velocity) VC (critical velocity).

Two types of maps were drawn depending on the freezing direction, Figure 2. One interesting observation which was that in downward freezing pores with a size larger than about 50 mm are all entrapped by the solid/liquid interface regardless of interface velocity. Effect of Solidification on the Pore Behavior When solidification occurs from the bottom to the top (upward freezing), feed metal is readily supplied to the solid/liquid interface. Therefore, all pores can be attributed to gas evolution from solidifying alloy. These pores will experience an increasing buoyancy force while the pore diameter increases. Figure 3 shows the percent porosity distribution changes in the castings quenched after different holding times (1, 3, & 5 min.) during upward solidification. Percent porosity was measured as total area percent of porosity by using an image analysis system. It is observed that pores floated and gathered in the liquid in the top portion of casting, while the solid/liquid interface was moving upward. Two peaks developed: the peaks at the left may occur due to pores nucleated during cooling and solidification of the liquid, while the peak to the right develops as porosity accumulates in the upper portion of the liquid. There is a region devoid of pores between the two peaks. Gas porosity could not escape by floating from the casting due to oxides existing at, and near, the melt surface. As the solid/liquid interface was moved upward, the amount of pores at the top of casting was significantly increased, as indicated in the case of the 5 min. specimen. Because the A356 alloy solidifies over a freezing range, a mushy zone, occurs

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Figure 3. Percent pore distribution changes through the castings quenched at different holding times (1, 3, & 5 min) on the upward freezing condition.

during solidification. No porosity was entrapped by the solid/liquid interface because the interface velocity was only about 100 mm/sec in this experiment. In the case of downward freezing, the formation of pores occurs due to both gas evolution and solidification contraction. The transfer of feed metal to the solid/liquid interface during solidification must take place against the effect of gravity. Although this gravity effect is minor, it should be considered as additive to pore formation due to gas evolution as a gas bubble will serve to nucleate shrinkage, and vise versa. Once pores are formed, they will be pushed or entrapped by the interface, depending on the pore size and the interface velocity. Figure 4 shows that the percent pore distribution changes in the casings quenched at different holding times (1, 2, & 3 min.) during downward freezing. It is seen that the amount of porosity ahead of the solid/liquid interface increases as the quenching time increases, due to porosity accumulating at the interface. Porosity entrapped in the solid was observed in the sample quenched after 3 min. The pore size and pore density distribution at the moving interface in the casting quenched after 2 min. of directional solidification is shown in Figure 5. Small pores are present in the mushy zone and large pores in front of the liquidus. The mechanism of pore growth in relation to the downward movement of the solid/liquid interface may be summarized as follows, based upon the experimental results observed. At an earlier stage, as the solid/liquid interface moves downward, porosity forms and grows in the mushy zone due to both gas evolution from solidifying metal and the influence of feed metal against gravity. These pores grow as they are pushed ahead of the proeutectic dendrites and the eutectic/liquid interface. The increasing concentration of pores ahead of the solid/liquid interface interferes with feed metal transfer so that porosity growth is accelerated. At a later stage, the large pores formed in the mushy zone finally become entrapped by the solid/liquid interface, or if near the casting surface the result may appear as the formation of a surface sink on the casting.

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Figure 4. Percent pore distribution changes through the castings quenched at different holding times (1, 2, & 3 min) on the downward freezing condition.

As discussed previously, a higher interface velocity and larger pore size favor engulfment of porosity. Figure 2b shows that large pores (with the diameter of 50 mm or more) may be expected to be engulfed by the interface regardless of its moving velocity (about 160 mm/s in this experiment). However, large pores (about 1 diameter) were observed to be entrapped in the solid. Although the critical pore size of entrapment was much larger than expected from the pore behavior map, the anticipation that large pores should be entrapped regardless of the interface velocity was validated considering that very slow interface velocity (160 mm/s) was used in these experiments.

Figure 5. Pore size and density distribution in the casting quenched at 2 min after downward freezing started.

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Summary In the case of upward freezing, pores formed at the interface are initially pushed as they grow. In this experiment no porosity was entrapped by the solid/liquid interface due to the slow interface velocity. Pores floated away from the interface when their size reached a critical size. They increased in size during floating by coarsening and coalescence of pores. In the case of downward freezing, pores are formed at the interface due to gas evolution and feed metal transfer against gravity. The pores are pushed by both the proeutectic dendrites and the eutectic front. Large pores having been pushed at the interface finally become entrapped in the solid. References 1. 2. 3. 4. 5. 6. 7. 8.

J. Campbell, Br. Foundaryman. April, pp. 147–158 (1969). B. Chalmers, Principles of Solidification, Wiley, New York (1964). H. Fredriksson and I. Svensson, Metall. Trans. B. 7B, 599 (1976). D. Shangguan and D. M. Stefanescu, Metall. Trans. B. 22B, 385 (1991). D. Shangguan, S. Ahuja, and D. M. Stefanescu, Metall. Trans. A. 23A, 669 (1992). D. M. Stefanescu, B. K. Dhindaw, S. A. Kacar, and A. Moitra, Metall. Trans. A. 19A, 2847 (1988). D. M. Stefanescu, A. Moitra, A. S. Kacar, and B. K. Dhindaw, Metall. Trans. A. 21A, 231 (1990). D. R. Gaskell, An Introduction to Transport Phenomena in Materials Engineering, Macmillan, Inc. (1992).