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Effects of Processing Parameters on the Fabrication of Copper Cladding Aluminum Rods by Horizontal Core-Filling Continuous Casting YA-JUN SU, XIN-HUA LIU, HAI-YOU HUANG, CHUN-JING WU, XUE-FENG LIU, and JIAN-XIN XIE Copper cladding aluminum (CCA) rods with a diameter of 30 mm and a sheath thickness of 3 mm were fabricated by horizontal core-filling continuous casting (HCFC) technology. The effects of key processing parameters, such as the length of the mandrel tube of composite mold, aluminum casting temperature, flux of the secondary cooling water, and mean withdrawing speed were optimized based on some quality criteria, including the uniformity of the sheath thickness, integrality of the rods, and thickness of the interface. The causes of internal flaws formation of CCA rods were also discussed. The results showed that the continuity of the liquid aluminum core-filling process and the interface reaction control between solid copper and liquid aluminum were two key problems that strongly affected the stability of the casting process and the product quality. Our research indicated that for the CCA rod with the previously mentioned size, the optimal length of mandrel tube was 210 mm. A shorter mandrel tube allowed of easier erosion at the interface, which led to a nonuniform sheath thickness. Conversely, it tended to result in a discontinuous filling process of liquid aluminum, which causes shrinkage or cold shuts. The optimal casting temperatures of copper and aluminum were 1503 K (1230 C) and 1043 K to 1123 K (770 C to 850 C), respectively. When the casting temperature of aluminum was below 1043 K (770 C), the casting process would be discontinuous, resulting in shrinkages or cold shuts. Nevertheless, when the casting temperature of aluminum was higher than 1123 K (850 C), a severe interface reaction between solid copper and liquid aluminum would occur. The proper flux of the secondary cooling water and the mean withdrawing speed were determined as 600 to 800 L/h and 60 to 87 mm/min, respectively. In the previously mentioned proper ranges of processing parameters, the interfacial shear strengths of CCA rods were 40.5 to 67.9 MPa. DOI: 10.1007/s11663-010-9449-2 The Minerals, Metals & Materials Society and ASM International 2010

I.

INTRODUCTION

COPPER has been used widely as an electrical material because of its excellent conductivity. However, because of the worldwide resource shortage and the large price fluctuation of copper, aluminum was adopted as an alternative material in the electric field since the 1950s for its relative abundant resource, lower price than that of copper, and good conductivity. But the poor reliability of the connection between aluminum conductors or between an aluminum conductor and a copper conductor restricted the wide application of the aluminum conductor. From the 1960s, a copper cladding aluminum (CCA) bimetallic laminar composite was developed as an ideal substitute for copper conductor and pure aluminum conductor because of its YA-JUN SU, PhD Student, XIN-HUA LIU and HAI-YOU HUANG, Lecturers, XUE-FENG LIU and JIAN-XIN XIE, Professors, are with the Key Laboratory of Advanced Materials Processing (MOE), University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] CHUN-JING WU, Professor, is with the School of Materials Science and Engineering, University of Science and Technology Beijing. Manuscript submitted May 2, 2010. Article published online November 5, 2010. 104—VOLUME 42B, FEBRUARY 2011

outstanding comprehensive performances, for example, low density, high conductivity, excellent corrosion resistance, and good brazing property. It has been used widely in the fields of signal transportation, power transmission, special electromagnetic wire, etc.[1–5] With the application of CCA in industry, more and more scientists and engineers have become interested in the development of the fabricating technologies of CCA wire. Several methods have been used to a produce a bar with a compound structure. So far, there are two categories of fabrication technologies of the CCA compound structure. The first one is solid–solid bonding technology, such as the corolling[6] method, and the overlay welding[7–9] method, which has been used to fabricate CCA wire for industrial production. The interface between copper and aluminum is formed based on mechanical contact and diffusion bonding. Therefore, both of these methods have some shortages, such as long process flow, high production costs, serious environmental issues during surface treatment, and difficulty in preparing composite conductors with the heteromorphic or large cross sections, e.g., a flat bus used for large electric current transmission. The second category of technology is solid–liquid bonding. Webber and Drescher[10] invented a method of METALLURGICAL AND MATERIALS TRANSACTIONS B

producing a nonferrous type of clad metallic product in lengths of desired size and, if desired, in continuous lengths. The advantage of the method is that a clad metallic product in continuous lengths can be fabricated easily. However, a critical problem to these methods is the partial oxidation on the interior surface of the cladding metal. An oxide layer located between the cladding and core metals severely affects the interface strength and physical properties of products, and it even results in failure at the interface during subsequent process, such as drawing or rolling. Additionally, it is difficult to obtain accurate control of the uniformity of cladding sheath. Furthermore, a composite bar with a large cross section might be inappropriate to fabricate by this method because of the restricted cooling capacity of the facilities. Neumann[11] invented a vertical continuous casting process by which the dual metal or clad metal castings can be produced. This method is applicable to almost any cladding metal products whose core metal has a lower melting point than that of cladding metal. Furthermore, the interface oxidation problem can be avoidable. However, the vertical casting method has several problems; for example, continuous production is inconvenient because of the limit of height of plant, and also this method is disadvantageous to the facility layout and automatic control. To resolve the problems of previous methods, Xie et al.[12] developed a horizontal core-filling continuous casting (HCFC) process to produce copper-cladding aluminum continuously. The previous investigations indicated that a key problem in fabricating CCA rods by HCFC technology is how to optimize processing parameters to ensure the filling of liquid aluminum into a presolidified copper tube and to avoid excess interface reaction between liquid aluminum and solid copper. In this work, the fabricating process of the CCA rods by HCFC was reported. The effects of key processing parameters, such as the mandrel tube length of composite mold, aluminum casting temperature, flux of secondary cooling water, and mean withdrawing speed

2

3

II.

EXPERIMENTAL PROCEDURES

A. Fabrication Process of CCA Rods In this work, 99.9 wt pct pure copper and 99.7 wt pct pure aluminum were used. CCA rods with a diameter of 30 mm and a sheath thickness of 3 mm were fabricated by an HCFC device, as shown in Figure 1. The detailed structure of the composite mold was illustrated in Figure 1(b). The fabricating process of a CCA rod was as follows: First, liquid copper in crucible 5 was insufflated into the compound mold continuously and solidified into a copper sheath tube. Then, liquid aluminum in crucible 1 was injected into the presolidified copper tube continuously through the mandrel tube and solidified into solid aluminum core. Thus, a CCA rod with a metallurgical bonding interface was fabricated. In the process of HCFC, many factors affect the stability of the fabricating process and the quality of CCA rod, including the uniformity of the sheath thickness, the structure of aluminum core, and the interface thickness. Previous investigations indicated that the key processing parameters that have remarkable effects mainly included the mandrel length (L), the aluminum casting temperature (TAl), the secondary cooling water flux (Q2), and the mean withdrawing speed (v). Based on the theoretical analysis and the previous experimental results, the main processing parameter ranges for our experiments were determined as follows: L = 190 to 230 mm, TAl = 1003 to 1123 K (730 C to 850 C), Q2 = 600 to 900 L/h, and v = 60 to 100 mm/ min. In all, 15 experiments in four groups were

3 17 16

5 2

on the quality of CCA rods, including the uniformity of the sheath thickness, the integrality of the rods, the interface thickness, and interfacial strength were analyzed, respectively. The optimal processing parameters for steady fabrication of CCA rods with good quality were determined.

18

19

20

15 14

4

13

2 6 7 8

L1

Withdrawing

L2

L

21

9 10 11

12

Withdrawing

1

12

(a)

22

(b)

Fig. 1—Schematic diagram of the HCFC device for preparing the CCA rod (a) Device; (b) composite mold; L—Length of the mandrel; L1—Distance from the mandrel orifice to copper solidification front; L2—Distance from the mandrel orifice to aluminum solidification front; 1—Molten aluminum holding furnace; 2—Thermal couple; 3—Stopper; 4—Runner; 5—Molten copper holding furnace; 6—Composite mold holding furnace; 7—Water in; 8—Water outlet; 9—Jacket water cooler; 10—Secondary cooler; 11—Pinch rolls; 12—CCA rod; 13—Liquid aluminum; 14—Mandrel tube; 15—Liquid copper; 16—Composite mold; 17—Thermal couple 1st; 18—Insulation pad; 19—Solidification front of liquid copper; 20—Thermal couple 2nd; 21—Solidification front of liquid aluminum; 22—Thermal couple 3rd. METALLURGICAL AND MATERIALS TRANSACTIONS B

VOLUME 42B, FEBRUARY 2011—105

Table I.

Serial No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Mandrel Tube Length L (mm) 190 210 230 210 210 210 210 210 210 210 210 210 210 210 210

Processing Parameters of Fabricating CCA Rods by HCFC Aluminum Casting Temperature TAl [K (C)] 1083 1083 1083 1003 1023 1043 1123 1063 1063 1063 1063 1063 1063 1063 1063

(810) (810) (810) (730) (750) (770) (850) (790) (790) (790) (790) (790) (790) (790) (790)

Flux of the Secondary Cooling Water Q2 (L/h)

Mean Withdrawing Speed v (mm/min)

600 600 600 600 600 600 600 600 700 800 900 700 700 700 700

60 60 60 60 60 60 60 67 67 67 67 60 75 87 100

* TCu = 1503 K (1230 C), Q1 = 600 L/h.

P 1 21

10

2

25

conducted to investigate the effects of the mandrel length (experiments 1–3), the aluminum casting temperature (experiments 4–7), the secondary cooling water flux (experiments 8–11), and the mean withdrawing speed (experiments 12–15), respectively. Designed processing parameters in all experiments were tabulated in Table I. In our experiments, the copper casting temperature TCu and the primary cooling water flux Q1 were fixed as 1503 K (1230 C) and 600 L/h, respectively, which ensures the solidification of a copper sheath tube. The intermittent withdrawing method was used to ensure the sufficient crystallization of copper and aluminum. In each experiment, an approximately 10-m length CCA rod was fabricated. Considering the stability of the HCFC technique, a 1-m length head was cut before sample preparation.

24 26

3 4 Length unit: mm

Fig. 2—Schematic diagram of the shear strength tester: 1—punch; 2—gland bush; 3—CCA rod sample; 4—die (gland bush 2 connects with die 4 by screwed connection).

B. Testing and Analysis Methods Cross-section samples with 10-mm thick and longitudinal section samples 70-mm long were cut from CCA rods by an electrical spark linear cutting machine. The section morphologies of the samples were observed after polishing by metallographic sandpaper. To analyze the uniformity of sheath thickness of CCA rods, the sheath thicknesses at the quartile of circumference of cross section were measured by a vernier caliper. The interface thicknesses of the samples were measured by scanning electron microscopy after polishing and etching by the reagent of HF(1 ml): HNO3(2.5 ml):H2O(95 ml). The average of the thickness values obtained from different positions of the cross-section samples was taken as the interface thickness of the sample. The interfacial shear strength was tested with a shear strength tester as shown in Figure 2 mounted in a 20-ton universal testing machine. A continuous load was applied on the samples with a thickness of 10 mm via

106—VOLUME 42B, FEBRUARY 2011

a punch 1 until the interface of CCA was failure. The maximum load Pmax was recorded and the diameter of the shear cylindrical of the deformed sample (d) was measured with a vernier caliper. The interfacial shear strength (s) was calculated according to the following equation: Pmax Pmax ¼ ½1 s¼ A pdh where A is the area of shear plane and h = 10 mm is the thickness of samples. The optimization criteria of processing parameters were selected based on the application requirements of the CCA rod. According to the quality requirements, the following properties were determined: the continuity of core-filling process of liquid aluminum during fabricating process, the uniformity of the sheath thickness, the integrality of CCA rod, the interface thickness, and the interfacial strength.

METALLURGICAL AND MATERIALS TRANSACTIONS B

III.

RESULTS AND DISCUSSION

A. Effects of the Mandrel Tube Length Table II shows the effects of the mandrel tube length of composite mold (L) on the quality of CCA rods in a condition of TAl = 1083 K (810 C), Q2 = 600 L/h, and v = 60 mm/min. The inner structure morphologies of CCA rods are shown in Figure 3. The photos of the cross section of samples are posted on the left side and those of the longitudinal section are posted on the right side correspondingly.

Table II and Figure 3 show that when L = 190 mm, the core-filling process of liquid aluminum is continuous, and there is no distinct hollow in the aluminum core. However, there are cracks in the core, and the sheath thickness is obviously asymmetrical; that is, the upper copper sheath of the rod is corroded seriously. When L = 210 mm, the core-filling process of liquid aluminum is also continuous, and there is no distinct shrinkage and cold shut in the core. The sheath thickness is uniform considering industry tolerance range. No erosion in the sheath occurs at the interface.

Table II. Effects of the Mandrel Tube Length on the Quality of CCA Rods Mandrel tube length (mm)

190

210

230

Liquid aluminum core-filling process

Continuous

Continuous

Discontinuous

0 3.1 3.0 3.1 — serious cracks

2.9 3.1 3.0 3.0 419 no no

2.9 3.0 3.0 3.1 — no hollows

Thickness of copper sheath (mm)

Upper Lower Left Right

Interface thickness (lm) Erosion in copper sheath Flaws in the core

Fig. 3—Internal morphologies of the CCA rods fabricated with different mandrel lengths (a) and (b) L = 190 mm; (c) and (d) L = 210 mm; (e) and (f) L = 230 mm. METALLURGICAL AND MATERIALS TRANSACTIONS B


Table III.

Effects of Aluminum Casting Temperature on the Quality of CCA Rods

Aluminum casting temperature [K (C)]

1003 (730)

1023 (750)

1043 (770)

1083 (810)

1123 (850)


Discontinuous

Discontinuous

Continuous

Continuous

Continuous

2.9 3.1 3.1 3.0 — no hollows

2.9 3.1 3.0 3.0 148 no shrinkages

2.9 3.1 3.0 3.0 160 no no

2.9 3.1 3.0 3.0 419 no no

2.9 3.1 3.1 3.0 487 no no




When L = 230 mm, the core-filling process of liquid aluminum becomes discontinuous. There are severe distinct hollows in the aluminum core. Based on the experimental results presented in Table II and Figure 3, the optimal mandrel tube length of L = 210 mm can be determined. B. Effects of Aluminum Casting Temperature Table III shows the effects of the aluminum casting temperature (TAl) on the quality of CCA rods. In this group of experiments, other processing parameters were fixed as L = 210 mm, Q2 = 600 L/h, and v = 60 mm/ min. The inner macrostructure morphologies of the CCA rods are presented in Figure 4. It can be observed from Table III and Figure 4 that when TAl £ 1023 K (750 C), the core-filling process of liquid aluminum is discontinuous, and there are some hollows, shrinkages, or cold shuts in the aluminum core. However, when 1043 K (770 C) £ TAl £ 1123 K (850 C), the core-filling process of liquid aluminum is continuous, and there is no flaw in the core. The sheath thickness is uniform and no erosion occurs at the interface. In addition, the interface thickness increases with the rise of the aluminum casting temperature. The interfacial reaction between liquid aluminum and solid copper tends to occur at a higher temperature, and some brittle intermetallic compounds (e.g., CuAl2, CuAl, and Cu9Al4,) will be generated, which has an adverse effect on the interfacial bonding strength. The results show that the higher the aluminum casting temperature, the thicker the interface and the more remarkable the adverse effect. A too high aluminum casting temperature should be avoided. The experimental results indicate that the optimal aluminum casting temperature lies in the range of 1043 K to 1063 K (770 C to 790 C), and under these temperatures, the interface thickness is approximately 160 to 363 lm. However, when TAl ‡ 1123 K (810 C), the interface thickness increases much faster and is thicker than 400 lm, which would have serious effects on the interfacial bonding strength of the CCA rod.

secondary cooling water flux of 600L/h. When we tried to increase the secondary cooling water flux to 700 L/h, 800 L/h, and 900 L/h, we found that a mean withdrawing speed of 60 mm/min does not match the secondary cooling water flux of 900 L/h. Therefore, to investigate the effect of the secondary cooling water flux, we changed the mean withdrawing speed to 67 mm/min in this group of experiments. Table IV shows the effects of the secondary cooling water flux (Q2) on the quality of the CCA rod, in a condition of L = 210 mm, TAl = 1063 K (790 C), and v = 67 mm/min. The inner structure morphologies of the CCA rods are presented in Figure 5. Table IV and Figure 5 show that when 600 L/ h £ Q2 £ 800 L/h, the core-filling process of liquid aluminum is continuous, and there is no significant macrosized flaw in the core, the sheath thickness is uniform, and no erosion occurs at the interface. Nevertheless, when Q2 = 900 L/h, some distinct shrinkages or cold shuts can be found in the core. However, the interface thickness of the rod decreases with the rise of the secondary cooling water flux. D. Effects of the Mean Withdrawing Speed Table V shows the effects of the mean withdrawing speed (v) on the quality of the CCA rods with other processing parameters set as L = 210 mm, TAl = 1063 K (790 C), and Q2 = 700 L/h, respectively. The inner structure morphologies of CCA rods are presented in Figure 6. It can be observed from Table V and Figure 6 that when 60 mm/min £ v £ 87 mm/min, the core-filling process of liquid aluminum is continuous, and there is no obvious flaw in the core, the sheath thickness is uniform, and no erosion occurs at the interface. However, when v = 100 mm/min, there is a crack in the aluminum core, and the sheath thickness is obviously uneven because significant erosion occurs at the upper sheath of the rod (shown in Figures 6(e) and (f)) even though the corefilling process of liquid aluminum is still continuous, and no distinct shrinkages or cold shuts are observed in the core.

C. Effects of Secondary Cooling Water Flux It can be observed that all the previously discussed results about the effects of the mandrel tube length or aluminum casting temperature were achieved under the 108—VOLUME 42B, FEBRUARY 2011

E. Interfacial Strength The interfacial strengths of the CCA rods fabricated under proper ranges of processing parameters METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 4—Internal morphologies of the CCA rods prepared at different aluminum casting temperatures. (a) and (b) TAl = 1003 K (730 C); (c) and (d) TAl = 1023 K (750 C); (e) and (f) TAl = 1043 K (770 C); (g) and (h) TAl = 1123 K (850 C).

Table IV.

Effects of the Secondary Cooling Water Flux on the Quality of CCA Rods

Secondary cooling water flux (L/h)

600

700

800

900


Continuous

Continuous

Continuous

Discontinuous

2.9 3.1 3.0 3.0 402 no no

2.9 3.1 3.0 3.0 276 no no

2.9 3.0 3.0 3.1 256 no no

2.9 3.0 3.0 3.1 235 no cold shuts




determined previously were measured and the results were tabulated in Table VI. Based on Table VI, the interfacial strengths of CCA rods could be determined as 40.5 to 67.9 MPa. METALLURGICAL AND MATERIALS TRANSACTIONS B

F. Discussion Previous investigations indicated that a fabrication process with the parameters of copper casting VOLUME 42B, FEBRUARY 2011—109

Fig. 5—Internal morphologies of the CCA rods prepared with different fluxes of the secondary cooling water (a) and (b) Q2 = 600 L/h; (c) and (d) Q2 = 800 L/h; (e) and (f) Q2 = 900 L/h.

Table V.

Effects of the Mean Withdrawing Speed on the Quality of CCA Rods

Mean withdrawing speed (mm/min)

60

67

75

87

100


Continuous

Continuous

Continuous

Continuous

Continuous

2.9 3.1 3.0 3.0 363 no no

2.9 3.1 3.0 3.0 276 no no

2.9 3.1 3.1 3.0 195 no no

3.0 3.0 3.1 3.0 75 no no

1.7 3.1 3.1 3.1 — serious cracks




temperature TCu = 1503 K (1230 C) and the primary cooling water flux Q1 = 600 L/h could produce a highquality presolidified copper sheath tube. According to the previously presented experimental results, two key issues need to be solved when the HCFC method is used to fabricate CCA rods with a uniform sheath thickness and a thinner interface (less than 300 to 400 lm) but without any apparent interior macrostructural flaws. One method is to ensure that the molten aluminum can be filled continuously into the presolidified copper tube through the mandrel tube of the composite mold, that is, to ensure the continuity of the 110—VOLUME 42B, FEBRUARY 2011

core-filling process of liquid aluminum. The other method is to control the interface reaction between liquid aluminum and the inner surface of copper tube to avoid forming excessive intermetallics, which could degrade the properties of CCA rod and even cause erosion in the presolidified sheath tube. To make the process of HCFC steady and continuous, and to fabricate CCA rods without macrostructural flaws, the position of copper solidification front should be controlled properly at first. That is to say, the distance from the solidification front of copper sheath to the outlet of the mandrel tube (L1) (shown in METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 6—Internal morphologies of the CCA rods prepared at different withdrawing speeds (a) and (b) v = 60 mm/min; (c) and (d) v = 87 mm/ min; (e) and (f) v = 100 mm/min.

Table VI.

The Interfacial Shear Strength of CCA Rods

Main Processing Parameters Serial No.* 2 6 7 8 9 10 12 13 14

TAl [K (C)] 1083 1043 1123 1063

(810) (770) (850) (790)

1063 (790)

Q2 (L/h)

v (mm/min)

600

60

600 700 800 700

67

Interface Thickness (lm)

60 75 87

419 160 487 402 276 256 276 195 75

Interfacial Strength s (MPa) 51.8 45.2 59.1 40.5 43.4 56.6 42.0 50.3 67.9

± ± ± ± ± ± ± ± ±

0.4 0.7 0.6 0.3 0.5 0.3 0.7 0.2 0.5

Other Processing Parameters TCu = 1503 K (1230 C) Q1 = 600 L/h L = 210 mm

* Corresponding to Table I.

Figure 1(b)) should be controlled properly during the process of preparing the presolidified sheath tube. The value of L1 is determined by the length of water-cooling mold, the intensity of the primary water cooling, and the mandrel tube length L. Because the former two parameters are fixed in our experiments, the size of L1 varies mainly with the length L of the mandrel tube. If the METALLURGICAL AND MATERIALS TRANSACTIONS B

distance of L1 is too short, then the contact temperature of copper sheath tube and liquid aluminum would be too high. Therefore, erosion would occur easily (Table II or Figure 3, L = 190 mm). Conversely, if the size of L1 is too long, then the contact temperature of copper sheath tube and liquid aluminum would be so low that it would make the distance from the aluminum VOLUME 42B, FEBRUARY 2011—111

solidification front to the mandrel outlet (L2 shown in Figure 1(b)) too short, or it might even cause the solidification front to be located in the mandrel tube. This result would cause the outlet of the mandrel tube to be jammed repeatedly and would cause the liquid aluminum core-filling process to be discontinuous under the withdrawing mode of draw-pause-draw. Therefore, some shrinkages or cold shuts, even hollows would be generated, for example, the state on the condition that L=230 mm is shown in Table II. Another key to fabricating CCA rods steadily without macrostructural defects is to control the position of solidification front of liquid aluminum properly, i.e., the distance from the solidification front to the mandrel outlet (L2) (Figure 1(b)) should be controlled properly. If the distance of L2 is too long, then the contact time of copper sheath tube and liquid aluminum at the higher temperature would be prolonged. Therefore, on the one hand, it is easy to form an excessive thicker diffusion layer or even to cause severe erosion in copper sheath. On the other hand, if the distance of L2 is too short, then the position of aluminum solidification front might be near the outlet or even inside the outlet of the mandrel tube. As mentioned previously, it would cause the mandrel tube to be jammed repeatedly under the withdrawing mode of draw-pause-draw. Therefore, some shrinkages or hollows would occur in an aluminum core of the rod because of the discontinuous corefilling process. The size of L2 is affected mainly by the aluminum casting temperature, the secondary watercooling intensity, and the mean withdrawing speed. It would increase with the rise of aluminum casting temperature, the decrease of the secondary water-cooling intensity, or the increase of the mean withdrawing speed. Conversely, L2 would decrease. Furthermore, the effect of the mean withdrawing speed (v) on the interface thickness of the CCA rod has its own duality. That is, on the one hand, the increase of the mean withdrawing speed (v) will lead to the increase of the size of L2, and consequently, it tends to cause the increase of the interface thickness of the rod. On the other hand, the contact time of liquid aluminum and the presolidified copper sheath tube would be shortened with the increase of the mean withdrawing speed, which tends to cause the decrease of the interface thickness of the rod. The experimental results and the previously mentioned analyses indicate that there is an interactional relationship between the parameters of L1 and L2. Therefore, to obtain a faster mean withdrawing speed and a thinner interface and, consequently, to improve the productivity of the CCA rods and the quality of the interface, the match relationship must be optimized among these processing parameters through a profound theoretical analysis and experimental research. Our experimental results indicate that in the conditions of a mandrel tube length L = 210 mm, copper casting temperature TCu = 1503 K (1230 C), and primary cooling water flux Q1 = 600 L/h given in this work, the proper ranges of main parameters are be as follows: aluminum casting temperature TAl = 1043 K to 1123 K (770 C to 850 C), secondary cooling water flux 112—VOLUME 42B, FEBRUARY 2011

Q2 = 600 to 800 L/h, and mean withdrawing speed v = 60 to 87 mm/min. In the previously presented ranges of processing parameters, the interfacial strengths of the CCA rods are in the range of 40.5 to 67.9 MPa, as shown in Table VI. Compared with the shear strength of 30 to 50 MPa and the tensile strength of 60 to 100 MPa for pure aluminum,[13] the results indicate that the interfacial strength of the fabricated CCA rods is even more than the shear strength of pure aluminum. A good metallurgical bonding between copper and aluminum was obtained. From the data presented within Table VI, no regular variation of interfacial strength with interface thickness is observed in the CCA rods. The interfacial strength may be influenced by the phase composition and microstructure of the interface rather than the interface thickness. More research needs to be done in the future.

IV.

CONCLUSIONS

In this work, the effects of the main processing parameters of the HCFC method on the stability of the fabrication process of the CCA rod, the inner macrostructure of the rod, the interface reaction, and the interfacial strength between copper and aluminum were investigated. The results are summarized as follows: 1. The optimal length of mandrel tube is approximately 210 mm. If the mandrel tube is too short, then it tends to generate the defect of interface erosion. Conversely, it easily results in the discontinuous corefilling process and consequently causes the formation of hollows, shrinkages, or cold shuts in the core. 2. When the casting temperature of aluminum is 1043 K to 1123 K (770 C to 850 C), CCA rods with good quality could be fabricated successfully. When the aluminum casting temperature is below 1043 K (770 C), the casting process would be discontinuous, and the core-filling defects would be generated. However, if the aluminum casting temperature is above 1123 K (850 C), then a severe interface reaction between solid copper and liquid aluminum would occur. 3. When the primary cooling water flux is 600 L/h, the proper secondary cooling water flux is 600 to 800 L/h. A too large flux of the secondary cooling water results in the discontinuous core-filling and casting process, but a too small flux of the secondary cooling water tends to make the interface reaction degree increase. 4. The mean withdrawing speed has a remarkable influence on the fabricating process of CCA rod, and the proper mean withdrawing speed is 60 to 87 mm/ min. 5. In the previously presented optimal ranges of processing parameters, the interfacial shear strengths of the CCA rods are in the range of 40.5 to 67.9 MPa. 6. The CCA rods with the thinnest interface thickness of approximately 75 lm and the maximum interfacial shear strength of 67.9 ± 0.5 MPa can be fabricated in METALLURGICAL AND MATERIALS TRANSACTIONS B

the following conditions: mandrel tube length of 210 mm; copper and aluminum casting temperature of 1503 K (1230 C) and 1063 K (790 C), respectively; mean withdrawing speed of 87 mm/min; and a primary and secondary cooling water flux of 600 L/h and 700 L/h, respectively.

ACKNOWLEDGMENTS The authors are grateful for the financial support of National Natural Science Foundation (Grant 50774009) and the National High Technology Research and Development Program (‘‘863’’Program) of China (Grant 2009AA03Z532). The authors also would like to thank Yong-Fu Wu, Yu Lei, Chao Wang, Jun Mei, and Bin Peng for their assistance in the experiments, and they are thankful to Professor Zhi-Hao Zhang for many helpful discussions.

METALLURGICAL AND MATERIALS TRANSACTIONS B

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