J. Cent. South Univ. (2012) 19: 882-886

DOI: 10.1007/s11771-012-1087-1

Casting-cold extrusion of Al/Cu clad composite by copper tubes with

different sketch sections

LUO Jun-ting(骆俊廷)¹, ZHAO Shuang-jing(赵双敬)^{1, 2}, ZHANG Chun-xiang(张春祥)¹

1. State Key Laboratory of Metastable Materials Science and Technology, School of Mechanical Engineering,

Yanshan University, Qinhuangdao 066004, China;

2. Tangshan Railway Vehicle Co., Ltd., Tangshan 063035, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Casting–cold extrusion technology was presented to fabricate aluminum/copper clad composite, and copper tubes with different sketch sections were designed. The technology of aluminum/copper clad composite fabricated by casting–cold extrusion was simulated by DEFORM software using tubes with four arc grooves. The stress and strain in different deformation zones were analyzed. The groove size reduces gradually and the groove shape drives to triangle during the extrusion procedure. The maximum values of equivalent effective stress and radial stress appear in groove zones, and the maximum equivalent effective strain firstly is obtained also in groove zones. The grain size in groove zones is less than that in other zones. The experimental results are consistent with simulation results, which prove that the copper tubes with sketch section are favorable to the metallurgy bond of boundary interface between aluminum and copper.

Key words: Al/Cu clad composite; casting-cold extrusion; sketch section; finite element simulation

1 Introduction

Good conductivity and strong corrosion resistance of copper have been favored, so it was regarded as the first choice of the inner conductor material for RF coaxial cable [1]. However, due to the lack and valuableness of copper, the RF coaxial cable with full copper as the inner conductor will cause waste of resources and high costs. Aluminum/copper clad composite was developed, taking into account the skin effect of high-frequency signal in the process of transmission, as well as the good electrical conductivity and thermal performance of aluminum. Compared with copper, the aluminum/copper clad composite has following characteristics: smaller density, lighter weight, more convenient for transportation and installation.

Aluminum/copper clad composite has several preparation methods, including plating aluminum wire with copper method, coated welding method, traditional extrusion method, hydrostatic extrusion method and continuous extrusion method [4-6] etc. Products produced by these processes either have poor metallurgical bonding properties between copper and aluminum (plating aluminum wire with copper method, coated welding method, traditional extrusion and continuous extrusion, etc.), or are fabricated by more complex technology, which have low productivity and higher costs (hydrostatic extrusion and hot extrusion). XIE et al [5-7] first proposed core-filling continuous casting technology. Conceptual equipment was developed and the composite bars of copper cladding aluminum with 24 mm in diameter of core material and 8 mm in thickness of cladding layer were successfully fabricated.

The cold extrusion forming technology of copper/ aluminum clad composite was reported [8-9]. The technology can achieve metallurgy combination at atom levels at copper/aluminum interface when the billets fabricated by casting technology are used. The technology effectively enhances solid combination at copper/aluminum interface, improves the quality of Al/Cu clad composite, and also makes production conditions simple and convenient. The technology was further reported in this work that copper tubes were designed into various sketch sections, which were more available for metallurgy combination at copper/ aluminum interface.

2 Section design of copper tubes

The designed section shapes of copper tubes are shown in Fig. 1.

Taking arc-shaped section copper tubes with four slots in inwall as an example, the forming technology of Al/Cu clad composite was investigated and the rules were analyzed by finite element simulation and experiments in this work. The cross-section size of copper tube is shown in Fig. 2.

Fig. 1 Copper tubes schemes with various section shapes:    (a) Ordinary copper tube; (b) Copper tube with rectangular convex reinforcing inwall; (c) Copper tube with U-shaped slot in inwall; (d) Copper tube with T-shaped slot in inwall;      (e) Copper tube with fan-shaped hole mesh section; (f) Copper tube with helical line convex reinforcing inwall

Fig. 2 Arc-shaped cross-section size of copper tube with four slots

3 Finite element simulation of forming technology

3.1 FES Model

Finite element analysis software DEFORM was used during simulation. The copper tube size for simulation is as follows: 60 mm in length, 29.8 mm in outer diameter and 3 mm in wall thickness. Pure aluminum is filled as core material. Material properties of pure aluminum and copper tube are given in Table 1. The flow stress-strain curves of materials used during simulation are from material storage model of DEFORM software, with a few work hardening.

The extrusion temperature is room temperature. The extrusion speed is 20 mm/s. The work belt length is   10 mm. The friction factor is set as 0.12 because good lubricant is used between outside surfaces of copper tube and dies. The friction factor of 0.1 is set between male die and the surface of billet. The extrusion angle of concave die is 30°, extrusion ratio is 5.45, and reduction of area is 81.66%.

Table 1 Performance parameters of materials

The finite element model is shown in Fig. 3. The 1/4 model for billet and whole model for die is used in simulation, which can further avoid inhomogeneous deformation in boundary and effectively reduce calculation time.

Fig. 3 Finite element model and schematic drawing: 1-Male die; 2—Copper tube; 3—Aluminum core; 4—Female die

3.2 Simulation results

The deformation process of Al/Cu clad composite during simulation is shown in Fig. 4. The products are of good quality in outer face without folds and cracks in deformation zones. The deformation procedure of billet includes four stages: upset stage, early extrusion stage, uniform extrusion stage and end extrusion stage.

The shape change procedure of groove cross-section during extrusion deformation is shown in Fig. 5. It can be seen from Fig.5 that groove diameter becomes smaller and smaller and gradually tends to be triangular shape by extrusion action of internal aluminum and external copper, which is very good for metallurgical combination of the two materials in groove parts.

The equivalent stress of billet during extrusion is analyzed by extracting the value in 82nd substeps, which is shown in Fig. 6. The equivalent stress is C=174 for the beginning stage of extrusion, which is caused by upsetting deformation. The value of equivalent stress increases gradually with the proceeding of extrusion. The value of equivalent stress in groove area increases more rapidly than that in other parts, which indicates that the stress state in groove area is more conducive to achieve metallurgical combination at the interface between copper and aluminum and improve the quality of Al/Cu clad composite. The value of equivalent stress is about F=435 in the near area of work tap, and the maximum value G=522 of equivalent stress appears in the top area of the groove. The value of equivalent stress rapidly decreases when billet is extruded out of the work zone.

Fig. 4 Deformation process of Al/Cu clad composite

Fig. 5 Shape change procedure of groove cross-section during extrusion deformation: (a) Initial groove shape; (b) Groove shape in cone-shaped deformation area; (c) Groove shape in work belt; (d) Groove shape after extrusion

Fig. 6 Equivalent stress of billet during extrusion in 82nd substep

The equivalent strain of billet during extrusion is analyzed by extracting the value for each stage, which is shown in Fig. 7. Figure 7(a) shows the equivalent strain nephogram for the initial stage of upset, and there is a little deformation in the bottom of billet. The maximum equivalent strain of billet in groove appears in cone-shaped area, which is shown in Fig. 7(b). The value of equivalent strain reaches the maximum with billet into work zone, which is shown in Fig. 7(c), and it is clear that the value of equivalent strain in groove region firstly reaches the maximum. The value of equivalent strain quickly decreases to zero when billet is extruded out of work zone, which is shown in Fig. 7(d), and the value of equivalent strain in groove region decreases more slowly than that in other parts.

Fig. 7 Equivalent strain in inwall and ectotheca of copper tube for each stage during extrusion process: (a) Upset stage;     (b) Early extrusion stage; (c) Uniform stage; (d) End extrusion stage

The strain value of copper in groove area is larger than that in non-groove area and with more time. The strain value is greater and more beneficial to the combination at interface, which can improve the quality of Al/Cu clad composite in groove area.

The radial stress of three different points (P1, P2, P3) in interface of copper is analyzed. The three points are shown in Fig. 8. P1 is the peak point of the groove; P2 is the mid point of half-groove section; P3 is the point at interface of copper excluding the area of groove.

Fig. 8 Schematic plan of point P1, P2 and P3

The curves of time versus radial stress in three points are shown in Fig. 9. The deformation procedure of these points in cone-shape area corresponds to the curves from start point to lowest point (the maximum compressive stress). The compressive stress becomes gradually smaller when these points are extruded into work zone. When the billet is extruded out of the work zone, the stress states of these points are tensile stress because of elastic recovery of the copper tube, then the value of stress is gradually stable and tends to be zero.

Fig. 9 Curves of radial stress versus time

The radial stress at P1, P2 is always larger than that at P3 from the sample initially extruded out of the work zone, and the radial stress at P3 is minimum during the whole stage, which indicates that the stress state is beneficial to the metallurgical combination at interface of two materials in groove. The value of the radial stress is about zero and no longer changes when the billet is extruded out of the work zone.

The curves of circumferential stress (σ_θ) and radial stress (σ_r) vs time are shown in Fig. 10 for P2. The stress state is two-dimension compressive stress in radial and circumferential directions for P2, and the vector sum of two-dimension compressive stress is vertical stress (σ_α) at P2. The stress analysis diagram for P2 is shown in  Fig. 11. The vertical stress at P2 is much larger than radial compressive stress at P1 and P3. The larger vertical stress is better for metallurgical combination at interface between copper and aluminum. Thus, the metallurgical combination at interface when groove tubes are used is better than that when common tubes are used, and the complex shapes of tubes are obtained, so the interface is more difficult for torsion, flake and separation.

Fig. 10 Curves of circumferential stress and radial stress vs time for P2

Fig. 11 Stress analysis diagram for P2

4 Experimental results

The changing procedure of cross-section shape of Al/Cu clad billet during extrusion is shown in Fig. 12. The cross sections are sequential for material in upsetting stage, in cone-shaped deformation stage and extruding stage from left to right. With the proceeding of extrusion, the groove shape tends to be triangular more and more, which is consistent with the simulation results.

Fig. 12 Changing procedure of cross-section shape of Al/Cu clad billet during extrusion: (a) Upsetting stage; (b) Cone- shaped deformation stage; (c) Extruding stage

The interface microstructures of different deformation stages are shown in Fig. 13. The microstructure of aluminum in con-shaped deformation zone is shown in Fig. 13(a). Aluminum grains at the interface are refined when materials begin to come into the con-shaped deformation stage, while the grains in the center remain the original state with large grain and non-uniform distribution [10]. The demarcation between the refined region and large grain region is obvious. The thickness of the interface is about 10 μm [11]. The microstructure of aluminum in extruding stage is shown in Fig. 13(b). The grain size becomes gradually fine from the central area of product to copper/aluminum interface. The grain size is more homogeneous than that in cone zone stage and without obvious demarcation between central area and near interface zone of aluminum. The grain size in central area is also refined obviously. The grain size in central area is about 15-20 μm, and the grain size near the interface zone is about 10-15 μm. The interface thickness is further reduced, which is difficult to identify by optical microscopy.

Fig. 13 Interface microstructures in different deformation stages: (a) Con-shaped deformation stage; (b) Extruding out stage

The microstructure of aluminum in groove is shown in Fig. 14. The grain size of copper and aluminum interface in groove is about 10 μm and finer than that in other areas of groove outside.

Fig. 14 Microstructure of aluminum in groove

5 Conclusions

1) The Al/Cu clad composite can be fabricated by casting-cold extrusion technology with copper tube of various sketch sections.

2) The better stress state can be obtained when copper tube is designed into groove cross-section shape, which is available for metallurgical combination at interface between copper and aluminum.

3) The grain size of copper/aluminum interface in groove is finer than that in other areas of groove outside, which is consistent with the result of theoretical analysis and simulation.

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(Edited by HE Yun-bin)

Received date: 2011-03-03; Accepted date: 2011-07-01

Corresponding author: LUO Jun-ting, PhD; Tel: +86-335-8052253; E-mail: ljtlyk@yahoo.com.cn