An investigation on diffusion bonding of aluminum to copper using equal channel angular extrusion process

Abstract

A new method for production of bimetallic rods, utilizing the equal channel angular extrusion (ECAE) process has been introduced before by previous researchers, but no attempt has been made to assess the effect of different temperatures and holding times in order to achieve a diffusional bond between the mating surfaces. In present research copper sheathed aluminum rods have been ECAEed at room temperature and subsequently held at a constant ECAE pressure, at different temperatures and holding times to produce a diffusional bond between the copper sheath and the aluminum core. The bonding quality of the joints was examined by shear strength test and a sound bonding interface was achieved. Based on the results, a bonding temperature of 200 °C and holding time of 60–80 min yielded the highest shear strength value.

Research highlights

► The highest joint shear strength is attained at 200 °C and 60–80 min. ► At a fixed temperature the joint strength rises with increasing the holding time. ► Temperature has a double effect on the joint strength.

1. Introduction

Bimetallic rods have advantages which are not achievable in a mono-metal rod. Copper coated aluminum rod is an instance of these materials widely used in electricity transmission and some aerospace applications. It has been reported that in comparison with a Cu rod, the bimetallic rod is 40–60% lighter and 30–40% cheaper [1].

Among different forming processes utilized to produce a bimetallic rod, equal channel angular extrusion (ECAE) is a unique one due to its several advantages such as dimensional precision, high shear bonding strength, simple shape of the die, and the general state of compressive stress which characterizes the ECAE process [2]. ECAE is a severe plastic deformation technique initially developed by Segal (1995) [3]. The circumstances of how to carry out this method has been adequately described elsewhere [4–6]. In the past decade, this technique has attracted much attention as a method to produce ultra fine grained materials [7].

Modern manufacturing technology requires a bonding process at low temperatures and low pressures in order to avoid undesired phase transformations and large deformations for the modern materials. In many circumstances, a metal and non-metal have to be bonded together. Diffusion bonding is such a process in which two matched surfaces are held together at an elevated temperature below the melting temperature of the materials under a low pressure which does not cause a macroscopic plastic deformation in the materials for a time required to form a metallurgical bond between the materials [8]. In addition, the waste created by this process is usually far less than the other joining processes and the final machining operations are minimized [9].

For solid-state bonding, the fine-grained microstructure is also important because the presence of many grain boundaries act as short-circuit paths for diffusion. Grain boundary migration is faster in fine-grained materials than in coarse-grained ones [10]. Since ECAE is a grain refining method, combining this method with diffusion bonding process would be a great challenge. Therefore, the aim of present study is to introduce another new application of the ECAE process which is the production of diffusion bonded Al/Cu bimetallic rods.

2. Materials and experimental procedures

2.1. Materials

Cylindrical shaped specimens were cut and machined from a 40 mm thickness aluminum plate (A1100) and a 16 mm diameter copper rod (commercially pure copper). The inner metal (the core) was a small cylindrical sample of 10.9 mm diameter and 75 mm length of the A1100 aluminum and the outer layer (the sheath) was a cylinder of the commercially pure copper with inner and outer diameters of 11 mm and 14.8 mm, respectively. The machining precision was so that the aluminum core could be fitted into the copper sheath with a small hand force.

2.2. ECAE die specifications

The ECAE die used in this study was a die consisting of two intersecting channels of 14.8 mm in diameter intersecting at a die angle of 120° and an outer curved corner of 0°. A split configuration die design was used to facilitate easy removal of the extruded specimen [2].

2.3. Experiment

The mating surfaces of the copper sheath and aluminum cylinder were prepared by conventional grinding and polishing techniques with ultimately 1 μm diamond finish. Subsequently, samples were cleaned in acetone and dried using compressed air [2]. The interval time between the surface preparation and ECAE test was kept less than 2 min to avoid the formation of a thick and continuous oxide layer on the mating surfaces [11].

ECAE was conducted on the billets using a 100 ton capacity hydraulic press. MoS₂ was used as the lubricant during pressing [12]. After pressing 2/3 of the specimen length, the ECAE process was stopped and subsequently the specimen was heated up to an elevated temperature by means of a furnace embracing the die while maintaining the ECAE pressure on the specimen. Finally, the whole system was timed to achieve a diffusional bond at the mating surfaces of the aluminum core and the copper sheath. The test was repeated for various bonding temperatures and holding times.

In order to examine the quality of the joint between the layers of the bimetallic rods, the shear strength tests were performed on the ECAEed bimetallic billets using sample and die design similar to that explained in Reference [2]. Three samples in each condition were tested and the average value was reported.

3. Results and discussion

3.1. Joint shear strength

Fig. 1 shows the joint produced at 200 °C and holding time of 100 min. The visual feature soundness of the joint is clear in this view. 30 joints were fabricated using different combinations of bonding temperature and holding time. The average shear strength results are displayed in Table 1.

Fig. 1.

View of the joint produced and the shear strength test specimen.

Table 1.

Experimental conditions and joint shear strength results.

Bonding condition		Shear strength (MPa)
Temperature (°C)	Time (min)
100	20	–
	40	–
	60	–
	80	–
	100	–
125	20	–
	40	–
	60	–
	80	–
	100	19.4
150	20	–
	40	–
	60	21.3
	80	23.3
	100	26.6
175	20	–
	40	22.3
	60	23.3
	80	27.2
	100	27.7
200	20	25.2
	40	31.1
	60	34.9
	80	34.9
	100	34.9
225	20	26.2
	40	32
	60	33.9
	80	33.6
	100	33.9

For all the specimens with bonding temperature of 100 °C and the specimens with bonding temperature of 125 °C, 150 °C, 175 °C and holding times lower than 100 min, 60 min and 40 min respectively, no bonding was occurred between the mating surfaces. This was due to the insufficient temperature to cause diffusion of atoms or insufficient holding times to achieve a sound bond [13,14].

As can be seen from Fig. 2, at a constant bonding temperature, the joint average shear strength increases with increasing the holding time and this is due to the increase in the extent of joint formed and the fact that prolonging the holding time would let the diffusion mechanisms to complete and leads to a better void closure [15]. However, after a specific time which depends on the bonding temperature (60 min for bonding temperatures of 200 °C and 225 °C), degradation of this increasing trend is observed to the extent that after a short time the shear strength values remain constant with increasing the holding time. It is well known that the condition of a surface which is to be joined by solid-state diffusion bonding plays a vital role in determining the minimum bonding time required and the integrity of the final bond [16]. It is obvious that the bonding time can be shortened greatly by improving the surface finish quality. The presence of large asperities on the bonding area prevents extensive plastic deformation of the surface asperities due to poor contact area. Accordingly, the bonding time will increase [17].

Fig. 2.

Dependence of the joint shear strength on holding time at a constant bonding temperature.

Fig. 3 exhibits the columnar diagram of joints average shear strength values versus holding times based on data represented in Table 1. Referring to the diagram, as it is expected, the bond strength increases with increasing the bonding temperature at a constant holding time. This was attributed to the affiliation of diffusion coefficient of atoms with temperature and therefore the acceleration and facilitation of the diffusion process at elevated temperatures. But increasing the temperature has a drawback too. The interface of solid-state welded Al/Cu is susceptible to the nucleation and growth of intermetallic compounds and oxide layers formation at temperatures greater than 120 °C. This is a thermally activated process and by increasing the temperature the nucleation and growth of intermetallic compounds and formation of oxide layers are accelerated. These compounds have a non-metallic covalence bond and therefore are brittle and can weaken the bonding strength [11,18].

Fig. 3.

Dependence of the joint shear strength on bonding temperature at a constant holding time.

As it can be seen in the same diagram, at temperatures higher than 200 °C at a holding time of 60 min or more, the extension of these intermetallic compounds and also the oxide formations due to the vacuum-free environment, leads to the joint strength degradation. It is apparent in above diagrams that a bonding temperature of 200 °C in conjunction whit a holding time of 60–80 min, leads to the highest shear strength value.

4. Conclusion

In this research, copper sheathed aluminum rods were ECAEed at room temperature and subsequently while tolerating a constant ECAE pressure, diffusion bonded at different temperatures and holding times. The bonding quality of the joints was checked by shear strength testing. The following conclusions can be derived from the achieved results:

• 1.A sound bonding interface can be achieved, applying this procedure. A bonding temperature of 200 °C and holding time of 60–80 min yields the highest shear strength value.
• 2.At a constant temperature the joint strength increases with increasing the holding time. But after a short time the shear strength values remains constant with further increase in holding time.
• 3.Temperature has a double effect on the joint strength. At a constant holding time the bond strength increases with increasing the bonding temperature, but on the other hand, by increasing the temperature the nucleation and growth of the intermetallic compounds and oxide layer formation are accelerated which it can weaken the bonding performance.