# Formation And Distribution of Brittle Structures in Friction Stir Welding of AA 6061 To Copper. Influence of Preheat

Seyed Vahid Safi, Hossein Amirabadi, Mohammad Besharati Givi Kazem

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Seyed Vahid Safi, Hossein Amirabadi, Mohammad Besharati Givi Kazem. Formation And Distribution of Brittle Structures in Friction Stir Welding of AA 6061 To Copper. Influence of Preheat. Mechanics, Materials Science & Engineering Journal, 2016, 10.13140/RG.2.1.2620.9684. hal-01306978

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# Formation And Distribution of Brittle Structures in Friction Stir Welding of AA 6061 To Copper. Influence of Preheat

Seyed Vahid Safi¹,ª, Hossein Amirabadi², Mohammad Kazem Besharati Givi³

1. M.Sc. student. Mech. Eng., University of Birjand, Birjand, Iran

2. Assoc. prof., Mech. Eng., University of Birjand, Birjand, Iran

3. Assoc. prof., Mech. Eng., University of Tehran, Tehran, Iran

a. vahid.safi@gmail.com

<figure><img src="image_2.png" /></figure>

DOI 10.13140/RG.2.1.2620.9684

**Keywords:** friction stir welding, mechanical properties, preheat, intermetallic.

**ABSTRACT.** In this paper, apart from introducing brand – new warm friction stir welding (WFSW) method, the effect of preheating on friction stir welded of copper and aluminum alloys sheets and its influence on improving the mechanical properties of the weld were investigated. Sheets of aluminum alloy 6061 and copper with thickness of 5mm were used. The tool was made of tool steel of grade H13 with a threaded cone shape. Rotational speeds (ω) of 1200-1400 rpm and traverse speeds (v) of 50-100 mm/min were used for better understanding the behavior of the tools during the heat input. The sheets were kept in furnace with temperature of 75 °C and 125°C and welding was done afterwards. At last, tensile and micro hardness tests were done to compare the mechanical properties of the welds. Considering to the high thermal conductivity of both copper and aluminum, the reason of increase in strength of the joints could be related to the low temperature gradient between the weld zone and base metal because the heat gets out of the stir zone with lower steep. A significant increase in hardness is observed in the SZ for the following reasons: (i) the presence of concentric grains with intensely refined recrystallization and (ii) the presence of intermetallic compounds. The tensile test results showed 85% increase in the strength of preheated joints. The maximum strength occurs for preheating of 75°C, rotational speed of 1200 rpm and traverse speed of 50 mm/min. In the present study, intermetallic compounds and the precipitates are moved to the grain boundaries during the welding process. These precipitates act as strong obstacles to the movements of dislocations and increase the deformation resistance of material. This phenomenon may result in locking of grain boundaries and consequently decrease of grain size. This grain refinement can improve the mechanical properties of welds. Accordingly, hardness and strength of the material will be increased.

**1. Introduction.** FSW is a process including many actions and interactions between a series of simultaneous thermodynamic processes. These reactions are the results of heat input rate, cooling rate, flow and deformation, dynamic recrystallization and integration of mechanical joints [1]. This method of welding was invented in 1991 by Thomas Wayne et al. in TWI in Cambridge, England [2]. FSW is a relatively new method in metal joining that because of solid state nature of this method (no melting happens at joining) it has some advantages comparing with other fusion welding methods. Low distortion and shrinkage, good mechanical properties, fewer defects and the ability of welding some metals that cannot be welded with fusion welding methods are among the most important advantages of this method [3]. In last few years, this process has been used in aerospace and aviation industries, automotive industries, fuel tanks etc. in most developed countries. During this process, imposes high lateral forces to the material during deformation which causes temperature to increase up to about 70-80% of its melting temperature [4, 5]. Copper has a wide range of applications because of its great thermal and electrical conductivity, corrosion and fatigue resistance and good flexibility. Copper alloys have various set of properties, which is dependent on the addition of elements and heat treatment [6].

Today, aluminum and its alloys have a wide range of applications in defense industry, aerospace, transportation, marine industry, construction industry, packaging and containers etc. some of the properties which made aluminum and its alloys as one of the most economical and popular group of
metals are: Light weight, flexibility, physical properties, mechanical properties, corrosion resistance [7]. During the improvement of FSW process, the conventional fusion welding methods for similar and dissimilar materials were used. There are a few studies about FSW for dissimilar materials at the moment. FSW of dissimilar materials such as aluminum and copper causes the formation of intermediate brittle phases of Al/Cu, which still needs to be studied in the future [8, 9].

Ouyang et al [10]. Also studied the FSW on dissimilar metals such as AA6061 (T6) and copper. They showed that joining of AA6061 to copper due to the brittle nature of intermetallic compounds in the weld nugget, is hard. Mechanical mixture zone of dissimilar joint of AA6061 to copper is mainly consisted of various intermetallic compounds including CuAl2, CuAl and Cu9Al4 with few amount of α-Al and FCC solid solution of aluminum in copper.

On the other hand Galvao et al [11]. observed that welding with higher ω/v, increase the heat input and causes the formation of mixture zones from materials with higher size and more homogenous. The morphology of stir zone, type and the amount of intermetallic structures which are formed because of thermo mechanical induction solid state are highly dependent to welding parameters. In FSW of dissimilar metals, pin is an important factor in compare to the two metals.

Ratnesh and Pravin [12] welded AA6061 to copper with FSW. They obtained good joints with changing the central line of the pin into the copper and in the advancing direction. They reported that ultimate tensile strength of aluminum in copper joint is low due to the presence of intermetallic compounds. Increase in the rotational speed lowers the tensile strength, which is mainly due to the increase in the amount of intermetallic compounds, which are formed in the aluminum-copper interface. Higher hardness in the stir zone in compare to the base metals is due to the formation of hard and brittle compounds such as CuAl2, CuAl and Cu9Al4.

Akinalbi et al [13]. observed that interface of the joint are recognized with lamellar mixture of aluminum and copper which are apparent in the structure and are due to heat input from the stir action of the pin during the FSW process. They also observed that decrease in grain size towards the stir zone of the welds increases.

Recent researches show that with placing the copper sheet, which has higher melting temperature in the advancing side, a weld with good compounds can be obtained. However, many of these studies show the presence of intermetallic compounds during FSW of aluminum with copper, but more study is needed on new formed phases for better understanding the weld joint. Optimizing the parameters of the process to decrease the formation of intermetallic compounds in the interface is also required.

**2. Experimental procedure.** Dissimilar FSW of 6061 aluminum alloy and pure copper sheets both with the thickness of 5 mm was performed. The specimens were cut to the dimension of 70 mm×200 mm by wire cut. Due to the different behaviors of aluminum alloy and pure copper, in addition to the welding and also complication of condition simultaneous with applying preheating condition, it has been attempted to obtain more appropriate analysis of condition dominant in dissimilar joints welding by considering a set of parameters affecting welding process. That is why a tool with frustum threaded pin and also pin tendency towards copper sheet (Fig. 1) in 3 different travel and rotation speeds have been used.

Parameters employed in dissimilar joining of copper and 6061 aluminum alloy sheets are shown in Table 1. The joints were performed in 3 different temperature domains including room temperature, 75 °C and 125 °C.

After welding, three tensile test samples were cut from each welded sheet in a vertical direction to the weld line using wire cut to investigate the strength of the joints. The tensile test samples were prepared according to ASTM E8M standard. The tests were carried out using a Hounsfield tensile testing machine under the crosshead speed of 1 mm min⁻¹ at room temperature. Microstructural analysis was performed on the cross section perpendicular to the welding direction. A solution of Fe (NO₃)₃ (5g) + HCL (25 ml) + H₂O (75 ml) was used to etch the copper side of the joints, while the aluminum side was etched by a reagent with a solution of 25ml HNO₃ and 75ml H₂O for 60 s. Optical
and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) were used in order to observe the microstructure and characterize the intermetallic compounds at the joint interface. Furthermore, EDS line scan analysis was performed to follow the compositional change across the intermetallic compounds existing at the interface of aluminum and copper sides of the interface. Micro hardness measurements were also taken from the cross sections of both aluminum and copper sheets at the welded area perpendicular to the welding direction by applying load of 25 gf for 20 s duration loading time.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic illustration showing set-up of dissimilar FSW.</figcaption></figure>

<table><caption>Table 1. Parameters employed for dissimilar joining of copper and 6061 aluminum alloy sheets</caption><thead><tr><td>Frustum threaded pin</td><td>Geometry of tool pin</td></tr></thead><tbody><tr><td>18-6-4.8 mm</td><td>Shoulder diameter, Pin diameter,</td></tr><tr><td>1400-1300-1200 rpm</td><td>Pin length</td></tr><tr><td>50-75-100 mm/min</td><td>Tool rotation speed</td></tr><tr><td>3 degree</td><td>Tool travel speed</td></tr><tr><td>2 mm toward copper sheet</td><td>Tilt angle</td></tr><tr><td></td><td>Tool offset</td></tr></tbody></table>

## 3. Result and Discussion.

**3.1 Joints appearance.** Fig. 2 shows surface appearances and cross sections of Al/Cu joints produced at different traverse and rotational speeds as it is observed, the weld surface does not contain any kind of defect and onion ring structure is clearly observable as well.

As can be observed in Fig. (2-e), within the stir zone, the dominant phase is aluminum and fine and coarse particles of copper have been distributed within the aluminum matrix. Full turbulence of material and displacement of copper toward aluminum is indicative of adequate flow in the stirring zone. Welding area of welded joints is similar to the common form of nugget zone in FSW process that is as elliptical. In the Fig. (2-d), an image of optimal cross sections (ω = 1200 ν = 50) can be observed. The boundary of Al/Cu is completely continuous and free of any hole or discontinuities. The presence of small particles of copper in the aluminum side indicates that conditions have been favorable for adequate stirring by means of tool in the weld zone. However, in this case, because of the placement of copper on the advance side, the situation is different (Fig. 2-f). In this case, the continuity of copper and stir zone is not desirable and in the border of copper, discontinuity and crack are observed. Moreover, there is several holes in the nugget zone. Because of the discontinuity in the boundary between the copper and the stir zone, the created joints is so weak in terms of mechanical property and can be easily fractured in the boundary of Al/Cu.
<figure><img src="image_4.png" /><figcaption>Fig. 2. Macroscopic view of weld surface and cross sections of joints at the welding conditions of: (a, d): ω=1200 v=50; (b, e): ω=1300 v=75 and (c, f): ω=1400 v=100.</figcaption></figure>

**3.2 Material flow.** In order to study the flow behavior of materials more precisely, the microstructures of the nugget zone were examined in detail by SEM and results are shown in Fig. 3. Different phases were evidenced and corresponding EDS analysis were carried out. The EDS results were presented in table 2.

<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM images of the dissimilar Al/Cu joints interface</figcaption></figure>
<table><caption>Table. 2. EDS analysis results of reaction layer pointed in Fig. 3 (at-%)</caption><thead><tr><th>Position</th><th>Al</th><th>Cu</th><th>Possible phases</th></tr></thead><tbody><tr><td>A</td><td>83/48</td><td>16/52</td><td>Al+Al<sub>2</sub>Cu</td></tr><tr><td>B</td><td>11</td><td>84/45</td><td>Solid Solution<br>Cu(Al)</td></tr><tr><td>C</td><td>39/43</td><td>60/57</td><td>Al<sub>2</sub>Cu<sub>3</sub></td></tr><tr><td>D</td><td>43/56</td><td>56/44</td><td>AlCu</td></tr><tr><td>E</td><td>32/15</td><td>64/44</td><td>Al<sub>4</sub>Cu<sub>9</sub></td></tr></tbody></table>

As can be observed in Fig. (3-a), rotation of tool in the interface of the joint has caused enough stirring and mixing of the copper and aluminum, that this mixture along with heat caused by the process of preheating sheets, plastic deformation and friction have caused the separation of coarse particles of copper and locating them in aluminum matrix. The result of this reaction is penetration of copper and aluminum into each other particles together and formation of intermetallic compounds. In Fig. (3-b), which is an image with a larger magnification compare with Fig. (3-a), the intermetallic compounds can be clearly observed. Compounds that as the small and scattered parts have been distributed in the boundary of the joint. In Fig (3-c), the sequential layers of copper and intermetallic compounds are observed that reflects this fact that the aluminum particles are isolated from the aluminum sheet and they convert to intermetallic compounds in the side of copper sheet because of suitable condition in the terms of the amount of heat and enough time to react. Fig (3-d) shows the presence of halos with different color besides copper layers that indicates the formation of different intermetallic compounds in those areas. By an exact investigation of the joint area using EDS of chemical composition of present components, the presence of fine particles of metallic compounds in aluminum matrix that have been distributed heterogeneously (Fig. 3-e). According to the results of the analysis carried out, the points of B and C (Fig. 3-f) are two different intermetallic compounds. Point B contains 5.13 wt-% of aluminum and 92.67 wt-% of copper, while point C includes 21.65 wt-% aluminum and 78.35 wt-% of copper that indicates the fact that the conditions in point C is much more favorable in terms of time and the amount of heat to create intermetallic compounds.

Compounds created in the D and E is directly related to the aluminum particles in the copper (Figure 3. g). The reason for this phenomenon and formation of different intermetallic compounds can be stated in a way, that atoms isolated from copper sheet in the aluminum side and isolated aluminum atoms in the copper side, according to the condition of joint such as the inlet temperature and time, react with each other in the interface of the joint and cause formation of intermetallic layers with different colors, that this difference in contrast is contributed to the differences in the chemical composition of these components. Then this difference in chemical composition occurs because of the penetration atoms into the interface of the joint. Therefore, it can be concluded that formation of intermetallic compounds depends on the permeability of atoms and welding conditions. The newly formed isolated structures and solid solution may be related to the local diffusion induced by the preheating effect on the nugget zone. Based on the EDS analysis results, typical intermetallic were observed in the lower part of the weld, including AlCu (D), Al<sub>2</sub>Cu<sub>3</sub> (C), and Al<sub>4</sub>Cu<sub>9</sub> (E).

## 3.3 Mechanical properties.

**3.3.1 Micro hardness distributions.** The results of the dissimilar Al/Cu joints hardness test can be observed in Fig. 4.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Hardness profiles along the traverse cross section of Al/Cu joint (ω=1200, v=50) at three different preheating temperatures</figcaption></figure>

The above values signify different welding states including constant traverse and rotational speed, and varied preheating conditions. The results suggest that in contrast to the base metal a significant increase in hardness is observed in the SZ for the following reasons: (i) the presence of concentric grains with intensely refined recrystallization and (ii) the presence of intermetallic compounds. TMAZ has less hardness compared to the SZ, but due to the presence of different grading, it enjoys more hardness compared to the base metal and the HAZ zone. Observations are indicative of the fact that, by the application of preheating conditions, the rate of hardness significantly increases in the stir zone, and reaches its maximum value at 75 degrees centigrade. One of the mechanisms increasing the hardness and strength of the materials is precipitation hardening. In this mechanism, some of the alloying elements combine with each other and form fine precipitates. These precipitates act as strong obstacles to the movements of dislocations and increase the deformation resistance of material. Accordingly, hardness and strength of the material will be increased. In the present study, intermetallic compounds and the precipitates are moved to the grain boundaries during the welding process. This phenomenon may result in locking of grain boundaries and consequently decrease of grain size. This grain refinement can improve the mechanical properties of welds. Locking of grain boundaries decreases the grain growth and grain boundary movement during the plastic deformation, which in turn delays the occurrence of recrystallization. Moreover, in this condition, the recrystallization temperature is probably higher than the temperature at which the precipitates are formed. Another possible explanation is that the recrystallization process needs noticeable amount of time, which is not provided during the welding process. Preheating of the specimens at 75 °C provides the driving force for precipitates to move to the metallurgical defects such as grain boundaries and prevent the grain growth during the welding process. Therefore, the joint strength will be increased due to the formation of finer grain by increasing of the preheating temperature from 75 to 125 °C.

**3.3.2 Tensile strength.** Tensile test results are presented in Fig. 5 according to the applied travel speed, rotational speed and preheating temperature. By considering to the tensile test results given in Fig. 5, it can be found that the maximum strength for the specimens welded at room temperature is related to the rotation and travel speeds of 1300 rpm and 100 mm/min, respectively. Fig. 5 shows the effect of traverse speed on UTS. Welding speed has an inverse effect on tensile strength. Lower
welding speed leads to greater amount of heat supplied to materials and therefore, it improves plastic
deformation of the material and the formation of the effective weld joint. However, increase in heat
caused loss of strength. Different behaviors of aluminum alloys and pure copper toward preheating
can be articulated as the main reason of this phenomenon. Because in a condition that heat input is
suitable for copper, aluminum alloy shows negative reaction to this heat or in the similar case, when
the generated heat to the welding area is appropriate for aluminum alloy, this amount of heat is
inadequate for copper stir and consequently suitable joint is not achieved. As a result, for dissimilar
joining of 6061 aluminum alloy and pure copper, a mediate condition toward different behaviors of
welding alloys should be considered. It was found from the result that the welding speed have more
signification effect on tensile strength.

<figure><img src="image_7.png" /><figcaption>Fig. 5. Tensile test result for dissimilar Al/Cu joints</figcaption></figure>
According to previous researches, ratio of tool rotation speed to its travel speed plays an important role on turbulence of the plasticized material and consequently on microstructure and mechanical properties of joints due to having direct impact on rates of generated heat. That is why this parameter is known as one of the most important process parameters [17]. Increase in $\omega/v$ ratio causes the decrease of tool advancing length in each rotation. In another word, in a specified length of joint, tool will have more rotation number. This issue will have an especial effect on materials flux in stir zone. In fact, stir of materials will be increased by decreasing the advancing speed and increasing the tool rotation speed. Changes in $\omega/v$ ratio in welding process will have a considerable impact on temperature peak and also its distribution. In fact, this is an important factor in microstructural changes and thereby the eventual properties of these kinds of joints. As a result, changes in $\omega/v$ variable are along with changes in joint quality.

The reasons of these changes can be justified in a way that reduction of $\omega/v$ ratio will lead to decrease in amount of heat interred in welding zone. Because the tool rotation number in one millimeter of weld will be decreased and so, friction heat and heat derived from plastic work (due to strain decreasing derived from $\omega$ decrease) will be reduced. In this case, material flux around the tool will be decreased and extent of turbulence zone will be reduced.

**Summary.** In this study, effect of preheating on mechanical properties of dissimilar friction stir welded Al/Cu joints have been investigated. The following results were obtained:

* Preheating of Al/Cu dissimilar joints improved the mechanical properties of the welds.

* The hardness at the copper side of the nugget was higher than that at the aluminum side and preheating has an effective role to improve it as well.

* Based on the EDS analysis results, typical intermetallic were observed in the lower part of the weld, including AlCu, Al₂Cu₃ and Al₄Cu₉.

* WFSW process is a novel method to introduce metallurgical and mechanical changes in materials because of the uniform distribution of the particles in the stir zone.

* Full stir of aluminum and copper was observed at the interface. Due to the presence of aluminum particles in copper on the advancing side, flow of materials is upward on the bottom of the welded plates, while it is downward in the retreating side.

**References**

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