3.1 Macrostructure analysis
The keyhole and flash are common phenomena during the FSSW process observed in all specimens. Figure 2 shows the cross-section morphologies with different magnification images of the joint at different rotational speeds and dwell times. In the stir zone (SZ), owing to the extrusion and stirring implemented by the pin tool, a Cu hook has been extruded upward from the lower copper sheet into the upper aluminum sheet (known as Cu-ring). This Cu-ring leads to interlocking between the two materials and results in adherence of dissimilar alloys during the joining process, and consequently, reaches a high strength before failure during tensile testing [27]. Based on the rotational speed and dwelling time, the shape of the hook changed during the joining process. Table. 2 presents the width of the weld zone of Al-Cu dissimilar welding under different rotational speeds and dwell times. At a dwell time of 2 s with a rotational speed of 1300 rpm, metals are not able to be fully softened and therefore only plastic flow happened in a small scope, leading to a small stir zone (SZ). The size of the weld zone expanded as rotational speed and dwell time improved (Fig. 2b and 2c). Wang et al. [28] showed that rotational speed is a prominent role to play in the thermal cycles of the joints and resulting in a wider weld zone. With the increase in rotational speed and dwell time, the amount of nanoparticle and copper elements in the stir zone increases resulting in the formation of laminated bands of wider size in the stir zones. Figure 2(d) briefly discusses the geometric parameters of the hook during the welding process under various conditions. As observed in this figure, hook height (HH) indicates the distance from the tip of the hook to the interface layer of the joint. A fully bonded region (FBR) is related to the horizontal distance between the margin of the exit hole and the tip of the hook. The ratio of hook height to FBR is signified as HH/FBR, and the larger HH/FBR the steeper copper hook penetrated into the aluminum plate resulting in a stronger interlocking between the dissimilar alloys. Table. 3 presents the quantitative analyses of the geometric parameters of the hook at various rotational speeds and dwell times. It is observed from the data that the geometric parameters increase as rotational speed and dwell time increase because of the high heat input, thermomechanical flow, and heterogeneous mixing. In addition, the effect of rotational speed compared to the dwelling time is obvious due to higher heat input from the rotating tool during the joining process. However, the HH/FBR dropped dramatically as the rotational speed and dwell time further increased to 1900 rpm and 6 s, respectively because of the curling of the hook [29].
Table 2
The width of the joining zone for different rotational speeds and dwell time.
Dwell time : 2 (s) | Dwell time : 4 (s) | Dwell time : 6 (s) |
Rotational speed (rpm) | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 |
Width (µm) | 552 | 667 | 686 | 862 | 873 | 1112 | 1400 | 1810 | 2021 |
Table 3
Geometric parameters of the joints at various rotational speeds and dwell time.
Joining parameters | HH (µm) | FBR (µm) | HH/FBR |
Rotational speed (rpm) | 1300 1600 1900 | 356.12 572.84 317.81 | 1211.63 1426.83 1026.47 | 0.2939 0.4014 0.3096 |
Dwelling time (s) | 2 4 6 | 356.12 519.23 304.26 | 1211.63 1364.95 983.84 | 0.2939 0.3804 0.3092 |
The top surface morphology of the weld joints with various rotation speeds and welding times is shown in Fig. 3. The quality of the weld zone in all samples is acceptable due to thermal conductivity inside the joint region improved, hence the forming property of the joint improves due to an increase in heat conduction. Furthermore, all top surfaces of joints have a shallow spot weld indentation in the stir zone (SZ) indicating the lowest position of the topography and a distinct flash of certain height at the outer edges of the joints. Due to the impression of the shoulder on the surface during the FSSW process, the formation of the flash is common. It has been accepted that elevated temperatures and high plastic deformation occurred as the heat input is increased during the joining process, resulting in the outward extension of the weld zone [30–32]. As shown in Fig. 3, the volume of the flash improved with rotational speed and joining time because of high friction heat and plastic strain. In terms of material flow, the materials at the top surface of the sheet (aluminum) flow in the tool rotation direction, and then this flow behavior has been transmitted downwards, consequently, a similar flow behavior happened in the bottom material (copper). During the welding with low heat input (rotational speed of 1300 and dwelling time of 2), the material flow is not sufficient to mix and joint dissimilar alloys owing to the high thermal conductivity of copper [33], consequently, the copper does not produce enough heat to soften the aluminum to generate sound joint. The intensity and area of the material flow enhanced as the underlying materials moved which have been driven by the materials above with the improvement of rotational speed and dwelling time.
3.2 Microstructure
It has been known that the input heat plays a vital role in the grain softening or grain hardening during the welding process. The grains undergo grain hardening or freezing and softening if the amount of heat generated is less and high, respectively [34–36]. Worth mentioning is that the quality weld of aluminum and copper dissimilar joints is determined not only by the macrostructure behavior but also by the interior quality of the joint. The optical microstructures of the stir zone (SZ) in the Al-Cu dissimilar joints are shown for the aluminum side and copper side of the weld in Figs. 4 and 5 under different welding conditions. Tables 4 and 5 summarized the grain size of the weld zone for three certain regions including the heat-affected zone (HAZ), thermos-mechanically affected zone (TMAZ), and stir zone (SZ) for both aluminum and copper sides of Al-Cu dissimilar welding under different joint conditions. It is obvious from figures. 4 and 5 that the grains in the stir zone increase as rotational speed and dwell time increase. It should be mentioned that FSW/FSSW is assumed to be a hot deformation process because of the presence of heat and deformation simultaneously during the welding. Therefore, the restoration mechanism of dynamic recrystallization is common during FSW/FSSW of various metals and alloys [37–39]. During this condition, low angle grain boundaries (LAGBs), because of severe plastic deformation and high heat generated by welding rotational tool changed to high angle grain boundaries (HAGBs) during the FSSW process. Therefore, the nucleation of new grain in preferred places in the matrix is possible resulting in grain size reduction [40–43]. Worth mentioning is that reinforcing particles play a significant role in microstructure refinement. That is, SiC nanoparticles restrict the movement of grain boundaries by pinning effect mechanism resulting in a reduction in grain growth. Based on the movement rate of grain boundary (V) equation [44]:
$$\begin{gathered} V=M\exp \left( { - \frac{Q}{{{R_g}T}}} \right)\left( {{P_{dis}} - {P_z} \pm {P_c}} \right) \hfill \\ {P_z}=\frac{{3{f_\upsilon }{\gamma _{AB}}}}{{4r}} \hfill \\ \end{gathered}$$
1
where M is a constant, Q is related to the apparent activation energy, T is the temperature, Pdis indicates the required force with respect to recrystallization and is evaluated as a function of Burgers vector (b), shear modulus (G), and dislocation density (q) as 0.5 qGb2, Pz is related to the Zener pinning pressure, Pc is associated with the geometrical required force owing to the curvature of the grain boundaries, fv and r are respectively related to volume fracture and radius of the particle, and γAB indicates the grain boundary tension. Karthikeyan and Mahadevan [45] studied the role of SiC nanoparticle addition in the joint zone during the FSW process. They showed that the nanoparticle restricts the growth of grain boundary by pinning and leads to an improvement in the mechanical characteristics to an extent of 33% in the as-welded condition over plain alloy. In addition, it has been known that reinforcing particles break up the initial grains into fine grains [46–48]. A similar mechanism with respect to the addition of SiC nanoparticles is observed in dissimilar Al-Mg joint alloys during the friction stir welding process [49]. SiC nanoparticles prevent the movement of the dislocation and grain boundary by pinning effect and also improved the recrystallization process owing to the development of nucleation sites. Regarding heat generation, owing to the friction between the rotating tool and workpiece as the main source of heat, the highest amount of heat is generated resulting in the annealing of metals. Consequently, the grain size increases in the stir zone. In other words, the heat input is at a high level while the cooling rate is at a low level as rotational speed and dwell time are increased, so there is sufficient energy and time with respect to recrystallized grains to grow.
From the distribution of nanoparticles, the tendency of SiC particles to agglomeration, owing to the high surface region, is high resulting in a reduction of their overall energy. The occurrence of agglomeration leads to an increase in inter-particle spacing to generate pores [50]. As observed in Fig. 4, the distribution of SiC in the matrix improves as rotational speed and dwell time decrease due to adequate input heat and time for break-up and interaction of reinforcing particles and matrix. These large reinforcing agglomerates are not able to pin the grain boundaries, consequently, they have no effective influence on the grain refinement of microstructure. Similar results were reported in the welding of dissimilar alloys with SiC nanoparticles by Moradi et al. [51]. They indicated the fact that there is a homogenous mixing action between SiC reinforcing particles with aluminum matrix without a cluster of nanoparticles due to the used optimized welding parameters.
Table 4
The grain size of three main zones of weld joint on Al side after welding.
Dwell time : 2 (s) | Dwell time : 4 (s) | Dwell time : 6 (s) |
Rotational speed (rpm) | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 |
Grain size (µm) | SZ TMAZ HAZ | 14.5 38.8 62.05 | 14.6 39.7 63.05 | 15.2 40.04 63.8 | 14.6 39.1 62.7 | 14.9 40.12 63.05 | 15.8 43.3 64.12 | 14.9 40.5 63.8 | 16.1 42.1 64.0 | 16.8 45.4 65.21 |
Table 5
The grain size of three main zones of weld joint on Cu side after welding.
Dwell time : 2 (s) | Dwell time : 4 (s) | Dwell time : 6 (s) |
Rotational speed (rpm) | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 | 1300 | 1600 | 1900 |
Grain size (µm) | SZ TMAZ HAZ | 10.5 35.8 39.05 | 11.6 36.7 40.5 | 12.2 37.04 43.09 | 11.02 36.2 41.7 | 11.8 37.1 42.5 | 12.7 39.3 44.1 | 11.9 37.5 42.4 | 13.2 38.2 43.0 | 14.4 41.2 45.1 |
3.3 Intermetallic compounds (IMCs) formation
The formation of brittle and hard intermetallic compounds (IMCs) layers in the FSSW process of dissimilar aluminum alloy and pure copper has been considered one of the most critical factors affecting the mechanical performance of joints [52]. In this condition, detachment of copper pieces from the copper alloy and distribution with aluminum matrix results in the formation of IMCs by the solid-state diffusion in the stir zone. Worth noticing is that temperature, strain rate, welding parameters, chemical compositions, and time have been considered the fundamental parameters in the formation of such [53–55]. Zuo et al. [56] investigated the role of IMCs formation in mechanical behaviors of weld joints during the dissimilar Al-Cu joint. It was found that three certain reaction layers namely AlCu, AlCu2, and Al2Cu/α-Al eutectic layer are formed at the Cu/Al welding interface. In addition, the thickness of the Al2Cu/α-Al eutectic layer increases, unlike the thickness of the continuous Al2Cu layer, as ultrasonic vibration is applied during the joining process. Xue et al. [57] optimized the joining parameters and indicated that in order to achieve a sound joint, a thin and continuous IMC layer at the interface joint is required. SEM images for the Al-Cu interface at the various rotational time and dwell times are shown in Figs. 6–8. It has been observed that the thickness of IMCs during the FSSW process increases as rotational speed and dwell time increase due to high interaction between Al and Cu materials. It should be noticed that a proper thickness joint is related to sufficient fluidity and stir in the weld zone resulting in good wettability of the weld layer on the Al-Cu substrates. Therefore, this phenomenon enhances the diffusion of Al and Cu atoms at the solid interfaces to fabricate strong Al-Cu metallurgical joints. The micro-crack at the interface joint of Al-Cu dissimilar welding is observed at high dwell time and rotational speed. This phenomenon may be associated with the dissimilarity in thermal expansion coefficients between IMCs and matrix resulting in incongruous deformation during the cooling process [58–60]. Liu et al. [61] indicated that the continued intermetallic compounds layers can lead to fast crack propagation during mechanical studies such as tensile shear tests. The EDS analysis results for possible IMCs in different zones are presented in Table. 6. Moreover, the XRD outcomes from the cross-section of the relevant weld samples are shown in Fig. 9 which verifies the EDS data. It is clear that the expanding of the IMC layer at the weld interface under high rotational speed and dwell time may be associated with the combined influences of higher heat input and severe plastic deformation. In all, the increase in the IMC formation at the joint interface shows variant trends of behaviors. Higher aluminum contents detected in the EDX spot examination of the weld interfaces could be owing to various factors such as the lower melting point of aluminum alloys compared to the copper alloys and welding tool pin offset towards the softer materials (aluminum) [62]. From Table. 6, the content of Si particles increases in the interface zone as the rational speed and dwell time increase during the FSSW process. This phenomenon may be attributed to the temperature effect on particle agglomeration. Bagheri et al. [63] indicated the fact that the SiC particles show a high tendency to formation of agglomeration at high-temperature values. It is proper to say that constant variation in different variables including time, complex grain nature, and temperature severely influences the formation of intermetallic compounds and expansion, and hence, transform the possible IMC [64]. Furthermore, the EDX spot analysis and EDX line scan are two distinct techniques applied in recognizing local chemical contents and elemental distribution of the aluminum and copper at the interfaces, respectively. However, the fundamental discrepancies in the joint interface depend on the thickness of the expanded IMC layer at each dwell time and rotational speed. Al-rich IMC layers improved in all welding conditions are AlCu and Al2Cu, while in the interface region, Al2Cu is the dominant IMC layer. Worth mentioning is that the formation of Al2Cu happens wherever adequate atomic concentrations have been locally achieved which is an important issue in terms of the therrmomechanically induced solid-state diffusion process. It has been also known as the most important IMCs layer in the Al-Cu system [65]. Based on results in Fig. 9, no Cu9Al4 was detected in the aluminum-copper interface under various welding conditions, unlike the previous studies. Bhattacharya et al. [66] reported the formation of this phase in the interface joint during FSW of HCP Cu and AA6063 dissimilar alloys. They found that the formation of the Cu9Al4 phase happened at high-peak temperature with high formation energy compared with CuAl2 and CuAl phases. Ouyang et al. [67] reported the occurrence of this phenomenon at the bottom of the stir zone due to mechanical integration. However, the absence of Cu9Al4 formation in this study can be associated with two critical reasons: Firstly, the heat input in Al-Cu alloys is exactly enough to plasticize the materials; and secondly, the peak temperature value under all welding conditions is lower than the formation temperature of this phase. These outcomes are in accordance with results reported by Fei et al. [68].
Table 6
The EDS analysis results for possible IMCs in different zones detected in Figs. 6–8.
Rotational speed : 1300 (rpm) | Rotational speed : 1600 (rpm) | Rotational speed : 1900 (rpm) |
Dwell time (s)/Point | 2 | 4 | 6 | 2 | 4 | 6 | 2 | 4 | 6 |
P1 | Al (%wt) Cu (%wt) Si (%wt) | 87.12 4.61 8.27 | 84.18 5.03 10.78 | 82.73 5.24 12.03 | 84.09 5.02 10.89 | 83.26 5.53 11.21 | 80.03 6.73 13.24 | 81.63 6.04 12.33 | 79.78 6.63 13.59 | 77.12 7.04 15.84 |
P1 | Al (%wt) Cu (%wt) Si (%wt) | 39.12 35.63 25.25 | 38.38 34.89 26.73 | 37.63 34.12 28.25 | 41.88 37.52 20.06 | 39.52 36.29 24.19 | 38.12 35.34 26.54 | 40.89 38.02 21.09 | 40.03 37.58 22.39 | 39.03 36.47 24.50 |
P3 | Al (%wt) Cu (%wt) Si (%wt) | 8.16 81.26 10.58 | 8.89 79.47 11.64 | 9.11 77.31 13.58 | 8.67 79.18 12.15 | 9.28 76.25 14.47 | 12.88 73.49 13.63 | 11.26 78.52 10.22 | 11.87 76.11 12.02 | 11.96 76.49 11.55 |
Fundamentally, Eq. (2) has been used to measure the thickness of the IMCs layer during thermal processing [69]:
W = ktn (2)
where W relates to the intermetallic thickness, k is the growth rate constant, n indicates the time exponent, and t expresses the thermal processing time. Several studies have reported that the IMCs show two distinct forms of solid-phase growth [70]. The first has been known the linear growth and the other has been known the parabolic growth. The linear growth rate is associated with the reaction rate of the growth interface. Parabolic growth is dependent on bulk diffusion. For IMCs layers with the parabolic growth mechanism, the value of n is 0.5 [71]. Figure 10 shows the variation of the interfacial IMCs layers thickness and void volume percentage regarding rotational speeds and dwell time for joint interfaces. It is clear from the results that the thickness of IMCs and void volume percentage in joints increase with an improvement in the rotational speed and dwell time. With the increase in rotational speed, the friction between the shoulder and the welding sample increases, therefore due to higher heat, the fluidity of the material in the interface increases resulting in acceleration of the atom diffusion to form IMCs layers. Given the long interaction between the FSSW tool and the workpiece at a high dwelling time, the frictional heat enhances causing the formation of large IMCs and stick of them to the welding tool or the workpiece. In other words, a higher welding temperature because of high rotational speed leads to the formation of a high-thick reaction layer.
3.4 Mechanical properties
Table. 7 presents the shear test results of joint samples under different welding conditions. According to these results, the shear strength of weld specimens increases as rotational speed and dwell time increase. This phenomenon may be associated with the formation of IMCs layers, as well as reinforcing particles’ role as mentioned in the microstructural evolution section. Worth mentioning that reinforcing particles in the formation of the IMCs layer are vital to the strength of weld samples. That is, the interaction between reinforcing particles and base metals, degree of particle distribution, and agglomeration in the matrix, as well as the interfacial bond between reinforcing particles and base metals, influences the strength properties [72, 73]. Adding the SiC nano-particle reinforcing leads to the transfer of the applied tension from the weak matrix into the strong reinforcing particle, resulting in an improvement of the strength. In other words, due to an incoherent reinforcement-matrix interface, load-bearing capacity could be improved between the base matrix and reinforcing agent in the composite structure. With increasing the input heat by increasing rotational speed or dwell time, the required energy for the formation of brittle intermetallic compounds is supplied. The IMC layer, at the lowest rotational speed and dwell time, formed discontinuously with low thickness at the joint interface, resulting in the weak metallurgical combination of the intermetallic. This phenomenon leads to a decrease in the tensile shear failure load of the joint. The continuous IMC layer with high thickness is formed as rotational speed and dwell time increase due to high input heat. Therefore, the weld zone is mechanically and metallurgically bonded which is studied in previous studies [74, 75]. In addition, SiC nanoparticle has been considered a prominent factor in impact energy absorption and dislocation destiny in weld samples due to mismatch in thermal coefficient [76].
The relationship between tensile shear failure load and the heat generation for weld specimens under different rotational speeds and dwell time is presented in Fig. 12. As shown, the load tensile shear increase as the heat generation is increased. This can be associated with the heat generation affected by the formation of the IMCs layer. These results indicate the fact that the range of used values for welding parameters is optimal with high mechanical properties. That is, the formation and thickness of IMCs at the joint interface do not show a negative effect on the quality of weld samples and play a positive role in the mechanical properties of joint specimens during the FSSW of Al-Cu alloys.
Table. 7 presents the microhardness profile of weld samples under different rotational speeds and dwell times. As seen, the microhardness values of joints increase as rotational speed and dwell time increase. It has been known that hardness mainly depends on the intermetallic formation, grain size, and density of dislocations in the weld region [76]. To be more specific, based on the Hall − Petch equation, there is a relationship between grain size and the microhardness profile of the joining zone to generate the dislocations because of the discrepancies between thermal expansion coefficients of base metal and reinforcing nano-particles. The introduction of reinforcing nano-particles due to the hard nature of SiC particles as well as the pinning effect of the grain boundaries results in an increase in microhardness. From the IMCs aspect, the high input heat due to an increase in rotational speed and dwell time, the formation of the IMCs is enhanced, so the microhardness is increased [77]. The stir zone, due to a high level of materials mixing between different elements and the formation of IMCs because of high-temperature values, shows higher hardness behaviors compared to two other regions (TMAZ and HAZ). Furthermore, large bulky copper fragments in the center zone of the weld region are reported by Mehta et al. [10] in which the level of severe plastic deformation, as well as strain hardening during the thermomechanical process, is high. It can be concluded from mechanical behaviors of weld samples with various rotational speeds and dwell time that the formation and thickness of continuous IMCs layer have a more prominent impact on the strength and hardness of the weld zone.
Table 7
Hardness profile of joint samples under different rotational speeds and dwell time. A) 1300 rpm, b) 1600 rpm, and c) 1900 rpm.
Rotational speed : 1300 (rpm) | Rotational speed : 1600 (rpm) | Rotational speed : 1900 (rpm) |
Dwell time (s) | 2 | 4 | 6 | 2 | 4 | 6 | 2 | 4 | 6 |
Hardness (HV) | SZ TMAZ HAZ | 122 ± 2 95 ± 2 74 ± 2 | 126 ± 2 98 ± 2 77 ± 2 | 132 ± 2 105 ± 2 82 ± 2 | 127 ± 2 115 ± 2 91 ± 2 | 133 ± 2 121 ± 2 98 ± 2 | 142 ± 2 134 ± 2 123 ± 2 | 131 ± 2 123 ± 2 104 ± 2 | 149 ± 2 131 ± 2 119 ± 2 | 156 ± 2 143 ± 2 125 ± 2 |