The experimental study of CFRP interlayer of dissimilar joint AA7075-T651/Ti-6Al-4V alloys by friction stir spot welding on mechanical and microstructural properties

1 Introduction

The demand of lightweight alloys in automotive and aerospace industries increases because of global resources and environment problems tending to be more and more serious; therefore more attention is paid on weight reduction. The use of lightweight alloys reduces the carbon emission and improves the fuel efficiency [1]. In automotive sector, the main aim for future years is to reduce the fuel consumption and consequently the harmful gas emission [2]. The 10% reduction in vehicle weight leads to reduce 5.5% of fuel consumption, i.e., one-pound reduction in vehicle weight can reduce 20-pound carbon dioxide over a life period of a vehicle [3]. Therefore, the requirement of lightweight alloys like aluminum, magnesium, titanium, and aluminum matrix composite increased instead of only steel alloys to reduce the weight [4]. Weight reduction is a critical challenge in automotive and aerospace industries to improve performance [5]. Nowadays, composites are playing a vital role in the modern society to fulfil the increasing demand of structural materials [6]. Carbon fiber belongs to the category of carbon material, which are considered very important reinforcement in advanced composite [7] due to their mechanical properties [8]. Carbon fiber-reinforced polymer (CFRP) is an ideal material for the designing and construction of hybrid structure of metal and composite [9]. Carbon fiber/aluminum composite have better strength and wear resistance in contrast of unreinforced aluminum matrix [10]. Carbon-based material can be added into nanocomposite to improve thermal conductivity [11], whereas carbon nanotubes are suitable for electrical and thermal application [12]. Porous composite material are widely used in energy absorbing system, sound absorber, heat exchanger, and construction materials due to its damage tolerance [13]. Recently, composite materials have prompt the worldwide investigation to manufacture advanced structure with superior mechanical properties [14]. CFRP composite exhibits excellent mechanical characteristics in terms of high level of stiffness-to-weight ratio and strength-to-weight ratio [15]. Consequently, CFRP composite are widely admire in many manufacturing sector application, such as aerospace, medical services, aeronautics, and automotive industries [16]. Conversely, laminated composite material has some issues such as matrix cracking, interlayer stress component, and interfacial debonding [17]. Therefore, only carbon composite material can be used to design a lightweight structure, which shows high performance in engineering applications [18]. However, carbon fiber reinforced are being used in concrete where it shows better fracture toughness and the enhanced compressive strength of the concrete [19]. At the elevated temperature, composite lacked the damage tolerance and it paved the way to nanocomposite [20]. The lack of ductility is a greatest concern in CFRP materials when used as prestressing reinforcements [21]. Application of CFRP in transportation industry, where energy efficiency is required, are rapidly growing due to inherent capacity to reduce the weight of an aircraft or car. It has outstanding corrosion resistance, environmental stability, and high fatigue performance which make it attractive for many industries [22]. Filament-wounded CFRP is the most effective solution for the high-pressure storage vessel [23]. Conversely, lightweight alloys like aluminum and titanium alloys play a vital role in automobile and aerospace industries: aluminum alloys reduce the weight and cost and titanium alloys enhance the strength, corrosion resistance, and high temperature workability [24].To use these materials for structure construction, a welding procedure is required. However, welding of these materials by conventional fusion welding is not possible because of high welding temperature [25]. In automotive industry, spot welding is a most well-known process, although welding of light alloys faces metallurgical problems [26]. A few decades ago, resistance spot welding (RSW) was the primary method of spot welding of aluminum sheet despite the excessive porosity and surface indentation, which severely affects the strength and fatigue performance of the weld joint. Higher electric power energy is required for the RSW of aluminum alloys, which restricts its extensive applications [27]. However, the joining of aluminum and titanium is a challenging process because of a huge difference in chemical, physical, and metallurgical properties [24].

Friction stir spot welding (FSSW) is a variant of linear friction stir welding (FSW) developed and carried out in the automotive industry as a substitute of resistance spot welding (RSW). FSW is a novel welding process invented and patented by TWI in 1991 [28]. Friction stir welding of aluminum alloys retains good mechanical and metallurgical properties [29]. Mazda Motor Corporation was the first company to report the use of FSSW in automotive assembly line in the production of rear door and bonnet of Mazda RX-8 in 2003 [30]. FSSW process has the capacity to eradicate the traditional fusion welding problems of low-density alloys like copper, aluminum, magnesium, titanium, and even metal matrix composites [31]. Some other solid-state joining processes were developed to joint dissimilar alloys, such as ultrasonic welding [32], laser welding [33], friction welding (FW) [34], FSW [35], friction stir interlocking [36], and FSSW [37]. In these methods, the joining proceeds at the solid state instead of the liquid state, and the formation of intermetallic compound (IMC) expected to be reduced because of slow diffusion. In friction stir welding of dissimilar aluminum and titanium alloys, different types of IMC formation were detected, namely, TiAl₃, TiAl, and Ti₃Al in AA6061/Ti-6Al-4V joint [34] and AA2024/Ti-6Al-4V alloy joint [38]. Plaine et al. [39] apply the friction stir spot weld on AA5754 and Ti-6Al-4V alloys and study the influence of dwell time on the microstructure of the interface and lap shear strength of the welded joint. On the interface, mainly TiAl₃ (IMC) layers were found. Dwell time has a significant influence on the diffusion process of FSSW, and it affects the mechanical properties of the joints. The optimizing thickness of the TiAl₃ layer is a key issue to obtain high-strength Ti/Al dissimilar weld joints. High dwell time resulted in an excessive increase of IMC thickness, which decreases the mechanical properties of the weld joint. Chen and Nakata [40] welded dissimilar ADC12 alloy and pure Ti by the friction stir welding technology. The maximum failure load of the lap joint reached 62% that of ADC12 Al alloy base metal. TiAl₃ intermetallic phase is formed at the weld interface due to the Al-Ti diffusion reaction, which strongly depends on the welding speed of the process and thus affects the mechanical properties of the joints. Choi et al. [41] studied the relationship between the microstructure of Ti and Al and IMC layers generated at the weld interface, mechanical properties, and fracture behavior of pure Ti and Al joint welded by FSSW. This finding shows that the increase of the probe offset and the rotational speed increased the thickness of the IMC layers. The formation of the IMC layer is TiAl₃, TiAl, and Ti₃Al. The TiAl₃ compound layer is formed at the lowest free energy, and the formation of TiAl and Ti₃Al is attributed to the Al diffusion from the TiAl₃ layer. Cheepu et al. [42] applied aluminum in FW of steel to titanium as an interlayer and investigated the mechanical properties of the weld joint. The obtained friction weld joint with aluminum inserts between steel and titanium alloy prevents the formation of IMC in the weld interface. The tensile strength of the aluminum inserted weld joint is higher than the direct weld joint.

Cao et al. [43] fabricate a carbon fiber-reinforced AA5052 bulk composite by multiple-pass friction stir processing (FSP) to improve the wear resistance of AA5052 and investigated the mechanical, microstructural, and tribological performance of the composite. The result showed 46.8% increase in the hardness of the composite compared to base metal and showed 18.6% higher tensile strength and 13.0% higher elongation in a composite fabricated at 1,000 rpm and 75 mm/min of welding speed. Goushegir et al. [44] reported the feasibility of friction spot joining of AA2024-T3/CF-PPS to investigate the microstructure of the joint and the process temperature. The maximum lap shear strength up to 31 and 43 MPa was achieved in bare and alclad specimens, respectively. Esteves et al. [45] applied Taguchi and analysis of variance to investigate the effect of process parameters (rotational speed, joining time, plunge depth, and joining force) on the microstructure and mechanical strength of AA6181-T4/CF-PPS double lap shear joints by friction spot joining. The joint showed good mechanical strength, which vary from 2,107 to 3,523 N. The obtained results indicated that the parameters that have more influence on the lap shear strength were rotational speed (34.77%) followed by joining time (32.37%), plunge depth (20.70%), and joining force (12.15%). Ageorges and Ye [46] applied the resistance welding between Al7075-T6 and carbon fiber (CF)-reinforced polyetherimide (PEI) to investigate the strength of a lap shear joints. The obtained results show that the lap shear strength of AA7075-T6/CF-PEI joint were larger than 20 MPa. SEM and atomic force measurement (AFM) revealed that the low welding time resulted in the low lap shear strength and induced incomplete filling of micro-porosity on the aluminum alloy or no bounding, and that the high welding time led to highest lap shear strength and also causes the thermal degradation in this process. Applications of interlayer help to enhance the mechanical properties and prevent the formation of the intermetallic compound during the FSSW process. Interlayer will produce diffusion barrier between the two plates and will deform during welding [42]. The CFRP is used as an interlayer due to its low thermal conductivity than aluminum; therefore, it can act as an obstacle between two plates and can absorb the temperature [47]. Madhusudhan Reddy and Venkata Ramana [48] studied the role of nickel as an interlayer to improve the properties of maraging steel and low alloy steel by FW. The obtained results reveals improvement in hardness, tensile strength, and impact toughness. Shojaei Zoeram and Akbari Mousavi [49] investigated the effect of copper interlayer thickness in dissimilar welding of nitinol shape memory alloy to Ti-6Al-4V by YAG laser beam and observed the effects on microstructure, chemical composition, mechanical properties, and fracture behavior. The results show that the thickness of 75 µm tensile strength and elongation of the joint increased and more increment in thickness resulted in deterioration in mechanical properties. Xu et al. [50] fabricated the joint of Mg-Al-Zn alloy sheet by FSSW with and without Zn interlayer and investigated the influence on microstructure and mechanical properties of the weld joint. The addition of the Zn interlayer weld joint shows a significant improvement in the tensile strength from 2.4 to 4 kN. Kalaf et al. [51] applied CFRP as an interlayer in similar aluminum alloy AA5052 to investigate the effect of the welding parameter (rotational speed and dwell time) on the mechanical properties, joint efficiency, and microstructure of the friction stir spot weld joint. The obtained result reveals that the joint efficiency increased by 39.5% at higher rotational and lower dwell time compared to the lower rotational speed. A significant improvement in hardness (29%) was founded in the key whole at high rotational speed and high dwell time.

In this present study, the aluminum alloy 7075-T651 and titanium alloy Ti-6Al-4V, which are widely used in industries, were welded with the CFRP interlayer as the lap-welded joint by FSSW. The effect of CFRP interlayer and welding parameters on mechanical and microstructural properties weld joint has been investigated. The main objective of this study is to evaluate the effect of CFRP welding parameters (rotational speed and dwell time) and CFRP interlayer on the performance of mechanical properties (tensile shear load and hardness) and microstructural properties (SEM-EDS) of the weld joints. The figure shows the graphical abstract of the process.

2 Experimental procedure

This study is on AA7075-T651, and Ti-6Al-4V alloy plates with 0.5 mm thickness of CFRP as an interlayer were welded by FSSW. This process is shown in Figure 1. The samples were selected according to the JIS Z3136 standard [52] with sample dimensions of 100 × 35 × 4 mm (Figure 2a). All experiments were conducted on semi-automatic milling machine (VERNIER-S.A. 06340-LA TRINITE) model type FV250E. To protect the metal from contaminants that can produce oxides, both faying surfaces of the plates were cleaned with alcohol. Aluminum AA7075-T651 alloy plate was placed at the top because of its low melting temperature which helps in easy penetration compared to titanium Ti-6Al-4V alloy plate. The overlap between the two plates are 35 × 35 mm with the CFRP interlayer. The joint is made at the center of the overlap points, which are marked by cross sign in Figure 2c. The high-strength H-13 steel tools were used with the shoulder diameter of 16 and 5 mm of cylindrical pin diameter and 5.8 mm long pin length (Figure 2a). A special assembly of two steel plates was created with guided rail that has 35 mm width and 2 mm depth at the middle of the upper and lower steel plates. Aluminum and titanium alloys with the CFRP interlayer were laid down in this assembly and then screwed tightly to keep the both plates stick together to avoid any misalignment in specimens during the welding process (Figure 2b). Full factorial design of experiment was conducted with two quantitative levels (rotational speed with three level and dwell time with two level) as welding parameters. These welding parameters were fully randomized by using Minitab 18.1 software (Table 1). The temperature evaluation of the top AA7075-T651 alloy plate was monitored by using (Thermometer PCE-T390) during the FSSW process. The maximum temperature obtained during the process considered a peak temperature on the top surface of metal plate.

Figure 1

Illustration of FSSW process.

Figure 2

(a) Schematic of lap joint and friction stir spot tool (mm), (b) specimen holder, (c) welded sample, and (d) sample for microstructural analysis.

The tensile shear load test was conducted by using universal testing machine (INSTRON 3385H). Specimens were mounted between the jaws, and the load rate was constant with cross head speed of 3 mm/min at ambient temperature for all specimens. Three identical specimens were prepared at each welding parameter. For weld characterization, samples were cut through the center of the weld joint (by using DK 7763 EDM wire cut machine) parallel to the loading direction. After the cutting process, specimens were mounted by using the cold mount method. In this method, 50% epoxy hardener and 50% epoxy resins were mixed together and then poured into the specimen. All mounted specimen’s passes through grinding and polishing processes to obtain mirror face surface. Grinding of all specimens was accomplished by using various numbers of sand papers (220, 320, 500, 1,000, 2,400, and 4,000) to obtain abrasive-free surface. After grinding, all specimens were washed using distilled water. Distilled clean specimen was subjected to the polishing process by using 1 µm grit size particle-impregnated carrier paste until mirror face surface is obtained. To reveal the microstructure of the weld coupons, the samples were etched with the killer reagent H₂O (95 mL) + HNO₃ (2.5 mL) + HCL (1.5 mL) + HF (1 mL). Samples for the micro-hardness test were prepared according to the ASTM E384 standard test method [53]. The micro-hardness values of all experiments were measured by using Vickers test machine (Tukon micro-hardness tester) with Vickers hardness of 0.5 HV at force of 4.903 N at dwell time of 20 s. Multiple indents were made along the cross-section of the weld joint (Figure 2d). Microstructural analysis was conducted by using stereo and optical microscope. Low magnification stereo microscope was used to observe the cross-sectional of the weld joint Figure 6. Olympus optical microscope was used to evaluate the internal structure and micro-cracks of the weld joint at the interface (Figure 7).

3 Result and discussion

3.1 Temperature evaluation

The elevated temperature plays a dominant role in the plastic flow of materials and diversified mixing during the FSSW process of dissimilar metal alloys, as well as the temperature plays a key role in nucleation of IMCs and the growth of these compound because the process of IMC is thermally activated [54]. Thus, the study of thermal history of dissimilar metal alloys during the FSSW welding process has significant importance. In this study, the thermal cycles of two dissimilar metal alloys were measured at the top and the bottoms of sheets. The temperature of these points is based to the selected parameters (rotational speeds and dwell times). The peek temperatures at the rotational speed of 2,000 rpm at the top sheet were 333.8 and 305.5°C at dwell time of 10 and 5 s respectively. The peak temperature was obtained in aluminum plates (Figure 3). The obtained results indicate that high rotational speed led to high heat input that leads to the peak temperature [55]. As the rotational speed increases with respect to the dwell time, temperature increases accordingly in both dwell times. As stated by Goushegir et al. [44] in AA2024-T3 with CF-PPS joint fabricated by friction spot welding, the average obtained peak temperature was between 350 and 400°C. The thermal degradation range of the polymer in the CF-PPS is well below this temperature. As the rotational speed increases, the value of peak temperature increases from 40 to 50°C during the welding process.

Figure 3

Temperature evaluation of Al7075-T651/Ti-6Al-4V alloy at different rotational speeds and dwell times.

3.2 Tensile shear load

The effect of welding parameters on tensile shear load was evaluated by manipulating the rotational speed and dwell time (Figure 4). It has been observed from the previous literature that in dissimilar FSSW welding of aluminum and titanium alloys without interlayer at high rotational speed and high dwell time, the tensile shear load was depreciated. However, with CFRP interlayer, this scenario is totally opposite and maximum tensile shear load was obtained at high rotational speed and high dwell time. At the dwell time of 10 s, the maximum tensile shear load value of the weld joints were 2597.8 and 2147.6 N cross ponding to their rotational speed 2,000 and 1,000 rpm, respectively. At the rotational speed of 2,000 rpm, tensile shear load value was enhanced by 17.3%. Increment in the rotational speed resulted in high plastically deformation zone (PDZ) area due to higher heat input [56]. Between the two plates, high heat input larger PDZ area causes higher adhesion force, which tends to increase the tensile shear load of the joints (see Figure 7d) [44]. Also, high rotational speed and high dwell time generate high heat, which decrease the viscosity of the interlayer; therefore, certain amount of molten interlayer CFRP squeezed out of bonding area as flash during this joining, which tend to establish a sound weld joint. High dwell time improves the joint length, and also high dwell time increases temperature, which increases the volume of the stirring material under the tool that leads to improve the tensile shear load [57]. The second high tensile shear load was obtained at the rotational speed of 1,000 rpm and 10 s of dwell time. Heat generation at 1,000 rpm was lower than 1,400 and 2,000 rpm. However, longer dwell time of 10 s plays an important role in improving the grain structure (grain size) and the joint length of the weld interface, which increase the tensile shear load. High dwell time led to high tensile shear load [58].

Figure 4

Variation in tensile shear load of FSSW joint as a function of rotational speed and dwell time.

However, a low tensile shear load value of 1403.4 N was obtained at the rotational speed of 1,400 rpm and 10 s dwell time due to heat generation, which resulted in the initiation micro-cracks in the periphery of the stir zone of the joint, thereby reducing the tensile shear load. The bond width in dissimilar aluminum-titanium alloy at the hook region plays a significant role in the tensile shear load [32]. According to Aydin et al. [59], lower tensile shear load is due to lower precipitate free zone (PFZ) along the grain boundaries emerged from the over aging effect with higher heat input owing to the higher dwell time. The weld with higher tensile shear load exhibits both brittle and ductile behaviors, while the joint with low tensile shear load value exhibits only brittle behavior [60].

At shorter dwell time of 5 s, the peak tensile shear load results were 1343.3, 986.4, and 1173.3 N at rotational speeds of 1,000, 1,400 and 2,000 rpm, respectively. The revealed results demonstrate that at low rotational speed and low dwell time, high tensile shear load increased compared to other process parameters, as the rotational speed increases, a drop in the tensile shear load was observed at the rotational speed of 1,400 and 2,000 rpm, which was mainly due to lower dwell time. Soften material did not gain enough time to stir and mixed very well at the interface of the weld joint and creates micro-cracks and IMC, which fairly depreciates the tensile shear load (Figure 7(b and c)) [41]. In FSSW of CFRP joint, three primary bonding mechanisms were founded by ref. [61]: at the interface, macro-mechanical interlocking formed during insertion from aluminum sheet to the composite layer, micromechanical interlocking because of the keying effect of aluminum and fiber entrapment, and finally adhesion forces between consolidated molten fiber interlayer and aluminum. This material mixing phenomena in aluminum/CFRP welding were observed by ref. [62], which led increase or decrease of the strength of the lap shear joint with the variation of parameters. Dwell time plays a vital role in the improvement of the tensile shear load. The effect of dwell time was clearly observed at the rotational speed of 2,000 rpm with 54.75% improvement in tensile shear load obtained from shorter dwell 5 s to higher dwell time of 10 s.

3.3 Micro-hardness

Figure 5 shows the micro-hardness profile of dissimilar Al 7075-T651 and Ti-6Al-4V with CFRP interlayer welded joints made at dwell time of 5 and 10 s and the rotational speed of 1,000, 1,400, and 2,000 rpm. Micro-hardness test was carried out after 40 days natural aging. The hardness profile of the weld joint in Al 7075-T651 side exhibits the smooth trend in the thermomechanical affected zone and the stir zone. A minor increase in hardness values was observed in some samples of stir zone areas. Moreover, rotational speed and dwell time imply a significant effect on hardness value. Especially at the rotational speed of 1,000 rpm and dwell time of 10 s, micro-hardness was very low in the thermomechanical affected zone and gradually increases until the stir zone region. The hardness results at shorter dwell time of 5 s were higher than 10 s dwell at rotational speed of 1,000 rpm [63]. The maximum hardness value in AA7075-T651 was obtained at 1,400 rpm at dwell time of 10 s. The hardness values of thermomechanical affected zone were higher than the stir zone due to grain size increment. The stir zone and thermomechanical affected zone are affected by thermomechanical parameters of welding, which include maximum temperature, cooling rate, and deformation [57]. Totally, the opposite trend of hardness result was observed at high rotational speeds of 1,400 and 2,000 rpm compared to the low rotational speed of 1,000 rpm. Higher hardness values were obtained at high dwell time 10 s compared to that at low dwell time of 5 s.

Figure 5

Micro-hardness values of weld interface at different rotational speeds and dwell times.

In Ti-6Al-4V alloy, maximum hardness values of 387.7 and 386.1 HV were obtained under the tool pin region at the rotational speed of 2,000 rpm at dwell time of 5 and 10 s, respectively. There is a minor difference in hardness between 5 and 10 s dwell time. The hardness values were higher under hook of the tool due to the dynamic recrystallization at high temperature, which causes severe plastic deformation [64]. At the rotational speed of 1,400 rpm and dwell time of 10 s, a sharp increment in hardness was observed in the thermomechanical affected zone region. The improvement in hardness is due to IMC, which causes the material to behave brittle [65]. FSSW joint exhibits various microstructural zones from the periphery of keyhole toward the base metal, dynamically recrystallized zone (stir zone), thermo-mechanical affected zone and base metal [66]. Tool rotational speed and dwell time have direct effect on the temperature of the weld joint [57].

3.4 Microstructural analysis

Figure 6 shows the cross-sectional view of welded specimens at different welding parameters evaluated by stereo and optical microscopy. A keyhole is formed due to the withdrawal of the tool after the formation of a weld joint. Two identical weld zones formed at the both sides of the keyhole. In Figure 6(a–c), a partially bonded region called hook is found [67]. It is the geometric defect, which is formed due to the insufficient metallurgical bonding between two sheets [68]. It has been reported that the plunge speed has also effect on hook geometry and tensile shear load of a weld joint [69]. In the conventional FSSW joint, the distance from the keyhole to the tip of the hook in the interface of a joint (including stir zone and hook region) is addressed as the bond width of the weld [70]. The bond width shows the minimum distance between the hook tip and pinhole boundary (Figure 6). The size of the bond width vary at each welding parameters, and also the size of hook region (stir zone and bonded width) [68] increased by increasing the dwell time of the weld joint. Upward material flow increases as the rotational speed increase (Figure 6c). High rotational speed during penetration of a tool pin and dwell time provides a driving force to material to move upward [71]. In the interface of the weld joint, micro-crack is observed along the keyhole. Micro-crack is formed due to incongruous deformation during the cooling process because of variation in the thermal expansion coefficient [72]. Residual stress also significantly affect the weld quality [73]. Aluminum and titanium alloys have low intersolubility during welding and produce brittle IMC, which easily propagate micro-crack at the weld interface, and reduce the tensile shear load of the weld joint [74].

Figure 6

Cross-sectional view of FSSW joint with carbon fiber (a) 1,000 rpm and 5 s dwell time, (b) 1,400 rpm and 10 s dwell time, and (c) 1,400 rpm and 5 s dwell time.

The Al/Ti dissimilar metal hybrid structure has numerous benefits compared to single material to fulfill the requirement of both performance and lightweight. Since in thermal welding, the inter solubility of aluminum and titanium is low, brittle intermetallic easily can form interface cracks, see Figure 7(a and b), which severely degrades the mechanical properties of Al/Ti welded joints [74]. Some additional irregularities were also observed in the near hook periphery due to the propagation of micro-cracks and nonuniform mixing of a material during the welding process. Crack propagation is because of IMC, which is formed at the weld interface [75]. Hook always acts as a failure source of the weld joint. Therefore, we contemplate that the first crack starts from the hook (Figure 6) and then breed through the IMC layer [76]. Both aluminum and titanium are active elements, which formed AlTi₃ [77] and Ti-Al-C (see Figure 9) [78], and other IMCs depends on the titanium-aluminum duality phase diagram [79]. Generally, in dissimilar welding of Al/Ti alloys, the strength of the joint relays on the formation and distribution of IMCs [80].

Figure 7

Optical microscopic results (a) 1,000 rpm 5 s, (b) 1,400 rpm 5 s, (c) 2,000 rpm 5 s, (d) 2,000 rpm 10 s, and (e) 1,400 rpm 10 s.

This article (SEM-EDS) analyzed the tool rotational speed of 2,000 rpm and dwell time of 5 s. Dissimilar metal alloys made a strong joint with the CFRP interlayer. According to Kattner et al. [81], the formation of IMC depends on the function of temperature in the Ti-Al binary system. At high tensile shear load weld joint, the maximum temperature was 333.8°C. In Figure 8, Ti₃Al IMC is seen, which is formed due to the large amount of Ti fragments. Increment in rotational speed increase the amount of Ti fragments in the weld interface which formed Ti₃Al IMC, due to that cracks initiated in the interface of the weld joint causes to reduce the tensile shear load. Also the thickness of IMC increased with the increasing of the rotational speed [41]. Cracks are shown in Figure 7, and the bonded regions are mainly formed in the aluminum side, which is denoted as hook. The flow of titanium toward aluminum makes reaction with aluminum, which produces Ti₃Al IMC [82]. However, aluminum carbide Al₄C₃ was not formed due to the low heat input (Max 333.8°C) during the welding process.

Figure 8

SEM-EDS analysis of friction stir spot welded joint: 2,000 rpm and 5 s dwell time.

Ti and Al are considered very appropriate materials as they provide sustainable strength to weight ratio and volume reduction. But when reacted with composite at the elevated temperature, they show high hardness and wear resistance [83]. Carbon fiber owes high modulus that exhibits good chemical compatibility with the Ti-Al compound. Therefore, carbon fibers may be good reinforcements of Ti-Al intermetallic [84]. Furthermore, the phase diagram of ternary Ti-Al-C IMC confirms the solubility of carbon in the α ₂ phase, which is higher than in the γ phase [85]. Figure 9 shows ternary (Ti-Al-C) IMC, in which carbon comes from the CFRP interlayer, and the mixing of these element occurred due to the high heat input during the FSSW process. Due to the presence of carbon reinforcement in the (Ti-Al-C) compound, the hardness increased by 18.90% in the keyhole of joint rather than in the stir zone and the thermal mechanical affected zone [86]. The presence of more carbon fiber as reinforcement will increase micro-hardness. The addition of carbonaceous particles can improve the strength-to-weight ratio [83]. Also, excessive amount of carbon reduces the distance between aluminum and fiber and increases stress, which reduce the strength of the composite [87]. In ternary Ti-Al-C IMC, carbon with titanium acts as a nucleation site during solidification and thus refines the grain size, which might improve the joint strength [88].

Figure 9

SEM-EDS analysis of friction stir spot welded joint: 2,000 rpm and 5 s dwell time.

4 Conclusion

In this present study, Al 7075-T651/Ti-6Al-4V alloy with CFRP as an interlayer was used to make a joint by the FSSW process. The effect of welding parameters such as rotational speed and dwell time on mechanical and microstructural properties was studied. Based on experimental results, the following conclusion are made.

• The maximum tensile shear load of 2597.8 N was obtained at the rotational speed of 2,000 rpm and dwell time of 10 s, which is 54.75% higher than shorter dwell time 5 s at the same rotational speed. The increase of the tensile shear load is due to heat input that melt the CFRP interlayer that squeezed out of the center, which enhances the bonding strength of the joint.

• The minimum tensile shear load value of 986.4 N increased at the rotational speed of 1,400 rpm at 5 s dwell time. As dwell time increases to 10 s, the tensile shear load values increased by 29.7%, which shows that the dwell time has a significant effect on the tensile shear load. Considerable irregularities (micro-cracks) were observed at the rotational speed of 1,400 rpm, which might be due to dynamic recrystallization of weld joint.

• The hardness value was mainly affected by the tool rotational speed. The maximum hardness value was achieved at the high rotational speed of 2,000 rpm and 5 s dwell time. The increase in hardness was due to the presence of ternary Ti-Al-C IMC. In aluminum side, high hardness was obtained at 1,400 rpm and 10 s dwell time. Higher hardness value causes to decrease the tensile shear load of a weld joint.

• Scanning electron microscopy coupled with energy-dispersive spectroscopy shows the formation of Ti₃Al and Ti-Al-C intermetallic compound, which affects the tensile shear load of a weld joint. The formation of IMC depends on the heat cycle of a weld joint, which can be controlled by the welding parameter (rotational speed and dwell time) of the FSSW process.