Enhancement of Mechanical Properties on Novel Friction Stir Welded Al-Mg-Zn Alloy Joints Reinforced with Nano-SiC Particles

Friction stir welding (FSW) is a solid-state technique used to join Al-Zn-Mg alloys e ﬀ ectively compared with other conventional welding methods. Al-Zn-Mg alloy was processed for welding because they signi ﬁ cantly demanded various engineering applications. A novel method of this research work is to characterize the unique mechanical properties of Al-Zn-Mg alloy reinforced with 1 to 3 wt% of nano silicon carbide (nano-SiC) particles developed by novel interlock friction-stir welding. The process parameters chosen for welding are rotational tool speed 1100rpm, weld speed 25 mm/min, and triangular pin pro ﬁ le. The weld joint properties such as tensile strength, yield strength, and hardness were tested per ASTM standard. The microstructure of weld joints was studied with XRD and optical and scanning electron microscopy. The existence of silica particles in the weld joints and uniformed and homogeneous distribution of the particulates in the weld was veri ﬁ ed by EDS analysis and microstructure. Al-Zn-Mg reinforced with nano-SiC joints has better static properties due to intensive softening in the stir region. Al-Zn-Mg with 3wt% nano-SiC exhibits maximum tensile strength, yield strength, and nugget hardness of 191 MPa, 165 MPa, and 171 HV. Weld microstructures showed a pinning mechanism because nano-SiC particles were used as reinforcement during friction stir welding.


Introduction
Welding is the most widely used fabrication technique in the manufacturing industry. Friction stir welding (FSW) was invented by Thomas WM at TWI UK in 1991 to overcome the fusion welding problems [1]. Kapil and Sharma investigated on few similar and dissimilar materials and identified some unsolved critical issues [2]. Few researchers extend the limits of FSW in welding of different materials such as Al-Mg [3], Cu-Al [4], Al-Cu [5], and plastics [6]. The material flow during the process was analytically studied [7]. Since its invention, the FSW process has been extensively used in various industrial applications, joining current and future advanced materials [8]. In this process, the shoulder/workpiece interface causes the frictional heat to form plastic deformation [9]. Welding parameters impact the material flow and heat generation, which affected the weld strength [10]. The surface roughness of the sample and the oxide layer presence were reported to influence the tensile shear force of the joint [11]. The rotational weld speeds of the tool and pin profile geometry affect the temperature and plastic flow field [12]. The filling stud is a single-stage process where the stud is placed onto the prefabricated hole in the joint. Weld joint strength was increased with an increase in TRS which was studied and reported in dissimilar AA7075-AA2024 Al-alloy FSW joints [13].
The weld quality was evaluated by the hardness of the joint; the relationship between peak temperature and hardness profile at the stir zone was investigated on AA6061-AA7075 Al-alloy joints [14]. The impact of weld speed on joint strength was studied in dissimilar AA6061-T6/AA7075-T6 Al-alloy joints. The welds were fabricated at different weld speeds of 80, 100, and 120 mm/min, where defect-free and superior strength joints were produced at 120 mm/min [15]. Employing lower plunge depth during FSSW produced weaker mechanical interlocking. The gradual rise in the plunge depth produced a stronger FSSW joint. However, a variation in the hook morphology was observed [16]. The noncircular tool profiles were reported to allow a smoother flow of plasticized material around the pin by breaking the oxide particles and producing fine grains at the stir zone [17]. The effects of the number of passes on microstructure and mechanical properties were discussed and reported with an increasing number of pass, the average grain size in the weld zone of aluminium 5083 decreases, and mechanical properties increased [18]. The effect of TRS on the stir zone of dissimilar AA6061/AA7050 Al-alloy joints revealed that the degree of material transfer was influenced by the rotational tool speed [19]. The study report on AA6061/AA5086 Al-alloy revealed that welds fabricated with threaded cylindrical pin profiles exhibit better mechanical properties than welds fabricated with other welds [20]. Dissimilar AA6061/AA5010 Al-alloy welds made with square pin profile exhibit superior strength compared to cylindrical pin profiles [21]. The hardness at the TMAZ-HAZ was observed to be low compared to the other weld regions, irrespective of the changes in the process parameters. The detailed analysis of the weld zone by TEM revealed that the size and shape of the strengthening precipitate influence the microstructure and the microhardness. The adherence of the plasticized material on the pin at the high dwell times [22]. The study reported on dissimilar AA6061/AA2014 revealed that joints fabricated by hybrid square pin profiles exhibit good material flow than other welds [23]. The thread profiles were observed to influence the material flow in the FSW process [24]. The increased upward material flow was observed with the right-hand threaded tool, which resulted in the tunnel defect formation. However, with left-hand threaded tools, excellent tensile properties were obtained due to the increased downward material flow, which resulted in defect-free weld joints with superior tensile strength [25].
Fabrication of different grade materials with FSW is challenging to the researchers and manufacturers. FSW of Al-Mg-Zn alloys was reported to produce stronger weld joints than conventional fusion welding methods by inhibiting the negative influence of intermetallic compound formation due to high heat input. Therefore, the present study was aimed at investigating the influence of nano-SiC particle reinforcement on mechanical and microstructural properties of 2 mm thick dissimilar Al-Mg-Zn alloy FSW interlock weldments. Al-Mg-Zn alloy has been widely employed to fabricate truck frames, aircraft structures, automotive parts, marine and shipbuilding, storage tanks, railroad cars, etc.

Experimental Procedure
Flat plates of Al-Mg-Zn alloy of 2 mm thick with various wt% of nanosilicon carbide particles (1 to 3 wt%) with 10 nm particle size were added in joint lap configuration. Sample numbers with reinforcement wt% are shown in Table 1. Initially, the specimens were chemically treated to remove grease and dirt particles at the weld site. Then, the process is carried out by a friction stir welding machine [26].
The interlock FSW of Al-Mg-Zn alloy was conducted at 1100 RPM, 25 mm/min triangular pin and varying wt% of nano-SiC particles from 1 to 3%. The base metal of 100 × 100 × 2 mm thickness was milled to a depth of 1 mm and width of 35 mm along the longitudinal direction [27]. The aluminium sheet top surface was milled to 0.75 mm width, as shown in Figure 1. Nano-SiC reinforcements were filled in the top groove that intersects to produce a hybrid seal between the welds [28]. The nonconsumable tool material used for this experiment is AISI-H13 tool steel, shown in Figure 2. During the process, a shoulder diameter of 25 mm and a triangular profile pin of 3 mm inscribed in a circle of 8 mm diameter were employed [29]. The tool is plunged to a depth of 3 mm and traversed along the longitudinal direction to a distance of 85 mm to form a weld joint with assistance from frictional heat and plastic deformation [30].
FSW interlock weldments joined at varying wt% of nano-SiC samples are shown in Figure 3. The hardness was tested on a Vickers hardness testing machine with a working load of 110 kgf and a dwell period of 10 sec [31]. Tensile test was conducted as per ASTM E8 on the precision universal testing machine (SHIMADZU) which has a test load of 50 kN at a crosshead velocity of 0.5 mm/min [16]. Microstructure analysis was studied with optical scanning electron microscopy.

Results and Discussion
3.1. X-Ray Diffraction Test. Figure 4 shows the XRD pattern for FSW interlock samples in the heat-affected zone. The Al-Mg-Zn/1.5 wt% nano-SiC Sample 3 Al-Mg-Zn/2.0 wt% nano-SiC Sample 4 Al-Mg-Zn/2.5 wt% nano-SiC Sample 5 Al-Mg-Zn/3.0 wt% nano-SiC 2 Journal of Nanomaterials weld cross section of the joint with the maximum tensile strength is analysed using XRD for different phases across the weld zone. In comparison to the normal diffraction data, the peaks were examined with the existence of various phases. 2θ reflections at 38.01, 52.5, and 75.6°correspond to the Al phase [32]. MgZn 2 and MgSi 2 phase is confirmed from the peaks at 21.45 and 24.32, respectively. The nano-SiC particle peak was observed at 62.03 and 88.6°confirmed with the JCPDS No. 29-1128. Peak intensity was found to vary with Si composition. It is observed that the peak shift was observed with increasing Si wt%.

Temperature Measurement.
During the FSW process, heat was generated from tool-workpiece interaction and the plastic deformation of the Al-Mg-Zn alloy. The temperature profile during the process was measured with the help of an infrared thermometer. The heat generated in the process is responsible for softening the material at the weld region, reducing the thermomechanical stresses on the tool during the process [33].
In the FSW process, the contact area between the pin profile and the weld surfaces was responsible for generating the frictional heat [34].
Q Tot is the total heat input, W; μ is the friction coefficient; P is the axial force, N; ω is the rotational speed; R is the shoulder radius, m; R p is the pin radius, m; and P l is the pin length. The heat input was primarily estimated by using equation 1 as The estimated heat generated of 9546.24 W during the process was calculated by using the following equation. The heat generated during the process was measured in terms of temperature with the help of an IR thermometer. The temperature transferred from NZ to the ends of the weld plates was observed to vary due to the influence of the tool shown in Figure 5. The maximum temperatures were observed during welding to be maximum around the edge of the tool. In the FSW process, the plastically deformed material in the weld zone was experienced by the thermomechanical cycle. The weld zone cross section has a fine-grained structure. In dissimilar material welding, the analysis of microstructure is complex due to the heterogeneous properties of materials. The microstructure obtained shows grains, grain boundaries are shown in Figures 6(a)-6(d) with intermetallic particles, and some grains are elongated. The microstructure analysis has reported that TMAZ consists of coarser and elongated grains because of the heat produced by the tool shoulder during rotation. Uniform distribution is observed in Figures 6(a)-6(d). Figures 7(a)-7(e). The microstructure at the weld zone was asymmetric due to different wt% of the composition of the silicon carbide particles (Figures 7(a) and 7(b)). The elements magnesium and silicon form the Mg 2 Si compound shown as bright regions that do not dissolve in parent materials (Figures 7(c) and 7(d)). This leads to conclude that dark and bright lamellar originates from Al-Mg-Zn alloys. On the other hand, the welds processed with nano-SiC 3 wt% have uniform distribution and good mixing as shown in Figure 7

SEM Analysis. SEM image of the interlock friction stir welded sample with different w.t% of nano-SiC is shown in
3.6. Hardness. Hardness test was conducted on the transverse section of welded samples with a spacing of 5 mm and load of 110 kgf with a dwell period of 10 sec. Al-Mg-Zn with various wt% of nano-SiC samples interlocked with friction stir welding using triangular pin profiles processed at 1100 rpm exhibits differences in hardness along the weld line as shown in Figure 9. The hardness of the weld joint was determined across the weld zone. The hardness variation across the weld zone was observed to possess a W profile, with hardness at nugget higher than thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and parent metal irrespective of changes in wt% of nano-SiC particles. The increased hardness at the nugget is credited to the grain refinement and nano-SiC particles. The intense stirring action during FSW and presence of nano-SiC particles resulted in severe plastic deformation resulting in increased hardness at the nugget. Moreover, nano-SiC particle addition resulted in grain boundary pinning [35]. The minimum hardness value was found at HAZ. The hardness at the HAZ is minimum due to precipitation phenomena [36]. The less hardness observed in the HAZ is due to the fact that IMCs may form Mg 2 Si and less Si compound on the Al-Mg-Zn/1 wt% nano-SiC (sample 1). The maximum hardness values of sample 5 (Al-Mg-Zn/5wt% nano-SiC) observed were 88 to 171 HV from NZ to HAZ. The weld zone hardness varied between 125 and 171 HV.

Tensile Strength.
Tensile test samples were prepared as per the standard of ASTM E8/ASTM B557. The test specimen shape was cut by wire EDM machine to get accurate dimensions shown in Figure 10. The test was conducted on the precision universal testing machine (SHIMADZU) at a test load of 50 kN and a crosshead velocity of 0.5 mm/min. The results observed from the experiment are tabulated in Table 2. The test results revealed that the interlock welded joints failed by tensile and shear mode of failure.
The tensile and yield strength was observed to increase with the wt% of nano-SiC particles. The increase in the 5 Journal of Nanomaterials mechanical strength of the weld joint with the increase in the wt% of nano-SiC reinforcements is credited to the increased material mixing between the base metal and the reinforcement, which led to the accumulation of large amounts of reinforcement particles at the grain boundaries due to decreased dislocation during dynamic recrystallization [35].
The NZ was composed of equiaxed and fine grains, and TMAZ has coarse bent recovered grains. The fracture location is between WN and TMAZ on AS. Maximum UTS and YS values were recorded as 191 MPa and 170 MPa, respectively. As per AWS D17 specifications, the joint efficiency factor for aerospace applications is 0.6 to 0.7,   Journal of Nanomaterials

Conclusions
In this research, Al-Zn-Mg alloy added with 1 to 3 wt% of silicon carbide (nano-SiC) particles was successfully developed by novel interlock friction-stir welding methods. The process parameters chosen for welding are rotational tool speed 1100 rpm, weld speed 25 mm/min, and triangular pin profile. FSW interlock weld samples were characterized, and the following findings were drawn: (i) Al-Zn-Mg/3 wt% nano-SiC (sample 5) shows better yield strength, tensile strength, and hardness (ii) SEM microstructure showed that very fine grains and nano-SiC particles uniformly mix between the base materials (iii) EDAX analysis confirmed the presence of silicon carbide particles in the interlock weld zone (iv) AWSD17 requirement attained with a common efficiency factor of 0.67 (v) The highest tensile strength was reported as 191 MPa, and hardness was reported as 171 HV

Data Availability
The data used to support the findings of this study are included in the article. Should further data or information be required, these are available from the corresponding author upon request.  Journal of Nanomaterials