Investigation of Weld Zone and Fracture Surface of Friction Stir Lap Welded 5052 Aluminum Alloy and 304 Stainless Steel Joints

: Dissimilar material joining of aluminum and steel in the present scenario is an important criterion in the manufacturing industry, especially because of their low weight and technical performance. In the present investigation, AA5052 and SS304 are friction stir welded in lap conﬁguration with di ﬀ erent tilt angles, welding speed, pin depth, and tool rotational speed, with aluminum as the top plate. A maximum of 3.16 kN shear strength was achieved at 2.5 ◦ tilt angle when the penetration depth was 4.3 mm. The shear strength samples were studied for fracture analysis and it was found that fracture of the samples mainly occurred on the aluminum side and the fracture demonstrated both brittle and ductile failure, consisting of quasi-cleavage, trans-granular, and intergranular fracture areas. Field emission scanning electron microscope images at the interfacial region of the weld show that di ﬀ erent intermetallic compounds were formed at various zones of the joint with respect to the change in process parameters. It was observed from energy dispersive spectroscopy that Al-rich intermetallic compounds were formed at the interfacial region of the welded samples. Amongst the process parameters, change in the tilt angle a ﬀ ected the weld zone signiﬁcantly. The thickness of the intermetallic compound (IMC) layer formed with 800 and 1000 rpm at 2.5 ◦ tilt angle was between 2.5 and 3 µ m, which resulted in achieving better joint strength. AlFe, AlFe 3 , Al 13 Fe 4 , and Al 5 Fe 2 were the di ﬀ erent intermetallic compounds detected using X-ray di ﬀ raction with di ﬀ erent process parameters. The hardness of the samples ranged between (300 and 630) HV, which further supports the formation of AlFe and AlFe 3 intermetallic compounds.


Introduction
Manufacturing industries are looking for cost-effective lightweight material production to increase the efficiency of vehicles in different sectors. Dissimilar material welding is one of the methods utilized to produce the needed materials. Friction stir welding (FSW) is one of the technologies that is used to join two different materials from among the likes of aluminum, magnesium, steel, and copper. Different process parameters and their variations have been used to produce defect-free welds to answer the need for dissimilar metal welding in the manufacturing industry. Aluminum and steel welding has opened new doors in the automobile and aerospace industry in reducing the overall weight of the components, which will improve efficiency [1][2][3].
Conventional welding techniques such as laser beam welding and tungsten inert gas welding were also used to join aluminum and steel joints [4,5] but friction stir welding is more advantageous because of its capability of welding the materials with lower heat. In the present scenario [6,7], friction stir experimental setup were according to the Taguchi's optimization of the design of experiments [35]. According to Taguchi's experimental design, the total degrees of freedom (DOF) for each factor with a set of three levels is 2. In the present study, we had four factors with three levels each, so the total DOF was 8. The orthogonal array satisfying the 8 DOF with three levels was L9. The process parameters and their levels for friction stir lap welding of aluminum 5052 and stainless steel 304 are given in Table 1 and the L9 orthogonal array is shown in Table 2. The welded joints were cut in 10 mm × 20 mm dimensions using an electrical discharge machine (EDM, Mitsubishi, Tokyo, Japan wire-cut for the microstructural analysis. Samples were mounted, polished, and then etched with Nital on the steel side and Kellar's reagent on the aluminum side. Etched samples were later analyzed by field emission scanning electron microscopy (FESEM, Zeiss Supra55 VP, Eindhoven, The Netherlands). X-ray diffraction analysis (X'Pert 3 , Malvern Panalytical, Malvern, UK) was conducted with Cu-Kα source radiation with a scan range of 0 • to 80 • to find the different phases of IMCs formed in the weld zone. The shear strength of the weld samples was measured according to ASTM-E8 standards. Fractography analysis of the broken samples at the interfacial region of aluminum and steel was conducted using FESEM. The hardness of the samples was determined using a Vickers hardness testing machine with a dwell time of 20 s and a load of 1000 gf.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 15 experiments [35]. According to Taguchi's experimental design, the total degrees of freedom (DOF) for each factor with a set of three levels is 2. In the present study, we had four factors with three levels each, so the total DOF was 8. The orthogonal array satisfying the 8 DOF with three levels was L9. The process parameters and their levels for friction stir lap welding of aluminum 5052 and stainless steel 304 are given in Table 1 and the L9 orthogonal array is shown in Table 2. The welded joints were cut in 10 mm × 20 mm dimensions using an electrical discharge machine (EDM, Mitsubishi, Tokyo, Japan wire-cut for the microstructural analysis. Samples were mounted, polished, and then etched with Nital on the steel side and Kellar's reagent on the aluminum side. Etched samples were later analyzed by field emission scanning electron microscopy (FESEM, Zeiss Supra55 VP, Eindhoven, The Netherlands). X-ray diffraction analysis (X'Pert³, Malvern Panalytical, Malvern, UK) was conducted with Cu-Kα source radiation with a scan range of 0° to 80° to find the different phases of IMCs formed in the weld zone. The shear strength of the weld samples was measured according to ASTM-E8 standards. Fractography analysis of the broken samples at the interfacial region of aluminum and steel was conducted using FESEM. The hardness of the samples was determined using a Vickers hardness testing machine with a dwell time of 20 s and a load of 1000 gf.   Table 2. Taguchi L9 orthogonal array for three levels.   LW-1  1  1  1  1  LW-2  1  2  2  2  LW-3  1  3  3  3  LW-4  2  1  2  3  LW-5  2  2  3  1  LW-6  2  3  1  2  LW-7  3  1  3  2  LW-8  3  2  1  3  LW-9  3  3  2  1 Coatings 2020, 10, 1062 4 of 15

Results and Discussion
It has been established that the tilt angle of the FSLW tool plays an important role in achieving better joint strength and eliminating the defects that occur during the lap welding of AA5052 and SS304 alloys [36]. In this study, the microstructural changes and the formation of intermetallic compounds at the interface of the weld were analyzed according to the change in tilt angle. The welded surfaces of the samples are shown in Figure 2. A tunnel defect, which is commonly seen in FSW, was formed because of insufficient heat during the welding process and could be observed on the surfaces of samples LW-5 and LW-9 when the experiments were conducted at 0 • tilt angle. The defects in the sample LW-1 were not visible on the surface of the weld but were visible through the keyhole formed at the end of the welding process. However, there were no surface defects when the tilt angle of the tool was further increased to 1.5 • and 2.5 • and the formation of the onion rings was visible on the surface.

Results and Discussion
It has been established that the tilt angle of the FSLW tool plays an important role in achieving better joint strength and eliminating the defects that occur during the lap welding of AA5052 and SS304 alloys [36]. In this study, the microstructural changes and the formation of intermetallic compounds at the interface of the weld were analyzed according to the change in tilt angle. The welded surfaces of the samples are shown in Figure 2. A tunnel defect, which is commonly seen in FSW, was formed because of insufficient heat during the welding process and could be observed on the surfaces of samples LW-5 and LW-9 when the experiments were conducted at 0° tilt angle. The defects in the sample LW-1 were not visible on the surface of the weld but were visible through the keyhole formed at the end of the welding process. However, there were no surface defects when the tilt angle of the tool was further increased to 1.5° and 2.5° and the formation of the onion rings was visible on the surface.  When the lap joint was performed with a 0° tilt angle with a welding speed of 20 mm/min, a penetration depth of 0.1 mm into the steel and tool rotational speed of 800 rpm, a huge tunnel defect was formed on the advancing side and a lot of cracks were detected at the interface of the two materials. Defects lead to improper mixing of the material, resulting in scattering of steel fragments into the aluminum side, as shown in Figure 3. The scattering of the steel particles was less on the advancing side because the rotating shoulder dragged the particles from the steel surface on the advancing side, which were later settled on the retreating side. The mixed stir zone of the weld clearly shows that the intermixing of the materials did not occur properly, which was the main reason behind the non-formation of the intermetallic layer between the two materials.
Energy dispersive spectroscopy (EDS) was performed at three different points in the mixed stir zone and the spectra are shown in Figure 4. The spectra show the presence of both Fe and Al elements in the interfacial region. Because of the improper mixing of the materials and diffusion between Al and Fe atoms, the IMCs are scattered instead of forming a layer at the mixed stir zone. The intermetallic compound cannot be predicted with the help of the EDS spectrum because of the spatial resolution and crystallographic information but the composition of Al and Fe can be determined. As the materials undergo severe plastic deformation during the process because of the mechanical and thermal reactions, the presence of intermetallic compounds is inevitable. Steel particles are scattered just above the interfacial region, as shown in Figure 4, because of the stirring action of the tool pin. The formation of tunnel defect is also responsible for the scattering of the particles because of the improper mechanical mixing in the stir zone. The weight percentage of aluminum at different points is less than 50%, which significantly shows that Fe-rich intermetallic compounds can be formed in the interfacial region.  When the lap joint was performed with a 0 • tilt angle with a welding speed of 20 mm/min, a penetration depth of 0.1 mm into the steel and tool rotational speed of 800 rpm, a huge tunnel defect was formed on the advancing side and a lot of cracks were detected at the interface of the two materials. Defects lead to improper mixing of the material, resulting in scattering of steel fragments into the aluminum side, as shown in Figure 3. The scattering of the steel particles was less on the advancing side because the rotating shoulder dragged the particles from the steel surface on the advancing side, which were later settled on the retreating side. The mixed stir zone of the weld clearly shows that the intermixing of the materials did not occur properly, which was the main reason behind the non-formation of the intermetallic layer between the two materials.
Energy dispersive spectroscopy (EDS) was performed at three different points in the mixed stir zone and the spectra are shown in Figure 4. The spectra show the presence of both Fe and Al elements in the interfacial region. Because of the improper mixing of the materials and diffusion between Al and Fe atoms, the IMCs are scattered instead of forming a layer at the mixed stir zone. The intermetallic compound cannot be predicted with the help of the EDS spectrum because of the spatial resolution and crystallographic information but the composition of Al and Fe can be determined. As the materials undergo severe plastic deformation during the process because of the mechanical and thermal reactions, the presence of intermetallic compounds is inevitable. Steel particles are scattered just above the interfacial region, as shown in Figure 4, because of the stirring action of the tool pin. The formation of tunnel defect is also responsible for the scattering of the particles because of the improper mechanical mixing in the stir zone. The weight percentage of aluminum at different points is less than 50%, which significantly shows that Fe-rich intermetallic compounds can be formed in the interfacial region.   The tunnel defect was eliminated when the materials were joined with a welding speed of 20 mm/min, penetration depth was increased to 0.3 mm, and a tool rotational speed of 1200 rpm at 1.5° tilt angle. It is evident from Figure 5 that a hook is formed on both sides of the stir zone, leaving some cracks at the interface of two materials. The scattering of steel particles is almost equal on the advancing side and retreating side, which shows that the intermixing of materials is better when compared to the joint formed with 0° tilt angle. By increasing the penetration depth, a mechanical link was obtained in the form of a hook. It is evident from the micro-structure that the stirred zone looks different from the aluminum and steel zone, which is considered as the mixed layer which leads to the formation of intermetallic compounds. Apart from the unevenly scattered steel particles, discontinuous thin wave shapes which were formed in the stirring zone can be considered as the intermetallic compounds formed according to Figner et al. [37] because of the interdiffusion produced during the weld.
The EDS spectra at three different points with the presence of both Al and Fe elements confirm that intermetallic compounds are formed at the interfacial region, as shown in Figure 6. The intermetallic compounds formed can be predicted with the help of the Al/Fe binary phase diagram of [38]. The weight percentage of aluminum in point 1 spectrum is 57.59%, which shows the presence of Fe2Al5 IMC, which is stable when the aluminum percentage is around (~55 wt.%). The other two spectra reveal that the weight percentage of aluminum varies from point to point, which reveals that the IMCs have different thicknesses and the IMC layer at the interfacial region is not stable. Elemental mapping was conducted in the interfacial region, and the distribution of aluminum and steel particles   The tunnel defect was eliminated when the materials were joined with a welding speed of 20 mm/min, penetration depth was increased to 0.3 mm, and a tool rotational speed of 1200 rpm at 1.5° tilt angle. It is evident from Figure 5 that a hook is formed on both sides of the stir zone, leaving some cracks at the interface of two materials. The scattering of steel particles is almost equal on the advancing side and retreating side, which shows that the intermixing of materials is better when compared to the joint formed with 0° tilt angle. By increasing the penetration depth, a mechanical link was obtained in the form of a hook. It is evident from the micro-structure that the stirred zone looks different from the aluminum and steel zone, which is considered as the mixed layer which leads to the formation of intermetallic compounds. Apart from the unevenly scattered steel particles, discontinuous thin wave shapes which were formed in the stirring zone can be considered as the intermetallic compounds formed according to Figner et al. [37] because of the interdiffusion produced during the weld.
The EDS spectra at three different points with the presence of both Al and Fe elements confirm that intermetallic compounds are formed at the interfacial region, as shown in Figure 6. The intermetallic compounds formed can be predicted with the help of the Al/Fe binary phase diagram of [38]. The weight percentage of aluminum in point 1 spectrum is 57.59%, which shows the presence of Fe2Al5 IMC, which is stable when the aluminum percentage is around (~55 wt.%). The other two spectra reveal that the weight percentage of aluminum varies from point to point, which reveals that the IMCs have different thicknesses and the IMC layer at the interfacial region is not stable. Elemental mapping was conducted in the interfacial region, and the distribution of aluminum and steel particles The tunnel defect was eliminated when the materials were joined with a welding speed of 20 mm/min, penetration depth was increased to 0.3 mm, and a tool rotational speed of 1200 rpm at 1.5 • tilt angle. It is evident from Figure 5 that a hook is formed on both sides of the stir zone, leaving some cracks at the interface of two materials. The scattering of steel particles is almost equal on the advancing side and retreating side, which shows that the intermixing of materials is better when compared to the joint formed with 0 • tilt angle. By increasing the penetration depth, a mechanical link was obtained in the form of a hook. It is evident from the micro-structure that the stirred zone looks different from the aluminum and steel zone, which is considered as the mixed layer which leads to the formation of intermetallic compounds. Apart from the unevenly scattered steel particles, discontinuous thin wave shapes which were formed in the stirring zone can be considered as the intermetallic compounds formed according to Figner et al. [37] because of the interdiffusion produced during the weld.
The EDS spectra at three different points with the presence of both Al and Fe elements confirm that intermetallic compounds are formed at the interfacial region, as shown in Figure 6. The intermetallic compounds formed can be predicted with the help of the Al/Fe binary phase diagram of [38]. The weight percentage of aluminum in point 1 spectrum is 57.59%, which shows the presence of Fe 2 Al 5 IMC, which is stable when the aluminum percentage is around (~55 wt.%). The other two spectra reveal that the weight percentage of aluminum varies from point to point, which reveals that the IMCs have different thicknesses and the IMC layer at the interfacial region is not stable. Elemental mapping Coatings 2020, 10, 1062 6 of 15 was conducted in the interfacial region, and the distribution of aluminum and steel particles can be perfectly seen. A thin discontinuous layer was detected at the interface of the joint which proves that improper mixing of materials has happened irrespective of the micro-cracks and voids. According to the elemental mapping images of Al and Fe in Figure 6, the interfacial region mainly consists of aluminum, which leads to the formation of Al-rich intermetallic compounds.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 15 can be perfectly seen. A thin discontinuous layer was detected at the interface of the joint which proves that improper mixing of materials has happened irrespective of the micro-cracks and voids. According to the elemental mapping images of Al and Fe in Figure 6, the interfacial region mainly consists of aluminum, which leads to the formation of Al-rich intermetallic compounds.  When the lap joint was performed with a 2.5° tilt angle with a welding speed of 40 mm/ min, a penetration depth of 0.3 mm into the steel, and tool rotational speed of 800 rpm, all the defects which were formed with 0° and 1.5° were eliminated, featuring a perfectly sound joint. The interface of the materials confirms that intermixing was sufficiently achieved, creating a streamline flow throughout the stirred zone, as shown in Figure 7a. The hooks were also formed on both sides of the mixed stirred zone, which improved the mechanical strength of the joint [36]. The scattering of steel particles was reduced, which could only be observed at the ends of the stirred zone region. The thin waves which formed because of the intermixing of the materials at the interface were the intermetallic layers formed between the two materials. Even the scattered steel particles on the advancing and retreating sides are surrounded by intermetallic layers. The thickness of the intermetallic layer measured with samples LW-3 and LW-4 was between 2.5 and 3 μm, as shown in Figure 7a,b. However, the thickness of the IMC layer increased to 4.5 μm with sample LW-8, as shown Figure 7c, because of the increase in the tool rotational speed, which leads to the increase in temperature, which results in a thicker IMC layer. IMC layer thickness of samples conducted at 2.5° tilt angle concludes that with tool rotational speeds of 800 and 1000 rpm, thinner IMCs can be achieved, which gives better joint strength when compared to the thicker IMCs formed with a tool rotational speed of 1200 rpm.  can be perfectly seen. A thin discontinuous layer was detected at the interface of the joint which proves that improper mixing of materials has happened irrespective of the micro-cracks and voids. According to the elemental mapping images of Al and Fe in Figure 6, the interfacial region mainly consists of aluminum, which leads to the formation of Al-rich intermetallic compounds.  When the lap joint was performed with a 2.5° tilt angle with a welding speed of 40 mm/ min, a penetration depth of 0.3 mm into the steel, and tool rotational speed of 800 rpm, all the defects which were formed with 0° and 1.5° were eliminated, featuring a perfectly sound joint. The interface of the materials confirms that intermixing was sufficiently achieved, creating a streamline flow throughout the stirred zone, as shown in Figure 7a. The hooks were also formed on both sides of the mixed stirred zone, which improved the mechanical strength of the joint [36]. The scattering of steel particles was reduced, which could only be observed at the ends of the stirred zone region. The thin waves which formed because of the intermixing of the materials at the interface were the intermetallic layers formed between the two materials. Even the scattered steel particles on the advancing and retreating sides are surrounded by intermetallic layers. The thickness of the intermetallic layer measured with samples LW-3 and LW-4 was between 2.5 and 3 μm, as shown in Figure 7a,b. However, the thickness of the IMC layer increased to 4.5 μm with sample LW-8, as shown Figure 7c, because of the increase in the tool rotational speed, which leads to the increase in temperature, which results in a thicker IMC layer. IMC layer thickness of samples conducted at 2.5° tilt angle concludes that with tool rotational speeds of 800 and 1000 rpm, thinner IMCs can be achieved, which gives better joint strength when compared to the thicker IMCs formed with a tool rotational speed of 1200 rpm. When the lap joint was performed with a 2.5 • tilt angle with a welding speed of 40 mm/ min, a penetration depth of 0.3 mm into the steel, and tool rotational speed of 800 rpm, all the defects which were formed with 0 • and 1.5 • were eliminated, featuring a perfectly sound joint. The interface of the materials confirms that intermixing was sufficiently achieved, creating a streamline flow throughout the stirred zone, as shown in Figure 7a. The hooks were also formed on both sides of the mixed stirred zone, which improved the mechanical strength of the joint [36]. The scattering of steel particles was reduced, which could only be observed at the ends of the stirred zone region. The thin waves which formed because of the intermixing of the materials at the interface were the intermetallic layers formed between the two materials. Even the scattered steel particles on the advancing and retreating sides are surrounded by intermetallic layers. The thickness of the intermetallic layer measured with samples LW-3 and LW-4 was between 2.5 and 3 µm, as shown in Figure 7a,b. However, the thickness of the IMC layer increased to 4.5 µm with sample LW-8, as shown Figure 7c, because of the increase in the tool rotational speed, which leads to the increase in temperature, which results in a thicker IMC layer. IMC layer thickness of samples conducted at 2.5 • tilt angle concludes that with tool rotational speeds It is evident from the EDS spectra that Al and Fe elements were present in the interfacial region of the weld, which can be observed in Figure 8. According to Balamagendiravarman et al. [21], when the aluminum weight percentage is (~70 wt.%) at point 1, this is evidence of the formation of Al3Fe intermetallic compound. From the point 2 EDS spectrum, it is evident that Fe-rich intermetallic compounds are formed in the interfacial region When the aluminum weight percentage is (~55 wt.%), as shown in the point 3 spectrum, there is a significant change in the formation of Fe2Al5 IMC. The distribution of aluminum and steel particles is shown in Figure 8, which consists of elemental mapping at the interface of the joints. The hook formation, intermetallic layer, and scattered steel particles can be observed from mapping analysis.

XRD Analysis of Intermetallic Compounds Formed in FSLW Joints
Friction stir welding between two different materials is complex because of their mechanical and thermal properties; regardless of the change in process parameters, it is hard to control the formation of the IMCs because of the uneven heat distribution which results in the formation of intermetallic It is evident from the EDS spectra that Al and Fe elements were present in the interfacial region of the weld, which can be observed in Figure 8. According to Balamagendiravarman et al. [21], when the aluminum weight percentage is (~70 wt.%) at point 1, this is evidence of the formation of Al 3 Fe intermetallic compound. From the point 2 EDS spectrum, it is evident that Fe-rich intermetallic compounds are formed in the interfacial region When the aluminum weight percentage is (~55 wt.%), as shown in the point 3 spectrum, there is a significant change in the formation of Fe 2 Al 5 IMC. The distribution of aluminum and steel particles is shown in Figure 8, which consists of elemental mapping at the interface of the joints. The hook formation, intermetallic layer, and scattered steel particles can be observed from mapping analysis. It is evident from the EDS spectra that Al and Fe elements were present in the interfacial region of the weld, which can be observed in Figure 8. According to Balamagendiravarman et al. [21], when the aluminum weight percentage is (~70 wt.%) at point 1, this is evidence of the formation of Al3Fe intermetallic compound. From the point 2 EDS spectrum, it is evident that Fe-rich intermetallic compounds are formed in the interfacial region When the aluminum weight percentage is (~55 wt.%), as shown in the point 3 spectrum, there is a significant change in the formation of Fe2Al5 IMC. The distribution of aluminum and steel particles is shown in Figure 8, which consists of elemental mapping at the interface of the joints. The hook formation, intermetallic layer, and scattered steel particles can be observed from mapping analysis.

XRD Analysis of Intermetallic Compounds Formed in FSLW Joints
Friction stir welding between two different materials is complex because of their mechanical and thermal properties; regardless of the change in process parameters, it is hard to control the formation of the IMCs because of the uneven heat distribution which results in the formation of intermetallic

XRD Analysis of Intermetallic Compounds Formed in FSLW Joints
Friction stir welding between two different materials is complex because of their mechanical and thermal properties; regardless of the change in process parameters, it is hard to control the formation of the IMCs because of the uneven heat distribution which results in the formation of intermetallic compounds. Tilt angle is the major contributing factor that decides the formation of defects in the weld zone. XRD patterns of three samples at different tilt angles are shown in Figure 9. AlFe is the common IMC formed irrespective of the change in tilt angle, which is mainly because of diffusion of aluminum and steel atoms. The formation of IMCs generally takes place between 500 and 1300 • C. Aluminum-rich intermetallic compounds are formed at lower temperatures, whereas Fe-rich intermetallic compounds are formed at elevated temperatures. Different intermetallic compounds, namely AlFe, AlFe 3 , Al 13 Fe 4 , and Al 5 Fe 2 , were detected in all the samples because of improper distribution of heat which was caused by the friction between the plates and the tool. AlFe and Al 13 Fe 4 are the more stable compounds that can be confirmed from three different peaks in all the samples. The proper mixing of the material at different temperature ranges without defects results in the formation of the equilibrium AlFe compound. Al 13 Fe 4 is formed when the melted Al reacts with undissolved Fe particles when the temperature crosses the melting point of aluminum [39]. AlFe 3 is formed because of low rotational speed and improper mixing of the material, which are responsible for the low temperature during the welding. When the tool pin depth, rotational speed, and welding speed have increased, the temperature at the weld zone rises. When the temperature reaches the melting point of aluminum, the Fe diffuses into Al and results in the formation of Al 5 Fe 2 . According to Movahedi et al. [40], Al 3 Fe is unstable with the rise in temperature; as a result, Al 5 Fe 2 is formed with the diffusion of Fe into aluminum. When the angle was increased to 2.5 • , the contact between the pin and plates was better and gives us a defect-free weld and AlFe 3 and Al 5 Fe 2 were formed at the interfacial region. From the XRD results and Al/Fe binary phase diagram, it is evident that different intermetallic compounds are formed based on the diffusion of Fe and Al at different temperatures.
Coatings 2020, 10, x FOR PEER REVIEW 8 of 15 compounds. Tilt angle is the major contributing factor that decides the formation of defects in the weld zone. XRD patterns of three samples at different tilt angles are shown in Figure 9. AlFe is the common IMC formed irrespective of the change in tilt angle, which is mainly because of diffusion of aluminum and steel atoms. The formation of IMCs generally takes place between 500 and 1300 °C. Aluminum-rich intermetallic compounds are formed at lower temperatures, whereas Fe-rich intermetallic compounds are formed at elevated temperatures. Different intermetallic compounds, namely AlFe, AlFe3, Al13Fe4, and Al5Fe2, were detected in all the samples because of improper distribution of heat which was caused by the friction between the plates and the tool. AlFe and Al13Fe4 are the more stable compounds that can be confirmed from three different peaks in all the samples. The proper mixing of the material at different temperature ranges without defects results in the formation of the equilibrium AlFe compound. Al13Fe4 is formed when the melted Al reacts with undissolved Fe particles when the temperature crosses the melting point of aluminum [39]. AlFe3 is formed because of low rotational speed and improper mixing of the material, which are responsible for the low temperature during the welding. When the tool pin depth, rotational speed, and welding speed have increased, the temperature at the weld zone rises. When the temperature reaches the melting point of aluminum, the Fe diffuses into Al and results in the formation of Al5Fe2. According to Movahedi et al. [40], Al3Fe is unstable with the rise in temperature; as a result, Al5Fe2 is formed with the diffusion of Fe into aluminum. When the angle was increased to 2.5°, the contact between the pin and plates was better and gives us a defect-free weld and AlFe3 and Al5Fe2 were formed at the interfacial region. From the XRD results and Al/Fe binary phase diagram, it is evident that different intermetallic compounds are formed based on the diffusion of Fe and Al at different temperatures.

Mechanical Properties of the FSLW Joints
The samples for the shear strength test were cut according to ASTM-E8 standards. The mechanical properties of the base materials are shown in Table 3. Three samples were tested from each weld and the average of them was calculated, and the load vs. displacement graph is plotted in Figure 10.

Mechanical Properties of the FSLW Joints
The samples for the shear strength test were cut according to ASTM-E8 standards. The mechanical properties of the base materials are shown in Table 3. Three samples were tested from each weld and the average of them was calculated, and the load vs. displacement graph is plotted in Figure 10. thicker IMCs formed at the interfacial region. It is noticeable from the shear strength values that tilt angle and penetration depth play an important role in determining the weld strength. Intermetallic compounds influence the ductility of the joint. The presence of thicker intermetallic compounds results in a brittle joint.  Intermetallic compounds present at the interfacial region can be recognized with the help of the hardness test. The hardness of the intermetallic compounds formed between Al and Fe approximately ranges from 350 to 1200 HV [41]. The hardness test was conducted at the cross-section of the lap joints from top to bottom. Hardness on the AA5052 kept increasing with the increase in penetration depth. The maximum hardness values for all the samples are in the welded region. The maximum hardness of 624 HV was achieved with the LW-3 sample, which was approximately an increase of 250% when compared to the SS304 base metal. The lowest hardness was obtained with the LW-9 sample, which has a huge tunnel defect. Hardness values are comparatively low for the samples that are welded at 0° tilt angle, because of the improper mixing of the aluminum and steel that leads to the formation of tunnel defect, whereas the hardness of the samples increased with the increase in tilt angle. The hardness values of the samples welded at 2.5° tilt angle are higher than those of the samples welded at 1.5°. Regardless of the defects, the hardness range for all the samples at the weld zone was between 300 and 630 HV, as shown in Figure 11. The higher hardness at the weld zone reflects the formation of IMCs in the interfacial region. According to Basariya et al. [30], AlFe has a hardness range of (491-667) HV and AlFe3 has a hardness range of (344-368) HV, which proves the presence of two intermetallic phases.  Intermetallic compounds present at the interfacial region can be recognized with the help of the hardness test. The hardness of the intermetallic compounds formed between Al and Fe approximately ranges from 350 to 1200 HV [41]. The hardness test was conducted at the cross-section of the lap joints from top to bottom. Hardness on the AA5052 kept increasing with the increase in penetration depth. The maximum hardness values for all the samples are in the welded region. The maximum hardness of 624 HV was achieved with the LW-3 sample, which was approximately an increase of 250% when compared to the SS304 base metal. The lowest hardness was obtained with the LW-9 sample, which has a huge tunnel defect. Hardness values are comparatively low for the samples that are welded at 0 • tilt angle, because of the improper mixing of the aluminum and steel that leads to the formation of tunnel defect, whereas the hardness of the samples increased with the increase in tilt angle. The hardness values of the samples welded at 2.5 • tilt angle are higher than those of the samples welded at 1.5 • . Regardless of the defects, the hardness range for all the samples at the weld zone was between 300 and 630 HV, as shown in Figure 11. The higher hardness at the weld zone reflects the formation of IMCs in the interfacial region. According to Basariya et al. [30], AlFe has a hardness range of (491-667) HV and AlFe 3 has a hardness range of (344-368) HV, which proves the presence of two intermetallic phases. that leads to the formation of tunnel defect, whereas the hardness of the samples increased with the increase in tilt angle. The hardness values of the samples welded at 2.5° tilt angle are higher than those of the samples welded at 1.5°. Regardless of the defects, the hardness range for all the samples at the weld zone was between 300 and 630 HV, as shown in Figure 11. The higher hardness at the weld zone reflects the formation of IMCs in the interfacial region. According to Basariya et al. [30], AlFe has a hardness range of (491-667) HV and AlFe3 has a hardness range of (344-368) HV, which proves the presence of two intermetallic phases.   When the AA5052 and SS304 were welded at 0 • tilt angle, the fracture was in the tunnel defect area, as shown in Figure 12a. The fracture was mainly on the aluminum side, with a brittle failure on at the edges of the nugget zone on both the advancing side and retreating side. It is evident from the fracture zone that on the aluminum side, the grains were peeled off, which led to the brittle fracture. Meanwhile, on the steel side, as shown in Figure 12b, the aluminum is deposited in the form of small precipitates in the stirred zone, and dimples were formed, which is a mixture of both brittle and ductile failure. The brittle precipitates are formed because of the uneven heat distribution which leads to improper mixing of the material. FESEM was used to study the type of fracture that happened at the interface of aluminum and steel. The fracture surface of aluminum and steel shows that onion rings are formed in the stirred zone because of the tool pin. The onion rings were visible on the steel side because of the penetration depth of the tool pin. The cracks on either side of the weld zone can be observed in all the samples. The fracture surface on the advancing side and retreating side show that grooves are formed on both sides because of the penetration depth of a tool pin which caused the mechanical mixing of the material, which lead to the formation of hooks.

Fractography Analysis of the Lap Joints
When the AA5052 and SS304 were welded at 0° tilt angle, the fracture was in the tunnel defect area, as shown in Figure 12a. The fracture was mainly on the aluminum side, with a brittle failure on at the edges of the nugget zone on both the advancing side and retreating side. It is evident from the fracture zone that on the aluminum side, the grains were peeled off, which led to the brittle fracture. Meanwhile, on the steel side, as shown in Figure 12b, the aluminum is deposited in the form of small precipitates in the stirred zone, and dimples were formed, which is a mixture of both brittle and ductile failure. The brittle precipitates are formed because of the uneven heat distribution which leads to improper mixing of the material.  When the AA5052 and SS304 plates were welded at 1.5 tilt angle, there was a cleavage formed in the stir zone on the aluminum side, as shown in Figure 13a, which confirms that the fracture is the result of brittle failure and it is a quasi-cleavage fracture. The brittle precipitates are unevenly distributed in the stir zone because of void defects at the interfacial region. From Figure 13b, it is evident that fracture on the steel side demonstrates both brittle and ductile failure. The fracture surface on the steel side shows that small dimples are formed along with cracks on the grain boundaries that show ductile failure. The presence of dimples is because of the dimpled intergranular fracture, which is caused by the micro-void nucleation. The dimples formed in the steel side are When the AA5052 and SS304 plates were welded at 1.5 tilt angle, there was a cleavage formed in the stir zone on the aluminum side, as shown in Figure 13a, which confirms that the fracture is the result of brittle failure and it is a quasi-cleavage fracture. The brittle precipitates are unevenly distributed in the stir zone because of void defects at the interfacial region. From Figure 13b, it is evident that fracture on the steel side demonstrates both brittle and ductile failure. The fracture surface on the steel side shows that small dimples are formed along with cracks on the grain boundaries that show ductile failure. The presence of dimples is because of the dimpled intergranular fracture, which is caused by the micro-void nucleation. The dimples formed in the steel side are smaller and occupy more area when compared to the dimples formed with 0 • tilt angle, because of better mechanical mixing of aluminum and steel. It is evident from Figure 14a,b that the stirred zone is much smoother when AA5052 and SS304 are welded at 2.5° tilt angle. The fracture surface shows that the fracture occurred at the edges of the nugget zone where the hooks were formed at the interfacial region. The fracture surface on the aluminum side is shown in Figure 14a; the fracture surface shows the formation of dimples, cleavage, and the cracks at the grain boundaries, which indicate that the fracture is both brittle and ductile. The dimples formed on the steel side are shown in Figure 14b, which are denser when compared to the samples welded at 0° tilt angle and 1.5° tilt angle, which is a sign of ductile failure on the steel side. Alongside the larger size of the dimples, the cracks are observed unevenly cutting through the grains, which is a trans-granular fracture.  It is evident from Figure 14a,b that the stirred zone is much smoother when AA5052 and SS304 are welded at 2.5 • tilt angle. The fracture surface shows that the fracture occurred at the edges of the nugget zone where the hooks were formed at the interfacial region. The fracture surface on the aluminum side is shown in Figure 14a; the fracture surface shows the formation of dimples, cleavage, and the cracks at the grain boundaries, which indicate that the fracture is both brittle and ductile. The dimples formed on the steel side are shown in Figure 14b, which are denser when compared to the samples welded at 0 • tilt angle and 1.5 • tilt angle, which is a sign of ductile failure on the steel side. Alongside the larger size of the dimples, the cracks are observed unevenly cutting through the grains, which is a trans-granular fracture.

Conclusions
In conclusion, this study highlights the investigation of weld surface, interfacial region, mechanical properties, and fracture surface of friction stir welded AA5052 and SS304 plates • FESEM analysis confirmed that the interfacial region is defect-free when the welds are performed at 2.5° tilt angle and a stable IMC layer. The thickness of the IMC layer formed with 800 and 1000 rpm is between 2.5 and 3 μm, which resulted in achieving better joint strength when compared to the 4.5 μm IMC layer formed with 1200 rpm. • XRD analysis further helped in detecting the IMCs formed in the weld zone. AlFe AlFe3, Al13Fe4, and Al5Fe2 are the IMCs detected in the interfacial region of the welds. Among the IMCs detected, AlFe and Al13Fe4 are the stable IMCs formed.

•
The maximum hardness of 624 HV was achieved with sample LW-4, which is a defect-free joint. The increase in hardness is approximately around 250% when compared to the SS304 base metal. The increase in hardness value ranges (300-630) HV supports the presence of AlFe and AlFe3 intermetallic compounds. Fracture surface analysis of the samples found that the fracture occurred on either side of the nugget zone. FESEM analysis conducted at different tilt angles shows that the fracture formed is both brittle and ductile. Three types of fracture, namely quasi-cleavage, trans-granular, and intergranular, were observed at different fracture zones.

Conclusions
In conclusion, this study highlights the investigation of weld surface, interfacial region, mechanical properties, and fracture surface of friction stir welded AA5052 and SS304 plates • FESEM analysis confirmed that the interfacial region is defect-free when the welds are performed at 2.5 • tilt angle and a stable IMC layer. The thickness of the IMC layer formed with 800 and 1000 rpm is between 2.5 and 3 µm, which resulted in achieving better joint strength when compared to the 4.5 µm IMC layer formed with 1200 rpm. • XRD analysis further helped in detecting the IMCs formed in the weld zone. AlFe AlFe 3 , Al 13 Fe 4 , and Al 5 Fe 2 are the IMCs detected in the interfacial region of the welds. Among the IMCs detected, AlFe and Al 13 Fe 4 are the stable IMCs formed.

•
The maximum hardness of 624 HV was achieved with sample LW-4, which is a defect-free joint. The increase in hardness is approximately around 250% when compared to the SS304 base metal. The increase in hardness value ranges (300-630) HV supports the presence of AlFe and AlFe 3 intermetallic compounds. Funding: This research received no external funding.