Abstract
The study presents the hybrid joining of the third generation AA2198-T8 aluminum lithium alloy to AA2024-T3 aluminum copper alloy, which has been highly demanded recently in the aerospace industry. This investigation aims to reduce the cost of production in the industrial sector. As a result, an affordable alternative is to use hybrid designs using AA2198-T8 alloy in crucial parts and AA2024-T3 alloy in the rest of the structure. A joining method is required to create hybrid structures composed of last-generation and standard aluminum alloys. The joining process was successfully friction stir-welded using five different welding travel speeds—36, 76, 102, 146, and 216 mm/min—with an invariable spindle speed of 960 rev/min. Two reversed steps, double-sided friction stir welding (DS-FSW) and single-sided friction stir welding (SS-FSW) techniques with two appropriate tool designs, were employed to investigate the dissimilar material mechanical properties and their morphological changes. The experimental outcomes show that DS-FSW of the reversed steps has a higher joining strength than SS-FSW for all the welding parameters studied. The variation in travel speeds provided the highest strength at 102 mm/min travel welding speeds for DS-FSW. Therefore, it is found that, from the three tensile samples, tensile strength, yield strength, and elongation of the joint were 407.1 MPa, 271.2 MPa, and 9.5%, respectively. The joint efficiency reached 87% compared with the base material tensile strength of AA2024-T3. Furthermore, fractures of the tensile samples were found in the vicinity of the thermomechanically affected zone (TMAZ) of the AA2198-T8 side. The microhardness and morphology results correspondingly have precise predictions for the fracture zone of the joints in this research examination.
1. Introduction
Aluminum alloys are the most desirable in several applications, especially in aerospace industries, because of their high strength-to-weight ratio and excellent corrosion resistance [1]. The recently developed third generation of aluminum alloys, such as AA2198 aluminum lithium alloys, become a choice for lightweight structural applications for the fuselage of aerospace industries [2]. AA2198 has lower density and improved mechanical characteristics than their conventional counterparts, AA2024 aluminum-copper alloys. Nevertheless, gaining these benefits costs material expenses. Meanwhile, a reasonable solution is using a hybrid joining of AA2198-T8 to AA2024-T3 alloys individually for critical regions through the remaining structure retaining AA2024-T3 alloys [3]. However, welding aluminum alloys via the conventional fusion welding process are inappropriate and not recommended as various weld defects such as porosity formation, cracking during solidification, significant loss of strength in the weld region, and high distortion. Friction stir welding (FSW) is a green hybrid solid-state joining welding process [3]. FSW offers considerable advantages for joining aluminum alloys in similar and dissimilar combinations over conventional welding [4]. FSW utilizes a rotating pin to mix the materials of the two sides of the joint locally below the melting point. Thus, welding flaws such as hot cracking are avoided from forming. Selecting the best FSW tool and process parameters is crucial for producing dependable joints for aerospace applications, particularly for dissimilar alloys with variable mechanical and thermal characteristics [5]. Friction stir welding tools can be made out of various materials. Each material has favorable and bad qualities depending on the material being welded. Steels, carbide particles, reinforced composites, tungsten carbide with cobalt, titanium carbide, tungsten carbide, nickel alloys, cobalt-base alloys, refractory metals, ceramic materials, and polycrystalline cubic boron nitride (PCBN) are among the materials in this category. Steel tools such as AISI H13 are used for soft welding materials like aluminum. Tungsten carbide with cobalt-based material and polycrystalline cubic boron nitride (PCBN) is employed for hard materials such as titanium and its alloys [6]. However, these materials have superior melting properties. They are more challenging to fabricate into the complex shapes that the tool may have. Difficulties in fabrication lead to increased tool costs; therefore, it is often desirable sometimes to sacrifice some durability for a reasonably priced tool [3, 7]. Hardened tool steel AISI (H13, D2) based tools are perfectly acceptable for welding workpiece thicknesses less than 12 mm in FSW of aluminum. However, in some case studies, the AISI H13-based steel tool is generally recommended for welding aluminum alloys within 0.5–50 mm but with more challenging or highly abrasive materials [8]. Many researchers in the tool geometry investigation have proved that FSW tool profile designs have crucial effects on enhancing welding sound joints [9]. Moreover, the discrepancy of the tool geometry proves that the parameters change, besides the type of material and the thickness used in FSW are further concerns in this regard. Therefore, the FSW tool geometry design proved in the most experimental research to be a threaded taper pin with concaved shoulders [10].
FSW is currently used as an alternative solution for the traditional welding process and riveting for the fuselages in the aerospace industry. Some weld defects such as excessive flashing, excavating (tunneling), and kissing bonds can also occur in friction stir welding if optimum weld parameters are not used [11]. The core of the work is to produce reliable hybrid joints via the friction stir welding process to select appropriate weld parameters, which are essential to improve the joining strength. Thus, the selection of the type of friction stir welding process, whether single-sided friction stir welded welding (SS-FSW) or double-sided friction stir welding (DS-SFW), should be done based on the experimental test results. Thus, mechanical behavior and microstructural changes should be inspected to determine the best technique for a set of fixed optimum weld parameters. In recent years, DS-FSW (double side friction stir welding) has drawn the attention of many industrial sectors and researchers to improve the mechanical properties of weldment materials. DS-FSW has been used as an additional parameter for friction stir welding techniques to improve joint efficiency and mechanical characteristics of the problematic welded materials. DS-FSW has become strongly recommended for material thickness exceeding 50 mm for a successful joining process [8]. Thus, the mechanical properties of reversed passes have been considered an additional parameter for the DS-FSW process [12]. Moreover, the effects of the tool size as geometrical design and the whole of the applicable mechanical parameters can be considered. Meanwhile, microstructural characterization and mechanical test results should be conducted to advise the applicable weld parameters [13].
This study aims to achieve the utmost efficient hybrid joints via FSW and reduce the cost of production in the industrial sector. As a result, an affordable alternative is to use hybrid designs using AA2198-T8 alloy in crucial parts and AA2024-T3 alloy in the rest of the aerospace structure. The examination of reversed DS-FSW and SS-FSW case techniques has been run into several crucial tests in compliance with particular international standards. Thus, the specimens were conducted into tensile, morphology, microhardness, and scanning electron microscope (SEM) examinations. The mechanical test as a central pillar for the work has been examined to specify the best defectless joint parameter.
2. Materials and Experimental Work
In this research examination, two base materials—AA2198-T8 aluminum lithium alloys and AA2024-T3 aluminum-copper alloys (300 mm × 75 mm × 6 mm)—were used to join via the FSW process. The chemical composition of the base material for AA2198-T8 and AA2024-T3 are analyzed by Spectro Maxx and shown in Tables 1 and 2, respectively. In this case study, the milling machine model (NK-IWASHITA) was used to perform FSW for both cases of welding techniques. The dissimilar plates are fixed on the specially made rigid base plate via a clamp to prevent slipping and shifting during FSW processes. The FSW tool selected for the hybrid joining process was made of hot-worked AISI H13 tool steel, which has been proven successful for the FSW process for aluminum alloys, and after heat treatment, the hardness reached 66 HRC [9]. Both FSW design cases are tapered threaded pins with concaved shoulders, per recommendations for experimental work case studies [3, 9, 14]. Simultaneously, the tool geometry for each method of FSW for hybrid welding joints has been designed in different pin and shoulder dimensions regarding material thickness and friction stir welding processes [2, 9]. The geometrical designs of the tool for reversed DS-FSW and SS-FSW are illustrated in Figure 1. Thus, the design of both techniques is upgraded with the fins section to improve tool life performance and reduce the heat rise of the tool. As a result, the research intends to find the best FSW process technique between reversed (advanced side AS and retreating side RS) DS-FSW and the traditional SS-FSW. In the meantime, many trials and experimental studies recommendations have been reviewed to address this aim. The rotation of the spindle speed of 960 rev/min, the tilt angle of 2o, the plunge-in depth of 0.3 mm, and the dwell time DT of 20 sec of welding parameters were kept constant through examining five different travel speeds or welding speeds of 36, 76, 102, 146, and 216 mm/min [15–17]. Before each joining process for reversed DS-FSW and SS-FSW, the base material intertouch surface has been machined 0.3 mm and swapped by ethanol to remove any oxide or dirt particles to perform a clean welding process.
Examination of reversed DS-FSW and SS-FSW case techniques has been run into several crucial tests in compliance with particular international standards. The mechanical test as a central pillar for the work has been examined to specify the best defectless joint parameter. Tensile machine model is HUALONG 600 kN. The tensile specimens have been prepared according to the American welding society standard specification AWS D17.3/17.3 M 2016. Meanwhile, the joint efficiency has been achieved through the ultimate tensile test compared with the base material of AA2024-T3. The Vickers hardness HV05 profile was measured via a Digital Vickers Hardness tester machine in compliance with ASTM E92. At the same time, it reveals the typical microstructure according to ASTM E3 standard. The specimen was sliced and ran into a sequence of grinding and polishing via grit papers (600, 800, 1000, and 3000 grits) and diamond suspension solutions (0.5 and 0.25 microns) to remove minor scratches on the surface during the polishing process. Thus, for etching purposes, the double etchant was employed, and two solutions were prepared: solution A, the composition of 25 ml of HNO3 and 75 ml of H2O; solution B, the composition of 0.5 g of NaF, 1 ml HNO3, 2 ml of HCL, and 97 ml of H2O. The prepared sample was immersed for 60 sec in solution A at 70°C and quenched significantly by cold water and then immersed in solution B for 30 sec and washed in a stream of warm water. However, Metkon Microscopic model M902 is used to capture the microstructure photos of the hybrid joint. Furthermore, the fracture surfaces of the utmost hybrid joint after the tensile test were examined through the scanning electron microscope (SEM) to reveal the propagation of defects and failure points.
3. Results and Discussion
3.1. Mechanical Characteristics
Tensile samples were sliced perpendicular to the welding line representing long gauge length. The ultimate tensile strength (UTS) yields tensile strength (YTS), and elongation percent (EL) has been considered for each welding speed in both reversed DS-FSW and SS-FSW. The joint efficiency for the reversed DS-FSW and SS-FSW has been compared with the parent metal. The tensile properties result in various welding speeds for reversed DS-FSW and SS-FSW, as shown in Figure 2. The hybrid joint tensile characteristics were significantly decreased compared to the base material. Furthermore, the tensile strength increased and decreased by increasing the welding speed from 36 mm/min to 216 mm/min in both cases of the welding process (reversed DS-FSW and SS-FSW). Since at lower welding speeds, the temperature rises cause a change in grain structure with excessive flash material, whereas higher welding speeds lead to propagation of defects like incomplete penetration in SS-FSW or misalignment of the double pass in reversed DS-FSW. The utmost joining efficiency in mechanical characteristics was carried out in reversed DS-FSW at 102 mm/min, where the maximum ultimate tensile (UTS), yield strength (YTS), and elongation (EL) are 407.1 MPa, 271.2 MPa, and 9.5%, respectively. The minimum values are obtained at 36 mm/min, 257.4 MPa, 160.1 MPa, and 3.7%, respectively. On the other hand, for the SS-FSW joints, the maximum UTS, YTS, and EL are found at 76 mm/min, 293.3 MPa, 190.2 MPa, and 4.8%, respectively. The minimum values are found at 216 mm/min, 210 MPa, 75.9 MPa, and 2.2%, respectively, as shown in Figure 2.
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3.2. Macrostructure Examination
Excessive flash material in friction stir welding has crucial side effects on the joining process. The flash material of the friction stir-welded material in both welding processes reversed DS-FSW and SS-FSW have been examined via macroexamination. As illustrated in Figure 3, the macrostructure examination was revealed via double etching solution according to the ASTM E3 standard. The sliced section surface has been prepared through the sequence of polishing grits and diamond suspension solutions to remove the minor scratches and glitter the surface. Macroexamination presented that the increase in the transverse speed of welding processes leads to misalignment of the tool axis, especially in the DS-FSW process, and the “S” shape curve appears in the weld center for SS-FSW. The root cracks were observed in the SS-FSW at 76 and 102 mm/min transverse welding speed. Through macrostructure examination at 36, 76, and 102 mm/min of transverse welding speeds, no misalignment was observed, and moderate flash material was noticed at 102 mm/min for DS-FSW. On the other hand, the heat-affected area in SS-FSW welding was more significant than the reversed DS-FSW. However, a larger heat-affected area increases the risk of losing mechanical properties and efficiency reduction in FSW of the hybrid joint AA2024-T3 to AA2198-T8.
3.3. Microstructure Examination
Double etching solutions revealed the microstructure of the reversed DS-FSW hybrid joint. Figure 4 shows the microstructure of the utmost hybrid joining process for reversed DS-FSW at 102 mm/min assembled after 372 captures. The grain size and distribution in SZ are found in refined equiaxed grains due to recrystallization behavior in the reversed DS-FSW process, as shown in Figure 4. This area corresponds closely to the route of the pin during welding. Compared to the base metal, the grain size of the substance in this zone is very small. This zone’s microstructure contains elements of both metals in the welding process. This zone is characterized by numerous concentric rings, often known as “onion ring structure.” On both sides of the stir zone, the TMAZ is located. During the welding process, plastic deformation of the base metals creates it. Here, the strain and temperature are lower than in the SZ, and as a result, the impact of welding on the microstructure is less distinct. In all welding procedures, the HAZ is created owing to the heat cycle of the weld. This area endures a heat cycle, but no plastic deformation occurs. Temperatures are lower than in the TMAZ but still significantly impact the zone's microstructure and mechanical behavior.
Meanwhile, parameters change significantly affect grain size and microstructure changes in three zones, SZ, TMAZ, and HAZ. The grain size measurement was done in compliance with international standard ASTM E112. Though it is noticed that the size of these equiaxed grains variants, the average grain size of the reversed DS-FSW for the SZ of unaltered spindle speed of 960 rev/min of the selected variable welding speeds 36, 76, 102, 146, and 216 mm/min are 4.2, 2.4, 2.1, 2.9, and 3.6 µm, respectively. The regular size in SZ was reduced when the welding speed increased from 36 to 102 mm/min and increased when the welding speed increased from 102 to 216 mm/min, which encircling 4.21 µm to 4.2 µm, and 2.1 µm to 3.6 µm, respectively. Consequently, the grain is significantly coarser at the lowest welding travel speed, as shown in Figure 5. The average grain size in SZ of the SS-FSW for the same selected welding speeds was 4.9, 2.4, 3.4, 4.1, and 6.2 µm, respectively. However, the overall average grain size in SZ for reversed DS-FSW was observed that it is severe and smaller than SS-FSW. Moreover, minor and major defects like tunnel defects and kissing bonds have been noticed in SS-FSW at welding speeds of 146 mm/min and 216 mm/min, as shown in Figure 6.
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3.4. Microhardness
The microhardness profile of the reversed DS-FSW and SS-FSW of the hybrid joints is fabricated at different welding speeds, showing the microstructure and grain size changes. Conferring to ASTM E92, the microhardness values were measured via HV05. Correspondingly, the microhardness distribution for the joints as a “W” shape specified a gradual reduction in TMAZ and HAZ regions [18, 19]. On the other hand, the HAZ for both cases at various speeds indicated a minimal microhardness value. In addition, the base material microhardness measured values were higher than the three weld zone values. Thus, this demonstrated that the three zones of the weld are softened: nugget zone (NZ), the thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ), as shown in Figures 7 and 8.
Figure 9 presents the mean microhardness values for reversed DS-FSW and SS-FSW hybrid joints for various welding speeds in the nugget zones. The maximum average microhardness value measured for reversed DS-FSW in the NZ is 127.4 HV at 102 mm/min welding speed. Meanwhile, the minimum microhardness value of the NZ is 113.6 HV at 36 mm/min welding speed. On the other hand, the microhardness mean values for the NZ of the SS-FSW hybrid joints are at the same welding speeds. Thus, the maximum microhardness value of SS-FSW was found to be 108.1 HV at 76 mm/min and the minimum 91 HV at 36 mm/min. Therefore, comparing reversed DS-FSW with the SS-FSW microhardness values in NZ, the measured values in reversed DS-FSW were higher than the SS-FSW at various welding speeds. Generally, microhardness values distribution in the three zones is correlated to the microstructure variations [20]. The main reason for the microhardness values of the NZ lower than the parent metals is the original particles that strengthen aluminum alloys which are dissolved in the FSW process; however, it precipitates in the NZ after the FSW process. Due to the dynamic recrystallization caused by the rotation tool in the stir zone forming refined equiaxed grains in NZ. The microhardness values in NZ are higher than the TMAZ and HAZ, as shown in Figures 7 and 8. Exclusively, when the welding speeds used 102 mm/min in reversed DS-FSW, the average microhardness value is higher due to fine equiaxed grain sizes. While due to the coarse grain size results of 36 mm/min, the lowest microhardness value was observed [21]. The grain size and microhardness values are closely related to the mechanical properties of precipitation strengthening of aluminum alloys. The strength and elongation of the base material scored a higher value due to the presence of a large amount of the precipitation phase. The superior tensile property and microhardness values in the NZ compared to the HAZ and TMAZ are due to the contribution of equiaxed fine grain size.
On the other hand, the grain size and reprecipitated phase distribution of various FSW joints generated by different welding speeds vary significantly. The favorable input of more refined equiaxed grains and more reprecipitated particles into the NZ in reversed DS-FSW at a welding speed of 102 mm/min may enhance the mechanical characteristics of FSW joints. The joints manufactured at 36 mm/min exhibit the reverse impact of coarse grain and reprecipitated particles in the NZ. Figures 7 and 8 demonstrate the prediction of the fractured locations of all welding joints for reversed DS-FSW and SS-FSW achieved during tensile testing at various welding speeds. The welding speeds of 36 mm/min, 146 mm/min, and 216 mm/min are used during the FSW process fractured in the NZ. It is evident to predict the fracture location via microhardness profile in Figure 8 that the hybrid joint generated at 102 mm/min and 76 mm/min breaks in the TMAZ with noticeable necking phenomena.
3.5. Fractured Locations
Figure 10 presents the fractured locations of the reversed DS-FSW and SS-FSW hybrid joints achieved during tensile testing for optimum and minimum strengths. As illustrated in Figure 10(a), it is evident that the joints generated in reversed DS-FSW at 36 mm/min break in the NZ with noticeable minimum tensile strength. However, the maximum tensile strength was achieved at 102 mm/min welding speed for reversed DS-FSW. Meanwhile, the fracture of the tensile test was in the TMAZ with noticeable necking phenomena, as presented in Figure 10(b). On the other hand, in SS-FSW, at welding speeds of 216 mm/min, as shown in Figure 10(c), the fracture locations were in NZ with minimum tensile strength. However, except for 76 mm/min, the fracture was in TMAZ, as illustrated in Figure 10(d), with higher tensile strength, nevertheless, still irresistible with the utmost tensile strength of the hybrid joint made by reversed DS-FSW.
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3.6. Fractured Surface
Figure 11 illustrates surface morphology SEM of the fractured surfaces of specific tension tests resulting from specific welding speeds, giving the optimum and minimum tensile properties.
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In Figure 11(a), the fractured surface of the hybrid joint reversed DS-FSW acquired at 36 mm/min has some river patterns and cleavage platforms with some dimples. It is a typical mixed fracture mode representing ductile and brittle fractures. Figure 11(b), the fractured surface of the hybrid joint of reversed DS-FSW, at 102 mm/min, has depicted many deep dimples and ripping ridges on the fractured surface, showing that this fracture mechanism is a complete ductile fracture. Figure 11(c), fractured surface of the joint SS-FSW at 36 mm/min, presents some river patterns and tearing edges with few deep dimples, indicating that it is a typical mixed fracture mode. Figure 11(d) shows that the fracture surface of 76 mm/min welding speed for SS-FSW joint presented fine dimples, and minimal tearing ridges on the flat facets suggest a mixed fracture mode in the NZ.
4. Conclusion
Hybrid joints of AA2024 to AA2198 have been successfully produced via reversed DS-FSW and SS-FSW using various welding speeds at a fixed rotational rate. The microstructure evolution and mechanical characteristics of the friction stir welded joints have been examined; thus, regarding experimental outcomes, the following have been concluded:(1)The welding speed increased from 36 mm/min to 216 mm/min, and the area of the heat-affected zone (HAZ) initially increased and then decreased due to different welding temperatures. At 102 mm/min, the size of the HAZ appeared to be the smallest for the joint of DS-FSW, and the “S” curves in the nugget zone (NZ) started to disappear due to the plastic flow. Generally, the DS-FSW typical HAZ size is narrower than the SS-FSW at different welding speeds.(2)The main reason for the microhardness values of the NZ lower than the parent metals is the original particles that strengthen aluminum alloys are dissolved in the FSW process; however, it precipitates in the NZ after the FSW process. Due to the dynamic recrystallization caused by the rotation tool in the stir zone forming refined equiaxed grains in NZ. At 102 mm/min in DS-FSW, the average microhardness value is recorded as the highest value, 127.4 HV, due to fine equiaxed grain sizes. While due to the coarse grain size results of SS-FSW at 36 mm/min, the lowest microhardness value was observed.(3)The utmost mechanical properties and microstructure evolution have been noticed at various welding speeds in reversed DS-FSW compared to the conventional SS-FSW. However, the joint efficiency of reversed double-sided friction stir welding DS-FSW recorded a higher value than the single-sided friction stir welding SS-FSW, reaching up to 87.3% compared to the base material of AA2024-T3.(4)The joints formed at 102 mm/min exhibit excellent tensile characteristics, such as ultimate tensile (UTS), yield strength (YTS), and elongation (EL) for the DS-FSW joints, recorded as 407.1 MPa, 271.2 MPa, and 9.5%, respectively. On the other hand, UTS, YTS, and EL for the SS-FSW joints were found maximum at 76 mm/min, 293.3 MPa, 190.2 MPa, and 4.8%, respectively. Only the joints cracked at 76 and 102 mm/min in the TMAZ, accompanied by severe necking phenomena. Thus, reversed DS-FSW has superior mechanical characteristics over traditional SS-FSW.
Data Availability
The data supporting the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was mainly supported by the Department of Mechanical and Mechatronics Engineering, College of Engineering, Salahaddin University-Erbil, Ministry of Higher Education and Scientific Research, Kurdistan Regional Government (KRG), Iraq.