Assessment of parameters windows and tool pin profile on mechanical property and microstructural morphology of FSWed AA2014 joints
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The role of optimized parameters in deciding the qualities of the friction stir welded joint of 2014 aluminium alloy in terms of the mechanical properties and microstructural features was investigated in this research. A wide range of process parameters viz: tool shoulder diameter, tool pin profile, plunge depth, tool rotation speed and welding speed were selected and their individual effects on the tensile as well as microstructural properties variations were studied in detail. Out of four various selected tool pin profiles the excellent mechanical as well as microstructural properties of the square pin (SQ) tool are due to the unique stirring effect by the pulsating action of the sharp edge of the pin and metal propagation process at the stir area with incorporation with the tool rotation as well as welding speed during the friction stir welding. The SQ pin tool of 24 mm shoulder diameter with a plunge depth of 0.06 mm and tool rotating at 815 rev/mm with welding performed at a speed of 63 mm/min yielded the best quality joint. The ultimate tensile strength is around 86% of the base metal (BM) with excellent metallurgical characteristics contributed by the 84% finer grains than the BM ensured from the detail microstructural analysis of the weld bead. The metallurgical study includes the variations in the grain size at the nugget zone with respect to the selected process parameters as well. Fractography of the welded specimen reveals a ductile and transgranular mode with cup and cone structures on the counterpart.
KeywordsFriction stir welding Aluminium alloy Process parameters Mechanical property Fractography Macro and microstructural characterization
Friction stir welding (FSW) method is a revolutionary process in solid form joining procedure invented at The Welding Institute by Thomas et al. . Being a solid form joining process, the FSW process has a numerous technical advantages, for example, high production rate, smooth weld bead surface, environment-friendly, fewer residual stress and distortion compared to the conventional fusion welding process. The process has brought a metamorphosis in the standard of welding in difficult-to-weld materials like aluminium (Al) and its alloys which are difficult to join through fusion welding process  and produces satisfactory mechanical properties. Aluminium and its alloy are widely applied in aviation, aeronautical structure panels  automotive and shipbuilding and many other industries. The demand for weight reduction, fuel consumption and cost in the aerospace industry can be fulfilled by the FSW process. FSW is a solid-state, less energy consuming and repeatable metal fabrication process capable of producing high strength joints, offers a potentially lower cost and is environment-friendly . The high strength 2014 aluminum alloys are extensively applied in aircraft structures and are prone to solidification cracking, void generation and dissolution of hardening precipitates welded by conventional fusion welding processes and are inferior in mechanical properties reported in [5, 6]. These difficulties can be solved by the FSW process which has key benefits over fusion welding methods . The FSW process yields less distortion of the workpiece, excellent metallurgical properties of joint, more reproducible properties, without shielding gas and surface cleaning requirement, etc., as the joint is fabricated by only plasticizing the metal by the rotating tool and advancement of the tool in welding direction . The plasticized material flow behaviour and the factors which influence it are mainly the tool shoulder diameter, plunge depth, pin profile of the tool and the tool rotation as well as welding speed that affect the quality of the FSW joint. This investigation aims to find out the mechanical attributes of the friction stir welded (FSWed) joint of AA2014 welded with different considered parameters.
The microstructure and mechanical qualities of the FSW joints of AA2139-T8 and AA7020-T651 were studied and co-related to the welding parameters viz; the rotation and welding speeds with the total and local mechanical behaviors and found that the joint hardly failed at the nugget zone (NZ) and most of the specimens failed at the heat affected zone (HAZ) which exhibited the lower hardness history due to the faster coarsening of η-precipitates present in the matrix . The tensile behaviour and corrosion resistance of AA2014 aluminium alloy FSWed at seven different speed combinations and found that the highest temperature was achieved at higher tool rotation speed and lower welding speed. Microhardness and corrosion resistance were found to be the best at low rotation speed and high traverse speed . The influences of various tools with a high ratio of depth-to-width on weld quality, hardness, grain structure, as well as crystallographic texture in the FSW process were also studied in [11, 12]. In the FSW of AA2198/AA2024, it is realized that the parameter tool traverse speed when increased the over-all hardness of the joint is increased and kissing bond defects are formed . In the FSW of AA2219-T6, it is possible to fabricate higher quality weld at much more higher speeds by hybrid tool pin profile without any harm to the tool pin than the conventional conical threaded tool . The mechanical and microstructural characteristics of both similar and dissimilar FSWed sheets of AA2219 and AA7475 AA were investigated and found that at considered process parameters of rotation speed, welding speed and tool tilt angle, grain refinement was observed at the NZ. The dissimilar metal FWS joint exhibited poor tensile strength due to improper metal transfer during welding. The retreating side of the thermo-mechanical affected zone (TMAZ) showed the lowest microhardness history due to thermal softening . During the FSW process heat generated by the tool due to friction has a remarkable effect in reducing hardness, heat affected area and its distance from the weld centre were studied in [16, 17]. Research on the effect of process parameters considered viz: tool rotation speed, welding speed, axial load and the shoulder to pin diameter ratio (D/d) for AA2024-T6 and AA7075-T6 in dissimilar FSW was carried out and found that (D/d) ratio affect the mechanical and microstructural features the most . The hardness variation at constant rotation and different welding speed of FSWed dissimilar joints of Mg–Zn–Gd and Mg–Al–Zn alloys were carried out and observed that the tensile properties of the joints are significantly influenced by the material’s positions, grain size and crystallographic orientation. Defects rose at lower heat input conditions due to insufficient metal plasticization . In FSW of AA2024 researchers can produce excellent quality weld by scroll shoulder and tri-flute pin at high welding speed. The welds achieved were defect-free and with a good ductility . The consequence of different process parameters along with the tool pin design chosen for the experiment on the mechanical aspects of AA 2014-T6 was studied and found that the quality of weld achieved by hexagonal tool pin profile was superior to the conical one . In dissimilar FSW of Ti–6Al–4V/AA6061, the effect of the most important process parameter, rotational speed on the mechanism of bonding at the interface is investigated and observed that the tensile properties and microhardness are increased due to the recrystallization process. The tool rotational speed causes the formation of the intermetallic at the joint line .
The aforementioned literature survey discovers the scarcity in the study of process parameters in a wide spectrum in single literature. It is understood that deep research is required to find the effect of different process parameters on material flow behaviour during FSW and microstructural evolution at the NZ of the FSWed AA 2014. There is a limited work regarding the impact of plunge depth and tool pin profile effect on the weld value. The information related to the flexural strength (FS) of FSWed AA2014 with different process parameters is very scant. Microhardness variation at the three different zones of the welded specimen such as upper, middle and bottom as well as the role of process parameters on the grain structure at the NZ and the influence of grain size variation on mechanical properties are still not studied. In this research work, the lacuna is filled and a detail investigation of the process parameters and their role in regulating the mechanical properties has been carried out.
2 Experimental procedure
Mechanical attributes of the base material
Bending angle (°)
Experiments conducted with different process parameter combinations
Shoulder diameter (mm)
Plunge depth (mm)
Tool pin profile
Tool rotation speed (rev/min)
Welding speed (mm/min)
3 Results and discussion
The weld quality can be judged by the output of mechanical and metallurgical quality attributes. In this section, tensile properties, FS, bend angle, maximum hardness at NZ were studied along with the metallurgical analysis like the grain size at different weld zones and variation of the same with respect to different combinations of the considered process parameters like tool shoulder diameter, plunge depth, tool pin profile, rotation and welding speed of the tool. The correlations of grain size at NZ with the output mechanical properties were established. The present investigation was concluded with the fractograph analysis of the tensile fractured specimens.
3.1 Tensile properties and flexural strength analysis
Mechanical properties of joints with various process parameters
Bending angle (°)
Hardness at NZ (HV)
Grain size of NZ (µm)
The impact of the shoulder diameter on weld qualities is depicted in Fig. 4b. The graph represents that the tensile properties increase with an increase in diameter of shoulder from 16 to 24 mm and then deteriorate at shoulder diameter 28 mm. At a lower shoulder diameter, the shoulder-workpiece rubbing surface is less which produces lower frictional heat to be conducted by the material to get plasticized. The inadequate frictional heat generated is not able to bring the metal to the desired level of plasticity for the proper mixing and filling to fabricate the joint . Hence, the tensile properties achieved were inferior. A fall in tensile properties was also recorded at diameter 28 mm for the local thinning effect of the weld area due to more frictional heat generation. The best tensile properties were achieved at a shoulder diameter of 24 mm due to the suitable contact surface of the shoulder and adequate heat generation and transfer to the material below the shoulder for proper softening and interlocking to form the joint. The UTS, YS and %E of the joint are 355.23 MPa, 236 MPa and 8.86 which are 77%, 67.4% and 68.15% of the BM respectively. More heat was generated and better weld was formed when the contact area is more. But when the contact area is too more i.e. at 28 mm shoulder diameter, the heat generated is more which makes the material softer and flowable and will be removed as an extra flash from the surface and results in to tear line defect. Also, a more contact area leads to the formation of wider TMAZ and HAZ and grain growth at the NZ due to extra heat input which resultantly deteriorates the tensile strength of the final weld . For less than 16 mm shoulder diameter, the heat generated is not enough for the material to achieve the preferred plasticity to get intimately mixed during the welding and recrystallization process at the NZ was not satisfactory resulting in low quality welds observed from the trial experiments. So the welds performed only from 16 to 28 mm shoulder diameters are only considered for the present study. The flexural properties and bending angle were also analyzed for the joint with the same process parameters. The effect of the shoulder diameter on FS was investigated and observed that the trend is similar to followed by the tensile properties shown in the same Fig. 4b. FS is improved up to a shoulder diameter of 24 mm and is 408 MPa and around 75% of the BM due to the proper heat generation and recrystallization of the grains at NZ and then degraded at a higher shoulder diameter for the grain coarsening due to unnecessary heat generation and grain growth .
Plunge depth is an important process parameter which regulates the final quality of the weld. There is a limited research article revealing the weld quality related to plunge depth. The plunge depth maintains the surface contact of the shoulder surface and base plate to transfer the frictional heat generated during the repeated rubbing of the shoulder on the workpiece . It regulates the grain refinement and the microstructural evolution at the welded area and so affects the qualities of the final product. From the present experimental conditions, the best tensile properties with respect to the plunge depth were achieved at 0.06 mm shown in Fig. 4c. There is a sharp decrease in the same with an increase in plunge depth which reduces the volume of the weld. The UTS, YS, %E and FS attained for the considered plunge depth are 370.05 MPa, 250 MPa, 9.03, and 420 MPa respectively, which are 80.45%, 71.40%, 69.50%, and 77% of the BM respectively. Tensile properties and FS are inferior at both less and more than the accepted value i.e. at 0.03 mm, at 0.09 mm, and 0.12 mm respectively. Low quality of the weld at lower plunge depth is due to the improper surface contact which transfers less frictional heat. At more plunge depth, due to more insertion of the shoulder on the workpiece surface, a volume of the material from the weld surface was trimmed out and induction of stress concentration at the NZ occurs .
The weld quality in terms of tensile properties and FS obtained from four different tool pins such as STC, TAC, THC and SQ as shown in Fig. 1 are represented in Fig. 4d. The SQ pin tool is found to yield the best tensile results as compared to the others. The %E and FS of the joint are also improved. The flat edge surface of the tool pin profile generates more frictional heat relative to the others with the same input energy. The plasticized metal gets swirled beneath the tool pin and the edges of the SQ pin tool pulsate the material in a regular manner and due to which an imposed plastic deformation takes place with fine recrystallized grains, which is the main cause of a defect-less joint with improvement in tensile, flexural as well as microstructural properties . SQ pin tool is better than the others in yielding superb mechanical as well as microstructural properties due to the plastically deformed and dynamically recrystallized grains at NZ . The UTS, YS, %E and FS achieved are 380.22 MPa, 265 MPa, 9.91, and 445 MPa respectively, and are 82.65%, 75.71%, 76.23% and 81.65% of BM respectively.
The effect of tool rotational speeds in different ranges such as 600, 815, 1100 and 1500 rev/min and welding speeds varying from 22, 36, 63 and 98 mm/min, on the weld quality were investigated and represented in Fig. 4e, f. The UTS, YS, %E and FS are found to follow an increasing path with an increase in these two process parameters and finally trace a fall in the trend. From the experimental results of considered tool rotational speeds as shown in Fig. 4e, the best weld quality is achieved at a tool rotation speed of 815 rev/min and the quality was deteriorated at 1100 rev/min and 1500 rev/min. At high tool rotation speed, excessive frictional heat was generated and was contributed to the unwanted grain growth at the NZ and makes the weld inferior. Also at tool rotational speed more than 1000 rev/min, the properties are degraded due to the disintegration and dissolution of the strengthening precipitates . So the graph follows a decreasing trend. Also due to more frictional heat, the metal became more plastic and is removed from the surface in combination with the high centrifugal force  generated due to the high tool rotational speed. So the weld metal volume is reduced and voids like the tunnel and micro pores in the NZ are produced which reduces the tensile as well as FS of the welded joint. Hence at a tool rotational speed of 815 rev/min, the best tensile, as well as flexural properties are obtained. The UTS, YS, %E and FS achieved are 389.34 MPa, 277 MPa, 10.58, and 474 MPa respectively, and are 84.63%, 79.14%, 81.38%, and 87% of the BM respectively.
3.2 Micro-hardness variation and relationship with process parameters
The variation in hardness with varying tool rotation speed and welding speed at NZ was analyzed and is represented in Fig. 8d, e respectively. The variation in average micro-hardness value depends on the stirring action, which is a function of the rotation speed of the tool. If the rotational speed is low the hardness is less due to improper stirring resulting from insufficient heat generation. More is the rotational speed (up to a certain limit) of the tool higher is the stirring of plasticized material and more is the dynamic recrystallization, due to which the grain size is reduced, voids are decreased, grains are compacted, and finally, the hardness is improved. But at higher rotational speed (1100 rev/min), the tool deteriorates the strengthening effects of the alloy due to more stirring action and results in a decrease in hardness  as well as due to grain growth for excess temperature generated the hardness is declined. From the figure, it is analyzed that the hardness at NZ is highest at a tool rotational speed 815 rev/min. A fall in the graph at higher tool rotational speed is observed due to the aforementioned cause. The measured hardness data as shown in Fig. 8e represents that lower welding speed gives rise to higher micro-hardness value as compared to the higher one as the exposure time for the weld to the temperature is more to get recrystallized. But after a certain extent, the hardness decreases due to overexposure to the temperature and grain growth  as shown in Fig. 8e. At lower welding speed, considering the tool rotational speed constant the hardness increases because of less pitch travel distance and finer grain.
3.3 Metallographic study
In this section microstructure of the welded joints was revealed through the optical microscope. The grain orientation and its size at various zones of the weld and the consequences of variation in process parameters on the grain size at NZ were analyzed. A detailed study on the mechanical property variation of the final weld with respect to the variation in grain size was also performed.
3.3.1 Weld bead analysis
3.3.2 Grains size variation at NZ with respect to process parameters
With the variation in plunge depth, the grain size also varies and hence, the mechanical properties. At a plunge depth of 0.03 mm, the average grain size is 23 μm shown in Fig. 10c. At a plunge depth of 0.06 mm, average grain size is lower and is 21 μm as in Fig. 10e. Finer grains were obtained in this case due to optimum heat transfer . At a plunge depth more than 0.06 mm heat generation is excessive which leads to grain growth. The design of the tool pin profile affects the grain size variation at NZ to a great extent. From the experimental results and microstructural analysis, the best tensile, flexural and microstructural properties were acquired from the SQ tool due to the unique material transportation and imposed plastic deformation as discussed in the earlier section. The grain developed at NZ of the SQ pin was compared with the STC pin based on mechanical properties obtained. The superior properties of the SQ is due to the smaller grain of 18 μm at the NZ as presented in Fig. 10j than the STC grain of 21 μm size at NZ shown in Fig. 10e. The same trend was repeated in case of welds done with parameters welding speed as well as tool rotation speed. There is declination in average grain size at NZ with a decline in welding speed and rise in tool rotation speed was noticed. It is found that the grain size variation depends on the ratio between the tool rotational speed and welding speed . At higher ratio (lower welding speed) grains are finer due to heat input per unit length is higher. At a higher ratio (high tool rotation speed) of 1100 rev/min and 1500 rev/min, finer grains were achieved, but tensile properties decreased significantly for local thinning due to flash formation. A comparison of the grain size at the NZ was established in case of 600 rev/min and 815 rev/min tool rotational speed as given in Fig. 10j, k respectively. The grains developed at 815 rev/min were 17 μm and are finer than the grains developed at 600 rev/min of 18 μm size. The grain size obtained was 17 μm at a welding speed of 22 mm/min and 15 μm at 63 mm/min as shown in Fig. 10 k, o respectively.
3.4 Fractograph analysis
In some of the experiments namely E1–2, E6–7, the fracture arose within the FSW joint area is also resembled the combination of ductile as well as brittle fracture mode. The feature of the fracture surface also includes the river like patterns and feather markings as found in Fig. 11c and its magnified view in Fig. 11d. The combination of ductile as well as brittle mode fracture starts dislocations movement at NZ. For most of the specimens with less tensile strength, a combination of ductile plus brittle mode fracture is observed at NZ which is also named as “quasi-cleavage” fracture  where voids without metallurgical bonds were also observed that lowers the strength. In some other experiments like E12–E13, the fracture occurred at TMAZ as revealed in Fig. 11e because of the variation in grain size. This type of fracture consists of intergranular fracture indicating the fracture line through the grain interface and as a result, some broken grains are also observed as indicated in the same Fig. 11e. The broken grains are not in regular shape and leads to inferior weld strength compared to E15.
FSW process is a suitable method to join AA 2014 of 6 mm thickness plate with a modified vertical milling machine with full penetration which yielded zero-defect joints by a 24 mm shoulder diameter SQ tool with 0.06 mm plunge depth at 815 rev/min tool rotational speed and 63 mm/min welding speed. The excellency of the joint is justified by the UTS and FS values which are 86% and 90% of the BM respectively. The microstructural feature is found to be the best with outstanding metallurgical characteristics contributed by the grains which are 84% finer than the BM.
Hardness profile of the FSWed specimen along the thickness represents an inverted V shape with the maximum value at NZ and the lowest at HAZ. The hardness measured at the upper zone of the joint is the highest than the middle and lower zone.
The metallurgical transformation at the NZ was revealed individually with respect to the considered process parameters by studying the grain size variation.
The present research work was supported by the Mechanical Engineering Department and the Central Instruments Facility, Indian Institute of Technology Guwahati, for availing facilities to conduct experiments and testing.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Human and animal rights
This research work was performed by human only and no involvement of any animal.
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