Results and discussion

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arranged by polishing and etching with Keller’s and Nital etchant for both aluminum and mild steel. Optical microscopy (OM; A1m Axio Imager, Carl Zeiss, Germany) with a load of 0.49 N for 10 seconds was then carried on the cross-section. For the estimation of mechanical properties of the joint, Vickers hardness was determined along the cross-section using a fully standardized Vickers Microhardness tester (A-1170, Leica, Germany).

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Figure 2.3. Z force response for travel speed of (a) 50 mm/min and (b) 75 mm/min.

Figure 2.4 Torque response for travel speed of (a) 50 mm/min and (b) 75 mm/min

Figure 2.5 Process response (a) Z-force, (b) Torque acquired from FSW machine

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Cz and Np correspond to the torque and the tool rotation speed (in rpm). The energy input values calculated using the above equation are listed in Table 2.3. The material flow and micrographs have been studied for four combinations of process parameters. However, to discuss the metallurgical aspects compactly, the remaining part of the manuscript will discuss the results of 1000/50 and 1500/50 combinations only.

Boosting the tool rotation speed from 800 to 1000 rpm maintains a constant weld speed of 75 mm/min together, the axial force and spindle torque reduce drastically. The force and torque descriptions documented during the FSW exhibit that the frictional heat increases enormously with the intensification in tool rotation speed at the constant weld speed; this impacts the plasticization of the materials. From the process responses in Fig. 2.9, the heat input through FSW can be approximated using the relation stated above.

2.3.2. Material flow behavior

The material flow behavior for the process parameter combination of 1000/50 was studied by optical microscopy (Fig. 2.10 (a)), and a schematic of the material flow is portrayed in Fig. 2.10 (b). In the material flow schematic (Fig. 2.10 (b)), the material flow of 409M FSS (in red) is indicated by yellow arrows, and green arrows indicate the material flow of LNASS (in blue). Fig. 2.10 (a) shows that the shoulder influence area is restricted to the top of the weld cross-section. This area promotes the transport of the FSS material from the retreating side to the advancing side of the joint. The quantity of FSS dragged from the retreating side to the advancing side of the joint diminished drastically under the tool pin. In the region under the tool pin, the bulk of the LNASS was extruded from advancing side and moved down to the lower part of the pin-influenced area due to the lower rotational speed and the lower resulting energy input.

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The bulk LNASS mixed with the FSS, creating a semi-elliptical banded zone with relatively higher hardness values. However, when the materials push upwards under the tool pin, the elongated LNASS (depicted by green arrows) hinders the material flow to the upper region of the joint. The LNASS has a higher torque and force response when exposed to lower energy input and less extensive plasticization; therefore, the sliding condition is a dominant factor that restricts the material flow to the upper region. The bulk LNASS, which was extruded from the advancing side and mixed with the FSS, remained in the middle regions of the stir zone and formed a complex structure. However, vortex flow was

seen just below the shoulder zone as marked by I in Fig. 2.10 (a) and as magnified in Fig. 2.10 (c). The result of EPMA mapping of the manganese in region II in Fig. 2.4 (a) validated the above description of the material flow as shown in Fig. 2.10 (d). It should be kept in mind that the 409M FSS and the LNASS was chosen in the present study have quite different manganese contents. The LNASS was squeezed out from the advancing side, as depicted by Figure 2.6.

Nevertheless, the LNASS cannot step upward due to the lesser energy input. Thus, the FSS was caught in the bulk LNASS, creating a blended zone with a vortex pattern.

Figs. 2.7 (a) and (b) exhibit the optical microscopy and the schematic of material flow for the 1500/50 combination in turn. For 1500/50 combinations, typical material flow behavior was observed. Detailed observation of regions III and IV reveals that the flow characteristics in these zones are distinct from those of regions I and II in Fig. 2.10. The optical microscopy in Fig.

2.9 (a) and the schematic of material flow in Fig. 2.9 (b) show that the extent of vortex flow increased considerably in region III contrasted to an area I for the 1000/50 combination. EPMA mapping of manganese for the area I showed that the LNASS and FSS materials were almost homogeneously mixed in the intermixing

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Figure 2.6 (a) Material movement, (b) representation, (c) enlarged view of region (I), and (d) EPMA of region (II) for Manganese of LN ASS and 409M FSS dissimilar joint for the 1000/50

combination of parameter.

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Figure 2.7 (a) Material movement, (b) representation, (c) enlarged view of region (III), and (d) EPMA of region (IV) for Manganese of LN ASS and 409M FSS dissimilar joint for the 1500/50

combination of parameter

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the region, as clearly identified in Fig. 2.9 (d). It is worth remembering that the heat input for the 1500/50 combination was 351.3 kJ, whereas, for the 1000/50 combination, it was only 189.4 kJ.

The different material mixing behaviors for the 1500/50 and 1000/50 combinations may be clarified by the greater heat input of the 1500/50 combination, which enhanced the plasticization of the two different ferrous alloys. It is hypothesized that the improved plasticization of the tougher extruded LNASS permitted further mixing of the softer FSS. Hence, identical quantities of mixing and vortex flow patterns in the advancing side were detected in the pin governed region of the 1500/50 combination. Vortex flow shapes of the swirl activities of the materials were also explored with the growing plasticity of LNASS, specifically in region III, as displayed in Fig. 2.9 (b).

The material flow in the FSW joint between mild steel and aluminum was analyzed for both FSW parameter arrangements, 800 rpm - 75mm/min and 1000 rpm - 75 mm/min, applying a differential etching contrast method and OM, as presented in Figs. 2.6 and 2.7. The bright and dark zones correspond to the aluminum alloy and the mild steel respectively inside the figures.

For the FSW parameter combinations, the OM of the cross-sections reveals that the shoulder influenced area (SIF) is limited to the top part of the SZ. In contrast, the pin controlled the bottom part of the SZ (the pin-influenced area: PIF). In the SZ, the material flow occurred along the horizontal and vertical paths, as indicated by the blue arrows. Material flow in the SIF suggests the transmission of aluminum alloy as of the retreating side towards the advancing side alongside the horizontal path. Inside the PIF, the material movement is nearly the contrary. The steel as of the advancing side was extruded and penetrated the aluminum alloy in the retreating side, demonstrating an elongated band structure in the bottom part of the SZ.

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Figure 2.8 (a) Material movement path and weld cross-section for 800/75 combination of parameters (b),(c)& (d) are magnified zones in advancing, central and retreating side as marked

in the weld cross section

Figure 2.9 (a) Material flow path and weld cross-section for 1000/75 combination of parameters (b), (c) & (d) are magnified zones in advancing, central and retreating sides is marked in the

weld cross-section

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Figure 2.10 (a) SEM image of the midpoint of SIF with 1000 rpm / 75 mm/min and the consequences of EDS elemental scan of the region presented in (a): (b) aluminum and (c) iron;

(d) SEM image of the center of PIF with 1000 rpm /75 mm/min and the results of EDS elemental scan of the region shown in (d): (e) aluminum and (f) iron

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In Figs. 2.6 (b)-(d) and Figs. 2.7 (b)-(d) intermixing of the steel and the aluminum alloy with the creation of lamellar patterns inside the SIF is clearly observed, as presented in the magnified views. The findings of the EDS elemental scan for the SIF and PIF with 1000 rpm / 75 mm/min (Fig. 4.5) validate the observation. This indicates that a certain amount of the steel extruded to the retreating side was again re-transferred to the advancing side. While the material flow paths for both FSW parameter combinations are generally related, the higher heat input by the parameter sequence with 1000 rpm spontaneously plasticizes the materials to a higher degree.

This causes, a greater volume of aluminum alloy transferring from the retreating side and a wider SIF for the parameter combination with 1000 rpm was generated inside the SZ.

2.3.3. Optical microscopy

The optical micrographs of the stir zone and thermomechanically affected zone (TMAZ) were noticed for the 1000/50 and 1500/50 combinations, as displayed in Figs. 2.11 and 2.12.

Very fine equiaxed ferrite grains with austenite were observed in the upper region of the stir zone in the FSS side (Figs. 2.11 (b) and 2.12 (b)). The TMAZ/stir zone border in the advancing side (Figs. 2.11 (a) and 2.12 (a)) showed a semi-elliptical layered mixed mode of patterns, which mainly consisted of ferrite and austenite, likely with some percentage of martensite for most of the cases. It is fascinating to note that a better degree of grain refinement was noticed in the FSS side stir zone for all joining situations than that observed on the LNASS side.

.

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Figure 2.11 SZ and TMAZ of 1000/50 combination of parameter (a) ASS and (b) FSS side

Figure 2.12 SZ and TMAZ of 1500/50 combination of parameter (a) ASS and (b) FSS side

30 2.3.4. EBSD assessment

EBSD study was carried out in case of 1000/50 and 1500/50 parameter groupings for better understanding the dynamic recrystallization and phase transformation behavior. Inverse pole figure maps of the base LNASS and the base 409M FSS perpendicular to the welding direction (WD) are shown in Figs. 2.13 (a) and (b). In the LNASS, the misorientation distribution in Fig. 2.13 (a) (ii) reveals that about 48% of the high-angle boundaries (HABs) display the first-order twin coincidence-site lattice (CSL) orientation of Σ3 order . The amount of low angle boundaries (fLABs) with misorientation angles between 2^o and 15^o was very less in the LNASS (11%), while the 409M FSS in Fig. 2.13 (b) (ii) included a substantial quantity of fLABs

(42%). LNASS and the 409M FSS average grain sizes of were 37 and 10 μm, respectively.

EBSD data related to the advancing side and the retreating side of the stir zone and the center of the stir zone, indicated SZ_AS, SZ_C, and SZ_RS, respectively, are displayed for the 1000/50 and 1500/50 blends in the form of phase maps perpendicular to WD in Fig. 2.14 Hybrid structures of ferrite and austenite were clearly observed for both 1000/ 50 and 1500/50 combinations in all three different regions, SZ-AS, SZ-C, and SZ-RS. {111} pole figures of the austenite and ferrite at SZ-AS, SZ-C, and SZ-RS are presented in Fig. 2.15 with individuals of the base LNASS and 409M FSS. The base LNASS has a stronger {111} <110> orientation while the base 409M FSS has a strong {110} <111> orientation. {h k l} is marked as the crystallographic plane normal to WD, and <u v w> is denoted to be a crystallographic direction parallel to WD. The texture components in SZ-AS, SZ-CN, and SZ-RS are distinct from those of the base metals for mutually austenite and ferrite phases. The various textures indicate that the materials underwent acute deformation during FSW [Field DP et al, Park SH et al, Sato YS et al].

The textures in SZ-AS, SZ-C, and SZ-RS are consistently possessing a strong simple shear

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texture, particularly in ferrite for the 1500/50 combination. These textures initiated by the FSW process are quite related to the shear texture initiated by simple shear deformation of bcc materials. In the case of 1000/50 combination, fLAB of austenite in SZ (26%) is larger than that in LNASS (11%), and the grain size of austenite in SZ (2 μm) is considerably smaller as compared to LNASS (37 μm) as displayed in Figs. 2.16 (a) and (b). The fLAB of ferrite in SZ (47%) is somewhat larger than that in 409M FSS (42%), and the grain size of the ferrite in SZ (2 μm) is smaller than that in 409M FSS (10 μm), as shown in Figs. 2.16 (a) and b). LABs can be developed by dynamic recrystallization or recovery by rearrangement of deformation-induced space of lattice dislocations. The findings indicate that dynamic recrystallization and recovery happened as a result of the high deformation and heat generation throughout the FSW method for the 1000/50 combination conditions. In the case, 1500/50 parameter set, the fLAB of austenite and ferrite in SZ are bigger as compared to the LNASS and 409M FSS base metals, respectively; in addition grain sizes of the austenite and ferrite in SZ are meaningfully smaller than those of the LNASS and 409M FSS base metals, respectively, as displayed in Figs. 2.16 (a) and (b).

Therefore, dynamic recrystallization and recovery may have also occurred for the 1500/50 combination. In comparison with the 1500/50 and the 1000/50 combinations, the fLAB of austenite in SZ for the 1500/50 combination (51%) is almost two times higher than that for the 1000/50 combination (26%), as shown in Fig. 2.16 (a). Also, the grain size of austenite in SZ for the 1500/50 combination (13 μm) is more than six times larger than that for the 1000/50 combination (2 μm) (Fig. 2.16 (b)). In the case of ferrite, a higher fLAB of 51% with larger grain size (8 μm in SZ) is observed for the 1500/50 as compared to the 1000/50 combination (Figs.

2.16 (a) and (b)). From these results, it can be proposed that higher heat generation and more severe deformation at the higher tool rotational speed of 1500 rpm induced additional grain

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growth with dynamic recrystallization and recovery. It is interesting to note that the degree of grain refinement in ferrite is remarkably higher than that of austenite, especially in the 1500/50 combination, as shown in Fig. 2.16 (b). This is probably because the stacking fault energy (SFE) of ferritic SS is larger than that of austenitic SS [Giossop BA et al, Bain EC et al]. It is well known that the partial dislocations become closer with increasing SFE and may readily recombine to facilitate cross-slip with the enhancement of dislocation cell formation [Giossop BA et al, Clark B et al]. As a result, ferritic SS, i.e., the 409M FSS in the present study, which has high SFE, may experience a strong continuous dynamic recrystallization, resulting in a very fine microstructure.

The pre-existing annealing twins within the LNASS rotated away from the ideal CSL orientation relationship (Fig. 2.11(c)) for both the 1000/50 and 1500/50 combinations, and the originally straight coherent twin boundaries were converted to general HABs. Such twin

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Figure 2.13 (i) Inverse pole figure maps corresponding to welding path (WD) and (ii) misorientation-angle distributions of base metal: (a) LNASS and(b) 409M FSS, respectively

Figure 2.14 Phase maps (austenite in green, ferrite in red) corresponding to welding path (WD) of the (a) 1000/50 and (b) 1500/50 combinations; For the 1000/50 and 1500/50 combinations, the

center point of the observation areas was about 370 and 870 μm away from the welded surface, respectively

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Figure 2.15 Pole figures of the 1000/50 and 1500/50 combinations at the regions marked as SZ (AS), SZ (RS) and SZ (center) in Fig. 2.14

Figure 2.16. (a) The fraction of low angle boundary (LAB), (b) grain size, and (c) the fraction of coincidence-site lattice (CSL) in austenite as a function of the distance from the weld center line

for the 1000/50 and 1500/50 combinations

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boundary distortion seems to be a result of complex interactions of twin boundaries with slip dislocations [Mironov s et al]. Hardness is an indication of tensile strength. The microhardness plot across the weld in Fig. 2.17 suggests that the average hardness in SZ is comparatively higher than that of both base metals. The average hardness in SZ is the highest for the 1000/50 combination among the process parameter combinations selected in the present study. Note that fLAB in both austenite and ferrite in SZ is clearly higher for the 1500/50 combination than that of the 1000/50, which suggests that the material underwent a higher degree of dynamic recovery and recrystallization under the 1500/50 combination than under the 1000/50 combination. Also, the grain size of both austenite and ferrite in SZ for the 1500/50 combination is larger than that for the 1000/50 combination, as shown in Fig. 2.16 (b). Therefore, the hardness in SZ for the 1500/50 combination is expected to be lower than that for the 1000/50 combination, as shown in Fig. 2.17.

The hardness value of around 400 Hv clearly indicates the formation of a martensite and mixed-mode microstructure. This is validated by the results of tensile tests that all the joints failed from the softer FSS base metal (not shown). A few scattered points with very high hardness were observed in the TMAZ, and these are probably due to hardened precipitations.

The martensite increase in the 1000/50 combination may be explained by a higher cooling rate induced by a lower heat input in comparison with the 1500/50 combination.

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Figure 2.17 Microhardness plot of the FSW joints

Figure 2.18 Vickers microhardness results of weld cross sections for 800/75 and 1000/75 parameters along line 1(b), 2 (c) & 3 (d)

37 2.4 Conclusion

Butt joints of LNASS and 409M FSS were successfully made by FSW. All the samples failed from the FSS base metal within the range of energy input selected in the present study.

EPMA mapping of the results of the 1000/50 and 1500/ 50 combinations reveals that the material movement pattern during FSW depends on the process parameter combination.

Dynamic recrystallization and recovery were observed in the FSW joints fabricated in the present study. Among the 1000/50 and 1500/50 combinations, additional grain growth in the stir zone was observed for the 1500/50 combinations due to the higher heat generation and more severe deformation compared to the lower tool rotational speed in the 1000/50 combination.

FSS with high SFE may affect the continuous dynamic recrystallization, resulting in a very fine microstructure.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (NO. NRF-2015R1A5A1037627). This research was also supported by the Ministry of Trade, Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT) through the Encouragement Program for The Industries of Economic Cooperation Region.

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