*3.3. Material Flow Features in the Weld Cross-Section*

The magnified micrographs in Figure 3 from the interface of the SZ and the external region illustrate a metallurgical bonding between the processed weld region and the parent metal. To distinguish the flow patterns from the grains structure, etchant reagent C was used, giving the results in Figure 4.

**Figure 4.** Photomontage microfeatures of the BFSW weld at the transverse cross-section (revealed by Reagent C). (**a**) AS border, (**b**) RS border. (1) flow-arms, (2) tunnel void, (3) finger patterns, and (4) kissing bond.

For this micro-flow evaluation, the sample of Figure 2a was chosen to reveal more details because of the existence of defects (e.g., tunnel void). As is shown in Figure 5a,b, the presence of the flow arms (banded patterns) is evident in the SZ at the hourglass borders for both sides of the weld. The flow arms of the AS are more compacted to each other.

**Figure 5.** Micrograph of the tunnel void and the microflow features around the defect at the circumferential side of the tunnel void (revealed by Reagent C). (**a**) Cascade flow bands, (**b**) hook lines, (**c**) finger pattern, (**d**) swirling lines.

The flow arms become dispersed when reaching the tunnel void (Figure 4a). In this case, the flow arms near the bottom surface of the weld follow a finger pattern, stretched from the hourglass boundary (at the circumferential edge of the tunnel void) towards the centre of the weld. It is more evident that the compaction effect from the top shoulder was higher than the bottom shoulder, while their diameters and contact surface conditions are the same. The branching pattern can also be observed when the plastic flow behaviour—between the bottom and top surfaces—is affected by the geometrical features of the tool (threads, flats, scrolls) during the stirring action. We attribute this to an unsteady internal flow, in turn caused by the inability of the flow to completely fill the void left by the tool moving forward. Behind the tool, the material moves from the RS towards the AS, and is deposited at the AS. If there are any insufficiencies in the flow, due perhaps to previous loss of material at entry, or early cooling of the material, then the forward movement of the tool creates a gap that is difficult for the flow to fill. We attribute the striation layers themselves to batches of material cut by the flats out of the base material.

The macro-section of the bobbin weld in Figure 4b reveals another distinct flow-based problem. The appearance of this linear discontinuity is similar to the kissing bond defect in CFSW; solid-state bonding with poor or no metallurgical adhesion. One of the major causes of kissing bond defects in friction stir welds is insufficient engagement of the tool pin into the plasticized material. We interpret the current findings as due to a recoil/backlash between the pin and the material during the stirring, (such as might be caused by vibration – which was prevalent), causing incomplete joining. It should be noted the kissing bond defect is more prominent in samples produced at lower rotational speeds which can be attributed to the inconsistency in strain distribution due to occurred flow-based condition. Furthermore, the position of the kissing bond defect is located exactly at the position of change in the flow direction from leading edge to the trailing edge of the pin.

It is very difficult to detect or accurately characterize the nature of this defect by the typical analysis methods, e.g., metallography or non-destructive testing (NDT). However, in Figure 4, we tried to delineate the exact location of the discontinuity line in the microstructure by Reagent C. As it is observable in Figure 4b, a continuous and uniform discontinuity flow was vertically delineated between the top surface and bottom surface through the SZ. Furthermore, this microscopic view confirms that the nature of the defect line is not a grain structure.

We propose that it is a kind of oxidation formed during the dynamic recrystallization of the deformed grains. The proposed explanation of is as follows: As the pin possesses a symmetrical thread-flats feature acting through the stirring zone, the rotating angle between the flat surface and the thread surface creates an empty space to allow air to enter into the weld region. During the stirring, which is at considerable temperature, this air causes a surface oxidation between the layers of stirred mass from the AS. Possibly the adiabatic compression of air pockets adds further heating.

The darker colour of the flow arms -visible in hourglass borders and around the tunnel void- is also attributed to this oxidation behaviour. In addition, it should be noted that this oxidized layer was formed in the sample containing the tunnel void defect, but not in defect-free samples (no tunnel void). Therefore, another flow-based relation can be assumed between the emergence of the tunnel void in the AS position, and formation of this oxidation bond at the RS proximity of the stirring zone.

While the pin-driven pressure force in the tunnel void region is insufficient to refill the position by a compensating flow, in the RS the pressure is high, which consequently squeezes the flow to the trailing edge of the tool. By revolution of the tool these oxidation bonding layers can be stretched/transferred also towards the RS. However, there is less compaction between the flow-arms at the RS and the refilling action and deposition of the stirred mass at the RS does not lead to a discontinuity at the RS.

We suggest the defects and oxidation layers have flow-based origins. Due to the clockwise rotation of the tool, the plastic flow behaviour differs in AS and RS of the SZ. As in the AS the rotation of the tool was in the direction of welding advance, the plastic deformation of the BM moves forward (and approaches the leading edge). Simultaneously in the opposite direction of the tool (RS), the extrusion nature of the process causes the plasticized flow to become squeezed through the region between the tool and base metal. This forms a channelized flow, which is transferred backwards on the trailing edge of the pin.

Similar to a kissing bond, this oxidation bonding likely causes a negative effect on mechanical properties of FSW joints. Typically, this brittle-nature defect can affect the fatigue/tensile strength of the weld as it is a stress raiser and eventually a place for initiating the fracture. It is suggested that in the presence of this oxidation layer, the crack initiation location and the crack growth would be mainly along the boundaries of this defect. This can cause a possibility for a failure mode, different to typical fracture in fusion welds.

One of the outcomes of the instability of the internal flow regimes is the emergence of tunnel void. Figure 5 shows micrographs of the AS region to evaluate the flow behaviour around the tunnel void defect.

The corresponding flow arms around the tunnel void and in proximity of AS border are shown in Figure 6 which reveals an intersecting layout from the top surface stretching downwards. The striation diagonal lines around the bay-shaped tunnel geometry (Figure 5) show the simultaneous effects of the pin (thread marks) and shoulders on the plastic deformation. The reason for the tunnel defect being at the bottom surface can be attributed to the action of the right-handed threads on the pin, which pump material upwards. There is also a mass deficit caused by loss of material at edge-entry. This mass deficit persists as a continuous void on the bottom surface.

**Figure 6.** Interlaced flow patterns at the AS border of SZ as the following of the cascade microflow (Reagent C).

Direction of the scrolls compared with the tool rotation provides a circulation towards the centre – this is deliberate and is intended to provide a dynamic sealing of the weld. In an ideal condition, threads and flats on the pin and scrolls on shoulders improve the uniformity of stirring by driving the lateral motion and pumping of material inwards, this reduces spilling [30]. To provide a uniform stirring condition in the sub-shoulder area, the spiral scrolled features started from the edge of the shoulders and ended at the proximity of the pin location. Upward pumping of material (driven by the thread effect) also increases the curvature of hourglass border close to the top shoulder. This shows the compaction of the plasticized flow in this region is more than by the bottom shoulder region.

This upwards flow potentially creates an additional frictional contact between the upper shoulder and the workpiece. If so, we would expect that the temperature on the top shoulder to be higher than that of the bottom shoulder. Consideration of grain sizes suggests that this indeed the case [21]. This may further contribute to cooling and stiffening of the flow at the bottom surface.

The aim of Figure 7 is to clarify the role of the pin features (threads/flats) and the welding parameters (ω/*V* ratio; rev/mm) in occurrence of the tunnel void. As is shown in Figures 5 and 6, a collection of striation lines accumulated at the AS border. These can be attributed to the simultaneous interaction of the pin threads/flats and the speed ratio (ω/*V*) to form the flow lines pattern. As a rough measure, the speed ratio by rev/mm is a unit of magnitude of the distance between the flow lines which can form the size of each nib in a saw-tooth like pattern (Figure 5) with the same distance between the flow arms. However, the flow complexities cause an interlaced condition for the flow arms which makes it unreliable as an accurate measurement. The pin threads/flats can cause the localization of deformation through the narrow shear zone to form this saw-tooth like pattern at the internal/inward bay of tunnel defect (Figures 5 and 7d).

**Figure 7.** Microflows, tangled in different region of the cascade flow region of the SZ (Reagent C). (**a**) swirling line (region d in Figure 5), (**b**) higher magnification of (**a**); hair-line micro-crack, (**c**) magnified finger pattern and the wavy flow lines (region c in Figure 5), and (**d**) hook line flow at the jagged edge of the tunnel void (region b in Figure 5).

It is clear that the tunnel void geometry emerged due to a large lack of bonding, presumably due to the influence of insufficient pushing of material flow during the stirring. Hence, the tunnel void formation is attributed to insufficiency in compaction of the material flow at the position of the defect. This indicates that for a good weld the material must have good flow forced by the tool geometry and process variables.

The typical periodic striation lines with the finite width at the border of the AS show the typical forging zone around the pin. These flow lines form a projected area at the circumferential side of the tunnel void where the bonding patterns interlaced abruptly. Our interpretation is that, as the bonding lines are separating, the flow velocity gradually decreases which eventually comes to a sharp decrease at the interface of the tunnel defect. This can also change the orientation of the material flow lines, as the interval of the flow lines is maximally separated (compared to the compacted banding pattern in the proximity of the AS border in Figure 6).

By comparing the lack of bonding defect with the characteristics of a proper joint geometry, the main reason for formation of the tunnel void appears to be insufficient frictional heat and integrity of material flow, due to the internal force and rotating speed during the process. Poor material flow is attributed to an insufficient heat input, which leads to more bonding defects. Also, the compression ratio is not enough to create a consistent forging/pressure force to extrude the plastic mass and fully/decently compensate/refill the defect position.

The curved striation lines in Figure 7a (region d in Figure 6) elucidate these flow-based problems where the connection of the flow lines with the main regime has been disrupted. This leads to formation of a tangled flow around the saw-tooth like pattern at the circumferential side of the tunnel void. As shown in Figure 7b, this tangled flow is a suitable place for stress concentration within the layers of the mass flow, in which may cause microcracking. The propagation of microcracks could eventually lead to the failure of the weld by coalescence of the macrosize voids. The interlaced flow-lines in

Figure 7c show a stretched finger pattern (region c in Figure 6) which was evident before in Figure 5a. This flow defect, similar to the curved flow pattern shown in Figure 7a (semi-circular swirling band), can affect the flow integrity as a suitable position for imitation of the micro-crack.

Another flow feature, the enlarged micro-flow shown in Figure 7d, reveals the hook line flow patterns positioned at the internal edge of the tunnel void. We interpret this feature as a lack of material consolidation during stirring, similar to the root flaw in CFSW.

From a metallurgical viewpoint, there is some debate about the nature of the flow-arms as the elongated flow bands with a different colour compared to the matrix. They are not metallographic stains or over-etching. Nor are they some type of intrinsic defects of the base metal formed during the rolling procedure. Rather they are specifically associated with the tunnel defect, and visible across multiple different welds. The literature generally terms them flow lines, i.e. attributes them to internal flow of the material. To further understand the nature of the flow-arms we applied elemental mapping via EDS to determine the composition of the texture. To observe the exact position of the flow-pattern, the samples after etching were observed by Backscattering Electron (BSE) imaging via SEM microscopy, also supported by the elemental mapping via the energy-dispersive X-ray spectrometer (EDS) detector. The scanning analysis was done for the flow-arms region, also the selected location of the hair-line micro-crack, both shown in Figure 8.

**Figure 8.** Analysis of the flow-arms region (region 1, Figure 4a) using (**a**) BSE, and (**b**) EDS elemental map, and similarly for (**c**,**d**) hair-line micro-crack position (demonstrated in Figure 7b). The oxygen-rich areas of the EDS elemental map have been delineated in dark (**b**,**d**).

In general, the BSE imaging can be used to show the different elements present in the sample. The microscopic analysis of the flow-arms region in Figure 8a,b confirms that the elongated flow-arms

are rich in oxygen. This region is delineated as the darker area at the OM etched samples. Additionally, the EDS analysis of the location of the hair-line micro-crack shows that the edge of the crack is rich in oxygen (Figure 8c,d).

A further SEM study for the Flow-arms pattern in Figure 4b were demonstrated in Figure 9. The scanned area in Figure 9a belongs to the tip of the one of flow-arm branched at the RS of the weld region. The bunch of elongated grains in Figure 9a all belong to one flow line pattern in Figure 4b. The scanned area reveals that the elongated grains are distinctly separated from each other by main grain boundaries; however there are some sub-grain details inside of the grains. The higher magnification of the inside of the grain (Figure 9b) shows that there is a high density of the sub-grain boundaries within the elongated grain. This is a main thermomechanical characteristic of dynamic recrystallization, suggesting that the shearing within the texture can activate the sub-grain boundaries during the re-cooling process after stirring. It should be noticed that the nature of the sub-grain boundaries can be analysed further by EBSD and TEM techniques, which is beyond the scope of the current work. From this result it is concluded that the presence of the shearing during the stirring induced a stored strain within the compacted deposited layers of the plasticized mass at the back of the tool. This activated the formation of the microscopic features at the hourglass border, which in RS is revealed as the sub-grain boundaries, appearing as the elongated flow patterns similar to the oxidation layers at the AS. The high density of the sub-grain boundaries can cause a darker band during the etching, delineating a flow-arm shaped pattern similar to the oxidation patterns at the AS of the hourglass border.

**Figure 9.** SEM analysis of the flow arm shaped branches at the RS. (**a**) A bunch of elongated grains at the position of the flow pattern, (**b**) higher magnification of the inside of the grain, representative of the high density of the sub-grain boundaries as a response to the stored strained during the DRX.
