Characteristics of friction stir welding

1.5.2.1 Microstructure of welding zone in friction stir welding

Based on the microstructural characterization of grains and precipitates in the FSW joint region, three distinct zones, the nugget zone (NZ) or termed as the stir zone (SZ) or dynamically recrystallized weld zone (DXZ), the thermo-mechanically affected zone (TMAZ), and the heat-affected zone (HAZ), have been identified as shown in Fig. 1-6. The part of base metal (BM) is the original metal materials, not undergoing any influence of welding process. The region between BM and TMAZ is HAZ. This zone experiences a thermal cycle with the occurrence of grain growth, but does not undergo any plastic deformation. The HAZ retains the same grain structure as the base metal. Beyond the nugget zone there is TMAZ which experiences both temperature and deformation during FSW. A typical micrograph of TMAZ is shown in Fig. 1-7 [89]. The TMAZ is characterized by a highly deformed structure. The base metal elongated grains were deformed in an upward flowing pattern around the nugget zone. Although the TMAZ underwent plastic deformation, recrystallization did not occur in this zone due to insufficient deformation strain. The part of DXZ is in the center location of welding zone. This DXZ has an apparent characteristic of fully recrystallized and equiaxed grains due to intense plastic deformation and frictional heating during FSW.

Generally, from the relation of the rotational direction of the pin tool and the direction of moving forward, the welding zone can be classified into the advancing side and retreating side, as shown in Fig. 1-8. The advancing side is on the position of the same direction of the rotational direction of the pin tool and forward direction. On the contrary, the retreating side is on the other position of the opposite direction of the rotation direction and forward direction.

In general, the difference of the transition region between DXZ and TMAZ is apparent

in aluminum alloys. However, there is no obvious difference between TMAZ and DXZ in the magnesium alloy. The microstructure in the TMAZ of Al alloy results from an insufficient heating temperature or strain for dynamic recrystallization during FSW. Mg alloys may experience dynamic recrystallization more easily than Al alloys, because Mg alloys have lower recrystallization temperature (about 523 K) than that of Al alloys [90]. Therefore, the deformed microstructure just outside the stir zone may be also dynamically recrystallized in Mg alloy. This may be the reason why the transition region between DXZ and TMAZ has roughly same microstructure as the nugget zone [91].

1.5.2.2 Recrystallization mechanisms

The fully recrystallized and equiaxed grains in DXZ comes from the occurrence of dynamical recrystallization due to the high strain and friction heat during the stirring process.

Generally, the friction heat between the shoulder, pin and workpieces can provide enough energy to make the nugget zone to reach 0.6~0.8 Tm, and this temperature range is higher than the common recrystallization temperature (~0.5 Tm).

There were some possible mechanisms proposed for dynamic recrystallization process during FSW, such as discontinuous dynamic recrystallization (DDRX) [92-94] and continuous dynamic recrystallization (CDRX) [95,96]. The DDRX is characterized by nucleation of new grains at old high-angle boundaries and gross grain boundary migration [97]. On the other hand, mechanisms of CDRX have been proposed whereby subgrains rotate and achieve a high misorientation angle with little boundary migration. For example, mechanisms include subgrain growth [98], lattice rotation associated with sliding [99,100], and lattice rotation associated with slip [101].

Jata and Semiatin [95] were the first to propose CDRX as operative dynamic nucleation mechanism during FSW. They suggested that low-angle boundaries in the parent metal are replaced by high-angle boundaries in the nugget zone by means of a continuous rotation of the original low-angle boundaries during FSW. In their model, dislocation glide gives rise to a gradual relative rotation of adjacent subgrains. Su et al. [96] conducted a detailed microstructural investigation of FSW 7050Al-T651. Based on microstructural observations, they suggested that the dynamic recrystallization in the nugget zone can be considered a CDRX on the basis of dynamic recovery. Subgrain growth associated with absorption of dislocation into the boundaries is the CDRX mechanism. Repeated absorption of dislocations into subgrain boundaries is the dominant mechanism for increasing the misorientation between adjacent subgrains during the CDRX.

Alternatively, DDRX has been recently proposed as an operative mechanism for dynamic nucleation process during FSW. Su et al. [94] reported generation of recrystallized grains of 0.1 μm in a FSP 7075Al by means of rapid cooling behind the tool. Similarly, Rhodes et al. [92] obtained recrystallized grains of 25–100 nm in FSP 7050Al-T76 by using

‘‘plunge and extract’’ technique and rapid cooling. These recrystallized grains were significantly smaller than the pre-existing subgrains in the parent alloy, and identified as non-equilibrium in nature, predominantly high-angled, relatively dislocation-free. Thus, Rhodes et al. [92] and Su et al. [94] proposed that DDRX mechanism is responsible for the nanostructure evolution.

Although it is still a debating issue, the phenomenon of the nugget zone experiencing severe deformation and elevated temperature to result in fully recrystallized and equiaxed grain is of no doubt. Moreover, it is worth mentioning that this nugget zone exhibits a high fraction of high misorientation angles [95,102,103].

1.5.2.3 Onion rings in nugget zone

Besides the fully recrystallized and equiaxed grains with a high fraction of high misorientation angle, the onion rings are the most prominent features of nugget zone, as shown in Figs. 1-9 and 1-10. Krishnan [104] reported that the appearance of onion rings is attributed to a geometrical effect in that a transverse cross-section through a stack of semicylinders would appear like onion rings with ring spacing being wider at center and narrower towards the edge. He also thought that the formation of the onion rings is due to the process of friction heating due to the rotation of the tool and the forward movement extrudes the metal around to the retreating side of the tool. He qualitatively proved that the ring spacing would decrease with increasing rotation speed and decreasing advancing speed.

Mahoney et al. [105] reported that the onion ring structure within the stir zone is characterized by alternating bands of different grain size. Material flow probably leads to repetitive introduction of strain and strain rate into the material, which probably produces an alternation of three-dimensional ellipsoids with different grain sizes in the stir zone, as shown in Fig. 1-11. The alternation of the ellipsoids with different grain sizes generates an onion ring structure on the cross section perpendicular to the advancing direction. Park et al. [106]

suggested that the formation of the elliptical nugget and that of the onion ring structure have their origin in shear deformation arising from the rotation of the threaded pin.

Yang et al. [107,108] reported that the precipitation-strengthening aluminum alloys (2024 and 2524) after FSW revealed the characteristic of banded microstructure. This bands consisted of band A containing a higher density of secondary particles and smaller grains and band B containing a lower density of secondary particles and larger grains. These banded

structure also affected performance of micro-hardness and mechanical characterization. For example, band A exhibited a higher value of micro-hardness and lower local-strain than band B, as shown in Fig. 1-12. In addition, they also pointed that these banded structure also affected the fracture behavior [109,110]. For some solid solution strengthening alloy, Chang [111] also found the similar but not apparent banded structure in the AZ31 Mg alloy after FSP.

This similar banded structure was comprised by bands with larger grains and bands with smaller grains. The information of the formation of banded structure is limited and still needed to explore.

1.5.2.4 Materials flow behavior in nugget zone

The material flow during friction stir welding is quite complex depending on the tool geometry, process parameters and materials. Reynolds and coworkers [112,113] investigated the material flow behavior using a marker insert technique (MIT). Based on the observations, they suggested that the friction stir welding process can be roughly described as an in situ extrusion process wherein the tool shoulder, the pin, the weld backing plate, and cold base metal outside the weld zone form an ‘‘extrusion chamber’’ which moves relative to the workpiece. They concluded that the extrusion around the pin combined with the stirring action at the top of the weld created within the pin diameter a secondary, vertical, circular motion around the longitudinal axis of the weld.

In the study of the material flow of FSW 6061Al by means of a faying surface tracer and a pin frozen in place at the end of welding, Guerra et al. [114] reported that the material was moved around the pin in FSW by two processes. First, material on the advancing side front of a weld entered into a zone that rotates and advances simultaneously with the pin. The material in this zone was very highly deformed and sloughed off behind the pin in arc shaped

features. Second, material on the retreating front side of the pin extruded between the rotational zone and the parent metal and in the wake of the weld fills in between material sloughed off from the rotational zone. Further, they pointed out that material near the top of the weld (approximately the upper one-third) moved under the influence of the shoulder rather than the threads on the pin.

In the study of the material flow behavior during FSW of aluminum alloys by means of steel shot tracer technique and ‘‘stop action’’ technique, Colligan [115] suggested that not all the material in the tool path was actually stirred and rather a large amount of the material was simply extruded around the retreating side of the welding tool pin and deposited behind. Murr and co-workers [116,117] investigated the solid-state flow visualization in friction stir butt welding of 2024Al to 6061Al and copper to 6061Al. The material flow was described as a chaotic–dynamic intercalation microstructures consisting of vortex-like and swirl features.

Recently, Arbegast [118] suggested that the resultant microstructure and metal flow features of a friction stir weld closely resemble hot worked microstructure of typical aluminum extrusion and forging. Therefore, the FSW process can be modeled as a metalworking process in terms of five conventional metal working zones: (a) preheat, (b) initial deformation, (c) extrusion, (d) forging, and (e) post heat/cool down, as shown in Fig.

1-13. It is widely accepted that material flow within the weld during FSW is very complex and still poorly understood. It has been suggested by some researchers that FSW can be generally described as an in situ extrusion process and the stirring and mixing of material occurred only at the surface layer of the weld adjacent to the rotating shoulder.

1.5.2.5 Hardness variation in the weld zone

FSPed materials often possess equiaxed finer grains in the nugget zone. According to Hall-Petch relationship, finer grain size has higher hardness value. However, a number of investigations demonstrated that the change in hardness in the friction stir welds is different.

Not all hardness profiles of FSPed materials follow the Hall-Petch relationship [119]. This is because of the different precipitation phenomenon. Aluminum alloys generally are classified into heat-treatable (precipitation-hardenable) alloys and non heat-treatable (solid-solution-hardened) alloys. FSW creates a softened region around the weld center in a number of precipitation-hardened aluminum alloys [116,120-124]. It was suggested that such a softening is caused by coarsening and dissolution of strengthening precipitates, such as Mg2Si or MgZn2, during the thermal cycle of the FSW. In the study of the hardness profiles associated with the microstructure in an FSW 6063Al-T5, Sato et al. [122] reported that hardness profile was strongly affected by precipitate distribution rather than grain size in the weld. In general, the mechanical properties of the FSWed precipitation-hardened alloys have been found to strongly depend on the volume fraction, size, and distribution of the strengthening precipitates and slightly on the grain size.

For the solution-hardened aluminum alloys, generally, FSW does not result in softening in the welds [125,126]. For 5083Al-O containing small particles, the hardness profile was roughly homogenous in the weld [126], whereas for 1080Al-O without any second-phase particles, the hardness in the nugget zone was slightly greater than that in the base material.

Sato et al. [126] reported that FSW created the fine recrystallized grains in the nugget zone and recovered grains in the TMAZ in 5083Al-O with the nugget zone and the TMAZ having slightly higher dislocation densities than the base material. Both small and large Al6(Mn,Fe) particles were detected in the nugget zone and the base material. They concluded

that the hardness profile could not be explained by the Hall–Petch relationship, but rather by the Orowan strengthening, namely, the hardness profile in the FSW 5083Al was mainly governed by the dispersion strengthening due to distribution of small particles. In this case, the interparticle spacing is likely to be much lower than the grain size.

It appears that excellent welds with no strength loss or other degradation of properties will occur in the FSW of aluminum or magnesium alloys where no reprecipitation or related aging/annealing effects occur. Therefore, the increase in hardness in the nugget zone of these materials can be explained by the Hall-Petch relationship, such as 1080Al and AZ31 Mg alloy [111]. The average hardness value in the nugget zone increases with decreasing grain size in the nugget zone.