Microstructure evolutions

heat to the total heat generation, the side of the pin is 11%, and the tip of the pin is 3%. The experiment results also demonstrated that the temperature difference was small no matter if the tool is attached with or without the pin.

The average heat input per unit area and time according to the research of Frigaad et al.

[108] is

3 2

0 3

4

Rp

P

q = π μ ω , (18)

where q0 is the net power (W, in unit of P), μ the friction coefficient, P the pressure (Pa), w the tool rotational speed (rot/s), and Rp is the tool radius (m). Frigaad et al. [108] suggested that the rotational speed and the shoulder radius are the main process variables to the heat input. The pressure P, in practice, cannot exceed the actual flow stress of the material at the operating temperature if a sound weld without depression is to be obtained.

As for the maximum temperature during FSW, Arbegast [106] speculated an experiential equation as a function of the rotation speed (w, in unit of rpm) and forward travel speed (vf, in unit of IPM, inch per minute) given by :

a

f

m v

K w T

T ⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

= ⋅² ₄

10 , (19)

where the exponent “a” is found to range between 0.04 and 0.06, the constant K lies the between 0.65 and 0.75, and T_m (^oK) is the melting point of the alloy. The maximum temperature observed during the FSW of aluminum alloys is between 0.6 Tm and 0.9 Tm.

The FSW joint region is normally divided into a heat affected zone (HAZ), a thermomechanically affected zone (TMAZ) and a stirred zone (SZ) or dynamically recrystallized weld nugget zone (DXZ), as shown in Fig. 1-15. The part of base metal is the original metal materials, not undergoing any influence of the FSW process. The HAZ is the part that only undergoes heat influence with the occurrence of grain growth; the average grain size is usually slightly larger than the base metal. The part of the TMAZ is a region that experiences heat influence and deformation. 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 the influence of heat and high strain. The microstructure characteristics of these different zones are introduced in the following section.

1.6.4.1 Nugget zone (or stirred zone)

(A) Onion rings in the stirred zone

The nugget zone often appears the characteristic feature, the onion rings, as shown in Figs. 1-16 and 1-17. Krishnan [109] thought 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 extruding 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. As for whether only the pin tool affects the formation of onion rings in the nugget zone or not, Guerra et al. [100] thought materials near the top of the weld (approximately the upper one-third) move under the

influence of the shoulder rather than the threads of the pin.

(B) Grain structures in the stirred zone

The stirred zone usually exhibits nearly recrystallized and equiaxed grains after FSW, and it is suggested that the recrystallized grains come from the occurrence of dynamical recrystallization due to the high strain during FSP and the friction heat between the shoulder, pin and workpieces. Generally, the latter heat can provide enough energy to make the nugget zone to reach 0.6~0.8 T_m, and this temperature range will have enough energy to induce the recrystallizaiton occurrence. About the mechanism of dynamic recrystallization, Jata and Semiatin, [110] and Su et al. [111] speculated it as the mechanism of continuous dynamic recrystallizaiotn for the Al alloys. However, Rhodes et al. [112] thought as the mechanism of discontinuous dynamic recrystallizaiton. 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 [110,113,114].

Recently, Prangnell and Heason [115] conducted an experiment of “freezing” the friction stir welding process by stopping the tool and immediately quenching the work piece in an Al-2195 plate to study the formation of grain structure. They found that, at the cold periphery of the deformation zone ahead of the tool, the parent grains first split into coarse primary deformation bands, as shown in Fig. 1-18 (a). As the strain increases, the original grain boundaries and new deformation band boundaries reduce their spacing due to the geometric requirements of the strain, forming elongated fibrous grains, as shown in Fig. 1-18 (b). The similar grain microstructures were reported by Fonda et al. [116]. Further grains subdivision continues to occur on a finer scale, probably encouraged by the dominant simple shear

deformation model. With increasing temperature closer to the tool, bands of fine grains in the stirred zone are first formed, from closely spaced parallel high angle grain boundaries that develop from finer scale deformation bands, as shown in Fig. 1-18(c). This process involves local boundary migration driven by the surface tension and transverse low angle boundaries.

A mixed microstructure develops consisting of a matrix of the stirred zone grains containing high aspect ratio fibrous grains, as shown in Fig. 1-19. The bands of fine grains are forced closer together and increase in volume fraction with strain. Finally, the remaining fibrous grains fragments become unstable when they thin to subgrain dimensions and break up to form an onion-like microstructure comprised of low aspect ratio ultra-fine grains. Following welding, the ultra-fine grain nugget structure formed by this process becomes more equiaxed and coarsens slightly due to static annealing in the thermal wake of the tool.

(C) Banded structure in the stirred zone

The FSW stirred zone frequently contains the microstructure characteristic of concentric

“onion rings” in the cross sectional plane. These onion rings will be the characteristic of concentric semi-circle bands in the horizontal cross-section, as shown in Fig. 1-17. The formation of these rings has been attributed to the difference in particle density [93] or grain size [117-119], but their origin has not yet been fully clarified. Sutton et al. [117] and Yang et al. [118,119] reported that the precipitation-strengthening aluminum alloys (2024 and 2524) after FSW revealed the characteristic of banded microstructure. These banded microstructure 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, as shown in Fig.

1-20. The banded structure also affected the performance of micro-hardness and mechanical properties. For example, band A exhibited a higher value of micro-hardness and lower local-strain than band B, as shown in Fig. 1-21. In addition, they also pointed out that these

banded structure also affected the fracture behavior [120,121]. For some solid solution strengthening alloys, Chang [93] 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.

Apart from the band characteristics in the stirred zone, the FSW process could result in the worse mechanical and hardness performances in the stirred zone than those in the parent material due to dissolution of precipitation in the matrix. It is well known that the friction between the tool and the workpiece, and plastic deformation around the rotating pin will generate a high amount of energy to rise the temperature up to 0.6-0.8 Tm. And this temperature range will be sufficient to dissolve the precipitation in the original material and decrease the initial strength. Sato et al. [94,122], Mahoney et al. [123] and Genevois et al.

[124] ever reported this phenomenon for the precipitation-hardening alloys. This

phenomenon will be a disadvantage for the precipitation-strengthening alloys. In contrast, the solution-hardening alloys do not exhibit this phenomenon of strength reduction and even upgrade the strength due to grain refinement.