Sound FSW butt joints of AA6061 T651 specimens were obtained in this research work. This shows the distinctive capabilities of the FSW process. In FSW, the welding of plates is accomplished by generated heat due to rotation of tool on AA6061 T651 plates and plastic deformation. The generated heat is consumed in softening the plates. This facilitates the flow of material. During FSW, the material is shifted from AS to the RS at the front of the tool. Material is moved from RS to AS at rear end of the tool. When the material moves from AS, it forms an opening. When the tool rotates, the material from RS fills the opening created in the AS. This occurs if the amount of material moved to AS from RS is less than the amount of material removed from AS. If heat generated is less, plasticization of material will slow; this will lead to reduced flow of material. Result will be defect in the stir zone. Otherwise, if surplus heat is generated, turbulent flow of materials takes place. This leads to defects. Therefore, the optimal heat generation is required to attain the sound joint. Same is the opinion of Humphreys and Hatherly (2004). In spite of the optimal heat generated, the material is to be stirred by the pin shape. This is necessary to have sound joints. The area in touch with pin shoulder is the top part of stir section. This faced the effects such as heat generation and material flow. These are exclusively produced by rubbing of tool. Stirred zone comprises fine and equiaxed grains for both situations of FSW. Fine recrystallized zone at weld nugget is because of substantial plastic deformation. This is followed by dynamic recrystallization because of thermo-mechanical processing.
The SZ in AA6061 T651 has recrystallized grains with substantially minor grain size in contrast to BM. The grains on SZ of AA6061 T651 are considerably finer, in spite of the bigger original grain size of AA6061 T651. This is due to the happening of heavy plastic deformation and robust dynamic recrystallization on the AA6061 T651. Similar were the observations of Murr, Liu, and Mcclure (1998). Al with FCC structure has additional slip planes present for distortion than Mg with HCP structure. This increases the tendency of Al to plastically deform. Hence, substantial grain modification witnessed in SZ of AA6061 T651 can be ascribed to major plastic deformation and subsequent heat input in AA6061 T651. Recrystallized microstructure in SZ would arise from a dynamic recovery (DRV) and a dynamic recrystallization (DRX). In rigorously deformed microstructure, subgrains are made by DRV and they create grains with HAGB in the process of DRX [Su, Nelson, & Sterling, 2005].The microstructures of the plates consists of coarse grains, a sufficient number of HAGBs, and large number of precipitates such as Mg2Si (for AA6061 T651). Hence, continuous grain growth (CGG) of dynamically recrystallized grains in stirred zone on AS is started. These grains are somewhat coarsened after plastic deformation. Reason is static annealing during weld cooling cycle. Distinctive boundary is there amid stirred zone and HAZ on the AS alike to results stated in previous articles [Kumar, Yuan, & Mishra, 2015; Venkateswarlu, Nageswararao, Mahapatra, Harsha, & Mandal, 2015; Threadgill, Leonard, & Shercliff, 2009]. This is in difference with RS of weld joint. The boundary is further diffusive and rather unclear. Hence, the two zones cannot be easily differentiated. This occurs because strain rate and temperature gradients are much sharper on AS than that on RS. Shear plastic deformation in AA6061 T651 takes place within a lesser time. Reasons are the torsion and circumventing velocity fields with opposed directions in AA6061 T651.
Surface roughness helps in deciding surface integrity. Roughness also helps in identifying function of surface. This is because an important share of material failure initiates at surface. It may be due to either the discontinuity or deterioration of the quality of the surface. Surface finish also plays a significant role in corrosion resistance. Surface finish improves performance and reduces costs of life cycle of component. At the interface between the AA6061-T651 joints, onion rings are seen. Space between the layers in onion ring structure is equivalent to advancing motion of tool in single rotation. Therefore, it can be concluded that reduction in surface roughness of FSW joints has a significant role in governing the quality of FSW joints. Flow of material on AS is unlike from the flow on RS. AA6061-T651 on the RS certainly not go in rotational zone nearby the pin. The reason is that the material on the AS forms fluidized bed nearby pin and revolves around it. In transition zone, AA6061-T651 movement takes place mainly on the RS. This phenomenon is also supported by Li, Murr, and McClure (1999). There is no flash on the RS. This is possibly owing to deficiency of heat generation triggered by decrease of surface roughness. Therefore, flexibility of AA6061-T651 reduces. Hence, it becomes difficult to extrude below the shoulder space. Width of the HAZ reduces steadily with the reduction in plate surface roughness. Reduction in FSW joint surface roughness leads to fall in the heat generation. Hence, less heat is transferred to HAZ region. This results in decrease in width of HAZ.
Amount of least FSW joint surface roughness shows the important influences on grain refinement. On one side, maximum grain size was attained with extreme FSW joint surface roughness. While on the other side, minimum grain size was attained with least FSW joint surface roughness. Therefore, it is inferred that grain size in NZ is reduced, when amount of AA6061-T651 surface roughness is reduced. Hirata et al. (2007) stated that grain size in NZ was reduced, when flow of friction heat was reduced. Therefore, the decrease in amount of plate surface roughness triggered the reduction in heat generation. This resulted in additional grain refinement.
Intermetallic formation, boundary energy, precipitate formation, and strain hardening of FSW joint affect the microhardness of joint. Extra hardness in SZ of FSW joints is due to the increase of grain boundaries and fine grains. Grain size is inversely proportional to hardness and strength. Hence, formation of fine recrystallized grains leads to increase in hardness in SZ. Similar results were also obtained by Guven and Cam et al. (2014). During welding, heat generated in SZ is transferred to neighboring regions (TMAZ and HAZ). For heat treatable AA6061 T651, strength and hardness mostly depend on availability and distribution of precipitates. Availability and distribution of precipitates in matrix are controlled by prevalent thermal conditions. Precipitation sequence of Al-Mg-Si 6xxx alloys is generally termed to be solid solution →GP→ß”→ß’→ß (Mg2Si). During solutionization, precipitates are dissolved in matrix and form super saturated solid solution upon cooling. Further aging leads to precipitation of a secondary phase which reinforces strength of aluminum alloy [Gallais, Denquin, Bréchet, & Lapasset, 2008].
Because AA6061 T651 is heat treatable Al alloy, hardness is largely ascribed to existence of precipitates. Thermal cycle in TMAZ region makes dissolution of strengthening precipitates, which shows decreased hardness in retreating and advancing side in the Table 5. This region is softer since solute additions trapped in second phases dissolve back into solid solution. It can be also said that heating and cooling thermal condition exists in TMAZ, making precipitation dissolve in matrix. According to Cam et al. (2014), a hardness reduction usually occurs in weld region of AA6061-T6 joints due to solution and/or coarsening of strengthening particles within TMAZ region and overaging in HAZ region. Scialpi, De Giorgi, De Filippis, Nobile, and Panella (2008) stated that HAZ has a peak temperature; therefore, results show decrease in hardness. The hardness of AA6061 T651 primarily is influenced by size and amount of precipitate particles. Maximum particles are mixed in TMAZ, and precipitates are partially mixed in stirred zone. Average grain size of TMAZ on AS was higher than that of SZ, whereas microhardness in SZ shows only a small enhancement as compared to that of TMAZ on AS. This is primarily because of existence of many precipitate particles in TMAZ. This outcome is in line with findings of Uzun, Dalle Donne, Argagnotto, Ghidini, and Gambaro (2005).
Kim, Fujii, Tsumura, Komazaki, and Nakata (2006) stated that at constant rotational speed, mechanical opposition of joints increases with rise of travel speed. This is due to decreased heat input. Therefore, this research work considered less heat generation due to low welding speed. It is observed from Table 6 that UTS of all the 08 samples of FSW joints is less than the AA6061-T651 (BM). This is because of two reasons. First reason is influence of dissolved precipitates formed in FSW process. This results in decrease of tensile strength of joints. Second reason is robust tendency for intergranular cracking. This is as a consequence of the combination of weak grain boundaries and concentration of grain boundary stress. Therefore, cracks can propagate speedily along grain boundary. Hence, strength of the BM is higher than strength of welded joints. Ipekoglu and Cam et al. (2014) found that tensile strength of FSW joints were further less than lower strength of AA6061-T6 BM. Reason for this is coarsening of strengthening particles within DXZ and TMAZ regions and overaging in HAZ region. Increase in speed from 710 to 1400 rpm (at constant translation speed of 16 mm/min) linearly increases UTS from 0.25 to 0.43 kN/mm2, whereas increase in speed from 710 to 1400 rpm (at constant translation speed of 20 mm/min) linearly increases UTS from 0.37 to 0.44 kN/mm2. Increase in rotary speed increases heat input per unit weld length monotonically which leads to better bonding. This is the reason for increases in UTS with rise in speed. No significant change is observed in UTS with increase of translation speed from 16 to 20 mm/min. Alike results were conveyed by Sharma, Dheerendra, and Pradeep (2012).