PSI - Issue 17

Madhav Raturi et al. / Procedia Structural Integrity 17 (2019) 495–502

498

Madhav Raturi et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussion

Friction stir welds are acknowledged as heterogeneous where different regions like base, mixed zone, interfaces can be easily spotted. Concentration of stress and hence failure is conventionally to occur in the weaker zone. The different weld zones, their shape and size within the nugget zone of dissimilar friction stir welds obtained using different combination of process parameters are shown in Fig. 2 and Fig. 3. The intermittent threads with flat faces in TIF pin profile resulted in augmented flow of material, thorough mixing and heterogeneous interlocking in the nugget zone as compared to TPZ pin profile (refer Fig. 2 and Fig. 3). Also, the width of the stir zone (SZ) was comparatively smaller at the bottom for the welded samples obtained using TPZ pin profile due to its taper along the pin length, thus reducing the fine grain portion. The SZ width reduces more steeply for welds obtained using TPZ pin profile creating more of a tapered shape SZ and for the joint made with TIF pin, the pot shaped (inflated bottom) nugget was observed. The representative stress-strain diagram for the AA6061-AA2014 FSW joint prepared with TIF tool pin of one sample each for four different process conditions are shown in Fig. 4(a). The comparison of UTS and impact toughness of the specimens welded with different process conditions (refer Table. 1) is represented in Fig. 4(c). Among the welds obtained using TIF pin profile, the maximum tensile UTS of the joint was observed for tool rotational speed of 1200 rpm and welding speed of 98 mm/min whereas, the minimum UTS was noted for the combination of 900 rpm and 36 mm/min. The relatively higher tool rotational speed with low welding speed can cause excessive heat accumulation, thereby causing softening and poor friction leads to slipping of plasticized material. Also at high heat input metallurgical changes like grain coarsening, coarsening of strengthening precipitates may also take place which adversely affect the UTS of the joint.

Fig. 2. Macrographs of weld cross section of AA6061-AA2014 FSW joint obtained using TIF pin profile at (a) 660 rpm, 63 mm/min; (b) 900 rpm, 36 mm/min; (c) 900 rpm, 63 mm/min; (d) 1200 rpm, 98 mm/min.

The tensile test of the samples showed that the specimen failed from the vicinity of the HAZ region on the AS (AA6061-T6) for all conditions except one obtained for TPZ pin profile with parameter setting of 1200 rpm tool rotation and 36 mm/min welding speed. The material on the AS (AA6061-T6) is relatively softer with lower strength than the material on the RS (AA2014-T6), thereby led to failure from the AS side. Additionally, the HAZ that undergoes the thermal cycle without strain/deformation and are observed to be softer than neighboring TMAZ region. Also, the accumulation, clustering and coarsening of Mg 2 Si precipitates can also take place near the advancing side HAZ. The magnitude of coarsening and clustering will depend on the whirling during plastic deformation which is process parameter dependent. The balanced heat input and suited metallurgical combinations of secondary phase reinforcing particles of Mg 2 Si and Al-Cu made the SZ intact from any fracture. For joints obtained with TPZ tool pin, the maximum tensile UTS was noted for joint prepared with tool rotation 660 rpm and 63 mm/min welding speed. The representative stress-strain plot of each one sample prepared with TPZ tool pin is shown in Fig. 4(b). It may be noted from Fig 4(b) that although lower UTS was obtained for joint prepared with TPZ tool pin at 660 rpm, tool rotation, 36 mm/min welding speed (W5) but the highest elongation was noted for this sample. As relatively low tool rotational and welding speed causes softening of weld nugget which reduces the strength but specimen breaks at comparatively higher deformation/elongation.